Down Syndrome (Trisomy 21)

This condition derives its name from Dr Langdon Down, who first described it in the Clinical Lecture Reports of the London Hospital in 1866. The chromosomal basis of Down syndrome was not established until 1959 by Lejeune and his colleagues in Paris.

Clinical Features

These are summarized in Box 18.1. The most common finding in the newborn period is severe hypotonia. Usually the facial characteristics of upward sloping palpebral fissures, small ears, and protruding tongue (Figures 18.2 and 18.3) prompt rapid suspicion of the diagnosis, although this can be delayed in very small or premature babies. Single palmar creases are found in 50% of children with Down syndrome (Figure 18.4), in contrast to 2% to 3% of the general population. Congenital cardiac abnormalities are present in 40% to 45% of babies with Down syndrome, with the three most common lesions being atrioventricular canal defects, ventricular septal defects, and patent ductus arteriosus.

Box 18.1

Common Findings in Down Syndrome

Newborn period

Hypotonia, sleepy, excess nuchal skin

Craniofacial

Brachycephaly, epicanthic folds, protruding tongue, small ears, upward sloping palpebral fissures

Limbs

Single palmar crease, small middle phalanx of fifth finger, wide gap between first and second toes

Cardiac

Atrial and ventricular septal defects, common atrioventricular canal, patent ductus arteriosus

Other

Anal atresia, duodenal atresia, Hirschsprung disease, short stature, strabismus

FIGURE 18.2 A child with Down syndrome.

FIGURE 18.3 Close-up view of the eyes and nasal bridge of a child with Down syndrome showing upward sloping palpebral fissures, Brushfield spots, and bilateral epicanthic folds.

FIGURE 18.4 The hands of an adult with Down syndrome. Note the single palmar crease in the left hand plus bilateral short curved fifth fingers (clinodactyly).

Patau Syndrome (Trisomy 13) and Edwards Syndrome (Trisomy 18)

These very severe conditions were first described in 1960 and share many features in common (Figures 18.5 and 18.6). The incidence for both is approximately 1 : 5000 and prognosis is very poor, with most infants dying during the first days or weeks of life, though most cases are detected prenatally, often leading to termination. In the unusual event of longer term survival, there are severe learning difficulties. Cardiac abnormalities occur in at least 90% of cases.

FIGURE 18.5 Facial view of a child with trisomy 13 showing severe bilateral cleft lip and palate.

FIGURE 18.6 A baby with trisomy 18. Note the prominent occiput and tightly clenched hands.

Chromosome analysis usually reveals straightforward trisomy. Both disorders occur more frequently with advanced maternal age, the additional chromosome being of maternal origin (see Table 3.4, p. 43). Approximately 10% of cases are caused by mosaicism or unbalanced rearrangements, particularly Robertsonian translocations in Patau syndrome.

Triploidy

Triploidy (69,XXX, 69,XXY, 69,XYY) is a relatively common finding in material cultured from spontaneous abortions, but is seen only rarely in a liveborn infant. Such a child almost always shows severe intrauterine growth retardation with relative preservation of head growth at the expense of a small thin trunk. Syndactyly involving the third and fourth fingers and/or the second and third toes is a common finding. Cases of triploidy resulting from a double paternal contribution usually miscarry in early to mid-pregnancy and are associated with partial hydatidiform changes in the placenta (p. 101). Cases with a double maternal contribution survive for longer but rarely beyond the early neonatal period.

Hypomelanosis of Ito

Several children with mosaicism for diploidy/triploidy have been identified. These can demonstrate the clinical picture seen in full triploidy but in a milder form. An alternative presentation occurs as the condition known as hypomelanosis of Ito. In this curious disorder, the skin shows alternating patterns of normally pigmented and depigmented streaks that correspond to the embryological developmental lines of the skin known as Blaschko’s lines (see Figure 18.7). Most children with hypomelanosis of Ito have moderate learning difficulties and convulsions that can be particularly difficult to treat. There is increasing evidence that this clinical picture represents a non-specific embryological response to cell or tissue mosaicism. A similar pattern of skin pigmentation is sometimes seen in women with one of the rare X-linked dominant disorders (p. 117) with skin involvement, such as incontinentia pigmenti (see Figure 7.18, p. 118). Such women can be considered as being mosaic, as some cells express the normal gene, whereas others express only the mutant gene.

FIGURE 18.7 Mosaic pattern of skin pigmentation on the arm of a child with hypomelanosis of Ito.

(Reproduced with permission from Jenkins D, Martin K, Young ID 1993 Hypomelanosis of Ito associated with mosaicism for trisomy 7 and apparent ‘pseudomosaicism’ at amniocentesis. J Med Genet 1993; 30:783–784.)

