Genetic Disorders and Dysmorphic Conditions

Incidence of Chromosomal Abnormalities

Data from Hook (1992) suggest that upward of 50% of human conceptions terminate in a spontaneous abortion, typically so early during gestation that the pregnancy is never recognized. The earlier the abortion occurs, the more likely it is that the miscarried embryo had a chromosomal abnormality. Of recognized first-trimester abortuses, 50% are chromosomally abnormal, compared with 5% of later embryos. Among the chromosomally abnormal miscarried embryos, the most frequent abnormalities are triploidy (69 chromosomes), trisomy 16, and 45,X (Turner syndrome) ( Table 1.2 ). Generally speaking, triploidy and trisomy 16 are not compatible with life and are only rarely seen among liveborn infants. Despite the fact that Turner syndrome is relatively common among liveborn infants, the majority of conceptuses with 45,X also abort spontaneously. The incidence of chromosomal abnormalities among liveborn infants in general is about 6 in 1000, but for infants who are stillborn or who die in the immediate perinatal period, the number is increased to approximately 50 in 1000.

Abnormalities of Chromosome Number—Aneuploidy

Aneuploidy refers to an abnormality in chromosome number, in humans a chromosome number different from an even multiple of 23 (the haploid number) ( Fig. 1.8 ). In aneuploidy, there are typically 45 or 47 chromosomes instead of the usual 46. Rarely, multiples of the X or Y chromosome result in individuals with 48 or 49 chromosomes.

If aneuploidy occurs in a gamete as a result of an error of chromosomal division (nondisjunction or anaphase lag) during meiosis, all cells are affected in the fertilized embryo. With subsequent pregnancies, the risk for another aneuploidy (same or different) is increased approximately 1% to 2% above the general population risk. The risk of nondisjunction increases independently with maternal age ( Table 1.3 ). Although the mechanism of this increased risk is unknown, preconception genetic counseling is recommended in those with a family history of aneuploidy, multiple miscarriages, or increasing maternal age.

Mosaic Aneuploidy States

Mosaicism, the presence of two or more genetically different cell lines within an individual, can result from an error in division during either meiosis (preconception) or mitosis (post conception). When aneuploidy is present at conception, in some cases the extra chromosome is dropped from one or more cells during post-conception cell division, leading to mosaicism in the fetus for normal and aneuploid cell lines. In other cases of mosaicism the one-celled embryo (zygote) is chromosomally normal and a division error occurs after fertilization, during mitosis of an embryonic somatic cell, resulting in aneuploidy. Balanced and unbalanced chromosome aberrations and uniparental disomy (UPD) can be encountered in this setting. Generally speaking, if parents have a child with aneuploidy, their recurrence risk in future children is 1% to 2%; the reason for the increased recurrence risk is not fully understood but may reflect an undetectable aneuploid mosaic cell line in a parental gonad ( Fig. 1.9 ).

In some cases, chromosomal mosaicism can have visible effects. For example, in hypomelanosis of Ito the skin manifests marbleized or mottled areas of hypopigmented whorls along the Blaschko ( Fig. 1.10 ). Karyotyping from normal, hypo, or hyperpigmented regions can reveal mosaic chromosomal abnormalities in each region. In addition, individuals with hypomelanosis of Ito can have multiple congenital anomalies, dysmorphic features, variable intellectual disability, and other neurologic findings.

Abnormalities of Chromosome Structure—Inversions, Deletions, and Translocations

Chromosomes can be normal in number (diploid) but still be abnormal in structure. Inversions ( Fig. 1.11 ), deletions ( Fig. 1.12 ), and translocations ( Fig. 1.13 ) of genetic material are examples of structural chromosomal abnormalities. These can arise as new (sporadic) mutations in the egg or sperm from which the embryo was formed, in which case the parents’ recurrence risk for another child with a chromosomal abnormality is again 1% to 2%. However, the abnormality may also be inherited from a phenotypically normal parent who is a “carrier” of a balanced structural chromosomal abnormality ( Fig. 1.14 ). About 1 in 520 normal individuals carries a balanced but structurally abnormal set of chromosomes, called a chromosome translocation. The term balanced, for the purposes of this chapter, means that on cytogenetic analysis the structural abnormality does not appear to have resulted in any net loss or gain of genetic material, simply its relocation to a different place in the genome. A familial balanced translocation can be transmitted through multiple generations, but in some cases the translocated genetic material is passed down unevenly during mitosis, resulting in a fetus with aneuploidy.

