A majority of circadian clock genes are expressed in estrogen receptor and progesterone receptor status-dependent manner in breast cancer.

Circadian clocks, biochemical oscillators that are regulated by environmental time cues including the day/night cycle, have a central function in the majority of biological processes. The disruption of the circadian clock can alter breast biology negatively and may promote the development of breast tumors. The expression status of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) were used to classify breast cancer into different molecular subtypes such as triple-negative breast cancer (TNBC). Receptor status-dependent expression of circadian clock genes have been previously studied in breast cancer using relatively small sample sizes in a particular population. Here, using TCGA-BRCA data (=1119), we found that the expressions of and were higher in ER+ breast cancer cells compared with those of ER- status. Similarly, we showed that transcript levels of and were higher in PR+ breast cancer cells than in PR- breast cancer cells. We report that the expressions of and were lower, and the expression of was higher, in HER2+ breast cancer cells compared with HER2- breast cancer cells. Moreover, we studied these receptor status-dependent changes in the expressions of circadian clock genes also based on the race and age of breast cancer patients. Lastly, we found that the expressions of and were higher in non-TNBC than in TNBC, which has the worst prognosis among subtypes. We note that our findings are not always parallel to the observations reported in previous studies with smaller sample sizes performed in different populations and organisms. Our study suggests that receptor status in breast cancer (thus, subtype of breast cancer) might be more important than previously shown in terms of its influence on the expression of circadian clock genes and on the disruption of the circadian clock, and that ER or PR might be important regulators of breast cancer chronobiology that should be taken into account in personalized chronotherapies.