Klinefelter Syndrome (47,XXY)

First described clinically in 1942, this relatively common condition with an incidence of 1 : 1000 male live births was shown in 1959 to be due to the presence of an additional X chromosome.

Turner Syndrome (45,X)

This condition was first described clinically in 1938. The absence of a Barr body, consistent with the presence of only one X chromosome, was noted in 1954 and cytogenetic confirmation was forthcoming in 1959. Although common at conception and in spontaneous abortions (see Table 18.1), the incidence in liveborn female infants is low, with estimates ranging from 1 : 5000 to 1 : 10,000.

Clinical Features

Presentation can be at any time from pregnancy to adult life. Increasingly, Turner syndrome is being detected during the second trimester as a result of routine ultrasonography, showing either generalized edema (hydrops) or swelling localized to the neck (nuchal cyst or thickened nuchal pad) (Figure 18.8). At birth many babies with Turner syndrome look entirely normal. Others show the residue of intrauterine edema with puffy extremities (Figure 18.9) and neck webbing. Other findings may include a low posterior hairline, increased carrying angles at the elbows, short fourth metacarpals, widely spaced nipples, and coarctation of the aorta, which is present in 15% of cases.

FIGURE 18.8 Ultrasonographic scan at 18 weeks’ gestation showing hydrops fetalis. Note the halo of fluid surrounding the fetus.

(Courtesy Dr. D. Rose, City Hospital, Nottingham, UK.)

FIGURE 18.9 The foot of an infant with Turner syndrome showing edema and small nails.

Intelligence in Turner syndrome is normal. However, studies have shown some differences in social cognition and higher order executive function skills according to whether the X chromosome was paternal or maternal in origin (p. 105). The two main medical problems are short stature and ovarian failure. The short stature becomes apparent by mid-childhood, and without growth hormone treatment the average adult height is 145 cm. This short stature is due, at least in part, to haploinsufficiency for the SHOX gene, which is located in the pseudoautosomal region (p. 118). Ovarian failure commences during the second half of intrauterine life and almost invariably leads to primary amenorrhea and infertility. Estrogen replacement therapy should be initiated at adolescence for the development of secondary sexual characteristics and long-term prevention of osteoporosis. In vitro fertilization using donor eggs offers the prospect of pregnancy for women with Turner syndrome.

XXX Females

Birth surveys have shown that approximately 0.1% of all females have a 47,XXX karyotype. These women usually have no physical abnormalities, but can show a mild reduction of between 10 and 20 points in intellectual skills and sometimes quite oppositional behavior. This is rarely of sufficient severity to require special education. Studies have shown that the additional X chromosome is of maternal origin in 95% of cases and usually arises from an error in meiosis I. Adults are usually fertile and have children with normal karyotypes.

As with males who have more than two X chromosomes, women with more than three X chromosomes show a high incidence of learning difficulties, the severity being directly related to the number of X chromosomes.

XYY Males

This condition shows an incidence of about 1 : 1000 in males in newborn surveys but is found in 2% to 3% of males who are in institutions because of learning difficulties or antisocial criminal behavior. However, it is important to stress that most 47,XYY men have neither learning difficulty nor a criminal record, although they can show emotional immaturity and impulsive behavior. Fertility is normal.

Physical appearance is normal and stature is usually above average. Intelligence is mildly impaired, with an overall IQ score of 10 to 20 points below a control sample. The additional Y chromosome must arise either as a result of non-disjunction in paternal meiosis II or as a post-zygotic event.

Fragile X Syndrome

This condition, which could equally well be classified as a single gene disorder rather than a chromosome abnormality, has the unique distinction of being both the most common inherited cause of learning difficulties and the first disorder in which a dynamic mutation (triplet repeat expansion) was identified (p. 24) in 1991. It affects approximately 1 : 5000 males and accounts for 4% to 8% of all males with learning difficulties. Martin and Bell described the condition in the 1940s before the chromosome era, and it has also been known as Martin-Bell syndrome. The chromosomal abnormality was first described in 1969 but the significance not fully realized until 1977.

Clinical Features

Older boys and adult males usually have a recognizable facial appearance with high forehead, large ears, long face, and prominent jaw (Figure 18.10). After puberty most affected males have large testes (macro-orchidism). There may also be evidence of connective tissue weakness, with hyperextensible joints, stretch marks on the skin (striae) and mitral valve prolapse. The learning difficulties are moderate to severe and many show autistic features and/or hyperactive behavior. Speech tends to be halting and repetitive. Female carriers can show some of the facial features, and approximately 50% of women with the full mutation show mild-to-moderate learning difficulties.

