Modified HIV May Be Effective For Delivering And Regulating Gene Therapy

New findings released June 24 in the journal Molecular Therapy provide the first evidence that a genetically stripped-down amalgam of HIV components not only can safely deliver genes to target cells in the body, but can be fashioned with a molecular switch system that turns these genes off in response to a common antibiotic.

This accomplishment was achieved without toxic affects in laboratory rats. It suggests that doctors may someday be able to control gene expression in people who are treated with gene therapy vectors based on HIV.

With such control, genes can be switched off when no longer needed, say, for example, in the production of growth hormone. And if adverse affects develop, gene expression can be curtailed.

The new study was led by Dr. Tal Kafri, assistant professor of microbiology and immunology at the UNC-CH School of Medicine. Kafri conducted the study while he was a postdoctoral scientist with Dr. Inder M. Verma at the Salk Institute in La Jolla, Calif. In earlier experiments, Kafri and others had studied the potential feasibility of gene therapy viral vectors based on lentiviruses, a subfamily of retroviruses. HIV is a lentivirus.

"One advantage of HIV and other lentiviruses is that they can introduce the gene of interest into cells that do not divide, and simple retroviruses cannot do that," Kafri said. "This makes them ideal for delivering genomic material directly into the body because most of our cells do not divide. This is very important for transducing (transferring genetic material to) hemopoietic stem cells (precursor blood cells) because most of them are quiescent and do not divide."

Kafri, a member of UNC-CH’s Gene Therapy Center, said retroviruses are unique in that they can integrate their genome into the host’s genome. "Unlike adenovirus – another virus used as a vector in gene therapy research – you don’t lose the genomic sequence that is incorporated into the host DNA following cell division," he said.

Thus, as hemopoietic stem cells develop, they will continue carrying the gene that was introduced therapeutically via vectors based on HIV.

"Another advantage in using a lentiviral vector is its large capacity," Kafri said. "It’s much larger than a simple retrovirus, which means you can put more genes and more genomic material in the same vector."

To produce his HIV-based vector, Kafri separated the viral genome into several components.

"Each of these alone cannot become a virus, but once together in the cell, one component contributes proteins, another contributes nucleic acid, and the third contributes non-HIV envelope proteins," Kafri said. The molecular switch system and the regulated therapeutic genes can be packaged in the form of RNA for transduction.

In the new study, the vector with its components and payload was injected into the brains of 12 rats. When expressed – switched on – a "reporter gene" for research purposes showed as green fluorescence in brain cells. After the antibiotic doxycycline was added to the animals’ drinking water, fluorescence dramatically diminished. Withdrawal of antibiotic from the drinking water was followed by return of fluorescence.

"These studies show that an inducible lentiviral vector can deliver and regulate transgene expression in vivo," the authors wrote in the journal article. "We believe that regulated gene expression is an essential tool for successful gene therapy approaches."

Kafri said the success of gene therapy would rise and fall with the vector.

"I believe that we have arrived at a point where gene therapists need to make their own basic science," he said. "And we need to direct the basic science toward questions relevant to vectors."

In terms of his lentivirus vector, Kafri said the huge advantage is that it’s the most efficient at transducing hemopoietic stem cells.

"This carries implications for treating blood disorders such as Fanconi anemia, thalassemia and sickle cell anemia; Gaucher’s disease, an inherited enzyme deficiency; and diseases of immune deficiency," he said.

And might that meant targeting HIV? "Sure, if you want to target HIV, a lentivirus vector would be the vector of choice," he said.