Applications of genome editing on laboratory animals.

For four decades, genetically altered laboratory animals have provided invaluable information. Originally, genetic modifications were performed on only a few animal species, often chosen because of the ready accessibility of embryonic materials and short generation times. The methods were often slow, inefficient and expensive. In 2013, a new, extremely efficient technology, namely CRISPR/Cas9, not only made the production of genetically altered organisms faster and cheaper, but also opened it up to non-conventional laboratory animal species. CRISPR/Cas9 relies on a guide RNA as a 'location finder' to target DNA double strand breaks induced by the Cas9 enzyme. This is a prerequisite for non-homologous end joining repair to occur, an error prone mechanism often generating insertion or deletion of genetic material. If a DNA template is also provided, this can lead to homology directed repair, allowing precise insertions, deletions or substitutions. Due to its high efficiency in targeting DNA, CRISPR/Cas9-mediated genetic modification is now possible in virtually all animal species for which we have genome sequence data. Furthermore, modifications of Cas9 have led to more refined genetic alterations from targeted single base-pair mutations to epigenetic modifications. The latter offer altered gene expression without genome alteration. With this ever growing genetic toolbox, the number and range of genetically altered conventional and non-conventional laboratory animals with simple or complex genetic modifications is growing exponentially.