Methionine adenosyltransferase I/III deficiency: Long-term follow-up and treatment of 3 adult siblings.

Methionine adenosyltransferase I/III deficiency, also known as Mudd's disease, is a rare inborn error of methionine metabolism. Because pathophysiological mechanisms of the disease remain poorly understood, the consequences of this disorder and the need for medical management remain uncertain; likewise, the effect of medical interventions on clinical outcomes in Mudd's disease is largely unknown due to a relative lack of published longitudinal clinical data. There are few reports of adults in the medical literature affected with this disease. Clinical symptoms of reported adults range from asymptomatic to individuals with neurological, developmental, or behavioral symptoms. Here we report three siblings affected with Mudd's disease that were ascertained following an abnormal newborn screen for hypermethioninemia in the case of our index patient. All three had a variable degree of longstanding neurologic or psychiatric symptoms which had not prompted a clinical investigation for a genetic or metabolic disorder prior to identification through our clinic. While the causal association of these symptoms to the metabolic disorder remains unclear in these cases, all three patients demonstrated a degree of amelioration of symptoms and/or improvement in measurements on standardized psychiatric ratings scales when specific therapy for the metabolic disorder was instituted. The symptoms, treatment, and outcomes over the course of six years of follow-up are presented here, expanding the possible natural history of Mudd's disease.

[Newborn screening and variant analysis for methionine adenosyltransferase I/III deficiency].

OBJECTIVE: To summarize newborn screening for methionine adenosyltransferase I/III (MAT I/III) deficiency in Quanzhou region of Fujian Province. METHODS: A total of 364 545 neonates were screened for inherited metabolic diseases by tandem mass spectrometry. High-throughput next generation sequencing combined with Sanger sequencing was used to detect potential variants in newborns with MAT I/III deficiency. Pathogenicity of suspected variants was predicted by using MutationTaster and HSF software. RESULTS: Three newborns were identified with MAT I/III deficiency by newborn screening, which yielded an incidence rate of 1 in 121 515. Amino acid and acylcarnitine analysis suggested that the serum methionine of the three patients have increased to various extents. All patients showed normal growth and development during follow-up, and were found to carry MAT1A gene variants including two missense variants [c.776C>T (p.Ala259Val) and c.791G>A (p.Arg264His)] and a synonymous variant [c.360C>T (p.Cys120Cys)]. Among these, c.776C>T (p.Ala259Val) and c.791G>A (p.Arg264His) were known to be pathogenic, whereas c.360C>T (p.Cys120Cys) was a novel variant. Bioinformatics analysis suggested that this variant may alter RNA splicing and affect the structure and function of the MAT1A protein. CONCLUSION: A systematic review of newborn screening for MAT I/III deficiency was provided. Discovery of the novel variant has enriched the variant profile of the MAT1A gene and provided a basis for the diagnosis of this disease.

Neonatal methionine adenosyltransferase I/III deficiency with abnormal signal intensity in the central tegmental tract.

Methionine adenosyltransferase I/III (MAT I/III) deficiency is characterized by persistent hypermethioninemia. The clinical manifestations in cases with MAT I/III deficiency vary from a complete lack of symptoms to neurological problems associated with brain demyelination. We experienced a neonatal case with MAT I/III deficiency, in which severe hypermethioninemia was detected during the newborn screening test. The patient gradually showed hyperreflexia, foot clonus, and irritability from the age of 1 month onwards, and his brain magnetic resonance imaging scans showed abnormal signal intensity in the bilateral central tegmental tracts. His neurological manifestations improved after the S-adenosylmethionine (SAMe) treatment, deteriorated after discontinuation of SAMe, and re-improved owing to re-administration of SAMe. He achieved normal neurodevelopment through SAMe and methionine restriction therapy. Lack of SAMe as well as severe hypermethioninemia were thought to contribute towards the clinical psychophysical state. Moreover, impaired MAT I/III activity contributed to the development of neurological disorder from the early neonatal period.

Complementation of a metK-deficient E. coli strain with heterologous AdoMet synthetase genes.

S-adenosyl-l-methionine (AdoMet) is an essential metabolite, playing a wide variety of metabolic roles. The enzyme that produces AdoMet from l-methionine and ATP (methionine adenosyltransferase, MAT) is thus an attractive target for anti-cancer and antimicrobial agents. It would be very useful to have a system that allows rapid identification of species-specific inhibitors of this essential enzyme. A previously generated E. coli strain, lacking MAT (∆metK) but containing a heterologous AdoMet transporter, was successfully complemented with heterologous metK genes from several bacterial pathogens, as well as with MAT genes from a fungal pathogen and Homo sapiens. The nine tested genes, which vary in both sequence and kinetic properties, all complemented strain MOB1490 well in rich medium. When these strains were grown in glucose minimal medium, growth delays or defects were observed with some specific metK genes, defects that were dramatically reduced if l-methionine was added to the medium.

