Biochemical Genetics

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CHAPTER 11 Biochemical Genetics

Life … is a relationship between molecules.

LINUS PAULING

The existence of chemical individuality follows of necessity from that of chemical specificity, but we should expect the differences between individuals to be still more subtle and difficult of detection.

ARCHIBALD GARROD (1908)

In this chapter, we consider single-gene biochemical or metabolic diseases, including mitochondrial disorders. The range of known disorders is vast, so only an overview is possible, but it is hoped that the reader will gain a flavor of this fascinating area of medicine. At the beginning of the twentieth century, Garrod introduced the concept of ‘chemical individuality’, leading in turn to the concept of the inborn error of metabolism (IEM). Beadle and Tatum later developed the idea that metabolic processes, whether in humans or any other organism, proceed by steps. They proposed that each step was controlled by a particular enzyme and that this, in turn, was the product of a particular gene. This was referred to as the one gene–one enzyme (or protein) concept.

Inborn Errors of Metabolism

In excess of 200 IEMs are known that can be grouped by either the metabolite, metabolic pathway, function of the enzyme, or cellular organelle involved (Table 11.1). Most follow autosomal recessive or X-linked recessive inheritance, with only a few being autosomal dominant. This is because the defective protein in most cases is a diffusible enzyme, and there is usually sufficient residual activity in the heterozygous state (loss-of-function, see p. 26) for the enzyme to function normally in most situations. If, however, the reaction catalysed by an enzyme is rate limiting (haploinsufficiency, see p. 26) or the gene product is part of a multimeric complex (dominant-negative, see p. 26), the disorder can manifest in the heterozygous state and follow dominant inheritance (p. 109).

Table 11.1 Characteristics of Some Inborn Errors of Metabolism

Disorders of Amino Acid Metabolism

There are a number of disorders of amino acid metabolism, the best known of which is phenylketonuria.

Phenylketonuria

Children with phenylketonuria (PKU), if untreated, are severely intellectually impaired and often develop seizures. There is a deficiency of the enzyme required for the conversion of phenylalanine to tyrosine, phenylalanine hydroxylase (PAH)—causing a ‘genetic block’ in the metabolic pathway (Figure 11.1).

FIGURE 11.1 Sites of ‘biochemical block’ in phenylketonuria, alkaptonuria, congenital hypothyroidism, and oculocutaneous albinism.

PKU was the first genetic disorder in humans shown to be caused by a specific enzyme deficiency, by Jervis in 1953. As a result of the enzyme defect, phenylalanine accumulates and is converted into phenylpyruvic acid and other metabolites that are excreted in the urine. The enzyme block leads to a deficiency of tyrosine, with a consequent reduction in melanin formation, and children therefore often have blond hair and blue eyes (Figure 11.2). In addition, areas of the brain that are usually pigmented, such as the substantia nigra, may also lack pigment.

FIGURE 11.2 Facies of a male with phenylketonuria; note the fair complexion.

Treatment of PKU

An obvious method of treating children with PKU would be to replace the missing enzyme, but this is not simply achieved (p. 349). Bickel, just 1 year after the enzyme deficiency had been identified, suggested that PKU could be treated by removal of phenylalanine from the diet and this has proved effective. If PKU is detected early enough in infancy, intellectual impairment can be prevented by giving a phenylalanine restricted diet. Phenylalanine is an essential amino acid and therefore cannot be removed entirely from the diet. By monitoring the level of phenylalanine in the blood, it is possible to supply sufficient amounts to meet normal requirements but avoiding toxic levels, resulting in mental retardation. After brain development is complete, dietary restriction can be relaxed—from adolescence onward.

The intellectual impairment seen in children with phenylketonuria is likely due to toxic levels of phenylalanine, and/or its metabolites, rather than a deficiency of tyrosine, of which adequate amounts are present in a normal diet. Both prenatal and postnatal factors may be responsible for developmental delay in untreated PKU.

