Defects in Metabolism of Amino Acids

Clinical Manifestations

The affected infant appears normal at birth. Profound intellectual disability develops gradually if the infant remains untreated. Cognitive delay may not be evident for the 1st few months. In untreated patients, 50–70% will have an IQ below 35, and 88–90% will have an IQ below 65. Only 2–5% of untreated patients may have normal intelligence. Vomiting, sometimes severe enough to be misdiagnosed as pyloric stenosis, may be an early symptom. Older untreated children become hyperactive with autistic behaviors, including purposeless hand movements, rhythmic rocking, and athetosis.

Untreated and undertreated infants are lighter in their complexion than unaffected siblings. Some may have a seborrheic or eczematoid rash, which is usually mild and disappears as the child grows older. These children have an odor of phenylacetic acid, which has been described as musty or “mousey.” Neurologic signs include seizures (approximately 25%), spasticity, hyperreflexia, and tremors; >50% have electroencephalographic (EEG) abnormalities. Microcephaly, prominent maxillae with widely spaced teeth, enamel hypoplasia, and growth retardation are other common findings in untreated children. Low bone mineral density and osteopenia have been reported in affected individuals of all ages. Although inadequate intake of natural proteins seems to be the major culprit, the exact pathogenesis of this sequela remains unclear. Long-term care of patients with PKU is best achieved by a team of experienced professionals (metabolic specialist, nutritionist, and psychologist) in a regional treatment center. The clinical manifestations of classic PKU are rarely seen in countries where neonatal screening programs for the detection of PKU are in effect.

Diagnosis

Because of the gradual and nonspecific nature of early clinical symptoms such as vomiting, developmental delay, or eczematoid rash, hyperphenylalaninemia is usually diagnosed through newborn screening in all developed countries. In infants with positive screening results, diagnosis should be confirmed by quantitative measurement of plasma phenylalanine concentration. Identification and measurement of phenylketones in the urine has no place in any screening program. In countries and places where such programs are not in effect, identification of phenylketones in the urine by ferric chloride may offer a simple test for diagnosis of infants with developmental and neurologic abnormalities. Once the diagnosis of hyperphenylalaninemia is established, additional studies for BH ₄ metabolism should be performed to rule out BH ₄ deficiency as the cause of hyperphenylalaninemia.

Treatment

The mainstay of treatment of PKU is a low-phenylalanine diet . The general consensus is to start diet treatment immediately in patients with blood phenylalanine levels >10 mg/dL (600 µmol/L). It is generally accepted that infants with persistent (more than a few days) plasma levels of phenylalanine ≥6 mg/dL (360 µmol/L) should also be treated with a phenylalanine-restricted diet similar to that in classic PKU. The goal of therapy is to reduce phenylalanine levels in the plasma and brain. Formulas free of, or low in, phenylalanine are commercially available. The diet should be started as soon as the diagnosis is established. Because phenylalanine is not synthesized endogenously, the diet should provide phenylalanine to prevent phenylalanine deficiency. Dietary phenylalanine tolerance is determined based on age and severity of the PAH deficiency. Phenylalanine deficiency is manifested by lethargy, failure to thrive, anorexia, anemia, rashes, diarrhea, and even death. Further, tyrosine becomes an essential amino acid in this disorder, and its adequate intake must be ensured. Special food items low in phenylalanine are commercially available for dietary treatment of affected children and adults.

There is no firm consensus concerning optimal levels of blood phenylalanine in affected patients either across different countries or among treatment centers in the United States. The current recommendation is to maintain blood phenylalanine levels between 2 and 6 mg/dL (120-360 µmol/L) throughout life. Discontinuation of therapy, even in adulthood, may cause deterioration of IQ and cognitive performance. Lifelong adherence to a low-phenylalanine diet is extremely difficult. Patients who maintain good control as children but discontinue the phenylalanine-restricted diet as teenagers or adults may experience significant difficulties with executive function, concentration, emotional liability, and depression. Executive dysfunction may also occur in early-treated children despite diet treatment.

