DNA repair mechanisms as molecular determinants of glioblastomachemoresistance: in vitro investigation and characterization of novel animalmodels

Despite maximal surgical resection, radiation therapy and concomitantchemotherapy with the DNA alkylating agent temozolomide (TMZ),glioblastoma (GBM) patients ultimately succumb, due in great part to theaction of DNA repair mechanisms that drive resistance and tumor relapse.The DNA-repair protein MGMT mediates the direct removal of O6-methylguanine (O6-meG), the most highly cytotoxic lesion induced by TMZ.In GBM, epigenetic silencing of MGMT is observed in about 45% of thepatients and confers a small but significant survival benefit in patientstreated with TMZ. However, it remains unclear whether the impact of MGMTsilencing results from the sole inactivation of MGMT or from more complexmechanisms. Furthermore, intricate pathways operate to repair the DNAdouble strand breaks (DSBs) triggered by O6-meG left unrepaired byMGMT. Targeting their components might help sensitize GBM cells to TMZ,and prevent resistance and tumor relapse. Using otherwise isogenic, MGMT-negative and MGMT-positive GBM stemcells, generated in the laboratory, we plan to address the following issues: i)Examine the impact of ectopic MGMT expression on GBM development andresponse to TMZ in orthotopic GBM xenograft models, including innovativemouse resection models which closely mimic the clinical setting of GBMpatients, ii) Investigate the impact of MGMT on the DNA damage responseand transcriptional programs elicited by TMZ in vitro, iii) characterize themolecular details of the recombinational repair of DSBs associated with O6-meG lesions left unrepaired by MGMT and in particular the role of XAB2 -identified in the lab as a novel gene required for TMZ resistance in GBMand iv) validate XAB2 as a potential target for the TMZ sensitization of GBMusing orthotopic GBM xenograft models.