The alpha;MHC-MerCreMer transgene has the mouse cardiac-specific alpha-myosin heavy chain promoter (αMHC or alpha-MHC; Myh6) directing expression of a tamoxifen-inducible Cre recombinase (MerCreMer) to juvenile and adult cardiac myocytes. Mice homozygous for the αMHC-MerCreMer transgene are viable and fertile prior to tamoxifen administration. Cre recombinase expression in heart tissue is confirmed by western blot. Southern blot confirmed heart cell-specificity compared to brain, kidney, lung, liver and skeletal muscle.

The MerCreMer double fusion protein has substantially greater Cre recombinase activity with less promiscuity compared with the CreMer single fusion protein. When αMHC-MerCreMer transgenic mice are bred with mice containing loxP-flanked sequences, tamoxifen-inducible Cre-mediated recombination is expected to result in deletion of the floxed sequences in heart cells of the offspring. These αMHC-MerCreMer transgenic mice allow the creation of double mutant/bitransgenic animals for Cre-lox studies of temporally regulated deletion of loxP-flanked targeted genes in cardiac tissues/cells.

View cre expression characterization.

A 2013 publication reported that αMHC-MerCreMer transgenic mice given multi-dose tamoxifen injections develop heart defects (Lexow et al. 2013 Dis Model Mech; PMID:23929940). Specifically, those defects were decreased cardiac function, focal fibrosis (left ventricle, septum and right ventricle), focal cell infiltration into the myocardium, significantly elevated proinflammatory markers (IL1β, IL6, TNFα, IFNγ and Ccl2) and significantly elevated cardiac remodeling markers (ANF, BNP and Col3A1). Importantly, a single tamoxifen injection (40 mg tamoxifen/kg body weight) resulted in the same uniform recombination in cardiomyocyte fibers without the cardiomyopathological effects. Therefore, it is strongly recommended that the most appropriate induction method for each model be determined prior to extensive experimentation, and the optimal dose of tamoxifen be adjusted to the lowest concentration required for achieving sufficient recombination. Please refer to Lexow et al. 2013 for a detailed discussion.

A 2016 publication extensively characterized the transgene insertion site (Harkins and Whitton 2016 Transgenic Res; PMID:27165291) using the C57BL/6-congenic αMHC-MerCreMer strain (Stock No. 005657, which was derived from Stock No. 005650). Their study revealed the insertion site on chromosome 19 was accompanied by a 19.5 kbp deletion of genomic DNA (chr19:31,852,891-31,872,392 bp). The deleted region spans a portion of the A1cf locus (including the promoter, exon 1 and part of intron 1), as well as several computer-predicted regulatory sites. This deletion eradicates A1cf protein expression in the expected gene-dosage fashion: hepatic A1CF protein levels were highest in non-carriers (C57BL/6J), intermediate in hemizygotes and undetectable in homozygotes. Importantly, in the tissue expressing MerCreMer (heart), no A1CF protein was detected in non-carriers (C57BL/6J), hemizygotes or homozygotes. To mitigate any effects of A1CF-deficiency, researchers should distinguish the zygosity of the αMHC-MerCreMer mice most suitable for their experiment.

Harkins and Whitton 2016 also described the transgene components and sequence rearrangements; these aspects are more fully reviewed in the strain development section. Regarding expression, these additional elements have not been reported to have any adverse effects on the cell specificity of transgene expression nor the efficacy of tamoxifen-inducible Cre recombinase activity.

The MerCreMer double fusion protein consists of Cre recombinase flanked on each end with a mutated murine estrogen receptor (mer) ligand binding domain (amino acids 281-599, G525R); 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, MerCreMer 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.

In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. It should be noted that the phenotype could vary from that originally described. We will modify the strain description if necessary as published results become available.