Alkaptonuria

Alkaptonuria was the original autosomal recessive IEM described by Garrod. Here there is a block in the breakdown of homogentisic acid, a metabolite of tyrosine, because of a deficiency of the enzyme homogentisic acid oxidase (see Figure 11.1). As a consequence, homogentisic acid accumulates and is excreted in the urine, which then darkens on exposure to air. Dark pigment is also deposited in certain tissues, such as the ear wax, cartilage, and joints, where it is known as ochronosis, which in joints can lead to arthritis later in life.

Oculocutaneous Albinism

Oculocutaneous albinism (OCA) is an autosomal recessive disorder resulting from a deficiency of the enzyme tyrosinase, which is necessary for the formation of melanin from tyrosine (see Figure 11.1). In OCA there is a lack of pigment in the skin, hair, iris, and ocular fundus (Figure 11.3), and the lack of eye pigment results in poor visual acuity and uncontrolled pendular eye movements—nystagmus. Reduced fundal pigmentation leads to underdevelopment of part of retina for fine vision—the fovea—and abnormal projection of the visual pathways to the optic cortex.

FIGURE 11.3 Oculocutaneous albinism in a child of Afro-Caribbean origin.

(Courtesy of Dr V. A. McKusick.)

Heterogeneity of OCA

OCA is genetically and biochemically heterogeneous. Cells from those with classic albinism have no measurable tyrosinase activity, the so-called tyrosinase-negative form. However, cells from some persons with albinism show reduced but residual tyrosinase activity and are termed tyrosinase positive. This is usually reflected clinically by variable development of pigmentation of their hair and skin with age. Both types are known as tyrosinase gene-related OCA type 1.

OCA type 1 is due to mutations in the tyrosinase gene on chromosome 11q. However, linkage studies in some families with tyrosinase-positive OCA have excluded the tyrosinase gene. Some of these have a mutation in the P gene, the human homolog of a mouse gene called pink-eyed dilution, or ‘pink-eye’, located on chromosome 15q. This has been termed OCA type 2. In addition, in a proportion of families with OCA, linkage to both of these two loci has been excluded, consistent with the existence of a third locus responsible for OCA.

Homocystinuria

Homocystinuria is a recessively inherited sulfur amino-acid IEM characterized by learning disability, seizures, thrombophilia, osteoporosis, scoliosis, pectus excavatum, long fingers and toes (arachnodactyly), and a tendency to dislocation of the lenses. The somatic features therefore resemble the autosomal dominant disorder Marfan syndrome (p. 300).

Homocystinuria results from deficiency of the enzyme cystathionine β-synthase and can be screened for by means of a positive cyanide nitroprusside test, which detects the presence of increased levels of homocystine in the urine. The diagnosis is confirmed by raised plasma homocystine levels. Treatment involves a low-methionine diet with cystine supplementation. A proportion of individuals with homocystinuria are responsive to the enzyme cofactor pyridoxine (i.e., the pyridoxine-responsive form). A small proportion of affected individuals have mutations in genes leading to deficiencies of enzymes involved in the synthesis of cofactors for cystathionine β-synthase.

Disorders of Branched-Chain Amino Acid Metabolism

The essential branched-chain amino acids leucine, isoleucine and valine have a part of their metabolic pathways in common. Deficiency of the enzyme involved results in maple syrup urine disease.

Maple Syrup Urine Disease

Newborn infants with this autosomal recessive disorder present in the first week of life with vomiting, then alternating tone, leading to death within a few weeks if untreated. The name derives from the odor of the urine—likened to that of maple syrup. The disorder is caused by a deficiency of the branched-chain ketoacid decarboxylase, producing increased urinary excretion of the branched-chain amino acids valine, leucine, and isoleucine, the presence of which suggests the diagnosis, confirmed by demonstration of the three essential branched-chain amino acids in blood. Treatment involves a diet restricting the intake of these three amino acids to the amounts necessary for growth. Affected individuals are particularly susceptible to deterioration, particularly in association with intercurrent illnesses leading to catabolic protein degradation.

