The Associations Between Single Nucleotide Polymorphism Of CLOCK Gene And The Susceptibility Of Breast Cancer

ObjectivesBreast cancer is one of the most frequent cancers among women. Its risk factors include menstrual status, parity, diet and family history, et al. Circadian disruption has been indicated as a risk factor for breast cancer in recent epidemiologic studies. The molecular bases of circadian rhythm are several clock genes that formed transcription-translation feedback loops. We hypothesized that there were the associations between single nucleotide polymorphism of CLOCK gene and susceptibility of breast cancer and intended to investigate other risk factors of breast cancer.MethodsA hospital-based case-control study was conducted in Tianjin Medical University Cancer Hospital and Institute from January 2006 to September 2007. A total of 814 breast cancer patients and 943 healthy controls enrolled in this study.(1) All information was collected by questionnaire in face-to-face interview, which included demographic data, menstrual status, and disease history, family history of cancer, behavior pattern, dietary habit, and social psychological character, clinical data and laboratory data.(2) We used the blood samples collected from cases and controls. Genomic DNA was extracted using TKM method and validated by electrophoresing on 0.8% agarose gel.(3) Polymorphism of CLOCK gene was assessed by Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP). PCR products were subjected to electrophoresis on 2% agarose gel.(4) Two-sided chi square test was used to identify Hardy-Weinberg equivalence and frequency differences between groups. Odds ratios and their 95% confidence intervals for breast cancer were derived using univariate and multivariate unconditional logistic regression analysis. Results(1) Frequency of genotype CC, CT and TT is 36.24%,49.63% and 14.13% in cases separately, and 39.34%,47.51% and 13.15% in controls. There's no significant difference between groups.(2) Results from univatiate logistic regression analysis showed that the selected polymorphism of CLOCK gene was not significantly associated with the risk of breast cancer.(3) Genotype and allele frequencies did not significantly differ between breast cancer patients and healthy controls except in higher parity strata (χ2=6.751,P=0.034).(4) The carriers of the TT genotype with more than one parity could increase the risk of breast cancer by 65% after adjusted by environmental factors (aOR=1.65, 95%CI=1.07-2.55); The carriers of the TT genotype with longer period of breast feeding could increase the risk of breast cancer by 68% after adjusted by environmental factors (aOR=1.68,95%CI=1.10-2.58).(5) Multiple analysis indicated that later married (OR=3.193), higher education level (OR=4.499), early age of menarche (OR=1.880), shorter period of breast feeding (OR=1.446), induced abortion (OR=2.637), history of benign disease (OR=4.915), family history (OR=3.719), smoking (OR=7.149), lack of physical activity (OR=2.076) and eating more meat (OR=1.955) had a higher increase of risk of breast cancer, while more parity (OR=0.125) and eating more vegetables (OR=0.603) may reduce breast cancer risk.ConclusionsThe results suggest that CLOCK (rs 3805151) polymorphism may have significant associations with the risk of breast cancer among the carriers of the TT genotype with more parity and longer period of breast feeding.