Sean Rodich

Introduction.   The circadian rhythm is an endogenously generated, roughly 24-hour cycle regulating various physiological, biochemical, and cellular changes that occur throughout any given day. At a cellular level, one important process impacted by circadian rhythm genes (or ‘clock genes’) is the cell cycle. Thus, disturbances to the circadian rhythm may impact cell cycle regulation, resulting in a hyper-proliferative, carcinogenic state within the cell.^{1, 2} Studies have revealed a link between altered expression patterns of clock genes such as BMAL1 and PER2 and increased incidence of numerous types of cancer, particularly breast cancer.^3-5 Expanding on this knowledge, other studies have focused on using specific clock genes (namely PER2⁶ and CRY1-2⁷) as targets for breast cancer treatment. Methods.   In pathogenic investigational studies, DNA microarrays were used to quantify mRNA levels for several circadian-controlled genes in four different subtypes of human breast cancer (HER2/neu, triple negative, and two different types of luminal) as well as one non-cancerous breast epithelial cell control throughout a 48-hour period. This data was used to identify clock genes potentially linked to breast cancer and to compare changes in specific clock gene expression common among the subtypes analyzed.³ In targeted treatment studies, either esiRNA⁶ or KS15⁷ was used to create PER2- and CRY1-2- knockdown human breast cancer cells, respectively. MTT cell viability assays and wound healing assays were then used to evaluate the chemosensitivity of the cancer cells upon exposure to a chemotherapeutic agent (doxorubicin) ineffective against similar PER+/CRY1-2+ controls. Results.    Pathogenic investigational studies show that the peak mRNA levels for BMAL1 and PER2 over a given 24-hour period were significantly lower in four different breast cancer subtypes than those in a non-cancerous control. Additionally, the times of the day at which those peaks were reached were reached were relatively independent of one another, unlike the control.³ Targeted treatment studies reveal a dose-dependent increase in chemosensitivity of otherwise-resistant human breast cancer cells to doxorubicin upon exposure to either esiRNA⁶ or KS15⁷ and subsequent genetic knockdown of PER2 or CRY1-2, respectively.  Conclusions.  Together, these studies suggest that circadian rhythmicity is significantly altered in multiple breast cancer subtypes, despite evidence that there is still some capacity for clock gene oscillation intact. Further, pharmacologic agents developed against clock genes such as PER2 or CRY1-2 may be useful in the treatment of breast cancer as an adjunct to doxorubicin, particularly in patients where doxorubicin monotherapy proves to be ineffective due to chemoresistance.

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3. Gutierrez-Monreal, M. A.; Trevino, V.; Moreno-Cuevas, J. E.; Scott, S. P., Identification of circadian-related gene expression profiles in entrained breast cancer cell lines. Chronobiol Int 2016, 33 (4), 392-405.
4. Reszka, E.; Przybek, M.; Muurlink, O.; Peplonska, B., Circadian gene variants and breast cancer. Cancer Lett 2017, 390, 137-145.
5. Kiessling, S.; Beaulieu-Laroche, L.; Blum, I. D.; Landgraf, D.; Welsh, D. K.; Storch, K. F.; Labrecque, N.; Cermakian, N., Enhancing circadian clock function in cancer cells inhibits tumor growth. BMC Biol 2017, 15 (1), 13.
6. Mitchell, M. I.; Engelbrecht, A. M., Circadian Rhythms and Breast Cancer: The Role of Per2 in Doxorubicin-Induced Cell Death. J Toxicol 2015, 2015, 392360.
7. Chun, S. K.; Chung, S.; Kim, H. D.; Lee, J. H.; Jang, J.; Kim, J.; Kim, D.; Son, G. H.; Oh, Y. J.; Suh, Y. G.; Lee, C. S.; Kim, K., A synthetic cryptochrome inhibitor induces anti-proliferative effects and increases chemosensitivity in human breast cancer cells. Biochem Biophys Res Commun 2015, 467 (2), 441-6.