Measuring the seeding activity of mutant huntingtin (mHTT) protein — its ability to trigger protein aggregation — could be an early biomarker of Huntington’s disease with the potential for monitoring disease onset and progression, according to preclinical findings in mouse and fly models.
The study, “mHTT Seeding Activity: A Marker of Disease Progression and Neurotoxicity in Models of Huntington’s Disease,” was published in the journal Molecular Cell.
Huntington’s disease is caused by mutations in the HTT gene, which provides instructions for making the huntingtin protein. Mutations underlying the disease are known as CAG repeat expansions, which consist of many small DNA repeats abnormally inserted within the HTT gene. These result in the production of a mutant huntingtin protein (mHTT) that is larger than normal because it carries additional protein (polyglutamine) stretches.
mHTT is known to affect many regions in the brain, eventually leading to the progressive breakdown of nerve cells and consequent deterioration of patients’ physical and mental abilities.
However, the chain of events at the level of cells and molecules that connects mHTT accumulation, nerve cell death, and disease onset are not completely understood and are clinically undetectable.
mHTT and its fragments tend to get misshapen and form aggregates that are able to self-propagate and spread between cells. The accumulation of these aggregates in the brain is thought to drive progression of Huntington’s disease, much like other neurological conditions such as Alzheimer’s and Parkinson’s, where toxic protein clusters seeded in the nervous system play a critical role.
In this study, researchers explored whether mHTT seeding activity, that is, its ability to initiate protein clustering, could be related to Huntington’s onset and progression. If so, identifying this phenomenon may have the potential to serve as an early biomarker for the disease.
Current monitoring of Huntington’s disease relies on clinical symptoms (e.g., chorea, movement disorders, cognitive decline, and depression), while reliable markers of early biological changes occurring before nerve cell loss are mostly unavailable.
The researchers developed a sensitive analysis called the fluorescence resonance energy transfer (FRET)-based mHTT aggregate seeding (FRASE) assay, a technique that enables the quantification of mHTT’s seeding activity.
Using this method in the brain tissue of both patients and mouse models of Huntington’s revealed the tool was sensitive and specific enough to quantify mHTT seeding activity in disease-affected tissues.
Importantly, they found that mHTT’s seeding capacity was already present in the brains of mice many weeks before they showed any symptoms, and this activity increased progressively throughout disease development.
A more detailed molecular characterization revealed the bulk of seeding activity mostly originated from small mHTT fibrils, “suggesting that such structures are responsible for this activity,” the researchers said.
Experiments on a fruit fly model of Huntington’s disease further reinforced the biological relevance of mHTT seeding activity. The formation of seeding-competent huntingtin fragments in adult nerve cells was associated with a shorter lifespan, supporting the notion that these structures are indeed neurotoxic.
“We have demonstrated that [mHTT seeding activity] is a robust, early disease biomarker in [Huntington’s disease] transgenic mice and flies. We propose that it also might be of high value for monitoring disease onset and progression in [Huntington’s disease] patients if [mHTT seeding activity] could be quantified in biosamples whose collection is technically and ethically possible, like cerebrospinal fluid, blood, or muscle tissue,” the researchers wrote.
Using FRASE to quantify mHTT seeding activity in patient samples could also be useful for determining the best time to start clinical trials and evaluate the effectiveness of therapeutic interventions.
Another short article in the same scientific journal focused on this particular study and stressed that it “creates a new angle in addressing multiple crucial issues in the [Huntington’s disease] field — from understanding the impact of mHtt aggregation to further elucidating that pathogenic [disease-causing] cascade at the biochemical level.”
Adapting “the novel FRASE assay for clinical purposes may prove useful in the development of much needed biomarkers,” the authors also wrote.