PURPOSE: Recent studies show that glioblastoma (GBM) is more sensitive to Temozolomide (TMZ) in the morning. In cells, inhibiting O6-Methylguanine-DNA-Methyltransferase (MGMT) abolished time-dependent TMZ efficacy, suggesting that circadian regulation of this DNA repair enzyme underlies daily TMZ sensitivity. Here, we tested the hypotheses that MGMT-promoter methylation and protein abundance vary with time-of-day in GBM, resulting in daily rhythms in TMZ efficacy.
METHODS: We assessed daily rhythms in-promoter methylation in GBMand retrospectively analyzed MGMT methylation status in human GBM biopsies collected at different times of day. Next, we measured MGMT and BMAL1 protein abundances in GBM cells collected at 4-hour intervals. To understand the therapeutic implications of circadian variations in MGMT, we incorporated its daily rhythms into anmathematical model capturing interactions between MGMT, TMZ, and GBM DNA. Mgmt in vitro in vitro
RESULTS: We found daily rhythms in-promoter methylation and protein levels in GBMand in patient biopsies peaking at midday. Further, MGMT protein levels peaked at CT4, corresponding to the time of maximal TMZ efficacy. When we incorporated cell-intrinsic circadian rhythms in MGMT protein into a mathematical model for GBM chemotherapy, we found that dosing when daily MGMT levels peaked and began to decline produced maximum DNA damage. Mgmt in vitro, in vitro
CONCLUSION: Our findings suggest that the likelihood of diagnosis of MGMT-promoter methylation varies with time of biopsy in GBM. Furthermore, we predict that efforts to deliver TMZ after the daily peak of MGMT activity, with exact time being dose-dependent, will significantly enhance its therapeutic efficacy.
Circadian Biology