Significance: The circadian clock mechanism is a daily rhythmic activation of circadian gene transcription by the CLOCK–BMAL1 heterodimer and repression by CRYs (CRY1 and CRY2) and PERs (PER1 and PER2). CRYs and PERs are highly expressed around predawn, when they begin repressing transcription of cry and per and other clock genes, leading to a nadir of CRYs and PERs in the afternoon. CRY represses by simply binding to the CLOCK–BMAL1 heterodimer on DNA, and PER represses by removing CLOCK–BMAL1 from DNA in a CRY-dependent manner. This work shows that removal of CLOCK–BMAL1 involves phosphorylation of CLOCK, which is accomplished by CK1δ when CRY and PER deliver CK1δ to the CLOCK–BMAL1 complex in the nucleus.

Study Objectives:. Most organisms have behavioral and physiological circadian rhythms, which are controlled by an endogenous clock. Although genetic an

The circadian clock controls the expression of nearly 50% of protein coding genes in mice and most likely in humans as well. Therefore, disruption of the circadian clock is presumed to have serious pathological effects including cancer. However, epidemiological studies on individuals with circadian disruption because of night shift or rotating shift work have produced contradictory data not conducive to scientific consensus as to whether circadian disruption increases the incidence of breast, ovarian, prostate, or colorectal cancers. Similarly, genetically engineered mice with clock disruption do not exhibit spontaneous or radiation-induced cancers at higher incidence than wild-type controls. Because many cellular functions including the cell cycle and cell division are, at least in part, controlled by the molecular clock components (CLOCK, BMAL1, CRYs, PERs), it has also been expected that appropriate timing of chemotherapy may increase the efficacy of chemotherapeutic drugs and ameliorate their side effect. However, empirical attempts at chronochemotherapy have not produced beneficial outcomes. Using mice without and with human tumor xenografts, sites of DNA damage and repair following treatment with the anticancer drug cisplatin have been mapped genome-wide at single nucleotide resolution and as a function of circadian time. The data indicate that mechanism-based studies such as these may provide information necessary for devising rational chronochemotherapy regimens.

Significance: A molecular clock controls circadian rhythmicity in mammals. The interplay of the clock with cellular growth and development is of interest regarding possible connections between cancer and clock disruption. The c-MYC oncoprotein is a transcriptional activator that has been reported to control and be controlled by the clock. We have used knockout mice mutated in genes controlling either the negative or positive arm of the molecular clock to clarify the role of the clock in c-MYC regulation. Results with both types of mutant mice consistently show that the clock controls c-MYC protein levels by controlling expression of the c-Myc oncogene. These findings will aid in understanding the interplay of the clock and clock disruption on cell growth control and cancer.

Cisplatin is the most commonly used chemotherapeutic drug for managing solid tumors. However, toxicity and the innate or acquired resistance of cancer cells to the drug limit its usefulness. Cisplatin kills cells by forming cisplatin-DNA adducts, most commonly the Pt-d(GpG) diadduct. We recently showed that, in mice, repair of this adduct 2 h following injection is controlled by two circadian programs. 1) The circadian clock controls transcription of 2000 genes in liver and, via transcription-directed repair, controls repair of the transcribed strand (TS) of these genes in a rhythmic fashion unique to each gene's phase of transcription. 2) The excision repair activity itself is controlled by the circadian clock with a single phase at which the repair of the nontranscribed strand (NTS) and the rest of the genome takes place. Here, we followed the repair kinetic for long periods genome-wide both globally and at single nucleotide resolution by the Excision Repair-sequencing (XR-seq) method to better understand cisplatin DNA damage and repair. We find that transcription-driven repair is nearly complete after 2 days, whereas weeks are required for repair of the NTS and the rest of the genome. TS repair oscillates in rhythmically expressed genes up to 2 days post injection, and in all expressed genes, we see a trend in TS repair with time from the 5′ to 3′ end. These findings help to understand the circadian- and transcription-dependent and -independent control of repair in response to cisplatin, and should aid in designing cisplatin chemotherapy regimens with improved therapeutic indexes.