Title: Novel Roles of Ataxia Telangiectasia Mutated (ATM) in DNA Repair and Tumor Suppression
Description: Mammalian cells possess a variety of different DNA repair pathways, which work together to safeguard genomic integrity upon encountering different types of DNA damage. Among all lesions, DNA double-strand breaks (DSBs) are most toxic and, if left unrepaired, results in loss of genetic information and genomic instability- a hallmark of tumorigenesis. Ataxia Telangiectasia Mutated (ATM) is a protein kinase, a master regulator of the DNA damage response, and is activated upon the formation of DSBs. ATM senses DNA DSBs through its accessory proteins and functions as a transducer of the DNA damage response (DDR), which entails the activation of genes involved in DNA repair, cell cycle checkpoint, and apoptosis. Consequently, loss of ATM results in increased genomic instability and compromised checkpoint regulation. Moreover, loss of ATM has been reported in various human cancers, and Atm-deficient mice uniformly develop thymic lymphomas, highlighting its role as a tumor suppressor. Although ATM has been extensively studied, much of its known functions to date pertained to its kinase activity, and the structural function of ATM remains elusive.

To investigate whether ATM possesses structural functions beyond its kinase activity, we generated a mouse model expressing kinase-dead (KD) ATM protein. Intriguingly, while Atm-/- are viable, AtmKD/KD and AtmKD/- mice were embryonic lethal and AtmKD/KD and AtmKD/- cells displayed greater genomic instability compared to ATM-null cells, suggesting that the presence of the ATM KD protein blocks additional DNA repair pathways that are not affected in ATM-null cells. In this context, we identified defects in homologous recombination, resolution of Camptothecin (CPT)-induced Topoisomerase-I lesions, and replication progression specifically in AtmKD/- cells beyond those observed in Atm-/-. Mouse model expressing KD ATM (AtmKD/-) in hematopoietic stem cells (HSCs) developed thymic lymphomas faster and more frequently than the corresponding model with the ATM-null HSCs, which was associated with increased genomic instability and loss of tumor-suppressor Pten. In collaboration with others, we showed that the majority of tumor-associated ATM mutations reported in TCGA are missense mutations and are highly enriched in the kinase domain, while Ataxia-Telangiectasia (A-T) associated germline ATM mutations are almost always truncating mutations leading to complete loss of ATM protein.

This result suggests that ATM KD protein might be expressed in a significant fraction of human cancer. These results, for the first time, identified a previously unknown phosphorylation-dependent, structural function of ATM in the maintenance of genomic integrity and tumor suppression. Furthermore, the tumorigenicity and vulnerability to particular DNA damaging agents caused by the expression of the ATM KD protein relative to the loss of ATM highlight the importance of distinguishing the types of ATM mutations in tumors, and provide novel insights into the clinical use of specific ATM kinase inhibitors, as well as the prognosis and treatments of ATM-mutated cancers. ATM has been reported to be frequently inactivated in human B-cell lymphomas, including up to 50% Mantle Cell Lymphoma (MCL), which represents around 6% of all Non-Hodgkins Lymphomas (NHLs). MCL is characterized by the recurrent t(11;14)(q13;q32) translocation, which juxtaposes CCND1/BCL-1 to the IGH enhancer, leading to deregulated expression of CyclinD1 (CCND1).

However, CyclinD1 overexpression in B cells alone is not sufficient to induce MCL in mouse models, and the role of ATM in the suppression of B-cell lymphomas is not well understood, in part due to the lack of ATM-deficient mature B-cell lymphoma models. To address this, we generated a mouse model that combines conditional deletion of ATM specifically in early progenitor B-cells via Mb1cre, and overexpressing CyclinD1 in lymphoid cells via EµCyclinD1 transgene. While ATM loss alone resulted in the development of indolent, clonal, mature B-cell lymphoma, combined ATM-loss and CyclinD1 overexpression accelerated and increased the incidence of B-cell lymphoma. Furthermore, ATM-loss combined with CyclinD1 overexpression led to greater genomic instability and the expansion of naïve ATM-deficient B-cells in the spleen. This study, for the first time, developed an ATM-deficient B-cell lymphoma model and demonstrated a synergistic function of ATM and CyclinD1 in pre-GC B-cell proliferation and lymphomagenesis.

Furthermore, the mice described here provide a prototypic animal model to study the pathogenesis of human MCL, for which there are no suitable mouse models.