A frequent way in which families with apparently balanced chromosome translocations come to medical attention occurs when a child is born with structural malformations and on karyotyping is found to have an unbalanced chromosome translocation. Data suggest that a small percentage of individuals with de novo apparently “balanced” translocations are actually mildly affected clinically by variable degrees of cognitive and physical deficits. Thus high-resolution chromosome analyses and molecular cytogenetics techniques may be indicated, particularly when assessing new (de novo) translocations in a child. Parental karyotypes are used to distinguish the etiology and are crucial in providing accurate genetic counseling regarding future pregnancies for that couple.

When to Suspect a Chromosomal Abnormality

Chromosomal abnormalities of either number or structure are likely to have a detrimental effect on the phenotype of an affected individual. Aneuploidy of an autosome, or non-sex chromosome, generally significantly impairs physical and cognitive development. However, aneuploidy of a sex chromosome may have little or no apparent effect on the phenotype. One should look for clustering of abnormalities in family members to suggest a problem, although their absence does not rule out a chromosomal abnormality.

Carriers of an inherited or a de novo reciprocal translocation are usually genetically balanced and are subsequently normal. However, their conceptuses can be genetically unbalanced and may abort spontaneously or be born with major congenital anomalies. A history of unexplained infertility, multiple spontaneous abortions (three or more), and particularly of a previous birth to the couple or to a close relative of a child with dysmorphic findings and/or major anomalies may be an indication that one of the parents carries a balanced chromosomal translocation or rearrangement. Thus a chromosome study on the couple is indicated, and if translocation is found, they should seek antenatal genetic counseling. This may also be advisable for extended family members. The risk for an individual carrying a balanced translocation to have a liveborn child with an unbalanced translocation varies from 4% to 20%, depending on the reproductive history of the individual and other family members who carry the balanced translocation.

Classic Autosomal Aneuploidies

Down Syndrome

The worldwide incidence of Down syndrome among liveborn infants is approximately 1 in 660, with 45% of affected individuals born to women older than 35 years old. The incidence of Down syndrome among conceptuses is far greater than among liveborns because the majority of affected fetuses spontaneously abort. No single physical stigma of Down syndrome exists; rather, the clinical diagnosis rests on finding a recognizable constellation of clinical characteristics, including a combination of major and minor anomalies (see Figs. 1.9 and 1.15 , Table 1.3 ). The etiology of Down syndrome is trisomy 21, the presence of an extra chromosome 21 either as a simple trisomy or as part of a chromosome 21 fused (translocated) with another chromosome. Chromosome studies should therefore be performed on the parents and appropriate family members of an individual with translocation Down syndrome. Cases of mosaicism, in which trisomy 21 cell lines coexist with cell lines with the standard 46 chromosomes, exist as well and may range in phenotype from normal to that typical of complete trisomy 21. An association between trisomy 21 and advanced maternal age is clear ( Table 1.4 ).

Table 1.4

Maternal Age–Specific Risk for Trisomy 21 at Live Birth

Maternal Age	Prevalence at Live Birth
25 years old	1/1350
30 years old	1/890
35 years old	1/355
40 years old	1/97
45 years old	1/23

Trisomy 13 and Trisomy 18

Trisomy 13 is a relatively rare (1 in 5000) genetic condition caused by the presence of additional chromosome material from all or a large part of chromosome 13. The vast majority of embryos with classic trisomy for a complete 13th chromosome abort spontaneously, but approximately 5% survive to be liveborn. Of these, about 5% survive the first 6 months of life with rare longer survival. They have a severe, recognizable pattern of malformation that allows clinicians to suspect this etiology immediately ( Fig. 1.16 , Table 1.5 ). The most recognizable features include holoprosencephaly with midline deformities, cutis aplasia, and post-axial polydactyly.

Table 1.5

Physical Abnormalities and Frequencies of Occurrence in Trisomy 13 and Trisomy 18 Syndromes

Abnormality	Trisomy 13	Trisomy 18
Severe developmental delay/intellectual disability	††††	††††
Approximately 90% die within first year	††††	††††
Cryptorchidism in males	††††	††††
Low-set, malformed ears	††††	††††
Multiple major congenital anomalies	††††	††††
Prominent occiput	†	††††
Cleft lip and/or palate	†††	†
Micrognathia	††	†††
Microphthalmos	†††	††
Coloboma of iris	†††	†
Short sternum	†	†††
Rocker-bottom feet	††	†††
Congenital heart disease	††	††††
Scalp defects	†††	†
Flexion deformities of fingers	††	††††
Polydactyly	†††	†
Hypoplasia of nails	††	†††
Hypertonia in infancy	†	†††
Apneic spells in infancy	†††	†
Midline brain defects	†††	†
Horseshoe kidneys	†	†††

Symbols: Relative frequency of occurrence ranges from †††† (usual) to † (rare).

