Principles of Human Genetics

Published on 06/06/2015 by admin

Last modified 22/04/2025

Print this page

This article have been viewed 1850 times

116 Principles of Human Genetics

Recent data have suggested that 71% of pediatric hospital admissions were for children with an underlying disorder with a significant genetic component, including 34% with a clear genetic etiology. These genetic disorders comprise chromosomal abnormalities (0.4%-0.6%), diseases caused by a disruption of a single gene (4%-7%), disorders with a genetic association (4%-22%), and malformations of unclear etiology (14%-19%).

Approximately 6500 specific genes have been associated with human disease. As a result of sequencing the human genome, our understanding of human genetics has grown at a rapid pace. Analysis of our genome indicates that there are 20,000 to 25,000 individual genes that determine structure and function of human fetuses, children, and adults, and theoretically, a mutation that disrupts the normal function of any one of these genes could result in human disease. As we learn more about the relationships among genes and human disease, it is clear that an understanding of modes of inheritance and the role of genetic testing will be critical for the care of pediatric patients.

Chromosome Structure and Nomenclature

The human genome is composed of deoxyribonucleic acid (DNA), the genetic code for all organisms. Chromosomes are located in the cell nucleus and are composed of DNA packaged with proteins that exist in 23 pairs in every human diploid cell—22 pairs of autosomes (numbered 1-22) and one pair of sex chromosomes (X and Y). Each pair of chromosomes is connected at a region called the centromere, creating two arms for each pair of chromosomes (Figure 116-1). In most chromosomes, the centromere is located closer to one end than the other, forming a short p arm and a longer q arm. Chromosomes can be visualized under a microscope when they are captured during the metaphase stage of the cell cycle. Using a special technique known as Giemsa staining, metaphase chromosomes can be used to determine a karyotype for an individual (see Figure 116-1). A karyotype result indicates the total number of chromosomes identified in the cell, typically 46, followed by the sex chromosomes observed, usually XX or XY. Therefore, a normal female karyotype is 46,XX, and a normal male karyotype is 46,XY.

Figure 116-1 Chromosome structure and karyotype.

Giemsa staining also allows chromosomal structural differences, and aneuploidy, or imbalance in genetic material, to be identified. The most easily identified forms of aneuploidy include monosomy, the presence of only one of two normal copies of a pair of chromosomes, and trisomy, three copies of a chromosome. Karyotypes are performed on 20 or more cells to determine whether aneuploidy is present in all cells from a patient. To denote aneuploidy, a “+” or “–” is used to indicate an extra or missing chromosome or portion of a chromosome. For example, a female with trisomy 21 would be denoted 47,XX,+21. Another monosomy involving the short arm of chromosome 4, causing Wolf-Hirschhorn syndrome, would be designated in an affected male as 46,XY,4p-. Smaller chromosomal alterations that result from duplicated or deleted region of a chromosome are designated as dup or del followed by the chromosome segment that is missing or extra [e.g., 46,XY del(1)(p36)].

Methods Of Detecting Aneuploidy

With advances in molecular biology, more specific techniques for evaluating aneuploidy have been developed. In the late 1980s, fluorescent in situ hybridization (FISH) was developed as a method to detect specific chromosomal deletions (Figure 116-2). A fluorescent-labeled DNA sequence that is complementary to a known region of a chromosome is hybridized to a set of chromosomes in a cell. If two copies of the chromosome sequence are present, two fluorescent probes are visualized; if only one probe is seen, it is inferred that one copy of the chromosomal segment is deleted. FISH is used to confirm karyotype findings or to detect deletions too small to be visualized with a standard karyotype. FISH also permits analysis of uncultured cells, permitting diagnoses to be made more rapidly.

Figure 116-2 Fluorescent in situ hybridization (FISH) detection of aneuploidy.

Duplicated or deleted regions of the genome must be more than 3 million base pairs in size to be visualized via karyotyping and more than 100 kilobases to be visualized by FISH. Recently, more sophisticated methods of identifying chromosomal alterations of 10 to 100 kilobases to be detected have been introduced. These array-based methods use bacterial artificial chromosome (BAC) probe comparative genomic hybridization (CGH) or synthetic oligonucleotide hybridization probes or single nucleotide polymorphism (SNP) probes. All approaches analyze DNA from a test subject and compare the amount of signal at 5000 with 2 million positions across the genome with that of a normal reference patient’s sample. Often, both pathologic and normal variants are noted and collectively are called copy number variants (CNVs). Those that are believed to cause disease have been termed copy number alterations (CNAs).

