title: Augmented HR Repair Mediates Acquired Temozolomide Resistance in Glioblastoma.
abstract: Glioblastoma (GBM) is the most common and aggressive primary brain tumor in adults and is universally fatal. The DNA alkylating agent temozolomide is part of the standard-of-care for GBM. However, these tumors eventually develop therapy-driven resistance and inevitably recur. While loss of mismatch repair (MMR) and re-expression of MGMT have been shown to underlie chemoresistance in a fraction of GBMs, resistance mechanisms operating in the remaining GBMs are not well understood. To better understand the molecular basis for therapy-driven temozolomide resistance, mice bearing orthotopic GBM xenografts were subjected to protracted temozolomide treatment, and cell lines were generated from the primary (untreated) and recurrent (temozolomide-treated) tumors. As expected, the cells derived from primary tumors were sensitive to temozolomide, whereas the cells from the recurrent tumors were significantly resistant to the drug. Importantly, the acquired resistance to temozolomide in the recurrent lines was not driven by re-expression of MGMT or loss of MMR but was due to accelerated repair of temozolomide-induced DNA double-strand breaks (DSB). Temozolomide induces DNA replication-associated DSBs that are primarily repaired by the homologous recombination (HR) pathway. Augmented HR appears to underpin temozolomide resistance in the recurrent lines, as these cells were cross-resistant to other agents that induced replication-associated DSBs, exhibited faster resolution of damage-induced Rad51 foci, and displayed higher levels of sister chromatid exchanges (SCE). Furthermore, in light of recent studies demonstrating that CDK1 and CDK2 promote HR, it was found that CDK1/2 inhibitors countered the heightened HR in recurrent tumors and sensitized these therapy-resistant tumor cells to temozolomide.
  
  
  
  Augmented HR repair is a novel mechanism underlying acquired temozolomide resistance in GBM, and this raises the possibility of improving the therapeutic response to temozolomide by targeting HR with small-molecule inhibitors of CDK1/2. Mol Cancer Res; 14(10); 928-40. ©2016 AACR.
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Carlos Rodrigo
authors.initials: CR
authors.lastname: Gil Del Alcazar
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Pavlina Krasimirova
authors.initials: PK
authors.lastname: Todorova
authors.affiliation: Department of Neurology and Neurotherapeutics, University of Texas Southwestern Medical Center, Dallas, Texas. North Texas VA Medical Center, Dallas, Texas.
authors.forename: Amyn A
authors.initials: AA
authors.lastname: Habib
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Bipasha
authors.initials: B
authors.lastname: Mukherjee
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas. sandeep.burma@utsouthwestern.edu.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Burma
mesh_terms: D000818:Animals; D001932:Brain Neoplasms / Q000188:drug therapy / Q000235:genetics*; D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded; D015254:DNA Modification Methylases / Q000235:genetics; D045643:DNA Repair Enzymes / Q000235:genetics; D003606:Dacarbazine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm; D015972:Gene Expression Regulation, Neoplastic; D005909:Glioblastoma / Q000188:drug therapy / Q000235:genetics*; D006801:Humans; D051379:Mice; D009368:Neoplasm Transplantation; D059767:Recombinational DNA Repair; D000077204:Temozolomide; D025521:Tumor Suppressor Proteins / Q000235:genetics
chemical_list: D025521:Tumor Suppressor Proteins; D003606:Dacarbazine; D015254:DNA Modification Methylases; C528331:MGMT protein, mouse; D045643:DNA Repair Enzymes; D000077204:Temozolomide