Cell-autonomous circadian DNA damage response

Keywords: :

• DNA damage
• apoptosis
• cancer
• cell cycle checkpoint
• circadian system
• transformation

This article refers to:

Effect of circadian clock mutations on DNA damage response in mammalian cells

The role of the circadian clock in cancer development and progression is intimately linked to the role of the circadian system in genotoxic stress response.¹ A few original reports (reviewed in ref. 2) support the idea that disruption of the circadian clock may cause the development of cancer. Several key cancer-related genes and signaling pathways have been found to be potential targets for the clock. Circadian clock-dependent regulation of proliferation, cell cycle, apoptosis, DNA damage response and DNA repair have been proposed as potential molecular mechanisms.³ Circadian clock proteins, specifically the periods, have been added to the list of tumor suppressors.² However, with a growing number of experimental works, more and more clouds are appearing in an originally clear sky of the circadian clock/cancer connection. No effect of circadian disruption on the rate of tumorigenesis has been reported in model systems, and in some cases, even the opposite effects were observed.³ It is necessary to mention that different groups were using different model systems, different experimental approaches and worked with different circadian clock genes, which adds to the growing discrepancy in the literature. Recent attempts to investigate the effect of several circadian clock genes on tumorigenesis in the same study⁴ has also been challenged;⁵ therefore, questions about the clock and cancer are still open. In agreement with the role of genotoxic stress response in cancer, original in vivo and in cell culture reports (reviewed in refs. 1 and 2) demonstrated the regulation of DNA damage-associated pathways by the clock proteins, but later reports (reviewed in ref. 3) made the picture more complicated.

In the present paper by Gaddameedhi and collegues,⁶ the authors made an attempt to study, systemically, the role of the circadian clock proteins in the DNA damage response. The authors used three different DNA damaging agents, which produce three different types of DNA damage and activate three different response pathways. Cells deficient for the components of the positive arm of the molecular circadian clockwork (Clock and Bmal1) and of the negative arm (Periods and Cryptochromes) have been treated with the indicated agents; cell survival, the effects on cell cycle checkpoints, apoptosis and DNA repair were all assayed. With minor exceptions, the authors did not find any significant difference between wild-type and circadian clock-deficient cells. Together, with recently published works^7,8 on the sensitivity of Crptochrome1,2^−/− and Bmal1^−/− cells to DNA damaging agents, the present work argues that in contrast to the observed difference in sensitivity to genotoxic agents in vivo, there is no effect of circadian disruption in the cell-autonomous response.

What is the reason for the difference between in vivo and in cell culture-based experiments? One possibility suggested by the authors is that in culture, the cells lose circadian rhythms very fast, due to the loss of synchronization between cells. Another interpretation, also discussed by the authors, is that the absence of circadian clock-dependent control of systemic factors, such as growth factors or hormones, is an inherent difficulty of cell culture, and may affect the response to genotoxic stress. There is at least one more interpretation—tissue specificity of the response. In most of the experiments,^6-8 the authors used mouse embryonic or adult fibroblasts; fibroblasts are one of the most popular models to study DNA damage response and results and conclusions obtained with this system have been often extrapolated as universal. However, fibroblasts are not a major contributor to an organism’s sensitivity to genotoxic stress in vivo. Lymphatic and epithelial tissues are two major sites of damage, which determine sensitivity in vivo. Tissue specificity of the response to DNA damage upon circadian clock disruption has been suggested as a potential interpretation of existing conflicting results.³ Another possibility is the difference in the effect on normal and transformed cells. The authors partially addressed this by demonstrating the absence of any effect of genetic manipulation with Per1 expression in cancer cell lines NCI-H460 and HCT-116. But suppression of Bmal1 expression in the same HCT-116 cell line resulted in an increased resistance to irradiation.⁹ Therefore, to “close the case,” it is necessary to study the consequences of circadian disruption on the response to DNA damage in cells of epithelial and lymphatic origin, fibroblasts in vivo and tumor cells.