Consensus recommendations for the diagnosis, treatment and follow-up of inherited methylation disorders.

Inherited methylation disorders are a group of rarely reported, probably largely underdiagnosed disorders affecting transmethylation processes in the metabolic pathway between methionine and homocysteine. These are methionine adenosyltransferase I/III, glycine N-methyltransferase, S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. This paper provides the first consensus recommendations for the diagnosis and management of methylation disorders. Following search of the literature and evaluation according to the SIGN-methodology of all reported patients with methylation defects, graded recommendations are provided in a structured way comprising diagnosis (clinical presentation, biochemical abnormalities, differential diagnosis, newborn screening, prenatal diagnosis), therapy and follow-up. Methylation disorders predominantly affect the liver, central nervous system and muscles, but clinical presentation can vary considerably between and within disorders. Although isolated hypermethioninemia is the biochemical hallmark of this group of disorders, it is not always present, especially in early infancy. Plasma S-adenosylmethionine and S-adenosylhomocysteine are key metabolites for the biochemical clarification of isolated hypermethioninemia. Mild hyperhomocysteinemia can be present in all methylation disorders. Methylation disorders do not qualify as primary targets of newborn screening. A low-methionine diet can be beneficial in patients with methionine adenosyltransferase I/III deficiency if plasma methionine concentrations exceed 800 µmol/L. There is some evidence that this diet may also be beneficial in patients with S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. S-adenosylmethionine supplementation may be useful in patients with methionine adenosyltransferase I/III deficiency. Recommendations given in this article are based on general principles and in practice should be adjusted individually according to patient's age, severity of the disease, clinical and laboratory findings.

Glycine N-methyltransferase expression in the hippocampus and its role in neurogenesis and cognitive performance.

The hippocampus is a brain area characterized by its high plasticity, observed at all levels of organization: molecular, synaptic, and cellular, the latter referring to the capacity of neural precursors within the hippocampus to give rise to new neurons throughout life. Recent findings suggest that promoter methylation is a plastic process subjected to regulation, and this plasticity seems to be particularly important for hippocampal neurogenesis. We have detected the enzyme GNMT (a liver metabolic enzyme) in the hippocampus. GNMT regulates intracellular levels of SAMe, which is a universal methyl donor implied in almost all methylation reactions and, thus, of prime importance for DNA methylation. In addition, we show that deficiency of this enzyme in mice (Gnmt-/-) results in high SAMe levels within the hippocampus, reduced neurogenic capacity, and spatial learning and memory impairment. In vitro, SAMe inhibited neural precursor cell division in a concentration-dependent manner, but only when proliferation signals were triggered by bFGF. Indeed, SAMe inhibited the bFGF-stimulated MAP kinase signaling cascade, resulting in decreased cyclin E expression. These results suggest that alterations in the concentration of SAMe impair neurogenesis and contribute to cognitive decline.

Spectrum of mutations associated with methionine adenosyltransferase I/III deficiency among individuals identified during newborn screening in Japan.

Methionine adenosyltransferase I/III deficiency (MAT I/III deficiency) is an inborn error of metabolism that results in isolated persistent hypermethioninemia. Definitive diagnosis is now possible by molecular analyses of the MAT1A gene. Based on newborn screening (NBS) data collected between 2001 and 2012 in Hokkaido, Japan, the estimated incidence of MAT I/III deficiency was 1 in 107,850. 24 patients (13 males, 11 females) from 11 prefectures in Japan were referred to our laboratory for genetic diagnosis of MAT I/III deficiency. They were all found between 1992 and 2012 by the NBS program in each region. In these 24 individuals, we identified 12 distinct mutations; 14 patients were heterozygous for an R264H mutation; R264H caused an autosomal dominant and clinically benign phenotype in each case. The mutations in the other 10 patients showed autosomal recessive inheritance and included eight novel MAT1A mutations. Putative amino acid substitutions at R356 were observed with six alleles (three R356P, two R356Q, and one R356L). MAT I/III deficiency is not always benign because three of our cases involved brain demyelination or neurological complications. DNA testing early in life is recommended to prevent potential detrimental neurological manifestations.

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.

Cell division, one-carbon metabolism and methionine synthesis in a metK-deficient Escherichia coli mutant, and a role for MmuM.

An E. coli K-12 mutant deficient in S-adenosylmethionine (SAM) synthesis, i.e ΔmetK, but expressing a rickettsial SAM transporter, can grow in glucose minimal medium if provided with both SAM and methionine. It uses the externally provided (R)-enantiomer of SAM as methyl donor to produce most but not all of its methionine, by methylation of homocysteine catalysed by homocysteine methyltransferase (MmuM). The ΔmetK cells are also altered in growth and are twice as long as those of the parent strain. When starved of SAM, the mutant makes a small proportion of very long cells suggesting a role of SAM and of methylation in the onset of crosswall formation.

