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The metabolic fate of amino acids conforms to one or more of the following: (i) incorporation into protein; (ii) conversion into messenger compounds, such as neurotransmitters and hormones; and (iii) oxidation to form carbon dioxide, water and ammonia.
Congenital defects of protein synthesis have not yet been described. If they occur, they probably are lethal early in development. Inherited defects in the synthesis of messenger compounds are known, such as the formation of thyroid hormone from tyrosine, but so little amino acid flux is directed toward synthesis of these compounds that no amino acid accumulation is noted.
Almost all amino acidurias reflect derangements of amino acid oxidation, or conversion to CO2, H2O and NH3. Before amino acids are so metabolized, they usually are converted to organic acids, that is, to relatively simple carboxylate anions, such as methylmalonic acid, which are transformed to tricarboxylic acid cycle (TCA) intermediates.
The oxidation of amino acids gives rise to ammonia, which in high concentration is neurotoxic. Most organisms have developed mechanisms for the disposal of this metabolite. In mammals, the urea cycle serves this function, abetting the excretion of 10 to 20 g of ammonia per day in the healthy adult. Congenital deficiencies of the urea cycle (see below) cause hyperammonemia and other evidence of nitrogen accumulation, such as elevations in the plasma concentration of glutamine, which is formed from ammonia.
Infants who succumb in the first days of life commonly manifest neuronal degeneration and reactive astrogliosis. Evidence of dysmyelination is common, particularly when the baby survives for a few weeks. Cortical atrophy is not unusual with long-standing disease. These findings are encountered in many other toxic encephalopathies [5,6].
Various biochemical changes occur in experimental models of amino acid metabolism disorders, including compromised energy metabolism and depletion of ATP. Several underlying processes probably are involved, including an uncoupling of oxidative metabolism, impaired glucose homeostasis and alterations of the intracellular redox potential.
The pathophysiology also may involve competitive inhibition of amino acid transport across the blood—brain barrier. Many amino acids are transferred into the CNS via specialized transport systems, for example, the L system mediating the uptake of neutral amino acids. Excessive plasma concentrations of one amino acid, such as phenylalanine, may inhibit the transport of others. An increase in the ratio of the concentration of tryptophan to that of other amino acids, a phenomenon that occurs in patients who are treated with low-protein diets, may lead to greater tryptophan entry into the brain and increased synthesis of serotonin.
Decreases of lipids, proteolipids and cerebrosides have been noted in several of these syndromes, notably maple syrup urine disease. As noted above, pathological changes in brain myelin are common, especially in infants who die early in life. The fundamental lesion may involve a failure of myelin protein synthesis as a consequence of the imbalanced brain amino acid content.
Finally, in some instances, the injury may be caused by the formation of oxygen radicals or by disturbances of ion channel function. Indeed, the probability is high that disordered amino acid metabolism damages the brain by several independent mechanisms, each of which contributes to the final pathophysiology.
, reaction 2), a mitochondrial enzyme. Decarboxylation of the branched-chain ketoacids, derived from transamination of branched-chain amino acids (BCAA), proceeds via a reaction for which the cofactors are thiamine pyrophosphate, lipoic acid, NAD, FAD and coenzyme A. The ketoacids are freely reaminated to the parent amino acids, the latter being readily measured in the blood and urine. Ketoacids impart to the urine a distinct odor that sometimes is compared with maple syrup or burnt sugar.
The decarboxylase is composed of four subunits: E1-α, E1-β, E2 and E3. A specific kinase and phosphatase activate and deactivate, respectively, the enzyme complex. Most MSUD patients have mutations involving the E1-α subunit, which catalyzes the actual decarboxylation of the ketoacid, although defects of the E1-β protein have been described [7]. The E1-α mutation usually causes faulty assembly of the heterotetrameric (α2β2) E1 protein. Lesions of either the E2 or E3 moiety are extremely rare. The E3 subunit is common to other decarboxylating systems, including pyruvate dehydrogenase and 2-oxyglutarate dehydrogenase. Hence, mutations in this protein can cause lactic acidosis and deranged TCA activity, as well as an accumulation of BCAA.
Infants are protected during gestation because the placenta clears most potential toxins. The classical form of the disease, therefore, does not become clinically manifest until a few days after birth. Initial periods of alternating irritability and lethargy progress over a period of days to coma and respiratory embarrassment. Irreversible brain damage is common in babies who survive, particularly those whose treatment is delayed until after the first week of life.
Survivors may suffer a metabolic relapse at any time. The most common cause of relapse is intercurrent infection, which often favors endogenous protein catabolism. As a consequence, the patient's limited capacity to oxidize BCAA is overwhelmed and these compounds, together with their cognate ketoacids, accumulate to a toxic level. Relapse also can occur in association with surgery, trauma and emotional upset.
Patients with partial enzymatic deficiencies may present later in life with intermittent ketoacidosis, prostration and recurrent ataxia. Plasma concentrations of BCAA are elevated during these episodes, but they may be normal or near normal during the periods when patients are metabolically compensated.
Rare patients respond to the administration of thiamine in large doses, 10 to 30 mg per day. In these patients, the clinical course is even more mild than in patients with intermittent disease. Thiamine is a cofactor for the branched-chain ketoacid dehydrogenase, and the presumed mutation in these patients involves faulty binding of the vitamin to the apoprotein.
In many localities, newborn screening has become standard for this disorder, which in the general population has an approximate incidence of 1 in 250,000 live births. Carrier detection is possible, either by measurement of enzymatic activity in cultured fibroblasts or by study of restriction endonuclease fragments of DNA via Southern blotting. Antenatal testing is possible.
Treatment entails continuous dietary restriction of the BCAA. This is accomplished by administration of a special formula from which these amino acids are removed. The outlook for intellectual development is favorable in youngsters whose diagnosis is made early and who do not suffer recurrent, severe episodes of metabolic decompensation [8].
Gene therapy for this metabolic defect may become available within the next few years. In vitro studies have demonstrated the feasibility of retrovirus-mediated gene transfer of both the E1-α and E2 subunits of the branched-chain decarboxylase complex [7,9].
, reaction 3) is a mitochondrial enzyme of approximately 175 kDa composed of four identical subunits that are coded on human chromosome 15. The enzyme first transfers electrons from isovaleryl-CoA to FAD and then to electron-transferring flavoprotein (ETF). A specific ETF dehydrogenase then shifts the electrons to coenzyme Q in the electron-transport chain.
Affected patients usually have <5% of control capacity to oxidize isovaleric acid. The clinical presentation includes both a fulminant syndrome of neonatal onset and an intermittent disorder that usually becomes manifest in the first year or two of life. In the former instance, the baby develops irritability, vomiting, convulsions and progressive loss of consciousness during the first week. The rancid odor of isovaleric acid, which often is apparent from the urine, saliva and ear cerumen, accounts for the unusual name, “sweaty socks syndrome.” Patients frequently exhibit hyperammonemia, ketoaciduria, metabolic acidosis, pancytopenia and hypocalcemia.
Youngsters with intermittent disease usually present with the characteristic odor, lethargy, ataxia and vomiting in association with an intercurrent infection or the administration of a relatively large amount of protein. Hyperammonemia is common. A family may have children with both the neonatal and intermittent forms of the disease, suggesting that several phenotypes may be related to one genotype.
, reaction 4). Patients excrete isovalerylglycine even when clinically compensated, thereby facilitating diagnosis. Formation of isovalerylglycine also detoxifies isovaleric acid during periods of stress since the conjugate is hydrophilic and excreted into urine more efficiently than isovaleric acid itself. Indeed, supplementation of the diet with glycine is beneficial, especially during a crisis [10].