FIGURE 18.10 A family affected by fragile X syndrome. Two sisters, both carriers of a small FRAXA mutation inherited from their father, have had affected sons with different degrees of learning difficulty.

The Fragile X Chromosome

The fragile X syndrome takes its name from the appearance of the X chromosome, which shows a fragile site close to the telomere at the end of the long arm at Xq27.3 (Figure 18.11). A fragile site is a non-staining gap usually involving both chromatids at a point at which the chromosome is liable to break. In this condition, detection of the fragile site involves the use of special culture techniques such as folate or thymidine depletion, which can result in the fragile site being detectable in up to 50% of cells from affected males. Demonstration of the fragile site in female carriers is much more difficult and cytogenetic studies alone are not a reliable means of carrier detection, in that, although a positive result confirms carrier status, the absence of the fragile site does not exclude a woman from being a carrier.

FIGURE 18.11 X chromosome from several males with fragile X syndrome.

(Courtesy Ashley Wilkinson, City Hospital, Nottingham, UK.)

The Molecular Defect

The fragile X locus is known as FRAXA and the mutation consists of an increase in the size of a region in the 5′-untranslated region of the fragile X learning difficulties (FMR-1) gene. This region contains a long CGG trinucleotide repeat sequence. In the DNA of a normal person, there are between 10 and 50 copies of this triplet repeat and these are inherited in a stable fashion. However, a small increase to between 59 and 200 renders this repeat sequence unstable, a condition in which it is referred to as a premutation. Alleles of 51 to 58 are referred to as intermediate.

A man who carries a premutation is known as a ‘normal transmitting male’, although it has been recognized that these premutation carriers are at increased risk of a late-onset neurological condition named ‘fragile X tremor/ataxia syndrome’ (FXTAS). All of his daughters will inherit the premutation and have normal intelligence, but they are also at small risk of FXTAS in later years. When they have sons, there is a significant risk that the premutation will undergo a further increase in size during meiosis, and if this exceeds 200 CGG triplets, it becomes a full mutation.

The full mutation is unstable not only during female meiosis but also in somatic mitotic divisions. Consequently, in an affected male gel electrophoresis shows a ‘smear’ of DNA consisting of many different-sized alleles rather than a single band (Figure 18.12). Note that a normal allele and premutation can be identified by polymerase chain reaction (PCR), whereas Southern blotting is necessary to detect full mutations as the long GCC expansion is often refractory to PCR amplification. At the molecular level, a full mutation suppresses transcription of the FMR-1 gene by hypermethylation, and this in turn is thought to be responsible for the clinical features seen in males, and in some females with a large expansion (Table 18.7). The FMR-1 gene contains 17 exons encoding a cytoplasmic protein that plays a crucial role in the development and function of cerebral neurons. The FMR-1 protein can be detected in blood using specific monoclonal antibodies.

FIGURE 18.12 Southern blot of DNA from a family showing expansion of the CGG triplet repeat being passed from a normal transmitting male through his obligate carrier daughter to her son with fragile X learning difficulties.

(Courtesy Dr. G. Taylor, St. James’s Hospital, Leeds, UK.)

Table 18.7 Fragile X Syndrome: Genotype-Phenotype Correlations

Number of Triplet Repeats (Normal Range 10–50)	Fragile Site	Intelligence Detectable
Males
51–58 (intermediate alleles)
59–200 (premutation)	No	Normal (normal transmitting male)
200–2000 (full mutation)	Yes (in up to 50% of cells)	Moderate-to-severe learning difficulties
Females
51–58 (intermediate alleles)
59–200 (premutation)	No	Normal
200–2000 (full mutation)	Yes (usually <10% of cells)	50% normal, 50% mild learning difficulties

Another fragile site adjacent to FRAXA has been identified at Xq28. This is known as FRAXE. The expansion mutations at FRAXE also involve CGG triplet repeats and occur much less frequently than FRAXA mutations. Some males with these mutations have mild learning difficulties, whereas others are just as severely affected as men with FRAXA. FRAXE may show up as a fragile site cytogenetically but the PCR test is separate. A third fragile site, FRAXF, has been identified close to FRAXA and FRAXE. This does not seem to cause any clinical abnormality.

Microdeletions

Through high-resolution prometaphase banding (p. 33) and FISH (p. 34), several previously unexplained syndromes have been shown to be due to submicroscopic or micro-deletions. Some microdeletions involve loss of only a few genes at closely adjacent loci, resulting in contiguous gene syndromes. For example, several boys with Duchenne muscular dystrophy (DMD) have been described who also have other X-linked disorders, such as retinitis pigmentosa and glycerol kinase deficiency. The loci for these disorders are very close to the DMD locus on Xp21. Examples of well known microdeletion syndromes are given in Table 18.8, all of them relatively rare.

