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.