Oncogenic IDH mutations increase heterochromatin-related replication stress without impacting tumor mutation burden

Oncogenic mutations in the metabolic enzyme isocitrate dehydrogenase 1 and 2 (IDH1/2) have been found in a number of liquid and solid tumors. Their pathogenic mechanism of action involves production of 2-hydroxyglutarate (2HG), an oncometabolite that acts in part by inhibiting members of a family of dioxygenases that modulate chromatin dynamics. Recent work has suggested that mutant IDH (mIDH) and 2HG also impact sensitivity to inhibitors of poly-ADP ribose polymerases (PARP) but the molecular basis for this sensitivity is unclear. Unlike PARP inhibitor-sensitive BRCA1/2 tumors which exhibit impaired homologous recombination, IDH-mutant tumors have a silent mutational profile and lack mutational signatures associated with impaired homologous recombination. Instead, 2HG-producing IDH mutations lead to heterochromatin-dependent slowing of DNA replication and increased replication stress, resulting in DNA double strand breaks. This replicative stress manifests as replication fork slowing but the breaks are repaired without a significant increase in the cellular mutation burden. Faithful resolution of replicative stress in IDH-mutant cells is dependent on poly-ADP ribosylation. Treatment with PARP inhibitors restores replication fork speed but results in incomplete repair of DNA breaks. These findings provide evidence of a requirement for PARP in the replication of heterochromatin and further validate PARP as a potential therapeutic target in IDH-mutant tumors.