PARP1 and DNA-PKcs synergize to suppress p53 mutation and telomere fusions during T-lineage lymphomagenesis

Poly(ADP-ribose) polymerase 1 (PARP1) interacts genetically with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to suppress early-onset T-lineage lymphomas in the mouse, but the underlying mechanisms have remained unknown. To address this question, we analyzed a series of lymphomas arising in PARP1(-/-)/DNA-PKcs(-/-) (P1(-/-)/D(-/-)) mice. We found that, despite defective V(D)J recombination, P1(-/-)/D(-/-) lymphomas lacked clonal reciprocal translocations involving antigen-receptor loci. Instead, tumor cells were characterized by aneuploidy driven by two main mechanisms: p53 inactivation and abnormal chromosome disjunction due to telomere fusions (TFs). Aberrant accumulation of p53 was observed in 13/19 (68.4%) lymphomas. Sequence analysis revealed five p53 mutations: three missense point mutations (one transition in exon 8 and two transversions in exons 5 and 8, respectively), one in-frame 5-11 microindel in exon 7 and a 410-bp deletion encompassing exons 5-8, resulting in a truncated protein. Analysis of tumor metaphases using sequential telomere fluorescent in-situ hybridization and spectral karyotyping revealed that nine out of nine lymphomas contained TFs. Mutant but not wild-type p53 status was associated with frequent clonal and nonclonal TFs, suggesting that p53 normally limits the extent of telomere dysfunction during transformation. Chromosomes involved in TFs were more likely to be aneuploid than chromosomes not involved in TFs in the same metaphases, regardless of the p53 status, indicating that TFs promote aneuploidy via a mechanism that is distinct from p53 loss. Finally, analysis of radiation responses in P1(-/-)/D(-/-), and control primary cells and tissues indicates that loss of PARP1 increases in vivo radiosensitivity and genomic instability in DNA-PKcs-deficient mice without impairing p53 stabilization and effector functions, suggesting a more severe defect in double-strand break (DSB) repair in double mutants. Together, our findings uncover defective DSB repair leading to tumor suppressor inactivation and abnormal segregation of fused chromosomes as two novel mechanisms promoting tumorigenesis in thymocytes lacking PARP1 and DNA-PKcs.