Patients usually fare well with a low-protein, that is, low-leucine, diet. Some suffer no relapses at all. The blood carnitine concentration usually is low, reflecting excessive excretion of isovalerylcarnitine. Carnitine therapy therefore has been suggested, but the utility of this approach still is uncertain.
, reaction 5) are rare and should be distinguished from the syndrome of 3-methylcrotonic aciduria, which occurs secondarily to defects of biotin metabolism (see below). Some patients present in early infancy with vomiting, metabolic acidosis, hyperlactatemia, convulsions and coma. Others remain well for 2 to 5 years, when they develop recurrent vomiting, metabolic acidosis, hypoglycemia and progressive lethargy leading to coma.
, reaction 6). It should be emphasized that 3-hydroxyisovaleric aciduria can be a nonspecific finding in ketotic patients. Excretion of 3-methylcrotonylglycine is elevated.
, reaction 7) are extremely rare. Patients may present only with delayed speech development or with relatively mild psychomotor retardation. The urine contains increased amounts of 3-methylglutaconate, 3-hydroxyisovalerate and 3-methylglutarate, the latter presumably being formed from hydrogenation of 3-methylglutaconic acid.
Several patients now have been described with an autosomal recessive disorder involving 3-methylglutaconic aciduria but with normal hydratase activity. Loading with oral leucine does not increase urinary excretion of 3-methylglutaconate. Most of these children have had a progressive course characterized by neurodegeneration, often beginning in the first few months of life, and death after a few months or years. These patients, unlike those with hydratase deficiency, do not excrete excessive amounts of 3-hydroxyisovaleric acid. The underlying biochemical defect is not yet known.
, reaction 8) become ill as neonates. In others, the symptoms are inapparent until 6 to 12 months. The most prominent findings are vomiting, lethargy, coma, convulsions and metabolic acidosis. An important finding is hypoglycemia without significant ketoaciduria, reflecting the significance of 3-hydroxy-3-methylglutaryl-CoA lyase to the synthesis of ketone bodies. The hypoglycemia may be referable to excessive consumption of glucose in the absence of the capacity to utilize an alternate fuel such as acetoacetate. Hepatomegaly with increased serum transaminases and hyperammonemia can occur and may lead to confusion with Reye's syndrome.
, reaction 7).
Patients must avoid fasting, which predisposes them both to developing hypoglycemia and, by favoring the synthesis of ketones from fatty acids, to the accumulation of 3-hydroxy-3-methylglutaric acid. Restriction of dietary protein and fat also may have a therapeutic role.
, reaction 9) is one of several 3-oxothiolases that catalyze formation of acetyl-CoA and the corresponding acyl-CoA. The reactions usually are reversible, and one such enzyme is a cytoplasmic protein that mediates the condensation of 2 mol of acetyl-CoA to form acetoacetyl-CoA, which then reacts with another mol of acetyl-CoA to generate 3-hydroxy-3-methylglutaryl-CoA in the pathway of cholesterol synthesis.
The mitochondrial thiolase is specific for 2-methylacetoacetyl-CoA. In the liver, this enzyme also abets ketogenesis by catalyzing synthesis of 3-hydroxy-3-methylglutaryl-CoA from acetoacetyl-CoA and acetyl-CoA. It is a tetramer of approximately 170 kDa and is stimulated by potassium, unlike the cytosolic enzyme.
The inherited disorder, sometimes termed β-ketothiolase deficiency, causes recurrent acidosis, ketosis, vomiting and even death. Patients respond to intravenous glucose and bicarbonate. Mental retardation is not unprecedented, but it is exceptional.
), also accumulates and usually is excreted as tiglylglycine. The ketosis is referable to inhibition of acetoacetyl-CoA metabolism by 2-methylaceto-acetyl-CoA. Excretion of these metabolites is variable when patients are not acutely ill.
, reaction 13), accumulates and forms ninhydrin-positive conjugates with cysteine and cysteamine, which can be detected with amino acid analysis. The urine organic acids are otherwise unremarkable. A single patient had multiple congenital anomalies, including tetralogy of Fallot, facial dysmorphism and dysgenesis of the brain. The infant died at 3 months of age. There is no treatment.
). Additional sources are methionine and threonine, as well as odd-chain fatty acids and cholesterol. Methylmalonic acid (MMA) can be formed also from the catabolism of thymine.
Children with propionic acidemia and methylmalonic acidemia may manifest an intense hyperglycinemia. Indeed, these disorders once were known as “ketotic hyperglycinemia.” As the underlying biochemistry became better understood, this description was discarded in favor of more specific terminology, that is, propionic acidemia and methylmalonic acidemia.
, reaction 11) is a tetramer of 540 kDa composed of two α and two β subunits. The α subunit has been mapped to human chromosome 13 and the β subunit to chromosome 3. Leader peptides, facilitating transport of the propeptides into the mitochondria, also have been identified. The α subunit contains the biotin-binding site, at which a specific enzyme, holocarboxylase synthetase, mediates binding of this cofactor to the carboxylase.
Patients with a near-total enzyme deficiency become sick as neonates with dehydration, lethargy progressing to coma, vomiting, ketoaciduria and hypotonia. The toxicity may involve the bone marrow, resulting in neutropenia and thrombocytopenia. Hyperammonemia and death from hemorrhage are not unusual. Hyperglycinemia occurs in many cases. Acidosis is not an invariant feature of the syndrome [11].
Infants may relapse, particularly in association with an infection. Permanent brain damage, seizures and mental retardation are frequent. A poor outcome is common even in patients who have enjoyed ostensibly good metabolic control [12]. Some patients remain well until later infancy or childhood, when their developmental retardation and failure to thrive are first discovered.
A variety of neuropathological features have been noted [6], including vacuolization of the white matter and patchy neuronal degeneration. The basal ganglia are a special target of injury. Hemorrhagic lesions have been reported at autopsy in the caudate, putamen, globus pallidus and thalamus. Endothelial cells in these regions tended to be swollen and hyperplastic, perhaps indicating a breakdown of the blood—brain barrier. This type of injury has been termed a metabolic stroke [13].
Various mutations have been described. These were formerly classified into two groups, pccA and pccBC, depending upon the thermostability of the mutant enzyme, its affinity for the effector K+ and the response to addition of avidin. The pccA group, which is more common, has little or no α chain and the pccBC mutants are defective in β chain activity. Inheritance is autosomal recessive.
Presumptive diagnosis requires demonstration of increased excretion of propionate derivatives, including propionylglycine, 3-hydroxypropionate, tiglylglycine and methylcitric acid, a condensation product of propionyl-CoA and oxaloacetate. Definitive diagnosis involves the measurement of enzymatic activity in peripheral blood leukocytes. Prenatal diagnosis is feasible. Hyperammonemia is a more common initial finding than metabolic acidemia.
Treatment entails the restriction of dietary protein to minimize propionate production. Most patients have growth failure. Propionyl-CoA carboxylase is stimulated by biotin, but supplementation with this vitamin is not of documented benefit. Blood carnitine is low, probably because of loss as propionylcarnitine, and some evidence points to clinical improvement with carnitine treatment. An important recent advance is orthotopic liver transplantation, which has been tried with some success in the most severe cases [14].
, reaction 12) is a dimer of approximately 150 kDa made of two identical subunits bound to 1 mol of adenosylcobalamin (vitamin B12). The gene has been mapped to the short arm of human chromosome 6. Patients can have a complete deficiency of the apoenzyme (mut0), partial deficiency (mut−) or various defects of vitamin B12 metabolism. The latter patients have homocystinuria and hypermethio-ninemia as well as methylmalonic aciduria.
Patients with the mut0 lesion present as neonates with vomiting, acidosis, hyperammonemia, hepatomegaly, hyperglycinemia and hypoglycemia. Neutropenia and thrombocytopenia can occur. Growth failure is very common in children. Mortality is high and prognosis is poor in children with an early onset (<2 months) of disease. The outlook is somewhat improved in children with a late-onset syndrome, although even in this group morbidity tends to be quite high [15].
