Reviewer #1 (Public Review):
Weiss et al have developed a novel model of Huntington's disease (HD) by injecting a mixture of recombinant adeno-associated viral vectors (AAVs) into the caudate and putamen of rhesus macaque monkeys. There is a significant need for relevant models of HD. While many mouse models exist, current models lack genetic relevance (with repeat lengths much longer than those found in humans being used) and mice lack the anatomical relevance to humans since they have small brains with important brain regions (in particular the neostriatum) being dissimilar to those seen in humans. The authors used non-human primates because they have large brains with anatomy similar to humans. They used a mixture of recombinant adeno-associated viral vectors (AAVs) in an attempt to overcome the shortcomings of previous models using AAVs. They studied their animals over 20 months using both behavioural tasks and MRI assessment. The animal served as their own controls for the imaging, which improves the power of the study. The methods of analysis, particularly the imaging, are modern and directly relevant to assessments that can be conducted on human patients.
Strengths<br />
The major strength of the paper is that the authors used Rhesus macaque, a species that is highly relevant to studies aimed at assessing therapies and drug delivery. As a stepping-stone to humans, the macaque has many advantages, including brain size, relevant anatomy and in particular, longevity compared to mice. The approach of mixing a number of recombinant AAVs is also interesting since it overcomes some of the limitations of individual AAVs as detailed in the Introduction. The tasks used for behavioural assessment to investigate the effect of the AAV on brain and behaviour also highlight the advantages of a monkey model, since human-relevant assessments were used. The study was very well controlled, with both vehicle and AAV containing non-pathogenic length CAG repeat (10Q) used.
The behavioural assessment was comprehensive, and the motor control measures are relevant to HD. MR imaging is also very relevant to what can be measured in humans. The imaging was comprehensive and of excellent quality. Overall, the study presents some important and interesting data, because an acute monkey model has not been studied in such depth previously. The range of approaches taken to assessing the animals is comprehensive and impressive.
This is a study that will be of interest to researchers who are developing methods for studying the role of the caudate/putamen in behaviour. I agree with the statement that the model will be useful for studying the effects/causes of disruption to the cortico-basal ganglia. The effect of the lesions on cortical regions of the brain are interesting, and well presented.
Weaknesses<br />
The major weakness of the study is that with the interpretation of the results. The changes in tractography, behavior and TBM are what would be expected following lesions of the neostriatum. Indeed, all the data point to this being an acute lesion model, and in my opinion, the authors have made an interesting novel neurotoxic model (using a very relevant neurotoxin). Unfortunately, there is no detailed pathology showing what is happening at the level of the striatum or associated cortical regions (see also below). The results have been interpreted as showing a progressive model, although evidence that there is progression is limited. The whole manuscript is written as though this is a genetically-relevant progressive model of HD. But the animals are normal, and so there is no genetic context relevant to HD. While the authors present this as a new model of HD with progressive motor and cognitive changes (as seen in the title of the paper) there is little evidence presented that there are major 'progressive' changes seen. Furthermore, the idea that the changes mirror those seen in HD patients (as stated in the discussion) is somewhat misleading. While their data may be similar to some findings in patients who have early degenerative changes, there are many differences that are either not seen or not explored in the new model. In particular, evidence that the changes seen in the monkeys are relevant to progression of disease in HD patients is missing. While this appears to be a sophisticated acute lesion model that has been assessed more thoroughly than previous models, nevertheless all evidence points to it being an acute rather than a progressive model. While I agree that the data show clearly that there are changes in motor function and to a lesser extent in cognitive function caused by the injection of the AAVs, it does not follow that the changes seen are 'progressive'. Even in rats following striatal lesions, changes in behaviour have been measured for months afterwards. For example, in acute lesions of monkeys, Deglon et al showed years ago that in NHPs the behavioral sequelae of acute neurotoxic lesions change with time. This model also does not show 'progression' in the sense of a progressive disorder in HD.
The authors state in the Abstract that the injection resulted in "robust expression of mutant huntingtin in the caudate and putamen". These data are not in the manuscript. This seems to be deduced, rather than measured experimentally.
