A Cre-loxP based mouse model for conditional somatic gene expression and knock-down in vivo using avian retroviral vectors

Abstract

Site and time-specific somatic gene transfer using the avian sarcoma-leucosis retrovirus RCAS, has been shown to be a powerful tool to analyze gene function in vivo. RCAS retroviruses transduce only mammalian cells genetically engineered to express the avian retroviral receptor, TVA. Here, we have generated a knock-in mouse line termed LSL-R26Tva-lacZ with concomitant conditional expression of TVA and lacZ by targeting the Rosa26 locus. A loxP flanked transcriptional stop cassette was used for conditional activation of TVA and LacZ expression in a Cre recombinase dependent manner. To demonstrate the ability of this system for site and time-specific somatic gene transfer in vivo, we directed TVA expression to the pancreas. Introduction of an RCAS vector carrying an oncogenic KrasG12D expression cassette induced focal ductal pancreatic lesions that recapitulate human pancreatic intraepithelial neoplasias which progress to pancreatic ductal adenocarcinomas. TVA mediated infection of genetically engineered mice with endogenous expression of KrasG12D in pancreatic progenitor cells using RCAS virus carrying a short hairpin RNA directed against murine TP53, resulted in dramatically enhanced progression to invasive adenocarcinomas. These results show that conditional expression of TVA enables spatio-temporal gene expression and knock-down in a small subset of somatic cells in vivo. Therefore, it closely models carcinogenesis in humans where tumors evolve from somatic gene mutations in developmental normal cells. Combined with the growing number of Cre expression models, RCAS-TVA based gene expression and knock-down systems open up promising perspectives for analysis of gene function in a time-controlled and tissue-specific fashion in vitro and in vivo.