Examination of the brain with magnetic resonance imaging shows some white matter attenuation during the first month of life. This progresses to a failure of myelination as the infant grows. The globus pallidus may display a disproportionate degree of injury, although this may regress to a variable degree following institution of therapy to lower the toxic concentration of methylmalonic acid (MMA) [5].
The urine contains excessive MMA, even when patients are well. The concentration in the CSF equals or even exceeds that in the blood.
Treatment involves a diet that is low in the amino acid precursors to MMA. Medical attention should be sought whenever the patient develops an acute infection. There may be a role for l-carnitine supplementation.
High concentrations of MMA, or propionate, adversely affect oxidative metabolism, resulting in a depletion of ATP. Several enzyme systems, for example, pyruvate carboxylase, are inhibited by these organic acids. In addition, the sequestration of CoA as methylmalonyl-CoA or propionyl-CoA probably causes a depletion of the free CoA pool, which would adversely affect the synthesis of myelin, urea and glucose.
Vitamin B12 is ineffective in patients with either the mut0 or mut− lesions, but it may help infants with defects of cobalamin synthesis and/or transport (see below). There is no hazard associated with giving 1 to 2 mg per day of vitamin B12, and this approach is warranted until the results of enzymatic studies are available.
, reaction 4; also see below, section on disorders of sulfur-containing amino acids). Neurological symptoms, frequently severe and presenting in early infancy, are common to most of these syndromes.
Intestinal absorption of cobalamin requires intrinsic factor, a glycoprotein that is synthesized in the gastric parietal cells. The cobalamin—intrinsic factor complex is taken up by cells in the ileum, where the complex dissociates. Cobalamin enters the circulation bound to transcobalamin II (TC-II), which also abets uptake into tissues. Methylcobalamin is the major circulating form, but the major intracellular species, including in the brain, appears to be adenosylcobalamin. The cobalamin—TC-II complex is broken down by a specific lysosomal protease. The free cobalamin then is methylated in the cytoplasm, or it enters the mitochondrion, where, as adenosylcobalamin, it serves as cofactor for the MMA-CoA mutase reaction.
Several genetic defects have been identified, including anomalies involving either cobalamin absorption, cellular uptake or intracellular handling. Transport defects are represented by inherited deficiency of intrinsic factor in juvenile pernicious anemia, cobalamin malabsorption syndrome and TC-II deficiency. These patients usually have megaloblastic anemia, methylmalonic aciduria and homocystinuria. Neurological signs, especially in infants with TC-II deficiency, tend to be quite severe. The pathogenesis of the CNS lesions probably involves a failure to generate an adequate amount of S-adenosylmethionine, the major methyl donor in the developing brain. This pivotal compound is formed from methionine, which is regenerated from homocysteine in a reaction that is dependent upon methylcobalamin (see below, section on disorders of sulfur amino acid metabolism).
Primary defects in the synthesis of methylcobalamin and adenosylcobalamin have been described. Patients with methylmalonic aciduria secondary to deranged vitamin B12 metabolism can be distinguished from those with the mutase deficiency by their response to pharmacological doses of cyanocobalamin or adenosylcobalamin, which sharply reduce methylmalonate excretion. At least two distinct inherited forms of faulty adenosylcobalamin synthesis, known as cblA and cblB, have been identified from clinical findings and complementation analysis in skin fibroblasts. Unlike infants with the mutase deficiency, who present in the first 1 to 2 weeks of life, these patients do not become clinically ill until after the first month of life. They may have ketonuria and metabolic acidemia, as well as a severe neurological syndrome involving coma and convulsions. Survivors commonly have mental retardation and microcephaly. Evidence of pathology to the cerebellum and the dorsal columns of the spinal cord is common.
, reaction 16). Patients may lack a specific mitochondrial cobalamin reductase (Fig. 44-2, reactions 14 and 15). Patients with cblB disease are missing a functional adenosyltransferase enzyme (Fig. 44-2, reaction 16).
The diagnosis of defective adenosylcobalamin synthesis is suggested by methylmalonic aciduria without megaloblastic changes, homocystinuria or hypomethioninemia. The blood vitamin B12 concentration is normal. Prenatal diagnosis is feasible, either through study of the mutase reaction in amniocytes or by quantitation of MMA in amniotic fluid.
More than 90% of patients with the cblA disease respond favorably to the administration of hydroxycobalamin, which reduces MMA excretion. They have a good prognosis, with most surviving into adulthood in an intact state. Only 40% of individuals with cblB disease show a positive response. Their prognosis is less sanguine, with neurological impairment often noted among youngsters who survive the initial insult. Protein restriction, which minimizes production of MMA, is also indicated.
). Patients with a congenital absence of glutaryl-CoA dehydrogenase (Fig. 44-3, reaction 3) seem normal at birth, although they may manifest macrocephaly. Normal development is common for the first 1 to 2 years, when they develop hypotonia, opisthotonus, seizures, rigidity, dystonia, facial grimacing and seizures. These signs may develop very abruptly, following an intercurrent illness, or in a more gradual manner. Developmental assessment is difficult because of severe motor involvement, but mental retardation may not occur. The neurological syndrome may be progressive, and death may occur during the first decade of life. The diagnostic hallmark is excretion of glutaric acid and 3-hydroxyglutaric acid. Imaging of the brain shows atrophy of the caudate and putamen and a loss of white matter in both frontal and occipital horns. Pathological examination of the brain also shows degenerative changes in the basal ganglia and the cortical white matter. A special diet low in tryptophan and lysine will reduce excretion of glutaric acid, but it may not improve the clinical status.
, reaction 3) involves transfer of electrons to FAD, in the process forming FADH2. Glutaric aciduria type II usually is caused by a congenital deficiency of either ETF or ETF:ubiquinone reductase. These proteins, which are encoded by nuclear genes, mediate the transfer of electrons from flavoproteins to the respiratory chain in mitochondria. Other substrates that donate electrons to these proteins are dimethylglycine dehydrogenase and sarcosine dehydrogenase.
Patients often present as neonates with hepatomegaly, hypoglycemia without significant ketonemia, lipid storage myopathy with hypotonia, metabolic acidosis and a rancid urine odor similar to that of isovaleric acidemia (see above). The kidneys commonly are enlarged. Cystic changes of both the liver and kidney are frequent. Facial dysmorphism also can occur. Magnetic resonance imaging of brain typically suggests a leukodystrophy. The outlook is almost uniformly fatal, and the few babies who survive have severely compromised development and a cardiomyopathy that usually proves fatal. In rare cases, a patient stays asymptomatic until after the neonatal period, when hepatomegaly, vomiting, metabolic acidosis, hypoglycemia and a proximal myopathy become evident.
Pathological examination of the brain reveals dysplasias and other evidence of aberrant neural migration. The gyri of the cortex generally are reduced. 1H-Magnetic resonance spectroscopy of the brain shows an increase of lactic acid concentration and of the choline:creatine ratio, suggesting dysmyelination. 31P-Spectroscopy of muscle shows a severe compromise of energy metabolism [16]. Occasionally, patients have responded to riboflavin therapy [17], but there is no effective therapy.
, reaction 1), many youngsters have hyperphenylalaninemia caused by a partial deficiency of the enzyme. They do not suffer mental retardation, but they may have more subtle neurological problems [18].
