Disruption of Circadian Rhythm and its Impact on the Progression of Colorectal Cancer in the Murine Animal Model

Colorectal cancer (CRC) is thought to be initiated by mutation of Adenomatous Polyposis Coli (APC), a tumour suppressor gene mutant in more than 80% of all human CRC cases. Loss of heterozygosity of the APC gene results in non-functioning alleles. This renders an APC protein incapable of maintaining proper Wnt signaling - critical in intestinal stem cell (ISC) homeostasis - preventing ISC differentiation and promoting a stem cell-like proliferative state. This increased Wnt activity raises Wnt target genes, including AXIN2, MYC, and the stem cell marker (LGR5) gene in tumours comparatively to adjacent tissue. Clock genes activate the transcription of their own repressors in a transcriptional/translational feedback loop (TTFL), in which CLOCK/BMAL1 drive the expression of CRY and PER whose products in turn suppress CLOCK/BMAL1. A second TTFL regulates BMAL1. The clock and its target genes confer rhythmic functions to many cellular processes including intestinal proliferation. I hypothesized that tumours will have disrupted circadian clock gene expression. To test this, APCmin/+ animal tumours and tissue were sampled over 24 hours using RT-qPCR. This revealed that circadian genes were less expressed in tumours, and that Wnt, cell cycle progression, and stem cell marker genes were more greatly expressed in tumours compared to adjacent tissue. Also, I hypothesized that tumorigenesis would increase under clock disruption. To test this, APCmin/+ were crossed with BMAL1 mice to create an APCmin; BMAL1-/- mice. These mice have a 2-fold increase in tumour counts while average size of tumours is smaller, implying disrupted clock fuels an increased induction of tumours. Together, these data suggest that tumours suppress circadian clock function, and that completely disrupted circadian clock function increases tumorigenesis