Defects in human methionine synthase in cblG patients

Abstract

Inborn errors resulting in isolated functional methionine synthase deficiency fall into two complementation groups, cblG and cblE. Using biochemical approaches we demonstrate that one cblG patient has greatly reduced levels of methionine synthase while in another, the enzyme is specifically impaired in the reductive activation cycle. The biochemical data suggested that low levels of methionine synthase activity in the first patient may result from mutations in the catalytic domains of the enzyme, reduced transcription, or generation of unstable message or protein. Using Northern analysis, we demonstrate that the molecular basis for the biochemical phenotype in this patient is associated with greatly diminished steady-state levels of methionine synthase mRNA. The biochemical data on the second patient cell line implicated mutations specific to reductive activation, a function that is housed in the C-terminal AdoMet-binding domain and the intermediate B12-binding domain, in the highly homologous bacterial enzyme. We have detected two mutations in a compound heterozygous state, one that results in conversion of a conserved proline (1173) to a leucine residue and the other a deletion of an isoleucine residue (881). The crystal structure of the C-terminal domain of the Escherichia coli MS predicts that the Pro to Leu mutation could disrupt activation since it is embedded in a sequence that makes direct contacts with the bound AdoMet. Deletion of isoleucine in the B12-binding domain would result in shortening of a beta-sheet. Our data provide the first evidence for mutations in the methionine synthase gene being culpable for the cblG phenotype. In addition, they suggest directly that mutations in methionine synthase can lead to elevated homocysteine, implicated both in neural tube defects and in cardiovascular diseases.

The Allele Frequencies of Three Polymorphisms in Genes Involved in Homocysteine Metabolism in a Group of Unrelated Healthy Singaporeans

“…The MS gene has been cloned, sequenced and located, and several polymorphisms have also been identified [4,[30][31][32]. The most prevalent polymorphism is the A2756G SNP that lead to a substitution of aspartic acid by glycine [4].…”

Investigation of inter-individual variability of the one-carbon folate pathway: a bioinformatic and genetic review

“…Mutations in the MTR gene result in methylcobalamin deficiency G (cblG) disorder, which is characterized by homocystinuria, and elevated homocysteine and depleted methionine levels. 52,53 A common non-synonymous variant (p.P1173L) has been detected in 465% of cblG patients. 54 In addition, 13 novel mutations, including 5 deletions and 2 nonsense mutations were identified, which resulted in the synthesis of truncated proteins that lacked portions critical for enzyme function.…”

Outcomes of four patients with homocysteine remethylation disorders detected by newborn screening

“…At 4 months of age, his parents gave consent to a skin biopsy for complementation studies of fibroblasts, the results of which were consistent with functional methionine synthase (MTR) deficiency. 7,8 The diagnosis was verified by the identification of two alterations in MTR encoding methionine synthase: a known mutation c.3518C>T (p.P1173L) 9 and a novel variant c.3109G>C (p.A1037P), which has not been described previously. The amino acid residue at position 1037 resides in one of the β-strands located in the activation domain of MTR protein.…”