Abnormal Hypermethylation at Imprinting Control Regions in Patients with S-Adenosylhomocysteine Hydrolase (AHCY) Deficiency

S-adenosylhomocysteine hydrolase (AHCY) deficiency is a rare autosomal recessive disorder in methionine metabolism caused by mutations in the AHCY gene. Main characteristics are psychomotor delay including delayed myelination and myopathy (hypotonia, absent tendon reflexes etc.) from birth, mostly associated with hypermethioninaemia, elevated serum creatine kinase levels and increased genome wide DNA methylation. The prime function of AHCY is to hydrolyse and efficiently remove S-adenosylhomocysteine, the by-product of transmethylation reactions and one of the most potent methyltransferase inhibitors. In this study, we set out to more specifically characterize DNA methylation changes in blood samples from patients with AHCY deficiency. Global DNA methylation was increased in two of three analysed patients. In addition, we analysed the DNA methylation levels at differentially methylated regions (DMRs) of six imprinted genes (MEST, SNRPN, LIT1, H19, GTL2 and PEG3) as well as Alu and LINE1 repetitive elements in seven patients. Three patients showed a hypermethylation in up to five imprinted gene DMRs. Abnormal methylation in Alu and LINE1 repetitive elements was not observed. We conclude that DNA hypermethylation seems to be a frequent but not a constant feature associated with AHCY deficiency that affects different genomic regions to different degrees. Thus AHCY deficiency may represent an ideal model disease for studying the molecular origins and biological consequences of DNA hypermethylation due to impaired cellular methylation status.

Molecular and biochemical investigations of patients with intermediate or severe hyperhomocysteinemia

A discrepancy has been identified between numbers of expected and identified patients with homocystinuria due to cystathionine beta-synthase (CBS) deficiency. Patients homozygous for the frequent c.833T>C (p.I278T) are most often responsive to vitamin B6, and can present with a total-homocysteine (tHcy) <100 μM on a normal diet. In Denmark, patients with tHcy <100 μM are not routinely sequenced for CBS(2) mutations. This study investigated the prevalence of CBS mutations and the common methylenetetrahydrofolate reductase (MTHFR) c.677C>T polymorphism in patients with tHcy ≥ 50 μM and the association with clinical manifestations. We studied a cohort of patients with intermediate and severe hyperhomocysteinemia (tHcy ≥ 50 μM) determined between 1996 and 2011. Among the 413 eligible patients, 184 (45%) patients agreed to participate in the present follow-up study. A MTHFR(3)c.677TT genotype was found in 49% of the patients. Eight patients were found to have mutations in CBS(2). Of those, two were homozygous for c.833T>C (p.I278T), and four were compound heterozygous for c.833T>C. One c.833T>C (p.I278T) compound heterozygote was identified by lowering the threshold for sequencing from tHcy at 100 μM to 50 μM. The most prominent clinical presentation among patients with a CBS(2) mutation was thrombosis presenting at a median age of 25 years. In case of arterial or venous thrombosis without any explanation in individuals below 40 years, tHcy should be part of the thrombophilia screening. When tHcy is between 50 and 100 μM genotyping for the MTHFR(3) c.677TT is relevant, and when tHcy >100 μM CBS should be genotyped.

Liver transplantation for treatment of severe S-adenosylhomocysteine hydrolase deficiency

A child with severe S-adenosylhomocysteine hydrolase (AHCY) deficiency (AHCY c.428A>G, p.Tyr143Cys; c.982T>G, p.Tyr328Asp) presented at 8 months of age with growth failure, microcephaly, global developmental delay, myopathy, hepatopathy, and factor VII deficiency. Plasma methionine, S-adenosylmethionine (AdoMet), and S-adenosylhomocysteine (AdoHcy) were markedly elevated and the molar concentration ratio of AdoMet:AdoHcy, believed to regulate a myriad of methyltransferase reactions, was 15% of the control mean. Dietary therapy failed to normalize biochemical markers or alter the AdoMet to AdoHcy molar concentration ratio. At 40 months of age, the proband received a liver segment from a healthy, unrelated living donor. Mean AdoHcy decreased 96% and the AdoMet:AdoHcy concentration ratio improved from 0.52±0.19 to 1.48±0.79 mol:mol (control 4.10±2.11 mol:mol). Blood methionine and AdoMet were normal and stable during 6 months of follow-up on an unrestricted diet. Average calculated tissue methyltransferase activity increased from 43±26% to 60±22%, accompanied by signs of increased transmethylation in vivo. Factor VII activity increased from 12% to 100%. During 6 postoperative months, head growth accelerated 4-fold and the patient made promising gains in gross motor, language, and social skills.

Clinical and metabolic findings in patients with methionine adenosyltransferase I/III deficiency detected by newborn screening

Persistent hypermethioninemia due to mutations in the MAT1A gene is often found during newborn screening (NBS) for homocystinuria due to cystathionine beta-synthase deficiency, however, outcomes and optimal management for these patients are not well established. We carried out a multicenter study of MAT I/III-deficient patients detected by NBS in four of the Spanish regional NBS programs. Data evaluated during NBS and follow-up for 18 patients included methionine and total homocysteine levels, clinical presentation parameters, genotypes, and development quotients. The birth prevalence was 1:1:22,874. At detection 16 of the 18 patients exhibited elevations of plasma methionine above 60 μmol/L (mean 99.9 ± 38 μmol/L) and the mean value in confirmation tests was 301 μmol/L (91-899) μmol/L. All patients were asymptomatic. In four patients with more markedly elevated plasma methionines (>450 μmol/L) total homocysteine values were slightly elevated (about 20 μmol/L). The average follow-up period was 3 years 7 months (range: 2-123 months). Most patients (83%) were heterozygous for the autosomal dominant Arg264His mutation and, with one exception, presented relatively low circulating methionine concentrations (<400 μM). Additional mutations identified in patients with mean confirmatory plasma methionines above 400 μM were Arg199Cys, Leu355Arg, and a novel mutation, Thr288Ala. During continued follow-up, the patients have been asymptomatic, and, to date, no therapeutic interventions have been utilized. Therefore, the currently available evidence shows that hypermethioninemia due to heterozygous MAT1A mutations such as Arg264His is a mild condition for which no treatment is necessary.

Neurologically normal development of a patient with severe methionine adenosyltransferase I/III deficiency after continuing dietary methionine restriction

BACKGROUND

There is not much information on established standard therapy for patients with severe methionine adenosyltransferase (MAT) I/III deficiency.

CASE PRESENTATION

We report a boy with MAT I/III deficiency, in whom plasma methionine and total homocysteine, and urinary homocystine were elevated. Molecular genetic studies showed him to have novel compound heterozygous mutations of the MAT1A gene: c.191T>A (p.M64K) and c.589delC (p.P197LfsX26). A low methionine milk diet was started at 31 days of age, and during continuing dietary methionine restriction plasma methionine levels have been maintained at less than 750 μmol/L. He is now 5 years old, and has had entirely normal physical growth and psychomotor development.

