Enter the knockout rat

The mouse has gained a lot of ground in neurobiological research owing to its strength as a genetic model. But the rat has a long history as an experimental model in neuroscience, and there is a large body of knowledge that has come from its study in fields such as behaviour and neurophysiology. In many cases, the emergence of the mouse as the model of choice has imposed on us the need to characterize in this species what we already knew in the rat, which is why many scientists still hope for the advent of the rat as a genetic model. This hope might soon become a reality, according to a paper recently published in Nature Biotechnology.

Zan et al. report on a two-step method to generate knockout rats. The first step entails the generation of germline mutations in male rats using N-ethyl-N-nitrosourea (ENU) such that their offspring carry the genetic alteration. In the second step, the preweanling offspring (F₁) from the ENU-treated males is individually screened for the mutation of interest using a yeast-based system. More specifically, the screening system depends on a vector in which there is a gap between the ADH1 yeast promoter and the ADE2 gene. The gene of interest from each of the F₁ rats is then cloned into this gap. If the cloned gene is wild type, it is transcribed and translated together with ADE2, generating a chimeric protein that enables the yeast to form large, white colonies. But if the inserted gene has a mutation that interferes with the formation of the chimeric protein, then the yeast colonies are small and red. Once an F₁ mutant rat is found, it can be used as a heterozygous founder, interbreeding its offspring (F₂) to generate homozygous mutant rats (F₃). Using this method, Zan et al. successfully knocked out the breast-cancer suppressor genes Brca1 and Brca2.

This new method should stimulate the generation of rats that lack genes that are important for neuronal and glial function. Although we are still a long way from achieving in rats the sophisticated temporal and spatial control over gene expression that we already have in mice, this strategy is an important step in the right direction.