Genetic Diseases

Sequence and Copy Number Variations (Polymorphisms)

A surprising revelation from the recent progress in genomics is that, on average, any two individuals share greater than 99.5% of their DNA sequences. Thus, the remarkable diversity of humans is encoded in less than 0.5% of our DNA. Though small when compared to the total nucleotide sequences, this 0.5% represents about 15 million base pairs. The two most common forms of DNA variations (polymorphisms) in the human genome are single-nucleotide polymorphisms (SNPs) and copy number variations (CNVs).

Epigenetic Changes

Epigenetic changes are those involving modulation of gene or protein expression in the absence of alterations in DNA sequence (i.e., mutation) or structure of the encoding gene. Epigenetic regulation is of critical importance during development, as well as in homeostasis of fully developed tissues. One central mechanism of epigenetic regulation is by alterations in the methylation of cytosine residues at gene promoters—heavily methylated promoters become inaccessible to RNA polymerase, leading to transcriptional silencing. Promoter methylation and silencing of tumor suppressor genes (Chapter 5) commonly are observed in many human cancers, leading to unchecked cell growth and proliferation. Another major player in epigenetic regulation of transcription involves the family of histone proteins, which are components of structures called nucleosomes, around which DNA is coiled. Histone proteins undergo a variety of reversible modifications (e.g., methylation, acetylation) that affect secondary and tertiary DNA structure, and hence, gene transcription. As expected, abnormalities of histone modification are observed in many acquired diseases such as cancer, leading to transcriptional deregulation. Physiologic epigenetic silencing during development is called imprinting, and disorders of imprinting are discussed later on.

Alterations in Non-Coding RNAs

It is worth noting that until recently the major focus of gene hunting has been discovery of genes that encode for proteins. Recent studies indicate, however, that a very large number of genes do not encode proteins. Instead, the non-encoded products of these genes—so-called “non-coding RNAs (ncRNAs)”—play important regulatory functions. Although many distinct families of ncRNAs exist, here we will only discuss two examples: small RNA molecules called microRNAs (miRNAs), and long non-coding RNAs (lncRNAs) (the latter encompasses ncRNAs >200 nucleotides in length). The miRNAs, unlike messenger RNAs, do not encode proteins but instead inhibit the translation of target mRNAs into their corresponding proteins. Posttranscriptional silencing of gene expression by miRNA is preserved in all living forms from plants to humans and is therefore a fundamental mechanism of gene regulation. Because of their profound influence on gene regulation, miRNAs are assuming central importance in efforts to elucidate normal developmental pathways, as well as pathologic conditions, such as cancer. Andrew Fire and Craig Mello were awarded the Nobel prize in physiology or medicine in 2006 for their work on miRNAs.

By current estimates, there are approximately 1000 genes in humans that encode miRNAs. Transcription of miRNA genes produces primary miRNA transcript (pri-miRNA), which is processed within the nucleus to form another structure called pre-miRNA (Fig. 6–1). With the help of specific transporter proteins, pre-miRNA is exported to the cytoplasm. Additional “cutting” by an enzyme, appropriately called Dicer, generates mature miRNAs that are about 21 to 30 nucleotides in length (hence the designation micro-). At this stage the miRNA is still double-stranded. Next, the miRNA unwinds, and single strands of this duplex are incorporated into a multiprotein complex called RNA-induced silencing complex (RISC). Base pairing between the miRNA strand and its target mRNA directs the RISC to either cause mRNA cleavage or repress its translation. In this way, the gene from which the target mRNA was derived is silenced (at a post-transcriptional state). Given that the numbers of miRNA genes are far fewer than those that encode proteins, it follows that a given miRNA can silence many target genes. All mRNAs contain a so-called seed sequence in their 3′ untranslated region (UTR), which determines the specificity of miRNA binding and gene silencing.

Figure 6–1 Generation of microRNAs and their mode of action in regulating gene function. pri-miRNA, primary microRNA transcript; pre-miRNA, precursor microRNA; RISC, RNA-induced silencing complex.

Another species of gene-silencing RNA, called small interfering RNAs (siRNAs), works in a manner quite similar to that of miRNA. Unlike miRNA, however, siRNA precursors are introduced by investigators into the cell. Their processing by Dicer and functioning via RISC are essentially similar to that described for miRNA. Synthetic siRNAs have become powerful tools for studying gene function in the laboratory, and are being developed as possible therapeutic agents to silence specific genes, such as oncogenes, whose products are involved in neoplastic transformation.

Recent studies have elucidated an untapped universe of lncRNAs (by some calculations, the number of lncRNAs may exceed coding mRNAs by ten-fold to twenty-fold), and their putative functions in the human genome might explain why humans are at the apex of the evolutionary pyramid despite the relatively modest number of coding genes. lncRNAs modulate gene expression in many ways; for example, they can bind to regions of chromatin, restricting access of RNA polymerase to the encompassed coding genes within the region. One of the best known examples of lncRNAs is XIST, which is transcribed from the X-chromosome, and plays an essential role in physiologic X chromosome inactivation (see later). XIST itself escapes X inactivation, but forms a repressive “cloak” on the X chromosome from which it is transcribed, resulting in gene silencing. Emerging studies are highlighting the roles of lncRNAs in various human diseases, from atherosclerosis to cancer.

With this brief review of the nature of abnormalities that contribute to the pathogenesis of human diseases, we can turn our attention to the three major categories of genetic disorders: (1) those related to mutant genes of large effect, (2) diseases with complex multigenic inheritance (sometimes known as multifactorial disorders), and (3) those arising from chromosomal aberrations. The first category, sometimes referred to as mendelian disorders, includes many uncommon conditions, such as the storage diseases and inborn errors of metabolism, all resulting from single-gene mutations of large effect. Most of these conditions are hereditary and familial. The second category includes some of the most common disorders of humans, such as hypertension and diabetes mellitus. Multifactorial, or complex, inheritance implies that both genetic and environmental influences condition the expression of a phenotypic characteristic or disease. The third category includes disorders that are the consequence of numeric or structural abnormalities in the chromosomes.

To these three well-known categories, it is necessary to add a heterogeneous group of genetic disorders that, like mendelian disorders, involve single genes but do not follow simple mendelian rules of inheritance. These single-gene disorders with nonclassic inheritance patterns include those resulting from triplet repeat mutations, those arising from mutations in mitochondrial DNA, and those in which the transmission is influenced by an epigenetic phenomenon called genomic imprinting. Each of these four categories is discussed separately.

Disorders of Autosomal Dominant Inheritance

Disorders of autosomal dominant inheritance are manifested in the heterozygous state, so at least one parent in an index case usually is affected; both males and females are affected, and both can transmit the condition. When an affected person marries an unaffected one, every child has one chance in two of having the disease. The following features also pertain to autosomal dominant diseases:

• With any autosomal dominant disorder, some patients do not have affected parents. Such patients owe their disorder to new mutations involving either the egg or the sperm from which they were derived. Their siblings are neither affected nor at increased risk for development of the disease.