The hydroxylase is a trimer of approximately 150 kDa of identical subunits and is located predominantly in the liver. The enzyme has been mapped to human chromosome 12q22-24.1, where the gene comprises 13 exons extending over 90 kb of genomic DNA. Deletions in the gene are not common. A frequent cause among northern Europeans (~40%) is a G-to-A transition at the 5′ donor splice site in intron 12, resulting in absence of the C-terminus. Another relatively common (~20%) mutation in northern Europeans involves a C-to-T transition in exon 12, resulting in substitution of a tryptophan for an arginine residue [19]. Over 70 different mutations have been described to date in the American population [20].
Mutations have been associated with specific haplotypes, the latter determined by analysis of restriction fragment length polymorphisms. This approach has been utilized for prenatal diagnosis. The study of haplotypes also has revealed that the majority (~75%) of northern European patients are compound PKU heterozygotes.
Affected babies are not retarded at birth, but almost all will be impaired if they are not treated by 3 months of age. Mass screening has largely eliminated the untreated PKU phenotype of eczema, poor growth, irritability, musty odor caused by phenylacetic acid and tendency to self-mutilation. Progressive motor dysfunction has been described in children with long-term hyperphenylalaninemia.
The clinical utility of dietary restriction of phenylalanine to 200 to 500 mg per day is clear. Well-controlled patients have normal intelligence, although there is an increased risk of perceptual learning disabilities, emotional problems and subtle motor difficulties [21]. Diet therapy is maintained throughout adolescence and, perhaps, indefinitely. Performance may deteriorate after the diet is discontinued.
Exposure to excessive (>1 mM) blood phenylalanine concentrations in early infancy can impair neuronal maturation and the synthesis of myelin. The responsible factor is excess phenylalanine, not a phenylalanine metabolite or tyrosine deficiency. One hypothesis suggests that excessive phenylalanine inhibits the transport of other neutral amino acids across the blood—brain barrier (see Chap. 32). Conversely, some have proposed that high intracerebral phenylalanine concentrations impair the transport of tyrosine from the brain to the blood. High brain phenylalanine concentrations can inhibit synaptosomal Na,K-ATPase activity and the synthesis of neurotransmitters. Excess phenylalanine also causes disaggregation of brain polysomes, which may explain the dysmyelination that has been described in the phenylketonuric brain. A loss of neurotransmitter receptors has been described in a murine model of hyperphenylalaninemia [22].
The genotypically normal offspring of an untreated mother may have microcephaly and irreversible brain injury, as well as cardiac defects. Scrupulous monitoring of dietary phenylalanine intake in these women has resulted in a much better outcome [23].
, reaction 2). BH4 is regenerated from QH2 in an NADH-dependent reaction (Fig. 44-4, reaction 2) that is catalyzed by dihydropteridine reductase (DHPR), which is widely distributed. In the brain, this enzyme also mediates hydroxylation of tyrosine and tryptophan. Human DHPR has been mapped to chromosome 4p15.1-p16.1 The coding sequence shows little homology to other reductases, for example, dihydrofolate reductase.
, reactions 3 and 4) [24]. BH4 functions also in the hydroxylation of tyrosine and tryptophan.
Even careful phenylalanine restriction fails to avert progressive neurological deterioration because patients are unable to hydroxylate tyrosine or tryptophan, the synthesis of which also requires BH4. Thus, neurotransmitters are not produced in sufficient amount.
Patients sustain convulsions and neurological deterioration. The urine contains low concentrations of the metabolites of serotonin, norepinephrine and dopamine. The reductase also plays a role in the maintenance of tetrahydrofolate concentrations in brain, and some patients have had low folate in the serum and CNS. Treatment has been attempted with tryptophan and carbidopa to improve serotonin homeostasis and with folinic acid to replete diminished stores of reduced folic acid. This therapy sometimes is effective. Diagnosis involves assay of DHPR in skin fibroblasts or amniotic cells. Phenylalanine hydroxylase activity is normal.
Other causes of PKU secondary to defective BH4 synthesis include GTP cyclohydrolase deficiency and 6-pyruvoyltetrahydrobiopterin synthase deficiency. Patients with either defect have psychomotor retardation, truncal hypotonia with limb hypertonia, seizures and a tendency to hyperthermia. Intravenous administration of BH4 may lower blood phenylalanine concentrations, but this cofactor may not readily cross the blood—brain barrier. Treatment with synthetic pterin analogs or supplementation with tryptophan and carbidopa may prove more efficacious, particularly if treatment is started early in life.
, reaction 1). Pyridoxal phosphate and tetrahydrofolate are cofactors in this reaction. Glycine is the precursor to the “one-carbon pool” of folic acid intermediates that is pivotal to many synthetic reactions (Fig. 44-5, reaction 1) [25].
Affected infants become ill by the first or second day of life. Seizures are very prominent and even may occur in utero. EEG often displays a hypsarrythmia or a burst-suppression pattern. Patients display myoclonic jerks, hiccuping and a profound hypotonia. The few patients who survive past the first week of life usually sustain profound mental retardation and neurological disability. Brain imaging shows atrophy and a loss of myelin.
Rare patients present later in life with psychomotor retardation and growth failure. Others have had initially normal development followed by a progressive loss of developmental milestones. Some patients have manifested spinocerebellar degeneration and other symptoms of motor dysfunction [26].
Glycine is extremely high in the blood, often rising to >1 mM, with normal being 150 to 350 μM. The concentration in the CSF almost always exceeds 100 μM, with normal being ~10 μM. The CSF:blood ratio of glycine usually is five to ten times the control value of 0.02, especially with the classical form of the disease.
The GCS is composed of four distinct subunits: pyridoxal-dependent decarboxylase (P); heat-stable, lipoic acid-binding carrier of the aminoethyl group (H); tetrahydrofolate-requiring (T); and lipoamide dehydrogenase (L). Most infants with the classical disease have had defects either in the P or the T proteins.
A transient form of nonketotic hyperglycinemia (NKH), probably reflecting delayed maturation of the GCS, has been described in neonates with seizures but an otherwise normal neurological examination. The seizures ceased by 8 weeks of age and did not recur. Glycine concentrations in both the blood and the CSF were high. Urine organic acid analysis was normal.
There is no specific therapy. Exchange transfusion and dialysis usually do not alter the progressive neurological deterioration. Sodium benzoate has been administered in the hope that glycine would react with it to form hippuric acid, but this approach is not helpful. It may be that a combination of benzoate and carnitine therapy is more availing [27]. Similarly, the restriction of dietary protein and the administration of pyridoxine or choline have not proved useful.
Glycine is a neurotransmitter, having a postsynaptic inhibitory activity in the spinal cord and in some central neurons (see Chap. 16). Therapy with strychnine, which blocks the action of glycine at postsynaptic receptors, has been unhelpful. Treatment with diazepam has been attempted because this drug displaces strychnine from its binding sites. The combination of benzoate and diazepam may be more effective since high doses of the former reduce glycine concentrations in the CNS, thereby potentiating the ability of strychnine to block the glycine effect.
A few infants have been treated with antagonists of the N-methyl-d-aspartate (NMDA) receptor, an excitatory glutamatergic receptor which is potentiated by glycine (see Chap. 15) [28]. Ketamine and dextromethorphan have been used with inconclusive results. Some infants may have had an improvement of their irritability and EEG. One infant, treated with both benzoate and dextromethorphan, was seizure-free by 12 months of age and had only moderately delayed development. However, this favorable experience has not always been duplicated. Treatment with dextromethorphan at the recommended maximal dosage of 5 mg/kg/day seems to be well tolerated.
) entails the transfer of the sulfur atom of methionine to serine with the ultimate formation of cysteine. The first step is activation of methionine, which reacts with ATP to form S-adenosylmethionine (SAM) (Fig. 44-2, reaction 1). This compound is a key methyl donor and plays a prominent role in the synthesis of several neurotransmitters and of creatine (Fig. 44-2, reaction 2). A portion of the carbon of spermidine and spermine is derived from SAM following the decarboxylation of that compound.
