Cardiac-Specific Gene Expression Facilitated by an Enhanced Myosin Light Chain Promoter

Background: Adenoviral gene transfer has been shown to be effective in cardiac myocytes in vitro and in vivo. A major limitation of myocardial gene therapy is the extracardiac transgene expression. Methods: To minimize extracardiac gene expression, we have constructed a tissue-specific promoter for cardiac gene transfer, namely, the 250-bp fragment of the myosin light chain-2v (MLC-2v) gene, which is known to be expressed in a tissue-specific manner in ventricular myocardium followed by a luciferase (luc) reporter gene (Ad. 4 x MLC250.Luc). Rat cardiomyocytes, liver and kidney cells were infected with Ad. 4 x MLC.Luc or control vectors. For in vivo testing, Ad. 4 x MLC250.Luc was injected into the myocardium or in the liver of rats. Kinetics of promoter activity were monitored over 8 days using a cooled CCD camera. Results: In vitro: By infecting hepatic versus cardiomyocyte cells, we found that the promoter specificity ratio (luc activity in cardiomyocytes per liver cells) was 20.4 versus 0.9 (Ad. 4 x MLC250.Luc vs. Ad.CMV). In vivo: Ad. 4 x MLC250.Luc significantly reduced luc activity in liver (38.4-fold), lung (16.1-fold), and kidney (21.8-fold) versus Ad.CMV (p = .01); whereas activity in the heart was only 3.8-fold decreased. The gene expression rate of cardiomyocytes versus hepatocytes was 7:1 (Ad. 4 x MLC.Luc) versus 1:1.4 (Ad.CMV.Luc). Discussion: This new vector may be useful to validate therapeutic approaches in animal disease models and offers the perspective for selective expression of therapeutic genes in the diseased heart. Mol Imaging (2004) 3, 69-75.