Cigarette smoking, cytochrome P450 1A1 polymorphisms, and breast cancer risk in the Nurses' Health Study

Environmental exposure to carcinogens may contribute to increasing breast cancer rates and geographic variation in breast cancer incidence in the United States. One class of chemicals that has received much attention are the polyaromatic hydrocarbons that are ubiquitous in the environment and occur in cigarette smoke. The cytochrome P450 1A1 (CYP1A1) gene codes for an enzyme that contributes to aryl hydrocarbon hydroxylase activity, which is involved in the metabolism of polyaromatic hydrocarbons. Genotypic variants of CYP1A1 have been associated with increased aryl hydrocarbon hydroxylase activity, and some epidemiological studies suggest that women with the variant genotype(s) are at increased risk for breast cancer. We prospectively evaluated the associations between the CYP1A1 polymorphisms and breast cancer risk, as well as the potential modification of these associations by cigarette smoking, in a case-control study nested within the Nurses' Health Study. We analyzed the T→C transition at nucleotide 6235 (MspI) and the A→G transition at nucleotide 4889 (exon 7) in CYP1A1 by PCR-RFLP assays among 466 incident breast cancer cases and 466 matched controls. Relative risks (RRs) and 95% confidence intervals (CIs) were used to quantify the risk of breast cancer among subjects who had at least one variant allele relative to subjects who were homozygous for the wild-type allele, using conditional logistic regression. No overall increase in breast cancer risk with the variant CYP1A1 genotypes was apparent (RR(MspI), 1.05; 95% CI, 0.74-1.50 and RR(exon7), 0.88; 95% CI, 0.58-133). However, a suggestive increase in breast cancer risk was observed among women who hod commenced smoking before the age of 18 and had the CYP1A1-MspI variant genotype compared to nonsmokers who were homozygous wild type for the polymorphism (RR, 5.65; 95% CI, 1.50-21.3; percentage of all breast cancer cases attributable to this risk factor, 2.5%). A similar gene-environment association was observed for the exon 7 polymorphism (RR, 3.61; 95% CI, 1.11-11.7; percentage of all breast cancer cases attributable to this risk factor, 2.2%). These data are compatible with the hypothesis that cigarette smoking early in life is a modifiable cause of breast cancer in a subpopulation of genetically susceptible women. However, the proportion of breast cancer attributable to cigarette smoking at a young age among Caucasian women with the variant form of the CYP1A1 polymorphisms is low.