Mechanisms of Acquired Temozolomide Resistance in Glioblastoma

The integration of temozolomide (TMZ) chemotherapy into therapy for patients newly diagnosed with glioblastoma multiforme (GBM) has a major impact on survival for a subset of patients.

Unfortunately, essentially all patients develop resistance to temozolomide, and consequently, understanding the mechanisms of TMZ resistance development is critically important.

Using primary glioblastoma multiforme xenograft models, we have demonstrated that acquired TMZ resistance can be induced by two distinct mechanisms within the same tumor, with one mechanism being driven by increased histone acetylation within the promoter of a critical DNA repair protein MGMT. Moreover, co-treatment with the histone deacetylase inhibitor SAHA promotes development of this epigenetically driven resistance mechanism.

Whole-exome sequencing and RNA sequencing in TMZ-resistant models without MGMT elevation identified mutations in DNA ligase IV, Ku70 and Ku80, which are key components in the nonhomologous end-joining (NHEJ) DNA repair pathway. Consistent with defects in this critical pathway, these mutations were associated with delayed DNA repair kinetics and a paradoxical increase in radiation responsiveness.

The focus of this application is to investigate these two distinct mechanisms of TMZ resistance emergence using our patient-derived xenograft models and patient tumor tissues.

The planned aims are:

  • Aim 1: Define the changes in the chromatin-modifying complexes regulating MGMT upregulation. The lab's preliminary data suggest that distinct chromatin modifying complexes suppress MGMT expression in different MGMT-methylated glioblastoma multiforme xenograft lines. We hypothesize that the specific resident complex within the MGMT promoter will define the effects of epigenetically targeted therapies on MGMT upregulation.
  • Aim 2: Test whether DNA damage-induced alterations in heterochromatin promote evolution of temozolomide resistance. DNA damage induced by temozolomide or radiation triggers a highly orchestrated DNA damage response that includes chromatin remodeling, and we hypothesize that MGMT upregulation requires focal chromatin opening within the MGMT promoter in association with damage repair.
  • Aim 3: Define whether deregulation of DNA repair contributes to mechanisms of temozolomide resistance in MGMT nonexpressing tumors.

Based on our previous work demonstrating that suppression of nonhomologous end-joining activity can abrogate the cytotoxicity of PARP inhibitors, we hypothesize that a similar disruption of NHEJ through acquired mutations contributes to TMZ resistance, while leading to an increase in radiation responsiveness.

Defining mechanisms of temozolomide resistance associated with radiation hypersensitivity may provide a strategy for selecting patients most likely to benefit from re-irradiation after tumor progression.