A disadvantage of the method used, that has been a bugbear of the field, is that the authors chose to use a fragment of the HD gene, with a very long repeat that is seen only in juvenile patients. While using the fragment rather than the whole gene is a sensible approach, since it is known to be toxic, I am not clear why they chose to use a juvenile length repeat rather than a repeat in the adult-onset pathological range. There are mouse models with a CAG repeat of 40-50, but short lifespan of mice has limited their usefulness. Longevity is one of the major advantages of using a monkey. Had they used a repeat of 45 or 50, this would have been a much more interesting paper, because there is little known about the toxicity of proteins with that length repeat in vivo. As it stands, the model is a non-human primate acute fragment model using a long repeat. The disadvantages of fragment models and long CAG repeats has been well discussed in the literature and is a major criticism of many of the mouse models. Consistent with the disadvantages of fragment models with long repeats, the onset of the symptoms of the monkeys is much more rapid that would be expected in either juvenile or adult-onset HD and is likely to be due to both the fragment nature of the vector and long CAG repeat. The rapid onset of phenotype is not discussed in the context of other models.
The use of a DA receptor agonist was an interesting idea, because DA agonists have been shown to exacerbate abnormal involuntary movements in HD patients. The mechanism for this is complicated, however, given that the balance of D1 and D2 receptors changes as HD pathology progresses. The authors chose to use a non-selective agonist, which caused transient changes in behaviour. However, the usefulness/relevance of the apomorphine data is unclear, particularly since the effect was only seen at the early timepoints and not at the later timepoints. If the AAV causes acute toxicity, then such changes in response to apomorphine would be expected, and this would be expected to resolve with time - as was seen.
For their cognitive testing, the authors used a task (delayed non-match to sample) that measures object recognition and familiarity. Before surgery, only 11/17 of the animals were successfully trained to complete this task. It is not clear how useful the data are when only 64% of the animals can be included. It would have been better to have choosen a task that all monkeys could perform at baseline.
The results of the motor task showed a clear deficit. This would be expected with an acute lesion of the striatum. It was interesting that there was a significant improvement from the 9-month point to the 20-month point in the 85Q lesioned monkeys, whereas the performance of the vehicle-treated monkeys plateaued. There appeared to be a deleterious effect on the 10Q monkey performance that was maintained. (This was not discussed in any detail but should be.) This begs the question as to whether or not the 85Q-lesioned monkeys would recover to a level similar to the 10Q animals if left for another 12 months.
The tractography and tensor-based morphology data are clear, and consistent with the idea that a Q85 fragment would be neurotoxic. The authors consistently suggest that this is relevant to early stages of HD, but there is little evidence presented to support this statement. The sensitivity of the MRI used shows that multiple regions were affected beyond the lesion sites. This would be expected and is new and interesting data in the non-human primate field, although it does not bring anything particularly new to the table with respect to HD (since similar findings have been shown in lesion studies using mice, rats and sheep).
For HD researchers investigating aspects of the disease, such as transition from prodromal to early symptomatic stages, or for developing treatments, the usefulness of this model will be limited. It is not clear how this monkey model will be useful for developing either disease biomarkers or therapeutic strategies for HD (as stated in the abstract). For studying biomarkers of the disease, this model lacks a number of critical parameters. First, the genetic context of the disease is missing. Second, it is known that HD has multiple sites of pathology, and that symptoms are not simply due to degeneration of the caudate/putamen and that multiple regions of the brain where mutant Htt is expressed become dysfunctional and eventually degenerate. Understanding how the caudate/putamen degenerate is important, but since last century HD researchers have been very aware that dysfunction in the HD brain occurs at many sites other than the caudate/putamen. Indeed, it is probably not only a brain disease since there is evidence of peripheral pathology in humans and other models.
The authors state that they hope the model will become a widely used resource. This seems an unlikely scenario, given the limitations of the current study and the challenges associated with using monkeys. They say that a major advantage of their technique is being able to generate large numbers of monkeys. But this is not a relevant argument if the usefulness of the model to investigate HD is not proven. Studying the role of the caudate/putamen in motor behaviour is interesting for a small field but limited in scope.
The authors suggested a number of experiments that could be done, for example, using a shorter HD-relevant CAG repeat length. But as stated above, this is a weakness of the current study, and it would be much more useful had the authors done this experiment themselves. It seems unlikely that until the authors prove its usefulness, this model will not become a widely used resource, since the disadvantages of the model outweigh the advantages. Using monkeys requires a specialist laboratory and facilities and a careful consideration of the ethics involved in animal experimentation. Unless the model offers clear advantages over other models, it is unlikely to become mainstream. It is also not clear what therapies could be tested in this model that could not be tested in other existing models. For example, given that there is no control over which cells are infected by the AAVs, or if any of the cortical pathology is due to spread of AAV from the initial sites of injections, it is not clear how antisense oligonucleotides efficacy could be tested.