Table 18.8 Microdeletion Syndromes

WAGR, Wilms’ tumor, aniridia, genitourinary malformations, and retardation of growth and development.

Microarray-CGH

The 1990s witnessed the development of FISH-based analysis of all chromosome telomeres using subtelomeric probes. This led to the diagnosis of some cases of learning difficulty/dysmorphic patients, not detected by multiplex ligation-dependent probe amplification (p. 66). In recent few years this has been rapidly superseded by extensive microarray comparative genomic hybridization (CGH) testing (p. 36) and a number of new microdeletion (and to a lesser extent microduplication) syndromes have emerged. At high resolution, this new technology is yielding significant results in about 20% of cases of well selected, previously unknown dysmorphic patients with developmental delay/learning disability. This compares to a positive pick-up rate of 4% to 5% from standard karyotyping on patients considered likely to have a chromosome disorder. Examples of these new and emerging syndromes are shown in the following section.

Lessons from Microdeletion Syndromes

Retinoblastoma

It was originally observed that approximately 5% of children presenting with retinoblastoma had other abnormalities, including learning difficulties. In some of these, a constitutional interstitial deletion of a region of chromosome 13q was identified. The smallest region of overlap was 13q14, which was subsequently shown to be the position of the locus for the autosomal dominant form of retinoblastoma due to mutations in the RB1 gene (p. 216).

Wilms’ Tumor

Some children with the rare renal embryonal neoplasm known as Wilms’ tumor (or hypernephroma) also have aniridia, genitourinary abnormalities, and retardation of growth and development. This combination is referred to as the WAGR syndrome. Chromosome analysis in these children often reveals an interstitial deletion of 11p13 (Figure 18.15). The deletion genes include PAX6, which is responsible for the aniridia (Figure 18.16). Confirmation is made by FISH probe analysis (or direct gene mutation analysis in cases of pure aniridia). Loss of the WT1 gene causes the development of Wilms’ tumor (see also p. 219). Knowing this, it can now be predicted whether a newly diagnosed child with deletion 11p13 is at high risk of developing a Wilms’ tumor, using a separate FISH analysis at the WT1 locus. It is important to note that, however, that the genetics of Wilms’ tumor is complex, with other loci sometimes involved.

FIGURE 18.15 A, Metaphase spread showing the number 11 chromosomes (double arrows). The chromosome indicated by the single arrow has an interstitial deletion in the short arm. See Figures 18.11 and 18.12.

(Courtesy Meg Heath, City Hospital, Nottingham, UK)

B, FISH showing failure of a PAX6 locus specific probe (red) to hybridize to the deleted number 11 chromosome shown in A from a child with WAGR syndrome. The green probe acts as a marker for the centromere of each number 11 chromosome.

(Courtesy Dr. John Crolla, Salisbury and Dr. Veronica van Heyningen, Edinburgh, UK.)

FIGURE 18.16 A baby with deletion 11p13 presenting with aniridia on routine neonatal examination.

Angelman and Prader-Willi Syndromes

These two conditions have special place in medical genetics as paradigms for genomic imprinting. Children with Angelman syndrome (see Figure 7.24; p. 123) have inappropriate laughter, convulsions, poor coordination (ataxia), and severe learning difficulties. Children with Prader-Willi syndrome (see Figure 7.22; p. 122) are extremely floppy (hypotonic) with poor feeding in infancy, and later develop hyperphagia and obesity, with mild-to-moderate learning difficulties. A large proportion of children with these disorders have a microdeletion involving 15q11-13.

In contrast, a deletion occurring at the same region on the maternally inherited chromosome 15 causes Angelman syndrome. Non-deletion cases also exist and are often due to uniparental disomy (p. 121), with both number 15 chromosomes being paternal in origin in Angelman syndrome, and maternal in origin in Prader-Willi syndrome. These parent-of-origin effects are explained by imprinting (see Figure 7.23; p. 123).

