Genetics and Prenatal Genetic Testing

Abnormalities of Chromosome Number

Aneuploidy is the most common clinically significant type of human chromosomal abnormality and occurs in 3% to 4% of recognized pregnancies. Nondisjunction in mitosis or meiosis is the cause of most aneuploidies. Maternal meiotic nondisjunction is known to increase with maternal age, and therefore the risk for aneuploid offspring also increases with advancing maternal age ( Table 2-1 ). Both trisomy, the presence of three copies of an individual chromosome, and monosomy, the presence of a single copy of a chromosome, typically have significant phenotypic consequences. Monosomy is seen less frequently than trisomy because this situation is generally not compatible with life. Trisomy or monosomy for any chromosome can occur theoretically, but in practice, some are seen much more frequently than others ( Table 2-2 ). This is because most aneuploidies are incompatible with life, and aneuploid embryos spontaneously abort very early in gestation. Some chromosome abnormalities, for example, trisomy 8, are seen at birth only in mosaic form, and the full trisomy is likely lethal. Mosaicism is defined as the presence of two different cell lines with different genotypes in the same individual. Mosaicism is not always deleterious but can result in an abnormal phenotype depending on the degree and type of affected tissue. An entire extra set or sets of chromosomes, polyploidy, is also possible but is not compatible with long-term survival.

Trisomy 21

The most common aneuploidy detected at birth is trisomy 21, which occurs in approximately 1/700 live births ( Fig. 2-3 ). Trisomy 21 occurs as a result of maternal nondisjunction during meiosis in 95% of cases of DS; the remainder of cases is due to translocations involving chromosome 21. Karyotype analysis from the affected individual can differentiate between nondisjunction and translocation causes. Individuals with DS from either cause have a characteristic phenotype that includes distinctive facial features including midface hypoplasia and upslanting palpebral fissures, short stature, brachycephaly, a short neck with redundant skin on the nape, short broad hands with a single transverse palmar crease, and hypotonia. Other findings include intellectual disability, an increased risk for acute myeloid leukemia in the first 3 years of life, and thyroid dysfunction. Congenital heart defects (CHDs) are reported in 44% to 58% of affected newborns and are important contributors to morbidity and fatality. Other structural birth defects associated with this diagnosis include hydronephrosis and duodenal atresia.

The recurrence risk of trisomy 21 (or other autosomal trisomies) is approximately 1% after one such child with nondisjunction-related aneuploidy is born to a couple. Although often requested, there is no indication for parental chromosomal evaluation after the birth of a child with trisomy 21 unless it is due to a translocation. A history of DS elsewhere in the family does not increase the risk of having a child with a chromosome abnormality, provided there is no familial translocation.

Abnormalities of Chromosome Structure

There are many types of abnormalities of chromosome structure, but all generally result from chromosome breakage, followed by recombination in an abnormal configuration. Overall, structural chromosome abnormalities occur in about 1/375 newborns. Reciprocal translocations involve an exchange of segments between nonhomologous chromosomes. These rearrangements are relatively common and can be balanced (that is, the appropriate amount of chromosomal material is present), typically resulting in a normal outcome, or unbalanced, in which some chromosomal material is gained or lost, therefore resulting in an abnormal outcome. There is an increased risk of unbalanced rearrangements in the offspring of people who carry balanced translocations, thereby leading to decreased fertility and an increased risk of offspring with structural malformations or intellectual disability.

A robertsonian translocation occurs when the long arms of two acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22) are fused and the corresponding two short arms are lost. Because the short arms of these chromosomes do not contain essential genetic material (they are made up of multiple copies of ribosomal RNA genes), the loss of this material does not result in an abnormal phenotype in a balanced robertsonian translocation carrier. The phenotype of an unbalanced robertsonian translocation is either miscarriage or a child with a trisomic condition such as trisomy 21 or trisomy 13 from the additional material. The most common robertsonian translocation involves chromosomes 14 and 21 and is responsible for about 5% of cases of DS. The recurrence of translocation DS depends on whether the translocation was inherited from a carrier parent. In 75% of cases, the translocation is de novo, and the recurrence risk is very low (<1%). If the translocation was inherited from a parent who carries a 14;21 balanced translocation, there is a 10% chance of recurrence if the mother is the carrier and about a 2% chance if the father is the carrier.

