A New Murine Model for B Cell Malignancies on ATM-Deficient Background Reveals Involvement of Multiple Pathogenic Mechanisms

Inactivation of Ataxia Telangiectasia Mutated (ATM) gene, a principal regulator of DNA damage responses, is a frequent event in a wide range of sporadic B and T cell lymphoid malignancies and gives rise to ataxia telangiectasia (A-T) characterized by predisposition to a similar tumor spectrum. Due to the defect in ATM/p53 dependent apoptosis, tumors with Atmalterations exhibit poor responses to standard chemotherapy and require novel therapeutic approaches that need testing in preclinical models.

Current animal models for ATM-deficient lymphoid malignancies are limited as they show bias for a single tumor type, mostly thymic lymphoma. We hypothesized that by crossing Atm-/- mice with athymic nude mice (nu-/-) that lack thymic development, the survival of animals would increase, permitting the development of ATM-deficient B cell lymphoid malignancies. As predicted, overall survival of Atm-/-nu-/- (median survival 21 months) mice greatly increased compared to Atm-/- animals that succumb to thymic lymphomas by 4 months (median survival 13 weeks; p<.0001). More importantly, the increased survival was coupled with a shift in the spectrum of malignancies with a predominance of B cell lymphomas. Altogether, neoplasms occurred in 24.3% (17 out of 70) of Atm-/-nu-/- mice revealing variable localization (spleen, liver, abdomen, thorax or axillary region) and immunochemistry consistent with diffuse large B cell lymphoma (DLBCL), follicular lymphoma (FL), Burkitt's lymphoma, CLL-like disease as well as T cell anaplastic large cell lymphoma (ALCL). Similar to A-T patients, DLBCL was the most frequent type of malignancy, observed in 58.8% (10 out of 17) of all cases. The phenotypic heterogeneity of B cell tumors was further supported through their differing cellular origin which encompassed both tumors with independent VDJ recombination as well as those with monoclonal VDJ recombination with or without VH somatic hypermutation. Thus, the development of Atm-/-nu-/-lymphomas was consistent with the role of the germinal centre together with remaining T cells in lymphomagenesis. Importantly, in three animals, lymphomas from different sites shared common VDJ recombination and lymphomas could be propagated into secondary immunodeficient recipients (NOG mice).

To further elucidate the mechanisms of Atm-/-nu-/- tumor development we employed two types of genome wide analysis: spectral karyotype and RNAseq. This analysis revealed participation of three principal pathogenic mechanisms a) c-myc driven lymphomagenesis b) lymphomagenesis driven by translocations involving immune system genes and c) genomic instability as a major driving force associated with translocations involving chromosome 18 and multiple partners.

In conclusion, by crossing Atm knockout mice with nude mice that have greatly diminished T cell development, we have generated a mouse model that faithfully recapitulates the range of Atm-deficient non Hodgkin's lymphomas observed in the general population as well as in A-T patients. Our model highlights the role of genomic instability in Atm-dependent lymphomagenesis, providing a powerful transplantable tool to study the principal pathways involved in Atm-deficient tumors and test their targetability by new compounds.