Heterozygous mice are viable and fertile, homozygotes are embryonic lethal. The HCN4CreERT2 allele expresses CreERT2 from the hyperpolarization-activated, cyclic nucleotide-gated K+ 4 (HCN4) promoter/enhancer elements. HCN4 is expressed in the first heart field of differentiating myocytes in the developing cardiac crescent. Cre-ERT2 fusion gene activity is observed in the same cells only following tamoxifen administration. When these mice are bred with mice containing loxP-flanked sequence, Cre-mediated recombination will result in deletion of the floxed sequences in the cardiac conduction system (CCS) of the offspring.
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 coadministered.
