Cytogenetics

Published on 27/03/2015 by admin

Last modified 22/04/2025

Print this page

This article have been viewed 5369 times

Chapter 76 Cytogenetics

Clinical cytogenetics is the study of chromosomes: their structure, function, inheritance, and abnormalities. Chromosome abnormalities are very common and occur in approximately 1-2% of live births, 5% of stillbirths, and ~50% of early fetal losses in the 1st trimester of pregnancy. Chromosome abnormalities are more common among persons with mental retardation, and they have a significant role in the development of some neoplasias.

Chromosome analyses are indicated in persons presenting with multiple congenital anomalies, dysmorphic features, and/or mental retardation. The specific indications for studies include advanced maternal age (>35 yr) or multiple abnormalities on fetal ultrasound (prenatal testing), multiple congenital anomalies, unexplained growth retardation in the fetus or postnatal problems in growth and development, ambiguous genitalia, unexplained mental retardation with or without associated anatomic abnormalities, primary amenorrhea or infertility, recurrent miscarriages (≥3) or prior history of stillbirths and neonatal deaths, a 1st-degree relative with a known or suspected structural chromosome abnormality, clinical findings consistent with a known anomaly, some malignancies, and chromosome breakage syndromes (Bloom syndrome, Fanconi anemia).

76.1 Methods of Chromosome Analysis

Carlos A. Bacino, Brendan Lee

Cytogenetic studies are usually performed on peripheral blood lymphocytes, although cultured fibroblasts may also be used. Prenatal (fetal) chromosome studies are performed with cells obtained from the amniotic fluid, chorionic villus tissue, and fetal blood or, in the case of preimplantation diagnosis, by analysis of a blastomere. Cytogenetic studies of bone marrow have an important role in tumor surveillance, particularly among patients with leukemia. These are useful to determine induction of remission and success of therapy or, in some cases, the occurrence of relapses.

Chromosome anomalies include abnormalities of number and structure and are the result of errors during cell division. There are 2 types of cell division: mitosis, which occurs in most somatic cells, and meiosis, which is limited to the germ cells. In mitosis, 2 genetically identical daughter cells are produced from a single parent cell. DNA duplication has already occurred during interphase in the S phase of the cell cycle (DNA synthesis). Therefore, at the beginning of mitosis the chromosomes consist of 2 double DNA strands joined together at the centromere known as sister chromatids. Mitosis can be divided into 4 stages: prophase, metaphase, anaphase, and telophase. Prophase is characterized by condensation of the DNA. Also during prophase, the nuclear membrane and the nucleolus disappear and the mitotic spindle forms. In metaphase, the chromosomes are maximally compacted and are clearly visible as distinct structures. The chromosomes align at the center of the cell and spindle fibers connect to the centromere of each chromosome and extend to centrioles at the 2 poles of the mitotic figure. In anaphase, the chromosomes divide along their longitudinal axes to form 2 daughter chromatids, which then migrate to opposite poles of the cell. Telophase is characterized by formation of 2 new nuclear membranes and nucleoli, duplication of the centrioles, and cytoplasmic cleavage to form the 2 daughter cells.

Meiosis begins in the female oocyte during fetal life and is completed years to decades later. In males, it begins in a particular spermatogonial cell sometime between adolescence and adult life and is completed in a few days. Meiosis is preceded by DNA replication so that at the outset each of the 46 chromosomes consists of 2 chromatids. In meiosis, a diploid cell (2n = 46 chromosomes) divides to form haploid cells (n = 23 chromosomes). Meiosis consists of 2 major rounds of cell division. In meiosis I, each of the homologous chromosomes pair precisely so that genetic recombination, involving exchange between 2 DNA strands (crossing over), can occur. This results in a reshuffling of the genetic information on the recombined chromosomes and allows further genetic diversity. Each daughter cell then receives 1 of each of the 23 homologous chromosomes. In oogenesis, 1 of the daughter cells receives most of the cytoplasm and becomes the egg, whereas the other smaller cell becomes the 1st polar body. Meiosis II is similar to a mitotic division but without a preceding round of DNA duplication (replication). Each of the 23 chromosomes divides longitudinally, and the homologous chromatids migrate to opposite poles of the cell. This produces 4 spermatogonia in males, or an egg cell and a 2nd polar body in females, each with a haploid (n = 23) set of chromosomes. Consequently, meiosis fulfills 2 crucial roles: It reduces the chromosome number from diploid (46) to haploid (23) so that upon fertilization a diploid number is restored, and it allows genetic recombination.

Two errors of cell division commonly occur during meiosis or mitosis, and either can result in an abnormal number of chromosomes. The 1st is nondisjunction, in which 2 chromosomes fail to separate during meiosis and thus migrate together into 1 of the new cells, producing 1 cell with 2 copies of the chromosome and another with no copy. The 2nd is anaphase lag, in which a chromatid or chromosome is lost during mitosis because it fails to move quickly enough during anaphase to become incorporated into 1 of the new daughter cells (Fig. 76-1).

Figure 76-1 Generation of mosaicism. A, Postzygotic nondisjunction in an initially normal conceptus. In this example, 1 cell line (monosomic 21) is subsequently lost, with the final karyotype 46,N/47,+21. B, Postzygotic nondisjunction in an initially 46,XX conceptus, resulting in 45,X/46,XX/47,XXX mosaicism. C, Postzygotic anaphase lag in an initially 47,+21 conceptus.

(From Gardner RJM, Sutherland GR: Chromosome abnormalities and genetic counseling, ed 3, New York, 2003, Oxford University Press, p 33.)

For chromosome analysis, cells are cultured (for varying periods depending on cell type), with or without stimulation, and then artificially arrested in mitosis during metaphase (or prometaphase), later on subjected to a hypotonic solution to allow proper dispersion of the chromosomes for analysis, fixed, banded, and finally stained. The most commonly used banding and staining method is the GTG banding (G-bands trypsin Giemsa) also known as G banding, which produces a unique combination of dark (G-positive) and light (G-negative) bands that permits recognition of all individual 23 chromosome pairs for analysis.

Other banding techniques such as Q-banding using quinacrine, reverse (R-banding) using acridine orange, and C-banding (constitutive heterchromatin) using barium hydroxide are available for use in certain circumstances but are losing ground to molecular technologies. Metaphase chromosome spreads are 1st evaluated microscopically, and then their images are photographed or captured by a video camera and stored on a computer to be later analyzed. Humans have 46 chromosomes or 23 pairs, which are classified as autosomes for chromosomes 1 to 22, and the sex chromosomes, often referred as sex complement: XX for females and XY for males. The homologous chromosomes from a metaphase spread can then be paired and arranged systematically to assemble a karyotype according to well-defined standard conventions like those established by International System for Human Cytogenetic Nomenclature (ISCN), with chromosome 1 being the largest and 22 the smallest. According to nomenclature, the description of the karyotype includes the total number of chromosomes followed by the sex chromosome constitution. A normal karyotype is 46,XX for females and 46,XY for males (Fig. 76-2). Abnormalities are noted after the sex chromosome complement.

Figure 76-2 Karyotype of a normal male at the 550-600 band level. The longer the chromosomes are captured at metaphase or sometimes prometaphase, the more bands that can be visualized.

Although the internationally accepted system for human chromosome classification relies largely on the length and banding pattern of each chromosome, the position of the centromere relative to the ends of the chromosome also is a useful distinguishing feature (Fig. 76-3). The centromere divides the chromosome in 2, with the short arm designated as the p arm and the long arm designated as the q arm. A plus or minus sign before the number of a chromosome indicate that there is an extra or missing chromosome, respectively. Table 76-1 lists some of the abbreviations used for the descriptions of chromosomes and their abnormalities. A metaphase chromosome spread usually shows 450-550 bands. Prophase and prometaphase chromosomes are longer, are less condensed, and often show 550-850 bands. High-resolution analysis is useful for detecting subtle chromosome abnormalities that might otherwise go unrecognized.

Figure 76-3 Example of different chromosome types according to the position of the centromere. On the left is a chromosome 1 pair with the centromere equidistant from the short and long arm (also known as metacentric). In the center is a chromosome 11 pair that is submetacentric. On the right is a chromosome 13 pair that is an example of an acrocentric chromosome. Acrocentric chromosomes contain a very small short arm, stalks, and satellite DNA. The black arrow indicates the position of the centromere. The blue arrow shows the long arm of a chromosome. The red arrow shows the short arm of a chromosome. The green arrow highlights the satellite region, which is made of DNA repeats. The light area between the short arm and the satellite is known as the stalk.

Table 76-1 SOME ABBREVIATIONS USED FOR DESCRIPTION OF CHROMOSOMES AND THEIR ABNORMALITIES

Molecular techniques such as fluorescence in situ hybridization (FISH) and comparative array genomic hybridization studies (conventional CGH and array CGH [aCGH]) have filled a significant void for the diagnosing cryptic chromosomal abnormalities. These techniques identify subtle abnormalities that are often below the resolution of standard cytogenetic studies. FISH is used to identify the presence, absence, or rearrangement of specific DNA segments and is performed with gene- or region-specific DNA probes. Several FISH probes are used in the clinical setting: unique sequence or single-copy probes, repetitive-sequence probes (alpha satellites in the pericentromeric regions), and multiple-copy probes (chromosome specific or painting). FISH involves using a unique known DNA sequence or probe labeled with a fluorescent dye that is complementary to the studied region of disease interest. The labeled probe is exposed to the DNA on a microscope slide, typically metaphase or interphase chromosomal DNA, that has been previously treated (denatured) to allow the DNA to become single stranded and to permit hybridization. When the probe pairs with its complementary DNA sequence, it can be then visualized by fluorescence microscopy (Fig. 76-4). In metaphase chromosome spreads, the exact chromosomal location of each probe copy can be documented and often the number of copies (deletions, duplications) of the DNA sequence as well, if they are not too close to each other; whereas in interphase cells, only the number of copies of a particular DNA segment can be determined. When the interrogated segments (as in genomic duplications) are close together, only interphase cells can accurately determine the presence of 2 or more copies or signals. In metaphase cells, some duplications might falsely appear as a single signal.

Figure 76-4 Fluorescence in situ hybridization (FISH) involves denaturation of double-stranded DNA as present in metaphase chromosomes or interphase nuclei on cytogenetic slide preparations (A) into single-stranded DNA (B). The slide-bound (in situ) DNA is then renatured or reannealed in the presence of excess copies of a single-stranded, fluorochrome-labeled DNA base-pair sequence or probe (C). The probe anneals or “hybridizes” to sites of complementary DNA sequence (D) within the chromosomal genome. Probe signal is visualized and imaged on the chromosome by fluorescent microscopy.

