Trans-complementation of vector replication versus Coxsackie-adenovirus-receptor overexpression to improve transgene expression in poorly permissive cancer cells

Gene therapy of cancer requires high-level expression of therapeutic transgenes in the target cells. Poor gene transfer is an important limitation to adenovector-mediated cancer gene therapy. We investigated two fundamentally different approaches to improve transgene expression in poorly permissive cancer cells. First, overexpression of the adenovirus attachment receptor CAR to facilitate receptor-mediated adenovector (AdV) uptake into the target cells; second, coinfection of this vector together with traces of replication competent adenovirus (RCA) accidentally arising by back-recombination during large-scale vector preparation. Among eight gastrointestinal cancer cell lines, the colorectal cancer lines showed particularly poor vector-mediated transgene expression (down to 67-fold lower than in HeLa cells). Expression of the adenovirus receptors CAR, alpha (v)beta (5)- and alpha (v)beta (3)-integrin were highly variable between cell lines. AdV uptake was significantly associated with CAR levels on the cell surface, but not with those of the integrins. AdV-mediated CAR overexpression increased CAR density on the surface of all investigated tumor cells and led to enhancement of transgene expression by 1.8- to 6.7-fold. The other principle to enhance transgene expression was 'trans-complementation' of the therapeutic vector, ie induction of ifs replication within the target cells. Traces of RCA in a vector preparation, as well as purified RCA were found to provide sufficient E1-region transcripts to induce replication of the therapeutic vector genome. The number of adenovector-based transgene expression cassettes was greatly amplified by this principle, notably without any influence on the rate of vector entry. Co-infection of four colorectal cancer cell lines with marker vector plus RCA (at around 240:1 particle ratio) resulted in far stronger enhancement of transgene expression (up to 46-fold) as compared with CAR overexpression, even in cancers almost refractory to standard adenovector-mediated gene transfer. Whereas RCAs need to be strictly avoided in gene therapy of non-malignant diseases for safety reasons, the magnitude of helper virus-induced therapeutic transgene expression could possibly warrant application of this principle to overcome the resistance of highly malignant cancers against gene therapy.