Unravelling the complexity of glioblastoma relapse and drug resistance

Glioblastoma (GBM) is highly fatal with a median survival of less than 15 months. In terms of years of life lost, the population burden from glioblastoma is the highest of all malignant cancers. The standard treatment plan involves surgical resection, followed by radiotherapy combined with temozolomide chemotherapy. Despite this intense multimodality therapy, essentially all tumours recur and invariably lead to patient death. Resistance, intrinsic or acquired, represents a formidable hurdle to overcome. Urgent attention to understand the drug resistance intrinsic to the tumour and the molecular changes that occur to the tumour as a result of treatment is required. Herein, the prognostic significance of MGMT promoter methylation in patients treated with temozolomide was confirmed. Pyrosequencing was a feasible test for assessing MGMT status. Through the testing of pre- and post-treated tumour specimens, the methylation percentage did not change. Intrinsically, DNA mismatch repair (MMR) proteins have been shown to mediate resistance to alkylating therapy. My in vitro data confirmed the depletion of MSH6 and MLH1 conferred resistance to temozolomide. However, the role of MMR was not mutually exclusive with a fraction of MMR proficient glioblastomas exhibiting poor response to temozolomide, suggesting additional mechanisms are involved. Whole exome sequencing (WES) was performed on DNA from matched primary and recurrent glioblatomas to novel somatic mutations acquired upon recurrence post-temozolomide treatment. My WES results clearly demonstrated that a large number of genes are altered as a result of temozolomide. Patients treated with long-term temozolomide typically showed higher numbers of somatic mutations, and those with hypermutation phenotype were predominantly characterized by C:G>T:A transitions in the context of CpC dinucleotide. Moreover, I discovered frequent mutations in genes involved in the epithelial-to-mesenchymal (EMT) pathway. EMT was a significant pathway linked to temozolomide treatment ...