Using the GEMM-ESC strategy to study gene function in mouse models

Preclinical in vivo validation of target genes for therapeutic intervention requires careful selection and characterization of the most suitable animal model in order to assess the role of these genes in a particular process or disease. To this end, genetically engineered mouse models (GEMMs) are typically used. However, the appropriate engineering of these models is often cumbersome and time consuming. Recently, we and others described a modular approach for fast-track modification of existing GEMMs by re-derivation of embryonic stem cells (ESCESCESCs) that can be modified by recombinase-mediated transgene insertion and subsequently used for the production of chimeric mice. This 'GEMM-ESCESCESC strategy' allows for rapid in vivo analysis of gene function in the chimeras and their offspring. Moreover, this strategy is compatible with CRCRISPRSPRSPR/Cas9-mediated genome editing. This protocol describes when and how to use the GEMM-ESCESCESC strategy effectively, and it provides a detailed procedure for re-deriving and manipulating GEMM-ESCESCESCs under feeder-and serum-free conditions. This strategy produces transgenic mice with the desired complex genotype faster than traditional methods: generation of validated GEMM-ESCESCESC clones for controlled transgene integration takes 9-12 months, and recombinasemediated transgene integration and chimeric cohort production takes 2-3 months. The protocol requires skills in embryology, stem cell biology and molecular biology, and it is ideally performed within, or in close collaboration with, a transgenic facility.