NAME: Hoa Nguyen, M.D
CURRENT POSITION: Head of the Huntington Disease Working Group and Head of Cancer Genetics at the Institute of Medical Genetics and Applied Genomics, University of Tübingen, Tübingen, Germany
EDUCATION: MD, Hannover Medical School, Hannover, Germany; research at Emory University, Atlanta, Georgia, USA, and McGill University, Montréal, Québec, Canada
Dr. Hoa Nguyen is a board-certified clinical geneticist who devotes his time to oncogenetics and predictive testing for HD. As a researcher, he leads the HD Working Group at the University of Tübingen. His team recently participated in the MitoTarget project, a three-year translational research program funded in part by the European Commission and led by Trophos, to study mitochondrial abnormalities in neurological diseases, and to test the company’s lead compound, olesoxime, in these diseases. He has also contributed to the development and description of the BACHD rat, a novel transgenic rat model of HD. Dr. Nguyen recently discussed his work with HD Insights. The following is an edited transcript of the conversation.
HD INSIGHTS: Dr. Nguyen, you are both a clinician and a researcher. Can you describe your clinical practice?
NGUYEN: I do most of the predictive testing and counseling here, including for HD patients and families. Currently I am working more with oncogenetics. As you may have heard, Angelina Jolie spoke publicly about her BRCA-1 mutation, and since then, a lot of requests have come in!
HD INSIGHTS: So Angelina Jolie is driving traffic to your center?
NGUYEN: Yes! The number of patients has increased by about 200 percent.
HD INSIGHTS: Tell us about your experiences with predictive testing for HD.
NGUYEN: Unlike the US, we do not have genetic counselors in Germany, so all the genetic testing and counseling is done by medical doctors. I do what a genetic counselor does in the US, but I also discuss potential treatments. I try not to talk too much about treatments with HD patients, because we don’t yet have any, but I try to give some hope, and some insight into ongoing research. We follow very strict guidelines for predictive testing. In the first session we usually just talk about HD, the family history, possible outcomes, treatment options, and so forth.
Patients also receive counseling with a psychiatrist and neurologist before we actually do the test. We give patients a bit of time to reflect on their decision to undertake predictive testing before they actually come in for the blood draw, and then a couple of weeks later, they can return for the results. I think predictive testing is more common here in Germany and in Europe compared with the US, possibly because of insurance issues. In Germany, testing is all paid for, and there is no possibility of people losing insurance. About 30 percent of patients who come in for predictive testing and counseling actually proceed with testing.
HD INSIGHTS: One of your primary research projects has been the development of the BACHD transgenic rat model. What are the benefits and disadvantages of this new model?
NGUYEN: Most people in the HD field work with mice, mainly because transgenic mouse models are easier to generate. However, a rat has several advantages compared with a mouse. First, rats are larger animals, so there are more possibilities for imaging studies in rats than in mice. Second, in rats you can do more biosampling, such as CSF, and you can also do longitudinal biosampling.
From a behavioral standpoint, most behavioral research has been done in rats and the research has just been adapted to mice. However, we actually don’t know that much about the physiology and neurological circuitry in mice. Early electrophysiology was also mainly done in rats, so we have a much better understanding of these kinds of things in rats than in mice. And last but not least, there are some compounds that have better pharmacokinetic dynamics in rats than in mice. For example, we were recently approached by a company that has an autophagy compound that they would like to test in rats rather than in mice because it was much more toxic in mice than in rats.
Of course there are disadvantages with rats. Cost is a problem, because rats are bigger animals and need more space, and treatment studies and compound testing may be more expensive in rats if you must use more compound. Another disadvantage is that it is much more difficult to generate a knock-in rat model than a knock-in mouse model. We are currently working with CHDI on generating a knock-in rat model, and that has been quite difficult.
HD INSIGHTS: Could you describe the differences between a knock-in animal model and a transgenic animal model?
NGUYEN: A knock-in animal model expresses the gene of interest in the same place as in the human genome. In the case of HD, this is on chromosome 4. A transgenic animal model can express the gene anywhere, and this may lead to disruption of other important genes, which may cause unexpected problems.
HD INSIGHTS: Has anyone looked to see whether current transgenic animal models have this problem?
