Mechanisms of Resistance to PARP Inhibitors—Three and Counting

(A) Poly(ADP-ribosyl)ation often referred to as PARylation is a post-translational protein modification catalyzed by poly(ADP-ribose) polymerases [PARPs]. PARylation involves the polymerization of ADP ribose units from donor nicotinamide adenine dinucleotide (NAD+) molecules and is catalytically activated by single strand breaks in DNA. PARylation forms heterogenous linear and branched chains of poly(ADP-ribose) or PAR. A chain consists of as many as 200 ADP-ribose units linked via glycosidic 1′ → 2′ bonds, covalently bound to acceptor proteins, that include PARP1 itself (auto-PARylation), histones and other proteins involved in the recognition and repair of DNA strand breaks. Each residue in a PAR chain contains an adenine moiety capable of stacking and forming hydrogen bonds and two negatively charged phosphate groups. PAR recruits additional proteins such as p53, the base excision repair protein XRCC-1 and others, most of which contain a PAR binding motif (PBM)≈ 20 amino acids in length. (B) PARP inhibitors (PARPi) in clinical development mimic the nicotinamide moiety of NAD+, and bind to the enzyme's catalytic domain. Inhibition of PARPs leads to persistence of DNA single strand breaks (SSBs) that collapse replication forks resulting in double strand breaks (DSBs) and in turn trigger homologous recombination (HR). Such DSBs are present in higher numbers when PARP has been inhibited. In cells with intact HR, DSBs are successfully repaired. However, in cells with defective HR such as those with dysfunctional BRCAs (asterisk, part C), DSB repair cannot occur efficiently leading to apoptosis or to the use of error prone non-homologous end joining (NHEJ), which may in turn lead to apoptosis. (C) Mechanisms of resistance to PARPi described to date include secondary BRCA mutations that restore BRCA function, increased drug efflux mediated by P-glycoprotein and reduced/absent 53BP1 expression resulting in partial restoration of HR.