Three letters, C-A-G, can make the difference between being healthy and having a genetic brain disorder called Huntington’s disease (HD). HD is a progressive neurodegenerative disease that affects movement, cognition and personality. Currently more than 30,000 Americans have HD and there is no cure or treatment to stop the disease from progressing.
A genetic mutation in the huntingtin gene. caused by an expanded repeat of CAG nucleotides, the building blocks of DNA that make our genes, is responsible for causing HD. Normal people have less than 26 CAG repeats while those with 40 or more repeats will get HD. The reasons are still unknown why this trinucleotide expansion causes the disease, but scientists hypothesize that the extra CAG copies in the huntingtin gene produce a mutant version of the Huntingtin protein, one that doesn’t function the way the normal protein should.
As with many diseases, things start to go wrong in the body long before symptoms of the disease reveal themselves. This is the case for HD, where symptoms typically manifest in patients between the ages of 30 and 50 but problems at the molecular and cellular level occur decades before. Because of this, scientists are generating new models of HD to unravel the mechanisms that cause this disease early on in development.
Induced pluripotent stem cells (iPSCs) derived from HD patients with expanded CAG repeats are an example of a cell-based model that scientists are using to understand how HD affects brain development. In a CIRM-funded study published today in the journal Nature Neuroscience, scientists from the HD iPSC Consortium used HD iPSCs to study how the HD mutation causes problems with neurodevelopment.
They analyzed neural cells made from HD patient iPSCs and looked at what genes displayed abnormal activity compared to healthy neural cells. Using a technique called RNA-seq analysis, they found that many of these “altered” genes in HD cells played important roles in the development and maturation of neurons, the nerve cells in the brain. They also observed differences in the structure of HD neurons compared to healthy neurons when grown in a lab. These findings suggest that HD patients likely have problems with neurodevelopment and adult neurogenesis, the process where the adult stem cells in your brain generate new neurons and other brain cells.
After pinpointing the gene networks that were altered in HD neurons, they identified a small molecule drug called isoxazole-9 (Isx-9) that specifically targets these networks and rescues some of the HD-related symptoms they observed in these neurons. They also tested Isx-9 in a mouse model of HD and found that the drug improved their cognition and other symptoms related to impaired neurogenesis.
The authors conclude from their findings that the HD mutation disrupts gene networks that affect neurodevelopment and neurogenesis. These networks can be targeted by Isx-9, which rescues HD symptoms and improves the mental capacity of HD mice, suggesting that future treatments for HD should focus on targeting these early stage events.
I reached out to the leading authors of this study to gain more insights into their work. Below is a short interview with Dr. Leslie Thompson from UC Irvine, Dr. Clive Svendsen from Cedars-Sinai, and Dr. Steven Finkbeiner from the Gladstone Institutes. The responses were mutually contributed.
Q: What is the mission of the HD iPSC Consortium?
To create a resource for the HD community of HD derived stem cell lines as well as tackling problems that would be difficult to do by any lab on its own. Through the diverse expertise represented by the consortium members, we have been able to carry out deep and broad analyses of HD-associated phenotypes [observable characteristics derived from your genome]. The authorship of the paper – the HD iPSC consortium (and of the previous consortium paper in 2012) – reflects this goal of enabling a consortium and giving recognition to the individuals who are part of it.
Q: What is the significance of the findings in your study and what novel insights does it bring to the HD field?
Our data revealed a surprising neurodevelopmental effect of highly expanded repeats on the HD neural cells. A third of the changes reflected changes in networks that regulate development and maturation of neurons and when compared to neurodevelopment pathways in mice, showed that maturation appeared to be impacted. We think that the significance is that there may be very early changes in HD brain that may contribute to later vulnerability of the brain due to the HD mutation. This is compounded by the inability to mount normal adult neurogenesis or formation of new neurons which could compensate for the effects of mutant HTT. The genetic mutation is present from birth and with differentiated iPSCs, we are picking up signals earlier than we expected that may reflect alterations that create increased susceptibility or limited homeostatic reserves, so with the passage of time, symptoms do result.
What we find encouraging is that using a small molecule that targets the pathways that came out of the analysis, we protected against the impact of the HD mutation, even after differentiation of the cells or in an adult mouse that had had the mutation present throughout its development.
Q: There’s a lot of evidence suggesting defects in neurodevelopment and neurogenesis cause HD. How does your study add to this idea?
Agree completely that there are a number of cell, mouse and human studies that suggest that there are problems with neurodevelopment and neurogenesis in HD. Our study adds to this by defining some of the specific networks that may be regulating these effects so that drugs can be developed around them. Isx9, which was used to target these pathways specifically, shows that even with these early changes, one can potentially alleviate the effects. In many of the assays, the cells were already through the early neurodevelopmental stages and therefore would have the deficits present. But they could still be rescued.
Q: Has Isx-9 been used previously in cell or animal models of HD or other neurodegenerative diseases? Could it help HD patients who already are symptomatic?
The compound has not been used that we know of in animal models to treat neurodegeneration, although was shown to affect neurogenesis and memory in mice. Isx9 was used in a study by Stuart Lipton in Parkinson’s iPSC-derived neurons in one study and it had a protective effect on apoptosis [cell death] in a study by Ryan SD et al., 2013, Cell.
We think this type of compound could help patients who are symptomatic. Isx-9 itself is a fairly pleiotropic drug [having multiple effects] and more research would be needed [to test its safety and efficacy].
Q: Have you treated HD mice with Isx-9 during early development to see whether the molecule improves HD symptoms?
Not yet, but we would like to.
Q: What are your next steps following this study and do you have plans to translate this research into humans?
We are following up on the research in more mature HD neurons and to determine at what stages one can rescue the HD phenotypes in mice. Also, we would need to do pharmacodynamics and other types of assays in preclinical models to assess efficacy and then could envision going into human trials with a better characterized drug. Our goal is to ultimately translate this to human treatments in general and specifically by targeting these altered pathways.