(From Lin RL, Cherry AM, Bangs CD, et al: FISHing for answers: the use of molecular cytogenetic techniques in adolescent medicine practice. In Hyme HE, Greydanus D, editors: Genetic disorders in adolescents: state of the art reviews. Adolescent medicine, Philadelphia, 2002, Hanley and Belfus, pp 305–313.)

Metaphase and interphase FISH are particularly useful for detecting very small deletions that might escape notice with G-band analysis. In most cases the probe used for identification is used in conjunction with a control probe with a known location nearby the region studied. This allows correct identification of the hybridized signal to the right chromosome, and in some cases identification to the rearranged chromosome. With high-resolution chromosome analysis it is very difficult to recognize deletions of <5 million bp (5 Mb); FISH can reliably detect deletions as small as 50 to 200 kb of DNA. This has allowed the clinical characterization of a number of microdeletion syndromes. In addition to gene- or locus-specific probes, complex mixtures of DNA from a chromosome arm or an entire chromosome are available for fluorescence staining of large chromosome sections or entire chromosomes. The probe mixtures are referred to as chromosome paints (Fig. 76-5A and B). Other probes hybridize to repetitive sequences located to the pericentromeric regions. These probes are useful for the rapid identification of certain trisomies in interphase cells of blood smears, or even in the rapid analysis of prenatal samples from cells obtained through amniocentesis. Such probes are available for chromosomes 13, 18, and 21 and for the sex pair X and Y (Fig. 76-5C and D).

Figure 76-5 A, Fluorescence in situ hybridization (FISH) analysis of interphase peripheral blood cells from a patient with Down syndrome using a chromosome 21-specific probe. The 3 red signals mark the presence of 3 chromosomes 21. B, FISH analysis of a metaphase chromosome spread from a clinically normal individual using a whole-chromosome paint specific for chromosome 5. Both chromosomes 5 are completely labeled (yellow) along their entire length. C, Fluorescence in situ hybridization studies (FISH) on metaphase cells using a unique sequence probe that hybridizes to the elastin gene on chromosome 7q11.23, inside the Williams syndrome critical region. The elastin probe is labeled in red, and a control probe on chromosome 7 is labeled in green. The left image shows normal hybridization to chromosome 7, with 2 signals for the elastin region and 2 for the control probe. The right image shows a normal chromosome on the right with control and elastin signals, and a deleted chromosome 7 on the left, evidenced by a single signal for the control probe. This image corresponds to a patient with a Williams syndrome region deletion. D, FISH in interphase cells using DNA probes that hybridize to repetitive α-satellite sequences in the pericentromeric region for the sex chromosomes. Left, interphase cells with 2 signals, 1 labeled in red for the X chromosome and green for the Y chromosome, consistent with a normal male chromosome complement. Right, interphase cell showing 2 red signals for the X chromosome, compatible with a normal female chromosome complement.

Spectral karyotyping (SKY) and multicolor FISH (M-FISH) are similar molecular cytogenetic techniques that use 24 different chromosome painting probes and 5 fluorochromes to simultaneously visualize every chromosome in a metaphase spread. Each of the 24 different chromosome paints is labeled with a different combination of the 5 fluorescent dyes, which emit at different wavelengths. Each of the 22 autosomes and the X and Y chromosomes has its own unique spectrum of wavelengths of fluorescence. Special filters, cameras, and image-processing software are required to identify each chromosome. SKY and M-FISH are especially useful for identifying the complex chromosome rearrangements found in many tumors. This technique requires very special and costly equipment and is being displaced by comparative aCGH.

Comparative genomic hybridization (CGH) is a molecular-based technique that involves differentially labeling the patient’s DNA with a fluorescent dye (green) and a normal reference DNA with another fluorescent dye (red) (Fig. 76-6). Equal amounts of the two-label DNA samples are mixed and then used as a painting probe for FISH with normal metaphase chromosomes. The ratio of green : red fluorescence is measured along each chromosome. Regions of amplification of the patient’s DNA display an excess of green fluorescence, and regions of loss show excess red fluorescence. If the patient’s and the control DNA are equally represented, the green : red ratio is 1 : 1 and the chromosomes appear yellow.

Figure 76-6 An example of a cryptic microdeletion at a translocation breakpoint of an apparently balanced translocation in a patient with DD and growth defect (with permission from the author and the publisher). A, Partial karyotype shows t(15;22)(q26.1;q11.2). B, FISH with clones 2O19 (green) and 354M14 (red) at 15q26.1; arrows indicate signals only present on the normal chromosome 15, suggesting a deletion on the der(15). C, Two-color aCGH with dye swap with 244 K oligo probes; arrowhead indicates a 3.3-Mb deletion at chromosome 15q26.1-q26.2, arrow points to the close-up view of the deletion.

(From Li MM, Andersson HC: Clinical application of microarray-based molecular cytogenetics: an emerging new era of genomic medicine, J Pediatr 155:311–317, 2009.)

A modified version of this technology, aCGH, uses DNA spotted onto a slide or microarray grid. In this case, instead of metaphase chromosomes, segments of DNA are represented by BACs (bacterial artificial chromosomes) and PACs (P1 artificial chromosomes) or oligonucleotides (short DNA segments of varied size) distributed in a microarray that resembles the chromosomes in a metaphase. The detection could reach the size of the BACs, PACs, and oligonucleotides and ranges in resolution from approximately 50 to 200 kb (even smaller if using cosmids or shorter polymerase chain reaction (PCR) products used as probes).

There are many advantages of aCGH. They can test all critical disease causing regions in the genome at once; FISH requires the clinical knowledge and tests only 1 area at a time. aCGH can detect duplications and deletions not currently recognized as recurrent disease-causing regions probed by FISH. aCGH can detect single and contiguous gene deletion syndromes. aCGH does not always require cell culture to generate sufficient DNA, something that may be important in the context of prenatal testing because of timing. There are disadvantages to aCGH: It does not detect balanced translocations, inversions, or very low-levels of mosaicism.

There are 2 different types of aCGH, targeted and whole-genome arrays. aCGH is the equivalent of doing thousands of FISH experiments in a single sample at once. Targeted aCGH is an effective and efficient technique for detecting clinically known cryptic chromosomal aberrations, which are typically associated with known disease phenotypes; many of these arrays have expanded detection to areas potentially susceptible to recurring deletion or duplication. Whole-genome array targets the entire genome. The advantage of this latter technique is that it allows better and denser coverage of the entire genome in evenly spaced portions; its disadvantage is that interpretation of deletions or duplications may be difficult if it involves areas not previously known to be involved in disease.

There are many copy number variations (CNVs) causing deletion or duplication in the human genome. Thus, most detected genetic abnormalities, unless associated with very well known clinical phenotypes, require parental investigations because a detected CNV that is inherited might turn out to be an incidental polymorphic variant. A de novo abnormality (i.e., 1 found only in the child and not the parents) is often more significant if it is associated with an abnormal phenotype and if it involves genes with important functions. aCGH is a very valuable technology alone or when combined with FISH and conventional chromosome studies (Fig. 76-7).

Figure 76-7 Array CGH in a female patient with Down syndrome. Each black dot represents a piece of DNA segment specific for different chromosome location. Most of the dots displayed between the 0.0 and 0.2 axis are considered within normal range. Exceptions are often due to polymorphic variations. A group of dots colored in green clusters on chromosome 21 and above 0.5. Those represent a gain in copy number of DNA segments for chromosome 21 as seen in Down syndrome and consistent with trisomy 21.

Bibliography

ACOG Practice Bulletin. Screening for fetal chromosomal abnormalities. Obstet Gynecol. 2007;109:217-227.

Bejjani BA, Saleki R, Ballif BC, et al. Use of targeted array-based CGH for the clinical diagnosis of chromosomal imbalance: is less more? Am J Med Genet. 2005;134A:259-267.

Li MM, Andersson HC. Clinical application of microarray-based molecular cytogenetics: an emerging new era of genomic medicine. J Pediatr. 2009;155:311-317.

Saugier-Veber P, Girard-Lemaire F, Rudolf G, et al. Genetic compensation in a human genomic disorder. N Engl J Med. 2009;360:1211-1216.

76.2 Down Syndrome and Other Abnormalities of Chromosome Number

Karen Summar, Brendan Lee

Aneuploidy and Polyploidy

Human cells contain a multiple of 23 chromosomes (n = 23). A haploid cell (n) has 23 chromosomes (typically in the ovum or sperm). If a cell’s chromosomes are an exact multiple of 23 (46, 69, 92 in humans), those cells are referred to as euploid. Polyploid cells are euploid cells with more than the normal diploid number of 46 (2n) chromosomes: 3n, 4n. Polyploid conceptions are usually not viable, but the presence of mosaicism with a karyotypically normal line can allow survival. Mosaicism is an abnormality defined as the presence of 2 or more cell lines in a single individual. Polyploidy is a common abnormality seen in 1st-trimester pregnancy losses. Triploid cells are those with 3 haploid sets of chromosomes (3n) and are only viable in a mosaic form. Triploid infants can be liveborn but do not survive long. Triploidy is often the result of fertilization of an egg by 2 sperm (dispermy). Failure of 1 of the meiotic divisions, resulting in a diploid egg or sperm, can also result in triploidy. The phenotype of a triploid conception depends on the origin of the extra chromosome set. If the extra set is of paternal origin, it results in a partial hydatidiform mole with poor embryonic development, but triploid conceptions that have an extra set of maternal chromosomes results in severe embryonic retardation with a small fibrotic placenta that is typically spontaneously aborted.

Abnormal cells that do not contain a multiple of haploid number of chromosomes are termed aneuploid cells. Aneuploidy is the most common and clinically significant type of human chromosome abnormality, occurring in at least 3-4% of all clinically recognized pregnancies. Monosomies occur when only 1, instead of the normal 2, of a given chromosome is present in an otherwise diploid cell. In humans, most autosomal monosomies appear to be lethal early in development, and survival is possible in mosaic forms or by means of chromosome rescue (restoration of the normal number by duplication of single monosomic chromosome). An exception to this rule is monosomy for the X chromosome (45,X), seen in Turner syndrome; it has been estimated that the majority of 45,X conceptuses are lost early in pregnancy for as yet unexplained reasons.