DiGeorge/Sedláčková/Velocardiofacial Syndrome

DiGeorge syndrome affects approximately 1 : 4000 births, is usually sporadic, and is characterized by heart malformations (particularly those involving the cardiac outflow tract), thymic and parathyroid hypoplasia, cleft palate and typical facies. The molecular defect is a 3-Mb microdeletion on chromosome 22 (22q11.2). Dr Eva Sedláčková from Prague reported a large series of children with a congenitally short palate in 1955, 10 years earlier than DiGeorge, and these patients clearly had the same condition. A similar phenotype was described by Shprintzen and referred to as velocardiofacial syndrome. Because of the confusion of eponyms and other terms given to this condition over the years, ‘deletion 22q11 syndrome’ now has the most widespread acceptance (at the molecular level the deleted DNA segment is still called the DiGeorge Critical Region). Figure 18.17 shows individuals with deletion 22q11.2 at different ages. Because it is the most common of the microdeletion syndromes, it has been intensely researched. It is variable and many affected individuals are able to reproduce, so the condition follows autosomal dominant inheritance in some families. The 3 Mb deletion occurs because this region is flanked by two identical sequences of DNA, known as low-copy repeats (LCRs), of the type that occur frequently throughout the genome. At meiosis the chromosomes can be ‘confused’ when they align, such that the downstream DNA sequence aligns with the upstream. If recombination occurs between these two flanking regions, a deletion of 3 Mb results on one chromosome 22. It is possible that the phenotypic features may be due largely to haploinsufficiency for the TBX1 gene that lies within the region.

FIGURE 18.17 Deletion 22q11 (DiGeorge/Sedláčková/velocardiofacial) syndrome. A, A young infant. B, A young child. C, An older child. D, The same individual (shown in C) as an adult age 49 years.

Those diagnosed should be investigated for cardiac malformations, calcium and parathyroid status, immune function, and renal anomalies. About half have short stature and a small proportion of these have partial growth hormone deficiency. About 25% have schizophrenia-like episodes in adult life.

Duplication 22q11.2

The misaligned pairing at meiosis, of the LCRs that flank the 3 Mb region at 22q11.2 that causes DiGeorge syndrome, predicts that gametes duplicated for this DNA segment would be present in equal numbers. However, duplication 22q11.2 syndrome is seldom seen in clinical practice, suggesting that it might be subclinical in its effects.

Some patients have been reported but there is no consistent phenotype. Some cases bear similarity to the deletion 22q11.2 phenotype, but marked variability occurs. The problems range from isolated mild learning difficulties to multiple abnormalities with non-specific dysmorphic features, congenital heart disease, cleft palate, hearing loss, and postnatal growth deficiency. More cases may be diagnosed using microarray-CGH.

Williams Syndrome

Williams syndrome occurs because of a microdeletion at chromosome 7q11 and diagnosis can be confirmed by FISH. The clinical phenotype was first reported by Williams in 1961 and later expanded by Beuren (hence, sometimes, Williams-Beuren syndrome). Hypocalcaemia is a variable feature in childhood and sometimes persists, whilst supravalvular aortic stenosis (SVAS) and peripheral pulmonary artery stenosis are congenital abnormalities of the great vessels. Haploinsufficiency at 7q11 leads to loss of one copy of the gene that encodes elastin, a component of connective tissue. This is probably the key factor causing SVAS and the vascular problems that are more common in later life. Individuals have a characteristic appearance (Figure 18.18) with mild short stature, a full lower lip, and sloping shoulders. Equally characteristic is their behavior. They are typically very outgoing in childhood—having a ‘cocktail party manner’—but become withdrawn and sensitive as adults. All are intellectually impaired to the extent that they cannot lead independent lives, and the majority do not reproduce, although parent-child transmission has been reported.

FIGURE 18.18 A person with Williams syndrome as a baby (A), a young child (B), an older child (C), and in his early forties (D).

Smith-Magenis Syndrome

This microdeletion syndrome is due to loss of chromosome material at 17p.11.2, often visible cytogenetically. As with DiGeorge syndrome, the mechanism of deletion in many cases involves homologous recombination between flanking LCRs. The physical characteristics are not highly distinctive (Figure 18.19), but congenital heart disease occurs in one-third, scoliosis develops in late childhood in more than half, and hearing impairment in about two-thirds. The syndrome is most likely to be recognized by the behavioral characteristics: as children, patients exhibit self-harming (head-banging, pulling out nails, and inserting objects into orifices), a persistently disturbed sleep pattern, and characteristic ‘self-hugging’. Some degree of learning difficulty is the norm. The sleep pattern can often be managed by judicious use of melatonin.

FIGURE 18.19 A young person with Smith-Magenis syndrome; the facial features are not highly distinctive, but the philtrum is usually short. As babies, chromosome studies are often requested because the possibility of Down syndrome is raised.

Deletion 1p36 Syndrome

A new microdeletion syndrome that emerged through improved cytogenetic techniques and the use of FISH in the 1990s is 1p36 syndrome which, in keeping with today’s approach to nomenclature, has no eponym. The features are hypotonia, microcephaly, growth delay, severe learning difficulties, epilepsy (including infantile spasms), characteristically straight eyebrows with slightly deep-set eyes, and midface hypoplasia (Figure 18.20). Some cases developed dilated cardiomyopathy.

FIGURE 18.20 A child with deletion 1p36 syndrome—very straight eyebrows, epilepsy, and learning difficulties.