The chromosomal disorder trisomy 18 occurs in approximately 3 in 10,000 newborns, and females are more likely to be liveborn. Affected infants exhibit some hallmark features including clenched hands with overlapping fingers and are typically small for gestational age with a frail appearance and failure to thrive ( Fig. 1.17 , see Table 1.5 ). Trisomy 18 was previously thought to be almost invariably fatal in the neonatal period, however more recent data suggest that a small percentage of children can live longer; between 5% and 10% survive at least 1 year, and a few long-term survivors have been reported (frequently females with fewer structural abnormalities of major organs). Embryonal cancers such as Wilms tumor appear increased in survivors.

Partial chromosome abnormalities involving extra material determined to originate from chromosome 13 or 18 must be identified and distinguished from classic trisomies, because the clinical phenotype and prognosis may be different and, in some cases, less severe. Children with mosaicism, that is, with a normal cell line and a trisomy cell line, as well as those with trisomy of part of chromosome 13 or 18, can be identified by chromosome analysis and often have a less severe course.

Even with optimal neonatal, pediatric, and surgical management and excellent home-based care, children with classic trisomies 13 and 18 often “fail to thrive” and have significant developmental and cognitive impairments. Discussions with parents about interventions must take into account the slim possibility of long-term survival and require sensitivity to the needs of the child and family. Great care must be taken in providing a balanced picture to the family when discussing treatment options. It has been our experience that parent support organizations can be extremely helpful to family members in the long process of adjustment to having a child with a chromosome problem, or to provide a source of support through the grieving and healing process if the child dies.

Abnormalities of Sex Chromosomes

Turner Syndrome

Turner syndrome is one of the three most common chromosomal abnormalities found in early spontaneous abortions. The phenotype is female. About 1 in 2000 liveborn females has Turner syndrome. Primary amenorrhea, sterility, sparse pubic and axillary hair, underdeveloped breasts, and short stature (4½ to 5 ft) are the usual manifestations. Other external physical features may include webbing of the neck; cubitus valgus; a low-set posterior hairline; a shield chest with widely spaced nipples; and malformed, often protruding, ears ( Fig. 1.18 ). Internally, renal anomalies may be present along with congenital heart disease, particularly bicuspid aortic valve (in 30% of cases) and coarctation of the aorta (in 10% of cases). Affected women have an underdeveloped uterus, and their ovaries consist only of strands of fibrous connective tissue. Newborns often have lymphedema of the feet and/or hands (see Fig. 1.18C ), which can reappear briefly during adolescence. Mental development is usually normal. Schooling and behavioral problems seem to be the same as in age-matched control subjects, although difficulty with spatial orientation (such as map reading) may be a problem. The classic physical findings of Turner syndrome may be absent, or the abnormalities may be so minimal in the newborn that the diagnosis is missed. The first indication may be unexplained short stature in later childhood or failure to develop secondary sex characteristics by late adolescence. Thus a chromosome study is indicated as part of the diagnostic workup of adolescent girls with these complaints.

The karyotype in the majority of individuals with Turner syndrome is 45,X. Most often, the missing sex chromosome is paternally derived; the risk of Turner syndrome does not increase with maternal or paternal age. Another 15% of individuals with Turner syndrome have mosaicism (X/XX, X/XX/XXX, or X/XY). The physical stigmata may be less marked in mosaicism, in some cases with preserved fertility. The short (p) arm of the X chromosome appears most critical; deletion of the long (q) arm usually produces only streak (fibrous) gonads with consequent sterility, amenorrhea, and immature secondary sex characteristics but without the other somatic stigmata of Turner syndrome. A smaller percentage of cases of Turner syndrome have 46 chromosomes, with one normal plus one structurally abnormal X (frequently a shortened or missing short [p] arm). These cases are more likely to have other, more serious major anomalies, including cognitive deficits. A structurally abnormal X chromosome may lead to abnormal X inactivation, resulting in a deleterious dosage effect for X-linked genes. Occasionally the patient will have a 46, XY karyotype, but vital segments of the p arm of the Y chromosome have been deleted. If an XY cell line is present, the intraabdominal gonads should be removed, because they are prone to malignant change.

