DDDR-15. miR-181d as a master-regulatory microRNA in glioblastomas that regulates RAD51 and MGMT expression to mediate acquired temozolomide resistance and cross-resistance to ionizing radiation

Abstract BACKGROUND Glioblastoma trials targeting base excision repair (BER) by inhibiting methyl-guanine methyl transferase (MGMT), the key mechanism for repairing temozolomide-induced DNA damage, have yielded disappointing results. One explanation for the disappointing results of these clinical trials of BER inhibitors involves the redundancy of the DNA repair process. A prime example of such redundancy is the redundancy of BER and homologous recombination (HR) as repair processes for repairing temozolomide-induced DNA damage. Here, we showed miR-181d as a master-regulator miRNA that caused simultaneous inhibition of both HR and MGMT. METHODS Preclinical studies. RESULTS In addition to MGMT, unbiased profiling of miR-181d targets revealed genes essential for HR, including RAD51. miR-181d transfection suppressed RAD51 expression and luciferase activity in a RAD51-3’UTR-luciferase assay. Moreover, the radiation sensitizing and HR suppressing effects of miR-181d were epistatic to RAD51 in vitro and in vivo. Notably, the temozolomide sensitizing effect of miR-181d was reconstituted by simultaneous silencing of RAD51 and methyl-guanine-methyl transferase (MGMT), the miR-181d-regulated BER gene. This finding demonstrates the key roles of BER and HR in temozolomide resistance. Treatment with temozolomide caused a decrease in steady-state miR-181d level, thereby derepressing MGMT and RAD51 expression. The de-repression leads to increased steady-state MGMT levels and consequent acquired temozolomide resistance. The parallel increase in RAD51 expression leads to cross-resistance to ionizing radiation. Both forms of resistance were reversed by the exogenous administration of miR-181d. CONCLUSIONS Our results support miR-181d’s role as a master-regulatory miRNA that coordinates the interplay between BER and HR to govern acquired resistance to temozolomide and cross-resistance to ionizing radiation after temozolomide treatment. The findings provide compelling rationale for a miR-181d-based strategy for future glioblastoma therapeutics development.