Increasing evidence has suggested that microRNAs (miRNA) play an important role in tumorigenesis. As transcriptional regulators, altered miRNA expression may affect many cancerrelated biological pathways, indicating that miRNAs can function as tumor suppressors and/or oncogenes. We first performed a genetic association analysis by screening genetic variants in 15 miRNA genes and detected that a common sequence variant in hsa-miR-196a-2 (rs11614913, C!T) was significantly associated with decreased breast cancer risk ( for homozygous variant: odds ratio, 0.44; 95% confidence interval, 0.28-0.70). Hypermethylation of a CpG island upstream (-700 bp) of the miR-196a-2 precursor was also associated with reduced breast cancer risk (odds ratio, 0.35; 95% confidence interval, 0.15-0.81).

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As transcriptional regulators, the genes responsible for maintaining circadian rhythm exert influence in a variety of biological processes. Recently, it has been suggested that the core circadian genes may play a role in breast tumorigenesis, possibly by influencing hormone regulation or other cancer-relevant pathways. Here, we examine the role of the central circadian regulator CLOCK in breast cancer by conducting a genetic and epigenetic association study, as well as transcriptional profiling arrays and a pathway-based network analysis. Significant associations were detected between CLOCK tagging SNPs and breast cancer risk, with apparent effect modification by ER/PR status. Furthermore, hypermethylation in the CLOCK promoter was found to reduce breast cancer risk, and these findings were corroborated by publicly available tissue array data, which showed lower levels of CLOCK expression in healthy controls relative to normal or tumor tissue from breast cancer patients. Finally, we silenced CLOCK in vitro and performed a whole genome expression microarray and pathway analysis, which identified a cancer-relevant network of transcripts with altered expression following CLOCK gene knockdown. These findings support the hypothesis that circadian genes may be relevant for tumorigenesis, and suggest that circadian gene variants may represent a novel panel of breast cancer susceptibility biomarkers.

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