Less than half of all breast cancers can be explained by known risk factors.
Results from recent epidemiological studies have been largely consistent in suggesting a positive association between night work and increased risk of breast cancer in women. Th ese findings suggest a role of circadian rhythms in breast cancer development. Results from animal studies have led to the hypothesis that genetic variations in genes responsible for maintaining circadian rhythms might influence cancer risk.
The objectiv e of this project is to describe the association between night work and breast cancer risk, and the influence on risk of factors associated with shift work. Secondly, to assess associations between circadian gene variants and breast cancer risk, and third ly, to evaluate the interaction between night work and specific variants in circadian genes with respect to breast cancer risk.
The proposed design is a nested case-control study within the Norwegian Nurses' Cohort (N= 49 000), including all first occurr ence breast cancer cases diagnosed between 1990 and 2006. Information on demographic data, occupational history including work schedules, and other factors with possible implications for breast cancer risk will be collected in telephone interviews, by Sta tistics Norway. Participants will receive a saliva collection kit including instructions. With ~1 000 cases and 1 000 controls, and ~40% who have worked at night, there will be enough power to reveal even moderate risks in this material.
Identification of night work as a risk factor for breast cancer may help prevent the disease directly. While night work is inevitable, circadian rhythm disruption may be eliminated or reduced by careful shift planning or the use of appropriate lighting.
Establishment o f associations between circadian gene variations and shift work requirements at work may help identify women more susceptible to breast cancer.