Increased breast cancer risk at high plasma folate concentrations among women with the MTHFR 677T allele.

Background: Folate is involved in DNA synthesis and methylation and may thereby influence carcinogenesis.

Objectives: We examined plasma folate (P-folate) concentration in relation to genotypes of the folate-metabolizing enzyme methylenetetrahydrofolate reductase [MTHFR 677C-->T (rs1801133) and 1298A-->C (rs1801131)]. We also explored whether P-folate was associated with risk of postmenopausal breast cancer overall and in subgroups with genetic variants of the MTHFR single nucleotide polymorphisms (SNPs).