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.

Methionine adenosyltransferase I/III deficiency: neurological manifestations and relevance of S-adenosylmethionine.

Methionine adenosyltransferase I/III (MAT I/III) deficiency, caused by mutations in the MAT1A gene, is an inherited metabolic disorder characterized by persistent hypermethioninemia, usually detected by newborn mass screening. There is a wide range of clinical manifestations, from completely asymptomatic to neurological problems associated with brain demyelination. Physiological role of S-adenosylmethionine (SAM), the metabolic product of methionine catalyzed by MAT, in the central nervous system has been investigated in vivo and in vitro, and case reports demonstrated an effectiveness of supplementary treatment of SAM in the improvement of neurological development and myelination. Methionine restriction can be an additional therapeutic strategy because hypermethioninemia alone may be neurotoxic; however, lowering methionine carries a risk to decrease the synthesis of SAM.

S-adenosylmethionine treatment in methionine adenosyltransferase deficiency, a case report.

Reported is a female patient with methionine adenosyltransferase I/III (MAT I/III) deficiency, who was found to have pronounced hypermethioninemia on newborn mass spectroscopy screening, and had two compound heterozygous missense mutations in the gene encoding human MAT1A protein. Hypermethioninemia persisted and her mental development was deficient. At 4 years and 8 months, we started with the supplementary treatment of S-adenosylmethionine, the metabolic product of methionine catalyzed by MAT, which was effective in her neurological development.

Fibroblast growth factor receptor-mediated activation of AKT-ß-catenin-CBP pathway regulates survival and proliferation of murine hepatoblasts and hepatic tumor initiating stem cells.

UNLABELLED: Fibroblast Growth Factor (FGF)-10 promotes the proliferation and survival of murine hepatoblasts during early stages of hepatogenesis through a Wnt-ß-catenin dependent pathway. To determine the mechanism by which this occurs, we expanded primary culture of hepatoblasts enriched for progenitor markers CD133 and CD49f from embryonic day (E) 12.5 fetal liver and an established tumor initiating stem cell line from Mat1a(-/-) livers in media conditioned with recombinant (r) FGF10 or rFGF7. FGF Receptor (R) activation resulted in the downstream activation of MAPK, PI3K-AKT, and ß-catenin pathways, as well as cellular proliferation. Additionally, increased levels of nuclear ß-catenin phosphorylated at Serine-552 in cultured primary hepatoblasts, Mat1a(-/-) cells, and also in ex vivo embryonic liver explants indicate AKT-dependent activation of ß-catenin downstream of FGFR activation; conversely, the addition of AKT inhibitor Ly294002 completely abrogated ß-catenin activation. FGFR activation-induced cell proliferation and survival were also inhibited by the compound ICG-001, a small molecule inhibitor of ß-catenin-CREB Binding Protein (CBP) in hepatoblasts, further indicating a CBP-dependent regulatory mechanism of ß-catenin activity. CONCLUSION: FGF signaling regulates the proliferation and survival of embryonic and transformed progenitor cells in part through AKT-mediated activation of ß-catenin and downstream interaction with the transcriptional co-activator CBP.

Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase.

ATP-dependent cob(I)alamin adenosyltransferase (ATR) is a bifunctional protein: an enzyme that catalyzes the adenosylation of cob(I)alamin and an escort that delivers the product, adenosylcobalamin (AdoCbl or coenzyme B(12)), to methylmalonyl-CoA mutase (MCM), resulting in holoenzyme formation. Failure to assemble holo-MCM leads to methylmalonic aciduria. We have previously demonstrated that only 2 equiv of AdoCbl bind per homotrimer of ATR and that binding of ATP to the vacant active site triggers ejection of 1 equiv of AdoCbl from an adjacent site. In this study, we have mimicked in the Methylobacterium extorquens ATR, a C-terminal truncation mutation, D180X, described in a patient with methylmalonic aciduria, and characterized the associated biochemical penalties. We demonstrate that while k(cat) and K(M)(Cob(I)) for D180X ATR are only modestly decreased (by 3- and 2-fold, respectively), affinity for the product, AdoCbl, is significantly diminished (400-fold), and the negative cooperativity associated with its binding is lost. We also demonstrate that the D180X mutation corrupts ATP-dependent cofactor ejection, which leads to transfer of AdoCbl from wild-type ATR to MCM. These results suggest that the pathogenicity of the corresponding human truncation mutant results from its inability to sequester AdoCbl for direct transfer to MCM. Instead, cofactor release into solution is predicted to reduce the capacity for holo-MCM formation, leading to disease.

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 ß-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.

Activation of LKB1-Akt pathway independent of phosphoinositide 3-kinase plays a critical role in the proliferation of hepatocellular carcinoma from nonalcoholic steatohepatitis.

