The MTHFR gene encodes the vitamin-dependent enzyme, methylenetetrahydrofolate reductase.
There are 3 common SNPs giving rise to MTHFR alleles:
- rs1801133, also known as C677T or A222V
- rs1801131, also known as A1298C or E429A
- rs2274976, also known as G1793A or R594Q
It is likely that there are numerous other variations that are much rarer. When the MTHFR genes of 564 individuals of diverse ethnicities were fully sequenced, in addition to the 3 common alleles mentioned above, 11 other nonsynonymous changes were found, each with a frequency under 1%. Four of these 11 rarer alleles affected enzyme function (as did A222V) based on tests in yeast, most of which could be fixed with higher folate supplementation (in yeast; this is not yet tested in humans). Since all five impaired alleles map to the N-terminal catalytic domain of the enzyme, it seems likely that additional SNPs causing nonsynonymous changes in this region could have similar effects.10.1073/pnas.0802813105
SNP rs1801133, A222V, is not thought to be a major risk factor for neural tube defects [PMID 17035141] except in certain populations. For example, Dutch and Irish populations indicate an increased risk for (T;T) carriers [PMID 10090889], but this same genotype (ie carriers of the thermolabile T allele) appears to have a protective effect in an Italian population studied [PMID 12111380].