In a new study in the journal Cell Reports, researchers show how they successfully engineered an ancient virus and used it to deliver gene therapy to the retina, liver and muscle tissue of mice.
Gene therapy is a relatively new and largely experimental approach that uses genes instead of drugs or surgery to prevent and treat disease.
The team, including members from the Massachusetts Eye and Ear Infirmary and the Schepens Eye Research Institute in Boston, MA, says the study should help make gene therapies safer, more potent and available to more patients.
They also hope the findings will increase scientists’ understanding of the complex structures of viruses that can be used as gene therapy “vectors” – vehicles that insert genes into cells.
Senior author Luk H. Vandenberghe, an assistant professor at Harvard Medical School who heads a lab at the Institute and Infirmary, says:
“We believe our findings will teach us how complex biological structures, such as AAVs (adeno-associated viruses), are built. From this knowledge, we hope to design next-generation viruses for use as vectors in gene therapy.”
Viruses make ideal delivery vehicles for genes. They survive by inserting themselves into the genetic material inside the cells of the organisms they invade. They then hijack the cell’s machinery to make copies of themselves and proliferate.
By inserting therapeutic genes into viruses, researchers can use them to ferry the genes into the cells or tissue of patients. Adeno-associated viruses (AAVs) are small viruses that infect humans but do not cause disease. This is one of the features that makes them ideal vectors for gene therapy.
So far, gene therapy developers have chosen AAVs that naturally circulate in the human population. But the problem with this is that when a person is exposed to such a virus, their immune system remembers it and tries to eliminate it next time it invades.
As a result, the effectiveness of gene therapy based on natural AAVs is limited if the patient’s immune system has seen them before and attacks the vectors before they have had a chance to insert sufficient genes into cells for the therapy to have effect.
The solution is to engineer new, benign AAVs that patients’ immune system will not recognize, giving them time to insert the therapeutic genes into the target cells. This would make the therapy available to many more patients.
But AAVs are not easy to engineer due their complex structure. The proteins of the virus shell fit snugly together in unique, intricate patterns, like a jigsaw.
The pattern is so intricate and intermeshed that tweaking a protein to achieve a benefit – such as more efficient transfer of the gene into the cell – could result in destruction of the whole shell.
To solve the problem of engineering AAVs that have the benefits without the drawbacks, the team looked to the ancestors of viruses that are around today.
By examining the lineages of viruses, the researchers were able to work backwards up their tree of ancestors, discovering the changes that have occurred in their evolution. From this knowledge, they engineered nine viruses that had structural integrity and also had features that might make them good vectors.
When they tested the engineered viruses in mice, they found Anc80, the most ancient of them, was able successfully to target and enter liver, muscle and retina cells without toxic side effects.
The next step for the researchers will be to look at the interaction between virus and host throughout evolution in an effort to find improved vectors for clinical use. The team also plans to examine whether Anc80 could be used to treat liver diseases and retinal forms of blindness.
Prof. Vandenberghe sums up their findings so far:
“The vectors developed and characterized in this study demonstrate unique and potent biology that justify their consideration for gene therapy applications.”
To deliver the healthy gene, the team created an AAV together with a promoter that only turned the gene on in certain cells of the inner ear.