UNLABELLED: LKB1, originally considered a tumor suppressor, plays an important role in hepatocyte proliferation and liver regeneration. Mice lacking the methionine adenosyltransferase (MAT) gene MAT1A exhibit a chronic reduction in hepatic S-adenosylmethionine (SAMe) levels, basal activation of LKB1, and spontaneous development of nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC). These results are relevant for human health because patients with liver cirrhosis, who are at risk to develop HCC, have a marked reduction in hepatic MAT1A expression and SAMe synthesis. In this study, we isolated a cell line (SAMe-deficient [SAMe-D]) from MAT1A knockout (MAT1A-KO) mouse HCC to examine the role of LKB1 in the development of liver tumors derived from metabolic disorders. We found that LKB1 is required for cell survival in SAMe-D cells. LKB1 regulates Akt-mediated survival independent of phosphoinositide 3-kinase, adenosine monophosphate protein-activated kinase (AMPK), and mammalian target of rapamycin complex (mTORC2). In addition, LKB1 controls the apoptotic response through phosphorylation and retention of p53 in the cytoplasm and the regulation of herpesvirus-associated ubiquitin-specific protease (HAUSP) and Hu antigen R (HuR) nucleocytoplasmic shuttling. We identified HAUSP as a target of HuR. Finally, we observed cytoplasmic staining of p53 and p-LKB1(Ser428) in a NASH-HCC animal model (from MAT1A-KO mice) and in liver biopsies obtained from human HCC derived from both alcoholic steatohepatitis and NASH. CONCLUSION: The SAMe-D cell line is a relevant model of HCC derived from NASH disease in which LKB1 is the principal conductor of a new regulatory mechanism and could be a practical tool for uncovering new therapeutic strategies.

CD133+ liver cancer stem cells from methionine adenosyl transferase 1A-deficient mice demonstrate resistance to transforming growth factor (TGF)-beta-induced apoptosis.

UNLABELLED: Methionine adenosyltransferase (MAT) is an essential enzyme required for S-adenosylmethionine biosynthesis. Hepatic MAT activity falls during chronic liver injury, and mice lacking Mat1a develop spontaneous hepatocellular carcinoma by 18 months. We have previously demonstrated that CD133(+)CD45(-) oval cells isolated from 16-month-old Mat1a(-/-) mice represent a liver cancer stem cell population. The transforming growth factor beta (TGF-beta) pathway constitutes a central signaling network in proliferation, apoptosis, and tumorigenesis. In this study, we tested the response of tumorigenic liver stem cells to TGF-beta. CD133(+)CD45(-) oval cells were isolated from premalignant 16-month-old Mat1a(-/-) mice by flow cytometry and expanded as five clone lines derived from a single cell. All clone lines demonstrated expression of both hepatocyte and cholangiocyte markers and maintained a small population (0.5% to 2%) of CD133(+) cells in vitro, and three of five clone lines produced tumors. Although TGF-beta1 inhibited cell growth equally in CD133(-) and CD133(+) cells from each clone line, the CD133(+) population demonstrated significant resistance to TGF-beta-induced apoptosis compared with CD133(-) cells. Furthermore, CD133(+) cells demonstrated a substantial increase in mitogen-activated protein kinase (MAPK) pathway activation, as demonstrated by phosphorylated extracellular signal-regulated kinase levels before and after TGF-beta stimulation. MAPK inhibition using mitogen-activated protein kinase kinase 1 (MEK1) inhibitor PD98059 led to a significant increase in TGF-beta-induced apoptosis in CD133(+) cells. Conversely, a constitutively active form of MEK1 blocked the apoptotic effects of TGF-beta in CD133(-) cells. CONCLUSION: CD133(+) liver cancer stem cells exhibit relative resistance to TGF-beta-induced apoptosis. One mechanism of resistance to TGF-beta-induced apoptosis in CD133(+) cancer stem cells is an activated mitogen-activated protein kinase/extracellular signal-regulated kinase pathway.

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 ß-synthase (CßS) 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.

Methionine metabolism and liver disease.

In the early 1930s, Banting and Best, the discoverers of insulin, found that choline could prevent the development of fatty liver disease (steatosis) in pancreatectomized dogs treated with insulin. Later work indicated that in rats and mice, diets deficient in labile methyl groups (choline, methionine, betaine, folate) produced fatty liver and that long-term administration of diets deficient in choline and methionine also caused hepatocellular carcinoma. These experiments not only linked steatosis and diabetes but also provided evidence, for the first time, of the importance of labile methyl group balance to maintain normal liver function. This conclusion is now amply supported by the observation of mice devoid of key enzymes of methionine and folate metabolism and in patients with severe deficiencies in these enzymes. Moreover, treatments with various methionine metabolites in experimental animal models of liver disease show hepatoprotective properties.