Researchers at the Stanford University School of Medicine have found a novel way to engineer key cells of the immune system so they remain resistant to infection with HIV, the virus that causes AIDS.
A new study describes the use of a kind of molecular scissors to cut and paste a series of HIV-resistant genes into T cells, specialized immune cells targeted by the AIDS virus. The genome editing was made in a gene that the virus uses to gain entry into the cell. By inactivating a receptor gene and inserting additional anti-HIV genes, the virus was blocked from entering the cells, thus preventing it from destroying the immune system, said Matthew Porteus, MD, an associate professor of pediatrics at Stanford and a pediatric hematologist/oncologist at Lucile Packard Children's Hospital.
"We inactivated one of the receptors that HIV uses to gain entry and added new genes to protect against HIV, so we have multiple layers of protection - what we call stacking," said Porteus, the study's principal investigator. "We can use this strategy to make cells that are resistant to both major types of HIV."
He said the new approach, a form of tailored gene therapy, could ultimately replace drug treatment, in which patients have to take multiple medications daily to keep the virus in check and prevent the potentially fatal infections wrought by AIDS. The work was done in the laboratory, and clinical trials would still be needed to determine whether the approach would work as a therapy.
"Providing an infected person with resistant T cells would not cure their viral infection," said Sara Sawyer, PhD, assistant professor of molecular genetics and microbiology at the University of Texas-Austin and a co-author of the study. "However, it would provide them with a protected set of T cells that would ward off the immune collapse that typically gives rise to AIDS."
The study will be published in the Jan. 22 issue of Molecular Therapy.
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.