GC is a major health issue and remains a leading cause of cancer death worldwide. Although early GC has a good prognosis, most patients are diagnosed at advanced stages with dismal outcome. To improve the survival in GC patients, early detection and subsequent surveillance are essential. Endoscopy with biopsy is the gold standard in currently available screening and diagnostic tools. Future studies should focus on the incorporation of molecular biomarkers into clinical management to forecast the cancer stage, prognosis and improve outcome, especially in advanced-stage GC patients.
Circadian rhythms are endogenously generated rhythms that occur with a periodicity of approximately 24 hours and play an important role in regulating the daily rhythms of human physiology and behaviors. The disruption of circadian rhythms is considered a contributory factor in many clinical conditions including sleeping disorders, gastrointestinal diseases, metabolic syndrome, inflammation and even cancers . Observational studies have revealed that working a rotating night shift at least three nights per month for 15 or more years increases the risk of CRC in women . Night shift work also increases the risk of breast cancer  and endometrial cancer  in women. Therefore, researchers have considered a possible link between molecular clock machinery and some aspects of carcinogenesis such as angiogenesis, cell proliferation, apoptosis and DNA repair . Indeed, aberrant expression of circadian clock genes has been observed in CRC , breast cancer , and endometrial cancer ; however, associations between the expression of circadian clock genes and GC have not been reported in the literature.
In this study, we observed an up-regulation of PER2 in GCs. PER2 play an important role in tumor suppression and DNA damage response in vivo. Our previous studies have revealed down-regulation of PER2 in HCC , CML [19, 20], HNSCC , and breast cancer  but not in endometrial cancer . Recently, reduced PER2 expression has also been reported in pancreatic cancer  and CRC . Down-regulated expression of PER2 has been found in many cancers in both humans and mice [30, 31] and often considered a tumor suppressor gene; however, we cannot explain why upregulated expression of PER2 has, to date, been found only in GC. Indeed, the roles of circadian clock genes in the mechanism of carcinogenesis remain to be clarified. The role of PER2 as a tumor suppressor may not be applicable in all cancers.
In this study, we also observed an up-regulation of CRY1 in more advanced stage GC but not in earlier stage. CRY1 is a component of the negative circadian feedback loop and is essential for the maintenance of circadian rhythm . CRY1 participates in cell cycle regulation and the cellular response to DNA damage by controlling the expression of certain cell cycle genes . Deregulated CRY1 expression has also been observed in CML [19, 20] and HNSCC  but not in HCC  or endometrial cancers . A 2013 study by Yu et al. found up-regulated expression of CRY1 in CRC cancer tissues compared with that in adjacent noncancerous tissues in 168 CRC patients . Higher CRY1 expression was found in patients with lymph node metastasis and more advanced stages. The authors also found higher expression of CRY1 correlated positively with poor patient outcomes. In vitro study, they found overexpressed CRY1 of CRC cells promote cell proliferation and migration. In mouse study, nude mice had more obvious tumor growth after subcutaneously injecting overexpressed CRY1 of human CRC cells compared to that in control group. Their results suggested CRY1 plays an important role in CRC development and progression both in humans and mice, and may be a prognostic biomarker in CRC . Similar to these findings in CRC, our study showed CRY1 overexpression in more advanced GC. A statistical significance was not reached for higher CRY1 expression indicating a poor prognosis, but the results may be limited by the small number of patients in our study. It is necessary to collect more cases in the future to validate the relationship of CRY1 expression and GC cancer stage. CRY1 expression may be considered a useful biomarker for determining cancer stage and prognosis in GC patients.
A correction between patients’ survival days and the expression level of PER3 was also observed in our study. PER1, PER2 and PER3 genes belong to the same Period gene family. PER1 and PER2 are important in regulating the circadian clock [7, 9, 27] but the exact role of PER3 has not been well described. It has been shown that the PER1, PER2, PER3 and Dec1 genes are expressed in a similar circadian manner in human peripheral blood mononuclear cells, with the peak level occurring during the habitual time of activity  suggesting that the oscillation of PER3 may also be an essential factor in maintaining circadian rhythm. Besides, altered PER3 expression has been reported in various cancers, including CML [19, 20], HNSCC , HCC , and CRC . Further investigations of PER3 function may reveal the direct links between deregulation of PER3 and prognosis in GC patients.
Down-regulation of one or more circadian clock genes has been found in most cancers, which is in contrast to our findings. Although an aberrant circadian rhythm in malignant tissues is commonly observed, what is the exact mechanism through disrupted circadian rhythm to carcinogenesis remains to be clarified. Gating of the cell division cycle by the circadian clock has been observed in some organisms [37, 38] and humans. A study by Bjarnason et al.  found correlation with the timing of circadian clock gene expression in oral mucosa and the timing of S phase of the cell cycle, suggesting that the circadian clock may control the timing of cell-cycle events in tissues. Alteration in the circadian clock genes expression, regardless up- or down-regulation, breaks the balance of cell division and results in proliferation of tumor cells. Disrupted circadian rhythm may therefore be is both a cause and an effect of cancer.
GC is a multistep and multifactorial disease. Helicobacter pylori (Hp) infection is the most important factor in the pathogenesis of chronic gastritis and is an essential factor in GC. Hp-related chronic gastritis often results in atrophic gastritis and intestinal metaplasia which are indicators of an increased risk of malignant transformation and serve as precancerous markers [40, 41]. Gastrointestinal disorders, mainly pain and alterations in bowel habits, are more common in shift workers than in day workers. Ulcers have been named the occupational disease of shift workers. Up to date, the association between circadian rhythm disruption and Hp-related gastritis, peptic ulcers or GC has not been well described. A recent study reported a weak correlation between shift work and Hp-positive gastritis or upper gastrointestinal complaints  but the results did not support the conclusion that shift work is related to gastric disorders. Studies in nocturnal animals have demonstrated that limiting food availability completely inverts the phase of the expression of circadian clock genes in peripheral tissues . During caloric restriction, both the suprachiasmatic nucleus (SCN) and peripheral oscillators exhibit resetting of circadian rhythms . Because circadian rhythms are directly dictated by food availability, we hypothesize that circadian rhythm disruption partly involved in the development of GC. Therefore, we first examined the expression of circadian clock genes in GC and in hopes of finding a link. Future studies analyzing the expression of circadian clock genes in Hp-positive and Hp-negative GCs would be interesting to investigate the role of Hp in gastric circadian rhythm disturbance.
Whether animal or human, studies have disrupted circadian rhythms and deregulated expressions of circadian clock genes in the cancer development and progression. We hope that the roles of circadian clock genes in the mechanism of carcinogenesis will be well clarified in the future.