NGUYEN: No, no one has done that research in detail.
HD INSIGHTS: There are also some other animal models of HD. What are your thoughts on those?
NGUYEN: There is a pig model that is especially interesting, and also non-human primate models developed by Anthony Chan at Emory. These models have the advantage of bigger brains compared with the rat and mouse model, and their brains are relatively similar to human brains. Again, the disadvantage is cost. You cannot breed pigs and non-human primates as easily as rats and mice, and they have longer gestation periods. With the non-human primate models, if you were to use the full human gene, you would have to wait years before you got the phenotype. So instead, you use a short fragment of the gene and over-express the fragment with a very high number of CAG repeats. These primates have a very severe phenotype and cannot reproduce. I think this model can be used for symptomatic and some treatment studies, but in the long run, you probably want a model that can reproduce from generation to generation, and that you don’t have to generate again and again, because every time you generate a transgenic animal, the gene may incorporate in a different region with different expression. Currently there is no non-human primate model that you can actually breed and keep for generations.
HD INSIGHTS: You have started to use your BACHD rat model to screen drugs. Can you tell us more about that?
NGUYEN: We are involved in some of the European projects funded by the European Commission, including one project called MitoTarget in which we are studying mitochondria-targeted drugs. A company called Trophos in Marseille, France, wanted us to test a compound called olesoxime they had hit upon that is actually already in the clinical phase for amyotrophic
lateral sclerosis (see “Meet the Compound,” p. 6). We knew that olesoxime somehow targets mitochondria and we thought we could test it in our rats. Surprisingly, we didn’t see much effect on the motor phenotype, but we saw improvement in the cognitive and anxiety-related phenotypes.
Olesoxime seems to work totally differently from what we had thought. It doesn’t work much on mitochondria – it made some changes to mitochondrial function, but what was more interesting to us was that at the biochemical and neuropathologic level, treatment with olesoxime increased the amount of full-length soluble huntingtin, which is mainly mutant huntingtin, and reduced the amount of fragmented huntingtin, and aggregation. This is one of the first compounds we have seen that decreases the cleavage of the full-length protein and actually changes the amount of mutant huntingtin, the main target of the disease.
Now we are working on understanding why this happens. We have found that calpain is activated in our transgenic rats, and use of olesoxime decreases calpain activity. There have been studies showing that calpain cleaves huntingtin into fragments. We showed that olesoxime’s inhibition of calpain activity reduces the cleavage of full-length huntingtin, which leads to the increased levels of full-length soluble huntingtin, reduced levels of huntingtin fragments, and reduced number of aggregates.This is our current area of research, and we are trying to figure out how it all works. It seems very promising to us.
HD INSIGHTS: What areas of science or therapeutic development are you most excited about?
NGUYEN: I’m very interested in treating patients, and in new treatments in general. In particular, I think the various gene silencing and huntingtin-lowering strategies have been quite successful in some studies.
These include the usual RNA interference or antisense oligonucleotide approaches, but we are also working with our collaborators in Leiden on some exon-skipping approaches that have been successful in some studies in Duchenne muscular dystrophy. The idea is that you actually skip an exon where you think there is an important cleavage site, so for example, you could think of skipping a whole region of the calpain cleavage site, or even the CAG repeats. There’s also some interesting work in post-translational modification coming from William Yang’s lab. I think these are some of the most interesting new strategies coming up now.
HD INSIGHTS: What about trials in premanifest HD? Do you think we are ready for those trials?
NGUYEN: I think that is the way to go. We probably have a better chance of modifying the disease if we treat sooner rather than later. Whether we are ready or not is a question for clinicians who actually run those clinics, but from what Sarah Tabrizi (see HD Insights, Vol. 1) and Ralf Reilmann (see HD Insights, Vol. 6) have found in TRACK-HD, there seem to be robust changes that could be used as biomarkers to track disease progression.
HD INSIGHTS: Any final thoughts on HD?
NGUYEN: I think we’re at a very exciting time right now for HD research. Clinical studies such as PREDICT and TRACK have given us a lot of information on the natural course of the disease, and prepared us very well for clinical trials. Now we can actually use this information to test more compounds. It is very exciting to be in HD at this moment; I hope that we can find something quite soon.