Recent findings have thrust poly(ADP-ribose) polymerases (PARPs) into the limelight as potential chemotherapeutic targets. To provide a framework for understanding these recent observations, we review what is known about the structures and functions of the family of PARP enzymes, and then outline a series of questions that should be addressed to guide the rational development of PARP inhibitors as anticancer agents.Current efforts to develop poly(ADP-ribose) polymerase (PARP) inhibitors as anticancer drugs represent the culmination of over 40 years of research. After Paul Mandel's research group first described a nuclear enzymatic activity that synthesizes an adenine-containing RNA-like polymer 1 , independent studies by French and Japanese teams demonstrated that this polymer, designated poly(ADP-ribose) (pADPr), is composed of two ribose moieties and two phosphates per unit polymer [2][3][4][5] . The purification of an enzyme that could generate large amounts of pADPr, PARP1 (REFS 6,7 ), led to the discovery that PARP1 is activated by DNA strand breaks [8][9][10] . Seminal work by Sydney Shall's group showed that PARP1 is involved in DNA repair and also suggested the potential use of PARP inhibitors to enhance the cytotoxic effects Correspondence to G.G.P. guy.poirier@crchul.ulaval.ca. Competing interests statementThe authors declare no competing financial interests. DATABASES NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript of alkylating agents 10 . Examination of knockout mouse models 11 strengthened the hypothesis that PARP1 participates in DNA repair and simultaneously provided the first evidence for the existence of PARP2 (REF. 12 ). A parallel set of experiments demonstrated that PARP1 hyperactivation leads to nicotinamide adenine dinucleotide (NAD + ) and ATP depletion after various types of DNA damage 13,14 (BOX 1), potentially contributing to a unique form of metabolic cell death, which is now termed parthanatos 15 . PARP was thrust into the limelight by the discovery that PARP inhibition is particularly toxic in cancer cell lines 16,17 and human tumours 18 that lack BRCA1 or BRCA2. Despite this progress, there is still much that we do not understand about the biology of the PARP family and pADPr, as detailed below. Box 1 PARP1 hyperactivation and cell deathNicotinamide adenine dinucleotide (NAD + ) is the source of ADP-ribose used by poly(ADPribose) polymerases (PARPs) to produce poly(ADP-ribose) (pADPr). Because hyperactivation of PARP1 consumes the cytosolic and nuclear pools of NAD + to generate pADPr, pADPr synthesis translates DNA damage intensity into changes in cellular energy. Low to moderate DNA damage triggers pADPr-dependent DNA repair. In the context of excessive DNA damage, however, PARP1 hyperactivation leads to extended pADPr synthesis and extensive NAD + consumption 8,13,14 . Depending on the cellular context, this intense pADPr synthesis can induce cell death through several mechanisms. Long and branched pADPr (60mers and longer) can directly trigg...

PARP-3 is a member of the ADP-ribosyl transferase superfamily of unknown function. We show that PARP-3 is stimulated by DNA double-strand breaks (DSBs) in vitro and functions in the same pathway as the poly (ADP-ribose)-binding protein APLF to accelerate chromosomal DNA DSB repair. We implicate PARP-3 in the accumulation of APLF at DSBs and demonstrate that APLF promotes the retention of XRCC4/DNA ligase IV complex in chromatin, suggesting that PARP-3 and APLF accelerate DNA ligation during nonhomologous end-joining (NHEJ). Consistent with this, we show that class switch recombination in Aplf(-/-) B cells is biased toward microhomology-mediated end-joining, a pathway that operates in the absence of XRCC4/DNA ligase IV, and that the requirement for PARP-3 and APLF for NHEJ is circumvented by overexpression of XRCC4/DNA ligase IV. These data identify molecular roles for PARP-3 and APLF in chromosomal DNA double-strand break repair reactions.

No abstract

After the generation of DNA double-strand breaks (DSBs), poly(ADP-ribose) polymerase-1 (PARP-1) is one of the first proteins to be recruited and activated through its binding to the free DNA ends. Upon activation, PARP-1 uses NAD+ to generate large amounts of poly(ADP-ribose) (PAR), which facilitates the recruitment of DNA repair factors. Here, we identify the RNA-binding protein NONO, a partner protein of SFPQ, as a novel PAR-binding protein. The protein motif being primarily responsible for PAR-binding is the RNA recognition motif 1 (RRM1), which is also crucial for RNA-binding, highlighting a competition between RNA and PAR as they share the same binding site. Strikingly, the in vivo recruitment of NONO to DNA damage sites completely depends on PAR, generated by activated PARP-1. Furthermore, we show that upon PAR-dependent recruitment, NONO stimulates nonhomologous end joining (NHEJ) and represses homologous recombination (HR) in vivo. Our results therefore place NONO after PARP activation in the context of DNA DSB repair pathway decision. Understanding the mechanism of action of proteins that act in the same pathway as PARP-1 is crucial to shed more light onto the effect of interference on PAR-mediated pathways with PARP inhibitors, which have already reached phase III clinical trials but are until date poorly understood.