CONCLUSIONS

Although some severely MAT I/III deficient patients have developed neurologic abnormalities, we report here the case of a boy who has remained neurologically and otherwise normal for 5 years during methionine restriction, suggesting that perhaps such management, started in early infancy, may help prevent neurological complications.

Two patients with hepatic mtDNA depletion syndromes and marked elevations of S-adenosylmethionine and methionine

This paper reports studies of two patients proven by a variety of studies to have mitochondrial depletion syndromes due to mutations in either their MPV17 or DGUOK genes. Each was initially investigated metabolically because of plasma methionine concentrations as high as 15-21-fold above the upper limit of the reference range, then found also to have plasma levels of S-adenosylmethionine (AdoMet) 4.4-8.6-fold above the upper limit of the reference range. Assays of S-adenosylhomocysteine, total homocysteine, cystathionine, sarcosine, and other relevant metabolites and studies of their gene encoding glycine N-methyltransferase produced evidence suggesting they had none of the known causes of elevated methionine with or without elevated AdoMet. Patient 1 grew slowly and intermittently, but was cognitively normal. At age 7 years he was found to have hepatocellular carcinoma, underwent a liver transplant and died of progressive liver and renal failure at age almost 9 years. Patient 2 had a clinical course typical of DGUOK deficiency and died at age 8 ½ months. Although each patient had liver abnormalities, evidence is presented that such abnormalities are very unlikely to explain their elevations of AdoMet or the extent of their hypermethioninemias. A working hypothesis is presented suggesting that with mitochondrial depletion the normal usage of AdoMet by mitochondria is impaired, AdoMet accumulates in the cytoplasm of affected cells poor in glycine N-methyltransferase activity, the accumulated AdoMet causes methionine to accumulate by inhibiting activity of methionine adenosyltransferase II, and that both AdoMet and methionine consequently leak abnormally into the plasma.

Hypermethioninemias of genetic and non-genetic origin: A review

This review covers briefly the major conditions, genetic and non-genetic, sometimes leading to abnormally elevated methionine, with emphasis on recent developments. A major aim is to assist in the differential diagnosis of hypermethioninemia. The genetic conditions are: (1) Homocystinuria due to cystathionine β-synthase (CBS) deficiency. At least 150 different mutations in the CBS gene have been identified since this deficiency was established in 1964. Hypermethioninemia is due chiefly to remethylation of the accumulated homocysteine. (2) Deficient activity of methionine adenosyltransferases I and III (MAT I/III), the isoenzymes the catalytic subunit of which are encoded by MAT1A. Methionine accumulates because its conversion to S-adenosylmethionine (AdoMet) is impaired. (3) Glycine N-methyltrasferase (GNMT) deficiency. Disruption of a quantitatively major pathway for AdoMet disposal leads to AdoMet accumulation with secondary down-regulation of methionine flux into AdoMet. (4) S-adenosylhomocysteine (AdoHcy) hydrolase (AHCY) deficiency. Not being catabolized normally, AdoHcy accumulates and inhibits many AdoMet-dependent methyltransferases, producing accumulation of AdoMet and, thereby, hypermethioninemia. (5) Citrin deficiency, found chiefly in Asian countries. Lack of this mitochondrial aspartate-glutamate transporter may produce (usually transient) hypermethioninemia, the immediate cause of which remains uncertain. (6) Fumarylacetoacetate hydrolase (FAH) deficiency (tyrosinemia type I) may lead to hypermethioninemia secondary either to liver damage and/or to accumulation of fumarylacetoacetate, an inhibitor of the high K(m) MAT. Additional possible genetic causes of hypermethioninemia accompanied by elevations of plasma AdoMet include mitochondrial disorders (the specificity and frequency of which remain to be elucidated). Non-genetic conditions include: (a) Liver disease, which may cause hypermethioninemia, mild, or severe. (b) Low-birth-weight and/or prematurity which may cause transient hypermethioninemia. (c) Ingestion of relatively large amounts of methionine which, even in full-term, normal-birth-weight babies may cause hypermethioninemia.

S-adenosylhomocysteine hydrolase deficiency: two siblings with fetal hydrops and fatal outcomes

This paper reports the clinical and metabolic findings in two sibling sisters born with fetal hydrops and eventually found to have deficient S-adenosylhomocysteine hydrolase (AHCY) activity due to compound heterozygosity for two novel mutations, c.145C>T; p.Arg49Cys and c.257A>G; p.Asp86Gly. Clinically, the major abnormalities in addition to fetal hydrops (very likely due to impaired synthetic liver function) were severe hypotonia/myopathy, feeding problems, and respiratory failure. Metabolic abnormalities included elevated plasma S-adenosylhomocysteine, S-adenosylmethionine, and methionine, with hypoalbuminemia, coagulopathies, and serum transaminase elevation. The older sister died at age 25 days, but the definitive diagnosis was made only retrospectively. The underlying genetic abnormality was diagnosed in the second sister, but treatment by means of dietary methionine restriction and supplementation with phosphatidylcholine and creatine did not prevent her death at age 122 days. These cases extend the experience with AHCY deficiency in humans, based until now on only the four patients previously identified, and suggest that the deficiency in question may be a cause of fetal hydrops and developmental abnormalities of the brain.

Enzymatic activity of methionine adenosyltransferase variants identified in patients with persistent hypermethioninemia

Methionine adenosyltransferases (MAT's) are central enzymes in living organisms that have been conserved with a high degree of homology among species. In the liver, MAT I and III, tetrameric and dimeric isoforms of the same catalytic subunit encoded by the gene MAT1A, account for the predominant portion of total body synthesis of S-adenosylmethionine (SAM), a versatile sulfonium ion-containing molecule involved in a variety of vital metabolic reactions and in the control of hepatocyte proliferation and differentiation. During the past 15years 28 MAT1A mutations have been described in patients with elevated plasma methionines, total homocysteines at most only moderately elevated, and normal levels of tyrosine and other aminoacids. In this study we describe functional analyses that determine the MAT and tripolyphosphatase (PPPase) activities of 18 MAT1A variants, six of them novel, and none of them previously assayed for activity. With the exception of G69S and Y92H, all recombinant proteins showed impairment (usually severe) of MAT activity. Tripolyphosphate (PPPi) hydrolysis was decreased only in some mutant proteins but, when it was decreased MAT activity was always also impaired.

A revisit to the natural history of homocystinuria due to cystathionine beta-synthase deficiency

We review the evidence that in Denmark and probably certain other European countries the number of individuals identified with homocystinuria due to homozygosity for the widespread c.833T>C (p.I278T) mutation in the gene that encodes cystathionine beta-synthase (CBS) falls far short of the number of such individuals expected on the basis of the heterozygote frequency for this mutation found by molecular screening. We conclude that the predominant portion of such homozygotes may be clinically unaffected, or may be ascertained for thromboembolic events occurring no sooner than the third decade of life. If so, there was significant ascertainment bias in the time-to-event curves previously published describing the natural history of untreated CBS deficiency Mudd et al. and these curves should be used with care.