, reaction 3).
, reaction 4), utilizes methylcobalamin as a cofactor. The kinetics of the reaction favor remethylation. One form of homocystinuria is caused by a defect of this remethylation process. Faulty remethylation can occur secondary to (i) dietary factors, such as vitamin B12 deficiency; (ii) a congenital absence of the apoenzyme; (iii) a congenital inability to convert folate or vitamin B12 to the methylated, metabolically active form (see below); or (iv) the presence of a metabolic inhibitor, for example, an antifolate agent that is used in an antineoplastic regimen.
, reaction 5) is pyridoxine-dependent and converts homocysteine to cystathionine via condensation with serine. SAM potently stimulates the reaction in the forward direction [29]. This enzyme has been mapped to human chromosome 21. The equilibrium favors cystathionine synthesis. Thus, homocysteine concentrations normally are very low since both the remethylation pathway and the cystathionine synthetase route efficiently dispose of this amino acid.
, reaction 6). The enzyme functions almost entirely to produce cysteine, there being virtually no reversal of the reaction.
A number of mutations that result in cystathionine synthetase deficiency have been described. The known mutations cause synthesis of an unstable enzyme; a protein that loosely binds either pyridoxal phosphate, serine or homocysteine; or an enzyme differing in size from the wild-type [30]. Cystathionine synthetase is present in many organs, including the brain, and homocystinuric patients typically manifest deficient enzyme activity in these tissues.
Blood homocysteine is elevated to 50 to 200 μM, with normal being <10 μM. The blood cysteine concentration tends to be low, reflecting the failure of cysteine synthesis. Increased remethylation of the homocysteine that is not converted to cystathionine results in elevated blood methionine, often in excess of 200 μM, with normal being 20 to 40 μM.
Some patients respond to the administration of pharmacological doses of pyridoxine, 25 to 100 mg daily, with a reduction of plasma homocysteine and methionine. Pyridoxine responsiveness appears to be hereditary, with siblings tending to show a concordant response. The clinical syndrome is milder in these individuals. Pyridoxine sensitivity can be documented by enzyme assay in skin fibroblasts. The precise biochemical mechanism of the pyridoxine effect is not well understood, but it may not reflect a mutation resulting in diminished affinity of the enzyme for cofactor because even high concentrations of pyridoxal phosphate do not restore mutant enzyme activity to a control level.
About half of the individuals who do not respond to pyridoxine (vitamin B6) will sustain ectopia lentis by age 5 to 10 years. Indeed, the diagnosis commonly is made by an ophthalmologist to whom a child with bilaterally displaced lenses has been referred.
The median IQ scores for vitamin B6-responsive and nonresponsive patients are 78 and 56, respectively. Some children may come to clinical attention at 1 to 2 years with psychomotor retardation. Other signs are convulsions, which occur in about 20% of patients, and psychiatric difficulties, notably depression and personality disorders, which occur in about half of cases.
The most striking feature is a thromboembolic diathesis. This can occur in virtually any vessel, with thrombi common in peripheral veins and arteries, the cerebral and renal vasculature and coronary arteries. Almost 25% of pyridoxine nonresponders sustain a major vascular insult during childhood. The comparable risk in untreated, pyridoxine-responsive subjects is 25% by age 20 years. Vascular insults sometimes occur in association with dehydration secondary to vomiting and diarrhea. The stress of major surgery and anesthesia increases the risk of thrombosis by ~5%. Homocystinuric patients who also have the relatively common Leiden mutation of clotting factor V are at sharply increased risk for developing a thrombosis [31].
Affected patients commonly manifest a marfanoid habitus with arachnodactyly, high-arched palate, tall stature and pes cavus. Bony abnormalities are common, with osteoporosis and scoliosis being frequent sources of clinical problems. The orthopedic findings are more common and more severe in patients who do not respond to pyridoxine treatment.
Demyelination and spongy degeneration of the white matter have been reported. Infarctions are relatively common in virtually all parts of the brain. The arterial wall shows thickening of the intima and splitting of the smooth musculature of the media. The changes are not dissimilar to those of atherosclerosis.
The probable cause of the pathology is excess homocysteine. Excess methionine is not thought to play an etiological role. The biochemical basis of homocysteine toxicity has been the subject of intense scrutiny. Homocysteine increases the adhesiveness of platelets in vitro, perhaps by favoring the synthesis of selected thromboxanes. Administration of homocysteine to rats or baboons can cause endothelial injury. Homocysteine may diminish the mean survival time of peripheral blood platelets, possibly by a direct toxic effect on the vascular endothelium, which becomes denuded and thereby provides an atherogenic nidus. A direct effect of homocysteine on the blood-clotting cascade also is possible. Thus, activation of factor V in cultured endothelial cells has been noted. This favors the conversion of prothrombin to thrombin.
Homocysteine also promotes accumulation of copper in the vascular endothelium. This induces the oxidation of ceruloplasmin and the concomitant release of sufficient H2O2 to injure endothelial cells. Supplementation of the medium with catalase protects against such an insult, thus confirming the role of oxidant injury.
High concentrations of homocysteine or one of its metabolites may directly affect brain function. Administration of homocysteine to rats induces grand mal convulsions, a phenomenon that is worsened by either methionine or pyridoxine. Homocysteine-induced blockade of the GABA receptor may be involved. In addition, brain can oxidize homocysteine to homocysteic acid, which has a glutamatergic activity.
A high intracerebral concentration of S-adenosylhomocysteine may inhibit methylation reactions involving SAM. The metabolic repercussions would be extensive, including deficient methylation of proteins and of phosphatidylethanolamine as well as an inhibition of catechol-O-methyltransferase and histamine-N-methyltransferase.
Patients who respond to large doses of vitamin B6, 250 to 500 mg per day for several weeks, have the best prognosis. Efficacy of treatment usually is reflected in a reduction of blood homocysteine and methionine to normal or near-normal levels. Since supplementation with pyridoxine can cause a deficiency of folic acid, the latter should be given at 2 to 5 mg daily at the same time. Any patient receiving pyridoxine should be monitored carefully for any signs of hepatotoxicity and for a peripheral neuropathy.
Management of the pyridoxine-nonresponsive patient is difficult. Dietary restriction of methionine would seem logical, but this often is unpalatable, especially to an adult patient who has adapted to a diet that has not been purposefully restricted of protein.
, reaction 4) [32]. A theoretical hazard of betaine treatment is increasing the blood methionine, sometimes to an extravagant degree, as high as 1 mM. Experience to date indicates that betaine administration is safe, with no major side effects except for a fishy odor to the urine.
Other therapeutic approaches have included the administration of salicylate and of dipyridamole to ameliorate the thromboembolic diathesis. Dipyridamole has been effective in animal studies at restoring platelet survival to a near-normal range. Patients also have been treated with dietary supplements of l-cystine since block of the trans-sulfuration pathway in theory could diminish the synthesis of this amino acid.
, reaction 4). Patients often present early in life with lethargy, poor feeding, psychomotor retardation and growth failure. Hematological abnormalities are common, including megaloblastosis, macrocytosis, thrombocytopenia and hypersegmentation of the leukocytes. Occasional patients are clinically silent until later life, when seizures, dementia, hypotonia, mental retardation, spasticity or a myelopathy become evident.
Biochemical findings are variable. Interestingly, blood cobalamin and folate concentrations often are normal. Many have had homocysteinemia with hypomethioninemia, the latter helping to discriminate this group from homocystinuria secondary to cystathionine β-synthase deficiency. Urinary excretion of MMA may be high, reflecting the fact that vitamin B12 serves as a cofactor for the methylmalonyl-CoA mutase reaction (see above).
