Gene-environment and gene-gene interaction in the determination at plasma homocysteine levels in healthy middle-aged men

Healthy middle-aged men (n = 1470) from eight general practices across Brit ain were examined for plasma total homocysteine levels and genotyped for th e A222V polymorphism in the methylene-tetrahydrofolate (MTHFR) gene, the 68 bp insertion polymorphism in exon 8 of the cystathionine b synthase (CBS) gene and the D919G polymorphism in the methionine synthase (MS) gene. The m edian value for plasma homocysteine was 11.90 mu mol/l (25-75% Interquartil e range 10.10-14.20) for the whole sample. Smokers had significantly higher homacysteine levels than non-smokers (12.90 vs 11.70 mu mol/l and p <0.000 05) and levels significantly differed according to folate (p-value <0.00005 ), with men in the lowest quartile of folate having the highest median homo cysteine levels. Genotype at all three loci was associated with differences in plasma homocysteine level, Individuals homozygous for the MTHFR V222 al lele had 1.6 mu mol/l higher median homocysteine levels when compared to th e other two genotypes (p <0.00005), while for the CBS and MS genes, individ uals carrying one or more of the ran alleles had lower median homocysteine than individuals homozygous for the common allele (0.80 <mu>mol/l, p <0.03, and 0.70 <mu>mol/l, p <0.04 respectively). The raising effect associated w ith homozygosity for the V222 allele was greater in men in the lowest quart ile of folate (interaction between folate and genotype p = 0.02), but none of the genotype effects was significantly modulated by B12 levels. While th e raising effects of V222 and MS D919 homozygosity on homocysteine level we re essentially additive, the homocysteine lowering effect associated with t he CBS 68bp allele was seen most strongly in men homozygous for the V222 al lele (MTHFR-CBS genotype interaction p = 0.03) and the D919 allele (MS-CBS interaction p = 0.09). Age, folate, B12 and smoking explained 13.48% of the variance while the three genotypes combined and with interaction terms exp lained only an additional 2.63%. This interaction between CBS genotype and MTHFR and MS genotype points to a key role of the CBS transulphuration path way in the metabolism of homocysteine that may be particularly important as a compensatory mechanism in subjects with low dietary folate.