shRNA-armed conditionally replicative adenoviruses: a promising approach for cancer therapy

Abstract

The small-interfering RNAs (siRNAs) have been employed to knockdown the expression of cancer-associated genes and shown some promise in cancer therapy. However, synthetic siRNA duplexes or plasmid mediated delivery of siRNAs have several problems, such as short half-life, low transfection efficiency and cytotoxicity associated with transfection. Conditionally replicating adenovirus (CRAds) as the delivery vector for short hairpin RNAs (shRNAs) could overcome these limitations and have shown augmented anti-tumor effects in experimental studies and preclinical trials. In this review, we summarize recent progress in the development of CRAds-shRNA for cancer treatment. Combination of CRAds-shRNA with chemotherapeutics, radiation, dendritic cells, monoclonal antibodies and small-molecule inhibitors will be necessary to eradicate cancer cells and cancer stem cells and achieve superior outcomes. The use of CRAd platform for efficient delivery of shRNAs and foreign genes will open a new avenue for cancer therapy.

Keywords: RNA interference; cancer; conditionally replicating adenoviruses; target therapy.

Figure 1. A schematic diagram of anti-tumor effects of CRAd-shRNA based therapy

After CRAds infect and replicate in tumor cells, shRNAs are expressed within the nucleus where they spontaneously form hairpin RNAs and are transported to the cytoplasm. Then shRNAs are cleaved by Dicer into active siRNAs. Binding of the siRNA to RISC results in the activation of this complex, and after subsequent duplex unwinding, RISC facilitates the binding of target homologous mRNAs. Perfect binding sequences result in the cleavage and silencing of target genes.

Figure 2. The schematic stucture of CRAd vector for shRNA

A. Wild type adenovirus. B. In recombinant adenovirus, E1B55KD gene was replaced by one shRNA sequence expression cassette. C. CRAd vector harboring double-cistronic shRNA expression construct. D. CRAd armed with shRNA and tumor specific promoter. E. CRAd armed with shRNA and exogenous therapeutic genes.