If the diagnosis is clinically suspected, a chromosome karyotype should be ordered. Should the affected child be 45,X or a mosaic, the parental risk for recurrence of a chromosomally abnormal liveborn is 1% to 2% but may be higher if a parent carries a structurally abnormal X chromosome. Antenatal diagnosis of chromosomally abnormal fetuses should be discussed with the parents, and the relatively good prognosis for Turner syndrome liveborns should not be overlooked. Girls with Turner syndrome should receive appropriate hormone therapy during adolescence to enable development of secondary sex characteristics and stimulate menses. Rarely, 45,X women with Turner syndrome have been fertile for a limited number of years.

Klinefelter Syndrome

One in 500 newborn boys has a 47,XXY karyotype, also known as Klinefelter syndrome. The physical stigmata are subtle and usually not obvious until puberty, at which time the normal onset of spermatogenesis is blocked by the presence of two X chromosomes. Consequently the germ cells die, the seminiferous tubules become hyalinized and scarred, and the testes become small. Testosterone levels are below normal adult male levels, although the level varies from case to case (the average being about half as much as normal). Hence there is a wide range in degree of virilization. Presentation can range from the classic tall male with a small penis and gynecomastia ( Fig. 1.19 ), to a sexually functional male with unexplained infertility. Scoliosis may develop during adolescence. The average full-scale IQ of men with Klinefelter syndrome is 98, which is about the same as the general population. Behavioral problems may be more common than in the population at large, however.

The karyotype in Klinefelter syndrome is XXY in 80% of cases and mosaic (XY/XXY) in the other 20%. Rarely the latter type may be fertile. About 60% of cases reflect a chromosome error in oogenesis, and an error in spermatogenesis occurs in 40%. The risk of having an affected child increases with maternal age. Males with more than two X chromosomes (XXXY, XXXXY) are usually cognitively impaired and are more likely to have skeletal and other major congenital anomalies, such as cleft palate, congenital heart disease (particularly a patent ductus arteriosus), and microcephaly. The parents’ recurrence risk for another chromosomally abnormal liveborn is 1% to 2%; antenatal diagnosis with subsequent pregnancies is possible.

XXX and XYY

Females with a karyotype of 47,XXX have triple X syndrome. The incidence is approximately 1 in 1000 liveborn females. Affected individuals have no characteristic abnormal physical features. Although usually within the normal range of intelligence, their IQ scores may be lower than those of their normal siblings. Delays in development of motor skills and coordination are common, and approximately 60% require some special education classes. Behavioral problems occur in approximately 30% and are usually mild. XXX women are fertile, and their children are usually chromosomally normal. The presence of additional X chromosomes (48,XXXX, or 49,XXXXX) typically results in cognitive impairment to some degree.

XYY males have a karyotype of 47,XYY. The incidence is 1 in 840 liveborn males. These males tend to be tall in comparison with their own family members, but generally their phenotypic appearance is normal. As for 47,XXX females, their IQ is usually within the normal range but may be lower than that of siblings. Affected boys often come to medical attention because of problems with fine motor coordination, speech disorders, and learning disabilities. Early reports raised concerns about significant behavioral problems; however, long-term prospective studies now suggest that these boys do not have any greater incidence of problem behaviors than the general population.

The risk of recurrence for a couple with a child with an XXX or XYY karyotype depends on many factors including the parents’ own karyotype results and advancing maternal age. Therefore it is recommended that they be referred for individualized genetic counseling when considering future pregnancies.

Fluorescence In Situ Hybridization

FISH is a laboratory technology that has revolutionized the diagnostic capabilities of clinical cytogenetic laboratories. In this technique, a DNA probe is tagged with a label that fluoresces when viewed under a special microscope. A cocktail of many repetitive DNA probes blanketing a specific chromosome from end to end can be obtained. Using special microscope filters, a clinician can simultaneously FISH paint a slide with probes fluorescing in two or three different colors. This specific FISH technique using multiple probe colors is used for specific concerns (e.g., marker chromosome identification in cancer cells). FISH and other molecular techniques are now used primarily to confirm the imbalances detected by array-based testing (see following section).

Some of the well-recognized syndromes described initially by FISH probes for chromosome microdeletion syndromes include the following: Prader-Willi/Angelman syndrome: del 15q11–13, DiGeorge sequence/velocardiofacial syndrome: del 22q11.2, Williams syndrome: del 7q11.23 and others ( Figs. 1.20–1.23 ; Table 1.6 ).

