Vitamin D-related genetic variants, interactions with vitamin D exposure, and breast cancer risk among Caucasian women in Ontario.

BACKGROUND Vitamin D, from diet and sunlight exposure, may be associated with reduced breast-cancer risk. This study investigated if candidate gene variants in vitamin D pathways are associated with breast cancer risk, or modify the associations between breast cancer and vitamin D exposure. METHODS Breast cancer cases aged 25 to 74 years were identified from the Ontario Cancer Registry (histopathologically confirmed and diagnosed 2002-2003) and population-based controls were identified through random digit dialing of Ontario households. Saliva (DNA) was available for 1,777 cases and 1,839 controls. Multivariate logistic regression was used to evaluate associations between 19 single nucleotide polymorphisms (SNP) in vitamin D related genes, including vitamin D binding protein (GC), vitamin D receptor (VDR), and cytochrome P450 type 24A1 (CYP24A1). Statistical interactions were assessed using the likelihood ratio test. RESULTS Some SNPs were found to be significantly associated with breast cancer risk. For example, breast cancer risk was associated with the GC rs7041 TT genotype (age-adjusted odds ratio (OR) = 1.23; 95% CI: 1.01, 1.51) and inversely with the VDR Fok1 (rs2228570) ff genotype (OR = 0.71; 95% CI: 0.57, 0.88). Few significant gene-environment interactions were observed between dietary vitamin D and genetic variants. CONCLUSION Our study suggests certain vitamin D related genetic variants may influence breast-cancer risk and we found limited evidence that genetic variants modify the associations between vitamin D exposure and breast cancer risk. IMPACT Variation in vitamin D-related genotypes may help to explain inconsistent results from previous epidemiologic studies and may lead to targeted prevention strategies.