The role of circadian clock genes in leukemia

Circadian rhythm is present in human and all eukaryotes whose 24-hour cycle of behavior and physiology changes are driven by an autonomous circadian clock within the organisms. The mammalian circadian clock is organized in a hierarchical manner. The master pacemaker, located at suprachiasmatic nucleus (SCN), and the core circadian clock genes constitute the circadian oscillator and circadian rhythms (1). The SCN clock is entrained to the 24-hour day by the daily light-dark cycle through the retina-to-SCN neural pathway (2) and the peripheral rhythms in the peripheral oscillators of body cells are possibly driven or synchronized by the SCN (3). Maintaining a proper circadian clock function is crucial for an organism to respond to light and dark cycle properly. Circadian oscillators use transcriptional-translational feedback loops that rely on positive and negative elements in oscillators, therefore the strong daily cycling of clock gene and clock controlled genes is characteristic of circadian systems. Epigenetic regulations (e.g., post-transcriptional regulation, posttranslational modifications, chromatin remodeling), availability of clock proteins, and regulation of intracellular localization are intricately regulated to ensure the precise periodicity of the clock to close to 24 hours (4).

It is believed the cell proliferation, DNA repair, angiogenesis and immune functions are controlled by the molecular clock (5). Indeed, many epidemiological studies have linked altered circadian rhythms in shift workers to cancer susceptibility, including breast (6), ovarian (7), and prostate (8) cancer. Therefore, it is not surprising to know the altered expression of circadian clock genes in almost all types of cancers including our previous reports of chronic myeloid leukemia (CML) (9,10), acute leukemia (11), head and neck squamous cell carcinoma (HNSCC) (12,13), and gastric cancer (14). Studies of animal models (15) and human cancers have established that disrupted circadian rhythm is an essential endogenous factor contributing to cancer development (16). Although the underlying mechanism has not been fully understood, they are generally considered as tumor suppressors. Mice deficient in the Per2 gene were cancer prone and showed a markedly increased rate in tumor development and reduced apoptosis after irradiation (15). In addition, epigenetic silencing of many core circadian clock genes has been described in various cancers, such as CML (9), breast cancer (17), hepatocellular carcinoma (18).

In hematopoietic cancers, the regulatory role of circadian clock genes has not yet been entirely clarified. In a recent study, the circadian rhythm transcription factors BMAL1 and CLOCK was shown to be required for the growth of leukemia stem cell of acute myeloid leukemia (AML) and disruption of circadian pathway components could produce anti-leukemic effects (19). The authors used shRNA silencing of BMAL1 and CLOCK or pharmacological inhibition of BMAL1 transcription to disrupt the integrity of circadian rhythms in both murine and human AML cells and demonstrated the induction of myeloid differentiation and impair cell-cycle progression (19). This study established a novel pro-tumorigenic role for circadian clock genes in AML. However, our previous reports have shown that core circadian clock genes are more likely to act as tumor-suppressors in leukemias, where down-regulated expression of most of the genes were observed (9-11). We have observed a decreased expression of BMAL1 in AML (11) and CML (9,10) but when patients achieved remission the expression of BMAL1 did not recover. Also, the daily pattern expression of BMAL1 was disrupted in patients with CML but the recovery of expression pattern was not observed in patients after imatinib mesylate treatment and achieved remission (10). Therefore, it is speculated that BMAL1 is essential for leukemia but may not be essential for normal hematopoiesis. Moreover, the expression of CLOCK in CML, AML and acute lymphoid leukemia (ALL) was not different from healthy individuals (9-11) and the expression of CLOCK did not display time-dependent variations in peripheral blood leukocytes of either healthy individuals or patients with CML (10). In consistent with our findings, CLOCK was found to be arrhythmic in peripheral blood mononuclear cells (20) and bone marrow CD34⁺ cells (21). Although BMAL1 and CLOCK were identified as essential transcription factors mediating leukemia stem cell growth and self-renewal, it seems to be conflict with the fact that expression of BMAL1 was down-regulated in patients with AML. It is possible that the degrees of dependence on core circadian clock genes are different for normal and leukemic stem cells.