Given the difficulty of maintaining a strict low-phenylalanine diet, there are continuing attempts to find other modalities for treatment of these patients. Administration of large neutral amino acids (LNAAs) is another approach to dietary therapy. LNAAs (tyrosine, tryptophan, leucine, isoleucine, valine, methionine, histidine, and phenylalanine) share the same transporter protein (LNAA type 1 or LAT-1) for transit through the intestinal cell membrane and blood-brain barrier (BBB). Binding of LNAAs to the transporter protein is a competitive process. The rationale for use of LNAA is that these molecules compete with phenylalanine for transport across the BBB; therefore, large concentrations of other LNAAs in the intestinal lumen and in the blood reduce the uptake of phenylalanine into bloodstream and the brain. Large, controlled clinical trials are necessary to establish the efficacy of this treatment.

Oral administration of BH ₄ , the cofactor for PAH, may result in reduction of plasma levels of phenylalanine in some patients with PAH deficiency. Plasma levels of phenylalanine in these patients may decrease enough to allow for considerable modification of their dietary restriction. In very rare cases the diet may be discontinued because the phenylalanine levels remain under 6 mg/dL (360 µmol/L). The response to BH ₄ cannot be predicted consistently based on the genotype alone, especially in compound heterozygous patients. Sapropterin dihydrochloride , a synthetic form of BH ₄ , which acts as a cofactor in patients with residual PAH activity, is approved by the U.S. Food and Drug Administration (FDA) to reduce phenylalanine levels in PKU. A sustained decrease of plasma phenylalanine by at least 30% is consistent with sapropterin responsiveness. Injectable PEGylated recombinant phenylalanine ammonia lyase is in development.

Pregnancy in Women With PAH Deficiency (Maternal Phenylketonuria)

Pregnant women with PAH deficiency who are not on a phenylalanine-restricted diet have a very high risk of having offspring with intellectual disability, microcephaly, growth retardation, congenital malformations, and congenital heart disease. These complications are directly correlated with elevated maternal blood phenylalanine levels during pregnancy. Prospective mothers who have been treated for PAH deficiency should be maintained on a phenylalanine-restricted diet before and during pregnancy. The best observed outcomes occur when strict control of maternal blood phenylalanine concentration is instituted before pregnancy. Plasma phenylalanine levels >6 mg/dL (360 µmol/L) after conception are associated with increased incidence of intrauterine growth restriction and congenital malformations, as well as lower IQ. However, there is strong evidence that phenylalanine control instituted after conception results in improved outcomes. The currently recommended phenylalanine concentration is 2-6 mg/dL (120-360 µmol/L) throughout the pregnancy, although some expert groups advocate plasma phenylalanine levels <4 mg/dL (<240 µmol/L). All women with PAH deficiency who are of childbearing age should be counseled properly regarding the risk of congenital anomalies in their offspring.

Clinical Manifestations

Infants with cofactor BH ₄ deficiency are identified during screening programs for PKU because of evidence of hyperphenylalaninemia. Plasma phenylalanine levels may be as high as those in classic PKU or may be in the milder range. However, the clinical manifestations of the neurotransmitter disorders differ greatly from those of PKU. Neurologic symptoms of the neurotransmitter disorders often manifest in the 1st few months of life and include extrapyramidal signs (choreoathetotic or dystonic limb movements, axial and truncal hypotonia, hypokinesia), feeding difficulties, and autonomic abnormalities. Intellectual disability, seizures, hypersalivation, and swallowing difficulties are also seen. The symptoms are usually progressive and often have a marked diurnal fluctuation. Prognosis and outcome strongly depend on the age at diagnosis and at introduction of treatment, but also on the specific nature of the pathogenic variant and resulting enzyme defect.

Diagnosis

Despite the low incidence of BH ₄ synthesis defects, all newborns with hyperphenylalaninemia detected through newborn screening must be screened for BH ₄ synthesis defects. BH ₄ deficiency and the responsible enzyme defect may be diagnosed by several studies.

Treatment

The goals of therapy are to correct hyperphenylalaninemia and to restore neurotransmitter deficiencies in the CNS. The control of hyperphenylalaninemia is important in patients with cofactor deficiency, because high levels of phenylalanine cause intellectual disability and interfere with the transport of neurotransmitter precursors (tyrosine and tryptophan) into the brain. Plasma phenylalanine should be maintained as close to normal as possible (<6 mg/dL or <360 µmol/L). This can be achieved by oral supplementation of BH ₄ (5-20 mg/kg/day). Sapropterin dihydrochloride, the synthetic form of BH _4, is commercially available but expensive.