• Clinical features can be modified by reduced penetrance and variable expressivity. Some persons inherit the mutant gene but are phenotypically normal. This mode of expression is referred to as reduced penetrance. The variables that affect penetrance are not clearly understood. In contrast with penetrance, if a trait is consistently associated with a mutant gene but is expressed differently among persons carrying the gene, the phenomenon is called variable expressivity. For example, manifestations of neurofibromatosis 1 range from brownish spots on the skin to multiple tumors and skeletal deformities.

• In many conditions, the age at onset is delayed, and symptoms and signs do not appear until adulthood (as in Huntington disease).

• In autosomal dominant disorders, a 50% reduction in the normal gene product is associated with clinical signs and symptoms. Because a 50% loss of enzyme activity can be compensated for, involved genes in autosomal dominant disorders usually do not encode enzyme proteins, but instead fall into two other categories of proteins:

Those involved in regulation of complex metabolic pathways, often subject to feedback control (e.g., membrane receptors, transport proteins). An example of this mechanism of inheritance is familial hypercholesterolemia, which results from mutation in the low-density lipoprotein (LDL) receptor gene (discussed later).

Key structural proteins, such as collagen and cytoskeletal components of the red cell membrane (e.g., spectrin, abnormalities of which result in hereditary spherocytosis)

The biochemical mechanisms by which a 50% reduction in the levels of such proteins results in an abnormal phenotype are not fully understood. In some cases, especially when the gene encodes one subunit of a multimeric protein, the product of the mutant allele can interfere with the assembly of a functionally normal multimer. For example, the collagen molecule is a trimer in which the three collagen chains are arranged in a helical configuration. Even with a single mutant collagen chain, normal collagen trimers cannot be formed, so there is a marked deficiency of collagen. In this instance the mutant allele is called dominant negative, because it impairs the function of a normal allele. This effect is illustrated in some forms of osteogenesis imperfecta (Chapter 20).

Disorders of Autosomal Recessive Inheritance

Disorders of autosomal recessive inheritance make up the largest group of mendelian disorders. They occur when both of the alleles at a given gene locus are mutants; therefore, such disorders are characterized by the following features: (1) The trait does not usually affect the parents, but siblings may show the disease; (2) siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for each birth); and (3) if the mutant gene occurs with a low frequency in the population, there is a strong likelihood that the affected patient (the proband) is the product of a consanguineous marriage.

In contrast with the features of autosomal dominant diseases, the following features generally apply to most autosomal recessive disorders:

X-Linked Disorders

All sex-linked disorders are X-linked. No Y-linked diseases are known as yet. Save for determinants that dictate male differentiation, the only characteristic that may be located on the Y chromosome is the attribute of hairy ears, which is not altogether devastating. Most X-linked disorders are X-linked recessive and are characterized by the following features:

Marfan Syndrome

In Marfan syndrome, a connective tissue disorder of autosomal dominant inheritance, the basic biochemical abnormality is a mutation affecting fibrillin. This glycoprotein, secreted by fibroblasts, is the major component of microfibrils found in the extracellular matrix. Microfibrils serve as scaffolding for the deposition of tropoelastin, an integral component of elastic fibers. Although microfibrils are widely distributed in the body, they are particularly abundant in the aorta, ligaments, and the ciliary zonules that support the ocular lens; these tissues are prominently affected in Marfan syndrome.

Fibrillin is encoded by the FBN1 gene, which maps to chromosomal locus 15q21. Mutations in the FBN1 gene are found in all patients with Marfan syndrome. However, molecular diagnosis of Marfan syndrome is not yet feasible, because more than 600 distinct causative mutations in the very large FBN1 gene have been found. Since heterozygotes have clinical symptoms, it follows that the mutant fibrillin protein must act as a dominant negative by preventing the assembly of normal microfibrils. The prevalence of Marfan syndrome is estimated to be 1 per 5000. Approximately 70% to 85% of cases are familial, and the rest are sporadic, arising from de novo FBN1 mutations in the germ cells of parents.

While many of the abnormalities in Marfan syndrome can be explained on the basis of structural failure of connective tissues, some, such as overgrowth of bones, are difficult to relate to simple loss of fibrillin. Recent studies indicate that loss of microfibrils gives rise to abnormal and excessive activation of transforming growth factor-β (TGF-β), since normal microfibrils sequester TGF-β, thereby controlling bioavailability of this cytokine. Excessive TGF-β signaling has deleterious effects on vascular smooth muscle development and the integrity of extracellular matrix. In support of this hypothesis, mutations in the TGF-β type II receptor give rise to a related syndrome, called Marfan syndrome type 2 (MFS2). Of note, angiotensin receptor blockers, which inhibit the activity of TGF-β, have been shown to improve aortic and cardiac function in mouse models of Marfan syndrome and currently are being evaluated in clinical trials.

Ehlers-Danlos Syndromes

Ehlers-Danlos syndromes (EDSs) are a group of diseases characterized by defects in collagen synthesis or structure. All are single-gene disorders, but the mode of inheritance encompasses both autosomal dominant and recessive patterns. There are approximately 30 distinct types of collagen; all have characteristic tissue distributions and are the products of different genes. To some extent, the clinical heterogeneity of EDS can be explained by mutations in different collagen genes.

At least six clinical and genetic variants of EDS are recognized. Because defective collagen is the basis for these disorders, certain clinical features are common to all variants.

As might be expected, tissues rich in collagen, such as skin, ligaments, and joints, frequently are involved in most variants of EDS. Because the abnormal collagen fibers lack adequate tensile strength, the skin is hyperextensible and joints are hypermobile. These features permit grotesque contortions, such as bending the thumb backward to touch the forearm and bending the knee upward to create almost a right angle. Indeed, it is believed that most contortionists have one of the EDSs; however, a predisposition to joint dislocation is one of the prices paid for this virtuosity. The skin is extraordinarily stretchable, extremely fragile, and vulnerable to trauma. Minor injuries produce gaping defects, and surgical repair or any surgical intervention is accomplished only with great difficulty because of the lack of normal tensile strength. The basic defect in connective tissue may lead to serious internal complications, including rupture of the colon and large arteries (vascular EDS); ocular fragility, with rupture of the cornea and retinal detachment (kyphoscoliotic EDS); and diaphragmatic hernias (classical EDS), among others.

The molecular bases for three of the more common variants are as follows:

Familial Hypercholesterolemia

Familial hypercholesterolemia is among the most common mendelian disorders; the frequency of the heterozygous condition is 1 in 500 in the general population. It is caused by a mutation in the LDLR gene that encodes the receptor for low-density lipoprotein (LDL), the form in which 70% of total plasma cholesterol is transported. A brief review of the synthesis and transport of cholesterol follows.

