Glycine N-methyltransferase (GNMT) is the main enzyme responsible for catabolism of excess hepatic S-adenosylmethionine (SAMe). GNMT is absent in hepatocellular carcinoma (HCC), messenger RNA (mRNA) levels are significantly lower in livers of patients at risk of developing HCC, and GNMT has been proposed to be a tumor-susceptibility gene for liver cancer. The identification of several children with liver disease as having mutations of the GNMT gene further suggests that this enzyme plays an important role in liver function. In the current study we studied development of liver pathologies including HCC in GNMTknockout (GNMT-KO) mice. GNMT-KO mice have elevated serum aminotransferase, methionine, and SAMe levels and develop liver steatosis, fibrosis, and HCC. We found that activation of the Ras and Janus kinase ( T he first steps in mammalian methionine metabolism are conversion to S-adenosylmethionine (SAMe) and transfer of the methyl group of SAMe to a large variety of substrates (including DNA, RNA, histones, and small molecules such as glycine, guanidinoacetate, and phosphatidylethanolamine) with the formation of S-adenosylhomocysteine (SAH), an inhibitor of many SAMe-dependent methyltransferases. 1 Although there are a large number of SAMe-dependent methyltransferases, 2 methylation of glycine by glycine Nmethyltransferase (GNMT) to form sarcosine (N-methylglycine) is one of the reactions that contribute most to total transmethylation flux. 3 The importance of GNMT is to remove excess SAMe and maintain a constant hepatic SAMe/SAH ratio to avoid aberrant methylation. 2 Consistent with this function, the activation of GNMT in rats by the administration of retinoic acid causes a reduction in plasma methionine and homocysteine levels, as well as in liver DNA methylation. 4,5 In GNMT-knockout (GNMT-KO) mice, liver SAMe content is elevated 35-fold, and the SAMe/SAH ratio increases about 100-fold, 6 and individuals with GNMT mutations, which leads to inactive forms of the enzyme, have elevated plasma levels of methionine and SAMe but a normal concentration of homocysteine. 7,8 GNMT is expressed in the liver, pancreas, and prostate 9 and is absent in hepatocellular carcinoma (HCC) 10 and down-regulated in the livers of patients at risk of Abbreviations: GNMT, HCC, hepatocellular carcinoma; H3K27me3, trimethylated Received September 10, 2007; accepted November 26, 2007. Supported by NIH grants AA12677, AA13847, and AT-1576 (to S.C.L. and J.M.M.); DK15289 (to C.W.), PN IϩD SAF 2005-00855, HEPADIP-EULSHM-CT-205, and ETORTEK 2005 (to J.M.M. and M.L.M.-C.); Program Ramón y Cajal (to M.L.M.-C.); and Fundación "La Caixa" (to M.L.M.-C., R.M., and A.M.A.).

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 M) without tyrosine elevation. Plasma total homocysteine was very slightly elevated for an infant to 14.5-15.9 M. In plasma, S-adenosylmethionine was 30-fold and AdoHcy 150-fold elevated. Activity of AdoHcy hydrolase was Ϸ3% 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.