About five years ago, George Church and his new postdoctoral fellow, Ying Kai Chan, sat hunched over a laptop in the Harvard pioneer genetics office and stared at an old paper in bewilderment.
The document documented Glybera’s first tests, the first and then the only gene therapy approved anywhere on the planet. Less than three dozen patients received it, but in the years before Luxturna and Zolgensma, he gave researchers an example that they could point to that gene therapy is really working.
Chan and Church, however, were shocked to see that researchers who conducted clinical trials gave patients a battery of high-dose immunosuppressive drugs, often reserved for organ transplants, names like mycophenolic acid and cyclosporine. And when they biopsied patients’ muscles, they were full of T cells, suggesting an immune response in action.
The results were particularly surprising because Glybera used the viral vector AAV, a delivery system that led to the resurgence of gene therapy precisely because it was commonly believed to prevent the over-immune reaction that condemned the field in the 1990s.
Ying Kai Chan“There are so many people working on gene therapy now and even when you say to them, oh, ‘Did you know that cyclosporine was used? Did you know that all these things were used? ‘People are like,’ Huh, what? “Chan said News from Endpoints. “Glybera was the poster boy, but it seems that people didn’t realize how much immunosuppression was needed.”
Chan graduated as a viral immunologist and entered the Church laboratory with the intuition that viral vectors, those leaked and spiked genetic taxis, were still viruses and treated by the body as such. Gradually, the field returned to its point of view. Several monkey studies have shown that high doses of AAV can be toxic to certain neurons, results that companies have reluctantly accepted. And last year, three deaths in a high-dose test raised concerns about the safety of AAV, even though they have not yet been conclusively linked to an immune reaction.
Meanwhile, Chan has been working on new methods of camouflaging AAV to make it safer and reduce the need for immunosuppressants. This week, he, Church and a larger team at the Wyss Institute published the work in Science, Translational Medicine, showing how to weave specific strands of human DNA into the vector can neutralize one of the body’s main defenses against foreign invaders.
“It was very inspired by nature,” said Chan.
One of the first ways gene therapy pioneer Jim Wilson showed that the body could react to AAV was through a set of sentinels called toll-like receptors. These sentinels provide one of the first layers of defense for the immune system, sounding an alarm if they detect something that looks strange. This means, however, that normal cells need a way to tell recipients that they are safe – an encryption key that only human cells know.
This encryption key is encoded in some strands of DNA at the ends of the telomeres, those strips in the form of paper clips at the ends of the chromosomes that are sometimes implicated in aging. Chan incorporated these strands into the DNA of an AAV2 vector, the serotype used in Luxturna. When the vector is injected, the filaments must attach to toll-like receptors throughout the body and tell the receptors not to sound an alarm.
When the team injected him into the muscles, liver and eyes of pig and mouse models, he triggered a markedly reduced immune reaction than a traditional vector, Chan reported in STM.
The results add up to a set of new technologies emerging from laboratories across the country to combat the immunogenicity of AAV. Wilson’s lab offered a way to use microRNAs – short strands that minimize the expression of a particular gene in a given cell – to mitigate neural effects. And Dyno Therapeutics, a laboratory developed by the Church, uses engineering and machine learning to create entirely new vectors, hoping to find some that can avoid the immune system.
Chan has now helped launch a new company, joining ARCH and a few other VCs to form Ally Therapeutics, a still-secret biotechnology that tries to minimize the immunogenicity of viral vectors.
Still, he acknowledges that he expected more radical results than he did in the end. Although its technology successfully contained the immune response in pigs and mice, the results were less profound in monkeys.
Chan’s team injected the vector into the eyes of non-human primates, a part of the body where much of the immune system cannot enter and, consequently, toll-like receptors are extremely important. They saw greater safety when administered below the retina, but injecting directly into the vitreous jelly in the center of the eye still triggered significant inflammation. Intravitreal injection is important to deal with various conditions and to make eye gene therapies safer and easier to administer, since only ophthalmic surgeons can administer sub-retinal.
The new article, however, is only version 1.0 of the approach, said Chan, and they have brought about significant improvements since then.
More broadly, the field still has a long way to go. Animal models, for example, are still poor predictors of the immune response in humans, making translation difficult and creating large gaps in safety testing. A vector that appears immune-silent in monkeys can still trigger reactions in humans and vice versa. Although their role in animals is well documented, it is not yet clear how large the role of toll-like receptors is in the human response to AAV.
Still, Chan says they did what they intended to do: they improved the vector and, in the process, helped the field to wake up to a problem that for years has been overlooked.
“There are still challenges,” said Chan. “What we really wanted to do was raise awareness and also find a promising solution. I would say that we have made progress on both fronts. “