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1 Radiology, University of California, San Francisco, San Francisco, CA, USA; Cardiology, Veterans Affairs Medical Center, San Francisco, CA, USA
2 Medicine, University of California, San Francisco, San Francisco, CA, USA; Cardiology, Veterans Affairs Medical Center, San Francisco, CA, USA
3 Gladstone Institute of Cardiovascular Disease, University of California, San Francisco, San Francisco, CA, USA
4 Medicine, University of California, San Francisco, San Francisco, CA, USA
5 Medicine, University of California, San Francisco, San Francisco, CA, USA; Cardiovascular Research Institute, University of California, San Francisco, San Francisco, CA, USA
6 Medicine, University of California, San Francisco, San Francisco, CA, USA; Gladstone Institute of Cardiovascular Disease, University of California, San Francisco, San Francisco, CA, USA
7 Medicine, University of California, San Francisco, San Francisco, CA, USA; Cardiovascular Research Institute, University of California, San Francisco, San Francisco, CA, USA; Cardiology, Veterans Affairs Medical Center, San Francisco, CA, USA
* To whom correspondence should be addressed. E-mail: ajbaker{at}itsa.ucsf.edu.
The cardiac-specific tetracycline-regulated gene expression system (tet-system) is a powerful tool using double-transgenic mice. The cardiac 
-myosin heavy chain promoter (MHC) drives lifetime expression of a tetracycline-inhibited transcription activator (tTA). Crossing 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 non-transgenic mice for the control group, while others use single transgenic 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 MHC-tTA mice compared to wild-type (WT) littermates. We monitored in-vivo and in-vitro function, and gene expression profiles for myocardium from WT- and MHC-tTA mice. Compared to WT littermates, MHC-tTA mice had a greater heart/body weight ratio (~10%), ventricular dilation and decreased ejection fraction suggesting mild cardiomyopathy. In-vitro, submaximal contractions were greater compared to 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 MHC-tTA versus WT myocardium. These findings demonstrate that introduction of the 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 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 MHC-tTA myocardium.
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