Coordinated signals from the DNA repair enzymes PARP-1 and PARP-2 promotes B-cell development and function.

Poly (ADP-ribose) polymerase (PARP)-1 and PARP-2 regulate the function of various DNA-interacting proteins by transferring ADP-ribose emerging from catalytic cleavage of cellular ß-NAD+. Hence, mice lacking PARP-1 or PARP-2 show DNA perturbations ranging from altered DNA integrity to impaired DNA repair. These effects stem from the central role that PARP-1 and PARP-2 have on the cellular response to DNA damage. Failure to mount a proper response culminates in cell death. Accordingly, PARP inhibitors are emerging as promising drugs in cancer therapy. However, the full impact of these inhibitors on immunity, including B-cell antibody production, remains elusive. Given that mice carrying dual PARP-1 and PARP-2 deficiency develop early embryonic lethality, we crossed PARP-1-deficient mice with mice carrying a B-cell-conditional PARP-2 gene deletion. We found that the resulting dually PARP-1 and PARP-2-deficient mice had perturbed bone-marrow B-cell development as well as profound peripheral depletion of transitional and follicular but not marginal zone B-cells. Of note, bone-marrow B-cell progenitors and peripheral mature B-cells were conserved in mice carrying either PARP-1 or PARP-2 deficiency. In dually PARP-1 and PARP-2-deficient mice, B-cell lymphopenia was associated with increased DNA damage and accentuated death in actively proliferating B-cells. Moreover, dual PARP-1 and PARP-2 deficiency impaired antibody responses to T-independent carbohydrate but not to T-dependent protein antigens. Notwithstanding the pivotal role of PARP-1 and PARP-2 in DNA repair, combined PARP-1 and PARP-2 deficiency did not perturb the DNA-editing processes required for the generation of a protective antibody repertoire, including Ig V(D)J gene recombination and IgM-to-IgG class switching. These findings provide key information as to the potential impact of PARP inhibitors on humoral immunity, which will facilitate the development of safer PARP-targeting regimens against cancer.

PARP-1/PARP-2 double deficiency in mouse T cells results in faulty immune responses and T lymphomas.

The maintenance of T-cell homeostasis must be tightly regulated. Here, we have identified a coordinated role of Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 in maintaining T-lymphocyte number and function. Mice bearing a T-cell specific deficiency of PARP-2 in a PARP-1-deficient background showed defective thymocyte maturation and diminished numbers of peripheral CD4+ and CD8+ T-cells. Meanwhile, peripheral T-cell number was not affected in single PARP-1 or PARP-2-deficient mice. T-cell lymphopenia was associated with dampened in vivo immune responses to synthetic T-dependent antigens and virus, increased DNA damage and T-cell death. Moreover, double-deficiency in PARP-1/PARP-2 in T-cells led to highly aggressive T-cell lymphomas with long latency. Our findings establish a coordinated role of PARP-1 and PARP-2 in T-cell homeostasis that might impact on the development of PARP-centred therapies.

Parp-2 is required to maintain hematopoiesis following sublethal γ-irradiation in mice.

Hematopoietic stem cells self-renew for life to guarantee the continuous supply of all blood cell lineages. Here we show that Poly(ADP-ribose) polymerase-2 (Parp-2) plays an essential role in hematopoietic stem/progenitor cells (HSPC) survival under steady-state conditions and in response to stress. Increased levels of cell death were observed in HSPC from untreated Parp-2-/- mice, but this deficit was compensated by increased rates of self-renewal, associated with impaired reconstitution of hematopoiesis upon serial bone marrow transplantation. Cell death after γ-irradiation correlated with an impaired capacity to repair DNA damage in the absence of Parp-2. Upon exposure to sublethal doses of γ-irradiation, Parp-2-/- mice exhibited bone marrow failure that correlated with reduced long-term repopulation potential of irradiated Parp-2-/- HSPC under competitive conditions. In line with a protective role of Parp-2 against irradiation-induced apoptosis, loss of p53 or the pro-apoptotic BH3-only protein Puma restored survival of irradiated Parp-2-/- mice, whereas loss of Noxa had no such effect. Our results show that Parp-2 plays essential roles in the surveillance of genome integrity of HSPC by orchestrating DNA repair and restraining p53-induced and Puma-mediated apoptosis. The data may affect the design of drugs targeting Parp proteins and the improvement of radiotherapy-based therapeutic strategies.

PARP-1 and PARP-2: New players in tumour development.

Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 belong to a family of enzymes that, using NAD(+) as a substrate, catalyze poly(ADP-ribosyl)ation of proteins. PARP-1 and PARP-2 catalytic activity is stimulated by DNA-strand breaks targeting mainly proteins involved in chromatin structure and DNA metabolism, providing strong support for a dual role of both PARP-1 and PARP-2 in the DNA damage response as DNA damage sensors and signal transducers to downstream effectors. The DNA damage response has important consequences for genomic stability and tumour development. In order to manipulate DNA damage responses to selectively induce tumour cell death, a considerable effort is centred on defining the molecular mechanisms that allow cells to detect, respond to, and repair DNA damage. PARP inhibitors that compete with NAD+ at the highly conserved enzyme active site are arisen as new potential therapeutic strategies as chemo- and radiopotentiation and for the treatment of cancers with specific DNA repair defects as single-agent therapies. In the present review, we highlight emerging information about the redundant and specific functions of PARP-1 and PARP-2 in genome surveillance and DNA repair pathways. Understanding these roles might provide invaluable clues to design new cancer therapeutic approaches. In addition, we provide an overview of ongoing clinical trials with PARP inhibitors and the value of PARP-1 and PARP-2 expression as prognostic biomarkers in cancer.

Multicenter phase I study of irinotecan plus raltitrexed in patients with 5-fluorouracil-refractory advanced colorectal cancer.

Irinotecan and raltitrexed are active against advanced colorectal cancer, act through different mechanisms, and have non-overlapping toxicity profiles. In vitro studies have shown a schedule-dependent synergism between both drugs. The aim of this multicenter study was to determine the maximum tolerated dose (MTD) of this combination. Patients with 5-fluorouracil-refractory, advanced colorectal cancer were eligible. Dose escalation consisted of irinotecan (250-350 mg/m(2) as a 60-min infusion) in combination with a fixed dose of raltitrexed (3 mg/m(2) as a 15-min infusion, 1 h after irinotecan). Courses were repeated every 21 days. Three to 6 patients were to be included at each dose level. Dose limiting (NCI-CTC grade 3-4) toxicities (DLT) were assessed during the first 2 cycles. Thirteen patients were recruited (4, 3 and 6 in levels I, II and III, respectively). Main toxicity was diarrhea and asthenia, whereas myelotoxicity was mild. At level III, 2/6 patients experienced DLT (grade 4 diarrhea and neutropenia). The MTD was not reached, but further dose escalation was not attempted. Among 12 patients with measurable disease, 2 partial responses were observed for an overall response rate of 17%. The combination of single-agent full doses of irinotecan (350 mg/m(2)) and raltitrexed (3 mg/m(2)) in a 3-weekly schedule is feasible, with mild toxicity and a promising clinical activity. Diarrhea is the DLT, but it is not more common or severe than that described with irinotecan alone.