Metabolism-focused CRISPR screen unveils Mitochondrial Pyruvate Carrier 1 as a critical driver for PARP inhibitor resistance in lung cancer

Homologous recombination (HR) and poly ADP-ribosylation are partially redundant pathways for repair of DNA damage in normal and cancer cells. In cell lines that are deficient in HR, inhibition of poly (ADP-ribose) polymerase (PARP1/2) is a proven target with several PARP inhibitors (PARPi) currently in clinical use. Resistance to PARP inhibitors often develops, usually involving genetic alterations in DNA repair signaling cascades, but also metabolic rewiring particularly in HR-proficient cells. PARP1/2 utilize NAD+ (nicotinamide adenine dinucleotide), an essential substrate not only for PARPs but also for multiple pathways in cellular metabolism, TCA cycle and mitochondrial functions. Thus, NAD+ is central to many key cellular functions. Both activation of PARPs by DNA damage and their inhibition by drugs such as olaparib affect NAD+ consumption. We surmised that alterations in NAD+ metabolism by cancer drugs such as Olaparib might be involved in the development of resistance to drug therapy. To test this hypothesis, we conducted a metabolism-focused CRISPR knockout (KO) screen to identify genes which undergo alterations during treatment of tumor cells with PARP inhibitors. Of about 3000 genes in the screen, our data revealed that mitochondrial pyruvate carrier 1 (MPC1) is an essential factor in desensitizing NSCLC lung cancer lines to PARP inhibition. In contrast to NSCLC lung cancer cells, triple negative breast cancer cells do not exhibit such desensitization following MPC1 loss and reprogram the TCA cycle and oxidative phosphorylation pathways to overcome PARP inhibitor treatment. Our findings unveil a previously unknown synergistic response between MPC1 loss and PARP inhibition in lung cancer cells.