HIV is an exceptional adversary. It is more diverse than any other virus, and it attacks the very immune cells that are meant to destroy it. If that wasn’t bad enough, it also has a stealth mode. The virus can smuggle its genes into those of long-lived white blood cells, and lie dormant for years. This “latent” form doesn’t cause disease, but it’s also invisible to the immune system and to anti-HIV drugs. This viral reservoir turns HIV infection into a life sentence.
When the virus awakens, it can trigger new bouts of infection – a risk that forces HIV patients to stay on treatments for life. It’s clear that if we’re going to cure HIV for good, we need some way of rousing these dormant viruses from their rest and eliminating them.
A team of US scientists led by David Margolis has found that vorinostat – a drug used to treat lymphoma – can do exactly that. It shocks HIV out of hiding. While other chemicals have disrupted dormant HIV within cells in a dish, this is the first time that any substance has done the same thing in actual people.
At this stage, Margolis’s study just proves the concept – it shows that disrupting HIV’s dormancy is possible, but not what happens afterwards. The idea is that the awakened viruses would either kill the cell, or alert the immune system to do the job. Drugs could then stop the fresh viruses from infecting healthy cells. If all the hidden viruses could be activated, it should be possible to completely drain the reservoir. For now, that’s still a very big if, but Margolis’s study is a step in the right direction.
HIV enters its dormant state by convincing our cells to hide its genes. It recruits an enzyme called histone deacetylase (HDAC), which ensures that its genes are tightly wrapped and cannot be activated. Vorinostat, however, is an HDAC inhibitor – it stops the enzyme from doing its job, and opens up the genes that it hides.
It had already proven its worth against HIV in the lab. Back in 2009, three groups of scientists (including Margolis’ team) showed that vorinostat could shock HIV out of cultured cells, producing detectable levels of viruses when they weren’t any before.
To see if the drug could do the same for patients, the team extracted white blood cells from 16 people with HIV, purified the “resting CD4 T-cells” that the virus hides in, and exposed them to vorinostat. Eleven of the patients showed higher levels of HIV RNA (the DNA-like molecule that encodes HIV’s genes) – a sign that the virus had woken up.
Eight of these patients agreed to take part in the next phase. Margolis gave them a low 200 milligram dose of vorinostat to check that they could tolerate it, followed by a higher 400 milligram dose a few weeks later. Within just six hours, he found that the level of viral RNA in their T-cells had gone up by almost 5 times.
These results are enough to raise a smile, if not an outright cheer. We still don’t know how extensively vorinostat can smoke HIV out of hiding, or what happens to the infected cells once this happens. At the doses used in the study, the amount of RNA might have gone up, but the number of actual viral particles in the patients’ blood did not. It’s unlikely that the drug made much of a dent on the reservoir of hidden viruses, so what dose should we use, and over what time?
Vorinostat’s actions were also very varied. It did nothing for 5 of the original 16 patients. For the 8 who actually got the drug, some produced 10 times as much viral RNA, while others had just 1.5 times more. And as you might expect, vorinostat comes with a host of side effects, and there are concerns that it could damage DNA. This study could be a jumping point for creating safer versions of the drug that are specifically designed to awaken latent HIV, but even then, you would still be trying to use potentially toxic drugs to cure a long-term disease that isn’t currently showing its face. The ethics of doing that aren’t clear.
Steven Deeks, an AIDS researcher from the University of California San Francisco, talks about these problems and more in an editorial that accompanies the new paper. But he also says that the importance of the study “cannot be overstated, as it provides a rationale for an entirely new approach to the management of HIV infection”.
Reference: Archin, Liberty, Kashuba, Choudhary, Kuruc, Crooks, Parker, Anderson, Kearney, Strain, Richman, Hudgens, Bosch, Coffin, Eron, Hazudas & Margolis. 2012. Administration of vorinostat disrupts HIV-1 latency in patients on antiretroviral therapy. Nature http://dx.doi.org/10.1038/nature11286
Image by Dr. A. Harrison; Dr. P. Feorino
More on HIV:
In 2004, evolution itself served as a witness for the prosecution in the case of the State of Washington versus Anthony Eugene Whitfield. Whitfield contracted HIV in an Oklahoma prison, and first learned about his infection in 1992. After his release in 1995, he had more than a thousand sexual encounters with 17 different women, even fathering children with three of them. He rarely wore a condom, never told any of his partners about his infection and flatly denied it when asked.
