Generation of a Transgenic Mouse with Inducible Constitutively Active Stat5a,
Authors: Nivarthi Harini , Tsyrulnyk Andriy , Warsch Wolfgang , Wang Zhengqi , Winkler Susanne , Kenner Lukas , Müller Mathias , Baum Christopher , Bunting Kevin D. , Sexl Veronika , Rülicke Thomas , Moriggl Richard 
Published: October 2019

Abstract
Abstract 3399
The Jak/Stat signalling pathway is essential for survival and proliferation of haematopoietic cells. The Stat5 transcription factors (Stat5a and Stat5b) play a crucial role in the development of various lineages of the hematopoietic system. Stat5 is activated by many cytokines and growth factors that regulate hematopoiesis. Persistent Stat5 activity is frequently found in hematopoietic cancers due to aberrant tyrosine kinase signalling. We have previously characterized a constitutively activated version of Stat5 (called oncogenic Stat5; cS5F), which mimics persistent tyrosine kinase signalling and promotes multi-lineage leukaemia development in a bone marrow transplantation model.
The aim of the study is to generate a novel inducible mouse model for oncogenic Stat5a using the BAC recombineering technology. A cassette expressing cS5F (with a C-terminal FLAG tag) and a reporter gene (truncated human CD2; hCD2) was cloned, sequenced and biochemically verified for persistent tyrosine phosphorylation in absence of cytokines or growth factors. The cassette is flanked by loxP sites in opposite orientation and was used to replace the original Stat5a gene in a BAC carrying the endogenous Stat3/5 locus using BAC recombineering. Therefore, the expression of the transgene is regulated by the endogenous Stat5a promoter. The transgene is in an inverted (off) orientation and can be switched into the transcribing (on) orientation by Cre activity.
Pronuclear injections were performed with the linearized and purified BAC. We obtained 5 transgenic founder lines which showed germ line transmission of the transgene. The copy number of the BAC was estimated to be 2 by Southern blot analysis. The transgenic founder lines were bred with the Rosa CreERT2 mice, and the Cre activity was induced by treating the mice with 1 mg of tamoxifen every day, for 5 consecutive days. The recombination of the transgene into the ‘on' orientation in liver, lungs, kidney, spleen and bone marrow was demonstrated by Southern blotting. The expression of the hCD2 marker was detected in various hematopoietic lineages by FACS and the expression of the transgenic protein in liver was confirmed by Western blotting with anti-Stat5a and anti-FLAG antibodies.
One week after induction of the transgene, all the mice induced with tamoxifen (n=6) developed atrophic thymii with nearly a 4-fold reduction in total thymocyte numbers (p=0.0003). However, the percentage of CD8+ cells was increased more than 3 fold in the thymus (p=0.0002). Moreover, there was a 2-fold reduction in the number of early hematopoietic progenitors; defined as lin−, Sca1+ and c-kit+, in the bone marrow (n=5, p=0.0217).
We have established an inducible mouse model for expression of constitutively active Stat5a under the endogenous promoter. We are currently monitoring these mice for development of cancer as they provide a competent tool to study the molecular mechanisms triggered by persistent Stat5a activity, including identification of other cooperating signalling pathways that contribute to generation of cancer. Moreover, it would allow one to test interference strategies, which could lead to potential therapeutics.

Disclosures:
No relevant conflicts of interest to declare.