Vigilant vector: heart-specific promoter in an adeno-associated virus vector for cardioprotection.

Repeated bouts of ischemia in the heart lead to fibrosis and eventually to heart failure. Although certain genes, such as SOD or hemoxygenase and antisense to AT(1)R, ACE, and (beta(1)-AR can provide short-term protection of the heart from ischemia, there is no known mechanism for constantly responding to repeated incidences of ischemia. We hypothesized that a "vigilant vector," designed to be expressed specifically in the heart and switch on therapeutic genes only during hypoxia, would provide cardioprotection. To attain cardiac specificity, we inserted an MLC2v promoter into an adeno-associated virus (AAV) designed to deliver AS to AT(1)R and gfp. In in vitro experiments in cardiomyocytes (H9C2 cells), the MLC2v-AAV-gfp drove gene expression in all cells at levels comparable to a cytomegalovirus (CMV) promoter. In in vivo experiments, the rAAV-MLC2v-gfp was injected intravenously into mice or rats. Green fluorescence protein (GFP) DNA was located in kidney, heart (right and left ventricle), lung, adrenal and spleen. GFP mRNA, however, was expressed only in the heart and absent in other tissues. To switch on the rAAV transgene during ischemia, we inserted a hypoxia response element (HRE). This upregulates transcription when O(2) levels are low. Thus, there are 4 components to the vigilant vector; a gene switch (HRE), a heart-specific promoter (MLC2v), a therapeutic gene (AS-AT(1)R) and a reporter gene (gfp). To silence or lower basal level of expression while retaining specificity, we have reduced the length of the MLC2v promoter from 3 kb to 1775 bp or 281 bp. The truncated promoter is equally effective in heart specific expression. Preliminary studies with the rAAV-HRE-gfp in vitro show an increased expression in 1% O(2) in 4 to 6 hours. By adding additional hypoxia-inducible factor (HIFalpha) (5 microg), the MLC2v-gfp expression is increased by 4-fold in 1% O(2). Further amplification of the gene to 400-fold in 1% O(2) can be achieved with a double plasmid. The construct may serve as a prototype "vigilant vector" to switch on therapeutic genes in specific tissue with physiological signals.