Associations between single nucleotide polymorphisms in folate uptake and metabolizing genes with blood folate, homocysteine, and DNA uracil concentrations

Abstract

Background: Folate is an essential nutrient that supports nucleotide synthesis and biological methylation reactions. Diminished folate status results in chromosome breakage and is associated with several diseases, including colorectal cancer. Folate status is also inversely related to plasma homocysteine concentrations -- a risk factor for cardiovascular disease.

Objective: We sought to gain further understanding of the genetic determinants of plasma folate and homocysteine concentrations. Because folate is required for the synthesis of thymidine from uracil, the latter accumulating and being misincorporated into DNA during folate depletion, the DNA uracil content was also measured.

Design: Thirteen single nucleotide polymorphisms (SNPs) in genes involved in folate uptake and metabolism, including folate hydrolase (FOLH1), folate polyglutamate synthase (FPGS), gamma-glutamyl hydrolase (GGH), methylene tetrahydrofolate reductase (MTHFR), methionine synthase (MTR), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC1), were studied in a cohort of 991 individuals.

Results: The MTHFR 677TT genotype was associated with increased plasma homocysteine and decreased plasma folate. MTHFR 1298A>C and RFC1 intron 5A>G polymorphisms were associated with significantly altered plasma homocysteine concentrations. The FOLH1 1561C>T SNP was associated with altered plasma folate concentrations. The MTHFR 677TT genotype was associated with a approximately 34% lower DNA uracil content (P = 0.045), whereas the G allele of the GGH -124T>G SNP was associated with a stepwise increase in DNA uracil content (P = 0.022).

Conclusion: Because the accumulation of uracil in DNA induces chromosome breaks, mutagenic lesions, we suggest that, as for MTHFR C677T, the GGH -124 T>G SNP may modulate the risk of carcinogenesis and therefore warrants further attention.

Figures

FIGURE 1

Proteins involved in the uptake, retention, and metabolism of folate. FOLH1, folate hydrolase 1 (γ carboxy peptidase II); FOLR1, folate receptor 1; FPGS, folate polyglutamate synthase; GGH, γ-glutamyl hydrolase; MTHFR, methylene tetrahydrofolate reductase; MTR, methionine synthase; PCFT (SLC46A1), proton-coupled folate transporter; RFC1 (SLC19A1), reduced folate carrier 1; 1-C, one-carbon.

FIGURE 2

Association of single nucleotide polymorphisms (SNPs) in methylene tetrahydrofolate reductase (MTHFR), reduced folate carrier 1 (RFC1), and folate hydrolase 1 (FOLH1) with plasma folate and homocysteine (Hcy) concentrations. Values are expressed as adjusted means ± SEMs. Hcy values were adjusted for age, sex, alcohol intake, plasma folate, plasma pyridoxal phosphate, vitamin B-12, and creatinine. Folate data were adjusted for age, sex, smoking status, alcohol intake, and dietary folate equivalent intake. *Log homocysteine and folate significantly different (P ≤ 0.05, general linear model) from other 2 genotypes combined (dominant/recessive) after adjustment for the abovementioned factors (n = 875–929). Note that for the FOLH1 1561C>T SNP (C), the CT and TT genotypes were combined because n = 2 for the TT genotype.

FIGURE 3

Association of the γ-glutamyl hydrolase (GGH) single nucleotide polymorphisms rs11545076 genotype with blood DNA uracil concentrations. Values are expressed as means ±SEMs. n = 128, 92, and 18 for the TT, TG, and GG genotypes, respectively. Uracil data were adjusted for age, sex, plasma folate, vitamin B-12, and plasma pyridoxal phosphate. *P for trend (additive model) = 0.022 and 0.058 for the untransformed and log-transformed models, respectively.