Genetic Manipulation of the Renin-Angiotensin System Using Cre-loxP-Recombinase

The advent of gene-targeting technology in embryonic stem cells has provided an important tool for the dissection of complex biological systems by allowing investigators to generate germ line mutations in selected genes. Since the introduction of this technology in the early 1980s, hundreds of genes have been targeted for systemic deletion (knocked out), including each gene of the renin-angiotensin system (RAS). Although the technique is very powerful, there are weaknesses that limit its usefulness for studying the RAS. For example, systemic deletion of several of the RAS genes leads to a phenotype, of varying severity depending on the gene in question, in which the mice suc5cumb to severe renal lesions and ultimately die before the age of weaning. This is observed in angiotensinogen (Agt-/-), angiotensin-converting enzyme (ACE-/-), and combined angiotensin receptor subtype 1A and 1B (At1a-/-, At1b-/-) deficient "knockout" mice (1-5). Mice deficient in At1a, but wild type at the At1b locus, are phenotypically normal on mixed genetic backgrounds, but exhibit the same renal lesions and reduced mortality when bred onto "pure" genetic backgrounds, suggesting that renal morphology in response to Ang-II may be under some complex genetic control (6). Presumably, Ang-II is required during the early neonatal period for the continued development of the kidney, and the mice die if they are unable to either generate or utilize An-II during this period.