Targeting isocitrate dehydrogenase mutations by enzyme hyperactivation - PROJECT SUMMARY/ABSTRACT Mutations in isocitrate dehydrogenase (IDH) enzymes are hallmarks of a variety of deadly cancers, including acute myeloid leukemia (AML), glioma, cholangiocarcinoma, chondrosarcoma, and T cell lymphoma. Mutant IDH enzymes drive cancer through an unusual mechanism – they produce a metabolite called 2- hydroxyglutarate (2HG) that poisons gene expression machinery and locks malignant cells in a stem cell-like state. Drugs that inhibit mutant IDH enzymes induce durable clinical responses in some patients with IDH- mutant cancers, leading to FDA approvals for AML and cholangiocarcinoma. However, despite near universal inhibition of 2HG production, over half of patients do not respond to IDH inhibitors. Even for patients who initially respond to IDH inhibitors, most eventually acquire resistance to the drugs. While the mechanisms of resistance to IDH inhibitors remain incompletely understood, emerging evidence suggests that acquisition of specific co-occurring mutations during tumor evolution results in a loss of dependence on 2HG. Therefore, we need new treatment approaches that target IDH mutations in different ways beyond simple inhibition of the enzyme. We previously identified an unusual pattern of resistance mutations in the dimer interface of mitochondrial IDH2 wherein they occur in trans (on the other allele) relative to the 2HG-producing active-site mutation. Here, we show that in contrast to the in trans mutations that drive drug resistance, forced expression of a dimer-interface mutation in cis with the active-site mutation resulted in mitochondrial dysfunction and impaired growth of leukemia cells. Biochemical and structural studies demonstrated that the in cis dimer- interface mutation enabled IDH2 to aberrantly use NADH as an additional cofactor and dramatically enhanced production of 2HG. Seeking to exploit the toxicity exerted by the in cis dimer-interface mutation, we performed a chemical screen and identified small molecules capable of mimicking this aberrant enzymatic activity with selective toxicity towards IDH2-mutant leukemia cells. Thus, we hypothesize that hyperactivation (rather than inhibition) of mutant IDH offers an unexpected and effective new strategy to target IDH-mutant cancers. This hypothesis will be rigorously tested in three Specific Aims. Aim 1 will use enzyme assays and structural approaches to elucidate the biochemical basis for mutant IDH2 hyperactivation. Aim 2 will employ in vitro and in vivo cancer models to define the mechanisms of toxicity arising from hyperactivation of mutant IDH2. Aim 3 will utilize biochemical and functional approaches to determine if the approach of hyperactivation can applied to cytosolic IDH1 mutations. The proposed studies will reshape our understanding of the oncogenic properties of 2HG and the biochemistry of neomorphic IDH activation. More fundamentally, this work will demonstrate the feasibility of an entirely novel hyperactivation approach as a strategy to harness oncogene-mediated toxicity that could be applied to a wide range of oncogenes and cancer contexts.