Cystathionine gamma-lyase: Clinical, metabolic, genetic, and structural studies

We report studies of six individuals with marked elevations of cystathionine in plasma and/or urine. Studies of CTH, the gene that encodes cystathionine gamma-lyase, revealed the presence among these individuals of either homozygous or compound heterozygous forms of a novel large deletion, p.Gly57_Gln196del, two novel missense mutations, c.589C>T (p.Arg197Cys) and c.932C>T (p.Thr311Ile), and one previously reported alteration, c.200C>T (p.Thr67Ile). Another novel missense mutation, c.185G>T (p.Arg62His), was found in heterozygous form in three mildly hypercystathioninemic members of a Taiwanese family. In one severely hypercystathioninemic individual no CTH mutation was found. Brief clinical histories of the cystathioninemic/cystathioninuric patients are presented. Most of the novel mutations were expressed and the CTH activities of the mutant proteins determined. The crystal structure of the human enzyme, hCTH, and the evidence available as to the effects of the mutations in question, as well as those of the previously reported p.Gln240Glu, on protein structure, enzymatic activity, and responsiveness to vitamin B(6) administration are discussed. Among healthy Czech controls, 9.3% were homozygous for CTH c.1208G>T (p.Ser403Ile), previously found homozygously in 7.5% of Canadians for whom plasma total homocysteine (tHcy) had been measured. Compared to wild-type homozygotes, among the 55 Czech c.1208G>T (p.Ser403Ile) homozygotes a greater level of plasma cystathionine was found only after methionine loading. Three of the four individuals homozygous or compound heterozygous for inactivating CTH mutations had mild plasma tHcy elevations, perhaps indicating a cause-and-effect relationship. The experience with the present patients provides no evidence that severe loss of CTH activity is accompanied by adverse clinical effects.

S-adenosylhomocysteine hydrolase (AHCY) deficiency: two novel mutations with lethal outcome

This paper reports studies of two novel, allelic missense mutations found in the S-adenosylhomocysteine hydrolase (AHCY) gene from a new case of AHCY deficiency in an infant girl who died at age four months. The mutations lead to replacement of arginine with cysteine (p.Arg49Cys) and aspartic acid with glycine (p.Asp86Gly). Functional analysis of recombinant proteins containing the mutations detected showed that both dramatically reduce AHCY activity. The p.Arg49Cys mutant protein forms intermolecular disulphide bonds, leading to macromolecular structures that can be prevented by reducing agent DTT. The p.Asp86Gly protein tends to form enzymatically inactive aggregates and the loss of a single negative charge as a result of the mutation is involved in enzyme inactivation. We show that replacing Gly86 with negatively charged Glu86 in mutant protein restores enzymatic activity to 70% of wild-type, whereas changing Gly86 to positively charged Lys86 or uncharged Leu86 does not improve enzyme activity, indicating that the negative charge is important for maintenance of such activity. These studies significantly extend knowledge about the importance of residue 86 for AHCY activity. Residue 86 has not been implicated before in this way and the results suggest that the present model of S- adenosylhomocysteine (AdoHcy) hydrolysis may need refinement. Our functional studies provide novel insight into the molecular defect underlying AHCY deficiency and reveal that both low enzyme activity and protein stability of AHCY contribute to the clinical phenotype.

Glycine N-methyltransferase and regulation of S-adenosylmethionine levels

Methylation is a major biological process. It has been shown to be important in formation of compounds such as phosphatidylcholine, creatine, and many others and also participates in epigenetic effects through methylation of histones and DNA. The donor of methyl groups for almost all cellular methylation reactions is S-adenosylmethionine. It seems that the level of S-adenosylmethionine must be regulated in response to developmental stages and metabolic changes, and the enzyme glycine N-methyltransferase has been shown to play a major role in such regulation in mammals. This minireview will focus on the latest discoveries in the elucidation of the mechanism of that regulation.

Hypermethioninaemia due to methionine adenosyltransferase I/III (MAT I/III) deficiency: diagnosis in an expanded neonatal screening programme

The Expanded Newborn Screening Program (MS/MS) in the region of Galicia (NW Spain) was initiated in 2000 and includes the measurement of methionine levels in dried blood spots. Between June 2000 and June 2007, 140 818 newborns were analysed, and six cases of persistent hypermethioninaemia were detected: one homocystinuria due to cystathionine β-synthase (CβS) deficiency, and five methionine adenosyltransferase I/III (MAT I/III) deficiencies. The five cases of MAT I/III deficiency represent an incidence of 1/28 163 newborns. In these five patients, methionine levels in dried blood spots ranged from 50 to 147 μmol/L. At confirmation of the persistence of the hypermethioninaemia in a subsequent plasma sample, plasma methionine concentrations were moderately elevated in 4 of the 5 patients (mean 256 μmol/L), while total homocysteine (tHcy) was normal; the remaining patient showed plasma methionine of 573 μmol/L and tHcy of 22.8 μmol/L. All five patients were heterozygous for the same dominant mutation, R264H in the MAT1A gene. With a diet not exceeding recommended protein requirements for their age, all patients maintained methionine levels below 300 μmol/L. Currently, with a mean of 2.5 years since diagnosis, the patients are asymptomatic and show developmental quotients within the normal range. Our results show a rather high frequency of hypermethioninaemia due to MAT I/III deficiency in the Galician neonatal population, indicating a need for further studies to evaluate the impact of persistent isolated hypermethioninaemia in neonatal screening programmes.

Diseases of sulphur metabolism: implications for the methionine-homocysteine cycle, and vitamin responsiveness

Sixteen inherited human diseases are now recognized, affecting most of the major steps in sulphur metabolism. Studies of patients with three types of homocystinuria have demonstrated unequivocally the major role of cystathionine formation in degradation of homocysteine, and the importance of homocysteine remethylation. Methionine balance studies of normal subjects and of a sarcosine oxidase-deficient subject have shown the predominant role of creatine synthesis in methionine utilization and permitted assessment of the rate of oxidation of the methyl group of methionine. Together, the results demonstrate that once regulatory adjustments have been made the rate of methylneogensis is nicely controlled so that labile methyl groups are made available in amounts just sufficient to meet the needs for methionine. When excess methionine is ingested the four-carbon moiety is diverted into cystathionine, the methyl group is oxidized via sarcosine and the flow of partially oxidized one-carbon units is diverted away from 5-methyltetrahydrofolate toward CO2. Studies of cystathionine synthase-deficient patients demonstrate that the capacity to respond or not to respond to pyridoxine administration is genetically controlled, probably through structural differences in mutant cystathionine synthases. However, the properties of the enzyme crucial in conferring responsiveness have not yet been identified.