Array-Based Technology: Microarray for Evaluation of Copy Number Variation

In addition to classic cytogenetics, molecular cytogenetic methods are being incorporated in clinical settings at an increased rate. More recently, conventional cytogenetics is being enhanced with high-resolution molecular karyotyping using array-CGH. Array-CGH analyses are proficient in detecting imbalances in the genome and enable detection of copy number changes at high resolution. The degree of detection for copy-number variants for various cytogenetic techniques is shown in Table 1.7 . Array-CGH has been recommended by the American College of Medical Genetics (ACMG) as the first step in the investigation of patients with idiopathic intellectual disability, developmental delay, autism spectrum disorders, and multiple congenital anomalies, and has a much higher diagnostic yield—up to approximately 15% to 28%, compared to traditional G-banded karyotypes (on the order of 3%, excluding Down syndrome and other recognizable chromosomal syndromes). In addition, molecular cytogenetic techniques, such as array-CGH, have demonstrated that approximately 20% of apparently balanced chromosome translocations, de novo or familial, have gain or loss of genetic material at the breakpoints. Thus, molecular cytogenetic studies can more completely characterize the location of the chromosome breakpoints and potentially identify additional genetic material that may be duplicated or deleted that would not otherwise be detected by the traditional cytogenetic methods. By combining the array-CGH technique with classic cytogenetic and confirmatory FISH and appropriate molecular analyses, we are also able not only to identify cryptic genomic alterations but also to further analyze gross genomic alterations, including marker chromosome or other rearrangements identified by the classic cytogenetic analysis.

The application of whole-genome array-CGH in establishing the specific genetic defect has important consequences for genetic counseling of the families and follow-up of the patients. Detailed molecular analysis of the rearranged regions may help to identify the gene(s) associated with a specific phenotypic presentation. With microarray testing, many new microdeletion and microduplication syndromes have emerged (e.g., deletion 1p36, deletion 1q21.1, and deletion 16p13.11 syndromes) (see Fig. 1.22 ; and eFigs. 1.1–1.3 ). These diagnoses impact the patients’ immediate and long-term clinical management.

Single nucleotide polymorphism (SNP) arrays are being used in clinical settings, usually in conjunction with array CGH, and enhance genome-wide copy number analysis. The identification of “copy-number neutral” regions of homozygosity can reveal consanguinity and suggest genes of interest in the homozygous regions. This molecular cytogenetics test therefore simultaneously interrogates and not only identifies copy number variations (CNVs) but is also capable of detecting region(s) with long or short stretches of homozygosity (“regions of homozygosity” [ROH]) known as DNA copy number neutral alterations. Genetic counseling is helpful in interpretation, particularly for variants of uncertain significance or in the unanticipated identification of non-paternity or possible consanguinity.

The impact of these newer methodologies continues to emerge, and their usefulness in providing information key to clinical prognosis is clearly becoming evident. Molecular karyotyping and SNP arrays are ultimately more cost-effective tests and have been extremely useful to clinicians in identifying necessary medical surveillance and treatment options, and they provide information on recurrence risks and prenatal options for families. Recommendations include but are not limited to specific surveillance, pharmacological treatment, cancer-related screening or exclusion of screening, contraindications, and referrals for further evaluation.

Etiologic Mechanisms

The concept of imprinting refers to the fact that the function of certain genes is dependent on their parental origin: maternal versus paternal. One prominent example is the 15q11–q13 region of chromosome 15, a region that contains several imprinted genes that, when abnormal, result in recognizable constellations of physical and behavioral problems. Prader-Willi and Angelman syndromes are disorders that derive from abnormalities of these imprinted genes. Absence of the paternally derived region of this chromosome results in Prader-Willi syndrome, while absence of the maternally derived region of this chromosome results in Angelman syndrome.

Mechanisms that can produce the Prader-Willi phenotype include the following:

• A chromosome deletion of 15q11–q13, including the Prader-Willi critical region of the paternally derived chromosome 15 (majority of cases)

• A structural chromosome abnormality involving the Prader-Willi critical region of 15q11–q13 (translocation, etc.)

