The Agc1tm(IRES-CreERT2) allele has a CreERT2 sequence, and a frt-flanked neo cassette, inserted in the 3' untranslated region of the aggrecan (Acan) promoter/enhancer elements. Homozygous mice are viable and fertile. Aggrecan is an extracellular matrix protein expressed in the growth plate and articular cartilage, and fibrocartilage of the meniscus, trachea, and intervertebral disks. When Agc1creERT2 mice are bred with mice containing loxP-flanked sequence, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the aggrecan-expressing cells of the offspring.
Recent studies have shown that the insertion of the CreERT2 cassette results in a deletion of 760 bp in the 3'UTR of the Acan gene. This results in a cre dosage-dependent reduction in body weight and body length by one month of age due to a reduction in the length of the growth plate and the thickness of articular cartilage. Due to this effect on skeletal growth, heterozygous Agc1+/cre should be used for conditional deletion of a target gene in the cartilage tissue.
The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor which does not bind its natural ligand (17β-estradiol) at physiological concentrations but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, Cre-ERT2 can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be co-administered.
