Phenotypically silent Cre recombination within the postnatal ventricular conduction system

The cardiac conduction system (CCS) is composed of specialized cardiomyocytes that initi- ate and maintain cardiac rhythm. Any perturbation to the normal sequence of electrical events within the heart can result in cardiac arrhythmias. To understand how cardiac rhythm is established at the molecular level, several genetically modified mouse lines expressing Cre recombinase within specific CCS compartments have been created. In general, Cre driver lines have been generated either by homologous recombination of Cre into an endog- enous locus or Cre expression driven by a randomly inserted transgene. However, haploin- sufficiency of the endogenous gene compromises the former approach, while position effects negatively impact the latter. To address these limitations, we generated a Cre driver line for the ventricular conduction system (VCS) that preserves endogenous gene expres- sion by targeting the Contactin2 (Cntn2) 3’ untranslated region (3’UTR). Here we show that Cntn23’UTR-IRES-Cre-EGFP/+ mice recombine floxed alleles within the VCS and that Cre expression faithfully recapitulates the spatial distribution of Cntn2 within the heart.Wefur- ther demonstrate that Cre expression initiates after birth with preservation of native Cntn2 protein. Finally, we show that Cntn23’UTR-IRES-Cre-EGFP/+ mice maintain normal cardiac mechanical and electrical function. Taken together, our results establish a novel VCS-spe- cific Cre driver line without the adverse consequences of haploinsufficiency or position effects.Weexpect that our new mouse line will add to the accumulating toolkit of CCS-spe- cific mouse reagents and aid characterization of the cell-autonomous molecular circuitry that drives VCS maintenance and function.