According to our experience in leukemias, PER3 might be more decisive than BMAL1 and CLOCK in maintaining circadian rhythm than we have expected. PER3 was the most down-regulated circadian clock genes in AML, ALL and CML and recovery of PER3 was correlated with better clinical outcome (11). In patients with CML, we found the daily pattern expression of PER3 was disrupted in patients with newly-diagnosed pre-imatinib mesylate-treated and crisis-phase patients and when patients achieved complete cytogenetic response or major molecular response, a partial recovery of PER3 expression was observed (10). PER3 has been demonstrated to be required for CHEK2 activation and PER3 overexpression led to increased apoptosis and inhibition of cell proliferation (22). Since circadian clock has been shown to control the expression of cell cycle-related genes (23), it is reasonable to hypothesize that PER3 may have regulatory functions in cell-cycle progression and coupled to circadian rhythms. Since asynchrony of cell proliferation between normal and malignant tissues was commonly observed (24) and loss of circadian rhythmicity was commonly seen in patients with advanced cancers, disrupted rhythms of circadian clock gene expression may disturb the balance of the restrains of cell division resulting in prosurvival and proliferation of tumor cells. Tumor cells may accelerate their own growth by disrupting circadian rhythms of normal cells and establish their own rhythms, so restoring circadian rhythms in cancer patients should improve their prognoses. It will be interesting to investigate if once PER3 is restored it will further adjust the disrupted circadian clock back to its normal restrains.

As circadian clock has been shown to be controlled by sumoylation of BMAL1 (25) and deregulated methylation status of circadian clock genes has also been demonstrated in various types of cancers, the mechanisms of transcriptional and epigenetic regulation are considered critical for better understanding of the circadian clock regulation and its deregulation during leukemogenesis. Therefore, the future goal for solving the leukemic mechanism will be to uncover the interactions between circadian clock and epigenome of leukemia. Hopefully, it will provide a better understanding for leukemogenesis as well as for better references for therapeutic designs.

Acknowledgments

Funding: This work was supported by the Ministry of Science and Technology of Taiwan (MOST 104-2320-B-182-018 to MYY); Chang Gung Memorial Hospital (CMRPD8C0911 to MYY); and Kaohsiung Medical University Hospital (KMUH102-2T03 to SFL).

Footnote

Provenance and Peer Review: This article was commissioned and reviewed by the Section Editor Xia Fang (Department of Hematology, Shanghai Tongji Hospital, Tongji University School of Medicine, Shanghai, China).