The most common cause of aneuploidy is nondisjunction, the failure of chromosomes to disjoin normally during meiosis (see Fig. 76-1). Nondisjunction can occur during meiosis I or II or during mitosis. After meiotic nondisjunction, the resulting gamete either lacks a chromosome or has 2 copies instead of 1 normal copy, resulting in a monosomic or trisomic zygote, respectively.

Trisomy is characterized by the presence of 3 chromosomes, instead of the normal 2, of any particular chromosome. Trisomy is the most common form of aneuploidy. Trisomy can occur in all cells or it may be mosaic. Most individuals with trisomy exhibit a consistent and specific phenotype depending on the chromosome involved.

FISH is a technique that can be used for rapid diagnosis in the prenatal detection of common fetal aneuploidies including chromosomes 13, 18, and 21 as well as sex chromosomes (see Fig. 76-5C and D). The most common numerical abnormalities in liveborn children include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and sex chromosomal aneuploidies: Turner syndrome (usually 45,X), Klinefelter syndrome (47,XXY), 47,XXX, and 47,XYY, By far the most common type of trisomy in liveborn infants is trisomy 21 (47,XX,+21 or 47,XY,+21). Trisomy 18 and trisomy 13 are relatively less common and are associated with a characteristic set of congenital anomalies and severe mental retardation (Table 76-2). The occurrence of trisomy 21 and other trisomies increases with advanced maternal age (≥35 yr). Owing to this increased risk, women who are ≥35 yr at the time of delivery should be offered genetic counseling and prenatal diagnosis (including serum screening, ultrasonography, and amniocentesis or chorionic villus sampling; Chapter 90).

Table 76-2 CHROMOSOMAL TRISOMIES AND THEIR CLINICAL FINDINGS

SYNDROME	INCIDENCE	CLINICAL MANIFESTATIONS
Trisomy 13, Patau syndrome	1/10,000 births	Cleft lip often midline; flexed fingers with postaxial polydactyly; ocular hypotelorism, bulbous nose; low-set, malformed ears; microcephaly; cerebral malformation, especially holoprosencephaly; microphthalmia, cardiac malformations; scalp defects; hypoplastic or absent ribs; visceral and genital anomalies Early lethality in most cases, with a median survival of 7 days; 91% die by 1 year

Trisomy 18, Edwards syndrome 1/6,000 births

Low birthweight, closed fists with index finger overlapping the 3rd digit and the 5th digit overlapping the 4th, narrow hips with limited abduction, short sternum, rocker-bottom feet, microcephaly, prominent occiput, micrognathia, cardiac and renal malformations, and mental retardation

95% of children die in the 1st year

Trisomy 8, mosaicism 1/20,000 births Long face, high prominent forehead, wide upturned nose, thick everted lower lip, microretrognathia, low-set ears, high arched, sometimes cleft palate; osteoarticular anomalies common (camptodactyly of 2nd to 5th digits, small patella); deep plantar and palmar creases; moderate mental retardation

Down Syndrome

Trisomy 21 is the most common genetic cause of moderate mental retardation. The incidence of Down syndrome in live births is approximately 1 in 733; the incidence at conception is more than twice that rate; the difference is accounted by early pregnancy losses. In addition to cognitive impairment, Down syndrome is associated with congenital anomalies and characteristic dysmorphic features (Figs. 76-8 and 76-9; Table 76-3). Although there is variability in the clinical features, the constellation of phenotypic features is fairly consistent and permits clinical recognition of trisomy 21. Affected individuals are more prone to congenital heart defects (50%) such as atrioventricular septal defects, ventricular septal defects, isolated secundum atrial septal defects, patent ductus arteriosus, and tetralogy of Fallot. Congenital and acquired gastrointestinal anomalies and hypothyroidism are common (Table 76-4). Other abnormalities include megakaryoblastic leukemia, immune dysfunction, diabetes mellitus, and problems with hearing and vision (see Table 76-4). Alzheimer disease–like dementia is a known complication that occurs as early as the 4th decade and has an incidence 2-3 times higher than sporadic Alzheimer disease. Most males with Down syndrome are sterile, but some females have been able to reproduce, with a 50% chance of having trisomy 21 pregnancies. Two genes (DYRK1A, DSCR1) in the putative critical region of chromosome 21 may be targets for therapy.

Figure 76-8 A, Face of a child with Down syndrome. B, Karyotype of a male with trisomy 21 as seen in Down syndrome. This karyotype reveals 47 chromosomes instead of 46, with an extra chromosome in pair 21.

Figure 76-9 Prehensile foot in a 1-mo-old child.

(From Wiedemann HR, Kunze J, Dibbern H: Atlas of clinical syndromes: a visual guide to diagnosis, ed 3, St Louis, 1989, Mosby.)

Table 76-3 CLINICAL FEATURES OF DOWN SYNDROME IN THE NEONATAL PERIOD

CENTRAL NERVOUS SYSTEM

Hypotonia *

Developmental delay

Poor Moro reflex *

CRANIOFACIAL

Brachycephaly with flat occiput

Flat face *

Upward slanted palpebral fissures *

Epicanthal folds

Speckled irises (Brushfield spots)

Three fontanels

Delayed fontanel closure

Frontal sinus and midfacial hypoplasia

Mild microcephaly

Short hard palate

Small nose, flat nasal bridge

Protruding tongue, open mouth

Small dysplastic ears *

CARDIOVASCULAR

Endocardial Cushing defects

Ventricular septal defect

Atrial septal defect

Patent ductus arteriosus

Aberrant subclavian artery

Pulmonary hypertension

MUSCULOSKELETAL

Joint hyperflexibility *

Short neck, redundant skin *

Short metacarpals and phalanges

Short 5th digit with clinodactyly *

Single transverse palmar creases *

Wide gap between 1st and 2nd toes

Pelvic dysplasia *

Short sternum

Two sternal manubrium ossification centers

GASTROINTESTINAL

Duodenal atresia

Annular pancreas

Tracheoesophageal fistula

Hirschsprung disease

Imperforate anus

CUTANEOUS

Cutis marmorta

* Hall’s criteria to aid in diagnosis.

Table 76-4 ADDITIONAL FEATURES OF DOWN SYNDROME THAT CAN DEVELOP OR BECOME SYMPTOMATIC WITH TIME

NEUROPSYCHIATRIC

Developmental delay

Seizures

Autism spectrum disorders

Behavioral disorders (disruptive)

Depression

Alzheimer disease

SENSORY

Congenital or acquired hearing loss

Serous otitis media

Refractive errors (myopia)

Congenital or acquired cataracts

CARDIOVASCULAR

Acquired mitral, tricuspid, or aortic valve regurgitation

Endocarditis

MUSCULOSKELETAL

Atlantoaxial instability

Hip dysplasia

Slipped capital femoral epiphyses

Avascular hip necrosis

Recurrent joint dislocations (shoulder, knee, elbow, thumb)

ENDOCRINE

Congenital or acquired hypothyroidism

HEMATOLOGIC

Transient lymphoproliferative syndrome

Acute lymphocytic leukemia

Acute myelogenous leukemia

GASTROINTESTINAL

Celiac disease

Delayed tooth eruption

Respiratory

Obstructed sleep apnea

Frequent infections (sinusitis, nasopharyngitis, pneumonia)

CUTANEOUS

Hyperkeratosis

Seborrhea

Xerosis

Perigenital folliculitis

Developmental delay is universal (Tables 76-5 and 76-6; Fig. 76-10). Cognitive impairment does not uniformly affect all areas of development. Social development is relatively spared, but children with Down syndrome have considerable difficulty using expressive language. Understanding these individual developmental strengths will maximize the educational process for children with Down syndrome. Persons with Down syndrome often benefit from programs aimed at stimulation, development, and education. These programs are most effective in addressing social skills that often appear advanced for the intellectual delay. Children with Down syndrome also benefit from anticipatory guidance, which establishes the protocol for screening, evaluation, and care for patients with genetic syndromes and chronic disorders (Table 76-7).

Table 76-5 DEVELOPMENTAL MILESTONES

Table 76-6 SELF-HELP SKILLS

Figure 76-10 The area shaded in yellow denotes the range of intellectual function of the majority of children with Down syndrome.

(From Levine MD, Carey WB, Crocker AC, editors: Developmental-behavioral pediatrics, ed 2, Philadelphia, 1992, WB Saunders, p 226.)

Table 76-7 HEALTH SUPERVISION FOR CHILDREN WITH DOWN SYNDROME

CONDITION	TIME TO SCREEN	COMMENT
Congenital heart disease	Birth; by pediatric cardiologist Young adult for acquired valve disease

50% risk of congenital heart disease. Increased risk for pulmonary hypertension Strabismus, cataracts, nystagmus

Birth or by 6 mo; by pediatric ophthalmologist

Check vision annually

Cataracts occur in 15%, refractive errors in 50% Hearing impairment or loss Birth or by 3 mo with auditory brainstem response or otoacoustic emission testing; check hearing q6mo up to 3 yrs if tympanic membrane is not visualized; annually thereafter Risk for congenital hearing loss plus 50-70% risk of serous otitis media. Constipation Birth Increased risk for Hirschsprung disease Celiac disease At 2 years or with symptoms Screen with IgA and tissue transglutaminase antibodies Hematologic disease At birth and in adolescence or if symptoms develop Increased risk for neonatal polycythemia (18%), leukemoid reaction, leukemia (<1%) Hypothyroidism Birth; repeat at 6-12 mo and annually Congenital (1%) and acquired (5%) Growth and development

At each visit

Use Down syndrome growth curves

Discuss school placement options

Proper diet to avoid obesity

Obstructive sleep apnea Start at ∼1 yr and at each visit Monitor for snoring, restless sleep Atlantoaxial subluxation or instability (incidence 10-30%)

At each visit by history and physical exam

Radiographs at 3-5 years or when planning to participate in contact sports

Radiographs indicated wherever neurologic symptoms are present even if transient (neck pain, torticollis, gait disturbances, weakness)

Many are asymptomatic

Special Olympics recommendations are to screen for high risk sports, e.g., diving, swimming, contact sports Gynecologic care Adolescent girls Menstruation and contraception issues Recurrent infections When present Check IgG subclass and IgA levels Psychiatric, behavioral disorders At each visit

Depression, anxiety, obsessive compulsive disorder, schizophrenia seem in 10-17%

Autism spectrum disorder in 5-10%

Early-onset Alzheimer disease

IgA, immunoglobulin A; IgG, immunoglobulin G.