Prevention of brain disease from severe 5,10-methylenetetrahydrofolate reductase deficiency

Over a four-year period, we collected clinical and biochemical data from five Amish children who were homozygous for missense mutations in 5,10-methylenetetrahydrofolate reductase (MTHFR c.1129C>T). The four oldest patients had irreversible brain damage prior to diagnosis. The youngest child, diagnosed and started on betaine therapy as a newborn, is healthy at her present age of three years. We compared biochemical data among four groups: 16 control subjects, eight heterozygous parents, and five affected children (for the latter group, both before and during treatment with betaine anhydrous). Plasma amino acid concentrations were used to estimate changes in cerebral methionine uptake resulting from betaine therapy. In all affected children, treatment with betaine (534+/-222 mg/kg/day) increased plasma S-adenosylmethionine, improved markers of tissue methyltransferase activity, and resulted in a threefold increase of calculated brain methionine uptake. Betaine therapy did not normalize plasma total homocysteine, nor did it correct cerebral 5-methyltetrahydrofolate deficiency. We conclude that when the 5-methyltetrahydrofolate content of brain tissue is low, dietary betaine sufficient to increase brain methionine uptake may compensate for impaired cerebral methionine recycling. To effectively support the metabolic requirements of rapid brain growth, a large dose of betaine should be started early in life.

Methyl balance and transmethylation fluxes in humans

Various questions have been raised about labile methyl balance and total transmethylation fluxes, and further discussion has been encouraged. This report reviews and discusses some of the relevant evidence now available. The fact that, if needed, labile methyl balance is maintained by methylneogenesis appears to be established, but several aspects of transmethylation remain uncertain: definitive measurements of the rate of total transmethylation in humans of both sexes on various diets and at various ages; the extent to which synthesis of phosphatidylcholine has been underestimated; and the relative contributions of the 2 pathways for the formation of sarcosine (ie, N-methylglycine). The available evidence indicates that the quantitatively most important pathways for S-adenosylmethionine-dependent transmethylation in mammals are the syntheses of creatine by guanidinoacetate methyltransferase, of phosphatidylcholine by phosphatidylethanolamine methyltransferase, and of sarcosine by glycine N-methyltransferase. Data presented in this report show that S-adenosylmethionine and methionine accumulate abnormally in the plasma of humans with glycine N-methyltransferase deficiency but not of those with guanidinoacetate N-methyltransferase deficiency or in the plasma or livers of mice devoid of phosphatidylethanolamine N-methyltransferase activity. The absence of such accumulations in the latter 2 conditions may be due to removal of S-adenosylmethionine by synthesis of sarcosine. Steps that may help clarify the remaining issues include the determination of the relative rates of synthesis of sarcosine, creatine, and phosphatidylcholine by rapid measurement of the rates of radiolabel incorporation into these compounds from L-[methyl-3H]methionine administered intraportally to an experimental animal; clarification of the intracellular hepatic isotope enrichment value during stable-isotope infusion studies to enhance the certainty of methyl flux estimates during such studies; and definitive measurement of the dietary betaine intake from various diets.

S-adenosylhomocysteine hydrolase deficiency in a 26-year-old man

This paper reports the third proven human case of deficient S-adenosylhomocysteine (AdoHcy) hydrolase activity. The patient is similar to the only two previously reported cases with this disorder in having severe myopathy, developmental delay, elevated serum creatine kinase (CK) concentrations, and hypermethioninaemia. Although he has been followed from infancy, the basic enzyme deficiency was established only at age 26 years. The diagnosis was based on markedly elevated plasma concentrations of both AdoHcy and S-adenosylmethionine, some 20% of the mean control activity of AdoHcy hydrolase activity in haemolysates of his red-blood cells, and two missense mutations in his gene encoding AdoHcy hydrolase. He had low values of erythrocyte phosphatidylcholine and plasma free choline and marginally elevated excretion of guanidinoacetate, suggesting that the elevated AdoHcy may have been inhibiting methylation of phosphatidylethanolamine and guanidinoacetate. His leukocyte DNA was globally more methylated than the DNA's of his parents or the mean extent of methylation measured in age-matched control subjects.

Dimethyl sulfone in human cerebrospinal fluid and blood plasma confirmed by one-dimensional (1)H and two-dimensional (1)H-(13)C NMR

(1)H-NMR spectroscopy at 500 MHz was used to confirm that a previously unidentified singlet resonance at 3.14 ppm in the spectra of cerebrospinal fluid and plasma samples corresponds to dimethyl sulfone (DMSO(2)). A triple resonance inverse cryogenic NMR probe, with pre-amplifier and the RF-coils cooled to low temperature, was used to obtain an (1)H-(13)C HSQC spectrum of CSF containing 8 microM (753 ng/ml) DMSO(2). The (1)H-(13)C correlation signal for DMSO(2) was assigned by comparison with the spectrum from an authentic reference sample. In plasma and CSF from healthy controls, the concentration of DMSO(2) ranged between 0 and 25 micromol/l. The concentration of DMSO(2) in plasma from three of four patients with severe methionine adenosyltransferase I/III (MAT I/III) deficiency was about twice the maximum observed for controls. Thus, DMSO(2) occurs as a regular metabolite at low micromolar concentrations in cerebrospinal fluid and plasma. It derives from dietary sources, from intestinal bacterial metabolism and from human endogenous methanethiol metabolism.

S-Adenosylhomocysteine hydrolase deficiency: a second patient, the younger brother of the index patient, and outcomes during therapy

S-Adenosylhomocysteine (AdoHcy) hydrolase deficiency has been proven in a human only once, in a recently described Croatian boy. Here we report the clinical course and biochemical abnormalities of the younger brother of this proband. This younger brother has the same two mutations in the gene encoding AdoHcy hydrolase, and has been monitored since birth. We report, as well, outcomes during therapy for both patients. The information obtained suggests that the disease starts in utero and is characterized primarily by neuromuscular symptomatology (hypotonia, sluggishness, psychomotor delay, absent tendon reflexes, delayed myelination). The laboratory abnormalities are markedly increased creatine kinase and elevated aminotransferases, as well as specific amino acid aberrations that pinpoint the aetiology. The latter include, most importantly, markedly elevated plasma AdoHcy. Plasma S-adenosylmethionine (AdoMet) is also elevated, as is methionine (although the hypermethioninaemia may be absent or nonsignificant in the first weeks of life). The disease seems to be at least to some extent treatable, as shown by improved myelination and psychomotor development during dietary methionine restriction and supplementation with creatine and phosphatidylcholine.