Normal Cholesterol Metabolism

Cholesterol may be derived from the diet or from endogenous synthesis. Dietary triglycerides and cholesterol are incorporated into chylomicrons in the intestinal mucosa, which drain by way of the gut lymphatics into the blood. These chylomicrons are hydrolyzed by an endothelial lipoprotein lipase in the capillaries of muscle and fat. The chylomicron remnants, rich in cholesterol, are then delivered to the liver. Some of the cholesterol enters the metabolic pool (to be described), and some is excreted as free cholesterol or bile acids into the biliary tract. The endogenous synthesis of cholesterol and LDL begins in the liver (Fig. 6–2). The first step in the synthesis of LDL is the secretion of triglyceride-rich very-low-density lipoprotein (VLDL) by the liver into the blood. In the capillaries of adipose tissue and muscle, the VLDL particle undergoes lipolysis and is converted to intermediate-density lipoprotein (IDL). In comparison with VLDL, the content of triglyceride is reduced and that of cholesteryl esters enriched in intermediate-density lipoprotein (IDL), but IDL retains on its surface two of the three VLDL-associated apolipoproteins B-100 and E. Further metabolism of IDL occurs along two pathways: Most of the IDL particles are directly taken up by the liver through the LDL receptor described later; others are converted to cholesterol-rich LDL by a further loss of triglycerides and apolipoprotein E. In the liver cells, IDL is recycled to generate VLDL.

Figure 6–2 Low-density lipoprotein (LDL) metabolism and the role of the liver in its synthesis and clearance. Lipolysis of very-low-density lipoprotein (VLDL) by lipoprotein lipase in the capillaries releases triglycerides, which are then stored in fat cells and used as a source of energy in skeletal muscles. IDL (intermediate-density lipoprotein) remains in the blood and is taken up by the liver.

Two thirds of the resultant LDL particles are metabolized by the LDL receptor pathway, and the rest is metabolized by a receptor for oxidized LDL (scavenger receptor), to be described later. The LDL receptor binds to apolipoproteins B-100 and E and thus is involved in the transport of both LDL and IDL. Although the LDL receptors are widely distributed, approximately 75% are located on hepatocytes, so the liver plays an extremely important role in LDL metabolism. The first step in the receptor-mediated transport of LDL involves binding to the cell surface receptor, followed by endocytotic internalization inside so-called “clathrin-coated pits” (Fig. 6–3). Within the cell, the endocytic vesicles fuse with the lysosomes, and the LDL molecule is enzymatically degraded, resulting ultimately in the release of free cholesterol into the cytoplasm. The cholesterol not only is used by the cell for membrane synthesis but also takes part in intracellular cholesterol homeostasis by a sophisticated system of feedback control:

• It suppresses cholesterol synthesis by inhibiting the activity of the enzyme 3-hydroxy-3-methylglutaryl–coenzyme A reductase (HMG-CoA reductase), which is the rate-limiting enzyme in the synthetic pathway.

• It stimulates the formation of cholesterol esters for storage of excess cholesterol.

• It downregulates the synthesis of cell surface LDL receptors, thus protecting cells from excessive accumulation of cholesterol.

Figure 6–3 The LDL receptor pathway and regulation of cholesterol metabolism. The yellow arrows show three regulatory functions of free intracellular cholesterol: (1) suppression of cholesterol synthesis by inhibition of HMG-CoA reductase, (2) stimulating the storage of excess cholesterol as esters, and (3) inhibition of synthesis of LDL receptors. HMG-CoA reductase, 3-hydroxy-3-methylglutaryl–coenzyme A reductase; LDL, low-density lipoprotein.

The transport of LDL by the scavenger receptors, alluded to earlier, seems to take place in cells of the mononuclear-phagocyte system and possibly in other cells as well. Monocytes and macrophages have receptors for chemically modified (e.g., acetylated or oxidized) LDLs. The amount catabolized by this “scavenger receptor” pathway is directly related to the plasma cholesterol level.

Pathogenesis of Familial Hypercholesterolemia

In familial hypercholesterolemia, mutations in the LDL receptor protein impair the intracellular transport and catabolism of LDL, resulting in accumulation of LDL cholesterol in the plasma. In addition, the absence of LDL receptors on liver cells also impairs the transport of IDL into the liver, so a greater proportion of plasma IDL is converted into LDL. Thus, patients with familial hypercholesterolemia develop excessive levels of serum cholesterol as a result of the combined effects of reduced catabolism and excessive biosynthesis (Fig. 6–2). In the presence of such hypercholesterolemia, there is a marked increase of cholesterol traffic into the monocyte-macrophages and vascular walls mediated by the scavenger receptor. This accounts for the appearance of skin xanthomas and premature atherosclerosis.

Familial hypercholesterolemia is an autosomal dominant disease. Heterozygotes have a two- to three-fold elevation of plasma cholesterol levels, whereas homozygotes may have in excess of a five-fold elevation. Although their cholesterol levels are elevated from birth, heterozygotes remain asymptomatic until adult life, when they develop cholesterol deposits (xanthomas) along tendon sheaths and premature atherosclerosis resulting in coronary artery disease. Homozygotes are much more severely affected, developing cutaneous xanthomas in childhood and often dying of myocardial infarction before the age of 20 years.

Analysis of the cloned LDL receptor gene has revealed that more than 900 different mutations can give rise to familial hypercholesterolemia. These can be divided into five categories. Class I mutations are uncommon, and they are associated with complete loss of receptor synthesis. With class II mutations, the most prevalent form, the receptor protein is synthesized, but its transport from the endoplasmic reticulum to the Golgi apparatus is impaired due to defects in protein folding. Class III mutations produce receptors that are transported to the cell surface but fail to bind LDL normally. Class IV mutations give rise to receptors that fail to internalize within clathrin pits after binding to LDL, while class V mutations encode receptors that can bind LDL and are internalized but are trapped in endosomes because dissociation of receptor and bound LDL does not occur.

The discovery of the critical role of LDL receptors in cholesterol homeostasis has led to the rational design of the statin family of drugs that are now widely used to lower plasma cholesterol. They inhibit the activity of HMG-CoA reductase and thus promote greater synthesis of LDL receptor (Fig. 6–3).

Summary

Familial Hypercholesterolemia

• Familial hypercholesterolemia is an autosomal dominant disorder caused by mutations in the gene encoding the LDL receptor.

• Patients develop hypercholesterolemia as a consequence of impaired transport of LDL into the cells.

• In heterozygotes, elevated serum cholesterol greatly increases the risk of atherosclerosis and resultant coronary artery disease; homozygotes have an even greater increase in serum cholesterol and a higher frequency of ischemic heart disease. Cholesterol also deposits along tendon sheaths to produce xanthomas.