Extracted from Committee on Genetics: Health supervision for children with Down syndrome, Pediatrics 107:442–449, 2001; and Baum RA, Spader M, Nash PL, et al: Primary care of children and adolescents with Down syndrome: an update, Curr Prob Pediatr Adolesc Health Care 38:235–268, 2008.

The majority of children with Down syndrome do not have behavior problems. It is estimated that psychiatric comorbidity is 18-38% in this population. These estimates are higher than in unaffected children, but they are lower than in children with similar levels of mental retardation from other etiologies. All maladaptive behaviors in persons with Down syndrome are thought to be inherently linked to cognitive impairment. Common behavioral difficulties that occur in children with Down syndrome include inattentiveness, stubbornness, and a need for routine and sameness. Aggression and self-injurious behavior are less common in this population. All of these behaviors can respond to educational or pharmacologic interventions.

The life expectancy for children with Down syndrome is reduced and is approximately 50 to 55 yr. Little prospective information about the secondary medical problems of adults with Down syndrome is known. Retrospective studies have shown premature aging and an increased risk of Alzheimer disease in adults with Down syndrome. These studies have also shown unexpected negative associations between Down syndrome and other medical comorbidities. Persons with Down syndrome have fewer than expected deaths caused by solid tumors and ischemic heart disease. This same study reported increased risk of adult deaths due to congenital heart disease, seizures, and leukemia. In 1 large study, leukemias accounted for 60% of all cancers in people with Down syndrome and 97% of all cancers in children with Down syndrome. There was decreased risk of solid tumors in all age groups, including neuroblastomas and nephroblastomas in children with Down syndrome and epithelial tumors in adults with Down syndrome.

Most adults with Down syndrome are able to perform activities of daily living. However, most adults with Down syndrome have difficulty with complex financial, legal, or medical decisions. In most circumstances, a conservator is appointed for the adult with Down syndrome.

This risk of having a child with trisomy 21 is highest in women who conceive at >35 yr of age. Even though younger women have a lower risk, they represent half of all mothers with babies with Down syndrome because of their higher overall birth rate. All women should be offered screening for Down syndrome in their 2nd trimester by means of 4 maternal serum tests (free β-human chorionic gonadotropin (β-hCG), unconjugated estriol, inhibin, and α-fetoprotein). This is known as the quad screen; it can detect up to 80% of Down syndrome pregnancies compared to 70% in the triple screen. Both tests have a 5% false-positive rate. There is a method of screening during the 1st trimester using fetal nuchal translucency (NT) thickness that can be done alone or in conjunction with maternal serum β-hCG and pregnancy-associated plasma protein-A (PAPP-A). In the 1st trimester, NT alone can detect ≤70% of Down syndrome pregnancies, but with β-hCG and PAPP-A, the detection goes up to 87%. If both 1st and 2nd trimester screens are combined using NT and biochemical profiles (integrated screen), the detection rate goes up to 95%. If only 1st trimester quad screening is done, α-fetoprotein (MSAFP, which is decreased in affected pregnancies) is recommended as a 2nd trimester follow-up. Detection of fetal DNA in maternal plasma may also be diagnostic. The prenatal screens are also useful for other trisomies, although the detection rates are different from those given for Down syndrome.

In approximately 95% of the cases of Down syndrome there are 3 copies of chromosome 21. The origin of the supernumerary chromosome 21 is maternal in 97% of the cases as a result of errors in meiosis. The majority of these occur in maternal meiosis I (90%). Approximately 1% of persons with trisomy 21 are mosaics, with some cells having 46 chromosomes, and another 4% of have a translocation that involves chromosome 21. The majority of translocations in Down syndrome are fusions at the centromere between chromosomes 13, 14, 15, 21, and 22 known as Robertsonian translocations. The translocations can be de novo or inherited. Very rarely is Down syndrome diagnosed in a patient with only a part of the long arm of chromosome 21 in triplicate (partial trisomy). Isochromosomes and ring chromosomes are other rarer causes of trisomy 21. Down syndrome patients without a visible chromosome abnormality are the least common. It is not possible to distinguish the phenotypes of persons with full trisomy 21 and those with a translocation. Representative genes on chromosome 21 and their potential effects on development are noted in Table 76-8. Patients who are mosaic tend to have a milder phenotype.

Table 76-8 GENES LOCALIZED TO CHROMOSOME 21 THAT POSSIBLY AFFECT BRAIN DEVELOPMENT, NEURONAL LOSS, AND ALZHEIMER TYPE NEUROPATHOLOGY

Chromosome analysis is indicated in every person suspected of having Down syndrome. If a translocation is identified, parental chromosome studies must be performed to determine whether 1 of the parents is a translocation carrier, which carries a high recurrence risk for having another affected child. That parent might also have other family members at risk. Translocation (21;21) carriers have a 100% recurrence risk for a chromosomally abnormal child, and other Robertsonian translocations, such as t(14;21), have a 5-7% recurrence risk when transmitted by females. Genomic dosage imbalance contributes through direct and indirect pathways to the Down syndrome phenotype and its phenotypic variation.

Tables 76-9 and 76-10 provide more information on other aneuploidies and partial autosomal aneuploidies (Figs. 76-11 to 76-14).

Table 76-9 OTHER RARE ANEUPLOIDIES AND PARTIAL AUTOSOMAL ANEUPLOIDIES

DISORDER	KARYOTYPE	CLINICAL MANIFESTATIONS
Trisomy 8	47,XX/XY,+8	Growth and mental deficiency are variable The majority of patients are mosaics Deep palmar and plantar furrows are characteristic
Trisomy 9	47,XX/XY,+9	The majority of patients are mosaics Clinical features include craniofacial (high forehead, microphthalmia, low-set malformed ears, bulbous nose) and skeletal (joint contractures) malformations and heart defects (60%)
Trisomy 16	47,XX/XY,+16	The most commonly observed autosomal aneuploidy in spontaneous abortion; the recurrence risk is negligible
Tetrasomy 12p	46,XX[12]/46,XX, +i(12p)[8] (mosaicism for an isochromosome 12p)	Known as Pallister-Killian syndrome. Sparse anterior scalp hair, eyebrows, and eyelashes, prominent forehead, chubby cheeks, long philtrum with thin upper lip and cupid-bow configuration, polydactyly, and streaks of hyper- and hypopigmentation

Table 76-10 FINDINGS THAT MAY BE PRESENT IN TRISOMY 13 AND TRISOMY 18

TRISOMY 13	TRISOMY 18
HEAD AND FACE
Scalp defects (e.g., cutis aplasia) Microphthalmia, corneal abnormalities Cleft lip and palate in 60%-80% of cases Microcephaly Microphthalmia Sloping forehead Holoprosencephaly (arhinencephaly) Capillary hemangiomas Deafness	Small and premature appearance Tight palpebral fissures Narrow nose and hypoplastic nasal alae Narrow bifrontal diameter Prominent occiput Micrognathia Cleft lip or palate Microcephaly
CHEST
Congenital heart disease (e.g., VSD, PDA, and ASD) in 80% of cases Thin posterior ribs (missing ribs)	Congenital heart disease (e.g., VSD, PDA, ASD) Short sternum, small nipples
EXTREMITIES
Overlapping of fingers and toes (clinodactyly) Polydactyly Hypoplastic nails, hyperconvex nails	Limited hip abduction Clinodactyly and overlapping fingers; index over 3rd, 5th over 4th; closed fist Rocker-bottom feet Hypoplastic nails
GENERAL
Severe developmental delays and prenatal and postnatal growth retardation Renal abnormalities Only 5% live >6 mo	Severe developmental delays and prenatal and postnatal growth retardation Premature birth, polyhydramnios Inguinal or abdominal hernias Only 5% live >1 yr

VSD, ventricular septal defect; PDA, patent ductus arteriosus; ASD, atrial septal defect.

From Behrman RE, Kliegman RM: Nelson essentials of pediatrics, ed 4, Philadelphia, 2002, WB Saunders, p 142.

Figure 76-11 Facial appearance of a child with trisomy 13.

(From Wiedemann HR, Kunze J, Dibbern H: Atlas of clinical syndromes: a visual guide to diagnosis, ed 3, St Louis, 1989, Mosby.)

Figure 76-12 Trisomy 18: overlapping fingers and hypoplastic nails.

(From Wiedemann HR, Kunze J, Dibbern H: Atlas of clinical syndromes: a visual guide to diagnosis, ed 3, St Louis, 1989, Mosby.)

Figure 76-13 Trisomy 18: rocker-bottom feet (protruding calcanei).

(From Wiedemann HR, Kunze J, Dibbern H: Atlas of clinical syndromes: a visual guide to diagnosis, ed 3, St Louis, 1989, Mosby.)

Figure 76-14 Male infant with trisomy 18 at age 4 days. Note prominent occiput, micrognathia, low-set ears, short sternum, narrow pelvis, prominent calcaneus, and flexion abnormalities of the fingers.

Bibliography

Alfirevic Z. Prenatal screening for Down’s syndrome. BMJ. 2009;338:421-422.

Baum RA, Nash PL, Foster JEA, et al. Primary care of children and adolescents with Down syndrome: an update. Curr Prob Pediatr Adolect Health Care. 2008;38:235-268.

Caine A, Maltby AE, Parkin CA, et al. Prenatal detection of Down’s syndrome by rapid aneuploidy testing for chromosomes 13, 18, and 21 by FISH or PCR without a full karyotype: a cytogenetic risk assessment. Lancet. 2005;366:123-128.

Carter M, McCaughey E, Annaz D, Hill CM. Sleep problems in a Down syndrome population. Arch Dis Child. 2009;94:308-310.

Chiu RWK, Akolekar R, Zheng YWL, et al. Non-invasive prenatal assessment of trisomy 21 by multiplexed maternal plasma DNA sequencing: large scale validity study. BMJ. 2011;342:217.

Cicero S, Bindra R, Rembouskos G, et al. Integrated ultrasound and biochemical screening for trisomy 21 using fetal nuchal translucency, absent fetal nasal bone, free β-hCG and PAPP-A at 11 to 14 weeks. Prenat Diagn. 2003;23:306-310.

Dennis J, Archer N, Ellis J, et al. Recognising heart disease in children with Down syndrome. Arch Dis Child Educ Pract Ed. 2010;95:98-104.

Dykens EM. Psychiatric and behavioral disorders in persons with Down syndrome. Mental Ret Dev Dis Res Rev. 2007;13:272-278.

Einfeld SL, Brown R. Down syndrome—new prospects for an ancient disorder. JAMA. 2010;303:2525-2526.