When a balanced translocation or other rearrangement is identified in a fetus at the time of prenatal diagnosis, testing of the parents is recommended. If the translocation was inherited from a phenotypically normal parent, the fetus would be predicted to be normal. If a reciprocal translocation occurs as a de novo event, there is approximately a doubling of the background risk of phenotypic abnormality. This is thought to be due to subtle, undetected chromosomal imbalance or disruption of genes at the breakpoint(s).

Other Types of Chromosomal Rearrangements

In addition to translocations, there are several other types of unbalanced chromosomal rearrangements. These types include deletions , which occur with the loss of a chromosome segment, resulting in partial monosomy. Such deletions can involve one or many genes and are responsible for a number of clinically distinct microdeletion syndromes ( Table 2-3 ). One of the most common is the 22q deletion syndrome, which occurs in about 1/4000 live births. The 22q deletion is a relatively common cause of CHDs, particularly conotruncal abnormalities, such as truncus arteriosus and tetralogy of Fallot. Other features include cleft palate, velopharyngeal incompetence, renal malformations, characteristic facial features, immune deficiency, hypocalcemia, and learning difficulties. The majority of individuals (93%) has a de novo deletion identified by microarray or FISH. When an individual is diagnosed with this condition, parental testing is recommended because of the variable expressivity within a family and the small chance that one of the parents is a more mildly affected carrier.

Duplications of a chromosome segment also occur, resulting in partial trisomy. The size and region of the duplication confer risk for abnormal phenotype ranging from normal to significant structural defects and intellectual disability.

Autosomal-Dominant Inheritance

In AD inheritance, affected individuals are heterozygous for an abnormal allele that they transmit to 50% of their offspring, independent of gender. Overall, approximately 1/200 individuals is affected with an AD condition, although individual conditions are generally rare.

In theory, AD inheritance is straightforward, but in actuality, there are numerous factors affecting gene expression that can make evaluation of such cases extremely complex. Counseling of families regarding AD disorders requires consideration of mechanisms such as the rate of new mutation, mosaicism, conditions with late or variable age of onset, incomplete penetrance, and variation in expressivity. AD disorders may be inherited from an affected parent or may result from a new mutation, with the risk varying with each specific condition. Paternal age has a demonstrable effect on the rate of such new mutations ( Fig. 2-6 ) and is associated with an increased risk for AD disorders, including achondroplasia and neurofibromatosis (NF). No current screening or diagnostic testing is recommended for men who are aged 45 years or more, but genetic counseling, routine ultrasound, and discussion of this phenomena are indicated.

If a disorder occurs as a result of a new mutation, the risk of recurrence in a sibling is generally low. However, an individual may be mosaic for an AD mutation, meaning that there are cells of more than one genetic constitution present. If germline mosaicism is present, an apparently unaffected individual will have an increased risk of having affected offspring. Germline mosaicism is thought to occur in approximately 16% of cases of the perinatal lethal disorder osteogenesis imperfecta type II and can also occur in X-linked disorders such as Duchenne muscular dystrophy and hemophilia A. This means that the rate of recurrence is increased and is about 1.3% to 4% with osteogenesis imperfecta.

