Huntington’s disease (HD) is an autosomal dominant disorder caused by a mutation known as a CAG repeat expansion in the Huntingtin gene (HTT). A CAG repeat expansion means that the codon ‘CAG’, which can be thought of as one of the words that makes up the book of DNA, is repeated more times that it is meant to be in one particular spot of the genome. The HTT gene typically has 10-35 CAG repeats, however when this number of repeats is expanded to 40 or higher, it causes HD. Repeats in the intermediate range, from 36-40, may or may not cause HD, but account for a very small portion of the population. More information on the background and history of Huntington’s disease can be found here.
Limitations of Studying HD in Animal Models
One challenge to researching Huntington’s disease is the scientific community’s reliance on animal models, particularly mouse models of HD. An animal model, or model organism, is a species that is used to understand the biological processes of humans. Animal models can be bred or genetically modified in order to produce a disease or condition that is similar to a human one, such as Huntington’s disease. In order to study HD many researchers use a mouse model, because mice have a Huntingtin gene that is homologous, or related, to the human huntingtin gene. However, study of an animal model with a homologous gene, though extremely useful, is not exactly the same as study of the disease or gene in a human.
There are several types of mouse models, most being either:
- Knock-outs, meaning the huntingtin gene is either partially or completely prevented from being expressed.
- Transgenic, and therefore contain part of or the whole human mutant huntingtin gene somewhere in the mouse genome, as opposed to the normal location of the mouse huntingtin gene on chromosome five; or
- Knock-in, in which the mouse huntingtin gene is either partially or entirely replaced with the human gene.
Using mouse, and other animal, models allows researchers to study aspects of the disease that would be impossible, and likely unethical, in humans.
However, mouse models cannot provide a completely accurate representation of a human disease because different species have unique cell types, proteins, and toxicity sensitivities. Even though humans share a lot of common genetic material with other animals, there are often significant variations from species to species in how that genetic material is processed, which means that the same DNA sequence can be used to make products which are very different from those in humans. Drug trials in particular can be hindered by differences in how two different species react to the same drug. What is safe in mice may not be safe in humans. More information on animal models in HD research can be found here.
Studying HD in Human Stem Cells
Due to the limitations of animal models, stem cells have offered an opportunity for researchers to ethically and efficiently study disease in human cells. In the paper “Genomic Analysis Reveals Disruption of Striatal Neuronal Development and Therapeutic Targets in Human Huntington’s Disease Neural Stem Cells” (Ring, 2015) researchers use induced pluripotent stem cells (iPSCs) and differentiated neural stem cells (NSC) in order to explore the role mutant HTT plays in neural development and in producing disease phenotypes. Induced pluripotent stem cells (IPSCs), like other stem cells, have the ability to develop into different cell types in the body. IPSCs are unique in that they are originally adult cells that are reprogrammed to an embryonic stem cell-like state. Human iPSCs are capable of generating any type of cell in the body(2).
Differentiated neural stem cells (NSCs) are multipotent and can develop into the cells that characterize the nervous system, including neurons, astrocytes, and oligodendrocytes2. The most significant difference between IPSCs and NSCs is that NSCs are further along developmentally in that they are a more specific cell type and closer to reaching their final cell fate. However, these stems cells are similar in that they both have the potential to be influenced by external cues to become a specific type of cell.
In a previous study, these researchers used iPSCs and NSCs derived from patients with HD and found that iPSCs do not show any of the disease phenotypes of HD, such as cell death, loss of brain-derived neurotrophic factor (BDNF), and reduction of energy production. These types of phenotypes only manifest at the NSC stage. Using the same cell types, researchers built on this information in order to perform an analysis of transcriptional changes that occur in the genome from mHTT. Using corrected iPSCs and NSCs as controls, which are cells that are genetically modified to no longer express mutant HTT; researchers found that 4,466 genes were expressed differently in HD NSCs compared to corrected NSCs. In comparison, HD iPSCs only expressed 370 genes differently from corrected iPSCs. This 10-fold difference in the number of differentially expressed genes between iPSCs and NSCs supports the idea that mHTT affects cells at a later stage on the pathway to neural differentiation.
Further analysis between HD NSCs and corrected NSCs revealed several proteins to be important regulators of these gene expression changes. These proteins, including transformation growth factor β (TGF- β), β-estradiol, and tumor necrosis factor alpha (TNF-α), have been found by previous studies to play a role in HD (3,4,5,6,7).