S-adenosylhomocysteine hydrolase deficiency in a human: a genetic disorder of methionine metabolism

We report studies of a Croatian boy, a proven case of human S-adenosylhomocysteine (AdoHcy) hydrolase deficiency. Psychomotor development was slow until his fifth month; thereafter, virtually absent until treatment was started. He had marked hypotonia with elevated serum creatine kinase and transaminases, prolonged prothrombin time and low albumin. Electron microscopy of muscle showed numerous abnormal myelin figures; liver biopsy showed mild hepatitis with sparse rough endoplasmic reticulum. Brain MRI at 12.7 months revealed white matter atrophy and abnormally slow myelination. Hypermethioninemia was present in the initial metabolic study at age 8 months, and persisted (up to 784 microM) without tyrosine elevation. Plasma total homocysteine was very slightly elevated for an infant to 14.5-15.9 microM. In plasma, S-adenosylmethionine was 30-fold and AdoHcy 150-fold elevated. Activity of AdoHcy hydrolase was approximately equal to 3% of control in liver and was 5-10% of the control values in red blood cells and cultured fibroblasts. We found no evidence of a soluble inhibitor of the enzyme in extracts of the patient's cultured fibroblasts. Additional pretreatment abnormalities in plasma included low concentrations of phosphatidylcholine and choline, with elevations of guanidinoacetate, betaine, dimethylglycine, and cystathionine. Leukocyte DNA was hypermethylated. Gene analysis revealed two mutations in exon 4: a maternally derived stop codon, and a paternally derived missense mutation. We discuss reasons for biochemical abnormalities and pathophysiological aspects of AdoHcy hydrolase deficiency.

Infantile hypermethioninemia and hyperhomocysteinemia due to high methionine intake: a diagnostic trap

Studies were carried out to identify the cause of combined severe hypermethioninemia and moderate hyperhomocysteinemia in a cluster of 10 infants ascertained between 1999 and early 2001. Although several were thought initially to have cystathionine beta-synthase (CBS) deficiency and treated accordingly, CBS deficiency and other known genetic causes of hypermethioninemia were ruled out by assay of CBS activity in fibroblasts of four patients and by assays of plasma cystathionine and S-adenosylmethionine. Retrospective data on dietary methionine intakes and plasma concentrations of methionine and related metabolites established that the hypermethioninemia in nine of the 10 babies was related to ingestion of an infant protein hydrolysate formula, the methionine content of which had been increased from May 1998 to February 2001. The formula in question has now been reformulated and is no longer available. The 10th infant manifested similar metabolic abnormalities while receiving TPN containing excessive methionine. Brain MRI abnormalities indicative of cerebral edema, most marked in the cerebral cortex and posterior brainstem, occurred in two patients near times of extreme hypermethioninemia. Metabolic and MRI abnormalities resolved when the methionine intake decreased. A third infant had a normal MRI 1 day after the formula was changed. The possible relationship between extreme hypermethioninemia and cerebral edema is discussed and a working hypothesis offered to explain the relative sensitivity of the inferior colliculi, based upon the facts that this is the region most active in glucose utilization and that Na(+),K(+)-ATPase is inhibited by methionine and related metabolites.

Maternal methionine adenosyltransferase I/III deficiency: reproductive outcomes in a woman with four pregnancies

Four pregnancies in a women with moderately severe deficiency of methionine adenosyltransferase I/III (MAT I/III) activity are reported. She is an apparent homozygote for a point mutation in MAT1A, the gene that encodes the catalytically active subunit of MAT I/III. This mutation reduces the activity of her expressed enzyme to some 11% of wild-type. She was the first such individual identified in the United States, and these are the first pregnancies known in anyone with this extent of MAT I/III deficiency. No adverse effects were noted in the mother. Three normal babies resulted, but fetal arrest was detected in one embryo at 10-11 weeks gestation. Plasma methionine concentrations remained virtually constant at their elevated levels of 300-350 micromol/L throughout the pregnancies. Plasma free choline was below the reference range. In view of the evidence that maternal choline delivery to the fetus is important for brain development, it was suggested the patient ingest two eggs daily from gestation week 17. Plasma choline and phosphatidylcholine tended to rise during such supplementation. Plasma cystathionine concentrations rose progressively to far above normal during these pregnancies, but not during pregnancies in control women. This may be explained by delivery of excessive methionine to the fetus, with consequent increased cystathionine synthesis by fetal tissues. Because fetal tissues lack gamma-cystathionase, presumably cystathionine accumulated abnormally in the fetus and was transferred in abnormal amounts back to the mother. Plasma and urinary concentrations of methionine transamination metabolites rose during pregnancy for reasons that remain obscure.

Glycine N -methyltransferase deficiency: a new patient with a novel mutation

We report studies of a Greek boy of gypsy origin that show that he has severe deficiency of glycine N -methyltransferase (GNMT) activity due to apparent homozygosity for a novel mutation in the gene encoding this enzyme that changes asparagine-140 to serine. At age 2 years he was found to have mildly elevated serum liver transaminases that have persisted to his present age of 5 years. At age 4 years, hypermethioninaemia was discovered. Plasma methionine concentrations have ranged from 508 to 1049 micro mol/L. Several known causes of hypermethioninaemia were ruled out by studies of plasma metabolites: tyrosinaemia type I by a normal plasma tyrosine and urine succinylacetone; cystathionine beta-synthase deficiency by total homocysteine of 9.4-12.1 micro mol/L; methionine adenosyltransferase I/III deficiency by S -adenosylmethionine (AdoMet) levels elevated to 1643-2222 nmol/L; and S -adenosylhomocysteine (AdoHcy) hydrolase deficiency by normal AdoHcy levels. A normal plasma N -methylglycine concentration in spite of elevated AdoMet strongly suggested GNMT deficiency. Molecular genetic studies identified a missense mutation in the coding region of the boy's GNMT gene, which, upon expression, retained only barely detectable catalytic activity. The mild hepatitis-like manifestations in this boy are similar to those in the only two previously reported children with GNMT deficiency, strengthening the likelihood of a causative association. Although his deficiency of GNMT activity may well be more extreme, his metabolic abnormalities are not strikingly greater. Also discussed is the metabolic role of GNMT; several additional metabolite abnormalities found in these patients; and remaining questions about human GNMT deficiency, such as the long-term prognosis, whether other individuals with this defect are currently going undetected, and means to search for such persons.

Methionine adenosyltransferase I/III deficiency: two Korean compound heterozygous siblings with a novel mutation

Two Korean sisters, one detected during neonatal screening, the other ascertained at age 3 years during family screening, have persistent hypermethioninaemia without elevation of plasma tyrosine or severe liver disease. Plasma total homocysteine (tHcy) is mildly elevated, but not so markedly as to establish a diagnosis of homocystinuria due to cystathionine beta-synthase (CBS) deficiency. CBS deficiency was ruled out by the presence of slightly elevated concentrations of plasma cystathionine. Although the plasma concentrations of methionine were markedly elevated, plasma S-adenosylmethionine (AdoMet) was not. This pattern of metabolic abnormalities suggested that the patients have deficient activity of methionine adenosyltransferase (MAT) in their livers (MAT I/III deficiency). Molecular genetic studies demonstrate that each patient is a compound heterozygote for two mutations in MAT1A, the gene that encodes the catalytic subunit that composes MAT I and MAT III: a previously known inactivating G378S point mutation, and a novel W387X truncating mutation. W387X mutant protein, expressed in E. coli and purified, has about 75% of wild-type activity. Negative subunit interaction between the mutant subunits is suggested to explain the hypermethioninaemia of these sisters. They have had normal growth and development and have no mental retardation, neurological abnormalities, or other clinical problems. They are the first individuals of Korean descent proven to have MAT I/III deficiency.

