53BP1 and p53 synergize to suppress genomic instability and lymphomagenesis

Abstract

p53-binding protein 1 (53BP1) participates in the cellular response to DNA double-stranded breaks where it associates with various DNA repair͞cell cycle factors including the H2AX histone variant. Mice deficient for 53BP1 (53BP1 ؊/؊ ) are sensitive to ionizing radiation and immunodeficient because of impaired Ig heavy chain class switch recombination. Here we show that, as compared with p53 ؊/؊ mice, 53BP1 ؊/؊ ͞p53 ؊/؊ animals more rapidly develop tumors, including T cell lymphomas and, at lower frequency, B lineage lymphomas, sarcomas, and teratomas. In addition, T cells from animals deficient for both 53BP1 and p53 (53BP1 ؊/؊ ͞p53 ؊/؊ ) display elevated levels of genomic instability relative to T cells deficient for either 53BP1 or p53 alone. In contrast to p53 ؊/؊ T cell lymphomas, which routinely display aneuploidy but not translocations, 53BP1 ؊/؊ ͞p53 ؊/؊ thymic lymphomas fall into two distinct cytogenetic categories, with many harboring clonal translocations (40%) and the remainder showing aneuploidy (60%). We propose that 53BP1, in the context of p53 deficiency, suppresses T cell lymphomagenesis through its roles in both cell-cycle checkpoints and double-stranded break repair.thymic lymphoma ͉ aneuploidy ͉ translocations D ouble-stranded break (DSB) repair is essential for genomic stability and tumor suppression. In mammalian cells, DSBs arise through exogenous means such as ionizing radiation, at stalled replication forks, and in the context of two genetically programmed processes that occur in lymphoid cells (1-3). DSBs are repaired by either of two nonmutually exclusive mechanisms: homologous recombination and nonhomologous end joining. During the development of the immune system, both B and T cells assemble the exons that encode Igs and T cell receptor variable regions by means of V(D)J recombination. In addition, mature B cells can express different Ig heavy chain constant regions through a DSB-mediated process known as Ig heavy chain class switch recombination (CSR). The DSBs that initiate V(D)J recombination are generated by the site-specific RAG endonuclease, whereas those that initiate CSR appear to involve the activation-induced deaminase. However, both V(D)J recombination and CSR are completed by nonhomologous end joining. In contrast to V(D)J recombination, CSR requires components of the DNA DSB repair response, including the histone variant H2AX and p53 binding protein 1 (53BP1) (4-7), potentially to juxtapose broken DSBs for nonhomologous end joining (8). Although 53BP1 is required for joining of S regions during CSR, like H2AX, it does not appear to be absolutely required for V(D)J recombination (6, 7), perhaps because of the fact that RAG also mediates synapsis functions (8).53BP1 is phosphorylated in response to ionizing radiation, accumulates at or near sites of DNA damage, and participates in the intra S and G 2 ͞M checkpoints (reviewed in ref. 9). Accumulation of 53BP1 at sites of irradiation-induced foci depends on

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

“…Genetic analysis of DLBCLs with 53BP1 copy loss Since 53bp1 À/À mice develop lymphoid malignancies with frequent chromosomal translocations (Ward et al, 2005;Morales et al, 2006), we compared the frequencies of t(14;18) and t(3;__) in primary DLBCLs with and without 53BP1 copy loss. Although three tumors with 53BP1 copy loss had either t(14;18) or t(3;y), these frequencies were similar to those of DLBCLs with normal 53BP1 copy numbers (data not shown).…”

“…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). Of note, when normal 53bp1 À/À B cells were activated for CSR, these lymphocytes exhibited dramatically increased genomic instability and frequent IgH chromosomal breaks and translocations (Franco et al, 2006a, b).…”

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).…”