• Maternal UPD in which the child has two maternally derived chromosome 15s and no paternally contributed chromosome 15 (25% of cases)

• Mutations of imprinting control center genes (1% of cases)

The critical region of chromosome 15 for Angelman syndrome is located adjacent to the Prader-Willi critical region. However, it is the maternally derived chromosome segment that is absent. The six currently identified etiologic mechanisms of Angelman syndrome include the following:

• 1.

  A large chromosome deletion of 15q11–q13 including the Angelman critical region of the maternally derived chromosome 15 (68% of cases)

• 2.

  A structural chromosome abnormality involving the Angelman critical region of 15q11–q13 (translocation, etc.)

• 3.

  Paternal UPD of chromosome 15 (7% of cases)

• 4.

  Mutations of imprinting control center genes (3% of cases)

• 5.

  Mutations of the ubiquitin-protein ligase gene (UBE3A) (11% of cases)

• 6.

  Classic phenotype, with no identifiable etiologic mechanisms but a positive family history of other affected individuals (11% of cases)

Because of etiologic variability and complexity of the diagnostic process, families of children suspected of having either of these disorders should be referred for genetic evaluation and diagnostic testing to ensure the most accurate determination of etiologic mechanism, and therefore, of recurrence risk.

Current diagnostic testing for these disorders includes the following:

• Methylation studies, which determine whether genes within the 15q11–q13 critical region are correctly marked (methylated) by their parent of origin (detects approximately 99% of PW and 80% of Angelman syndrome cases)

• Chromosome microarray or FISH probes of the chromosome region (detects approximately 65% to 75% of PW and 68% of Angelman syndrome cases)

• SNP array (detects 80% to 90% of Prader-Willi syndrome (PWS) and 68% to 78% of Angelman syndrome cases)

• In some cases of Angelman syndrome, sequence analysis of the UBE3A gene (11%)

Note: Some other examples of imprinting disorders include Beckwith-Wiedemann syndrome ( Fig. 1.25 ) and Russell-Silver syndrome.

Clinical Findings in Prader-Willi Syndrome

Newborns affected with Prader-Willi syndrome usually are markedly hypotonic with a weak cry, poor sucking and swallowing, decreased deep tendon reflexes, and often have a history of decreased fetal movement in utero and breech fetal position and are small for gestational age. Hypotonia can predispose to choking episodes that can cause respiratory problems. Failure to thrive is virtually ubiquitous. Subsequently motor development is delayed, speech even more so, and most patients have cognitive impairment in the mild to moderate range. Hypotonia abates over the first 2 to 3 years, and patients develop an insatiable appetite that rapidly results in morbid obesity ( Fig. 1.26 ). The distribution of excess fat is particularly prominent over the lower trunk, buttocks, and proximal limb. Subtle facial features can include narrow bifrontal diameter, “almond shaped” eyes, and strabismus. Other features include skin picking, small hands and feet, short stature, hypogenitalism, and hypogonadism. In patients with chromosome deletions, hypopigmentation is common, with blond to light brown hair, blue eyes, and sun-sensitive fair skin.

Older children with Prader-Willi syndrome can exhibit extreme emotional lability and temper tantrums. These conditions and the overeating often can be partly ameliorated by intensive inpatient behavioral modification programs followed by longitudinal parental support and follow-up in the home. Growth hormone therapy can exacerbate sleep apnea, but significantly improves weight control, and reduces associated morbidities such as diabetes or risk of Pickwickian syndrome. Interestingly, despite a normal basal metabolic rate, weight reduction requires significantly more severe caloric restriction than usual. In untreated individuals, cardiorespiratory complications from morbid obesity can reduce life expectancy, therefore early diagnosis and treatment are crucial.

Clinical Findings in Angelman Syndrome

Angelman syndrome, first recognized in 1956, has an incidence of 1 in 15,000 to 1 in 20,000 live births. Except for the shared tendency to have hypopigmentation, the clinical phenotypes of Prader-Willi and Angelman syndromes are quite different. The latter have severe cognitive deficits, speech is impaired or absent, and inappropriate paroxysms of laughter are common. Physical features include microbrachycephaly, maxillary hypoplasia, large mouth, prognathism, and short stature (in adults). The gait is ataxic, with toe-walking and jerky arm movements. Akinetic or major motor seizures are common and can begin in the neonatal period. Although survival to adulthood is possible, to date only one patient with Angelman syndrome has been known to reproduce.

