Meet The Scientist

DAVIDSON: I recently woke up in the middle of the night thinking, “It has been 12 years since the publication of our knockdown studies.”1 I had no idea that it would take us this long to get to the clinic. How did I originally get into HD? I first started working in inborn errors of metabolism that affect the CNS. As a graduate student, I worked on Lesch-Nyhan syndrome. It always fascinated me how seemingly ambiguous mutations would induce such profound changes in the brain. I became fascinated by learning how mutations induce neuropathology, and developing ways to mitigate that neuropathology. I started early on in gene replacement strategies, always thinking in the back of my mind how we could apply some of the things we learn about getting genes into cells, and use this in some of the dominant neurodegenerative disorders.

We started working on RNA interference (RNAi) shortly after the seminal discovery by Fire and Mello in the late 1990s.2 We really did not have much success in achieving very selective knockdown until Tom Tuschl’s paper came out some time later, showing us that the best way to accomplish selective knockdown was by using very small RNA fragments that were complementary to the gene you were trying to knock down.

We started working on this with two collaborators at the time. We had developed the technology, but we needed an animal model, and in the laboratory next door to mine was Henry Paulson, who worked on spinocerebellar ataxia.

Through close interactions with Dr. Paulson we came to know Dr. Harry Orr, Dr. Nancy Bonini, and others working in the polyglutamine repeat field. We collaborated with Drs. Orr and Paulson to test some of our ideas in cell and mouse models. That is how we came to work in polyglutamine repeat diseases. It was really by chance that we began to focus on these sorts of therapeutics for spinocerebellar ataxia, and then also HD, both of which are classical polyglutamine repeat disorders.

Actually, my interest in HD goes back a bit further. My very close friend’s father had HD, and I recall seeing her grandmother cross the street when I was a child. I was with my dad, who was the local physician in our small community in south central Nebraska. I asked dad what was wrong with Shelly’s grandmother – she acted as if she could not walk very well. I was too young to know what a drunk looked like, and dad had told me that she had HD, and of course that meant nothing to me. But now we have come full circle. Many of Shelly’s family have succumbed to HD, so maybe I was destined from a very young age to work on this disease.

DAVIDSON: As with any new technology, in 2004-2005 we were using state-of-the-art methods for the time. As we began to learn more and more about the biology of the system that we were co-opting to perform RNAi in cells, we learned that there were better and safer ways to accomplish this.

The next six to seven years were to perfect the system and understand the basic ingredients of what it was we were making, and understand the impact of what we were making on the cell. The reason for taking this stepwise, careful, methodical approach is that HD is already terrible, and the last thing I want to do is take something that is very debilitating and make it worse.

We focused early on gene-silencing strategies, the idea being that the delivery of the viral vector to the affected cell would provide a one-time treatment to the brain. Sometime after our studies were published, Frank Bennett, at what is now known as Ionis Pharmaceuticals, began a fantastic collaboration with Dr. Don Cleveland’s group. Initially, it was to study ASOs for an inherited form of amyotrophic lateral sclerosis. Then, they transitioned to testing the ASOs in animal models of HD. These were the same models that many of us were using to test therapies at the time. His data showed that just as in our RNAi experiments, he could positively impact the disease course with doses of the

Ionis Pharmaceuticals, began a fantastic collaboration with Dr. Don Cleveland’s group. Initially, it was to study ASOs for an inherited form of amyotrophic lateral sclerosis. Then, they transitioned to testing the ASOs in animal models of HD. These were the same models that many of us were using to test therapies at the time. His data showed that just as in our RNAi experiments, he could positively impact the disease course with doses of the ASOs, and I think that really excited the community. Fortunately for Ionis, they have already been into the clinic with many of these molecules. Ionis is a large company that was well set to do the basic pharm tox that needs to be done to advance these molecules into therapies. Ionis has already started early phase-testing in HD patients, which is exciting. So, while we still remained poised to do our studies, they were already doing some safety studies in HD patients. It is an exciting moment for the gene-silencing community.

DAVIDSON: All the work that was developed while I was at the University of Iowa was licensed to Spark Therapeutics, and remains right now in IND-enabling studies.

Animal data suggest that you need not rid every cell of the mutated huntingtin gene, but to provide benefit, you need to hit a sufficient number of cells. The viral-vector delivered RNAi approach that we are taking, initially, is set to target the striatal region of the brain. This is in contrast to ASO therapies, in which the targeting is best achieved in the cortical structures. You could think of these as two complementary approaches to achieve widespread gene-silencing in the brain, in which the ASO could provide benefit to cortical structures, and the RNAi approach could provide benefit to sub-cortical structures. Our hope is that we can extend the period that HD patients live productively with the disease, significantly delaying its progression.

DAVIDSON: That was a study that we felt was really important not only for the RNAi community focusing on HD, but also for the ASO community. The study asked the simple question, can you reduce normal huntingtin in normal monkeys without deleterious effect? These were normal rhesus monkeys, not an HD model, but being a nonhuman primate, their brain closely approximated that of humans.

Our data showed that you could partially reduce huntingtin expression, and it was not deleterious to the animal.

That study, published by my group and Dr. Jodi McBride’s group at Oregon Health Sciences University, was the first to show that this approach was safe in a nonhuman primate. That was followed up by a longer-term study published by a group at Medtronic,4 in which they evaluated a very similar approach for six months, and came to the same conclusion as us. That suggested to us all that this could work, and really propelled us to move forward with a non – allele-specific silencing strategy as a potential treatment for HD patients.

DAVIDSON: The other condition we work on is spinocerebellar ataxia type 1, which, like HD, is a polyglutamine repeat disorder. We have also done some limited preclinical work in spinocerebellar ataxia type 7. Spinocerebellar ataxia type 1 was the first model in which we tested the RNAi approach, and spinocerebellar ataxia type 7, a second model. We have done more extensive dosing studies in spinocerebellar ataxia type 1 mouse models, and also tested some reversal studies in those models. I would not say that they are ahead of the HD program, I would say they are pretty close together.

We are also looking at the pathogenesis of HD and lysosomal storage diseases. We have some exciting data in HD, and this is really following close on the heels of some exciting work that has come out of Dr. Laura Ranum’s lab, where she has found that there are some funny transcripts that arise from the HD locus as a result of the CAG expansion.5 She sees transcripts in the nucleus that are not toxic in a normal-length allele, but in the setting of a disease-causing mutation, you can see these transcripts arising from that region, and if they are transported to the cytoplasm, they can be expressed as toxic proteins known as RAN translation products.

I think understanding how those products contribute to disease will be very interesting, as well as understanding other global changes that go on in the HD brain. I wonder whether we can take advantage of that information to develop small molecule therapies. Will gene-silencing approaches be the be-all and end-all? I think the best approach might be a combination therapy where we tackle some of the downstream cellular impacts of mHTT and the mutant transcript, and in addition try to lower the insult through gene-silencing.

DAVIDSON: I really thank HD Insights for the opportunity to let people know how we have reached where we are, and how excited we are to be so close to providing benefit to HD patients.