Altered DNA methylation of repetitive elements may explain the modified risk of cervical intraepithelial neoplasia (CIN) associated with genetic polymorphisms of the folate metabolic pathway in the US post folic acid fortification era

Abstract

Despite the controversy regarding the adverse effects of exposure to higher folate concentrations on health outcomes particularly cancer, there has been paucity of research to understand the biological mechanisms affected by such exposures in relation to the risk of developing pre-cancer or cancer. We have recently addressed this concern by demonstrating that higher plasma folate concentrations were not associated with greater risk of being diagnosed with higher grades of cervical intraepithelial neoplasia (CIN 2+) especially in women with sufficient plasma vitamin B12 concentrations. Higher plasma folate was also associated with a pre-cancer protective level of peripheral blood mononuclear cells (PBMC) long interspersed nuclear elements-1 (LINE-1) methylation, a validated surrogate marker of global DNA methylation status. However, it is unknown whether these results could be reproduced using homocysteine (Hcy), a functional indicator of both folate and vitamin B12 status and whether genetic polymorphisms in the folate metabolic pathway (FMP) may have an effect on these associations in a population of women exposed to FA fortification program. The objectives of this research were to determine the (1) association between Hcy and risk of CIN 2+ (2) the association between MTHFR C677T, TS 3’UTR 6bp deletion and DHFR 19bp deletion polymorphisms and the risk of CIN 2+ (3) modifying effects of plasma folate and vitamin B12 on the association between the genetic polymorphisms and the risk of CIN 2+ (4) the association between the genetic polymorphisms and pre-cancer/cancer associated epigenetic biomarkers and DNA damage markers (5) modifying effects of plasma folate and vitamin B12 on the association between the genetic polymorphisms and these biomarkers. Higher Hcy was associated with increased risk of CIN 2+ and a lower degree of PBMC LINE-1 methylation. Women with MTHFR 677CT or TT genotype and lower plasma folate were at a reduced risk for CIN 2+ and had lower degree of PBMC LINE-1methylation compared to women with MTHFR 677 CT or TT genotype and higher plasma folate. In conclusion, even in the FA fortification era, not all women have adequate folate status to overcome the adverse effects of higher Hcy and MTHFR C677T. PBMC LINE-1 methylation is a useful marker to assess the risk of CIN 2+ in individuals having MTHFR C677T polymorphism.

Keywords: folate, fortification, tHcy, genetic polymorphisms, CIN, PBMC LINE-methylation