53BP1 cooperates with p53 and functions as a haploinsufficient tumor suppressor in mice

p53 binding protein 1 (53BP1) is a putative DNA damage sensor that accumulates at sites of double-strand breaks (DSBs) in a manner dependent on histone H2AX. Here we show that the loss of one or both copies of 53BP1 greatly accelerates lymphomagenesis in a p53-null background, suggesting that 53BP1 and p53 cooperate in tumor suppression. A subset of 53BP1^-/- p53 ^-/- lymphomas, like those in H2AX^-/- p53^-/- mice, were diploid and harbored clonal translocations involving antigen receptor loci, indicating misrepair of DSBs during V(D)J recombination as one cause of oncogenic transformation. Loss of a single 53BP1 allele compromised genomic stability and DSB repair, which could explain the susceptibility of 53BP1 ^+/- mice to tumorigenesis. In addition to structural aberrations, there were high rates of chromosomal missegregation and accumulation of aneuploid cells in 53BP1^-/- p53^+/+ and 53BP1^-/- p53^-/- tumors as well as in primary 53BP1^-/- splenocytes. We conclude that 53BP1 functions as a dosage-dependent caretaker that promotes genomic stability by a mechanism that preserves chromosome structure and number.