Elevated plasma total homocysteine in severe methionine adenosyltransferase I/III deficiency

Abnormal elevation of plasma methionine may result from several different genetic abnormalities, including deficiency of cystathionine beta-synthase (CBS) or of the isoenzymes of methionine adenosyltransferase (MAT) I and III expressed solely in nonfetal liver (MAT I/III deficiency). Classically, these conditions have been distinguished most readily by the presence or absence, respectively, of elevated plasma free homocystine, detected by amino acid chromatography in the former condition, but absent in the latter. During the present work, we have assayed methionine, S-adenosylmethionine, S-adenosylhomocysteine, total homocysteine (tHcy), cystathionine, N-methylglycine (sarcosine), and total cysteine (tCys) in groups of both MAT I/III- and CBS-deficient patients to provide more evidence as to their metabolite patterns. Unexpectedly, we found that MAT I/III-deficient patients with the most markedly elevated levels of plasma methionine also had elevations of plasma tHcy and often mildly elevated plasma cystathionine. Evidence is presented that methionine does not inhibit cystathionine beta-synthase, but does inhibit cystathionine gamma-lyase. Mechanisms that may possibly underlie the elevations of plasma tHcy and cystathionine are discussed. The combination of elevated methionine plus elevated tHcy may lead to the mistaken conclusion that an MAT I/III-deficient patient is instead CBS-deficient. Less than optimal management is then a real possibility. Measurements of plasma cystathionine, S-adenosylmethionine, and sarcosine should permit ready distinction between the 2 conditions in question, as well as be useful in several other situations involving abnormalities of methionine and/or homocysteine derivatives.

Adverse event associated with methionine loading test: a case report

The death of a control subject after an oral load of methionine for a study of the possible relationship between homocysteine and Alzheimer's disease is reported. The subject developed postload plasma concentrations of methionine far beyond those reported previously in humans given the usual oral loading dose of methionine (100 mg/kg body wt). Her preload plasma metabolite values rule out known genetic diseases that might predispose one to unusually high methionine concentrations. The most likely explanation for these events is that the subject received a substantial overdose of methionine. The possibility that extremely high methionine concentrations may lead to severe cerebral effects is discussed, and it is recommended that any move to increase the sensitivity of the usual methionine loading test by increasing the dose of methionine either not be undertaken or be taken only with extreme care.

Progressive cerebral edema associated with high methionine levels and betaine therapy in a patient with cystathionine beta-synthase (CBS) deficiency

Cystathionine beta-synthase (CBS) deficiency, the most common form of homocystinuria, is an autosomal recessive inborn error of homocysteine metabolism. Treatment of B6-nonresponsive patients centers on lowering homocysteine and its disulfide derivatives (tHcy) by adherence to a methionine-restricted diet. However, lifelong dietary control is difficult. Betaine supplementation is used extensively in CBS-deficient patients to lower plasma tHcy. With betaine therapy, methionine levels increase over baseline, but usually remain below 1,500 micromol/L, and these levels have not been associated with adverse affects. We report a child with B6-nonresponsive CBS deficiency and dietary noncompliance whose methionine levels reached 3,000 micromol/L on betaine, and who subsequently developed massive cerebral edema without evidence of thrombosis. We investigated the etiology by determining methionine and betaine metabolites in our patient, and several possible mechanisms for her unusual response to betaine are discussed. We conclude that the cerebral edema was most likely precipitated by the betaine therapy, although the exact mechanism is uncertain. This case cautions physicians to monitor methionine levels in CBS-deficient patients on betaine and to consider betaine as an adjunct, not an alternative, to dietary control.

Glycine N-methyltransferase deficiency: a novel inborn error causing persistent isolated hypermethioninaemia

This paper reports clinical and metabolic studies of two Italian siblings with a novel form of persistent isolated hypermethioninaemia, i.e. abnormally elevated plasma methionine that lasted beyond the first months of life and is not due to cystathionine beta-synthase deficiency, tyrosinaemia I or liver disease. Abnormal elevations of their plasma S-adenosylmethionine (AdoMet) concentrations proved they do not have deficient activity of methionine adenosyltransferase I/III. A variety of studies provided evidence that the elevations of methionine and AdoMet are not caused by defects in the methionine transamination pathway, deficient activity of methionine adenosyltransferase II, a mutation in methylenetetrahydrofolate reductase rendering this activity resistant to inhibition by AdoMet, or deficient activity of guanidinoacetate methyltransferase. Plasma sarcosine (N-methylglycine) is elevated, together with elevated plasma AdoMet in normal subjects following oral methionine loads and in association with increased plasma levels of both methionine and AdoMet in cystathionine beta-synthase-deficient individuals. However, plasma sarcosine is not elevated in these siblings. The latter result provides evidence they are deficient in activity of glycine N-methyltransferase (GNMT). The only clinical abnormalities in these siblings are mild hepatomegaly and chronic elevation of serum transaminases not attributable to conventional causes of liver disease. A possible causative connection between GNMT deficiency and these hepatitis-like manifestations is discussed. Further studies are required to evaluate whether dietary methionine restriction will be useful in this situation.

Biochemical basis for the dominant inheritance of hypermethioninemia associated with the R264H mutation of the MAT1A gene. A monomeric methionine adenosyltransferase with tripolyphosphatase activity

Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine (AdoMet), the main alkylating agent in living cells. Additionally, in the liver, MAT is also responsible for up to 50% of methionine catabolism. Humans with mutations in the gene MAT1A, the gene that encodes the catalytic subunit of MAT I and III, have decreased MAT activity in liver, which results in a persistent hypermethioninemia without homocystinuria. The hypermethioninemic phenotype associated with these mutations is inherited as an autosomal recessive trait. The only exception is the dominant mild hypermethioninemia associated with a G-A transition at nucleotide 791 of exon VII. This change yields a MAT1A-encoded subunit in which arginine 264 is replaced by histidine. Our results indicate that in the homologous rat enzyme, replacement of the equivalent arginine 265 by histidine (R265H) results in a monomeric MAT with only 0.37% of the AdoMet synthetic activity. However the tripolyphosphatase activity is similar to that found in the wild type (WT) MAT and is inhibited by PP(i). Our in vivo studies demonstrate that the R265H MAT I/III mutant associates with the WT subunit resulting in a dimeric R265H-WT MAT unable to synthesize AdoMet. Tripolyphosphatase activity is maintained in the hybrid MAT, but is not stimulated by methionine and ATP, indicating a deficient binding of the substrates. Our data indicate that the active site for tripolyphosphatase activity is functionally active in the monomeric R265H MAT I/III mutant. Moreover, our results provide a molecular mechanism that might explain the dominant inheritance of the hypermethioninemia associated with the R264H mutation of human MAT I/III.

