Biological and Functional Consequences of Single Nucleotide Polymorphisms of the O6-Methylguanine-DNA-Methyltransferase Gene

Single nucleotide polymorphisms (SNPs) in DNA repair genes could alter the transcriptional levels, structure and function of DNA repair proteins and alter DNA repair proficiency. Consequently, these SNPs could significantly influence the level of genetic damage, which is an early critical factor in the cascade of events leading to cancer. Molecular epidemiological studies indicate that SNPs in the 06-Methylguanine-DNA-Methyltransferase (MGMT) gene, which repairs alkyl adducts at the 06-position of guanine, may be associated with an increased risk of lung cancer. However, the functional and biological significance of these SNPs has yet to be systematically characterized. We used two biologically relevant endpoints in an exposed population of 350 individuals to determine the association between genetic damage and SNPs in MGMT; chromosome aberrations to examine macrolesions, and mutation frequency to examine microlesions. In addition, we used the luciferase reporter assay to determine effects of SNPs in the promoter/enhancer region of MGMT on promoter activity. Coding SNPs had a marginal effect on macrolesion damage after exposure to alkylating agents. However a strong effect of coding SNPs on microlesion damage was observed. The luciferase expression data demonstrated a significant increase in promoter activity in the presence of the enhancer SNP compared to the wild-type form. This indicates that, in a biological system, inheritance of one copy of these SNPs could affect the level of genetic damage, especially after accumulated exposure to alkylating agents. While the coding SNPs evaluated are predicted to be detrimental, as indicated by the accumulation of genetic damage, the promoter/enhancer SNP evaluated may be protective as levels of cellular protein would be increased.