Deletion 9q34 Syndrome

Another of the relative new microdeletion syndromes, this was first reported as a condition featuring significant learning difficulties, hypotonia, obesity, brachycephaly, arched eyebrows, synophrys, anteverted nostrils, prognathism, sleep disturbances, and behavioural problems. Many patients have severe speech delay and not all manifest obesity. The case pictured here (Figure 18.21) bears a passing resemblance to Angelman syndrome. Some patients with the phenotypic features but no microdeletion have been shown to have mutations in the Euchromatin histone methyl transferase 1 (EHMT1) gene, which lies within the region. The syndrome might therefore be mainly due to haploinsufficiency for this gene.

FIGURE 18.21 A child with deletion 9q34 syndrome. She has arched eyebrows, narrow upslanting palpebral fissures, brachycephaly, prognathism, and severe learning difficulties. She was initially investigated for possible Angelman syndrome.

Deletion 17q21.31 Syndrome

This new condition has a prevalence of approximately 1 : 16,000 and is probably significantly underdiagnosed. The main features are severe developmental delay, hypotonia, and characteristic facial dysmorphisms including a long face with a high forehead and tubular or pear-shaped nose, a bulbous nasal tip, large ears, and everted lower lip (Figure 18.22). Individuals tend to be friendly. Other clinically important features include epilepsy, heart defects, kidney anomalies, and long slender fingers.

FIGURE 18.22 This person shows the characteristic facial features of deletion 17q21.31 syndrome. The face is long and the nose somewhat tubular or pear-shaped, and the nasal tip bulbous. There is developmental delay.

(Courtesy Dr. David Koolen, Nijmegen, Netherlands.)

Deletion 1q21.1 Syndrome

This condition was first identified in three individuals from a cohort of 505 with congenital heart disease. The phenotype is broad and includes mild-moderate mental retardation, small head size, growth retardation, heart defects, cataracts, hand deformities and skeletal problems, learning disabilities, seizures, and autism. On the other hand, however, some individuals with the aberration are only slightly affected, or apparently unaffected. A mother and her child, both with the deletion, are shown in Figure 18.23. Variable penetrance and lack of highly distinctive features make genetic counseling for this aberration highly problematic. Some experts regard it as a susceptibility locus rather than a clinically distinct syndrome.

FIGURE 18.23 A, This mother and child have deletion 1p21.1 syndrome. They bear a resemblance to each other and there is evidence of mild development delay and small head size. B, The same child nearly a year after the first picture.

Chromosome disorders and behavioral phenotypes. The distinctive behavior of children with Williams syndrome—their outgoing ‘cocktail party manner’—has been recognized as part of the condition for a long time. As the microdeletion conditions have emerged, it has been increasingly clear that patterns of behavior can reliably be attributed to certain disorders. This is very striking in Smith-Magenis syndrome, but also apparent to a lesser extent in deletion 22q11, cri-du-chat, and Angelman and Prader-Willi syndromes. It is also apparent in the aneuploidies (Down and Klinefelter syndromes), as well as in 47,XXX and 47,XYY and fragile X syndromes. Behavioral phenotypes have therefore become an area of considerable interest to clinical scientists and the observations lend support to the belief that behavior, to some extent at least, is genetically determined. In studying chromosome disorders we are of course looking at genetically abnormal situations, and from this we cannot necessarily extrapolate directly to ‘normal’ situations. For the latter, twin studies have provided substantial and valuable information. This field of study remains complex and understandably controversial. However, most now accept that behavior is a complex interaction of genetic background, physical influences during early development (e.g., fetal well-being), nurturing experiences, family size, culture, and belief systems.

True Hermaphroditism

In this extremely rare condition, an individual has both testicular and ovarian tissue, often in association with ambiguous genitalia. When an exploratory operation is carried out in these patients, an ovary can be found on one side and a testis on the other. Alternatively, there can be a mixture of ovarian and testicular tissue in the gonad, which is known as an ovotestis. Most patients with true hermaphroditism have a 46,XX karyotype, and in many of these individuals the paternally derived X chromosome carries Y chromosome–specific DNA sequences as a result of illegitimate crossing over between the X and Y chromosomes during meiosis I in spermatogenesis (Figure 18.24).

FIGURE 18.24 FISH showing hybridization of a Y chromosome paint to the short arm of an X chromosome in a 46,XX male.

(Courtesy Nigel Smith, City Hospital, Nottingham, UK.)

A small proportion of patients with true hermaphroditism are found to be chimeras with both 46,XX and 46,XY cell lines, a situation analogous to freemartins in cattle (p. 51).

