V(D)J RECOMBINATION ACTIVATES A P53-DEPENDENT DNA-DAMAGE CHECKPOINT IN SCID LYMPHOCYTE PRECURSORS

Double-stranded DNA breaks (DSBs) trigger p53-mediated cell cycle arre st or apoptosis pathways that limit the oncogenic consequences of expo sure to genotoxic agents, but p53-mediated responses to DSB generated by normal physiologic events have not been documented. ''Broken'' V(D) J coding ends accumulate in scid lymphocyte precursors as a consequenc e of a mutation in DNA-dependent protein kinase (DNA-PK). The ensuing failure to rearrange efficiently antigen receptors arrests lymphoid de velopment. Here we show that scid thymocytes express high levels of p5 3 protein, attributable to recombinase activating gene (RAG)-dependent generation of DSB adjacent to V, D, and J gene segments. To examine t he functional importance of p53 expression in vivo, we bred p53(-/-) s cid mice. The absence of p53 facilitated production of in-frame V(D)J beta coding joints and developmental progression of scid thymocytes, i n addition to a dramatic accumulation of pro-B cells. All mice develop ed disseminated pro-B or immature T cell lymphoma/leukemia by 7-12 wee ks of age. We present evidence that p53 deficiency prolongs the surviv al of scid lymphocyte precursors harboring broken V(D)J coding ends, a llowing the accumulation of aneuploid cells. These results demonstrate that a p53-mediated DNA damage checkpoint contributes to the immune d eficiency characteristic of the scid mutation and limits the oncogenic potential of DSBs generated during V(D)J recombination.