Autosomal-Recessive Disorders

AR disorders are classically described as those for which two gene mutations must be present for the disorder to manifest. Such disorders occur most commonly in persons whose healthy parents each carry a mutation in the same recessive gene. The recurrence risk to such carrier parents is 25% in each pregnancy. Because the carrier frequency of a given condition is usually low, these disorders most commonly occur with no prior family history. Multiple affected individuals may be identified in a sibship but not typically in multiple generations unless there is consanguinity or the disorder is particularly common. Consanguinity increases the risk of having offspring with AR disorders because of the shared genetic material within a family. The increased risk to parents who are first cousins to have a child with a major genetic or congenital abnormality is about 6%, or twice the background rate.

Again, with advances in genetics, the exceptions to the traditional rules of AR inheritance are increasingly evident. It is now appreciated that carriers of many of these conditions may have subtle symptoms. Examples of this are cystic fibrosis (CF), in which male carriers have increased susceptibility to chronic pancreatitis and congenital bilateral absence of the vas deferens. Similarly, sickle cell carriers may suffer splenic infarct at high altitudes and are at increased risk for urinary tract infection in pregnancy.

AR disorders demonstrate much less variation in expression, and lack of penetrance is rarely encountered, so counseling is more straightforward with these conditions. Genetic heterogeneity due to more than one causative locus, or multiple alleles at a single locus, is the major cause of variation in severity of a single disorder.

X-Linked Inheritance

X-linked disorders can be dominant or recessive. Most X-linked conditions are recessive and are traditionally thought of as affecting only men, who have unaffected carrier mothers. Because women carry two X chromosomes whereas men carry only one, one X chromosome is randomly inactivated early in embryonic development, thereby allowing women to produce X-linked gene products in the same quantities as men (Lyon hypothesis). As X inactivation is random, some female heterozygote carriers of X-linked disorders will inactivate primarily their X chromosomes carrying the normal allele and therefore be symptomatic due to skewed X inactivation. This occurs in hemophilia A, in which some carriers can have a mild bleeding disorder because of reduced levels of factor VIII. Such affected women have symptoms that are usually, but not always, milder than their male counterparts. With isolated X-linked recessive disorders, it is helpful when possible to determine if an isolated case represents a new mutation or if the mother is a carrier. When molecular diagnosis or other methods of carrier detection are available, the situation can be clarified. However, such tests are not available for all conditions, and risks must often be empirically determined based on pedigree analysis and consideration of other factors such as the number of unaffected men in the pedigree. Increasingly, genomic sequencing is available to identify genetic mutations in individuals in whom a genetic disorder is suspected.

X-linked dominant disorders are less common than X-linked recessive diseases, and some such conditions are lethal in men. Examples include Rett syndrome, incontinenti pigmenti, and Aicardi syndrome. Aicardi syndrome is a rare disorder primarily of the CNS, including agenesis of the corpus callosum, microphthalmia, and infantile spasms. When an affected child with such a condition is born to healthy parents, it is likely due to a new mutation and the recurrence risk is low, although consideration must be given to germline mosaicism.

Fragile X syndrome, the most common inherited form of mental retardation, is an important disorder that demonstrates X-linked inheritance. Fragile X syndrome is characterized not only by mental retardation but also by behavioral difficulties including autism, and specific dysmorphic features. The condition affects males (approximately 1/4000) more commonly and more severely than females. Fragile X syndrome is a trinucleotide repeat disorder, characterized by expansion of DNA triplets beyond the normal, stable threshold, which may occur in certain genes and result in a condition or disease. Trinucleotide repeat disorders are typically neuromuscular diseases and include Huntington disease, myotonic dystrophy, Friedreich ataxia, and spinocerebellar ataxia. The number of repeats can expand due to chromosomal instability during meiosis, which can cause the phenotype of the condition to change and become more severe. Because of this, trinucleotide repeat disorders generally show genetic anticipation, in which the severity increases with each successive generation that inherits them. Individuals are considered to be carriers for fragile X syndrome when they have an intermediate number of trinucleotide (CGG) repeats in the FMR1 gene (also called a premutation) that can expand during oogenesis to a larger number of repeats (full mutation) (see Fig. 2-5 ). Full-mutation men have moderate to severe mental retardation and behavioral problems such as autism, whereas women are affected by a more variable phenotype, ranging from normal to as severe as affected men. Premutation female carriers have a relatively high rate of premature menopause (15% to 25%), whereas male premutation carriers can have a tremor ataxia condition with an onset later in life. For any patient with a family history of mental retardation, early menopause, or autism without an attributable diagnosis, or adults with parkinsonian features, carrier testing for fragile X syndrome is indicated.

