HD Insights™

Nguyen, cont... Copyright © Huntington Study Group 2014. All rights reserved. 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. (continued on Page 6...) NGUYEN: 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. H D I N S I G H T S HD Insights, Vol. 8 5

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