NORMAL BRAIN MYELINATION IN A PATIENT HOMOZYGOUS FOR A MUTATION THAT ENCODES A SEVERELY TRUNCATED METHIONINE ADENOSYLTRANSFERASE I III/

Two isozymes of mammalian methionine adenosyltransferase, MAT I and MA T III, are expressed solely in adult liver. They are, respectively, te tramers and dimers of a single subunit encoded by the gene MAT1A. A th ird isozyme, MAT II, contains a catalytic subunit encoded by a separat e gene, MAT2A, and is expressed in a variety of tissues, including (to a slight extent) adult liver. Based on a recent finding that 2 childr en with isolated hypermethioninemia and brain demyelination were homoz ygous for MAT1A mutations predicted to produce severely truncated prot eins, and devoid of activity when expressed, it was concluded that com plete lack of MAT I/III activity may be associated with neurological s ymptoms and demyelination, We now report that a 43-year-old man with p ersistent isolated hypermethioninemia, previously demonstrated to have deficient MAT activity in his liver, has normal brain myelination on MRI and normal neurological function, despite being homozygous for a 5 39 TG insertion in exon V of MAT1A, so that the gene is predicted to e ncode a protein of only 184 rather than the normal 395 amino acids. Th is patient's exon V mutation was demonstrated by SSCP analysis and ver ified by sequencing, Both parents are heterozygous for the same insert ion. This suggests that MAT1A mutations producing severely truncated p roteins do not necessarily produce brain demyelination, This finding h as relevance to a previously reported 4-year-old girl who was also hom ozygous for the 539insTG mutation, Finally, our patient's 7% residual hepatic MAT activity, measured at 1 mM methionine, may reflect the hep atic activity of the more ubiquitous enzyme form. (C) 1998 Wiley-Liss, Inc.(1)