Cystic Fibrosis

With an incidence of 1 in 3200 live births in the United States, cystic fibrosis (CF) is the most common lethal genetic disease that affects white populations. It is uncommon among Asians (1 in 31,000 live births) and African Americans (1 in 15,000 live births). CF follows simple autosomal recessive transmission, and does not affect heterozygote carriers. There is, however, a bewildering compendium of phenotypic variation that results from diverse mutations in the CF-associated gene, the tissue-specific effects of loss of this gene’s function, and the influence of newly recognized disease modifiers. It is, fundamentally, a disorder of epithelial transport affecting fluid secretion in exocrine glands and the epithelial lining of the respiratory, gastrointestinal, and reproductive tracts. Indeed, abnormally viscid mucous secretions that block the airways and the pancreatic ducts are responsible for the two most important clinical manifestations: recurrent and chronic pulmonary infections and pancreatic insufficiency. In addition, although the exocrine sweat glands are structurally normal (and remain so throughout the course of this disease), a high level of sodium chloride in the sweat is a consistent and characteristic biochemical abnormality in CF.

Pathogenesis

The primary defect in CF is abnormal function of an epithelial chloride channel protein encoded by the CF transmembrane conductance regulator (CFTR) gene at chromosomal locus 7q31.2. The changes in mucus are considered secondary to the disturbance in transport of chloride ions. In normal epithelia, the transport of chloride ions across the cell membrane occurs through transmembrane proteins, such as CFTR, that form chloride channels. Mutations in the CFTR gene render the epithelial membranes relatively impermeable to chloride ions (Fig. 6–4). However, the impact of this defect on transport function is tissue-specific. The major function of the CFTR protein in the sweat gland ducts is to reabsorb luminal chloride ions and augment sodium reabsorption through the epithelial sodium channel (ENaC). Therefore, in the sweat ducts, loss of CFTR function leads to decreased reabsorption of sodium chloride and production of hypertonic (“salty”) sweat (Fig. 6–4, top). In contrast with that in the sweat glands, CFTR in the respiratory and intestinal epithelium forms one of the most important avenues for active luminal secretion of chloride. At these sites, CFTR mutations result in loss or reduction of chloride secretion into the lumen (Fig. 6–4, bottom). Active luminal sodium absorption through ENaCs also is increased, and both of these ion changes increase passive water reabsorption from the lumen, lowering the water content of the surface fluid layer coating mucosal cells. Thus, unlike the sweat ducts, there is no difference in the salt concentration of the surface fluid layer coating the respiratory and intestinal mucosal cells in normal persons and in those with CF. Instead, the pathogenesis of respiratory and intestinal complications in CF seems to stem from an isotonic but low-volume surface fluid layer. In the lungs, this dehydration leads to defective mucociliary action and the accumulation of concentrated, viscid secretions that obstruct the air passages and predispose to recurrent pulmonary infections.

Figure 6–4 Top, In cystic fibrosis (CF), a chloride channel defect in the sweat duct causes increased chloride and sodium concentration in sweat. Bottom, Patients with CF have decreased chloride secretion and increased sodium and water reabsorption in the airways, leading to dehydration of the mucus layer coating epithelial cells, defective mucociliary action, and mucous plugging. CFTR, cystic fibrosis transmembrane conductance regulator; ENaC, epithelial sodium channel responsible for intracellular sodium conduction.

Since the CFTR gene was cloned in 1989, more than 1300 disease-causing mutations have been identified. They can be classified as severe or mild, depending on the clinical phenotype: Severe mutations are associated with complete loss of CFTR protein function, whereas mild mutations allow some residual function. The most common severe CFTR mutation is a deletion of three nucleotides coding for phenylalanine at amino acid position 508 (ΔF508). This causes misfolding and total loss of the CFTR. Worldwide, ΔF508 mutation is found in approximately 70% of patients with CF. Since CF is an autosomal recessive disease, affected persons harbor mutations on both alleles. As discussed later, the combination of mutations on the two alleles influences the overall phenotype, as well as organ-specific manifestations. Although CF remains one of the best-known examples of the “one gene–one disease” axiom, there is increasing evidence that other genes modify the frequency and severity of organ-specific manifestations. One example of a candidate genetic modifier is mannose-binding lectin, a key effector of innate immunity involved in phagocytosis of microorganisms. In the setting of CF, polymorphisms in one or both mannose-binding lectin alleles that produce lower circulating levels of the protein are associated with a three-fold higher risk of end-stage lung disease, due to chronic bacterial infections.

Morphology

The anatomic changes are highly variable and depend on which glands are affected and on the severity of this involvement. Pancreatic abnormalities are present in 85% to 90% of patients with CF. In the milder cases, there may be only accumulations of mucus in the small ducts, with some dilation of the exocrine glands. In more advanced cases, usually seen in older children or adolescents, the ducts are totally plugged, causing atrophy of the exocrine glands and progressive fibrosis (Fig. 6–5). The total loss of pancreatic exocrine secretion impairs fat absorption, so avitaminosis A may contribute to squamous metaplasia of the lining epithelium of the ducts in the pancreas, which are already injured by the inspissated mucus secretions. Thick viscid plugs of mucus also may be found in the small intestine of infants. Sometimes these cause small bowel obstruction, known as meconium ileus.

Figure 6–5 Mild to moderate changes of cystic fibrosis in the pancreas. The ducts are dilated and plugged with eosinophilic mucin, and the parenchymal glands are atrophic and replaced by fibrous tissue.

The pulmonary changes are the most serious complications of this disease (Fig. 6–6). These changes stem from obstruction and infection of the air passages secondary to the viscous mucus secretions of the submucosal glands of the respiratory tree. The bronchioles often are distended with thick mucus, associated with marked hyperplasia and hypertrophy of the mucus-secreting cells. Superimposed infections give rise to severe chronic bronchitis and bronchiectasis. Development of lung abscesses is common. Staphylococcus aureus, Haemophilus influenzae, and Pseudomonas aeruginosa are the three most common organisms responsible for lung infections. Even more sinister is the increasing frequency of infection with another pseudomonad, Burkholderia cepacia. This opportunistic bacterium is particularly hardy, and infection with this organism has been associated with fulminant illness (“cepacia syndrome”). The liver involvement follows the same basic pattern. Bile canaliculi are plugged by mucinous material, accompanied by ductular proliferation and portal inflammation. Hepatic steatosis is a common finding in liver biopsies. Over time, cirrhosis develops, resulting in diffuse hepatic nodularity. Such severe hepatic involvement is encountered in less than 10% of patients. Azoospermia and infertility are found in 95% of the affected males who survive to adulthood; bilateral absence of the vas deferens is a frequent finding in these patients. In some males, this may be the only feature suggesting an underlying CFTR mutation.

Figure 6–6 Lungs of a patient who died of cystic fibrosis. Extensive mucous plugging and dilation of the tracheobronchial tree are apparent. The pulmonary parenchyma is consolidated by a combination of both secretions and pneumonia; the greenish discoloration is the product of Pseudomonas infections.