, reaction 11). SAM inhibits the reaction. The enzyme normally is present in human brain, where it may play a role in the reduction of dihydropteridines (see above, section on disorders of phenylalanine metabolism).
Patients typically present with severe developmental retardation, convulsions and microcephaly by age 6 to 12 months. A few individuals also have had psychiatric disturbances.
Homocysteinemia, usually ~50 μM, is the rule, with a relatively low blood methionine concentration of less than 20 μM. The blood concentration of vitamin B12 is normal, and, unlike individuals with defects of cobalamin metabolism, these patients manifest neither anemia nor methylmalonic aciduria. Blood folic acid is usually low.
A thromboembolic diathesis is not unusual, and thromboses have been reported in the brain vasculature. Other pathological changes have included microgyri, demyelination, gliosis and brain atrophy. Lipid-laden macrophages have been described.
A relatively large number of agents have been utilized to treat this intractable disorder: folinic acid (5-formyltetrahydrofolic acid), folic acid, methyltetrahydrofolic acid, betaine, methionine, pyridoxine, cobalamin and carnitine. Betaine, which provides methyl groups to the betaine:homocysteine methyltransferase reaction, appears to be a nontoxic approach that lowers blood homocysteine and increases methionine.
, reaction 4).
In cobalmin-E (cblE) disease, there is a failure of methyl-vitamin B12 to bind to methionine synthetase. It is not known if this reflects a primary defect of methionine synthase or the absence of a separate enzymatic activity. Patients manifest megaloblastic changes with a pancytopenia, homocystinuria and hypomethioninemia. There is no methylmalonic aciduria. Patients usually become clinically manifest during infancy with vomiting, developmental retardation and lethargy. They respond well to injections of hydroxocobalamin.
The activity of methionine synthetase is restored to normal in vitro by addition of large amounts of thiols to the incubation mixture. In contrast, in cblG disease, the enzymatic activity remains low even with thiol supplementation of the assay.
Complementation analysis allows the classification of patients with primary defects in the metabolism of vitamin B12 into one of three groups: cblC, cblD and cblF. The most common variant is cblC. Most individuals become ill in the first few months or weeks of life with hypotonia, lethargy and growth failure. Optic atrophy and retinal changes can occur. Methylmalonate excretion is excessive, although less than in methylmalonyl-CoA mutase deficiency. Patients do not display ketoaciduria or overwhelming metabolic acidosis.
Fibroblasts do not convert cyanocobalamin or hydroxocobalamin to methylcobalamin or adenosylcobalamin. The activities of both N5-methyltetrahydrofolate:homocysteine methyltransferase and methylmalonyl-CoA mutase are consequently diminished. These biochemical lesions can be rectified by supplementation of the medium with hydroxocobalamin. The precise nature of the underlying defect remains obscure, although it appears to involve a step in the activation of B12.
The diagnosis should be suspected in a child with homocystinuria, methylmalonic aciduria, megaloblastic anemia, hypomethioninemia and normal blood concentration of folate and vitamin B12. A definitive diagnosis requires demonstration of these abnormalities in fibroblasts. Prenatal diagnosis is possible.
Treatment involves the administration of large doses (as much as 1 mg) of intramuscular hydroxocobalamin. Administration of folate and betaine (see above) may be helpful, as is a reduction of protein intake.
It may become clinically manifest only in later life with mild mental retardation and behavioral abnormality.
Most patients have presented with megaloblastic anemia, seizures and a progressive syndrome of neurological deterioration. Folate, in both the blood and the CSF, is very low. The anemia is correctable with injections of folate or with the administration of large oral doses, but the concentration in the CSF remains low, suggesting that a distinct carrier system mediates folate uptake into the brain and that this system is the same as that facilitating intestinal transport.
) mediates the removal of ammonia as urea in the amount of 10 to 20 g per day in the healthy adult. The absence of a fully functional urea cycle may result in hyperammonemic encephalopathy and irreversible brain injury in severe cases. A failure of ureagenesis occurs because of acquired disease, such as cirrhosis secondary to alcoholism, or secondary to an inherited defect, usually a congenital enzymopathy.
, reaction 1). N-acetylglutamate (NAG), formed from glutamate and acetyl-CoA via NAG synthetase (Fig. 44-6, reaction 9), is an obligatory effector of CPS and an important regulator of ureagenesis. A variety of influences, including dietary protein, arginine and corticosteroids, augment the concentration of NAG in mitochondria.
Following condensation with ornithine, carbamyl phosphate is converted to citrulline in the ornithine transcarbamylase (OTC) reaction. OTC is coded on band p21.1 of the X chromosome, where the gene contains 8 exons and spans 85 kb of DNA. The activity of this enzyme is directly related to dietary protein. There may be “tunneling” of ornithine transported from the cytosol to OTC, with the availability of intramitochondrial ornithine serving to regulate the reaction.
, reaction 3). This enzyme is coded on human chromosome 9, where a 63-kb gene comprising 14 exons is located. The mRNA is markedly increased by starvation, treatment with corticosteroids or cAMP. Citrulline itself is a potent inducer of the mRNA.
, reaction 4). The products of the reaction are fumarate, which is oxidized in the TCA cycle, and arginine, which is rapidly cleaved to urea and ornithine via hepatic arginase. Both AL and arginase are induced by starvation, dibutyryl cAMP and corticosteroids. Several isozymes of arginase have been described.Clinical confusion with septicemia is common, and many infants are treated futilely with antibiotics. Hyperammonemia usually is severe, even in excess of 1 mM; normal in term infants is up to 100 μM.
Diagnosis usually is made from the blood aminogram. Plasma concentrations of glutamine and alanine, the major nitrogen-carrying amino acids, usually are high and that of arginine is low. Patients with citrullinemia, caused by a deficiency of AS, or argininosuccinic aciduria, caused by a deficiency of AL, will manifest marked increases of blood citrulline and argininosuccinate, respectively.
Urinary orotic acid generally is very elevated in babies with OTC deficiency and normal or even low in infants with CPS deficiency. Patients with OTC deficiency have increased excretion of orotic acid because carbamyl phosphate spills into the cytoplasm, where it enters the pathway of pyrimidine synthesis.
Diagnosis of the infant with either CPS or OTC deficiency may not always be apparent from the blood aminogram. Ornithine concentrations typically are normal in the latter disorder. The presence of hyperammonemia, hyperglutaminemia, hyperalaninemia and orotic aciduria in a critically ill infant affords strong presumptive evidence for OTC deficiency. Conversely, the presence of this pattern on the aminogram in the absence of an untoward orotic aciduria is suggestive of CPS deficiency.
Diagnosis of an urea cycle defect in the older child can be more problematic. Patients may present with psychomotor retardation, growth failure, vomiting, behavioral abnormalities, perceptual difficulties, recurrent cerebellar ataxia and headache. Thus, it is essential to monitor the blood ammonia in any patient with unexplained neurological symptoms. Measurement of blood NH3 alone may not be sufficient for diagnosis since hyperammonemia can be an inconstant finding with partial enzymatic defects. In the latter group, quantitation of blood amino acids and of urinary orotic acid is indicated.
Hyperammonemia also occurs in some organic acidurias, particularly those that affect neonates. Thus, the urine organic acids should be quantitated in all patients with significant hyperammonemia.
A variety of biochemical changes in brain metabolism have been described in experimental models of hyperammonemia. High ammonia concentrations impair the malate—aspartate shuttle, which mediates the transport of NADH from the cytosol to mitochondria. Changes also occur in the rate of oxidation of glucose and/or pyruvate. The intracellular ATP pool may be depleted, especially in the reticular activating system.
Hyperammonemia also may affect brain volume control; cell swelling is sometimes observed, perhaps because of the marked increase of brain glutamine. This change probably is most prominent in the astrocytes, where it would be expected to have an osmotic effect. Glial swelling is a common pathological finding in hyperammonemic patients.
