Circadian Transcription Profile of Mouse Breast Cancer Under Light-Dark and Dark-Dark Conditions

The circadian clock exists in virtually every cell and regulates key biological processes in cells and tissues. Even in cancer cells, DNA synthesis, cell division and tumor growth are gated by the circadian clock. This study examined the gene expression profiles of transplanted mouse breast tumor cells under normal light-dark (LD) as well as constant dark (DD) conditions. It was found that the overall percentage of rhythmic transcripts in breast tumor tissue was lower than that in normal tissue. Few transcripts had unaltered rhythmic expression patterns under both LD and DD conditions. Most rhythmic transcripts in DD displayed 12h or shorter periods. These results suggest that in addition to the circadian clock control of gene transcription, altering light, feeding, physical activity and other factors characteristically affect the expression of many genes. Circadian organization is highly conserved across species and is essential to an organism's well-being (1-3). Circadian rhythms are generated both directly and indirectly by a set of core circadian clock genes. The mammalian circadian clockwork mechanism includes both negative and positive transcription-translation feedback loops which generate approximately 24 h physiological cycles. Two transcription factors, CLK (or NPAS2), and BMAL1, form complexes that activate the transcription of Per (Per1, 2) and Cry (Cry1, 2) genes. The resultant PER and CRY proteins form dimers