Lifelong supplementation with neurotransmitter precursors such as l -dopa and 5-hydroxytryptophan, along with carbidopa to inhibit degradation of l -dopa before it enters the CNS, is necessary in most of these patients even when treatment with BH ₄ normalizes plasma levels of phenylalanine. BH ₄ does not readily enter the brain to restore neurotransmitter production. To minimize untoward side effects (especially l -dopa–induced dyskinesia), the treatment should be started with low doses of l -dopa/carbidopa and 5-hydroxytryptophan and should be gradually adjusted based on response to therapy and clinical improvement for each individual patient. Supplementation with folinic acid is also recommended in patients with dihydropteridine reductase deficiency. Unfortunately, attempting to normalize neurotransmitter levels using neurotransmitter precursors usually does not fully resolve the neurologic symptoms, because of the inability to attain normal levels of BH ₄ in the brain. Patients often demonstrate intellectual disability, fluctuating abnormalities of tone, eye movement abnormalities, poor balance and coordination, decreased ability to ambulate, and seizures despite supplementation with neurotransmitter precursors.

Hyperprolactinemia occurs in patients with BH ₄ deficiency and may be the result of hypothalamic dopamine deficiency. Measurement of serum prolactin levels may be a convenient method for monitoring adequacy of neurotransmitter replacement in affected patients.

Some drugs, such as trimethoprim/sulfamethoxazole, methotrexate, and other antileukemic agents, are known to inhibit dihydropteridine reductase enzyme activity and should be used with great caution in patients with BH ₄ deficiency.

Genetics and Prevalence

All defects causing hyperphenylalaninemia are inherited as autosomal recessive traits. The prevalence of PKU in the United States is estimated at 1 in 14,000 to 1 in 20,000 live births. The prevalence of non-PKU hyperphenylalaninemia is estimated at 1 in 50,000 live births. The condition is more common in whites and Native Americans and less prevalent in blacks, Hispanics, and Asians.

The gene for PAH is located on chromosome 12q23.2, and many disease-causing mutations have been identified in different families. Most patients are compound heterozygotes for 2 different mutant alleles. The gene for 6-pyruvoyl-tetrahydropterin synthase (PTS) , the most common cause of BH ₄ deficiency, resides on chromosome 11q23.1, the gene for dihydropteridine reductase (QDPR) is located on chromosome 4p15.32, and those of pterin-4-α-carbinolamine dehydratase (PCBD1) and GTP cyclohydrolase I (GCH1) are on 10q22.1 and 14q22.2, respectively. Prenatal diagnosis is possible if causative mutations are known.

Clinical Manifestations and Natural History

Without treatment, affected infants appear normal at birth and develop symptoms in the 1st yr of life. Most patients present between 2 and 6 mo of age but rarely may become symptomatic in the 1st mo or appear healthy beyond the 1st yr of life. Earlier presentation confers poorer prognosis. The 1-yr mortality of untreated children, which is approximately 60% in infants developing symptoms before 2 mo of age, decreases to 4% in infants who become symptomatic after 6 mo.

An acute hepatic crisis typically heralds the onset of the disease and is usually precipitated by an intercurrent illness that produces a catabolic state. Fever, irritability, vomiting, hemorrhage, hepatomegaly, jaundice, elevated levels of serum transaminases, hypoglycemia, and neuropathy are common. An odor resembling boiled cabbage may be present, resulting from increased methionine metabolites. Hepatic crises may progress to liver failure and death. Between the crises, varying degrees of failure to thrive, hepatomegaly, and coagulation abnormalities often persist. Cirrhosis and eventually hepatocellular carcinoma occur with increasing age.

Episodes of acute peripheral neuropathy resembling acute porphyria occur in approximately 40% of affected children. These crises, often triggered by a minor infection, are characterized by severe pain, often in the legs, associated with extensor hypertonia of the neck and trunk, vomiting, paralytic ileus, and occasionally self-induced injuries of the tongue or buccal mucosa. Marked weakness occurs in about 30% of episodes, which may lead to respiratory failure requiring mechanical ventilation. Crises typically last 1-7 days, but recuperation from paralytic crises can require weeks to months.

