Association of methylenetetrahydrofolate reductase polymorphism C677T and dietary folate with the risk of cervical dysplasia.

Epidemiological studies have been inconsistent regarding a role for folate in the etiology of cervical dysplasia. Methylenetetrahydrofolate reductase (MTHFR) catalyzes the synthesis of 5-methyltetrahydrofolate, which is involved in the methylation of homocysteine to methionine. A common variant of this enzyme, resulting from a 677C-->T (Ala-->Val) substitution in the gene, has been shown to have reduced activity and is associated with mild hyperhomocysteinemia. A multiethnic case-control study was used to examine the association of dietary folate and MTHFR genotype with the odds ratios (ORs) for cervical dysplasia among women identified from several clinics on Oahu, Hawaii, between 1992 and 1996. We collected blood samples for DNA extraction, cervical smears for cytological diagnosis, exfoliated cervical cells for human papillomavirus (HPV) DNA testing, and personal interviews from 150 women with squamous intraepithelial lesions (SILs) and from 179 women with cytologically normal (Pap) smears. We found a positive, monotonic trend (P = 0.02) in the ORs for cervical SILs associated with the number of variant MTHFR T alleles, after multivariate adjustment. Women with the heterozygous CT genotype had twice the risk of cervical SILs [OR, 2.0; 95% confidence interval (CI), 1.1-3.7], and women with the homozygous TT genotype had almost three times the risk of SILs (OR, 2.9; 95% CI, 1.0-8.8) compared to women with the homozygous MTHFR CC genotype. The dietary intakes of folate, vitamin B(6), and vitamin B(12) were inversely related to the ORs for cervical SILs, after adjustment for HPV DNA and other confounders. The OR among women in the highest quartile compared with women in the lowest quartile of folate intake was 0.3 (95% CI, 0.1-0.7; P for trend = 0.002). Women with the variant T allele and folate intakes below the median were at significantly elevated risk of cervical SILs (OR, 5.0; 95% CI, 2.0-12.2) compared to women with CC alleles and folate intakes above the median. HPV infection was a strong risk factor for cervical dysplasia, particularly among women with the variant T allele (OR, 46.6; 95% CI, 15.9-136.2). All associations of MTHFR genotype with the ORs for cervical SILs were independent of other risk factors under study. These findings suggest that the MTHFR T allele and reduced dietary folate may increase the risk for cervical SILs.