Urea Cycle Disorders

The urea cycle is a five-step metabolic pathway that takes place primarily in liver cells for the removal of waste nitrogen from the amino groups of amino acids arising from the normal turnover of protein. It converts two molecules of ammonia and one of bicarbonate into urea (Figure 11.4). Deficiencies of enzymes in the urea cycle result in intolerance to protein from the accumulation of ammonia in the body—hyperammonemia. Increased ammonia levels are toxic to the central nervous system and can lead to coma and, with some untreated urea cycle disorders, death. They are collectively and individually rare and, with the exception of X-linked ornithine transcarbamylase deficiency, inherited as autosomal recessive traits.

FIGURE 11.4 Diagram indicating the position of the various inborn errors of the urea cycle.

Disorders of Carbohydrate Metabolism

The inborn errors of carbohydrate metabolism can be considered in two main groups: disorders of monosaccharide metabolism and the glycogen storage disorders.

Disorders of Monosaccharide Metabolism

Two examples of disorders of monosaccharide metabolism are galactosemia and hereditary fructose intolerance.

Galactosemia

Galactosemia is an autosomal recessive disorder resulting from a deficiency of the enzyme galactose 1-phosphate uridyl transferase, necessary for the metabolism of the dietary sugar galactose. Newborns with galactosemia present with vomiting, lethargy, failure to thrive, and jaundice in the second week of life. If untreated, they develop complications that include mental retardation, cataracts, and liver cirrhosis. Complications can be prevented by early diagnosis and feeding infants with milk substitutes that do not contain galactose or lactose—the sugar found in milk that is broken down into galactose. Early diagnosis is essential and galactosemia can be screened for by the presence of reducing substances in the urine with specific testing for galactose.

Hereditary Fructose Intolerance

Hereditary fructose intolerance is an autosomal recessive disorder resulting from a deficiency of the enzyme fructose 1-phosphate aldolase. Dietary fructose is present in honey, fruit, and certain vegetables, and in combination with glucose in the disaccharide sucrose in cane sugar. Individuals with hereditary fructose intolerance present at different ages, depending on when fructose is introduced into the diet. Symptoms can be minimal but might also be as severe as those seen in galactosemia, which include failure to thrive, vomiting, jaundice, and seizures. The diagnosis is confirmed by the presence of fructose in the urine and enzyme assay on an intestinal mucosal or a liver biopsy sample. Dietary restriction of fructose is associated with a good long-term prognosis.

Glycogen Storage Diseases

Glycogen is the form in which the sugar glucose is stored in muscle and liver as a polymer, acting as a reserve energy source. In the glycogen storage diseases (GSDs) glycogen accumulates in excessive amounts in skeletal muscle, cardiac muscle, and/or liver because of a variety of inborn errors of the enzymes involved in synthesis and degradation of glycogen. In addition, because of the metabolic block, glycogen is unavailable as a normal glucose source. This can result in hypoglycemia, impairment of liver function and neurological abnormalities.

In each of the six major types of GSD, there is a specific enzyme defect involving one of the steps in the metabolic pathways of glycogen synthesis or degradation. The various types can be grouped according to whether they affect primarily the liver or muscle. All six types are inherited as autosomal recessive disorders, although there are variants of the hepatic phosphorylase that are X-linked.

Glycogen Storage Diseases that Primarily Affect Liver

von Gierke disease (GSD-I)

von Gierke disease was the first described disorder of glycogen metabolism and results from a deficiency of the enzyme glucose-6-phosphatase, which is responsible for degradation of liver glycogen to release glucose. Infants present with an enlarged liver (hepatomegaly) and/or sweating and a fast heart rate due to hypoglycemia, which can occur after fasting of only 3 to 4 hours duration. Treatment is simple—frequent feeding and avoidance of fasting to maintain the blood sugar concentration.