Renal involvement is manifested as a Fanconi-like syndrome with hyperphosphaturia, hypophosphatemia, normal–anion gap metabolic acidosis, and vitamin D–resistant rickets. Nephromegaly and nephrocalcinosis may be present on ultrasound examination. Glomerular failure may occur in adolescents and older patients.

Hypertrophic cardiomyopathy and hyperinsulinism are seen in some infants.

Laboratory Findings

Elevated levels of succinylacetone in serum and urine are diagnostic for tyrosinemia type I (see Fig. 103.1 ). Succinylacetone levels may fall below the diagnostic threshold in patients treated with nitisinone. In untreated patients, the blood level of α-fetoprotein is increased, often greatly, and liver-synthesized coagulation factors are decreased in most patients. Increased levels of α-fetoprotein are present in the cord blood of affected infants, indicating intrauterine liver damage. Serum transaminase levels are often increased, with marked increases possible during acute hepatic episodes. Serum concentration of bilirubin is usually normal but can be increased with liver failure. Plasma tyrosine levels are usually elevated at diagnosis, but this is a nonspecific finding and depends on dietary intake. Plasma levels of other amino acids, particularly methionine, may also be elevated in patients with liver damage. Hyperphosphaturia, hypophosphatemia, and generalized aminoaciduria may occur. The urinary level of 5-aminolevulinic acid (also known as delta aminolevulinic acid) is elevated because of inhibition of 5-aminolevulinate dehydratase by succinylacetone (see Fig. 110.1 ).

Diagnosis is usually established by demonstration of elevated levels of succinylacetone in urine or blood. Neonatal screening for hypertyrosinemia using tyrosine alone detects only a fraction of patients with tyrosinemia type I. Succinylacetone, which is assayed by many neonatal screening programs, has higher sensitivity and specificity than tyrosine and is the preferred metabolite for screening. Tyrosinemia type I should be differentiated from other causes of hepatitis and hepatic failure in infants, including galactosemia, hereditary fructose intolerance, neonatal iron storage disease, giant cell hepatitis, and citrullinemia type II (see Chapter 103.12 ).

Treatment and Outcome

A diet low in phenylalanine and tyrosine can slow but does not halt the progression of the condition. The treatment of choice is nitisinone (NTBC), which inhibits 4-HPPD and reduces the flux of tyrosine metabolites to FAH, thus decreasing the production of the offending compounds, fumarylacetoacetate and succinylacetone (see Fig. 103.1 ). The dose of nitisinone is titrated to the lowest, most effective dose (usually targeting the blood range of 20-40 µmol/L) to suppress production of succinylacetone while maintaining plasma tyrosine level <400 µmol/L (7.2 mg/dL). This treatment prevents acute hepatic and neurologic crises. Although nitisinone greatly slows disease progression, some pretreatment liver damage is not reversible. Therefore, patients must be followed for development of cirrhosis or hepatocellular carcinoma . On imaging, the presence of even a single liver nodule usually indicates underlying cirrhosis. Most liver nodules in tyrosinemic patients are benign, but current imaging techniques do not accurately distinguish all malignant nodules. For patients with severe liver failure not responding to nitisinone, liver transplantation is an effective therapy, which can also alleviate the risk of hepatocellular carcinoma. The impact of nitisinone treatment on the need for liver transplantation is still under study, but the greatest effect is in patients treated early, such as children detected by neonatal screening, prior to developing clinical symptoms. In early-treated patients, nitisinone has greatly reduced the need for liver transplantation. Because nitisinone treatment increases plasma tyrosine, a low-tyrosine, low-phenylalanine diet is recommended. Rarely, nitisinone-treated patients develop corneal crystals, presumably of tyrosine, which are reversible by strict dietary compliance. This finding, combined with observations of developmental delay in some patients with tyrosinemia type II who chronically have elevated tyrosine levels, suggest that a diet low in phenylalanine and tyrosine should be continued in patients treated with nitisinone. The dietary treatment of patients with tyrosine and phenylalanine restriction necessitates surveillance of growth and development by ensuring adequate intakes of amino acids and other nutrients.

