Cancer Therapy Resistance and Drug Target Discovery Program
Gasper J. Kitange, MD, PhD
The overarching goal of Dr. Kitange’s lab is to identify novel molecular drug targets for treatment of patients with glioblastoma (GBM), which is the most aggressive primary brain tumor and an incurable disease. The standard of treatment for newly diagnosed GBM patients is surgery followed by radiation (RT) in combination with temozolomide (TMZ). However, the impact of RT and/or TMZ therapy on the overall survival of GBM patients has always been limited, largely because of pre-existing or rapid evolution of acquired resistance. Thus, better understanding of the mechanisms underlying resistance is essential to generate more effective therapies for GBM treatment. Resistance to TMZ therapy is primarily linked to expression of the DNA repair protein O6-methyguanine-DNA-methytransferase (MGMT), which prevents cells from dying by removing the TMZ-induced cytotoxic DNA lesion, O6-methylguanine (O6-MG). MGMT expression is silenced in almost 50% of GBM patients by promoter hypermethylation, and lack of promoter methylation (MGMT promoter unmethylated) is linked to high-level MGMT expression and poor survival as compared to tumors with low or no expression (MGMT promoter hypermethylation). Even though GBM patients with MGMT promoter hypermethylated tumors initially respond to TMZ, they rapidly develop resistance due to poorly elucidated mechanisms and most patients die within 3 years from initial diagnosis. We have uncovered that both the epigenetic and non-epigenetic mechanisms may drive TMZ resistance in GBM. Therefore, our goal is to identify modulators of therapy sensitivity in GBM, especially the targets mediating resistance to TMZ. To this end, my lab uses whole genome screening as the main research strategy to identify novel molecular targets for GBM therapy. Using this strategy, we have identified the chromatin modifying retinoblastoma binding protein 4 (RBBP4) as a potential modulator of TMZ sensitivity in cells cultured from GBM PDXs. RBBP4 protein is a component of several chromatin-modifying protein complexes. In follow up experiments, we found that RBBP4 may modulate TMZ sensitivity in both MGMT promoter hypermethylated and unmethylated GBM in vitro and in vivo. Interestingly, silencing RBBP4 significantly suppressed the expression of DNA damage repair (DDR) genes, including MGMT, RAD51, FIGNL1 and EYA1 in GBM cells. Since RAD51, FIGNL1 and EYA1 are critical for efficient repair of TMZ-induced double-stranded breaks (DSBs), and MGMT activity repairs TMZ-induced O6-MG DNA lesions that lead to DSBs, RBBP4 may control TMZ sensitivity through regulation of these genes. Thus, RBBP4/p300 complex may occupy the promoter/enhancer regions of a cassette of DDR genes that are critical for recovery from therapy-induced DNA damage, and that this complex may be a robust chemo-/radio-sensitizing target in GBM and possibly in other malignancies. My lab is currently conducting further NIH-supported mechanistic investigations that will directly link RBBP4/p300 complex and the recovery from therapy-induced DNA damage in GBM. Besides RBBP4/p300 complex, my lab has identified over 600 druggable modulators of TMZ sensitivity in GBM cells. Pharmacologic inhibitors are already available for some of these targets while others are potential targets for developing novel inhibitors, especially to the candidates belonging to the protein kinase family. Therefore, ongoing research in my lab is focused on characterization of these targets to identify the best candidates for therapy either alone or in combination with TMZ and/or RT. My lab will continue testing novel inhibitors targeting our candidates as they emerge from pharmaceutical companies, and our goal is to synthesize some inhibitors locally or through external collaborations.