(Courtesy of Dr. Eduardo Yunis, Children’s Hospital of Pittsburgh, Pittsburgh, Pennsylvania.)

Phenylketonuria

There are several variants of phenylketonuria (PKU), an inborn error of metabolism that affects 1 in 10,000 live-born white infants. The most common form, referred to as classic phenylketonuria, is quite common in persons of Scandinavian descent and is distinctly uncommon in African American and Jewish populations.

Homozygotes with this autosomal recessive disorder classically have a severe lack of the enzyme phenylalanine hydroxylase (PAH), leading to hyperphenylalaninemia and PKU. Affected infants are normal at birth but within a few weeks exhibit a rising plasma phenylalanine level, which in some way impairs brain development. Usually, by 6 months of life, severe mental retardation becomes all too evident; less than 4% of untreated phenylketonuric children have intelligence quotients (IQs) greater than 50 or 60. About one third of these children are never able to walk, and two thirds cannot talk. Seizures, other neurologic abnormalities, decreased pigmentation of hair and skin, and eczema often accompany the mental retardation in untreated children. Hyperphenylalaninemia and the resultant mental retardation can be avoided by restriction of phenylalanine intake early in life. Hence, several screening procedures are routinely performed to detect PKU in the immediate postnatal period.

Many female patients with PKU who receive dietary treatment beginning early in life reach child-bearing age and are clinically normal. Most of them have marked hyperphenylalaninemia, because dietary treatment is discontinued after they reach adulthood. Between 75% and 90% of children born to such women are mentally retarded and microcephalic, and 15% have congenital heart disease, even though the infants themselves are heterozygotes. This syndrome, termed maternal PKU, results from the teratogenic effects of phenylalanine or its metabolites that cross the placenta and affect specific fetal organs during development. The presence and severity of the fetal anomalies directly correlate with the maternal phenylalanine level, so it is imperative that maternal dietary restriction of phenylalanine be initiated before conception and continued throughout pregnancy.

The biochemical abnormality in PKU is an inability to convert phenylalanine into tyrosine. In normal children, less than 50% of the dietary intake of phenylalanine is necessary for protein synthesis. The remainder is converted to tyrosine by the phenylalanine hydroxylase system (Fig. 6–7). When phenylalanine metabolism is blocked because of a lack of PAH enzyme, minor shunt pathways come into play, yielding several intermediates that are excreted in large amounts in the urine and in the sweat. These impart a strong musty or mousy odor to affected infants. It is believed that excess phenylalanine or its metabolites contribute to the brain damage in PKU. Concomitant lack of tyrosine (Fig. 6–7), a precursor of melanin, is responsible for the light color of hair and skin.

Figure 6–7 The phenylalanine hydroxylase system. NADH, nicotinamide adenine dinucleotide, reduced form.

At the molecular level, approximately 500 mutant alleles of the PAH gene have been identified, only some of which cause a severe deficiency of the enzyme. Infants with mutations resulting in a lack of PAH activity present with the classic features of PKU, while those with approximately 6% residual activity present with milder disease. Moreover, some mutations result in only modest elevations of blood phenylalanine levels without associated neurologic damage. This latter condition, referred to as benign hyperphenylalaninemia, is important to recognize, because affected persons may well have positive screening tests but do not acquire the stigmata of PKU. Because of the numerous disease-causing alleles of the phenylalanine hydroxylase gene, molecular diagnosis is not feasible, and measurement of serum phenylalanine levels is necessary to differentiate benign hyperphenylalaninemia from PKU; the levels in the latter disorder typically are five times (or more) higher than normal. Once a biochemical diagnosis is established, the specific mutation causing PKU can be determined. With this information, carrier testing of at-risk family members can be performed.

While 98% of cases of PKU are attributable to mutations in PAH, approximately 2% arise from abnormalities in synthesis or recycling of the cofactor tetrahydrobiopterin (Fig. 6–7). Clinical recognition of these variant forms of PKU is important to establish a prognosis, because the patients cannot be treated by dietary restriction of phenylalanine.

Galactosemia

Galactosemia is an autosomal recessive disorder of galactose metabolism that affects 1 in 60,000 live-born infants. Normally, lactase splits lactose, the major carbohydrate of mammalian milk, into glucose and galactose in the intestinal microvilli. Galactose is then converted to glucose in several steps, in one of which the enzyme galactose-1-phosphate uridyltransferase (GALT) is required. Lack of this enzyme, due to homozygous mutations in the encoding gene GALT, is responsible for galactosemia. As a result of this transferase deficiency, galactose-1-phosphate and other metabolites, including galactitol, accumulate in many tissues, including the liver, spleen, lens of the eye, kidney, and cerebral cortex.

The liver, eyes, and brain bear the brunt of the damage. The early-onset hepatomegaly is due largely to fatty change, but in time widespread scarring that closely resembles the cirrhosis of alcohol abuse may supervene (Chapter 15). Opacification of the lens (cataract) develops, probably because the lens absorbs water and swells as galactitol, produced by alternative metabolic pathways, accumulates and increases its tonicity. Nonspecific alterations appear in the central nervous system (CNS), including loss of nerve cells, gliosis, and edema. There is still no clear understanding of the mechanism of injury to the liver and brain.

Almost from birth, affected infants fail to thrive. Vomiting and diarrhea appear within a few days of milk ingestion. Jaundice and hepatomegaly usually become evident during the first week of life. Accumulation of galactose and galactose-1-phosphate in the kidney impairs amino acid transport, resulting in aminoaciduria. Fulminant Escherichia coli septicemia occurs with increased frequency. The diagnosis of galactosemia can be suspected from demonstration in the urine of a reducing sugar other than glucose, but tests that directly identify the deficiency of the transferase in leukocytes and red cells are more reliable. Antenatal diagnosis is possible by assay of GALT activity in cultured amniotic fluid cells or determination of galactitol level in amniotic fluid supernatant.

Many of the clinical and morphologic changes of galactosemia can be prevented or ameliorated by early removal of galactose from the diet for at least the first 2 years of life. Control instituted soon after birth prevents the cataracts and liver damage and permits almost normal development. Even with dietary restrictions, however, it is now established that older patients frequently are affected by a speech disorder and gonadal failure (especially premature ovarian failure) and, less commonly, by an ataxic condition.

