Parallel In Vivo Screening of Gene Knockout and Activation in the Mouse Mammary Gland

Forward genetics screens are routinely employed to perturb thousands of genetic elements in a pooled fashion with the goal of producing large-scale genotype-to-phenotype maps. While often carried out in cell culture systems, accumulating evidence supports that in vivo screens have the power to unveil new biology that cannot be recapitulated in vitro. However, the widespread application of this approach has been limited by two major challenges: a predominant focus on loss-of-function perturbations rather than gene activation and the significant technical hurdles of delivering complex genetic libraries to specific tissues in vivo. To overcome these challenges, we describe a simple and versatile intraductal injection strategy that enables efficient and rapid functional genomic screening in the mouse mammary gland, by generating tens of thousands of discrete epithelial clones. Furthermore, we provide all the details necessary for library generation, intraductal injection, screen deconvolution, and analysis of CRISPR-Knockout and Activation libraries for comprehensive in vivo screens. Using these tools, which we termed CRISPR-KOALA (Knockout and Activation Linked Assay), we have identified new tumor suppressors and oncogenes within the coding and non-coding genome in pooled libraries ranging from 46 loci to one-fifth of the genome. Importantly, this approach and analysis can be applied to other organs to study the biological function of any gene during homeostasis or disease.