Molecular Sequencing Technologies

Next generation sequencing (NGS) methodologies are rapidly becoming a mainstay of clinical diagnosis for heritable disorders. The first clinical application of NGS was for disease gene discovery in cancer involving analyses of patients’ tissue samples with suspected inherited monogenic aberrations in the nuclear and mitochondrial genome. Newer applications of NGS with immense promise for clinical diagnostic utility include whole genome sequencing (WGS) and diagnostic whole exome sequencing (WES). WES allows the unbiased simultaneous interrogation of the coding regions, or exons, and the exon boundaries of approximately 20,000 genes by massive parallel sequencing and is thus a powerful tool for making a diagnosis in complex clinical cases with a suspected heritable etiology that is becoming increasingly applicable to the clinical setting. Most disease-causing changes in the DNA sequence are found in the exons (exome) consisting of about 3% of the genome. WES must be interpreted in the context of familial segregation of any changes detected in the patient’s sequence, the patient’s medical and family history, and detailed knowledge of clinical features including other lab testing. WES has technological limitations in that it does not detect certain types of deleterious genetic changes, such as deletions, duplications, trinucleotide repeats, or methylation defects, and can lack sensitivity near highly repetitive regions, such as the first exon within a gene. Although mapping of the genes is complete, our understanding of the function of most genes is in its infancy. Ultimately, WES provides profound diagnostic and economic advantages in the diagnosis of pediatric and adult heritable disease presenting in the medical genetics clinic. The impact of achieving a definitive diagnosis and identification of the heritable etiology helps to streamline the multidisciplinary medical needs and implication(s) on the patient and his or her family. Diagnosis in patients with a comorbid diagnosis and blended phenotypes may only be possible through utilization of WES. Similarly, WGS and WES are critical in the identification of new mutations and genetic causes of novel phenotypic presentations ( Fig. 1.27 ).

Genomic Sequencing

Molecular sequencing can identify many genetic disorders, particularly if the suspected gene or disorder in question is known. Exome sequencing is increasingly utilized for the diagnosis of genetic disorders when the specific underlying genetic defect is not clearly identifiable. This technique sequences the exons (coding regions) of approximately 20,000 genes at once, with an estimated diagnostic rate of 40%. Some clinical laboratories have initiated the combined technology for exome sequencing with chromosomal array, or to replace chromosomal array as the first-line test for individuals with intellectual disability, autism, or congenital anomalies. Expanded sequencing techniques including WGS may be more efficient at detecting copy number variants simultaneously with detection of genetic mutations in highly repetitive parts of the genome, in early exons within a gene, and in deeply intronic regions (e.g., that can affect splicing, etc.), but is still emerging from the research phase to bedside genetics.

Single Gene Disorders

A mutation (pathogenic variant) in a gene typically results in either alteration of the amount of gene product produced, failure to produce the product at all, and/or compromise of its functional integrity. The loss of gene product or function is also referred to as haploinsufficiency . The greater the degree of functional loss, the more severe the clinical manifestations of the disorder and often the earlier their onset. A single-gene disorder is the result of a mutation altering the DNA sequence within a single gene on a chromosome. If mutation in the gene on only one chromosome is sufficient to cause disease, the disorder is dominant . If mutation of the gene on both chromosomes is required to manifest disease, the disorder is recessive . Genes on the X chromosome are called X-linked , and expression can depend on how many active X chromosomes express the gene. Figs. 1.27 through 1.46 are representative of single-gene disorders.

Pathogenic variant(s) of gene(s) within the nuclear genome are also recognized as mendelian disorders. The occurrence and/or recurrence are in fixed proportions (Mendel’s laws). These disorders are compiled in a catalog, the Online Mendelian Inheritance in Man (OMIM; http://www.ncbi.nlm.nih.gov/omim ), which is an excellent resource. Phenotype/genotype correlations are unfolded by detailed clinical evaluation, recognition at a clinical level, and confirmation by molecular diagnostics confirming the genotype.

Pedigree analyses are usually helpful in recognizing autosomal, X-linked, recessive, and dominant patterns. Gene penetrance, disease expressivity, genetic (locus) heterogeneity, and allelic heterogeneity are some of the well-recognized complexities characterizing mendelian disorders.

