GENETIC INTERACTION BETWEEN PARP AND DNA-PK IN V(D)J RECOMBINATION AND TUMORIGENESIS

Poly(ADP-ribose) polymerase (PARP) and DNA-dependent protein kinase (D NA-PK) are DNA break-activated molecules(1,2). Although mice that lack PARP display no gross phenotype and normal DNA excision repair(3), th ey exhibit high levels of sister chromatid exchange(4), indicative of elevated recombination rates. Mutation of the gene for DNA-PK catalyti c subunit (Prkdc) causes defective antigen receptor V(D)J recombinatio n and arrests B-and T-lymphocyte development in severe combined immune -deficiency (SCID) mice(5,6). SCID V(D)J recombination can be partly r escued in T-lymphocytes by either DNA-damaging agents (gamma-irradiati on and bleomycin)(7,8) or a null mutation of the p53 gene(9-10), possi bly because of transiently elevated DNA repair activity in response to DNA damage or to delayed apoptosis in the absence of p53. To determin e whether the increased chromosomal recombination observed in PARP-def icient cells affects SCID V(D)J recombination, we generated mice lacki ng both PARP and DNA-PK. Here, we show that thymocytes of SCID mice ex press both CD4 and CD8 co-receptors, bypassing the SCID block. Double- mutant T-cells in the periphery express TCR beta, which is attributabl e to productive TCR beta joints. Double-mutant mice develop a high fre quency of T-cell lymphoma. These results demonstrate that increased re combination activity after the loss of PARP anti-recombinogenic functi on can rescue V(D)J recombination in SCID mice and indicate that PARP and DNA-PK cooperate to minimize genomic damage caused by DNA strand b reaks.