Engineering a Cure for HIV
Twelve people in Pennsylvania are taking part in a highly experimental gene therapy clinical trial. Their blood is drawn, their immune cells isolated and then altered. Molecular scissors precisely cut the DNA in their cells. This transformation shuts the door on HIV. When the cells are re-infused into the Pennsylvania patients’ bodies they create an immune system that looks like that of an elite group of people. But the future of gene therapy for HIV does not rest on the results of this study alone. Instead, to understand where this therapy is headed, we have to look the big picture.
They were a medical mystery. A group of men in New York City who had been exposed many times to HIV but inexplicably remained unaffected by the virus responsible for so many deaths. In 1995 AIDS was the leading cause of death in Americans aged 25-44. All over New York City, people were dying of AIDS yet somehow this group of men was spared. A team of scientists at the Aaron Diamond AIDS research Center was determined to learn why. When the results were published in Cell in 1996 it would set forward a strange series of events, one leading to a man cured of HIV in 2008, to promising results in twelve men receiving a radical new gene therapy, to a possible future where HIV can be cured.
These men were able to hold off HIV because of a single mutation. This mutation is called Δ32. It’s 32 missing pieces of a gene. What the men in New York City all had in common was this unusual mutation. In an attempt to understand it, scientists drew their blood, cultured their cells in incubators and then challenged them with HIV. But the virus couldn’t infect them. Because of this one mutation, HIV couldn’t enter a single cell in their bodies. Sitting on the surface of all our immune cells are two proteins that the vast majority of HIV strains need: CD4 and CCR5. To break into human cells the virus needs to grasp both proteins. People with the Δ32 mutation don’t make a functional CCR5 protein. Without it HIV is left standing at the door of the cell with the wrong key.
This mutation is naturally found in a small number of people. It’s found at unusually high levels in Western Europe where it’s estimated that 1% of the population has two copies of this mutation rendering them, like the men in New York City, resistant to HIV. People with this mutation live normal, healthy lives (although there has been some concern that they may have increased susceptibility to West Nile Virus). CCR5 seems to be mostly unnecessary, like an appendix.
Gero Hütter was a medical student sitting in the library when he read the first Nature paper describing the Δ32 mutation in Western European populations. He was astounded. He wasn’t interested in infectious disease or HIV. Yet, he confidently assumed that for people working on HIV, this exciting data would result in new therapies to tackle the virus. What he didn’t expect was that a decade later he’d be the first person to use the power of the mutation in a person with HIV.
In 2007 Gero Hütter was an oncologist at Charité Hospital in Berlin, Germany. He had never treated a person with HIV. When he met Timothy Brown, a man newly diagnosed with acute myeloid leukemia who had been HIV positive for a decade, he immediately thought of the paper he read in medical school. What if, he reasoned, he could wipe out Timothy’s HIV along with his cancer? The plan was a bold one: give Timothy a stem cell transplant from a person who, because of the Δ32 mutation, was naturally resistant to HIV. Gero Hütter was told why the approach wouldn’t work: HIV is too good at hiding in the body, new strains would emerge in Timothy able to get around the mutation and the virus would mutate beyond their grasp. Despite these obstacles, he pursued the novel therapy, never attempted before, not even in an animal model. The gamble paid off. Today Timothy is free of both diseases and has been off HIV therapy since 2007. A study published last year was unable to find any HIV hiding in his body and postulated that he may have a sterilizing cure.
Dr. Carl June, a researcher at the University of Pennsylvania, was already pursuing a gene therapy for HIV when he learned of Timothy’s dramatic case. It was 2008 and no one was interested in funding HIV cure research. All the arguments Gero Hütter heard when attempting Timothy’s cure had long been repeated to Carl June. However, with the news of Timothy’s cases, the fantastic story of the Berlin patient, the medical field underwent a revolution in thought. Suddenly it was no longer crazy to pursue a cure for HIV. After all, someone had already been cured.
By the time Timothy’s case was published in the New England Journal of Medicine in 2009 the field was irrevocably changed. Because of his case, funding from amFAR, the California Institute of Regenerative Medicine, and the NIH to support HIV cure research became available. In their reports the funding agencies directly cited the Berlin patient as the reason for backing what had once been known only as the “c-word."
With this new surge in funding, Carl June was able to take the promising results in his lab, and with a humanized mouse model, and bring them to a small group of people living with HIV. June tested them in two groups. The first had healthy levels of immune cells, relatively untouched by the virus. The second wasn’t quite so lucky. Although being on antiviral therapy for HIV for between 2-20 years their immune cells had never quite recovered from the wallop of HIV.
Both groups had their blood drawn. Their T-cells, part of the essential immune cell army against invaders, were separated. They were then treated with an adenoviral vector modified to express a unique protein: a zinc finger nuclease or ZFN. A small biotechnology company in California called Sangamo biosciences developed these ZFNs. They’re composed of a sticky piece that can specifically bind to a piece of DNA and an enzyme that cuts the DNA. Only when two ZFNs line up around their DNA target does the enzyme become activated, causing a devastating double stranded break in the DNA.
In this case, the ZFNs were designed to target the CCR5 gene, the same mutated gene that protected a group of men from HIV in New York City in the 1990s and cured Timothy Brown in 2007. 10 billion of the patient’s own cells, a percentage of which were now modified by the ZFNs, were re-infused back into the Philadelphia patients. A month later, half of them stopped taking HIV therapy.
When the six patients went off therapy the virus came roaring back. At first blush this might seem like a failure. Why couldn’t the gene therapy protect them? In fact, the rebound of virus was all part of the plan. Carl June was using the virus against itself. When the virus came back it gave the genetically modified cells an advantage. The virus killed off the normal cells giving a selective advantage to those transformed by the ZFNs. The treatment interruption was not about clearing HIV; it was about priming the immune system to gear up for a cure. The study itself is an essential step in proving that this therapy can be taken safely, no small feat for a pair of molecular scissors like the ZFNs that have not been tested in humans before.
In addition to safety, the study showed that interrupting therapy briefly could boost the number of HIV resistant cells in the body. Timothy Brown had a leg up in this department. He started out with one copy of the Δ32 mutation. When he received the stem cell transplant in 2007, his body was already halfway to the finish line. Researchers wondered if there was any way to give people without this genetic advantage a similar leg up?
One idea was to use a drug called cyclophosphamide. Scientists had already borrowed so much from oncology in these clinical trials; why not add in a drug that is known to enhance the transplant of T-cells? The results, presented last week at the conference on retroviruses and opportunistic infections, were encouraging. Patients on the highest dose of the drug had a significant advantage at the starting line. Combining the drug with CCR5 ZFNs is capable of making a person’s immune system look like that of someone with one copy of the Δ32 mutation. In fact, of the 3 patients receiving the high dose of this drug, 2 of them haven’t gone back on therapy. These two patients have seen their virus drop significantly, more than a log lower than their viral set point (this is the stable level of virus in a person). These two patients have now been off therapy and controlling their virus for a few months. We can’t yet know whether HIV will come back or whether they may experience a functional cure.
These two studies on their own are hopeful. But what is most exciting is combining their lessons. They teach us that a successful clinical trial is within our grasp. We can combine the capability of a drug that boosts the population of genetically modified cells in the body with the experience of leveraging the virus against itself. Put together, these studies show us what a large scale clinical trial of HIV gene therapy will look like. Gene therapy for HIV once seemed wildly improbable. When Carl June first heard of ZFNs he didn’t take them seriously. He jokingly called it the “star wars approach.” Today, the wildly improbable is inching towards reality. And it’s all thanks to a student reading in a library.