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

Abstract

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.

Figure 1

RT-PCR analysis of transcripts encoded by minigenes containing wild-type or mutant splice-donor site at exon III. A, Schematic showing four minigene constructs in pSVL. MAT1A sequence is represented by boxes (exons) and solid lines (introns). Oligonucleotide primer is represented by arrows; vector sequence is represented by dashes. The constructs contained 46 bp of exon III 3′-region with either a G (wild type [WT]) or an A (mutant [M]) as the last base of this exon, either 378 bp (set I) or 1,017 bp (set II) of intron III, and 88 bp of exon IV 5′-region, as described in the Methods section. The deleted intron III regions are denoted by forked lines. B, Agarose-gel electrophoretic analysis of RT-PCR products of transcripts expressed by minigene constructs. Lane 1, Products of minigene I-M. Lane 2, Products of minigene II-M. Lane 3, Products of minigene I-WT. Lane 4, Products of minigene II-WT. Lane 5, Products from COS-1 cells transfected with the vector alone. Correctly spliced products (291 bp) will include 46 bp of exon III, 88 bp of exon IV, 148 bp of vector sequence, and a 9-bp linker sequence at the 5′ end. Lane M, HaeIII digest of ΦX174 DNA. Sizes are listed to the left.

Figure 2

SSCP analyses of PCR-amplified MAT1A exons on MDE gels. Exons containing mutations, as confirmed by sequencing analyses, are shown here. A, Patient 1. B, Patient 5. C, Patient 7. D, Patient 14. E, Patients 31 and family B. F = father; M = mother.

Figure 3

Pedigree of family B, showing dominant inheritance of hypermethioninemia. Blackened symbols represent individuals carrying the single-allelic R264H mutation; unblackened symbols represent normal subjects. Values in parentheses are plasma methionine concentrations (in μM [reference range 5–35 μM]). The MAT1A exon VII haplotype, described by Chamberlin et al. (1997), is indicated at the upper right corner of each symbol.