Genetics and Prevalence

Tyrosinemia type I is inherited as an autosomal recessive trait. The FAH gene maps to chromosome 15q25.1. DNA analysis is useful for molecular prenatal diagnosis if the familial pathogenic variants are known and for carrier testing in groups at risk for specific mutations, such as French-Canadians from the Saguenay-Lac Saint-Jean region of Quebec. The prevalence of the condition is estimated to be 1 in 1,846 live births in the Saguenay-Lac Saint-Jean region and approximately 1 in 100,000 live births worldwide. Prenatal screening can be performed by measurement of succinylacetone in amniotic fluid. Prenatal diagnosis is possible using DNA analysis of amniocytes or of chorionic villi, if the familial pathogenic variants are known.

Oculocutaneous (Generalized) Albinism

Lack of pigment is generalized, affecting skin, hair, and eyes. At least 4 genetically distinct forms of oculocutaneous albinism ( OCA ) have been identified: OCA ₁ , OCA ₂ , OCA ₃ , and OCA ₄ . The lack of pigment is complete in patients with OCA ₁ A; the other types may not be clinically distinguishable from one another. All affected individuals have ocular manifestations of albinism. All forms are inherited as autosomal recessive traits.

Ocular Albinism

Patients with ocular albinism ( OA ) present in the 1st months of life with nystagmus, hypopigmentation of iris and fundus, foveal hypoplasia, and decreased visual acuity. Electron microscopy demonstrates characteristic macromelanosomes in skin biopsies or hair root specimens. Most patients affected by ocular albinism have ocular albinism type 1 ( OA ₁ ), an X-linked disorder caused by pathogenic variants in the GPR143 gene. A rare form of OA with late-onset sensorineural deafness and apparent autosomal dominant inheritance has also been reported.

Syndromic Forms of Generalized Albinism

Localized Albinism

Localized albinism refers to localized patches of hypopigmentation of skin and hair, which may be evident at birth or develop with time. These conditions are caused by abnormal migration of melanocytes during embryonic development.

Homocystinuria Caused by Cystathionine β-Synthase Deficiency (Classic Homocystinuria)

This is the most common inborn error of methionine metabolism. Approximately 40% of affected patients respond to high doses of vitamin B ₆ and usually have milder clinical manifestations than those who are unresponsive to vitamin B ₆ therapy. These patients possess some residual enzyme activity.

Infants with classic homocystinuria appear normal at birth. Clinical manifestations during infancy are nonspecific and may include failure to thrive and developmental delay. Without newborn screening, the diagnosis can be delayed and is usually made after 3 yr of age, when subluxation of the ocular lens ( ectopia lentis ) occurs. This causes severe myopia and iridodonesis (quivering of the iris). Astigmatism, glaucoma, staphyloma, cataracts, retinal detachment, and optic atrophy may develop later in life. Progressive intellectual disability is common. Normal intelligence has been reported. In an international survey of >600 patients, IQ scores ranged from 10-135. Higher IQ scores are seen in vitamin B ₆ –responsive patients. Psychiatric and behavioral disorders have been observed in more than 50% of affected patients. Seizures are seen in approximately 20% of untreated patients. Affected individuals with homocystinuria manifest skeletal abnormalities resembling those of Marfan syndrome (see Chapter 722 ): tall with elongated limbs and arachnodactyly. Scoliosis, pectus excavatum or pectus carinatum, genu valgum, pes cavus, high-arched palate, and crowding of the teeth are typically seen. These children usually have fair complexions, blue eyes, and a peculiar malar flush. Generalized osteoporosis, especially of the spine, is the main x-ray finding. Thromboembolic episodes involving both large and small vessels, especially those of the brain, are common and may occur at any age. Optic atrophy, paralysis, cor pulmonale, and severe hypertension (from renal infarcts) are among the serious consequences of thromboembolism, which is likely caused by elevated homocysteine levels leading to abnormal angiogenesis and inhibition of fibrinolytic activity. The risk of thromboembolism increases after surgical procedures. Spontaneous pneumothorax and acute pancreatitis are rare complications.