Mitosis, Meiosis, and Gamete Formation

Diploid cells are maintained through the process of mitosis in dividing somatic cell types. Typical cells have four stages to their growth and division cycle (Figure 116-3). The first stage, G1 (for growth 1 or gap 1), is largely composed of growth and has no active cell division. During the next stage (S), DNA is replicated or synthesized to form a four-copied, or tetraploid, genome. After another growth phase (G2) during which the replicated genome is surveyed to correct any aberrations, a division phase (D), during which mitosis occurs, results in two new daughter cells. During mitosis, the replicated tetraploid genome is divided so that each daughter cell receives a diploid complement of chromosomes after cell division. Mitosis is divided into four stages—prophase, metaphase, anaphase, and telophase. At the beginning of prophase, the duplicated chromosomes are elongated and appear as a single chromosome, although two sister chromatids have already been formed by this stage. At the end of prophase, the sister chromatids have become more condensed, and the centrioles are seen at opposite poles of the nucleus. During metaphase, the nuclear membrane disintegrates, and the formation of the mitotic spindle made up of fibers that connect each chromatid of a sister chromatid pair to opposite centrioles, causing them to align in the middle of the nucleus. The centromere of each chromosome also divides in preparation for migration of sister chromatids to opposite centriole spindles. It is during metaphase that chromosomes are typically viewed under the microscope because this is when the chromosomes are most condensed and the mitotic spindle can be arrested with the chemical colchicine. During anaphase, sister chromatics are segregated and migrate toward opposite poles along the spindle. Telophase is marked by the arrival of chromosomes at opposite poles, the disintegration of the spindle, and reformation of a nuclear membrane.

Figure 116-3 Cell division and meiosis.

Haploid cells are generated via a special type of germ cell division called meiosis, so that each egg or sperm contributes half of the genetic material to an embryo (see Figure 116-3). A haploid complement of chromosomes consists of one of every autosome (a non-sex chromosome) and an X or Y. In this way, each parent contributes 23 chromosomes to produce a full complement of 46 in an embryo. Meiosis also permits some exchange of genetic material, thus creating genetic variability between gametes. Recombination events, usually at least one per chromosome, result in no two haploid gametes being identical. Recombination events are thought to occur twice as frequently in females as in males. Meiosis is divided into two stages, meiosis I and II. In meiosis I, similar to mitosis, a cell replicates its genome and then divides to create two cells with a full complement of 23 chromosome pairs. In meiosis II, daughter chromatids are divided among daughter cells, such that each contains a single copy of each of the 22 autosomes and an X or Y sex chromosome. In males, all four resulting haploid cells differentiate into sperm. In females, only one of these four haploid set of chromosomes ends up as an ova; the other products become polar bodies. A common cause of aneuploidy is thought to arise from nondisjunction, or failure of the chromosomes to divide equally between daughter cells during meiosis.

Gene Structure

DNA is a double-stranded macromolecule composed of complementary pairs of the purine bases, adenosine and guanine, and the pyrimidine bases, thymine and cytosine (Figure 116-4). The entire haploid genome is composed of some 3 billion base pairs, with each chromosome composed of 50 to 150 million base pairs. Of the entire genome, only about 2% is coding sequence (i.e., encodes genes that are transcribed into proteins). The remainder of the genome is noncoding sequence presumably involved in structural integrity of chromosomes or control of gene expression. A typical gene is transcribed into RNA (RNA), which in turn is translated into a functional protein composed of amino acids. Elements of the genomic DNA surrounding a gene, known as promoters and enhancers, serve to regulate the location, timing. and level of transcription. Before translation, transcribed RNA is processed to form messenger RNA (mRNA) by removing or splicing out intervening sequences known as introns to result in a mature mRNA containing only exons. Mature mRNA moves from the nucleus to the cytoplasm of a cell, where it can be translated into protein in a structure called the ribosome. The ribosome scans the mRNA codons, triplets of nucleotides that encode a specific amino acid, or a start or stop signal. Using the mRNA as a template, ribosomal RNA (rRNA), ribosomal proteins, and transfer RNA (tRNA), the ribosome delivers the appropriate amino acid into the chain of amino acids in the formation of a translated protein. At any stage in the process of transcription or translation, several types of errors can occur, resulting in alterations of protein structure, function, or both (see Figure 116-4).

