Suicide gene therapy is a cancer treatment strategy that targets tumors with toxic genes, or genes encoding enzymes that produce toxic compounds. However, since restricting suicide gene expression to the tumor is difficult, this approach can damage non-cancerous cells.
Now, Shu Wang of the Institute of Bioengineering and Nanotechnology of A*STAR, Singapore, and co-workers have successfully treated glioma in the mouse brain, while confining suicide gene expression to the tumor tissue1. They developed a novel approach that uses small nucleic acids called microRNAs, which regulate gene expression by binding to specific target DNA sequences.
Wang’s group used DNA microarrays to examine global microRNA expression in human glioma cell lines and healthy astrocytes, the brain cells from which gliomas arise. They identified three microRNAs that have increased expression in normal astrocytes and reduced expression in glioma cells.
The researchers constructed genetically engineered insect viruses containing the gene encoding the fluorescent firefly protein luciferase. The gene is controlled by the regulatory DNA sequence of a gene that is normally expressed only in normal astrocytes. They also inserted multiple copies of the target sequences of the three previously identified microRNAs. When introduced into healthy cultured astrocytes, the constructs containing two or three copies of each microRNA target sequence inhibited luciferase expression by 80%.
Wang and his co-workers then inserted the herpes simplex thymidine kinase (HSVtk) gene into these constructs. HSVtk metabolizes the drug ganciclovir into a toxic product that destroys cells by blocking DNA synthesis. When they added ganciclovir to the cell cultures, these constructs effectively blocked expression of HSVtk only in healthy cells. Consequently, pronounced cell death was only observed to occur in the cancerous cells. In contrast, the constructs lacking the microRNA target sequences killed both cell types.
Finally, Wang’s group tested the therapeutic effect of the constructs in live animals. They grafted modified, human glioblastoma cells expressing luciferase into the brains of mice, and then injected the constructs into these sites. Confirming the cell culture results, the constructs suppressed luciferase expression and eradicated the tumors upon addition of ganciclovir; healthy cells were unaffected. In mice given salt water instead of ganciclovir, however, the tumors continued to grow.
“HSVtk suicide gene therapy has been tested many times in clinical trials, but we are the first to use this type of virus for gene delivery,” says Wang. “Last year we tested the safety of our viruses in monkeys, but we need to perform more pre-clinical studies before entering clinical trials.”
The A*STAR affiliated researchers mentioned in this highlight are from the Institute of Bioengineering and Nanotechnology