One of the big challenges in treating AIDS is that the virus is notorious for mutating, so patients must be treated with a cocktail of drugs — known as highly active antiretroviral therapy or HAART — which hit it at various stages of the replication process. The researchers were able to get around that problem with a new, multi-pronged genetic attack that blocks HIV on several fronts. Essentially, they hope to mimic HAART through genetic manipulation.
The technique hinges on the fact that the virus typically enters T cells by latching onto one of two surface proteins known as CCR5 and CXCR4.
Some of the latest drugs now used in treatment work by interfering with these receptors’ activity. A small number of people carry a mutation in CCR5 that makes them naturally resistant to HIV. One AIDS patient with leukemia, now famously known as the Berlin patient, was cured of HIV when he received a bone marrow transplant from a donor who had the resistant CCR5 gene.
Scientists at Sangamo BioSciences in Richmond, Calif., have developed a technique using a protein that recognizes and binds to the CCR5 receptor gene, genetically modifying it to mimic the naturally resistant version. The technique uses a zinc finger nuclease, a protein that can break up pieces of DNA, to effectively inactivate the receptor gene. The company is now testing its CCR5-resistant genes in phase-1 and -2 trials with AIDS patients at the University of Pennsylvania.
The Stanford scientists used a similar approach but with an added twist. They used the same nuclease to zero in on an undamaged section of the CCR5 receptor’s DNA. They created a break in the sequence and, in a feat of genetic editing, pasted in three genes known to confer resistance to HIV, Porteus said. This technique of placing several useful genes at a particular site is known as “stacking.”
Incorporating the three resistant genes helped shield the cells from HIV entry via both the CCR5 and CXCR4 receptors. The disabling of the CCR5 gene by the nuclease, as well as the addition of the anti-HIV genes, created multiple layers of protection.
Blocking HIV infection through both the CCR5 and CXCR4 receptors is important, Porteus said, as it hasn’t been achieved before by genome editing. To test the T cells’ protective abilities, the scientists created versions in which they inserted one, two and all three of the genes and then exposed the T cells to HIV.
Though the T cells with the single- and double-gene modifications were somewhat protected against an onslaught of HIV, the triplets were by far the most resistant to infection. These triplet cells had more than 1,200-fold protection against HIV carrying the CCR5 receptor and more than 1,700-fold protection against those with the CXCR4 receptor, the researchers reported. The T cells that hadn’t been altered succumbed to infection with 25 days.