"To make sure CRISPR-Cas9 hits its HIV target, the team coupled it with an appropriately named escort: guide RNA (gRNA). The gRNA attaches to a very specific section of DNA, which is found only in HIV, ensuring that CRISPR-Cas9 can’t miss. The team screened for accidental, “off-target” editing and found virtually none. At least in the laboratory, the drug homes to and inactivates viral genes while leaving host DNA unchanged. Perhaps best of all, healthy cells containing the Cas9/gRNA complex were also immune to HIV infection in the experiment."
Revolutionary Biotech May Offer HIV Cure
By David Shultz
Earlier this month, an HIV infection reemerged in a 4-year-old Mississippi girl believed to have been cured of the virus. Researchers thought that by administering antiviral drugs quickly after she was born, they could destroy the virus before it could insert itself into her DNA.
For four years, it seemed the treatment had worked, but on July 10th officials announced that they had detected levels of the virus in her blood. Her story is representative of one of the tragic difficulties in curing HIV: The virus can hide in host DNA for long periods of time, evading drugs and the immune system alike. But a powerful new protein, known as CRISPR-Cas9, now has HIV’s clandestine genes in its sights.
HIV is often depicted as a spherical, free-floating virus coated with spiky protein receptors. This is accurate, but it only represents part of the virus’ life cycle. Like only imagining frogs as tadpoles, our depictions of HIV often fail to tell the whole story. Much of HIV’s life is spent inside our cells, lying dormant amidst our DNA. The reason we so rarely see an image of this latent version of the virus is because, at this point in its life, it’s made of nothing more than DNA itself. There is no viral shell, or membrane, or spiky protein receptors—just the genetic instructions for making them, sort of like a sleeper terrorist cell waiting to be activated.
It’s estimated that, even when no HIV particles are detectable in the body, around ten million cells carry genetic copies of the virus. As a result, the symptoms can be treated, but the infection can’t be cured.
That all stands to change if Kamel Khalili has his way. He and his team at Temple University have directed a protein called CRISPR-Cas9 to sniff out and remove latent HIV genes. The experiment took place in myeloid cells growing in culture -- an apt model, as these nervous system cells have proven to be a particularly good reservoir for HIV. When applied, Cas9 acts like a pair of molecular scissors that cut both ends of the HIV DNA, slicing it out of our chromosomes and preventing it from being used to make more viruses.
“It’s extremely specific, very efficient, and surprisingly, it does what you anticipate it should do,” said Khalili with a laugh. “We’ve actually converted the cell lines which carried the virus to be virus-free cells.”
One of the hardest parts of removing viral genes is finding them. The HIV-1 genome is 9,181 base pairs; our own contains around 3 billion. Finding those viral genes is tantamount to finding a fire ant on a mile-long stretch of highway. The cost of missing the mark can be dangerous too. Accidentally altering important, healthy genes can result in cancer or other harmful side effects.
To make sure CRISPR-Cas9 hits its HIV target, the team coupled it with an appropriately named escort: guide RNA (gRNA). The gRNA attaches to a very specific section of DNA, which is found only in HIV, ensuring that CRISPR-Cas9 can’t miss. The team screened for accidental, “off-target” editing and found virtually none. At least in the laboratory, the drug homes to and inactivates viral genes while leaving host DNA unchanged. Perhaps best of all, healthy cells containing the Cas9/gRNA complex were also immune to HIV infection in the experiment.
The power of the CRISPR-Cas9 system has been awe-inspiring since its potential as a gene-editing tool was realized in 2012. Now, it seems these proteins may be capable of removing HIV infection at its root.
There are, of course, large differences between curing HIV in the lab, and doing it in the body. “It has the potential, but it’s a tall order to get [CRISPR-Cas9] into every cell of the human body,” says Khalili. “We should be able to develop a strategy to effectively deliver this technology to infected individuals, and we would hope that it does the same thing.”
If his team is able to do that, then they may very well cure HIV.
Source: Hu W et al. "RNA-directed gene editing specifically eradicates latent and prevents new HIV-1 infection." PNAS. Published online before print. doi: 10.1073/pnas.1405186111
David Shultz is a freelance writer and an editorial intern at Nautilus Magazine. He tweets @dshultz14