Researchers also used gene co-expression analysis in order to identify modules of co-expressed genes in HD NSCs that share similar functions or cellular components. They found seven distinct modules that represented co-expressed genes in HD NSCs. They focused on two of these modules, labeled “Red” and “Black”, due to their relevance to the HD phenotype. These two modules were enriched with genes associated with human striatal tissue development, an area of the brain that experiences extensive cell loss in HD.
In addition to these modules, researchers found a number of other striatal-specific genes that were expressed at lower levels in HD NSCs than in corrected NSCs (CTIP2, DARPP-32, ISL1, TBR1, FOXP1, and PAX6).
Finally, researchers explored the question of whether, and how, development sets the stage for early pathophysiology in HD. They also looked into whether there were pathways or genes that could be identified as potential candidates in restoring neuronal health and protecting striatal medium spiny neurons (MSNs) in order to halt disease progression. Medium spiny neurons are the cells most acutely lost from the striatum in Huntington’s disease and play an important role in regulating a wide range of behaviors, including initiation and control of movement, motivation, and reinforcement learning. Researchers focused on two signaling pathways, TGF- β and netrin, as potential therapeutic targets for HD and found both to be altered in HD. They found that TGF- β was expressed at higher levels in HD NSCs than in wild-type controls. TGF- β could also be neuroprotective in HD NSCs, alleviating certain HD phenotypes such as high levels of caspase activity, which is involved in cell death pathways, and low respiratory capacity. This suggests a possible compensation mechanism in which HD NSCs express higher levels of TGF- β in order to compensate for negative effects of the HD mutation.
The other pathway studied was netrin-1, which is typically involved in axonal guidance and was altered in HD NSCs. Axonal guidance is the developmental mechanism through which neurons are able to reach their correct targets in the body. Researchers found that the addition of an axonal guidance molecule similar to netrin-1 was neuroprotective, suggesting that the alteration of netrin-1 and its receptors is a negative effect of the HD mutation.
In conclusion, these researchers brought forward several important details about the effects that the HD mutation has on gene expression and regulatory pathways involved in brain development. They also provide strong support for the use of a neural stem cell (NSC) model in order to study HD. Their HD NSC model provides evidence that early disruption of signaling pathways at the NSC stage is critical for determining striatal development in HD and that restoration of these pathways may be a potential option for developing therapeutics.
1) Original Paper: Ring, Karen L., et al. “Genomic Analysis Reveals Disruption of Striatal Neuronal Development and Therapeutic Targets in Human Huntington’s Disease Neural Stem Cells.” Stem Cell Reports 5.6 (2015): 1023-1038.
3) Battaglia, G., Cannella, M., Riozzi, B., Orobello, S., Maat-Schieman, M.L., Aronica, E., Busceti, C.L., Ciarmiello, A., Alberti, S., Amico, E., et al. (2011). Early defect of transforming growth factor b1 formation in Huntington’s disease. J. Cell. Mol. Med. 15, 555–571.
4) Kandasamy, M., Reilmann, R., Winkler, J., Bogdahn, U., and Aigner, L. (2011). Transforming growth factor-beta signaling in the neural stem cell niche: a therapeutic target for Huntington’s disease. Neurol. Res. Int. 2011, 124256.
5) Bode, F.J., Stephan, M., Suhling, H., Pabst, R., Straub, R.H., Raber, K.A., Bonin, M., Nguyen, H.P., Riess, O., Bauer, A., et al. (2008). Sex differences in a transgenic rat model of Huntington’s disease: decreased 17b-estradiol levels correlate with reduced numbers of DARPP32+ neurons in males. Hum. Mol. Genet. 17, 2595–2609.
6) Bjo¨rkqvist, M., Wild, E.J., Thiele, J., Silvestroni, A., Andre, R., Lahiri, N., Raibon, E., Lee, R.V., Benn, C.L., Soulet, D., et al. (2008). A novel pathogenic pathway of immune activation detectable before clinical onset in Huntington’s disease. J. Exp. Med. 205, 1869–1877.
7) Hsiao, H.Y., Chiu, F.L., Chen, C.M., Wu, Y.R., Chen, H.M., Chen, Y.C., Kuo, H.C., and Chern, Y. (2014). Inhibition of soluble tumor necrosis factor is therapeutic in Huntington’s disease. Hum. Mol. Genet. 23, 4328–4344.