Hyperammonemia also affects neurotransmitter metabolism. Major effects on the handling of GABA and serotonin have been observed. In the latter instance, a possible mechanism may be increased passage of tryptophan across the blood—brain barrier and consequent increased synthesis of serotonin. Treatment of patients with blockers of serotonergic receptors may alleviate the anorexia that is common in this population. Extracellular glutamic acid tends to increase, and recent experimental evidence suggests excitotoxic injury. Ammonia also has been shown to affect ion flux, in particular that of C1−. This might cause hyperpolarization of membranes.
Surprisingly little is known about the changes in the brain of patients dying with hyperammonemia. Abnormal myelination with cystic degeneration of neurons has been described. Cell swelling, particularly of the astrocytes, is common. Cortical atrophy may occur in youngsters with long-standing disease.
Except for patients with argininosuccinic aciduria, who may demonstrate varying degrees of hepatic fibrosis, there is very little evidence of pathological changes outside of the CNS.
CPS deficiency is relatively rare. Neonates quickly develop lethargy, hypothermia, vomiting and irritability. The hyperammonemia typically is severe, even exceeding 1 mM. Occasional patients with partial enzyme deficiency have had a relapsing syndrome of lethargy and irritability upon exposure to protein. Brain damage can occur in both neonatal and late-onset groups.
A deficiency of CPS activity also can arise because of the congenital absence of NAG synthetase, which catalyzes the formation of NAG from glutamate and acetyl-CoA. NAG is an obligatory effector of CPS. The few patients reported have had a malignant course of neonatal onset.
Presentation is variable, ranging from a fulminant, fatal disorder of neonates to a schizophrenia-like illness in an otherwise healthy adult. Affected men characteristically fare more poorly than women with this X-linked disorder. This difference reflects the random inactivation of the X chromosome, termed Lyon's hypothesis. If the inactivation affects primarily the X chromosome bearing the mutant OTC gene, then a more favorable outcome can be anticipated. Conversely, if the wild-type X chromosome is inactivated, the woman is expected to have a much more active disease.
The human OTC gene spans 73 kb, comprising 10 exons and nine introns. In the mouse, a 750-bp promoter 5′ to the transcription initiation codon confers tissue specificity.
The diagnosis has been aided by the use of genetic markers based on intragenic restriction fragment length polymorphisms (RFLPs) (see also Chap. 40). More than 80% of carriers can be detected in this manner, and antenatal diagnosis is possible in many cases. Approximately one-third of the mothers of boys and two-thirds of the mothers of girls have been found to be noncarriers, reflecting the greater propensity for mutation in the male gamete.
Diagnosis of carriers can be made with protein-loading tests, in which the excretion of urinary orotic acid has been used as a marker. This approach detects 85 to 90% of carriers. A recent elaboration of these tests involves the administration of allopurinol to favor orotic acid excretion. Loading studies with 15NH4C1 as metabolic tracer indicate that symptomatic female carriers for OTC produce less 15N urea compared with a control population. Asymptomatic heterozygotes form urea at a normal rate, but they produce excessive [5-15N]glutamine. Thus, whole-body nitrogen metabolism is abnormal even in this group [33].
Animal models for OTC deficiency have been developed. These include the sparse fur (spf) mouse and the sparse fur—abnormal skin and hair (spf-ash) mouse. In the former model, a histidine residue replaces an asparagine at position 117 of the gene, resulting in an enzymatic activity that is 15% of control. The spf-ash mutation entails a base change in exon 4, resulting in a splicing mutation and a reduction of OTC activity to 5% of normal. Both kinds of mutant mice manifest hyperammonemia, orotic aciduria, growth failure and sparse fur.
OTC deficiency must be suspected in any patient, male or female, with unexplained neurological symptoms. The absence of hyperammonemia in a casual sample should not rule out the diagnosis, especially if the history is positive for protein intolerance or an untoward reaction to infections. Family history also may be suggestive. Blood amino acids and urinary orotic acid should be quantitated in such individuals.
, reaction 3) usually die, and most survivors suffer major brain injury. Patients with a partial deficiency may have a milder course, and a few individuals with citrullinemia have been phenotypically normal.
The diagnosis usually is apparent from the hyperammonemia and the extreme hypercitrullinemia. The activity of AS can be determined in both fibroblasts and chorionic villus samples, thus simplifying the problem of antenatal diagnosis.
Patients with arginosuccinic aciduria excrete an enormous amount of argininosuccinate in their urine. The CSF also contains this polar molecule in high concentration. Neonates have a stormy clinical course, and almost all die or sustain severe brain injury. A peculiar finding in many cases is trichorrhexis nodosa, or dry brittle hair with nodular protrusions, which are best visible with light microscopy. The precise cause is unknown.
Most patients are thought to have psychomotor retardation during the first year of life. Seizures and growth failure may occur, although some patients are of normal size. The motor dysfunction usually comes to clinical attention by age 2 to 3 years. Leukodystrophic changes are seen. Blood NH3 is elevated less than in neonatal-onset disorders. The plasma arginine concentration usually is two to five times normal. Urine orotic acid excretion is extremely high, perhaps because arginine stimulates flux through the CPS reaction by favoring the synthesis of NAG.
Electron microscopy of the liver has shown irregularities of mitochondrial shape. This results in a failure of citrulline synthesis and a consequent hyperammonemia. Urinary orotic acid is high, presumably because of underutilization of carbamyl phosphate. In contrast, excretion of creatine is low, reflecting the inhibition of glycine transamidinase by excessive concentrations of ornithine.
These conditions may result in growth failure and varying degrees of mental retardation. Sometimes symptoms are deferred until adulthood. Vomiting, lethargy and hypotonia are noted after protein ingestion. Recurrent hospitalizations for hyperammonemia are the rule. Some patients have manifested a bleeding diathesis and hepatomegaly.
The clinical course in neonates usually is not severe. After weaning or upon exposure to foods high in protein, the infants manifest growth failure, hepatomegaly, splenomegaly, vomiting, hypotonia, recurrent lethargy, coma, abdominal pain and, in rare instances, psychosis. Rarefaction of the bones is common, and both fractures and vertebral compression have been reported. Most patients are not mentally retarded, although this may occur. Some patients have died with interstitial pneumonia, which may respond to corticosteroid therapy.
The dibasic aminoaciduria reflects a failure of reabsorption of lysine, ornithine and arginine by the proximal tubule. There also is a failure to absorb these compounds by the intestinal mucosa. The transport defect occurs at the basolateral, rather than the luminal, membrane. Hyperammonemia reflects a deficiency of intramitochondrial ornithine. An effective treatment is oral citrulline supplementation, which corrects the hyperammonemia by allowing replenishment of the mitochondrial pool of ornithine.
In patients with very severe disease, tolerance for dietary protein may be so limited that it is not possible to support normal growth.
Treatment with sodium benzoate and sodium phenylacetate represents an important advance in the management of urea cycle defects. Benzoyl-CoA reacts rapidly in the liver with glycine to form hippurate, and phenylacetyl-CoA reacts with glutamine to yield phenylacetylglutamine. Thus, waste nitrogen is eliminated from the body not as urea but as amino acid conjugates of benzoate and phenylacetate [34–36]. Excretion of ammonia as phenylacetylglutamine is more efficient than excretion as hippurate because 2 mol of ammonia are excreted with each mole of phenylacetylglutamine. The clinical utility of phenylacetate is limited by its objectionable odor. Sodium phenylbutyrate, which is less malodorous and is converted in the liver to phenylacetate, has been used with success in place of phenylacetate. Acylation therapy has greatly improved the survival and morbidity for selected patients. Thus, the outlook is favorable for heterozygote girls with OTC deficiency treated from an early age [36].