Figure 116-4 Gene structure and expression.

Modes of Inheritance

An individual inherits genetic material from the mother and father when an egg and sperm fuse during fertilization. Thus, both a maternal and paternal copy of every gene, or alleles, are present in the genome. There are exceptions to this rule, as is evident in males, because the Y chromosome does not carry a matching allele for every gene on the X chromosome. The combination of two specific alleles determines the genotype. The nature of the interaction between two alleles of a given gene determines the mode of inheritance of a disorder caused by mutations in that gene.

Genetic diseases can also be partially inherited as illustrated by the principles of penetrance and expressivity. Penetrance is the likelihood that an individual who carries a disease-causing gene will manifest signs and symptoms of the disorder. A highly penetrant disorder will manifest in nearly all individuals who carry the gene. Expressivity describes the variation in the phenotype, or clinical presentation, that results from a gene alteration, or genotype. Expressivity can be variable, meaning that individuals with the same genotype may have significantly different manifestations in their phenotype or in the severity of their disease.

Autosomal Dominant Inheritance

In some genetic disorders, a single altered copy of a gene is sufficient to cause disease. This is called dominant inheritance and often means that the parent and the progeny who inherited the defective gene are affected. Autosomal dominant implies that the gene is on a non-sex chromosome. Although one allele of the gene may be functional, the allele with the disease-causing mutation is “dominant” over the normal allele. Examples of autosomal dominant disorders include neurofibromatosis type I, achondroplasia, and Huntington’s disease. In addition, the affected individual is typically heterozygous, meaning the two alleles of the gene are not identical (i.e., one normal and one mutated).

Autosomal Recessive Inheritance

Autosomal recessive disorders result from alterations present in both alleles of a gene. Most autosomal recessive disorders are the result from inheritance of one defective gene from each parent. Individuals with an autosomal recessive disorder, therefore, typically have parents who are heterozygous carriers for the disease-causing gene. Carriers are usually unaffected because one functional copy of the gene is often sufficient to mask the effect of the mutant allele. When each allele of the gene has exactly the same mutation, the individual is homozygous for the mutation. However, unique disease-causing mutations can be inherited from each parent as well. Such an individual would manifest disease as the result of being a compound heterozygote for the recessive disorder. An example of an autosomal recessive disorder is sickle cell disease. In sickle cell disease, one can be homozygous for a disease-causing mutation as in sickle cell SS disease. One can also be a compound heterozygote as in sickle cell SC disease.

Sex-Linked Inheritance

A majority of inherited diseases are the result of disease-causing mutations in autosomal genes. However, a number of diseases are known to occur from a defective gene carried on one of the sex chromosomes, typically the X chromosome. X-linked disorders primarily affect males because the Y chromosome does not carry a matching allele for every gene present on the X chromosome. Therefore, a mutation in a gene present on the X chromosome in a male often behaves like a recessive trait because there is only one allele, termed X-linked recessive. X-linked disorders do not typically affect females because their normal allele masks a recessive trait. However, it is common for females to manifest signs and symptoms of an X-linked disorder as a result of skewed X inactivation. In every cell that carries two (or more) X chromosomes, one of the X chromosomes is inactivated, such that most of the alleles on that chromosome are not expressed. If in a female carrier of an X-linked disorder, a disproportionate number of normal or non-mutation-carrying X chromosomes are inactivated, she may manifest mild symptoms of the disease. The phenotype of a female carrier of an X-linked disorder with skewed X inactivation is usually mild compared with an affected male.

Mitochondrial Inheritance

The vast majority of human genes reside in the nucleus of the cell. However, 37 genes are found within mitochondria in a double-stranded DNA molecule (mtDNA). Each mitochondrion carries two to 10 copies of mtDNA encoding 13 proteins, 22 transfer RNAs, and two subunits of ribosomal RNA. As in nuclear-encoded DNA, mutations causing disease can be found in mitochondrial DNA. A unique feature of disorders caused by mitochondrial DNA mutations is that they are inherited maternally because the mitochondria in an embryo are almost universally derived from the 100,000 to 1 million copies of mtDNA in the egg. Another unique feature of mitochondrial genetics is called heteroplasmy, in which both normal and abnormal mitochondria can be present within the same cell, which can lead to a high degree of variability in disorders caused by defects in the mitochondria.

