Isolation and characterization of the cDNA encoding bovine poly(ADP-ribose) glycohydrolase

Poly(ADP-ribose) polymerase null mouse cells synthesize ADP-ribose polymers

Poly(ADP-ribose) polymerase (PARP) (EC 2.4.2.30), the only enzyme known to synthesize ADP-ribose polymers from NAD+, is activated in response to DNA strand breaks and functions in the maintenance of genomic integrity. Mice homozygous for a disrupted gene encoding PARP are viable but have severe sensitivity to gamma-radiation and alkylating agents. We demonstrate here that both 3T3 and primary embryo cells derived from PARP-/- mice synthesized ADP-ribose polymers following treatment with the DNA-damaging agent, N-methyl-N'-nitro-N-nitrosoguanidine, despite the fact that no PARP protein was detected in these cells. ADP-ribose polymers isolated from PARP-/- cells were indistinguishable from that of PARP+/+ cells by several criteria. First, they bound to a boronate resin selective for ADP-ribose polymers. Second, treatment of polymers with snake venom phosphodiesterase and alkaline phosphatase yielded ribosyladenosine, a nucleoside diagnostic for the unique ribosyl-ribosyl linkages of ADP-ribose polymers. Third, they were digested by treatment with recombinant poly(ADP-ribose) glycohydrolase, an enzyme highly specific for ADP-ribose polymers. Collectively, these data demonstrate that ADP-ribose polymers are formed in PARP-/- cells in a DNA damage-dependent manner. Because the PARP gene has been disrupted, these results suggest the presence of a previously unreported activity capable of synthesizing ADP-ribose polymers in PARP-/- cells.

DNA repair defect in poly(ADP-ribose) polymerase-deficient cell lines

To investigate the physiological function of poly(ADP-ribose) polymerase (PARP), we used a gene targeting strategy to generate mice lacking a functional PARP gene. These PARP -/- mice were exquisitely sensitive to the monofunctional-alkylating agent N -methyl- N -nitrosourea (MNU) and gamma-irradiation. In this report, we have analysed the cause of this increased lethality using primary and/or spontaneously immortalized mouse embryonic fibroblasts (MEFs) derived from PARP -/- mice. We found that the lack of PARP renders cells significantly more sensitive to methylmethanesulfonate (MMS), causing cell growth retardation, G2/M accumulation and chromosome instability. An important delay in DNA strand-break resealing was observed following treatment with MMS. This severe DNA repair defect appears to be the primary cause for the observed cytoxicity of monofunctional-alkylating agents, leading to cell death occurring after G2/M arrest. Cell viability following MMS treatment could be fully restored after transient expression of the PARP gene. Altogether, these results unequivocally demonstrate that PARP is required for efficient base excision repair in vivo and strengthens the role of PARP as a survival factor following genotoxic stress.

XRCC1 is specifically associated with poly(ADP-ribose) polymerase and negatively regulates its activity following DNA damage

Poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) is a zinc-finger DNA-binding protein that detects and signals DNA strand breaks generated directly or indirectly by genotoxic agents. In response to these breaks, the immediate poly(ADP-ribosyl)ation of nuclear proteins involved in chromatin architecture and DNA metabolism converts DNA damage into intracellular signals that can activate DNA repair programs or cell death options. To have greater insight into the physiological function of this enzyme, we have used the two-hybrid system to find genes encoding proteins putatively interacting with PARP. We have identified a physical association between PARP and the base excision repair (BER) protein XRCC1 (X-ray repair cross-complementing 1) in the Saccharomyces cerevisiae system, which was further confirmed to exist in mammalian cells. XRCC1 interacts with PARP by its central region (amino acids 301 to 402), which contains a BRCT (BRCA1 C terminus) module, a widespread motif in DNA repair and DNA damage-responsive cell cycle checkpoint proteins. Overexpression of XRCC1 in Cos-7 or HeLa cells dramatically decreases PARP activity in vivo, reinforcing the potential protective function of PARP at DNA breaks. Given that XRCC1 is also associated with DNA ligase III via a second BRCT module and with DNA polymerase beta, our results provide strong evidence that PARP is a member of a BER multiprotein complex involved in the detection of DNA interruptions and possibly in the recruitment of XRCC1 and its partners for efficient processing of these breaks in a coordinated manner. The modular organizations of these interactors, associated with small conserved domains, may contribute to increasing the efficiency of the overall pathway.

[Radiation-induced DNA fragmentation evaluated by anti-poly(ADP-ribose)immunolabeling in CHO cells. Standardization with pulsed-field electrophoresis]

The poly (ADP-ribose) polymerase is an ubiquitous nuclear protein capable of binding specifically to DNA strand breaks. It synthesizes ADP-ribose polymers proportionally to DNA breaks. The actual method of reference to determine DNA double strand breaks is pulsed-field gel electrophoresis, but this requires many cells. It thus appeared of interest to use poly (ADP-ribos)ylation to follow and estimate gamma-ray-induced DNA fragmentation at the level of isolated cells after gamma-irradiation in chinese hamster ovary cells (CHO-K1). The results obtained by the immunolabelling technique of ADP-ribose polymers were compared to those obtained by pulsed-field gel electrophoresis. They show that poly (ADP-ribos)ylation reflects the occurrence of radiation-induced DNA strand breaks. A clear relationship exists between the amount of ADP-ribose polymers detected and DNA double strand breaks after gamma-irradiation.