Polymorphisms in genes involved in oxidative stress and their interactions with lifestyle factors on skin cancer risk.

Ultraviolet (UV)-induced oxidative stress has been implicated in skin carcinogenesis [1]. Several antioxidant enzymes, such as glutathione peroxidase (GPX) and catalase (CAT), counteract oxidative damage and constitute a primary defense against oxidative stress. GPX is a soluble selenoprotein that reduces H2O2 and organic hydroperoxides to H2O, and GPX1 is the most abundant and ubiquitous intracellular isoform [1]. GPX1 activity is not strongly affected by UV and is considered to be the most important antioxidant enzyme defense mechanism in the skin [2]. CAT is a heme enzyme that neutralizes reactive oxygen species by converting H2O2 to H2O and O2. CAT activity in the skin is significantly reduced after exposure to UV [2], which suggests its effect may be prone to effect modification by environmental factors. Inherited variants in the encoding genes that affect the activity or expression of these antioxidant enzymes are hypothesized to modulate oxidative stress and thus influence skin cancer risk. A polymorphism in the GPX1 gene (Pro198Leu, rs1050450) and a polymorphism in the promoter region of the CAT gene (C-262T, rs1001179) have been shown to be associated with lower enzyme activities of their encoded enzymes [3, 4]. To test our main hypothesis that these two genetic polymorphisms are associated with skin cancer risk, we conducted a nested case-control study of Caucasians (218 melanoma, 285 squamous cell carcinoma (SCC), and 300 basal cell carcinoma (BCC) cases, and 870 age-matched controls) within the Nurses’ Health Study. We further investigated potential gene-environment interactions between these polymorphisms and lifestyle factors such as dietary antioxidant intake and sun exposure related risk factors. A detailed description of the characteristics of cases and controls was published previously [5]. Information on dietary intake was collected prospectively by food-frequency questionnaires, and total-energy-adjusted cumulative average of dietary intake was used to reduce within-person variation and represent long-term dietary intake [6]. We genotyped the two single nuclear polymorphisms (SNPs) (rs1050450 and rs1001179) by the 5′ nuclease assay (TaqMan®) in 384-well format, using the ABI PRISM 7900HT Sequence Detection System (Applied Biosystems, Foster City, CA). The distributions of genotypes for the two SNPs were in Hardy-Weinberg equilibrium among controls (p=0.94, 0.83, respectively). We compared the cases of each type of skin cancer to the common control series. We used unconditional multivariate logistic regression to model the association between genetic polymorphisms and skin cancer risk and to estimate multivariate Odds Ratios (ORs) and 95% Confidence Intervals (CIs). To test statistical significance of gene-environmental interactions, we used dominant model for genotypes and dichotomized environmental exposures as low versus high based on median values among controls. We tested the statistical significance of a single multiplicative interaction term. In the main effect analysis (Table 1), we observed that the GPX1 198 Leu/Leu genotype was significantly associated with a two-fold increased risk of melanoma (OR, 2.14; 95% CI, 1.22–3.72), after adjustment for age and other covariates. No association was found between this polymorphism and SCC or BCC risk, which was consistent with one previous study [7]. This polymorphism has been shown to be associated with lung cancer [8] and breast cancer [3] previously. We did not observe significant association between the CAT C-262T polymorphism and the risk of any type of skin cancer. Table 1 Association between GPX Pro198Leu and CAT C-262T genetic polymorphisms and skin cancer risk a As exploratory analyses, we further tested gene-environment interactions between the genetic variants and lifestyle factors that modulate oxidative stress. We found the association between the CAT C-262T polymorphism and melanoma risk was significantly modified by history of severe sunburns (p for interaction, 0.008, Table 2), a variable combining exposure intensity and biological response to sun exposure. The positive association between history of severe sunburns and melanoma risk was restricted to T carriers (OR, 1.73; 95% CI, 1.02–2.92), compared to women with CC genotype (OR, 1.03; 95% CI, 0.63–1.69). We also observed a significant gene-diet interaction between the CAT C-262T polymorphism and total carotenoid intake on melanoma risk (p for interaction, 0.01). The inverse association of total carotenoid intake with melanoma risk was limited among women with CC genotype (OR, 0.63; 95% CI, 0.41–0.97), whereas no association was observed among T carriers (OR, 1.23; 95% CI, 0.77–1.97). Inconsistent results were reported on the relationship between dietary carotenoid intake and melanoma risk in several previous case-control studies. An inverse association between the intake and the risk of melanoma was observed in some studies [9], but not in other studies [10]. Our results suggest that the inconsistency in the literature may reflect a potential gene-diet interaction. As we tested different genetic polymorphisms, multiple environmental exposures and dietary factors, and three types of skin cancer, multiple testing in our study may lead to false positive results. Replications in independent studies are needed to validate these results. No significant interactions were observed between the GPX1 polymorphism and these lifestyle factors on melanoma risk. We did not observe any significant interaction between these genetic variants and environmental exposures on the risk of SCC or BCC. Table 2 Interaction between the CAT C-262T genetic polymorphism and history of severe sunburns and total carotenoid intake on melanoma risk In summary, we first observed the GPX1 198 Leu/Leu genotype was significantly associated with a two-fold increased risk of melanoma, and the association between the CAT C-262T polymorphism and melanoma risk was significantly modified by history of severe sunburns and total carotenoid intake. Further research is needed to confirm these possible associations and illustrate the underlying molecular mechanisms.