Lysosomal Storage Diseases

Lysosomes, the digestive system of the cells, contain a variety of hydrolytic enzymes that are involved in the breakdown of complex substrates, such as sphingolipids and mucopolysaccharides, into soluble end products. These large molecules may be derived from the turnover of intracellular organelles that enter the lysosomes by autophagy, or they may be acquired from outside the cell by phagocytosis. With an inherited lack of a lysosomal enzyme, catabolism of its substrate remains incomplete, leading to accumulation of the partially degraded insoluble metabolites within the lysosomes (Fig. 6–8). Approximately 40 lysosomal storage diseases have been identified, each resulting from the functional absence of a specific lysosomal enzyme or proteins involved in their function. Traditionally, lysosomal storage disorders are divided into broad categories based on the biochemical nature of the substrates and the accumulated metabolites, but a more mechanistic classification is based on the underlying molecular defect (Table 6–4). Within each group are several entities, each resulting from the deficiency of a specific enzyme. Despite this complexity, certain features are common to most diseases in this group:

Figure 6–8 Pathogenesis of lysosomal storage diseases. In this example, a complex substrate is normally degraded by a series of lysosomal enzymes (A, B, and C) into soluble end products. If there is a deficiency or malfunction of one of the enzymes (e.g., B), catabolism is incomplete, and insoluble intermediates accumulate in the lysosomes.

Table 6–4 Lysosomal Storage Disorders

Data from Jeyakumar M, Dwek RA, Butters TD, Platt FM: Storage solutions: treating lysosomal disorders of the brain. Nat Rev Neurosci 6:1, 2005.

Fortunately for the potential victims of the diseases, most of these conditions are very rare, and their detailed description is better relegated to specialized texts and reviews. Only a few of the more common conditions are considered here. Type II glycogen storage disease (Pompe disease), also a lysosomal disorder, is discussed later in the chapter.

Tay-Sachs Disease (G_M2 Gangliosidosis: Deficiency in Hexosaminidase β Subunit)

Gangliosidoses are characterized by accumulation of gangliosides, principally in the brain, as a result of a deficiency of a catabolic lysosomal enzyme. Depending on the ganglioside involved, these disorders are subclassified into G_M1 and G_M2 categories. Tay-Sachs disease, by far the most common of all gangliosidoses, is characterized by a mutation in and consequent deficiency of the β subunit of the enzyme hexosaminidase A, which is necessary for the degradation of G_M2. More than 100 mutations have been described; most affect protein folding or intracellular transport. The brain is principally affected, because it is most involved in ganglioside metabolism. The storage of G_M2 occurs within neurons, axon cylinders of nerves, and glial cells throughout the CNS. Affected cells appear swollen and sometimes foamy (Fig. 6–9, A). Electron microscopy reveals whorled configurations within lysosomes composed of onion-skin layers of membranes (Fig. 6–9, B). These pathologic changes are found throughout the CNS (including the spinal cord), peripheral nerves, and autonomic nervous system. The retina usually is involved as well, where the pallor produced by swollen ganglion cells in the peripheral retina results in a contrasting “cherry red” spot in the relatively unaffected central macula.

Figure 6–9 Ganglion cells in Tay-Sachs disease. A, Under the light microscope, a large neuron has obvious lipid vacuolation. B, A portion of a neuron under the electron microscope shows prominent lysosomes with whorled configurations. Part of the nucleus is shown above.

(A, Courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas Southwestern Medical Center, Dallas, Texas. B, Courtesy of Dr. Joe Rutledge, Children’s Regional Medical Center, Seattle, Washington.)

The molecular basis for neuronal injury is not fully understood. Because in many cases the mutant protein is misfolded, it induces the so-called “unfolded protein” response (Chapter 1). If such misfolded proteins are not stabilized by chaperones, they trigger apoptosis. These findings have spurred clinical trials of molecular chaperone therapy for this and similar lysosomal storage diseases. Such therapy involves use of small molecules that increase chaperone synthesis or reduce degradation of misfolded proteins by the proteosomes.

In the most common acute infantile variant of Tay-Sachs disease, infants appear normal at birth, but motor weakness begins at 3 to 6 months of age, followed by neurologic impairment, onset of blindness, and progressively more severe neurologic dysfunctions. Death occurs within 2 or 3 years. Tay-Sachs disease, like other lipidoses, is most common among Ashkenazi Jews, among whom the frequency of heterozygous carriers is estimated to be 1 in 30. Heterozygote carriers can be reliably detected by estimation of the level of hexosaminidase in the serum or by DNA analysis.

Niemann-Pick Disease Types A and B

Type A and type B Niemann-Pick disease are related entities characterized by a primary deficiency of acid sphingomyelinase and the resultant accumulation of sphingomyelin. In type A, characterized by a severe deficiency of sphingomyelinase, the breakdown of sphingomyelin into ceramide and phosphorylcholine is impaired, and excess sphingomyelin accumulates in all phagocytic cells and in the neurons. The macrophages become stuffed with droplets or particles of the complex lipid, imparting a fine vacuolation or foaminess to the cytoplasm (Fig. 6–10). Electron microscopy confirms that the vacuoles are engorged secondary lysosomes that often contain membranous cytoplasmic bodies resembling concentric lamellated myelin figures, sometimes called “zebra” bodies. Because of their high content of phagocytic cells, the organs most severely affected are the spleen, liver, bone marrow, lymph nodes, and lungs. The splenic enlargement may be striking. In addition, the entire CNS, including the spinal cord and ganglia, is involved in this tragic, inexorable process. The affected neurons are enlarged and vacuolated as a result of the storage of lipids. This variant manifests itself in infancy with massive visceromegaly and severe neurologic deterioration. Death usually occurs within the first 3 years of life. By comparison, patients with the type B variant have organomegaly but no neurologic manifestations. Estimation of sphingomyelinase activity in the leukocytes or cultured fibroblasts can be used for diagnosis of suspected cases, as well as for detection of carriers. Antenatal diagnosis is possible by enzyme assays or DNA probe analysis.

Figure 6–10 Niemann-Pick disease in liver. The hepatocytes and Kupffer cells have a foamy, vacuolated appearance resulting from deposition of lipids.

(Courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas Southwestern Medical Center, Dallas, Texas.)

Gaucher Disease

Gaucher disease results from mutation in the gene that encodes glucocerebrosidase. There are three autosomal recessive variants of Gaucher disease resulting from distinct allelic mutations. Common to all is variably deficient activity of a glucocerebrosidase that normally cleaves the glucose residue from ceramide. This deficit leads to an accumulation of glucocerebroside, an intermediate in glycolipid metabolism, in the mononuclear phagocytic cells and their transformation into so-called Gaucher cells. Normally the glycolipids derived from the breakdown of senescent blood cells are sequentially degraded by the phagocytic cells of the body particularly in the liver, spleen, and bone marrow. In Gaucher disease, the degradation stops at the level of glucocerebrosides, which accumulate in the phagocytes. These phagocytes—the Gaucher cells—become enlarged, with some reaching a diameter as great as 100 µm, because of the accumulation of distended lysosomes, and acquire a pathognomonic cytoplasmic appearance characterized as “wrinkled tissue paper” (Fig. 6–11). No distinct vacuolation is present. It is evident now that Gaucher disease is caused not just by the burden of storage material but also by activation of the macrophages. High levels of macrophage-derived cytokines, such as interleukins (IL-1, IL-6) and tumor necrosis factor (TNF), are found in affected tissues.