Isolated hypermethioninemia: measurements of S-adenosylmethionine and choline

The concentrations of methionine and S-adenosylmethionine (AdoMet) in plasma and free choline and phospholipid-bound choline in both plasma and red blood cells from individuals with isolated hypermethioninemia have been measured. The only genetic abnormalities identified in these individuals have been inactivating mutations in MAT1A, the gene that encodes the subunit of the isozymes of methionine adenosyltransferase (MAT), MAT I, and MAT III, expressed only in adult liver. These measurements were performed to learn more about AdoMet metabolism and to test the working hypotheses that inadequate delivery of AdoMet, or of choline or a choline derivative, from liver to brain might be a cause of the neurologic disease often found in humans with the most severe losses of MAT I/III activity. In striking contrast to the elevations of plasma AdoMet reported in control humans with hypermethioninemia resulting from methionine loading, plasma AdoMet levels were generally below the mean reference value in the MAT I/II-deficient hypermethioninemic patients. This is interpreted as a result of subnormal formation of AdoMet in liver due to the deficient activity of MAT I/III and resultant lower-than-normal delivery of AdoMet from liver to plasma. A low plasma AdoMet concentration in the presence of an elevated methionine provides a useful diagnostic tool that pinpoints the cause of a case of hypermethioninemia as defective MAT I/III activity. Plasma-free choline concentrations were also generally somewhat below normal in the hypermethioninemic patients. However, neither plasma AdoMet nor plasma choline concentrations were strikingly lower in MAT I/III-deficient individuals with neurologic abnormalities than in those without. These results thus fail to provide support for the working hypotheses in question.

Methionine transamination in patients with homocystinuria due to cystathionine beta-synthase deficiency

To assess the ability of patients with homocystinuria due to cystathionine beta-synthase (CBS) deficiency to perform the reactions of the methionine transamination pathway, the concentrations of the products of this pathway were measured in plasma and urine. The results clearly demonstrate that CBS-deficient patients develop elevations of these metabolites once a threshold near 350 micromol/L for the concurrent plasma methionine concentration is exceeded. The absence of elevated methionine transamination products previously reported among 16 CBS-deficient B6-responsive patients may now be attributed to the fact that in those patients the plasma methionine concentrations were below this threshold. The observed elevations of transamination products were similar to those observed among patients with isolated hypermethioninemia. Plasma homocyst(e)ine did not exert a consistent effect on transamination metabolites, and betaine appeared to effect transamination chiefly by its tendency to elevate methionine. Even during betaine administration, the transamination pathway does not appear to be a quantitatively major route for the disposal of methionine.

Methionine adenosyltransferase I/III deficiency: novel mutations and clinical variations

Methionine adenosyltransferase (MAT) I/III deficiency, caused by mutations in the MAT1A gene, is characterized by persistent hypermethioninemia without elevated homocysteine or tyrosine. Clinical manifestations are variable and poorly understood, although a number of individuals with homozygous null mutations in MAT1A have neurological problems, including brain demyelination. We analyzed MAT1A in seven hypermethioninemic individuals, to provide insight into the relationship between genotype and phenotype. We identified six novel mutations and demonstrated that mutations resulting in high plasma methionines may signal clinical difficulties. Two patients-a compound heterozygote for truncating and severely inactivating missense mutations and a homozygote for an aberrant splicing MAT1A mutation-have plasma methionine in the 1,226-1,870 microM range (normal 5-35 microM) and manifest abnormalities of the brain gray matter or signs of brain demyelination. Another compound heterozygote for truncating and inactivating missense mutations has 770-1,240 microM plasma methionine and mild cognitive impairment. Four individuals carrying either two inactivating missense mutations or the single-allelic R264H mutation have 105-467 microM plasma methionine and are clinically unaffected. Our data underscore the necessity of further studies to firmly establish the relationship between genotypes in MAT I/III deficiency and clinical phenotypes, to elucidate the molecular bases of variability in manifestations of MAT1A mutations.

Maternal gamma-cystathionase deficiency: absence of both teratogenic effects and pregnancy complications

gamma-Cystathionase deficiency (cystathioninemia-cystathioninuria) is a disorder of the transsulfuration pathway characterized by the accumulation of cystathionine in blood and urine. There are probably no clinical consequences. However, maternal gamma-cystathionase deficiency has not been reported. We studied 2 pregnancies and the offspring of these pregnancies in a woman with the pyridoxine-nonresponsive form of the disorder. The outcomes were favorable, suggesting that maternal gamma-cystathionase deficiency may not be deleterious to the pregnant woman or the fetus.

Dominant inheritance of isolated hypermethioninemia is associated with a mutation in the human methionine adenosyltransferase 1A gene

Methionine adenosyltransferase (MAT) I/III deficiency, characterized by isolated persistent hypermethioninemia, is caused by mutations in the MAT1A gene encoding MAT(alpha)1, the subunit of major hepatic enzymes MAT I ([alpha1]4) and III([alpha1]2). We have characterized 10 MAT1A mutations in MAT I/III-deficient individuals and shown that the associated hypermethioninemic phenotype was inherited as an autosomal recessive trait. However, dominant inheritance of hypermethioninemia, also hypothesized to be caused by MAT I/III deficiency, has been reported in two families. Here we show that the only mutation uncovered in one of these families, G, is a G-->A transition at nt 791 in exon VII of one MAT1A allele that converts an arginine at position 264 to a histidine (R264H). This single allelic R264H mutation was subsequently identified in two hypermethioninemic individuals in an additional family, C. Family C members were also found to inherit hypermethioninemia in a dominant fashion, and the available affected members analyzed carried the single allelic R264H mutation. Substitution of R-264 with histidine (R264H, the naturally occurring mutant), leucine (R264L), aspartic acid (R264D), or glutamic acid (R264E) greatly reduced MAT activity and severely impaired the ability of the MAT(alpha)1 subunits to form homodimers essential for optimal catalytic activity. On the other hand, when lysine was substituted for R-264 (R264K), the mutant alpha1 subunit was able to form dimers that retain significant MAT activity, suggesting that amino acid 264 is involved in intersubunit salt-bridge formation. Cotransfection studies show that R264/R264H MAT(alpha)1 heterodimers are enzymatically inactive, thus providing an explanation for the R264H-mediated dominant inheritance of hypermethioninemia.

Demyelination of the brain is associated with methionine adenosyltransferase I/III deficiency

Individuals deficient in hepatic methionine adenosyltransferase (MAT) activity (MAT I/III deficiency) have been demonstrated to contain mutations in the gene (MATA1) that encodes the major hepatic forms, MAT I and III. MAT I/III deficiency is characterized by isolated persistent hypermethioninemia and, in some cases, unusual breath odor. Most individuals with isolated hypermethioninemia have been free of major clinical difficulties. Therefore a definitive diagnosis of MAT I/III deficiency, which requires hepatic biopsy, is not routinely made. However, two individuals with isolated hypermethioninemia have developed abnormal neurological problems, including brain demyelination, suggesting that MAT I/III deficiency can be deleterious. In the present study we have examined the MATA1 gene of eight hypermethioninemic individuals, including the two with demyelination of the brain. Mutations that abolish or reduce the MAT activity were detected in the MATA1 gene of all eight individuals. Both patients with demyelination are homozygous for mutations that alter the reading frame of the encoded protein such that the predicted MATalpha1 subunits are truncated and enzymatically inactive. The product of MAT, S-adenosylmethionine (AdoMet), is the major methyl donor for a large number of biologically important compounds including the two major myelin phospholipids, phosphatidylcholine and sphingomyelin. Both are synthesized primarily in the liver. Our findings demonstrate that isolated persistent hypermethioninemia is a marker of MAT I/III deficiency, and that complete lack of MAT I/III activity can lead to neurological abnormalities. Therefore, a DNA-based diagnosis should be performed for individuals with isolated hypermethioninemia to assess if therapy aimed at the prevention of neurological manifestations is warranted.