Other Novel Genetic Mechanisms

Some genetic mutations have been noted to have a very different effect depending on the parent of origin, a phenomenon known as genetic imprinting. Imprinting is an epigenetic phenomenon by which certain genes are expressed or not expressed depending on the parent of origin ( Fig. 2-7 ). As depicted in Figure 2-8 , the imprinting cycle takes places in primordial germ cells early in development. A number of genetic disorders, including Prader-Willi syndrome (PWS) and Beckwith-Wiedemann syndrome (BWS), occur because of abnormalities of imprinted genes. Prader-Willi syndrome is characterized by neonatal and infantile hypotonia, hypogonadism, typical facial features, and feeding problems; these are followed in early childhood by excessive caloric intake resulting in morbid obesity and intellectual disability. The disorder occurs when there is no functional paternal copy of the SNRPN gene on chromosome 15. This can happen when both copies of chromosome 15 are maternally inherited, and no paternal copy is present, or when there is a deletion on the paternally inherited copy of chromosome 15. Several recent reports have indicated an association between imprinting disorders and in vitro fertilization (IVF) with intracytoplasmic sperm injection.

Mitochondria, which produce adenosine triphosphate (ATP), have their own unique DNA in addition to nuclear DNA. Mitochondria are all maternally inherited and have a high mutation rate. Many mitochondrial diseases have now been identified, and they are most commonly maternally inherited such that affected women pass the condition to all of their offspring and affected men do not pass the condition to any of their offspring. Mitochondrial disorders demonstrate variable expressivity within a family. Typical symptoms include neuromuscular abnormalities (such as loss of motor control, muscle weakness, and pain), gastrointestinal disorders and swallowing difficulties, poor growth, cardiac disease, liver disease, diabetes, respiratory complications, seizures, visual/hearing problems, lactic acidosis, developmental delays, and susceptibility to infection.

Multifactorial Inheritance

Many common congenital malformations, including NTDs, cleft lip and palate, and CHDs, have an increased risk of recurrence in families above the baseline population risk. A small percentage of such defects have a specific cause, such as single gene disorders, chromosomal abnormalities, or teratogens. Most, however, are isolated defects that result from complex interactions among a number of factors, including genotype at one or more loci, and a variety of environmental exposures that trigger, accelerate, or exacerbate the disorder. Therefore, occurrence does not match simple mendelian inheritance but is complex and multifactorial. Recurrence risk of such disorders is increased by the presence of more than one affected relative, a severe or early-onset form of disease, an affected person of less common sex (in disorders in which persons of one sex are more likely to be affected), high heritability of the disorder, and consanguineous parentage.

NTDs have long been thought to follow a typical complex inheritance pattern determined by multiple genetic and environmental factors. Known environmental risk factors include maternal obesity, diabetes, and medications such as valproic acid. The association between lower socioeconomic status and NTD led investigators to consider nutritional deficiencies as risk factors. Therefore, it was a remarkable discovery that folate supplementation reduced the risk for NTD by approximately 80%, although most women with a fetal NTD diagnosed during pregnancy have normal range folate levels. Public health initiatives to educate reproductive age women about the importance of folate and fortification of the food supply (cereals and breads) with folic acid have been instituted in the United States for the purpose of decreasing the incidence of NTDs. The mechanism for the protective action of folic acid in prevention of NTDs is not well understood. Recent data suggest folate deficiency is not problematic unless present in combination with a mutated gene or genes; together these risk factors result in the embryologic malformation.