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/tcr.2016.07.46). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Albrecht U. Timing to perfection: the biology of central and peripheral circadian clocks. Neuron 2012;74:246-60. [Crossref] [PubMed]
2. Weaver DR. The suprachiasmatic nucleus: a 25-year retrospective. J Biol Rhythms 1998;13:100-12. [Crossref] [PubMed]
3. Reppert SM, Weaver DR. Coordination of circadian timing in mammals. Nature 2002;418:935-41. [Crossref] [PubMed]
4. Cermakian N, Sassone-Corsi P. Multilevel regulation of the circadian clock. Nat Rev Mol Cell Biol 2000;1:59-67. [Crossref] [PubMed]
5. Lévi F, Okyar A, Dulong S, et al. Circadian timing in cancer treatments. Annu Rev Pharmacol Toxicol 2010;50:377-421. [Crossref] [PubMed]
6. He C, Anand ST, Ebell MH, et al. Circadian disrupting exposures and breast cancer risk: a meta-analysis. Int Arch Occup Environ Health 2015;88:533-47. [Crossref] [PubMed]
7. Bhatti P, Cushing-Haugen KL, Wicklund KG, et al. Nightshift work and risk of ovarian cancer. Occup Environ Med 2013;70:231-7. [Crossref] [PubMed]
8. Zhu Y, Zheng T, Stevens RG, et al. Does "clock" matter in prostate cancer? Cancer Epidemiol Biomarkers Prev 2006;15:3-5. [Crossref] [PubMed]
9. Yang MY, Chang JG, Lin PM, et al. Downregulation of circadian clock genes in chronic myeloid leukemia: alternative methylation pattern of hPER3. Cancer Sci 2006;97:1298-307. [Crossref] [PubMed]
10. Yang MY, Yang WC, Lin PM, et al. Altered expression of circadian clock genes in human chronic myeloid leukemia. J Biol Rhythms 2011;26:136-48. [Crossref] [PubMed]
11. Yang MY, Lin PM, Hsiao HH, et al. Up-regulation of PER3 Expression Is Correlated with Better Clinical Outcome in Acute Leukemia. Anticancer Res 2015;35:6615-22. [PubMed]
12. Hsu CM, Lin SF, Lu CT, et al. Altered expression of circadian clock genes in head and neck squamous cell carcinoma. Tumour Biol 2012;33:149-55. [Crossref] [PubMed]
13. Hsu CM, Lin PM, Lai CC, et al. PER1 and CLOCK: potential circulating biomarkers for head and neck squamous cell carcinoma. Head Neck 2014;36:1018-26. [Crossref] [PubMed]
14. Hu ML, Yeh KT, Lin PM, et al. Deregulated expression of circadian clock genes in gastric cancer. BMC Gastroenterol 2014;14:67. [Crossref] [PubMed]
15. Fu L, Pelicano H, Liu J, et al. The circadian gene Period2 plays an important role in tumor suppression and DNA damage response in vivo. Cell 2002;111:41-50. [Crossref] [PubMed]
16. Evans JA, Davidson AJ. Health consequences of circadian disruption in humans and animal models. Prog Mol Biol Transl Sci 2013;119:283-323. [Crossref] [PubMed]
17. Chen ST, Choo KB, Hou MF, et al. Deregulated expression of the PER1, PER2 and PER3 genes in breast cancers. Carcinogenesis 2005;26:1241-6. [Crossref] [PubMed]
18. Lin YM, Chang JH, Yeh KT, et al. Disturbance of circadian gene expression in hepatocellular carcinoma. Mol Carcinog 2008;47:925-33. [Crossref] [PubMed]
19. Puram RV, Kowalczyk MS, de Boer CG, et al. Core Circadian Clock Genes Regulate Leukemia Stem Cells in AML. Cell 2016;165:303-16. [Crossref] [PubMed]
20. Kusanagi H, Hida A, Satoh K, et al. Expression profiles of 10 circadian clock genes in human peripheral blood mononuclear cells. Neurosci Res 2008;61:136-42. [Crossref] [PubMed]
21. Tsinkalovsky O, Smaaland R, Rosenlund B, et al. Circadian variations in clock gene expression of human bone marrow CD34+ cells. J Biol Rhythms 2007;22:140-50. [Crossref] [PubMed]
22. Im JS, Jung BH, Kim SE, et al. Per3, a circadian gene, is required for Chk2 activation in human cells. FEBS Lett 2010;584:4731-4. [Crossref] [PubMed]
23. Matsuo T, Yamaguchi S, Mitsui S, et al. Control mechanism of the circadian clock for timing of cell division in vivo. Science 2003;302:255-9. [Crossref] [PubMed]
24. Granda TG, Lévi F. Tumor-based rhythms of anticancer efficacy in experimental models. Chronobiol Int 2002;19:21-41. [Crossref] [PubMed]
25. Cardone L, Hirayama J, Giordano F, et al. Circadian clock control by SUMOylation of BMAL1. Science 2005;309:1390-4. [Crossref] [PubMed]