Garrison MM, Jeffries H, Christakis DA. Risk of death for children with Down syndrome and sepsis. J Pediatr. 2005;147:748-752.

Gibson PA, Newton RW, Selby K, et al. Longitudinal study of thyroid function in Down’s syndrome in the first two decades. Arch Dis Child. 2005;90:574-578.

Irving C, Basu A, Richmond S, et al. Twenty-year trends in prevalence and survival of Down syndrome. Eur J Hum Genet. 2008;16:1336-1340.

Juj H, Emery H. The arthropathy of Down syndrome: an undiagnosed and under-recognized condition. J Pediatr. 2009;154:234-238.

Malone FD, Canick JA, Ball RH, et al. First-trimester or second-trimester screening, or both, for Down’s syndrome. N Engl J Med. 2005;353:2001-2010.

McDowell KM, Craven DI. Pulmonary complications of Down syndrome during childhood. J Pediatr. 2011;158:319-325.

Morris JK, Alberman E. Trends in Down’s syndrome live births and antenatal diagnosis in England and Wales from 1989 to 2008: analysis of data from the national Down syndrome cytogenetic register. BMJ. 2009;339:b3794.

Papageorgiou EA, Karagrigoriou A, Tsaliki E, et al. Fetal-specific DNA methylation ratio permits noninvasive prenatal diagnosis of trisomy 21. Nat Med. Mar 6, 2011. epub, doi: 10.1038/nm.2312

Rasmussen SA, Whitehead N, Collier SA, et al. Setting a public health research agenda for Down syndrome: summary of a meeting sponsored by the CDC and the national Down syndrome society. Am J Med Genet. 2008;46A:2998-3010.

Reynolds T. Giving antioxidants to infants with Down’s syndrome. BMJ. 2008;336:568-570.

Shin M, Besser LM, Kucik JE, et al. Prevalence of Down syndrome among children and adolescents in 10 regions of the United States. Pediatrics. 2009;124:1565-1571.

Shott SR, Amin R, Chini B, et al. Obstructive sleep apnea: should all children with Down syndrome be tested? Arch Otolaryngol Head Surg. 2006;132:432-436.

Webb D, Roberts I, Vyas P. Haematology of Down syndrome. Arch Dis Child. 2007;92:F503-F507.

Weijerman ME, Van Furth M, Noordegraaf AV, et al. Prevalence, neonatal characteristics, and first-year mortality of Down syndrome: a national study. J Pediatr. 2008;152:15-19.

Wouters J, Weijerman ME, Van Forth AM, et al. Prospective human leukocyte antigen, endomysium immunoglobulin A antibodies, and transglutaminase antibodies testing for celiac disease in children with Down syndrome. J Pediatr. 2009;154:239-242.

76.3 Abnormalities of Chromosome Structure

Carlos A. Bacino, Brendan Lee

Translocations

Translocations, which involve the transfer of material from 1 chromosome to another, occur with a frequency of 1/500 liveborn human infants. They may be inherited from a carrier parent or appear de novo, with no other affected family member. Translocations are commonly reciprocal or Robertsonian, involving 2 chromosomes (Fig. 76-15).

Figure 76-15 A, Schematic diagram (left) and partial G-banded karyotype (right) of a reciprocal translocation between chromosome 2 (blue) and chromosome 8 (pink). The breakpoints are on the long (q) arm of both chromosomes at bands 2q33 and 8q24.1, with the reciprocal exchange of material between the derivative (der) chromosomes 2 and 8. This translocation is balanced, with no net gain or loss of material. The nomenclature for this exchange is t(2;8)(q33:q24.1). B, Schematic diagram (left) and partial G-banded karyotype (right) of a Robertsonian translocation between chromosomes 13 (blue) and 14 (pink). The breakpoints are at the centromere (band q10) of both chromosomes, with fusion of the long arms into a single derivative chromosome and loss of the short (p) arm material. The nomenclature for this exchange is der(13;14)(q10;q10).

Reciprocal translocations are the result of breaks in non-homologous chromosomes, with reciprocal exchange of the broken segments. Carriers of a reciprocal translocation are usually phenotypically normal but are at an increased risk for miscarriage due to transmission of unbalanced reciprocal trans-locations and for bearing chromosomally abnormal offspring. Unbalanced translocations are the result of abnormalities in the segregation or crossover of the translocation carrier chromosomes in the germ cells.

Robertsonian translocations involve 2 acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22) that fuse near the centromeric region with a subsequent loss of the short arms. Because the short arms of all 5 pairs of acrocentric chromosomes have multiple copies of genes for ribosomal RNA, loss of the short arm of 2 acrocentric chromosomes has no deleterious effect. The resulting karyotype has only 45 chromosomes, including the translocated chromosome that is made up of the long arm of the 2 fused chromosomes. Carriers of Robertsonian translocations are usually phenotypically normal. However, they are at increased risk for miscarriage and unbalanced abnormal offspring.

In some rare instances, translocations can involve 3 or more chromosomes, as seen in complex rearrangements. Another, less-common type is the insertional translocation. Insertional translocations result from a piece of chromosome material that breaks away and later is reinserted inside the same chromosome at a different site or inserted in another chromosome.

Inversions

An inversion requires that a single chromosome break at 2 points; the broken piece is then inverted and joined into the same chromosome. Inversions occur in 1/100 live births. There are 2 types of inversions: pericentric and paracentric. In pericentric inversions, the breaks are in the 2 opposite arms of the chromosome and include the centromere. They are usually discovered because they change the position of the centromere. The breaks in paracentric inversions occur in only 1 arm. Carriers of inversions are usually phenotypically normal, but they are at increased risk for miscarriages, typically in paracentric inversions, and chromosomally abnormal offspring in pericentric inversions.

Deletions and Duplications

Deletions involve loss of chromosome material and, depending on their location, they can be classified as terminal (at the ends of chromosomes) or interstitial (within the arms of a chromosome). They may be isolated or they may occur along with a duplication of another chromosome segment. The latter typically occurs in unbalanced reciprocal chromosomal translocation secondary to abnormal crossover or segregation in a translocation or inversion carrier.

A carrier of a deletion is monosomic for the genetic information of the missing segment. Deletions are usually associated with mental retardation and malformations. The most commonly observed deletions in routine chromosome preparations include 1p-, 4p-, 5p-, 9p-, 11p-, 13q-, 18p-, 18q-, and 21q- (Table 76-11 and Fig. 76-16), all distal or terminal deletions of the short or the long arms of chromosomes. Deletions may be observed in routine chromosome preparations, and deletions and translocations larger than 5-10 Mb are usually visible microscopically.

Table 76-11 COMMON DELETIONS AND THEIR CLINICAL MANIFESTATIONS

DELETION	CLINICAL ABNORMALITIES
4p-	Wolf-Hirschhorn syndrome. The main features are a typical “Greek helmet” facies secondary to ocular hypertelorism, prominent glabella, and frontal bossing; microcephaly, dolichocephaly, hypoplasia of the orbits, ptosis, strabismus, nystagmus, bilateral epicanthic folds, cleft lip and palate, beaked nose with prominent bridge, hypospadias, cardiac malformations, and mental retardation.
5p-	Cri-du-chat syndrome. The main features are hypotonia, short stature, characteristic shrill cry in the first few weeks of life (cat like cry), microcephaly with protruding metopic suture, hypertelorism, bilateral epicanthic folds, high arched palate, wide and flat nasal bridge, and mental retardation.
9p-	The main features are craniofacial dysmorphology with trigonocephaly, slanted palpebral fissures, discrete exophthalmos secondary to supraorbital hypoplasia, arched eyebrows, flat and wide nasal bridge, short neck with low hairline, genital anomalies, long fingers and toes with extra flexion creases, cardiac malformations, and mental retardation.
13q-	The main features are low birthweight, failure to thrive, microcephaly, and severe mental retardation. Facial features include high wide nasal bridge, hypertelorism, ptosis, micrognathia. Ocular malformations are common (retinoblastoma). The hands have hypoplastic or absent thumbs and syndactyly.
18p-	A few patients (15%) are severely affected and have cephalic and ocular malformations: holoprosencephaly, cleft lip and palate, ptosis, epicanthal folds, and varying degrees of mental retardation. Most (80%) have only minor malformations and mild mental retardation.
18q-	Growth deficiency, hypotonia with “froglike” position with the legs flexed, externally rotated, and in hyperabduction. The face is characteristic with depressed midface and apparent protrusion of the mandible, deep-set eyes, short upper lip, everted lower lip (“carplike” mouth); antihelix of the ears is very prominent; varying degrees of mental retardation and belligerent personality. Myelination abnormalities in the CNS.

Figure 76-16 A, Child with velocardiofacial syndrome (deletion 22q11.2). B, Child with Prader-Willi syndrome (deletion 15q11-13). C, Child with Angelman syndrome (deletion 15q11-13). D, Child with Williams syndrome (deletion 7q11.23).

High-resolution banding techniques, FISH, and molecular studies like aCGH can reveal deletions that are too small to be seen in ordinary or routine chromosome spreads (see Fig. 76-7). Microdeletions involve loss of small chromosome regions, the largest of which are detectable only with prophase chromosome studies and/or molecular methods. For submicroscopic deletions, the missing piece can only be detected using molecular methodologies such as FISH or DNA-based studies like aCGH. The presence of extra genetic material from the same chromosome is referred to as duplication. Duplications can also be sporadic or result from abnormal segregation in translocation or inversion carriers.

Microdeletions and microduplications usually involve regions that include several genes, so that the affected individuals can have a distinctive phenotype depending on the number of genes involved. When such a deletion involves more than a single gene, the condition is referred to as a contiguous gene deletion syndrome (Table 76-12). With the advent of clinically available aCGH, a large number of duplications, most of them microduplications, have been uncovered. Most of those microduplication syndromes are the reciprocal duplications of the known deletions or microdeletion counterparts and have distinctive clinical features (Table 76-13).

