53BP1 Cooperates with p53 and Functions as a Haploinsufficient Tumor Suppressor in Mice

Abstract

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.p53 binding protein 1 (53BP1) was identified in a yeast two-hybrid screen as a protein that binds to the central DNA binding domain of the tumor suppressor protein p53 (15). It contains two tightly packed tudor domains and a C-terminal tandem BRCT motif (9, 19). BRCT domains are thought to be protein-protein interaction domains and are found in many DNA damage response proteins (5, 19). The observation that, upon exposure of cells to ionizing radiation (IR), 53BP1 rapidly localizes in a dose-dependent manner to discrete nuclear foci supported the proposed role of 53BP1 in the DNA damage response (2,16,25,29,36). Moreover, following irradiation, 53BP1 interacts with and becomes phosphorylated by ataxia telangiectasia mutated (ATM), a central kinase in the DNA double-strand break (DSB) response (2,25,36). Colocalization of 53BP1 foci with those of phosphorylated histone H2AX and the Mre-11/Nbs1/Rad50 complex mapped the IRinduced accumulation of 53BP1 to regions of DNA DSBs (2, 25, 29). Subsequent studies using small interfering RNAs indicated that 53BP1 is involved in DNA damage checkpoint control by facilitating the phosphorylation of ATM substrates like Chk2 or SMC1 (12, 31). Moreover, 53BP1 was reported to be required for p53 accumulation in response to IR (31).Defects in DNA damage checkpoint control were also observed in cells derived from 53BP1 knockout mice (13); however, the more striking observations obtained with these mice were of sensitivity towards ionizing radiation and greatly impaired class switch recombination (CSR) combined with immunodeficiency and an increased lymphoma predisposition (23,24,33,34). Since CSR involves the generation and rejoining of DNA DSBs, a function of 53BP1 in DSB repair has been proposed (23, 34). It is well established that defects in the joining of DSBs can lead to chromosome translocations and gene amplifications that promote tumorigenesis, particularly in the co...

Integrative analysis reveals 53BP1 copy loss and decreased expression in a subset of human diffuse large B-cell lymphomas

“…In these studies, there was a close association between 53bp1 protein abundance and gene copy number, with 53bp1 þ /À cells from hemizygous animals expressing intermediate levels of the damage response protein (Ward et al, 2003b). With additional long-term follow-up, both 53bp1 À/À and 53bp1 þ /À mice had an increased incidence of late-onset tumors (Ward et al, 2005). Furthermore, in a p53 null background, the loss of one or both copies of 53bp1 dramatically increased both the incidence and rapidity of onset of lymphoid malignancies including thymic and B-cell tumors (Ward et al, 2005).…”

“…With additional long-term follow-up, both 53bp1 À/À and 53bp1 þ /À mice had an increased incidence of late-onset tumors (Ward et al, 2005). Furthermore, in a p53 null background, the loss of one or both copies of 53bp1 dramatically increased both the incidence and rapidity of onset of lymphoid malignancies including thymic and B-cell tumors (Ward et al, 2005). In an additional murine model of 53bp1 and p53 deficiency, only 53bp1 À/À /p53 À/À animals had an increased incidence of T-and B-cell lymphoma (Morales et al, 2006).…”

Breaking down cell cycle checkpoints and DNA repair during antigen receptor gene assembly

“…Ku70, DNA-PKcs, Artemis) or factors that survey V(D)J DSB intermediates (that is H2AX, 53BP1, Nbs1) leads to the rapid development of lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). Thus, we should consider that different cell types and/or cell cycle phases may have distinct thresholds for the DSB checkpoint, with the activation threshold in lymphocytes being exquisitely sensitive to a single DSB generated in the G 0 /G 1 phase of the cell cycle.…”

“…Deficiency in Nbs1, H2AX and 53BP1 also increases the frequency of TCRa translocations (Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). However, the defect is less severe than in ATM À/À mice, and Nbs1, H2AX and 53BP1-deficient mice are not highly prone to spontaneous lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006).…”

“…However, the defect is less severe than in ATM À/À mice, and Nbs1, H2AX and 53BP1-deficient mice are not highly prone to spontaneous lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). Moreover, in contrast to ATM knockouts, no impairment in the joining of V(D)J recombination associated breaks have been detected in H2AX À/À and 53BP1 À/À mice (Bassing et al, 2002;Manis et al, 2004;Ward et al, 2004).…”