Exploiting Mutant IDH1/2-induced Homologous Recombination Defects in Cancer

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Only a handful of therapies are available for treatment in the front-line for glioma: radiotherapy, temozolomide, and PCV (procarbazine, CCNU, and vincristine). These therapies have not changed in over 3 decades, and in most cases they are not curative, nor are they targeted to the underlying mutations driving these tumors. 2-Hydroxyglutarate (2HG) exists as two enantiomers, R-2HG and S-2HG, and both are implicated in tumor progression via their inhibitory effects on α-ketoglutarate (αKG)-dependent dioxygenases. The former is an oncometabolite that is induced by the neomorphic activity conferred by isocitrate dehydrogenase-1 and -2 (IDH1/2) mutations, while the latter is produced under pathologic process such as hypoxia. Our laboratory recently discovered that IDH1/2 mutations induce a homologous recombination (HR) defect which renders tumor cells exquisitely sensitive to Poly (ADP-Ribose) polymerase (PARP) inhibitors. Remarkably, this “BRCAness” phenotype can be completely reversed by small molecule mutant IDH1 inhibitors, and it can be entirely recapitulated by treatment with either 2HG enantiomer in cells with intact IDH1/2. We demonstrated IDH1-dependent PARP inhibitor sensitivity in a range of clinically relevant models, including primary patient-derived glioma cells in vitro and genetically-matched tumor xenografts in vivo. These findings directly challenge the current therapeutic strategy to block IDH1/2 mutant function by direct inhibition, and they instead provide a novel approach to treat these tumors with DNA repair inhibitors. Furthermore, our results uncover an unexpected link between oncometabolites, DNA repair and genetic instability. Based on the preliminary data presented above, our central hypothesis is that IDH1/2-mutant tumors harbor intrinsic double-strand break (DSB) repair defects, which can be exploited for a therapeutic gain. The overall goals of this application are (1) to understand how R-2HG and related oncometabolites, which are induced by IDH1/2 mutations and other processes, suppress DSB repair, (2) how DSB repair specifically is affected by these oncometabolites, and (3) the most effective way to exploit this defect using DNA repair inhibitor-based treatment regimens. In Aim 1 of this application, we will test the hypothesis that specific αKG- dependent dioxygenases mediate the observed phenotype of oncometabolite-induced DSB repair suppression. In Aim 2, we will perform a comprehensive evaluation of key nodes in the DNA damage response network, in order to localize the exact mechanism(s) of action by which DSB repair is suppressed. Finally, in Aim 3, we will test the hypothesis that the oncometabolite-induced DSB repair defect can be targeted by combination treatment with DNA repair inhibitors and DNA damaging agents. The long term goal of this study is to translate our novel findings into a clinical trial, in which we will test the efficacy of the combination therapies that are identified in this proposal. This trial would be tissue-agnostic and biomarker-driven, focusing on tumors that produce high levels of oncometabolites, which can be exploited with DNA repair inhibitor-based regimens.