Figure 6–11 Gaucher disease involving the bone marrow. A, Gaucher cells with abundant lipid-laden granular cytoplasm. B, Electron micrograph of Gaucher cells with elongated distended lysosomes.

(Courtesy of Dr. Matthew Fries, Department of Pathology, University of Texas Southwestern Medical Center, Dallas, Texas.)

One variant, type I, also called the chronic non-neuronopathic form, accounts for 99% of cases of Gaucher disease. It is characterized by clinical or radiographic bone involvement (osteopenia, focal lytic lesions, and osteonecrosis) in 70% to 100% of cases. Additional features are hepatosplenomegaly and the absence of CNS involvement. The spleen often enlarges to massive proportions, filling the entire abdomen. Gaucher cells are found in the liver, spleen, lymph nodes, and bone marrow. Marrow replacement and cortical erosion may produce radiographically visible skeletal lesions, as well as a reduction in the formed elements of blood. Bone changes are believed to be caused by the aforementioned macrophage-derived cytokines. Type I is most common in Ashkenazi Jews; unlike other variants, it is compatible with long life. Types II and III variants are characterized by neurologic signs and symptoms. In type II, these manifestations appear during infancy (acute infantile neuronopathic form) and are more severe, whereas in type III, they emerge later and are milder (chronic neuronopathic form). Although the liver and spleen also are involved, the clinical features in types II and III are dominated by neurologic disturbances, including convulsions and progressive mental deterioration. The level of glucocerebrosides in leukocytes or cultured fibroblasts is helpful in diagnosis and in the detection of heterozygote carriers.

Current therapy is aimed at lifelong enzyme replacement by infusion of recombinant glucocerebrosidase. A newer form of therapy involves reducing the substrate (glucocerebroside) by oral administration of drugs that inhibit glucocerebroside synthase. Since glucosylceramide is reduced, its accumulation also is reduced. Recent clinical trials in humans have shown considerable promise for this modality of therapy, with decrease in splenomegaly and improvements in skeletal disease. On the horizon is glucocerebrosidase gene therapy involving infusion of autologous hematopoietic stem cells transfected with the normal gene.

Glycogen Storage Diseases (Glycogenoses)

An inherited deficiency of any one of the enzymes involved in glycogen synthesis or degradation can result in excessive accumulation of glycogen or some abnormal form of glycogen in various tissues. The type of glycogen stored, its intracellular location, and the tissue distribution of the affected cells vary depending on the specific enzyme deficiency. Regardless of the tissue or cells affected, the glycogen most often is stored within the cytoplasm, or sometimes within nuclei. One variant, Pompe disease, is a form of lysosomal storage disease, because the missing enzyme is localized to lysosomes. Most glycogenoses are inherited as autosomal recessive diseases, as is common with “missing enzyme” syndromes.

Approximately a dozen forms of glycogenoses have been described in association with specific enzyme deficiencies. On the basis of pathophysiologic findings, they can be grouped into three categories (Table 6–5):

Table 6–5 Principal Subgroups of Glycogenoses

Figure 6–12 Top, A simplified scheme of normal glycogen metabolism in the liver and skeletal muscles. Middle, The effects of an inherited deficiency of hepatic enzymes involved in glycogen metabolism. Bottom, The consequences of a genetic deficiency in the enzymes that metabolize glycogen in skeletal muscles.

Trisomy 21 (Down Syndrome)

Down syndrome is the most common of the chromosomal disorders. About 95% of affected persons have trisomy 21, so their chromosome count is 47. As mentioned earlier, the most common cause of trisomy, and therefore of Down syndrome, is meiotic nondisjunction. The parents of such children have a normal karyotype and are normal in all respects. Maternal age has a strong influence on the incidence of Down syndrome. It occurs in 1 in 1550 live births in women younger than 20 years, in contrast with 1 in 25 live births in women older than 45 years. The correlation with maternal age suggests that in most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum. Indeed, in 95% of cases the extra chromosome is of maternal origin. The reason for the increased susceptibility of the ovum to nondisjunction is not fully understood. No effect of paternal age has been found in those cases in which the extra chromosome is derived from the father.

In about 4% of all patients with trisomy 21, the extra chromosomal material is present not as an extra chromosome but as a translocation of the long arm of chromosome 21 to chromosome 22 or 14. Such cases frequently (but not always) are familial, and the translocated chromosome is inherited from one of the parents, who typically is a carrier of a robertsonian translocation. Approximately 1% of patients with trisomy 21 are mosaics, usually having a mixture of 46- and 47-chromosome cells. These cases result from mitotic nondisjunction of chromosome 21 during an early stage of embryogenesis. Clinical manifestations in such cases are variable and milder, depending on the proportion of abnormal cells.

The diagnostic clinical features of this condition—flat facial profile, oblique palpebral fissures, and epicanthic folds (Fig. 6–15) —are usually readily evident, even at birth. Down syndrome is a leading cause of severe mental retardation; approximately 80% of those afflicted have an IQ of 25 to 50. Ironically, these severely disadvantaged children may have a gentle, shy manner and may be more easily directed than their more fortunate normal siblings. Of interest, some mosaics with Down syndrome have mild phenotypic changes and often even have normal or near-normal intelligence. In addition to the phenotypic abnormalities and the mental retardation already noted, some other clinical features are worthy of mention:

Despite all of these problems, improved medical care has increased the longevity of persons with trisomy 21. Currently the median age at death is 47 years (up from 25 years in 1983). Although the karyotype of Down syndrome has been known for decades, the molecular basis for this disease remains elusive. Data from the human genome project indicate that chromosome 21 carries about 500 annotated genes, including approximately 170 that are conserved protein-coding genes and 5 miRNAs. It is unclear whether the phenotype of Down syndrome arises as a consequence of increased gene dosage of protein coding genes on chromosome 21 itself or of the effects of deregulated miRNA expression on target genes located on other chromosomes (as described previously, miRNAs act through inhibition of target gene expression). Two chromosome 21 candidate genes, DYRK1A, which codes for a serine-threonine kinase, and RCAN1 (regulator of calcineurin 1), which codes for a protein that inhibits a critical cellular phosphatase enzyme called calcineurin, have emerged as the “top culprits” in the pathogenesis of Down syndrome.

