Measuring Success: How Do You Know If Your Treatment is Working? Some of the most promising therapies for HD are based on the same idea: reducing the amount of mutant huntingtin in brain cells. Huntingtin (sometimes called HTT) is the malfunctioning protein made by the mutant HD gene that clumps up inside neurons. There’s lots of evidence from lab experiments that those clumps make the cells malfunction and eventually die. So it seems logical that clearing away the faulty protein will help brain cells function better, live longer, and reduce or prevent the symptoms of HD. That idea has guided the development of many different types of huntingtin-lowering therapies (you may have heard of them referred to as anti-sense oligonucleotides (ASOs), siRNA, or RNA interference), some of which are now beginning to be tested in people in clinical trials. Neurologist Ed Wild donates cerebrospinal fluid to help test a new assay that can measure mutant huntingtin protein. But even though the gene was identified back in 1993 and the faulty huntingtin protein’s toxic effects are well documented, there still isn’t a good assay—a test to measure how much of the mutant protein is actually in one human brain. Measuring it is tricky: It sometimes floats around alone and sometimes forms small fibers or clumps. It is also very similar to the normal healthy huntingtin protein. “Five or six years ago, it was obvious that the field needed assays,” says Douglas Macdonald, PhD, CHDI’s Director, Drug Discovery and Development. If the drug is working, it should stop people’s symptoms from getting worse (or even show some improvement)—but that may take a while to be obvious, and that’s a very indirect measure of what’s happening in the brain. If it doesn’t work, the researchers wouldn’t have any idea why—did the therapy fail to lower the amount of huntingtin, or did it lower huntingtin levels but there was no beneficial effect, and is the whole idea of reducing huntingtin misguided? Not having a good assay posed a problem for researchers developing the new huntingtin-lowering therapies: How will they know quickly whether or not the drug is doing its job? “It’s really important when you perform a clinical trial to know that the drug or intervention you’re giving has the intended effect,” says Macdonald. “To be able to say, ‘I gave this intervention, and it’s actually lowering mutant huntingtin in the brain’—that’s a very valuable piece of information.” These direct measures are sometimes referred to as biomarkers. There are already assays that can measure the protein in animal models and in cells in dishes, and there’s even one that can detect the protein in blood samples taken from people with HD. But what’s going on in blood cells isn’t necessarily the same as what’s going on the brain. So a group of researchers began working on a much more sensitive test that would be able to detect the tiny amounts of huntingtin that float around in the cerebrospinal fluid (CSF), which is produced by the brain. CSF can be safely sampled with a spinal tap. A team eﬀort This project is a cooperative effort between a small Italian company called IRBM Promidis, neurologist-scientists at the University of British Columbia and University College London, CHDI, and volunteers who donated samples of blood and cerebrospinal fluid via lumbar puncture. “Collecting CSF is the only way we have of directly sampling the chemical state of the nervous system in living humans, ” says neurologist Ed Wild, MRCP, PhD, of the UCL Institute of Neurology and the National Hospital for Neurology and Neurosurgery. “It’s an incredibly valuable way to understand how the mutation causes the disease, and how we can detect and measure the effects of the treatments we’re working on.” The team compared the amount of mutant huntingtin protein in the CSF samples from people with HD, people who have the gene but are not yet diagnosed with HD, and control volunteers who do not have the disease—including Wild himself, who chronicled his experience getting a lumbar puncture on Twitter. The assay detected higher levels of the protein in people with diagnosed HD, as might be expected. The assay is now being further tested in an even larger and more diverse group of people. As another part of this validation process, it will be used to measure huntingtin in the same people over time to see if it can pick up subtle changes in the amount of mutant huntingtin in the body. If it does it could be useful in monitoring how the disease progresses. The hope is that it will also be used to help figure out whether the new huntingtin-lowering therapies work. “We’d like to treat a patient with something that lowers mutant huntingtin, sample their CSF, and see how much we’ve reduced the protein,” says Macdonald. “That’s the ultimate utility here.” This story was originally published in the Autumn 2014 issue of Enroll!