Isolated persistent hypermethioninemia

New information has been obtained on 30 patients with isolated persistent hypermethioninemia, most of them previously unreported. Biopsies to confirm the presumptive diagnosis of partially deficient activity of ATP: L-methionine S-adenosyltransferase (MAT; E.C.2.5.1.6) in liver were not performed on most of these patients. However, none showed the clinical findings or the extreme elevations of serum folate previously described in other patients with isolated hypermethioninemia considered not to have hepatic MAT deficiency. Patients ascertained on biochemical grounds had no neurological abnormalities, and 27/30 had IQs or Bayley development-index scores within normal limits or were judged to have normal mental development. Methionine transamination metabolites accumulated abnormally only when plasma methionine concentrations exceeded 300-350 microM and did so more markedly after 0.9 years of age. Data were obtained on urinary organic acids as well as plasma creatinine concentrations. Patterns of inheritance of isolated hypermethioninemia were variable. Considerations as to the optimal management of this group of patients are discussed.

Molecular mechanisms of an inborn error of methionine pathway. Methionine adenosyltransferase deficiency

Methionine adenosyltransferase (MAT) is a key enzyme in transmethylation, transsulfuration, and the biosynthesis of polyamines. Genetic deficiency of alpha/beta-MAT causes isolated persistent hypermethioninemia and, in some cases, unusual breath odor or neural demyelination. However, the molecular mechanism(s) underlying this deficiency has not been clearly defined. In this study, we characterized the human alpha/beta-MAT transcription unit and identified several mutations in the gene of patients with enzymatically confirmed diagnosis of MAT deficiency. Site-directed mutagenesis and transient expression assays demonstrated that these mutations partially inactivate MAT activity. These results establish the molecular basis of this disorder and allow for the development of DNA-based methodologies to investigate and diagnose hypermethioninemic individuals suspected of having abnormalities at this locus.

Persistent hypermethioninaemia with dominant inheritance

A clinically benign form of persistent hypermethioninaemia with probable dominant inheritance was demonstrated in three generations of one family. Plasma methionine concentrations were between 87 and 475 mumol/L (normal mean 26 mumol/L; range 10-40 mumol/L); urinary methionine and homocystine concentrations were normal. Plasma homocystine, cystathionine, cystine and tyrosine were virtually normal. The concentrations in serum and urine of metabolites formed by the methionine transamination pathway were normal or moderately elevated. Methionine loading of two affected family members revealed a diminished ability to catabolize methionine, but the activities of methionine adenosyltransferase and cystathionine beta-synthase were not decreased in fibroblasts from four affected family members. Fibroblast methylenetetrahydrofolate reductase activity and its inhibition by S-adenosylmethionine were also normal, indicating normal regulation of N5-methyltetrahydrofolate-dependent homocysteine remethylation. Serum folate concentrations were not increased. The findings in this family differ from those previously described for known defects of methionine degradation. Since the hepatic and fibroblast isoenzymes of methionine adenosyltransferase differ in their genetic control, this family's biochemical findings appear consistent with a mutation in the structural gene for the hepatic methionine adenosyltransferase isoenzyme.

The S-Methylmethionine Cycle in Lemna paucicostata

The metabolism of S-methylmethionine has been studied in cultures of plants of Lemna paucicostata and of cells of carrot (Daucus carota) and soybean (Glycine max). In each system, radiolabeled S-methylmethionine was rapidly formed from labeled l-methionine, consistent with the action of S-adenosyl-l-methionine:methionine S-methyltransferase, an enzyme which was demonstrated during these studies in Lemna homogenates. In Lemna plants and carrot cells radiolabel disappeared rapidly from S-methylmethionine during chase incubations in nonradioactive media. The results of pulse-chase experiments with Lemna strongly suggest that administered radiolabeled S-methylmethionine is metabolized initially to soluble methionine, then to the variety of compounds formed from soluble methionine. An enzyme catalyzing the transfer of a methyl group from S-methylmethionine to homocysteine to form methionine was demonstrated in homogenates of Lemna. The net result of these reactions, together with the hydrolysis of S-adenosylhomocysteine to homocysteine and adenosine, is to convert S-adenosylmethionine to methionine and adenosine. A physiological advantage is postulated for this sequence in that it provides the plant with a means of sustaining the pool of soluble methionine even when overshoot occurs in the conversion of soluble methionine to S-adenosylmethionine. The facts that the pool of soluble methionine is normally very small relative to the flux into S-adenosylmethionine and that the demand for the latter compound may change very markedly under different growth conditions make it plausible that such overshoot may occur unless the rate of synthesis of S-adenosylmethionine is regulated with exquisite precision. The metabolic cost of this apparent safeguard is the consumption of ATP. This S-methylmethionine cycle may well function in plants other than Lemna, but further substantiating evidence is neeeded.

Phosphatidylcholine synthesis in the rat: the substrate for methylation and regulation by choline

Two lines of evidence led us to reexamine the possibility that methylation of phosphoethanolamine and its partially methylated derivatives, in addition to methylation of the corresponding phosphatidyl derivatives, plays a role in mammalian phosphatidylcholine biosynthesis: (a) Results obtained by Salerno and Beeler with rat [Salerno, D.M. and Beeler, D.A. (1973) Biochim. Biophys. Acta 326, 325-338] appear to strongly support such a role for methylation of phosphobases; (b) Such reactions have recently been shown to play major roles in phosphatidylcholine synthesis by higher plants [see Datko, A.H. and Mudd, S.H. (1988) Plant Physiol. 88, 854-861 and references therein]. We found that, following continuous labeling of rat liver with L-[methyl-3H]methionine for 10.4 min (intraperitoneal administration) or for 0.75 min (intraportal administration), virtually no 3H was detected in methylated derivatives of phosphoethanolamine, but readily detectable amounts of 3H were present in the base moiety of each methylated derivative of phosphatidylethanolamine. Thus, there was no indication that phospho-base methylation makes a significant contribution. Studies of cultured rat hepatoma cells showed definitively for the first time in a mammalian system that choline deprivation up-regulates the rate of flow of methyl groups originating in methionine into phosphatidylethanolamine and derivatives. Even under these conditions, methylation of phosphoethanolamine bases appeared to play a negligible role.