Male Pseudohermaphroditism

In pseudohermaphroditism there is gonadal tissue of only one sex. The external genitalia can be ambiguous or of the sex opposite to that of the chromosomes. Thus in male pseudohermaphroditism there is a 46,XY karyotype with ambiguous or female genitalia.

The most widely recognized cause of male pseudohermaphroditism is androgen insensitivity (p. 175). In this condition, which is also known as testicular feminization syndrome, the karyotype is normal male and the external phenotype is essentially that of a normal female. Internally the vagina ends blindly and the uterus and fallopian tubes are absent. Testes are located in the abdomen or in the inguinal canal, where they can be mistaken for inguinal herniae. This condition is caused by the absence of androgen receptors in the target organs, so that, although testosterone is formed normally, its peripheral masculinizing effects are blocked. Androgen receptors are encoded by a gene on the X chromosome in which both deletions and point mutations have been identified. Curiously, expansion of a CAG repeat in the first exon of this androgen receptor gene causes a neurological disorder known as Kennedy disease, or spino-bulbar muscular atrophy. This is a rare example of the phenomenon sometimes called a ‘gene within a gene’.

Other causes of male pseudohermaphroditism include:

Female Pseudohermaphroditism

In female pseudohermaphroditism, the karyotype is female and the external genitalia are virilized so that they either resemble those of a normal male or are ambiguous.

Congenital adrenal hyperplasia (CAH) is by far the most important cause of female pseudohermaphroditism (p. 174). This can be caused by several different enzyme defects in the adrenal cortex, all of which show autosomal recessive inheritance. Reduced cortisol production leads to an increase in adrenocorticotrophic hormone secretion, which in turn causes hyperplasia of the adrenal glands. In the most common form of CAH, because of a 21-hydroxylase deficiency, hormone synthesis switches from the manufacture of cortisol and aldosterone to the androgen pathway (see Figure 11.5; p. 174), leading to striking virilization of a female fetus (see Figure 11.6; p. 175). The lack of cortisol and aldosterone usually leads to rapid collapse within days of birth, which can prove fatal unless appropriate hormone and electrolyte supplementation is initiated.

Rarer causes of female pseudohermaphroditism include an androgen-secreting tumor and maternal androgen ingestion during pregnancy.

Ataxia Telangiectasia

This is an autosomal recessive disorder that presents in early childhood with ataxia, oculocutaneous telangiectasia (Figure 18.25), radiation sensitivity, and susceptibility to sinus and pulmonary infection (p. 204). There is a 10% to 20% risk of leukemia or lymphoma. Cells from patients show an increase in spontaneous chromosome abnormalities, such as chromatid gaps and breaks, which are enhanced by radiation. The gene for ataxia telangiectasia is called ATM and maps to chromosome 11q23. The protein product is thought to act as a ‘checkpoint’ protein kinase, which interacts with the TP53 and BRCA1 gene products to arrest cell division and thereby allow repair of radiation-induced chromosome breaks before the S phase in the cell cycle (p. 39).

FIGURE 18.25 Ocular telangiectasia in a child with ataxia telangiectasia.

Bloom Syndrome

Children with this autosomal recessive disorder are small with a light-sensitive facial rash and reduced immunoglobulin levels (IgA and IgM). The risk of lymphoreticular malignancy is approximately 20%. Cultured cells show an increased frequency of chromosome breaks, particularly if they are exposed in vitro to ultraviolet light. The gene for Bloom syndrome maps to chromosome 15q26, where it encodes one member of a group of enzymes called the DNA helicases (p. 14). These are responsible for unwinding double-stranded DNA before replication, repair, and recombination. Normally the Bloom syndrome gene plays a major role in maintaining genome stability. When defective in the homozygous state, DNA repair is impaired and the rate of recombination between sister chromatids is increased dramatically. This can be demonstrated by looking for sister chromatid exchanges (see the following section).

Fanconi Anemia

This autosomal recessive disorder is associated with upper limb abnormalities involving the radius and thumb (Figure 18.26), increased pigmentation, and bone marrow failure leading to deficiency of all types of blood cells (i.e., pancytopenia). There is also an increased risk of neoplasia, particularly leukemia, lymphoma, and hepatic carcinoma. Multiple chromosomal breaks are observed in cultured cells (Figure 18.27) and the basic defect lies in the repair of DNA strand cross-links. There are five known subtypes of Fanconi anemia, each caused by recessive mutations at different autosomal loci. The most common, type A, maps to chromosome 16q24.

FIGURE 18.26 Bilateral radial aplasia with absent thumbs in an infant with Fanconi anemia.

FIGURE 18.27 Multiple chromosome breaks and gaps in a metaphase spread prepared from a child with Fanconi anemia.

