Genome editing and animal models

Transgenic technology allows a gene of interest to be introduced into the genome of a laboratory animal,and provides an extremely powerful tool to dissect the molecular mechanisms of disease.Transgenic mouse models made by microinjection of DNA into zygotic pronuclei in particular have been widely used by the genetics community for 30 years.However,it remains a rather crude approach:injected sequences randomly insert in multiple copies as concatamers,they can be mutagenic,and they have variable or silenced expression depending on the site of integration,a phenomenon called position effects.As a result,multiple lines are required in order to confirm appropriate transgene expression.This can be partially overcome by flanking transgenes with insulator sequences to protect the transgene from the influence of the surrounding regulatory elements.Large(\300 kb)BACbased transgenic vectors have also been shown to be more resistant to position effects.However,animals carrying extra copies of fairly large regions of the genome could have unpredictable phenotypes.The most effective method used to control for position effects is to target transgene insertion to specific genomic loci,the so-called targeted transgenesis;for instance,the fast,site-specific transgenic technology Targatt TM.The purpose of this review is to provide an overview on the current existing methods for making targeted transgenic mouse models.