Genome-wide forward genetic screens in mouse ES cells

Mouse embryonic stem (ES) cells are an attractive model system for investigating mammalian biology. Their relatively stable genome and high amenability to genome modification enables the generation of large-scale mutant libraries, which can be subsequently used for phenotype-driven genetic screens. While retroviral vectors have traditionally been used to generate insertional mutations in ES cells, their severe distribution-bias in the mammalian genome substantially limits genome-wide mutagenesis. The recent development of the DNA transposon piggyBac offers an efficient and highly versatile alternative for achieving more unbiased mutagenesis. Furthermore, heterozygous mutations created by insertional mutagens can be converted in parallel to homozygosity by using Blm-deficient ES cells, allowing genome-wide loss-of-function screens to be conducted. In this chapter, we describe the principles underpinning genetic screens in mouse ES cells with examples of previously successful screens. Protocols are provided for piggyBac transposon-mediated mutagenesis, production of the corresponding homozygous mutants in a Blm-deficient genetic background, and methods for mapping and validation of mutations recovered from screens of such libraries.