Chromosome Breakage and Sister Chromatid Exchange

Strong evidence of increased chromosome instability is provided by the demonstration of an increased number of sister chromatid exchanges (SCEs) in cultured cells. An SCE is an exchange (crossing over) of genetic material between the two chromatids of a chromosome in mitosis, in contrast to recombination in meiosis I, which is between homologous chromatids. SCEs can be demonstrated by differences in the uptake of certain stains by the two chromatids of each metaphase chromosome after two rounds of cell division in the presence of the thymidine analogue 5-bromodeoxyuridine (BUdR), which becomes incorporated in the newly synthesized DNA (Figure 18.28). There are normally about 10 SCEs per cell, but the number is greatly increased in cells from patients with Bloom syndrome and xeroderma pigmentosa. In the latter condition, this is apparent only after the cells have been exposed to ultraviolet light.

FIGURE 18.28 Chromosome preparation showing sister chromatid exchanges (arrow).

It is not clear how SCEs relate to the increased chromosome breakage observed in these two disorders, but it is thought that the explanation could involve one of the steps in DNA replication. It is also of interest that the number of SCEs in normal cells is increased on exposure to certain carcinogens and chemical mutagens. For this reason the frequency of SCEs in cells in culture has been suggested as a useful in vitro test of the carcinogenicity and mutagenicity of chemical compounds (p. 26).

Multiple Congenital Abnormalities

Every child with multiple congenital abnormalities should have chromosome studies undertaken. This is important for several reasons:

Although it can be very distressing for parents to be told that their child has a chromosome abnormality, they will often be relieved that an explanation for their child’s problems has been found.

Unexplained Learning Difficulties

Chromosome abnormalities cause at least one-third of the 50% of learning difficulties that are attributable to genetic factors. Although most children with a chromosome abnormality have other features such as growth retardation and physical anomalies, this is not always so. If the fragile X syndrome is a possibility, it is important that the cytogenetics laboratory is informed so that the correct culture conditions are used (p. 279), although in most centers the fragile X syndrome is now diagnosed by molecular methods rather than chromosome analysis. If standard karyotyping and fragile X syndrome testing is negative, great reliance will increasingly be placed on microarray CGH.

Sexual Ambiguity

The birth of a child with ambiguous genitalia should be regarded as a medical emergency, not only because of the inevitable parental anxiety, but also because of the importance of ruling out the potentially life-threatening diagnosis of salt-losing congenital adrenal hyperplasia (p. 174). A chromosome analysis should be among the first investigations undertaken.

Disorders of sexual development presenting in later life with problems such as delayed puberty, primary amenorrhea or male gynecomastia are also strong indications for chromosome analysis as a first-line investigation. This can provide a diagnosis such as Turner syndrome (45,X) or Klinefelter syndrome (47,XXY). Alternatively a normal karyotype will stimulate a search for other possible explanations, such as an endocrine abnormality.

Infertility and Recurrent Miscarriage

Unexplained involuntary infertility should prompt a request for chromosome studies, particularly if investigations reveal evidence of azoospermia in the male partner. At least 5% of such men are found to have Klinefelter syndrome. More rarely a complex chromosome rearrangement such as a translocation can cause such severe mechanical disruption in meiosis that complete failure of gametogenesis ensues.

At least 15% of all recognized pregnancies end in spontaneous miscarriage; in 50% of cases this is because of a chromosome abnormality (p. 273). Unfortunately, some couples experience recurrent pregnancy loss—usually defined as more than three spontaneous miscarriages. Often no explanation is found and many such couples go on to have successful pregnancies. However, in 3% to 6% one partner is found to carry a chromosome rearrangement that predisposes to severe imbalance through malsegregation at meiosis (p. 39). Consequently it is now standard practice to offer chromosome analysis to all such couples.

Unexplained Stillbirth/Neonatal Death

The presence of growth retardation and at least one congenital abnormality in a stillbirth or neonatal death would be an indication for chromosome studies based on analysis of blood or skin collected from the baby before or as soon after death as possible. Skin fibroblasts continue to be viable for several days after death. Chromosome abnormalities account for 5% of all stillbirths and neonatal deaths, and not all of these babies have multiple abnormalities that would immediately suggest a chromosomal cause.

Malignancy and Chromosome Breakage Syndromes

Certain types of leukemia and many solid tumors, such as retinoblastoma (p. 215) and Wilms’ tumor (p. 282), are associated with specific chromosomal abnormalities that can be of both diagnostic and prognostic value. Clinical features suggestive of a chromosome breakage syndrome (p. 288), such as a combination of photosensitivity and short stature, should also lead to appropriate chromosome fragility studies, such as analysis of sister chromatid exchanges.