Abstract A08: Fast and efficient generation of conditional ROSA26-based mouse models that recapitulate oncogene activation in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemias (T-ALLs) are aggressive hematologic tumors that result from the malignant transformation of T-cell progenitors. Due to intensified chemotherapy, the prognosis of T-ALL has gradually improved. Nevertheless, this clinical improvement is most pronounced in pediatric treatment protocols, whereas adult patients more often present with primary resistant or relapsed disease. Our aim is to develop new mouse models that mimic oncogenic lesions identified in human disease. These mouse models are critically required to further enhance our knowledge on the molecular mechanisms that drive T-cell leukemogenesis. Moreover, they serve as important preclinical models to evaluate new therapeutic strategies for the treatment of human T-ALL. Here, we generated a fast method for embryonic stem cell (ESC) targeting and mouse chimera production. We constructed gateway-compatible vectors that allow tailor-made targeting vector design, including conditional expression of a transgene combined with an eGFP/luciferase reporter from the ROSA26(R26)-promoter. The final targeting vector is recombination-mediated cassette exchange (RMCE)-compatible and can be inserted in a genetically engineered R26-locus via RMCE. Correct integration of the incoming construct reactivates the NeoR gene and results in up to 100% ESC targeting efficiencies. Using our technology, we generated R26-based conditional knock-in mouse models for putative oncogenes that have previously been implicated in T-ALL disease biology. More specifically, the MYB leucine zipper transcription factor is aberrantly activated in a subset of T-ALL patients through T-cell receptor driven translocations (t(6;7)(q23;q34)) or genomic duplications of the MYB locus itself. To study the in vivo roles of cMyb in the pathogenesis of T-ALL, we used the above-mentioned genomic engineering technology to generate cMyb conditional knock-in mouse models. These mice were crossed with VaviCre mice to obtain hematopoietic specific expression of Myb and the EGFP/luciferase from the ROSA26-promoter. We demonstrated that forced expression of Myb in hematopoietic precursors is sufficient to drive T-cell leukemia in mice. To allow comparative analysis of tumors with and without T-cell specific Myb expression, we also genetically deleted the tumor suppressor Pten in these mice and found that cMyb expression synergizes with Pten deletion and resulted in fully penetrant and accelerated T-ALL formation. These data demonstrate an important role for Myb in the pathologenesis of T-ALL. Finally, we use this novel murine T-ALL model to find new therapies for T-ALL. Importantly, the tumor cells from the cMyb knock-in mice are luciferase positive and are therefore suitable for in vivo drug testing using bioluminescence. Because cMYB activation drives an oncogenic transcription program by regulation of super-enhancer activity in human T-ALL will test drugs that impede Myb protein stability or Myb-mediated transactivation. Citation Format: Tim Pieters, Sara TSas, Beatrice Lintermans, Sofie Peirs, Filip Mathijssens, Jody Haigh, Geert Berx, Steven Goossens, Pieter Van Vlierberghe. Fast and efficient generation of conditional ROSA26-based mouse models that recapitulate oncogene activation in T-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr A08.