Cori disease (GSD-III)

Cori disease is caused by deficiency of the enzyme amylo-1,6-glucosidase, which is also known as the debrancher enzyme. Deficiency results in glycogen accumulation in the liver and other tissues because of the inability to cleave the ‘branching’ links of the glycogen polymer. Infants may present with hepatomegaly because of glycogen accumulation and/or muscle weakness. Treatment involves avoiding hypoglycemia by frequent feeding and avoiding prolonged periods of fasting.

Anderson disease (GSD-IV)

Anderson disease results from deficiency of glycogen brancher enzyme leading to the formation of abnormal glycogen consisting of long chains with few branches that cannot be broken down by the enzymes normally responsible for glycogen degradation. Infants present with hypotonia and abnormal liver function in their first year, progressing rapidly to liver failure. No effective treatment is available apart from the possibility of a liver transplant.

Hepatic phosphorylase deficiency (GSD-VI)

Hepatic phosphorylase is a multimeric enzyme complex with subunits coded for by both autosomal and X-linked genes. Deficiency of hepatic phosphorylase obstructs glycogen degradation, which results in children presenting in the first 2 years of life with hepatomegaly, hypoglycemia, and failure to thrive. Treatment is with carbohydrate supplementation.

Glycogen Storage Diseases that Primarily Affect Muscle

Pompe disease (GSD-II)

Infants with Pompe disease usually present in the first few months of life with floppiness (hypotonia) and delay in the gross motor milestones because of muscle weakness. They then develop an enlarged heart and die from cardiac failure in the first or second year. Voluntary and cardiac muscle accumulates glycogen because of a deficiency of the lysosomal enzyme α-1,4-glucosidase, which is needed to break down glycogen. The diagnosis can be confirmed by enzyme assay of white blood cells or fibroblasts. Early reports of enzyme replacement therapy appear promising.

McArdle disease (GSD-V)

People with McArdle disease present with muscle cramps on exercise in the teenage years. The condition is caused by a deficiency of muscle phosphorylase, which is necessary for degradation of muscle glycogen. There is no effective treatment, although in some the muscle cramps tend to decline if exercise is continued, probably as a result of other energy sources becoming available from alternative metabolic pathways.

Disorders of Steroid Metabolism

The disorders of steroid metabolism include a number of autosomal recessive inborn errors of the biosynthetic pathways of cortisol. Virilization of a female fetus may occur together with salt loss in infants of either sex from a deficiency of the hormone aldosterone. In addition, defects of the androgen receptor result in lack of virilization of chromosomally male individuals (Figure 11.5).

FIGURE 11.5 Steroid biosynthesis indicating the site of the common inborn errors of steroid biosynthesis.

Congenital Adrenal Hyperplasia (Adrenogenital Syndrome)

The diagnosis of congenital adrenal hyperplasia (CAH) should be considered in any newborn female infant presenting with virilization of the external genitalia, because this is the most common cause of ambiguous genitalia in female newborns (p. 288) (Figure 11.6). 21-Hydroxylase deficiency accounts for more than 90% of cases. Approximately 25% have the salt-losing form, presenting in the second or third week of life with circulatory collapse, hyponatremia, and hyperkalemia. Less commonly, CAH is a result of deficiency of the enzymes 11β-hydroxylase or 3β-dehydrogenase, and very rarely occurs as a result of deficiencies of enzymes 17α-hydroxylase and 17,20-lyase. Desmolase deficiency is very rare, with all pathways blocked, causing a reversed phenotype of ambiguous genitalia in males, and severe addisonian crises. Males with the rare 5α-reductase deficiency are significantly under-masculinized but do not suffer other metabolic problems and are likely to be raised as females. At puberty, however, the surge in androgen production is sufficient to stimulate growth of the phallus, making gender identity and assignment problematic.