C677T and Al298C polymorphisms of the methylenetetrahydrofolate reductase gene: Incidence and effect of combined genotypes on plasma fasting and post-methionine load homocysteine in vascular disease

Background: Moderately increased plasma concentrations of total homocystein e (tHcy) have been shown to be an important risk factor for vascular diseas es. Two common polymorphisms of the methylenetetrahydrofolate reductase (MT HFR) gene, the thermolabile C677T and a more recently reported A1298C polym orphism, may con tribute to hyperhomocysteinemia. Methods: Using PCR and restriction fragment length polymorphism analysis, w e studied the prevalence oi: the C677T and A1298C MTHFR genotypes and the c ombined effect of these polymorphisms on plasma tHcy concentrations, as mea sured by HPLC with fluorometric detection, both fasting and post-methionine load (PML), in 1238 individuals. Results: The prevalences of the C677T and A1298C genotypes did not differ s ignificantly in 772 individuals with documented coronary artery disease (CA D), 137 individuals with deep-vein thrombosis (DVT), and 329 individuals wi thout documented vascular disease. Individuals homozygous for the 677T alle le had significantly increased fasting tHcy, particularly in the presence o f low folate, compared with individuals homozygous for the wild-type allele . Neither the 1298AC nor the 1298CC genotype was associated with significan tly increased fasting or PML tHcy concentrations irrespective of serum fola te. Of the nine combined MTHFR genotypes, six were present in >10% of the p opulation. Of these, the difference in mean fasting tHcy reached statistica l significance (P <0.005) only in individuals with the 677TT/1298AA genotyp e compared with individuals with the wild-type 677CC/1298AA genotype. Diffe rences in mean fasting tHcy did not reach statistical significance in indiv iduals heterozygous for both MTHFR variants. We detected two 677CT/1298CC a nd three 677TT/1298AC individuals; only one, an 677TT/1298AC individual, ha d increased tHcy (both fasting and PML). No individuals had the 677TT/1298C C genotype. Conclusions: The prevalences of the C677T and A1298C polymorphisms did not differ among individuals with CAD, DVT, or those without documented vascula r disease. In contrast to the C677T polymorphism, the A1298C polymorphism i s nor associated with increased fasting tHcy. Although the two polymorphism s usually exist in trans configuration, crossover may occur rarely to form recombinant chromosomes. <(c)> 2001 American Association for Clinical Chemi stry.