Elevations of both methionine and homocystine (or homocysteine) in body fluids are the diagnostic laboratory findings . Freshly voided urine should be tested for homocystine because this compound is unstable and may disappear after prolonged storage. Cysteine is low or absent in plasma. Total plasma homocysteine is the preferred analyte for management of classic homocystinuria. Free plasma homocysteine may normalize or remain normal when total plasma homocysteine is lowered. The diagnosis may be established by molecular analysis of cystathionine β-synthase (CBS) or by assay of the enzyme in cultured fibroblasts, phytohemagglutinin-stimulated lymphocytes, or liver biopsy specimens.

Treatment with high doses of vitamin B ₆ (100-500 mg/24 hr) causes dramatic improvement in patients who are responsive to this therapy. The degree of response to vitamin B ₆ treatment may vary across families. Some patients may not respond because of folate depletion; a patient should not be considered unresponsive to vitamin B ₆ until folic acid (1-5 mg/24 hr) has been added to the treatment regimen. For patients who are unresponsive to vitamin B ₆ , restriction of methionine intake in conjunction with cysteine supplementation is also recommended. The need for dietary restriction and its extent remains controversial in patients with vitamin B ₆ –responsive form. In some patients with this form, addition of betaine may obviate the need for any dietary restriction. Betaine (trimethylglycine, 6 g/24 hr for adults or 200-250 mg/kg/day for children) lowers homocysteine levels in body fluids by remethylating homocysteine to methionine (see Fig. 103.3 ), which may result in elevation of plasma methionine levels. This treatment has produced clinical improvement (preventing vascular events) in patients who are unresponsive to vitamin B ₆ therapy. Cerebral edema has occurred in a patient with vitamin B ₆ –nonresponsive homocystinuria and dietary noncompliance during betaine therapy.

More than 100 pregnancies in women with classic homocystinuria have been reported with favorable outcomes for both mothers and infants. The majority of infants were full-term and normal. Postpartum thromboembolic events occurred in a few mothers.

The screening of newborn infants for classic homocystinuria has been performed worldwide, with an estimated prevalence of 1 in 200,000 to 1 in 350,000 live births, although it can be more common in some parts of the world (e.g., 1 : 1,800 in Qatar). Early treatment of patients identified by screening has produced favorable results. The mean IQ of patients with vitamin B ₆ –unresponsive form treated in early infancy was in the normal range. Dislocation of the lens seemed to be prevented in some patients.

Classic homocystinuria is inherited as an autosomal recessive trait. The gene for cystathionine β-synthase (CBS) is located on chromosome 21q22.3. Prenatal diagnosis is feasible by DNA analysis or by performing an enzyme assay of cultured amniotic cells. Most affected patients are compound heterozygotes for 2 different alleles. Heterozygous carriers are asymptomatic.

Homocystinuria Caused by Defects in Methylcobalamin Formation

Methylcobalamin is the cofactor for the enzyme methionine synthase, which catalyzes remethylation of homocysteine to methionine. At least 7 distinct defects in the intracellular metabolism of cobalamin may interfere with the formation of methylcobalamin. (To better understand the metabolism of cobalamin, see Methylmalonic Acidemia in Chapter 103.6 and Figs. 103.3 and 103.4 .) The 7 defects are designated as cbl C, cbl D (including cbl D variant 1), cbl E (methionine synthase reductase), cbl G (methionine synthase), cbl F, cbl J, and cbl X. Patients with cbl C, cbl D, cbl F, cbl J, and cbl X defects have methylmalonic acidemia in addition to homocystinuria, because the formation of both adenosylcobalamin and methylcobalamin is impaired.

Patients with cbl E, cbl G, and cbl D variant 1 defects are unable to form methylcobalamin and develop homocystinuria without methylmalonic acidemia ( Fig. 103.4 ). The clinical manifestations are similar in patients with these 3 defects. Nonspecific symptoms such as vomiting, poor feeding, failure to thrive, lethargy, hypotonia, seizures, and developmental delay may occur in the 1st few months of life. Late-onset forms of these disorders may present with neurocognitive defects, psychosis, and peripheral neuropathy. Laboratory findings include megaloblastic anemia, hyperhomocysteinemia, homocystinuria, and hypomethioninemia. The absence of hypermethioninemia differentiates these conditions from cystathionine β-synthase deficiency (see earlier). Renal artery thrombosis, hemolytic uremic syndrome, pulmonary hypertension, and optic nerve atrophy have been reported in some patients with these defects.