However, Whitfield did confess to various people that if he had HIV, he would give it to as many people as possible. He got his wish – five of his 17 partners became HIV-positive. Whitfield was finally arrested in 2004 and convicted on 17 counts of first-degree assault with sexual motivation, among other offences. His total sentence came to 178 years and a month.
To demonstrate Whitfield’s guilt, the prosecution had to show that he had wilfully exposed women to HIV, that his five HIV-positive partners contracted their infections from him. Fortunately, David Hillis from the University of Texas and Michael Metzker from Baylor College of Medicine knew exactly how to do that. They had evolutionary biology on their side.
Immunity to viral infections sounds like a good thing, but it can come at a price. Millions of years ago, we evolved resistance to a virus that plagued other primates. Today, that virus is extinct, but our resistance to it may be making us more vulnerable to the present threat of HIV.
Many extinct viruses are not completely gone. Some members of a group called retroviruses insinuated themselves into our DNA and became a part of our genetic code. Indeed, a large proportion of the genomes of all primates consists of the embedded remnants of ancient viruses. Looking at these remnants is like genetic archaeology, and it can tell us about infections both past and present.
When retroviruses (such as HIV, right) infect a cell, they insert their own DNA into their host’s genome, using it as a base of operations. From there, the virus can pop out again and make new copies of itself, re-infect its host or move on to new cells.
If it manages to infect an egg or sperm cell, the virus could pass onto the next generation. Hidden inside the embryo’s DNA, it becomes replicated trillions of times over and ends up in every single one of the new individual’s cells.
These hitchhikers are called ‘endogenous retroviruses‘. While they could pop out at any time, they quickly gain mutations in their DNA that knocks out their ability to infect. Unable to move on, they become as much a part of the host’s DNA as its own genes.
In 2005, a group of scientists led by Evan Eichler compared endogenous retroviruses in different primates and found startling differences. In particular, chimps and gorillas have over a hundred copies of the virus PtERV1 (or Pan troglodytes endogenous retrovirus in full). Our DNA has none at all, and this is one of the largest differences between our genome and that of chimps.
HIV is an elusive adversary. The virus is so good at fooling the immune system that the quest for an HIV vaccine, or even a countermeasure to resist infections, has spanned two fruitless decades. But maybe a defence has been lurking in our genomes all this time.
Nitya Venkataraman from the University of Central Florida has managed to reawaken a guardian gene that has been lying dormant in our genomes for 7 million years. These genes, known as retrocyclins, protect monkeys from HIV-like viruses. The hope is that by rousing them from their slumber, they could do the same for us. The technique is several safety tests and clinical trials away from actual use, but it’s promising nonetheless.
Retrocyclins are the only circular proteins in our bodies, and are formed from a ring of 18 amino acids. They belong to a group of proteins called defensins that, as their name suggests, defend the body against bacteria, viruses, fungi and other foreign invaders. There are three types: alpha-, beta- and theta-defensins. The last group is the one that retrocyclins belong to. They were the last to be discovered, and have only been found in the white blood cells of macaques, baboons and orang-utans.
In previous experiments, Venkataraman’s group, led by Alexander Cole, showed that retrocyclins were remarkably good at protecting cells from HIV infections. They are molecular bouncers that stop the virus from infiltrating a host cell. The trouble is that in humans, the genes that produce retrocyclins don’t work. Over the course of human evolution, these genes developed a mutation that forces the protein-producing machinery of our cells to stop early. The result is an abridged and useless retrocyclin.
But aside from this lone crippling mutation, the genes are intact and 90% identical to the monkey versions. Now, Venkataraman has awakened them. She found two ways to fix the fault in human white blood cells, one involving gene transfer and the other using a simple antibiotic. Either way, she restored the cells’ ability to manufacture the protective proteins. And the resurrected retrocyclins did their job well – they stopped HIV from infecting a variety of human immune cells.