Table 76-12 MICRODELETION AND CONTIGUOUS GENE SYNDROMES AND THEIR CLINICAL MANIFESTATIONS

DELETION	SYNDROME	CLINICAL MANIFESTATIONS
1p36	1p deletion	Growth retardation, dysmorphic features with midface hypoplasia, straight thin eyebrows, pointy chin, sensorineural hearing loss, progressive cardiomyopathy, hypothyroidism, seizures, mental retardation
5q35	Sotos (50% are deletions of NSD1 gene in Asians but only 6% in whites)	Overgrowth, macrocephaly, prominent forehead, prominence of extra-axial fluid spaces on brain imaging, large hands and feet, hypotonia, clumsiness, mental disabilities
6p25	Axenfeld-Rieger	Axenfeld-Rieger malformation, hearing loss, congenital heart defects, dental anomalies, developmental delays, facial dysmorphism
7q11.23	Williams	Round face with full cheeks and lips, long philtrum, stellate pattern in iris, strabismus, supravalvular aortic stenosis and other cardiac malformations, varying degrees of mental retardation, friendly personality
8p11	8p11	Kallman syndrome 2 (hypogonadotropic hypogonadism and anosmia), spherocytosis (deletions of ankyrin 1), multiple congenital anomalies, mental retardation
8q24.1-q24.13	Langer-Giedion or trichorhinophalangeal type II	Sparse hair, multiple cone-shaped epiphyses, multiple cartilaginous exostoses, bulbous nasal tip, thickened alar cartilage, upturned nares, prominent philtrum, large protruding ears, mild mental retardation
9q22	Gorlin	Multiple basal cell carcinomas, odontogenic keratocysts, palmoplantar pits, calcification falx cerebri
9q34	9q34 deletion	Distinct face with synophrys, anteverted nares, tented upper lip, protruding tongue, midface hypoplasia, conotruncal heart defects, mental retardation
10p12-p13	DiGeorge 2	Many of the DiGeorge 1 and velocardiofacial 1 features (conotruncal defects, immunodeficiency, hypoparathyroidism, dysmorphic features)
11p11.2	Potocki-Shaffer	Multiple exostoses, parietal foramina, craniosynostosis, facial dysmorphism, syndactyly, mental retardation
11p13	WAGR	Hypernephroma (Wilms tumor), aniridia, male genital hypoplasia of varying degrees, gonadoblastoma, long face, upward slanting palpebral fissures, ptosis, beaked nose, low-set poorly formed auricles, mental retardation
11q24.1-11qter	Jacobsen	Mental and growth retardation, cardiac and digit anomalies, thrombocytopenia
15q11-q13 (pat)	Prader-Willi	Severe hypotonia and feeding difficulties at birth, voracious appetite and obesity in infancy, short stature (responsive to growth hormone), small hands and feet, hypogonadism, mental retardation
15q11-q13 (mat)	Angelman	Hypotonia, feeding difficulties, GE reflux, fair hair and skin, midface hypoplasia, prognathism, seizures, tremors, ataxia, sleep disturbances, inappropriate laughter, poor or absent speech, severe mental retardation
16p13.3	Rubinstein-Taybi	Microcephaly, ptosis, beaked nose with low-lying philtrum, broad thumbs and large toes, mental retardation
17p11.2	Smith-Magenis	Brachycephaly, midfacial hypoplasia, prognathism, myopia, cleft palate, short stature, severe behavioral problems, mental retardation
17p13.3	Miller-Dieker	Microcephaly, lissencephaly, pachygyria, narrow forehead, hypoplastic male external genitals, growth retardation, seizures, profound mental retardation
20p12	Alagille syndrome	Bile duct paucity with cholestasis; heart defects, particularly pulmonary artery stenosis; ocular abnormalities (posterior embryotoxon); skeletal defects such as butterfly vertebrae; long nose
22q11.2	Velocardiofacial-DiGeorge syndrome	Conotruncal cardiac anomalies, cleft palate, velopharyngeal incompetence, hypoplasia or agenesis of the thymus and parathyroid glands, hypocalcemia, hypoplasia of auricle, learning disabilities, psychiatric disorders
22q13.3 deletion		Hypotonia, developmental delay, normal or accelerated growth, severe expressive language deficits, autistic behavior
Xp21.2-p21.3		Duchenne muscular dystrophy, retinitis pigmentosa, adrenal hypoplasia, mental retardation, glycerol kinase deficiency
Xp22.2-p22.3		Ichthyosis, Kallman syndrome, mental retardation, chondrodysplasia punctata
Xp22.3	Microphthalmia with linear defects (MLS)	Microphthalmia, linear skin defects, poikiloderma, congenital heart defects, seizures, mental retardation

pat, paternal; mat, maternal; GE, gastroesophageal.

Table 76-13 MICRODUPLICATIONS AND THEIR CLINICAL MANIFESTATIONS

DUPLICATION CHROMOSOME REGION	DISEASE REGION	CLINICAL FEATURES
1q21.1		Macrocephaly, DD, learning disabilities
3q29		Mild to moderate MR, microcephaly
7q11.23	Williams syndrome	DD and severe expressive language disorder, autistic features, subtle dysmorphisms
15q13.3	Prader-Willi/Angelman syndrome region	DD, MR, autistic features in duplications of maternal origin
15q24		Growth retardation, DD, microcephaly, digital anomalies, hypospadias, connective tissue abnormalities
16p11.2		FTT, severe DD, short stature, GH deficiency, dysmorphic features
17p11.2	Potocki-Lupski syndrome	Hypotonia, cardiovascular anomalies, FTT, DD, verbal apraxia, autism, anxiety
17q21.31		Severe DD, microcephaly, short and broad digits, dysmorphic features
22q11.2	Velocardiofacial-DiGeorge syndrome	Cardiovascular defects, velopharyngeal insufficiency
Xq28	MECP2 gene region (Rett syndrome)	In males: infantile hypotonia, immune deficiency, dysmorphic features, DD, speech delay, autistic behavior, regression in childhood

DD, developmental delay; MR, mental retardation; FTT, failure to thrive; GH, growth hormone.

Subtelomeric regions are often involved in chromosome rearrangements that cannot be visualized using routine cytogenetics. Telomeres, which are the distal ends of the chromosomes, are gene-rich regions. The distal structure of the telomeres is essentially common to all chromosomes, but proximal to those, there are unique regions known as subtelomeres, which typically involved in deletions and most other chromosome rearrangements. Small subtelomeric deletions, duplications, or rearrangements (translocations, inversions) may be relatively common in nonspecific mental retardation with minor anomalies. Subtelomeric rearrangements have been found in 3-7% of children with moderate to mild mental retardation and 0.5% of children with mild mental retardation.

Clinical features (>30%) include short stature, microcephaly, hypertelorism, nose and ear abnormalities, and cryptorchidism. This group is also characterized by a family history of mental retardation and an increased likelihood of retarded growth beginning in the prenatal period. Telomere mutations have also been associated with dyskeratosis congenita and other aplastic anemia syndromes as well as pulmonary or hepatic fibrosis. Both the subtelomeric rearrangements and the microdeletion and microduplication syndromes are typically diagnosed by molecular techniques like FISH, multiple ligation-dependent primer amplification (MLPA) and/or aCGH. Many studies show that aCGH can detect 14-18% of abnormalities in patients who are previously known to have normal chromosome studies.

Insertions

Insertions occur when a piece of a chromosome broken at 2 points is incorporated into a break in another part of a chromosome. A total of 3 breakpoints are then required, and they can occur between 2 or within 1 chromosome. A form of nonreciprocal translocation, insertions are rare. Insertion carriers are at risk of having offspring with deletions or duplications of the inserted segment.

Isochromosomes

Isochromosomes consist of 2 copies of the same chromosome arm joined through a single centromere and forming mirror images of one another. The most commonly reported autosomal isochromosomes tend to involve chromosomes with small arms. Some of the more common chromosome arms involved in this formation include 5p, 8p, 9p, 12p, 18p, and 18q. This is also a common abnormality seen in long arm of the X chromosome and associated with Turner syndrome. Individuals who have 1 isochromosome in 46 chromosomes are monosomic for genes in the lost arm and trisomic for the genes present in the isochromosome.

Marker and Ring Chromosomes

Marker chromosomes are rare and are usually chromosome fragments that are too small to be identified by conventional cytogenetics; they usually occur in addition to the normal 46 chromosomes. Most are sporadic (70%); mosaicism is often (50%) noted because of the mitotic instability of the marker chromosome. The incidence in newborn infants is 1 in 3,300, and the incidence in persons with mental retardation is 1 in 300. The associated phenotype ranges from normal to severely abnormal depending on the amount of chromosome material and number of genes associated with the fragment.

Ring chromosomes, which are found for all human chromosomes, are rare. A ring chromosome is formed when both ends of a chromosome are deleted and the ends are then joined to form a ring. Depending on the amount of chromosome material that is lacking or in excess (if the ring is in addition to the normal chromosomes), a patient with a ring chromosome can appear normal or nearly normal or can have mental retardation and multiple congenital anomalies.

Bibliography

Battaglia A, Hoyme HE, Dallapiccola B, et al. Further delineation of deletion 1p36 syndrome in 60 patients: a recognizable phenotype and common cause of developmental delay and mental retardation. Pediatrics. 2008;121:404-410.

Calado RT, Young NS. Telomere diseases. N Engl J Med. 2009;361:2353-2365.

Fillion M, Deal C, Van Vliet G. Retrospective study of the potential benefits and adverse events during growth hormone treatment in children with Prader-Willi syndrome. J Pediatr. 2009;154:230-233.

Gicquel C, Rossignol S, Cabrol S, et al. Epimutation of the telomeric imprinting center region on chromosome 11p15 in Silver-Russell syndrome. Nat Genet. 2005;37:1003-1007.

Mefford H, Shapr A, Baker C, et al. Recurrent rearrangements of chromosome 1q21.1 and variable pediatric phenotypes. N Engl J Med. 2008;359:1685-1698.

Peters J. Prader-Willi and snoRNAs. Nat Genet. 2008;40:688-689.

Rappold GA, Shanske A, Saenger P. All shook up by SHOX deficiency. J Pediatr. 2005;147:422-424.

Robin NH, Shprintzen RJ. Defining the clinical spectrum of deletion 22q11.2. J Pediatr. 2005;147:90-96.

Sahoo T, del Gaudio D, German JR, et al. Prader-Willi phenotype caused by paternal deficiency for the HBII-85 C/D box small nucleolar RNA cluster. Nat Genet. 2008;40:719-721.

Sharp AJ, Mefford HC, Li K, et al. A recurrent 15q13.3 microdeletion syndrome associated with mental retardation and seizures. Nat Genet. 2008;40:322-328.

Stafler P, Wallis C, et al. Prader-Willi syndrome: who can have growth hormone? Arch Dis Child. 2008;93:341-345.

