Tumour modelling using viral vectors

Human tumours typically harbour multiple mutations in tumour suppressor genes and oncogenes. Histologically identical tumours in different patients often exhibit different combinations of genetic alterations and it is now also evident that considerable genetic heterogeneity can exist between cells within individual tumours. A major ongoing research challenge remains to determine the functional significance of this enormous genetic diversity and heterogeneity in terms of impact on tumour phenotypes and responses to therapies. Autochthonous mouse models of human tumours have provided many insights into the combinations of genetic alterations that are causal to tumour initiation and progression in different tissues. However, conventional germline-based genetic approaches, such as the generation of compound mutant mice based on transgenics and knockouts, are limited by the time and cost associated with extensive intercrossing of mouse lines. Faster and more powerful genetic tools are required to accelerate the functional annotation of cancer mutational cataloguing studies. Excitingly, several new viral vector-mediated genetic approaches offer the ability to directly modify the genome of somatic cells in mouse tissues and these have recently been applied to the rapid generation of complex mouse tumour models that harbour multiple genetic changes. The use of lentiviral gene delivery vectors for cancer modelling studies offers the advantage that the proviral DNA integrates into the genome of infected cells, providing heritability of the introduced genetic alterations. Injections of lentiviruses that co-express shRNAs against Trp53 and Nf1 or that co-express oncogenic Hras and shRNA against Trp53 were employed to generate autochthonous mouse models of glioma [1]. This study provided proof-of-principle that the lentiviral approach is a feasible tool for cancer modelling. Building upon this idea, we recently generated the MuLE lentiviral system a highly flexible genetic toolbox allowing combinatorial gene knockdown, knockout, mutation or overexpression, together with tagging of infected cells with useful markers such as luciferase or fluorescent proteins [2]. We demonstrated the utility of this system for generating genetically complex autochthonous tumour models by showing that intramuscular injection of ecotropic MuLE lentiviruses expressing luciferase, oncogenic Hras and shRNAs against Cdkn2a or Trp53 or against both Trp53 and Pten, induced the formation of pleomorphic sarcomas that could be quantitatively monitored using luciferase-based imaging [2]. The histology of these tumours was indistinguishable from the pleomorphic sarcomas that arose through transgenic or adenoviral Cre-mediated genetic deletion of Trp53 and activation of oncogenic Kras in germline-modified mice [3,4], validating that the MuLE-mediated somatic genetics approach can rapidly and …