Deletion of Atm in the Hematopoetic Stem Cell As a Mouse Model for Human T Cell Acute Lymphoblastic Leukemia/Lymphoma.

Abstract 2418

Acute lymphoblastic leukemia (ALL) is diagnosed in approximately 2500 children per year. Although high cure rates have been achieved for ALL, these cancers account for the highest number of non-brain tumor cancer-related deaths in children. T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of immature TCRβ⁻CD4⁺/CD8⁺ T-cells that represents ∼15% of pediatric ALL diagnoses, comprises most of the therapy-resistant ALL tumors, and exhibits a high frequency of relapse.

The Ataxia Telangiectasia mutated (ATM) protein kinase activates the cellular response to DNA double strand breaks (DSBs) to coordinate DNA repair with cell survival, proliferation, and differentiation. Somatic inactivating ATM mutations occur in 10–20% of T-ALL and T cell lymphoblastic lymphoma (T-LL) tumors and are associated with resistance to genotoxic chemotherapy drugs and therapy relapse, likely driven by increased genomic instability in cells lacking functional ATM. The impaired DSB response of ATM-deficient cells can be exploited to design combinations of genotoxic drugs that specifically kill these cells in vitro. However, the in vivo potential of such drug combinations to treat T-ALL have not been reported.

We sought to develop a pre-clinical mouse model that could be used to test effectiveness of such drug combinations to treat T-ALLs and T-LLs with somatic ATM inactivation. Although germline ATM-deficient (Atm^−/−) mice succumb by six months of age to immature CD4⁺/CD8⁺ T-cell lymphomas containing genomic instability analogous to human T-ALL tumors, we sought a more physiologic model that would avoid potential complications due to ATM-deficiency in thymic epithelial cells. Thus, we generated and characterized VavCre:Atm^flox/flox mice with conditional Atm inactivation restricted to hematopoietic cell lineages. These mice contain reduced numbers of TCRβ⁻CD4⁺/CD8⁺, TCRβ⁺CD4⁺/CD8⁻, and TCRβ⁺CD4⁻/CD8⁺ thymocytes and of TCRβ⁺CD4⁺ and TCRb⁺CD8⁺ splenic T-cells, mirroring the phenotype of Atm^−/− mice. We have found that VavCre:Atm^flox/flox mice succumb at an average of 95 days (range 53–183 days) to clonal TCRβ⁻CD4⁺/CD8⁺ or TCRβ⁺CD4⁻/CD8⁺ thymic lymphomas. Evaluation of the bone marrow in a subset of these mice indicates that the lymphoma has disseminated and are classified as leukemia. Our initial cytogenetic analyses of these tumors indicate that they contain both clonal translocations involving chromosome 12 and/or chromosome 14 and deletion of one allelic copy of the haploinsufficient Bcl11b tumor suppressor gene. Hemizygous BCL11B inactivation occurs in ∼20% of human T-ALL tumors, indicating the clinical relevance of VavCre:Atm^flox/flox mice as a model for human T-ALL. Our ongoing studies include complete cytogenetic and molecular characterization of VavCre:Atm^flox/flox tumors and in vivo testing of chemotherapeutics targeting the Atm pathway in this mouse model of T-ALL/T-LL.