C677T polymorphism of methylenetetrahydrofolate reductase gene affects plasma homocysteine level and is a genetic factor of late-onset Alzheimer's disease

Background: Elevated plasma homocysteine levels are known as a risk for atherosclerotic vascular disease and venous thrombosis and have been shown as a risk for late‐onset Alzheimer's disease (LOAD). Method: To examine the effect of genetic factors predisposing to elevated plasma homocysteine levels on the occurrence of LOAD, we determined the genotype of a C677T polymorphism of methylenetetrahydrofolate reductase (MTHFR) gene and a variable number tandem repeat (VNTR) spanning exon 13–intron 13 boundary of cystathionine β‐synthase (CBS) gene in patients with LOAD and community‐based control subjects. Results: Logistic regression indicated that the MTHFR‐T allele was a risk for LOAD (P < 0.05), independently from apolipoprotein E‐?4 (APOE‐?4) allele. Kaplan–Meier tests showed that in APOE‐?4 non‐carriers, individuals with the MTHFR‐TT genotype have occurences of LOAD earlier than those with the MTHFR‐CC genotype (P < 0.05). Multiple regression analysis indicates that MTHFR‐T allele increases plasma homocysteine levels (P = 0.0002), while the number of X chromosomes decreases (P = 0.01). Plasma homocysteine level was not correlated with age, plasma albumin reflecting nutritional condition, and the dose of APOE‐?4 allele. The CBS‐20 VNTR allele showed the same trend to increase plasma homocysteine level as the MTHFR‐T allele, but a risk effect for LOAD was not evident. Conclusion: A genetic propensity for elevated plasma homocysteine levels, explained by the MTHFR‐T allele encoding defective enzymatic function, is involved in the development of LOAD, particularly in APOE‐?4 non‐carriers, and that homocysteine metabolism could be a preventive target to LOAD in the elderly.