ATM and 53BP1 Function in Nonoverlapping Pathways to Suppress T Lineage Lymphomagenesis.

In response to DNA double-strand breaks (DSB), mammalian cells activate the DNA Damage Response (DDR), a network of factors that promotes their detection, signaling and repair. Among DDR factors, the Ataxia-Telangiectasia Mutated (ATM) kinase and the 53BP1 adaptor localize to chromatin surrounding DSBs, where 53BP1 is thought to transduce a subset of ATM-dependent reactions. In addition, 53BP1 may have ATM-independent functions via its interactions with the MRE11/RAD50/NBS1 (MRN) complex and others to protect DNA ends from degradation and to promote synapsis of distant DSBs. Both ATM and 53BP1 are tumor suppressors in humans and mice, but whether the combined mutation may aggravate the DSB repair defect and/or accelerate oncogenesis in vivo is currently unknown.

1) To generate a compound mouse model deficient for both ATM and 53BP1 and investigate their genetic interaction during growth and development; 2) To determine whether 53BP1 plays ATM-independent functions in DSB repair and cell cycle checkpoint activation in primary thymocytes; 3) To determine the effect of 53BP1 deficiency on the penetrance and latency of T cell lymphomas in ATM^−/− mice.

We have generated 53BP1/ATM double knock out (DKO) mice and assessed the kinetics of DSB signaling and repair in primary thymocytes and mature B cells. We have also followed cohorts of DKO and control mice for tumor development and evaluated mechanisms of tumorigenesis using a combination of molecular, biochemical and cytogenetic approaches.

When compared to ATM^−/− mice, DKO mice showed no differences in organismal growth and development, or in lymphoid differentiation. However, DKO mice developed T cell lymphomas with higher penetrance and at younger age than ATM^−/− mice, and their survival was markedly reduced (p=0.0001; log-rank test). Like ATM^−/− lymphomas, DKO lymphomas contained clonal translocations with breakpoints at the TCRα/δ locus and amplification of upstream DNA sequences, indicating that 53BP1 deficiency markedly accelerates the acquisition of oncogenic translocations in an ATM^−/− background. In primary cells, loss of 53BP1 further increases the frequency of ionizing irradiation (IR)-induced chromosomal aberrations in ATM^−/− cells. We will present a model for how 53BP1 actions at DSBs suppress genomic instability in ATM-deficient thymocytes, including its effects on checkpoint activation and DSB repair.

Our studies in a mouse model of T cell lymphomagenesis suggest that mutations in 53BP1 and ATM may be nonmutually exclusive in human lymphomas. The combined mutation may define a subgroup of patients with more aggressive hematological malignancies, and may have implications for their response to therapy with DNA damaging agents.

No relevant conflicts of interest to declare.