Cardiac transgenesis with the tetracycline transactivator changes myocardial function and gene expression

The cardiac-specific tetracycline-regulated gene expression system (tet-system) is a powerful tool using double-transgenic mice. The cardiac alpha-myosin heavy chain promoter (alpha MHC) drives lifetime expression of a tetracycline-inhibited transcription activator (tTA). Crossing alpha MHC-tTA mice with mice containing a tTA-responsive promoter linked to a target gene yields double-transgenic mice having tetracycline-repressed expression of the target gene in the heart. Using the tet-system, some studies use nontransgenic mice for the control group, whereas others use single-transgenic alpha MHC-tTA mice. However, previous studies found that high-level expression of a modified activator protein caused cardiomyopathy. Therefore, we tested whether cardiac expression of tTA was associated with altered function of alpha MHC-tTA mice compared with wild-type (WT) littermates. We monitored in vivo and in vitro function and gene expression profiles for myocardium from WT and alpha MHC-tTA mice. Compared with WT littermates, alpha MHC-tTA mice had a greater heart-to-body weight ratio (approximate to 10%), ventricular dilation, and decreased ejection fraction, suggesting mild cardiomyopathy. In vitro, submaximal contractions were greater compared with WT and were associated with greater myofilament Ca2+ sensitivity. Gene expression profiling revealed that the expression of 153 genes was significantly changed by > 20% when comparing alpha MHC-tTA with WT myocardium. These findings demonstrate that introduction of the alpha MHC-tTA construct causes significant effects on myocardial gene expression and major functional abnormalities in vivo and in vitro. For studies using the tet-system, these results suggest caution in the use of controls, since alpha MHC-tTA myocardium differs appreciably from WT. Furthermore, the results raise the possibility that the phenotype conferred by a target gene may be influenced by the modified genetic background of alpha MHC-tTA myocardium.