Zinc Finger on the Trigger

Imperial College London researchers have used synthetic zinc finger proteins to switch off the Huntington’s disease gene in mice for up to six months using a single viral injection (1). We spoke with Mark Isalan, lead researcher, and reader in Gene Network Engineering at Imperial College London, to find out more about the implications of the work.

Why did you decide to investigate the huntingtin gene?

First of all, Huntington’s is a terrible disease and a cure is desperately needed. Second, the huntingtin disease mutation contains expanded poly-glutamine (CAG codon) DNA sequences, which are ideal for zinc fingers to stick to.

Back in 2001 I had the embryonic idea. I had already been working on zinc fingers for a long time and I was well aware that they could easily be re-engineered to bind guanine–cytosine-rich sequences, but I made the leap to Huntington’s after I had been working near Max Perutz in the MRC Laboratory of Molecular Biology. In the last few years of his life, he developed a great interest in poly-glutamine diseases, and Huntington’s is the most prevalent of these. He kept mentioning poly-glutamines and the idea got stuck in my head.

How do the zinc fingers work?

Our zinc fingers are designed, DNA-binding proteins that recognize and bind to poly-glutamine in the mutant huntingtin gene. They are gene switches, and by attaching another protein called a repression domain, they can shut down a gene in a long-term, stable manner. Back in 2012, we were the first to show that we could use zinc fingers to shut down the “bad” mutant huntingtin gene for a couple of weeks in mice – delaying neurological symptoms after injecting their brains with a virus that expresses the artificial zinc fingers. We’ve had to work quite hard to make the effects last longer, which is the breakthrough reported in this latest paper. By making the zinc finger protein generally more invisible to the immune system, and by producing it with a different promoter gene expression system, we were able to shut down the mutant gene very effectively for several months. In fact, the effect lasted up to six months at levels that we previously showed to alleviate symptoms in mice.

How close are you to creating clinically applicable zinc fingers?

We still have some work to do before moving into humans, although I believe we are getting closer. We need to answer important questions around the safety of the intervention, whether repeat treatments are effective, whether there might be longer-term side effects, and whether we can extend and increase the benefits beyond six months. We also need an industrial partner to take this very exciting result towards the clinic and we are actively seeking one out, while trying to secure follow-on funding. Securing funding has been surprisingly difficult all the way through this project.

What’s next?

We think that higher doses and repeat doses are likely to give even longer-term effects, so we’re planning to try this out in mice. We are also doing more safety and specificity testing to confirm promising early results which indicate that we are targeting the mutant gene specifically without affecting other related genes in the cells.