PS925 LMO2‐INDUCED MURINE T‐CELL ACUTE LYMPHOBLASTIC LEUKEMIA FREQUENTLY EVOLVES TO ONCOGENE INDEPENDENCE
Authors: H. Abdulla, T. Davies, J. Jackson, A. Vo, R. Alserihi, E. Viney, T. Wong, W. Shi, W. Alexander, R. Dickins, B. Shields, M. McCormack
Journal: HemaSphere (2019)

Background: LMO2 is an oncogenic transcription factor that is frequently overexpressed in T‐cell acute lymphoblastic leukemia (T‐ALL) including poor prognosis Early T‐cell Precursor‐like ALL (ETP‐ALL). In transgenic mouse models, LMO2 overexpression causes thymocyte self‐renewal resulting in T‐cell leukemia with long latency. However, the requirement for LMO2 for leukemia maintenance is poorly understood. Aims: 1. To develop a regulatable mouse model of LMO2‐induced T‐ALL. 2. To determine the molecular features associated with LMO2‐dependence in T‐ALL. Methods: To develop a regulatable model of LMO2‐induced T‐ALL, LMO2 was cloned downstream of the Tetracycline‐regulated element (TRE) promoter (see Figure). To allow us to track LMO2 expression, a Green Fluorescence Protein (GFP) reporter followed by a self‐cleaving peptide (P2A) was cloned upstream of LMO2. This construct was knocked into the Col1a1 locus to generate knock‐in mice (referred to as TRE‐LMO2 mice). To achieve broad overexpression of LMO2 throughout the hematopoietic system, TRE‐LMO2 mice were bred with Vav‐tTA transgenic mice, which express the tetracycline transactivator (tTA) downstream of the pan‐hematopoietic Vav promoter, to generate Vav‐tTA;TRE‐LMO2 mice (referred to as VTL mice), in which LMO2 is overexpressed throughout the hematopoietic system and can be repressed by administration of Doxycycline (Dox) (see Figure). Results: Vav‐tTA;TRE‐LMO2 mice showed widespread LMO2 overexpression throughout the hematopoietic system. This led to a specific block in T‐cell development and to the development of transplantable, self‐renewing T‐cell progenitors in the thymus from a young age, referred to as preleukemic stem cells (pre‐LSCs). This was followed by development of fully penetrant T‐lymphoblastic leukemia resembling human T‐ALL from 5 months of age. In preleukemic mice, Dox‐induced repression of LMO2 rapidly overcame the T‐cell developmental block and eliminated self‐renewing pre‐LSCs in the thymus. In contrast, LMO2 was dispensable for many LMO2‐induced T‐cell leukemias and leukemia‐derived cell lines, implying an evolution of oncogene addiction in many T‐ALLs. LMO2‐dependent T‐ALL correlated with an immature gene expression profile similar to human ETP‐ALL, but could not be predicted by immunophenotype or examination of Notch pathway activation. Summary/Conclusion: We have successfully created a regulatable model of LMO2‐driven murine T‐ALL. image Whilst continuous LMO2 expression is essential for its preleukemic effects, including T‐cell developmental blockade and thymocyte self‐renewal, LMO2 is frequently dispensable for maintenance of LMO2‐induced murine T‐ALL. LMO2‐dependent murine T‐ALL has an ETP‐like gene expression profile. The regulatable model presented here will be useful to determine the molecular features associated with LMO2‐dependence and critical components of LMO2‐induced self‐renewal pathways in poor prognosis ETP‐ALL.