Uniparental Disomy

Normal embryos are derived from maternal and paternal copies of each chromosome. Occasionally, both chromosomes and allelic regions of chromosome pairs can be derived from either the maternal or paternal contribution. This is called uniparental disomy (UPD) and can be either isodisomy, in which both chromosomes inherited from one parent are identical, or heterodisomy, in which both of the different chromosomes from one parent are inherited. UPD may result in autosomal recessive disease if the isodisomic chromosome carries a recessive allele. UPD can also result in disease when imprinting, or the parent-specific expression of the maternal or paternal alleles of a gene is required for normal expression. For example, Prader-Willi syndrome can be caused by maternal UPD of the q arm of chromosome 15, and Angelman’s syndrome is caused by paternal UPD of the same region.

Mosaicism

Mosaicism is a condition in which not all cells in an individual are genetically identical. Mosaicism can occur when chromosomal aneuploidy or a gene mutation arises after conception in a single cell of an early-stage embryo. As this single cell divides, it creates a subset of cells in the developing fetus that carry the chromosomal abnormality or mutation, but the rest do not. This results in heterogeneity of genotype within the various tissues of the individual. An example of a disorder that is only seen in mosaic form is tetrasomy 12p, or Pallister-Killian syndrome. Mosaicism is diagnosed when aneuploidy is identified in some tissues, such as skin fibroblasts, but is absent from other tissues, such as white blood cells from peripheral blood. Mosaicism can thus be missed on a routine karyotype. Down syndrome can also be found in a mosaic form. At times, the severity of the disorder can be inferred by the percentage of mosaic cells found.

Polygenic-Multifactorial Inheritance

Not all congenital anomalies can be explained by a chromosomal alteration or single gene mutation yet the anomalies are still related to genetic differences found within families. These anomalies are thought to be the result of multiple genes that are important in embryologic development of organs and physical features. In the same way that polygenic traits determine height, facial features, and age of pubertal changes, groups of genes inherited from parents can influence the likelihood of organ malformation and craniofacial abnormalities. Examples of polygenic traits that can have a risk of recurrence in families are congenital heart defects, urogenital tract and renal anomalies, cleft lip and palate, and spina bifida.

Evaluation and Diagnosis of Genetic Syndromes

Categories of Dysmorphic Disorders

A genetic syndrome is often suspected in children who have multiple congenital anomalies or who have dysmorphic, or atypically formed, facial or physical features. A pattern of changes that results from a common cause is termed a syndrome. Genetic syndromes result from the expression of a gene in more than one tissue of the body, and mutations subsequently affect different organs of different embryologic origins. A pattern of malformations arising from embryologically distinct tissues in nonrandom fashion with no clear cause is known as an association. This is different from a field defect or complex, which describes a series of malformations that arise from disruption of a single embryologic tissue. Malformation of specific organ systems can also be the result of dysplasia, the disorganization of a specific tissue on a cellular level that can be localized or general and can be system specific or organ specific.

When a primary embryologic defect leads to multiple additional and consequent defects, it is called a sequence. A well-recognized malformation sequence is Pierre-Robin sequence, which involves a small jaw, a large U-shaped cleft palate, and a large protruding tongue. The jaw malformation is thought to lead to upward displacement of the tongue and failure of the palate to form appropriately. Although Pierre-Robin sequence can be an isolated sequence not related to a genetic syndrome, it can also be found in syndromes with genetic causes.

Congenital malformations and dysmorphology can be genetic in origin, but it is important to recognize that they can also be caused by nongenetic mechanisms. A group of anomalies and dysmorphic features can occur as the result of environmental in utero exposures, or teratogens. Common exposures that result in recognized dysmorphology and malformations include alcohol, valproic acid, and TORCH (toxoplasmosis or Toxoplasma gondii, other infections, rubella, cytomegalovirus, and herpes simplex virus) infections.

Malformations can also be caused by an isolated disruption or deformation during embryogenesis unrelated to a genetic defect or teratogens (Figure 116-5). Disruption is a mechanical factor that can occur in utero, resulting in an isolated or in a series of structural abnormalities. An example of a disruption event that can lead to congenital anomalies is amniotic band syndrome. Limb and craniofacial defects are thought to arise from either a vascular disruption or strangulation of fetal digits or extremities by strands of a partially disrupted amniotic sac. Deformation describes an abnormal shape or form caused by mechanical forces from within the fetus or outside the fetus in the intrauterine environment. Examples of deformation include clubfoot and congenital dislocation of the hip. Deformations can occur as the result of crowding in the uterus, as in twin gestations.