Vissers LELM, van Ravenswaaji CMA, Admiraal R, et al. Mutations in a new member of the chromodomain gene family cause CHARGE syndrome. Nat Genet. 2004;36:955-957.

Walter S, Sandig K, Hinkel GK, et al. Subtelomere FISH in 50 children with mental retardation and minor anomalies, identified by a checklist, selects 10 rearrangements including a de novo balanced translocation of chromosomes 17p13.3 and 20q13.33. Am J Med Genet. 2004;128A:364-373.

Youings S, Ellis K, Ennis S, et al. A study of reciprocal translocations and inversions detected by light microscopy with special reference to origin, segregation, and recurrent abnormalities. Am J Med Genet. 2004;126A:46-60.

76.4 Sex Chromosome Aneuploidy

Carlos A. Bacino, Brendan Lee

About 1/400 males and 1/650 females have some form of sex chromosome abnormality. Considered together, sex chromosome abnormalities are the most common chromosome abnormalities seen in liveborn infants, children, and adults. Sex chromosome abnormalities can be either structural or numerical and can be present in all cells or in a mosaic form. Those affected with these abnormalities might have few or no physical or developmental problems (Table 76-14).

Table 76-14 SEX CHROMOSOME ABNORMALITIES

DISORDER	KARYOTYPE	APPROXIMATE INCIDENCE
Klinefelter syndrome	47,XXY	1/575-1/1,000 males
	48,XXXY	1/50,000-1/80,000 male births
	Other (48,XXYY; 49,XXXYY; mosaics)
XYY syndrome	47,XYY	1/800-1,000 males
Other X or Y chromosome abnormalities		1/1,500 males
XX males	46,XX	1/20,000 males
Turner syndrome	45,X	1/2,500-1/5,000 females
Turner syndrome	Variants and mosaics
Trisomy X	47,XXX	1/1,000 females
Trisomy X	48,XXXX and 49,XXXXX	Rare
Other X chromosome abnormalities		1/3,000 females
XY females	46,XY	1/20,000 females

Turner Syndrome

Turner syndrome is a condition characterized by complete or partial monosomy of the X chromosome and defined by a combination of phenotypic features (Table 76-15). Half of the patients with Turner syndrome have a 45,X chromosome complement. The other half exhibits mosaicism and varied structural abnormalities of the X or Y chromosome. Maternal age is not a predisposing factor for children with 45,X. Turner syndrome occurs in approximately 1/5,000 female live births. In 75% of patients, the lost sex chromosome is of paternal origin (whether an X or a Y). 45,X is 1 of the chromosome abnormalities most often associated with spontaneous abortion. It has been estimated that 95-99% of 45,X conceptions are miscarried.

Table 76-15 SIGNS ASSOCIATED WITH TURNER SYNDROME

Short stature

Congenital lymphedema

Horseshoe kidneys

Patella dislocation

Increased carrying angle of elbow (cubitus valgus)

Madelung deformity (chondrodysplasia of distal radial epiphysis)

Congenital hip dislocation

Scoliosis

Widespread nipples

Shield chest

Redundant nuchal skin (in utero cystic hygroma)

Low posterior hairline

Coarctation of aorta

Bicuspid aortic valve

Cardiac conduction abnormalities

Hypoplastic left heart syndrome and other left heart abnormalities

Gonadal dysgenesis (infertility, primary amenorrhea)

Gonadoblastoma (increased risk if Y chromosome material is present)

Learning disabilities (nonverbal perceptual motor and visuospatial skills) (in 70%)

Developmental delay (in 10%)

Social awkwardness

Hypothyroidism (acquired in 15-30%)

Type 2 diabetes mellitus (insulin resistance)

Strabismus

Cataracts

Red-green colorblindness (as in males)

Recurrent otitis media

Sensorineural hearing loss

Inflammatory bowel disease

Celiac disease (increased incidence)

Clinical findings in the newborns can include small size for gestational age, webbing of the neck, protruding ears, and lymphedema of the hands and feet, although many newborns are phenotypically normal (Fig. 76-17). Older children and adults have short stature and exhibit variable dysmorphic features. Congenital heart defects (40%) and structural renal anomalies (60%) are common. The most common heart defects are bicuspid aortic valves, coarctation of the aorta, aortic stenosis, and mitral valve prolapse. The gonads are generally streaks of fibrous tissue (gonadal dysgenesis). There is primary amenorrhea and lack of secondary sex characters. These children should receive regular endocrinologic testing (Chapter 580). Most patients tend to be of normal intelligence, but mental retardation is seen in 6% of affected children. They are also at increased risk for behavioral problems and deficiencies in spatial and motor perception. Guidelines for health supervision for children with Turner syndrome are published by the American Academy of Pediatrics (AAP).

Figure 76-17 Redundant nuchal skin (A) and puffiness of the hands (B) and feet (C) in Turner syndrome.

Patients with 45,X/46,XY mosaicism, can have Turner syndrome, although this form of mosaicism can also be associated with male pseudohermaphroditism, male or female genitalia in association with mixed gonadal dysgenesis, or a normal male phenotype. This variant is estimated to represent approximately 6% of patients with mosaic Turner syndrome. Some of the patients with Turner syndrome phenotype and a Y cell line exhibit masculinization. Phenotypic females with 45,X/46,XY mosaicism have a 15-30% risk of developing gonadoblastoma. The risk for the patients with a male phenotype and external testes is not so high, but tumor surveillance is nevertheless recommended. The AAP has recommended the use of FISH analysis to look for Y-chromosome mosaicism in all 45,X patients. If Y chromosome material is identified, laparoscopic gonadectomy is recommended.

Noonan syndrome shares many clinical features with Turner syndrome, although it is an autosomal dominant disorder resulting from mutations in several genes that are involved in the RAS-MAPK (mitogen activated protein kinase) pathway. The most common of these is PTPN11 (50%), which encodes a nonreceptor tyrosine kinase (SHP-2) on chromosome 12q24.1. Other genes include SOS1 in 10-15%, RAF1 in 3-8%, and KRAS in 5%. Features common to Noonan syndrome include short stature, low posterior hairline, shield chest, congenital heart disease, and a short or webbed neck (Table 76-16). In contrast to Turner syndrome, Noonan syndrome affects both sexes and has a different pattern of congenital heart disease typically involving right-sided lesions.

Klinefelter Syndrome

Persons with Klinefelter syndrome are phenotypically male; this syndrome is the most common cause of hypogonadism and infertility in males and the most common sex chromosome aneuploidy in humans (Chapter 577). Eighty percent of children with Klinefelter syndrome have a male karyotype with an extra chromosome X-47,XXY; the remaining 20% have multiple sex chromosome aneuploidies (48,XXXY; 48,XXYY; 49,XXXXY), mosaicism (46,XY/47,XXY), or structurally abnormal X chromosomes. The greater the aneuploidy, the more severe the mental impairment and dysmorphism. Early studies showed that the birth prevalence is approximately 1/1,000 males. The current prevalence of 47,XXY appears to have increased to approximately 1/580 liveborn boys; the reasons for this are still unknown. Errors in paternal nondisjunction in meiosis I account for half of the cases.

Puberty occurs at the normal age, but the testes remain small. Patients develop secondary sex characters late; 50% develop gynecomastia. They have taller stature. Because many patients with Klinefelter syndrome are phenotypically normal until puberty, the syndrome often goes undiagnosed until they reach adulthood, when their infertility aids in their clinical identification. Patients with 46,XY/47,XXY have a better prognosis for testicular function. Their intelligence shows variability and ranges from above to below average. Persons with Klinefelter syndrome can show behavioral problems, learning disabilities, and deficits in language. Problems with self-esteem are often the case with adolescents and adults. Substance abuse, depression, and anxiety have been reported in adolescents with Klinefelter syndrome. Those who have higher X chromosome counts show impaired cognition. It has been estimated that each additional X chromosome reduces the IQ by 10-15 points, when comparing these persons with their normal siblings. The main effect is seen in language skills and social domains.

47,XYY

The incidence of 47,XYY is approximately 1 in 800-1,000 males, with many cases remaining undiagnosed, because most affected individuals have a normal appearance and normal fertility. The extra Y is the result of nondisjunction at paternal meiosis II (MII). Those with this abnormality have normal intelligence but are at risk for learning disabilities. Behavioral abnormalities including hyperactive behavior, pervasive developmental disorder, and aggressive behavior have been reported. Early reports that assigned stigmata of criminality to this disorder have long been disproved.

Bibliography

Lanfranco F, Kamischke A, Zitzmann M, et al. Klinefelter’s syndrome. Lancet. 2004;364:273-283.

Lin AE. Focus on the heart and aorta in Turner syndrome. J Pediatr. 2007;150:572-574.

Massa G, Verlinde F, De Schepper J, et al. Trends in age at diagnosis of Turner syndrome. Arch Dis Child. 2005;90:267-268.

Mazzanti L, Cicognani A, Baldazzi L, et al. Gonadoblastoma in Turner syndrome and Y-chromosome–derived material. Am J Med Genet. 2005;135A:150-154.

Roberts AE, Araki T, Swanson KD, et al. Germline gain-of-function mutations in SOS1 cause Noonan syndrome. Nat Genet. 2007;39:70-74.

Schubbert S, Zenker M, Rowe SL, et al. Germline KRAS mutations cause Noonan syndrome. Nat Genet. 2006;38:331-336.

Sybert VP, McCauley E. Turner’s syndrome. N Engl J Med. 2004;351:1227-1238.

Tartaglia M, Pennacchio LA, Zhao C, et al. Gain-of-function SOSI mutations cause a distinctive form of Noonan syndrome. Nat Genet. 2007;39:75-79.

Zeger MPD, Zinn AR, Lahlou N, et al. Effect of ascertainment and genetic features on the phenotype of Klinefelter syndrome. J Pediatr. 2008;152:716-722.

Zenker M, Gernot Buheitel G, Rauch R, et al. Genotype-phenotype correlations in Noonan syndrome. J Pediatr. 2004;144:368-374.

76.5 Fragile Chromosome Sites

Carlos A. Bacino, Brendan Lee

Fragile sites are regions of chromosomes that show a tendency for separation, breakage, or attenuation under particular growth conditions. They appear as a gap in the staining. At least 120 chromosomal loci, many of them heritable, have been identified as fragile sites in the human genome (see Table 75-1).

