Sex-specific association of sequence variants in CBS and MTRR with risk for promoter hypermethylation in the lung epithelium of smokers

Abstract

Gene promoter hypermethylation is now regarded as a promising biomarker for the risk and progression of lung cancer. The one-carbon metabolism pathway is postulated to affect deoxyribonucleic acid (DNA) methylation because it is responsible for the generation of S-adenosylmethionine (SAM), the methyl donor for cellular methylation reactions. This study investigated the association of single nucleotide polymorphisms (SNPs) in six one-carbon metabolism-related genes with promoter hypermethylation in sputum DNA from non-Hispanic white smokers in the Lovelace Smokers Cohort (LSC) (n = 907). Logistic regression was used to assess the association of SNPs with hypermethylation using a high/low methylation cutoff. SNPs in the cystathionine beta synthase (CBS) and 5-methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR) genes were significantly associated with high methylation in males [CBS rs2850146 (-8283G > C), OR = 4.9; 95% CI: 1.98, 12.2, P = 0.0006] and low methylation in females [MTRR rs3776467 (7068A > G), OR = 0.57, 95% CI: 0.42, 0.77, P = 0.0003]. The variant allele of rs2850146 was associated with reduced gene expression and increased plasma homocysteine (Hcy) concentrations. Three plasma metabolites, Hcy, methionine and dimethylglycine, were associated with increased risk for gene methylation. These studies suggest that SNPs in CBS and MTRR have sex-specific associations with aberrant methylation in the lung epithelium of smokers that could be mediated by the affected one-carbon metabolism and transsulfuration in the cells.

Fig. 1.

Plot of SNPs in linkage disequilibrium with rs2850146 (large diamond). (A) Only one other SNP is in LD (R ² > 0.8) with rs2850146 within 500kb upstream and downstream. (B) Plot of rs2850146 and SNPs within 10kb indicates lack of known SNPs in CBS promoter region. Solid line indicates recombination rates.

Fig. 2.

CBS SNP rs2850146 (-8283G > C) correlates with CBS gene expression. Box plots show the range of expression in HBECs with common homozygous alleles of rs2850146 compared with HBECs with heterozygous alleles. Solid line indicates median expression; diamond indicates mean expression.