Marfan Syndrome

Marfan syndrome is a genetic disorder of connective tissue that is inherited as an autosomal dominant trait, although approximately 25% to 30% of cases represent new mutations. The site of the genetic abnormality or mutation is the fibrillin gene (FBN1) located at band 15q21.1 on chromosome 15. As a result, the molecular structure of the protein fibrillin, an intrinsic component of connective tissue, is abnormal. Clinical consequences are most notable in the musculoskeletal, cardiovascular, and ocular systems and in the dura. Approximately 70% of cases are familial. Classic phenotypic findings include arachnodactyly (see Fig. 1.32A and B ); elbow contractures with other joint hyperextensibility due to ligamentous laxity (see Chapter 5 ); tall stature with long, thin extremities (termed a “Marfanoid” habitus); a decreased upper-to-lower segment ratio; an arm span to height ratio that exceeds 1.05; and moderate to severe pectus excavatum or carinatum (see Fig. 1.32C ). Pes planus and thoracolumbar kyphoscoliosis are other common skeletal features (see Fig. 1.32D ). A defect in the suspensory ligaments of the eye is responsible for subluxation of the lens (seen in 50% to 60% by 10 years old), which is usually displaced in an upward direction. Myopia and astigmatism are common, and affected individuals are also at risk for developing glaucoma, cataracts, and retinal detachment in adulthood. Mitral valve prolapse may progress to mitral insufficiency (at times associated with arrhythmias). Of great concern is progressive aneurysmal dilatation of the ascending aorta and, less commonly, the thoracic or abdominal aorta. The latter is the major source of morbidity and mortality because it can result in acute dissection and death. Dural ectasia in the lumbosacral region, assessed by computed tomography (CT) or magnetic resonance imaging (MRI) of the spine, is observed in 65% of cases. The presence of a high arched palate is common. The incidence of hernias, both inguinal and femoral, is increased, and patients often have striae of the skin in unusual places such as the shoulder. Although most Marfan individuals are of normal intelligence, an occasional patient may have learning disabilities, which should prompt additional investigation.

The disorder is currently primarily suspected on clinical grounds, but family history and multiorgan manifestations are variable and can have age-dependent expressivity. Attempts have been made to create diagnostic criteria by establishing major and minor manifestations (first established in Berlin; Beighton et al., 1988) and revised as the Ghent criteria, most recently revised in 2010 ( ). The diagnostic criteria are based on cardiovascular, ocular, and skeletal features; the presence of a dural ectasia; and family history and have approximately 92% sensitivity in adults. These revisions have placed an increasing emphasis on the cardinal features of Marfan syndrome. Because it takes time for a number of the major abnormalities to develop or to become clinically evident, a firm diagnosis is generally impossible in early childhood without molecular testing, especially in the absence of a positive family history. Molecular testing is being used more frequently in children with an emerging clinical phenotype, especially in the absence of family history and to assess for genotype-phenotype correlations. A molecular diagnosis also provides for the diagnosis of family members who might have variable expressivity and fail to meet the diagnostic criteria on their own. The recurrence risk for affected individuals to their offspring is 50%; prenatal diagnosis is possible if the underlying mutation is known. Pregnant women with Marfan Syndrome require high risk management for cardiovascular complications and potential teratogenicity of some medication treatments.

The differential diagnosis of Marfan syndrome includes Loeys-Dietz syndrome (LDS); Beals congenital contractural arachnodactyly (see Figs. 1.33 through 1.35 ); homocystinuria; the MASS phenotype; familial ectopia lentis; Klinefelter syndrome (47,XXY), triple X syndrome (47,XXX), and many syndromes characterized by joint hypermobility, such as Stickler syndrome (see Figs. 1.38 and 1.39 ), classic EDS (cEDS) (see Fig. 1.36 ), and vascular EDS (vEDS); familial thoracic aortic aneurysm and aortic dissection (TAAD); neuromuscular disorders; fragile X syndrome; and some of the rare dysmorphologic entities, such as Shprintzen-Goldberg syndrome.

When the diagnosis of Marfan syndrome is strongly suspected or confirmed, patients should be monitored closely during growth spurts for signs of onset and progression of kyphoscoliosis; in addition, they should undergo regular ophthalmologic evaluations, and have regular echocardiograms and electrocardiograms. When aortic dilatation is detected, administration of β-blockers can slow progression by decreasing blood pressure and the force of myocardial contractions. Research has found transforming growth factor-β (TGF-β) to be involved in the formation of aortic aneurysms. Losartan, an angiotensin II type 1 receptor blocker, inhibited TGF-β in a mouse model of Marfan syndrome and has now become standard of care for pediatric and adult Marfan and LDS patients. Subacute bacterial endocarditis (SBE) prophylaxis may or may not be indicated for patients with evidence of cardiovascular involvement. Patients also should be cautioned to avoid weightlifting and contact sports.