One fragile site that has clinical significance is the one on the distal long arm of chromosome Xq27.3 associated with the fragile X syndrome. Fragile X accounts for 3% of males with mental retardation. There is another fragile site on the X chromosome (FRAXE on Xq28) that has also been implicated in mild mental retardation. The FRA11B (11q23.3) breakpoints are associated with Jacobsen syndrome (condition caused by deletion of the distal long arm of chromosome 11). Fragile sites can also play a role in tumorigenesis.

The main clinical manifestations of fragile X syndrome in affected males are mental retardation, autistic behavior, macro-orchidism, and characteristic facial features (Table 76-17). The macro-orchidism may not be evident until puberty. The facial features, which include a long face, large ears, and a prominent square jaw, become more obvious with age. Females affected with fragile X show varying degrees of mental retardation and/or learning disabilities. Diagnosis of fragile X is possible by DNA testing that shows an expansion of a triplet DNA repeat inside the FMR1 gene on the X chromosome. The expansion involves an area of the gene that contains a variable number of trinucleotide (CGG) repeats. The larger the triplet repeat expansion, the more significant the mental retardation. In cases where the expansion is large, females can also manifest different degrees of mental retardation. Therapy of the diverse neuropsychiatric manifestations associated with fragile X syndrome is noted in Table 76-18. Inhibitors of the metabolic glutamate receptor (overexpressed in fragile X) are undergoing clinical trials.

Table 76-17 CLINICAL FEATURES OF FULL AND PREMUTATION FMR1 ALLELES

Table 76-18 THERAPY FOR FMR1 RELATED DISORDERS

Bibliography

Dobkin C, Radu G, Ding XH, et al. Fragile X prenatal analyses show full mutation females at high risk for mosaic Turner syndrome: fragile X leads to chromosome loss. Am J Med Genet. 2009;149A:2152-2157.

Hagerman RJ, Berry-Kravis E, Kaufman WE, et al. Advances in the treatment of fragile X syndrome. Pediatrics. 2009;123:378-390.

Jacquemont S, Curie A, des Portes V, et al. Epigenetic modification of the FMR1 gene in fragile X syndrome is associated with differential response to the mGluR5 antagonist AFQ056. Sci Transl Med. 2011;3:64ra1.

Jacquemont S, Hagerman RJ, Hagerman PJ, et al. Fragile-X syndrome and fragile X associated tremor/ataxia syndrome: two faces of FMRI. Lancet Neurol. 2007;6:45-55.

Kuehn BM. Scientists find promising therapies for fragile X and Down syndrome. JAMA. 2011;305:344-346.

Paribello C, Tao L, Folino A, et al. Open-label add-on treatment trial of minocycline in fragile X syndrome. BMC Neurol. 2010;10:91.

Saul RA, Friez M, Eaves K, et al. Fragile X syndrome detection in newborns—pilot study. Genet Med. 2008;10:714-719.

76.6 Mosaicism

Carlos A. Bacino and Brendan Lee

Mosaicism describes an individual or tissue that contains ≥2 different cell lines typically derived from a single zygote and the result of mitotic nondisjunction (see Fig. 76-1). Study of placental tissue from chorionic villus samples collected at or before the 10th wk of gestation has shown that 2% or more of all conceptions are mosaic for a chromosome abnormality. With the exception of chromosomes 13, 18, and 21, complete autosomal trisomies are usually nonviable; the presence of a normal cell line might allow these other trisomic conceptions to survive to term. Depending on the point at which the new cell line arises during early embryogenesis, mosaicism may be present in some tissues but not in others. Germline mosaicism, which refers to the presence of mosaicism in the germ cells of the gonad, may be associated with an increased risk for recurrence of an affected child whether the germ cells are affected with a chromosomal abnormality or specific gene mutation.

Pallister-Killian Syndrome

Pallister-Killian syndrome is characterized by coarse facies (prominent full cheeks), abnormal ear lobes, localized alopecia, pigmentary skin anomalies, diaphragmatic hernia, cardiovascular anomalies, supernumerary nipples, seizures, and profound mental retardation. The syndrome is due to mosaicism for an isochromosome 12p. The presence of the isochromosome 12p in cells gives 4 functional copies for the short arm of chromosome 12 in the affected cells. The isochromosome 12p is preferentially cultured from fibroblasts that can be readily obtained from a skin punch biopsy and is seldom present in lymphocytes. The abnormalities seen in affected persons probably reflect the presence of abnormal cells during early embryogenesis.

Hypomelanosis of Ito

Hypomelanosis of Ito is characterized by unilateral or bilateral macular hypo- or hyperpigmented whorls, streaks, and patches (Chapter 645). Sometimes these pigmentary defects follow the lines of Blaschko. Hair and tooth anomalies are common. Abnormalities of the eyes, musculoskeletal system (growth asymmetry, syndactyly, polydactyly, clinodactyly), and central nervous system (microcephaly, seizures, mental retardation) may also be present. Patients with hypomelanosis of Ito might have 2 genetically distinct cell lines. The mosaic chromosome anomalies that have been observed involve both autosomes and sex chromosomes and have been demonstrated in about 50% of patients. The mosaicism might not be visible in lymphocyte-derived chromosome studies; it is more likely to be found when chromosomes are analyzed from skin fibroblasts. The distinct cell lines might not always be due to observable chromosomal anomalies but might result from single gene mutations or other mechanisms.

76.7 Chromosome Instability Syndromes

Carlos A. Bacino and Brendan Lee

Chromosome instability syndromes, formerly known as chromosome breakage syndromes, are characterized by an increased risk of malignancy and specific phenotypes. They display autosomal recessive inheritance and have an increased frequency of chromosome breakage and/or rearrangement, either spontaneous or induced. They result from specific defects in DNA repair, cell cycle control, and apoptosis. The resulting chromosomal instability leads to the increased risk of developing neoplasms. The classic chromosome instability syndromes are Fanconi anemia, ataxia telangiectasia, Nijmegen syndrome, ICF (immunodeficiency, centromere instability, and facial anomalies) syndrome, Roberts syndrome, Werner syndrome, and Bloom syndrome.

76.8 Uniparental Disomy and Imprinting

Carlos A. Bacino and Brendan Lee

Uniparental Disomy

Uniparental disomy (UPD) occurs when both chromosomes of a pair or areas from 1 chromosome in any individual have been inherited from a single parent. UPD can be of 2 types: uniparental isodisomy or uniparental heterodisomy. Uniparental isodisomy means that both chromosomes or chromosomal regions are identical (typically the result of monosomy rescue by duplication). Uniparental heterodisomy means that the 2 chromosomes are different members of a pair, both of which were still inherited from 1 parent. This results from a trisomy that is later reduced to disomy, leaving 2 copies from 1 parent. The phenotypic result of UPD varies according to the chromosome involved, the parent who contributed the chromosomes, and whether it is isodisomy or heterodisomy. Three types of phenotypic effects are seen in UPD: those related to imprinted genes (i.e., the absence of a gene that is normally expressed only when inherited from a parent of a specific sex), those related to the uncovering of autosomal recessive disorders, and those related to a vestigial aneuploidy producing mosaicism (Chapter 75).

In uniparental isodisomy, both chromosomes or regions (and thus the genes) in the pair are identical. This is particularly important when the parent is a carrier of an autosomal recessive disorder. If the offspring of a carrier parent has UPD with isodisomy for a chromosome that carries an abnormal gene, the abnormal gene will be present in 2 copies and the phenotype will be that of the autosomal recessive disorder; the child has an autosomal recessive disorder even though only 1 parent is a carrier of that recessive disorder. It is estimated that all human beings carry approximately 20 abnormal autosomal recessive genes. Some autosomal recessive disorders like spinal muscular atrophy, cystic fibrosis, cartilage-hair hypoplasia, α- and β-thalassemias, and Bloom syndrome have been reported in cases of UPD. The possibility of uniparental isodisomy should also be considered when a person is affected with >1 recessive disorder because the abnormal genes for both disorders could be carried on the same isodisomic chromosome. Uniparental isodisomy is a rare cause of recessively inherited disorders.

Maternal UPD involving chromosomes 2, 7, 14, and 15 and paternal UPD involving chromosomes 6, 11, 15, and 20 are associated with phenotypic abnormalities of growth and behavior. UPD maternal 7 is associated with a phenotype similar to Russell-Silver syndrome with intrauterine growth restriction. These phenotypic effects may be related to imprinting (see under Imprinting, next).

UPD for chromosome 15 is seen in some cases of Prader–Willi syndrome and Angelman syndrome. In Prader-Willi syndrome, about 25-29% of cases have maternal UPD (missing the paternal chromosome 15). In Angelman syndrome, paternal UPD of chromosome 15 is rarer and is observed in approximately 5% of the cases (missing the maternal chromosome 15). The phenotype for Prader-Willi syndrome (Fig. 76-18) and Angelman syndrome in cases of UPD is thought to result from the lack of the functional contribution from a particular parent of chromosome 15. In Prader-Willi syndrome the paternal contribution is missing, and the maternal contribution is missing in Angelman syndrome. Prader-Willi may be due to paternal deficiency of HB11-85 snoRNAs (small nucleolar RNAs). These findings suggest that there are differences in function of certain regions of chromosome 15, depending on whether it is inherited from the mother or from the father.

Figure 76-18 A and B, Individual showing morbid obesity with facial features as shown. C, Upper extremities are notable for small hands relative to body size. D, External genitalia after laparoscopic orchiopexy at 13 months. Parental informed consent, as approved by the Baylor College of Medicine Institutional Review Board, was obtained to publish the photographs.

(From Sahoo T, del Gaudio D, German JR, et al: Prader-Willi phenotype caused by paternal deficiency for the HBII-85 C/D box small nucleolar RNA cluster, Nat Genet 40:719–721, 2008.)

UPD most commonly arises when a pregnancy starts off as a trisomy by trisomy rescue. Because most trisomies are lethal, the fetus can only survive if a cell line loses 1 of the extra chromosomes to become disomic. One third of the time, the disomic cell line is uniparental. This is the typical mechanism for Prader-Willi syndrome, and it is often associated with advanced maternal age. The embryo starts off as trisomy 15 secondary to maternal meiosis I nondisjunction, followed by random loss of the paternal chromosome. In this case, the disomic cell line becomes the more viable one and outgrows the trisomic cell line. When mosaic trisomy is found at prenatal diagnosis, care should be taken to determine whether UPD has resulted and whether the chromosome involved is 1 of the disomies known to be associated with phenotypic abnormalities. There must always be concern that some residual cells that are trisomic are present in some tissues, leading to malformations or dysfunction. The presence of aggregates of trisomic cells might account for the spectrum of abnormalities seen in persons with UPD.