Impact of MGMT on tumor development and resistance to temozolomide inglioblastoma: in vitro and in vivo characterization using novel GBM models

Glioblastoma multiforme (GBM) is the most frequent and aggressiveprimary brain tumor in humans. Its poor prognosis is a tragic reminder of thelimitations in our current arsenal against GBM; in spite of maximal surgicalresection, radiation therapy and concomitant chemotherapy with the DNAalkylating agent TMZ (1), GBM patients ultimately succumb, due in greatpart to the action of DNA repair mechanisms that drive resistance and tumorrelapse. The DNA-repair protein O6-methylguanine-DNA methyltransferase(MGMT) mediates the direct removal of O6-methylguanine (O6-meG), themost highly cytotoxic lesion induced by TMZ. In GBM, epigenetic silencingof MGMT is observed in about 45% of the patients and confers a small butsignificant survival benefit in patients treated with TMZ. However, it remainsunclear whether the impact of MGMT silencing results from the soleinactivation of MGMT or from more complex mechanisms. Using otherwise isogenic, MGMT-negative and MGMT-positiveGBM stem cells, generated in the laboratory, we plan to address thefollowing issues: i) Investigate the impact of MGMT on the DNA damageand transcriptional responses elicited by TMZ in vitro, in order to identifywhich MGMT-dependent and –independent cellular programs are inducedby TMZ, ii) Examine the impact of MGMT expression on GBM tumordevelopment, aggressiveness and response to TMZ in xenograft mousemodels as well as in novel rat resection models which closely mimic theclinical setting of GBM patients, iii) Further exploit the rat resection model tomeasure the impact of depleting selected DNA repair genes that displaysynthetic lethality with MGMT, on tumor development and TMZ resistance.