Most patients who survive the neonatal period can be successfully maintained with a diet low in protein and treatment with sodium benzoate. A useful adjunct to treatment in cases of citrullinemia and argininosuccinic aciduria is supplementation of the diet with arginine, which enhances the ability to eliminate ammonia as either citrulline or argininosuccinate. In addition, maintenance of arginine concentrations in the normal range facilitates protein synthesis.
Liver transplantation has been utilized in children with urea cycle defects. The long-term utility is still uncertain. It appears to afford good metabolic correction, although some abnormalities of amino acid metabolism persist even after transplantation. The high morbidity of organ transplantation restricts the utility of this approach.
Dialysis, including hemodialysis and peritoneal dialysis, relieves acute toxicity during fulminant hyperammonemia. Exchange transfusions also have been performed, but this technique has not been equally useful in removing ammonia.
The possibility of gene therapy for these disorders has been a subject of intense scrutiny [37]. An adenoviral vector containing a cDNA for the OTC gene has been given to mice with a congenital deficiency of OTC. The result was complete correction of hepatic OTC activity over a 2-month period. Transient correction of serum glutamine and urine orotic acid was reported. This experimental approach holds enormous promise for the management of this enzymopathy and other inborn errors of intermediary metabolism.
). Biotin also is a cofactor for the pyruvate carboxylase reaction in the gluconeogenic pathway and for acetyl-CoA carboxylase in the pathway of fatty acid synthesis. Hence, dietary deficiencies of biotin or congenital anomalies of biotin metabolism lead to the accumulation of several organic acids (Fig. 44-1, reactions 5 and 11).
Biotin is covalently bound to these enzymes via an amide linkage with ϵ-NH2 groups of lysine residues. A specific enzyme, holocarboxylase synthetase, mediates this attachment. Another enzyme, biotinidase, cleaves biotinyl residues from enzymes, thereby facilitating the recycling of free biotin. Inherited defects of both biotinidase and holocarboxylase synthetase have been described. Prompt clinical recognition of these syndromes is essential because treatment with pharmacological doses of biotin dramatically improves outcome.
Most patients become symptomatic early in life. The blood pH is typically quite low, often less than 7, and the blood lactate is high. Many infants also have hyperammonemia. Quantitation of urinary organic acids typically shows a marked ketoaciduria with excretion of lactate, 3-methylcrotonylglycine, tiglylglycine, 3-hydroxypropionate, methylcitrate and 3-hydroxyisovalerate, inter alia. If the disorder is not treated promptly, patients can develop a skin rash, alopecia and varying degrees of psychomotor retardation. Direct assay of holocarboxylase synthetase in fibroblasts is possible. Antenatal diagnosis is feasible, either by determination of enzyme activity or by quantitation of organic acids in the amniotic fluid.
Hearing loss is common. Quantitation of the urinary organic acids shows increased excretion of lactate, 3-hydroxyisovalerate, methylcitrate and 3-hydroxypropionate; however, these are not invariant findings, and the measurement of biotinidase activity in fibroblasts or peripheral blood cells may be necessary. Biotinidase activity in the serum of affected children usually is <10% that of control values. Antenatal diagnosis is possible.
Pathological lesions in the brain include cystic changes and demyelination. The cerebellum is especially vulnerable. A few patients have manifested changes suggesting meningoencephalitis. Virtually all patients respond favorably to oral biotin at a dose of 10 to 40 mg daily. Many of the clinical findings are reversible, even including some of the neurological abnormalities, although the hearing loss tends to persist.
).
) and the glutamate derived from reaction 5 are converted to γ-glutamylcysteine via γ-glutamylcysteine synthetase (Fig. 44-7, reaction 1). The most important congenital defects in glutathione metabolism are glutathione synthetase deficiency (Fig. 44-7, reaction 2), γ-glutamylcysteine deficiency (Fig. 44-7, reaction 1), γ-glutamyltranspeptidase deficiency (Fig. 44-7, reaction 3) and 5-oxoprolinase deficiency (Fig. 44-7, reaction 5).
, reaction 1), thereby augmenting the concentration of γ-glutamylcysteine and the subsequent conversion of this dipeptide to cysteine and 5-oxoproline in the cyclotransferase pathway (Fig. 44-7, reaction 4).
This lesion has been diagnosed in a young adult with mental retardation, severe metabolic acidosis and evidence of a spastic quadriparesis and cerebellar disease. It also has been observed in patients who were first diagnosed in infancy, and who enjoyed a period of normal psychomotor development until late childhood, when a progressive loss of intellectual function became appreciated. Patients also may manifest a mild hemolysis. Pathological changes in the brain have included atrophy of the cerebellum and lesions in the cortex and thalamus. There is no specific therapy.
Patients with 5-oxoprolinase deficiency excrete increased amounts of oxoproline and have a somewhat elevated concentration. They do not have significant neurological problems.
Patients with this very rare disorder have displayed spinocerebellar degeneration, peripheral neuropathy, myopathy and an aminoaciduria secondary to renal tubular dysfunction. Psychosis and a hemolytic anemia have been features in some patients.
These patients also have shown varying degrees of mental retardation. The precise relationship of the neurological signs to the biochemical lesion is problematic. The enzyme is present in the brain, primarily in the capillaries, where it may facilitate amino acid transport. No specific treatment is available.
GABA is formed via the action of glutamate decarboxylase (see Chap. 16). The metabolism of this neurotransmitter is mediated first by uptake into neurons and glia and second by transamination to succinic semialdehyde via GABA transaminase (GABA-T). The semialdehyde is oxidized to succinate via succinic semialdehyde dehydrogenase.
Patients respond dramatically to the parenteral administration of pyridoxine at a dose of 10 to 100 mg with a cessation of convulsions and a marked amelioration of the EEG. Speculation has centered on the possibility that the disease involves faulty binding of pyridoxine, a cofactor in the glutamate decarboxylase reaction, to the enzyme protein.
Patients with this very rare disorder have severe psychomotor retardation and hyperreflexia. Concentrations in the CSF and blood of GABA and β-alanine are much greater than normal, as is the concentration of homocarnosine in the CSF. GABA-T activity is much diminished in blood lymphocytes and in the liver. A curious finding is increased stature, perhaps reflecting the ability of GABA to evoke release of growth hormone.
Affected patients have mental retardation, cerebellar disease and hypotonia. They excrete large amounts of both succinic semialdehyde and 4-hydroxybutyric acid. There is no known therapy.
Infants with Canavan's disease seem normal at birth, but a delay in development usually is apparent by 3 months of age. An increased head circumference that is greater than the 98th percentile is common, and hydrocephalus sometimes is suspected. Neurological function deteriorates rapidly over the next several months. Optic atrophy ultimately leads to blindness. These infants manifest minimal interest in their environment. Spasticity is frequent and seizures may occur. Imaging of the brain shows demyelination and brain atrophy with enlargment of the ventricles and widening of the sulci. Pathological examination shows swelling of the astrocytes with elongation of the mitochondria. Vacuoles appear in the myelin.
Excretion of N-acetylaspartate is grossly elevated, and the concentration of this amino acid in the CSF may be 50 times control values. The cause is a deficiency of aspartoacylase, which mediates the formation of aspartate and acetyl-CoA from N-acetylaspartate. Aspartoacylase normally is found primarily in the white matter, but N-acetylaspartate is most abundant in the gray matter. The defect is expressed in cultured skin fibroblasts. N-acetylaspartate is among the most abundant amino acids in the brain, although its precise function remains elusive. Putative roles have included osmoregulation and the storage of acetyl groups that subsequently are utilized for myelin synthesis. The relationship of the enzyme defect to the clinical findings remains problematic. No specific therapy is yet available.