22q11.2 Deletion Syndrome

The 22q11.2 deletion syndrome encompasses a spectrum of disorders that result from a small interstitial deletion of band 11 on the long arm of chromosome 22. The clinical features of this deletion syndrome include congenital heart disease affecting the outflow tracts, abnormalities of the palate, facial dysmorphism, developmental delay, thymic hypoplasia with impaired T cell immunity (Chapter 4), and parathyroid hypoplasia resulting in hypocalcemia (Chapter 19). Previously, these clinical features were believed to represent two different disorders: DiGeorge syndrome and velocardiofacial syndrome. However, it is now known that both are caused by 22q11.2 deletion. Variations in the size and position of the deletion are thought to be responsible for the differing clinical manifestations. When T cell immunodeficiency and hypocalcemia are the dominant features, the patients are said to have DiGeorge syndrome, whereas patients with the so-called velocardiofacial syndrome have mild immunodeficiency but pronounced dysmorphology and cardiac defects. In addition to these malformations, patients with 22q11.2 deletion are at particularly high risk for psychoses such as schizophrenia and bipolar disorder. The molecular basis for this syndrome is not fully understood. The affected region of chromosome 11 encodes many genes. Among these, a transcription factor gene called TBX1 is suspected to be responsible, since its loss seems to correlate with the occurrence of DiGeorge syndrome.

The diagnosis of this condition may be suspected on clinical grounds but can be established only by detection of the deletion by fluorescence in situ hybridization (FISH) (Fig. 6–37, B).

Klinefelter Syndrome

Klinefelter syndrome is best defined as male hypogonadism that develops when there are at least two X chromosomes and one or more Y chromosomes. Most affected patients have a 47,XXY karyotype. This karyotype results from nondisjunction of sex chromosomes during meiosis. The extra X chromosome may be of either maternal or paternal origin. Advanced maternal age and a history of irradiation in either parent may contribute to the meiotic error resulting in this condition. Approximately 15% of the patients show mosaic patterns, including 46,XY/47,XXY, 47,XXY/48,XXXY, and variations on this theme. The presence of a 46,XY line in mosaics usually is associated with a milder clinical condition.

Klinefelter syndrome is associated with a wide range of clinical manifestations. In some persons it may be expressed only as hypogonadism, but most patients have a distinctive body habitus with an increase in length between the soles and the pubic bone, which creates the appearance of an elongated body. Also characteristic is eunuchoid body habitus. Reduced facial, body, and pubic hair and gynecomastia also are frequently seen. The testes are markedly reduced in size, sometimes to only 2 cm in greatest dimension. In keeping with the testicular atrophy, the serum testosterone levels are lower than normal, and urinary gonadotropin levels are elevated.

Klinefelter syndrome is the most common cause of hypogonadism in males. Only rarely are patients fertile, and presumably such persons are mosaics with a large proportion of 46,XY cells. The sterility is due to impaired spermatogenesis, sometimes to the extent of total azoospermia. Histologic examination reveals hyalinization of tubules, which appear as ghostlike structures on tissue section. By contrast, Leydig cells are prominent, as a result of either hyperplasia or an apparent increase related to loss of tubules. Although Klinefelter syndrome may be associated with mental retardation, the degree of intellectual impairment typically is mild, and in some cases, no deficit is detectable. The reduction in intelligence is correlated with the number of extra X chromosomes. Klinefelter syndrome is associated with a higher frequency of several disorders, including breast cancer (seen 20 times more commonly than in normal males), extragonadal germ cell tumors, and autoimmune diseases such as systemic lupus erythematosus.

Turner Syndrome

Turner syndrome, characterized by primary hypogonadism in phenotypic females, results from partial or complete monosomy of the short arm of the X chromosome. With routine cytogenetic methods, the entire X chromosome is found to be missing in 57% of patients, resulting in a 45,X karyotype. These patients are the most severely affected, and the diagnosis often can be made at birth or early in childhood. Typical clinical features associated with 45,X Turner syndrome include significant growth retardation, leading to abnormally short stature (below the third percentile); swelling of the nape of the neck due to distended lymphatic channels (in infancy) that is seen as webbing of the neck in older children; low posterior hairline; cubitus valgus (an increase in the carrying angle of the arms); shieldlike chest with widely spaced nipples; high-arched palate; lymphedema of the hands and feet; and a variety of congenital malformations such as horseshoe kidney, bicuspid aortic valve, and coarctation of the aorta (Fig. 6–16). Cardiovascular abnormalities are the most common cause of death in childhood. In adolescence, affected girls fail to develop normal secondary sex characteristics; the genitalia remain infantile, breast development is minimal, and little pubic hair appears. Most patients have primary amenorrhea, and morphologic examination reveals transformation of the ovaries into white streaks of fibrous stroma devoid of follicles. The mental status of these patients usually is normal, but subtle defects in nonverbal, visual-spatial information processing have been noted. Curiously, hypothyroidism caused by autoantibodies occurs, especially in women with isochromosome Xp. As many as 50% of these patients develop clinical hypothyroidism. In adult patients, a combination of short stature and primary amenorrhea should prompt strong suspicion for Turner syndrome. The diagnosis is established by karyotyping.

Figure 6–16 Clinical features and karyotypes of Turner syndrome.

Approximately 43% of patients with Turner syndrome either are mosaics (one of the cell lines being 45,X) or have structural abnormalities of the X chromosome. The most common is deletion of the short arm, resulting in the formation of an isochromosome of the long arm, 46,X,i(X)(q10). The net effect of the associated structural abnormalities is to produce partial monosomy of the X chromosome. Combinations of deletions and mosaicism are reported. It is important to appreciate the karyotypic heterogeneity associated with Turner syndrome because it is responsible for significant variations in the phenotype. In contrast with the patients with monosomy X, those who are mosaics or have deletion variants may have an almost normal appearance and may present only with primary amenorrhea.

The molecular pathogenesis of Turner syndrome is not completely understood, but studies have begun to shed some light. As mentioned earlier, both X chromosomes are active during oogenesis and are essential for normal development of the ovaries. During normal fetal development, ovaries contain as many as 7 million oocytes. The oocytes gradually disappear so that by menarche their numbers have dwindled to a mere 400,000, and when menopause occurs fewer than 10,000 remain. In Turner syndrome, fetal ovaries develop normally early in embryogenesis, but the absence of the second X chromosome leads to an accelerated loss of oocytes, which is complete by age 2 years. In a sense, therefore, “menopause occurs before menarche,” and the ovaries are reduced to atrophic fibrous strands, devoid of ova and follicles (streak ovaries). Because patients with Turner syndrome also have other (nongonadal) abnormalities, it follows that some genes for normal growth and development of somatic tissues also must reside on the X chromosome. Among the genes involved in the Turner phenotype is the short stature homeobox (SHOX) gene at Xp22.33. This is one of the genes that remain active in both X chromosomes and is unique in having an active homologue on the short arm of the Y chromosome. Thus, both normal males and females have two copies of this gene. One copy of SHOX gives rise to short stature. Indeed, deletions of the SHOX gene are noted in 2% to 5% of otherwise normal children with short stature. Whereas one copy of SHOX can explain growth deficit in Turner syndrome, it cannot explain other important clinical features such as cardiac malformations and endocrine abnormalities. Clearly, several other genes located on the X chromosome also are involved.