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Investigation of the role of alcohol- metabolizing genes and DNA repair genes in the increase of levels of N2-ethylidenedeoxyguanosine

Background: Studies have shown a correlation between alcohol drinking and formation of N2-ethylidenedeoxyguanosine (N2-ethylidene-dG), a specific acetaldehyde derived DNA adduct. These results contribute to the hypothesis of a major role of acetaldehyde in the mechanisms related to alcohol drinking carcinogenesis. Association of the levels of this adduct with ALDH2 genes has also been demonstrated. In this study we investigated possible influences on the levels of N2-ethylidene-dG of variants in genes encoding for enzymes involved in ethanol metabolism and in DNA repair. Methods: 127 subjects with different drinking habits were selected from controls of ARCAGE a large multicenter case-control study on head and neck cancers conducted in Europe. N2-ethylidene-dG was measured as its reduced form N2-ethyldG by LC-ESI-MS/MS-SRM, performing the enzyme-catalyzed hydrolysis of DNA in the presence of NaBH3CN. On the same samples, nine polymorphisms in four genes involved in ethanol metabolism (ADH1B, ADH1C ADH7 ADH4) were analysed by TaqMan assay and 19 polymorphisms in 12 DNA repair genes (XRCC1, XRCC2, XRCC3, XRCC9, LIG3, PCNA, MGMT, OGG1, ERCC1, ERCC4, POLB, APEX), were analysed by APEX. Genes were selected according to preliminary data of an association with alcohol related cancer susceptibility. The effect of SNPs on the risk of having high levels of DNA adducts was investigated. Results: Levels of N2-ethyldG showed a dose response relationship across categories of alcohol drinking (p = 0.02). No association between SNPs and the risk of having high levels of N2-ethyldG was observed when considering variants in genes involved in ethanol metabolism. In addition, no association between these SNPs and high alcohol intake was detected. Within the group of SNPs in DNA repair genes analysed, variant alleles in the XRCC1 gene showed a significant influence on the levels of N2-ethyldG. When adjusting for sex, age, alcohol intake and smoking status, an increased risk of high adduct levels was observed for XRCC1 rs915927 on exon 8 (OR 2.54, 95% CI 1.08-5.92, p=0.03) and XRCC1 rs762507 on intron 5 (OR 2.77, 95% CI 1.16-6.58, p=0.02), while a decreased risk was observed for XRCC1 rs25487 on exon 10 (OR 0.38, 95% CI 0.16-0.83, p=0.01). When considering the combination of risk alleles, subjects with more than one risk allele had an increased risk (OR 3.9, 95% CI 1.48-10.48, p=0.006). Conclusion: These results suggest a role of XRCC1 in the repair pathway relevant to N2-ethylidene-dG formation. Further studies with larger number of subjects are needed to better understand the influence on the formation of N2-ethylidene-dG of ADH genes alone and in combination with ALDH genes. Additional investigation of the repair pathway for N2-ethylidene-dG is needed focusing also on possible synergistic or additive effects of different SNPs involved.

alcohol-metabolising genes; DNA-repair genes; N2-ethylenedeoxyguanosine

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