Diagnosis is established by DNA testing or by complementation studies performed in cultured fibroblasts. Prenatal diagnosis has been accomplished by studies in amniotic cell cultures. cbl E, cbl G, and cbl D variant 1 deficiencies are inherited as autosomal recessive traits. The gene for cbl E is MTRR , encoding methionine synthase reductase (located on chromosome 5p15.31). The gene for cbl G is MTR , encoding methionine synthase (located on chromosome 1q43). The cbl D variant 1 deficiency is caused by pathogenic variants affecting the C-terminal of the MMADHC gene (located on chromosome 2q23.2).

Treatment with vitamin B ₁₂ in the form of high-dose hydroxycobalamin helps improve the clinical and biochemical findings. Results vary among both diseases and sibships.

Homocystinuria Caused by Deficiency of Methylenetetrahydrofolate Reductase (MTHFR Deficiency)

This enzyme reduces 5,10-methylenetetrahydrofolate to form 5-methyltetrahydrofolate, which provides the methyl group needed for remethylation of homocysteine to methionine (see Fig. 103.3 ). The severity of the enzyme defect and the clinical manifestations vary considerably in different families. Clinical findings vary from apnea, seizure, microcephaly, coma, and death to developmental delay, ataxia, motor abnormalities, peripheral neuropathy, and psychiatric manifestations. Thromboembolism has also been observed. Exposure to the anesthetic nitrous oxide (which inhibits methionine synthase) in patients with MTHFR deficiency may result in neurologic deterioration and death.

Laboratory findings include moderate homocystinemia and homocystinuria. The methionine concentration is low or low-normal. This finding helps differentiate this condition from classic homocystinuria caused by cystathionine β-synthase deficiency. The diagnosis may be confirmed by molecular analysis of MTHFR or by the enzyme assay in cultured fibroblasts or leukocytes.

MTHFR deficiency should be differentiated from mild hyperhomocysteinemia due to two common polymorphisms in the MTHFR gene. Two “thermolabile” polymorphisms have been extensively studied, c.665C>T (p.Ala222Val, previously referred to as c.677C>T) and c.1286A>C (p.Glu429Ala, formerly referred to as c.1298A>C). These polymorphisms may minimally affect levels of plasma total homocysteine in some patients and are often confounded by dietary folate deficiency. Both polymorphisms have been studied as possible risk factors for a wide variety of medical conditions, including birth defects, autism, vascular disease, stroke, pregnancy loss, cancer, and response to chemotherapy. Population-based studies revealed a surprisingly high prevalence of homozygosity for these polymorphisms in the general population: up to 10–15% of the North American Caucasians and >25% in some Hispanics. It is hypothesized that fortification of flour with folate may have decreased the strength of associations observed in the past. To date, the best data support a role for the c.665C>T polymorphism (formerly c. 677C>T) as a risk factor for neural tube defects. Although a clinical test for this polymorphism is widely available, recent meta-analyses have not supported the association between the MTHFR polymorphism and risk for venous thromboembolism or between mild hyperhomocysteinemia and an increased risk for coronary heart disease.

The condition is inherited as an autosomal recessive trait. The diagnosis can be confirmed by MTHFR gene analysis. Prenatal diagnosis can be achieved by molecular analysis of MTHFR of the known familial pathogenic variants or by measuring MTHFR enzyme activity in cultured chorionic villus cells or amniocytes.

Treatment of MTHFR deficiency with a combination of folic acid, vitamin B ₆ , vitamin B ₁₂ , methionine supplementation, and betaine has been tried. Of these, early treatment with betaine appears to have the most beneficial effect.

Primary (Genetic) Hypermethioninemia

Elevation of plasma level of methionine occurs in several genetic conditions.

Acquired (Nongenetic) Hypermethioninemia

Hypermethioninemia occurs in premature and some full-term infants receiving high-protein diets, in whom it may represent delayed maturation of the enzyme MAT. Lowering the protein intake usually resolves the abnormality. It is also commonly found in patients with various forms of liver disease.