By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Benedette Cuffari, M.Sc.Jan 24 2025

Study uncovers how gradual CAG repeat expansion in neurons drives the onset of Huntington’s disease, offering new insights for potential therapeutic interventions.

Study: Long somatic DNA-repeat expansion drives neurodegeneration in Huntington’s disease. Image Credit: Shutterstock AI / Shutterstock.com

A recent study published in Cell reveals that the repeated DNA sequence that leads to Huntington’s disease (HD) expands slowly over several decades in disease-specific neurons, eventually leading to generation of these neurons and onset of the disease.

What causes HD?

HD is a life-threatening neurodegenerative disease that typically arises due to the inheritance of a DNA triplet repeat (CAG)n in exon 1 of the huntingtin (HTT) gene. This CAG repeat encodes for a polyglutamine tract within the HTT protein, which is involved in regulating gene expression, transporting materials between neurons, and protecting cells from death.

As compared to otherwise healthy individuals with 15-30 consecutive CAGs, people with HD will typically have a germline allele of 36-55 consecutive CAGs, with about 90% of these individuals inheriting 40-49 of these CAGs.

For several decades, people with HD will not exhibit any symptoms. However, typically between the ages of 40 and 50 years, HD patients will develop chorea, which describes the uncontrolled movements that are characteristic of HD that often progress to severe impairment, rigidity, and lethality, along with cognitive and psychiatric symptoms.

Gradual neuronal degeneration driven by CAG expansion

Despite existing evidence on the genetic basis of HD, it remains unclear why the disease-causing mutation induces degeneration in specific neurons only during midlife after decades of biological latency.

To explore this phenomenon, the researchers of the current study utilized single-nucleus ribonucleic acid (RNA) sequencing (snRNA-seq) to analyze RNA expression in 581,273 nuclei isolated from the anterior region of the caudate nucleus, the largest part of the striatum and area of the brain that is most often affected in HD patients. Caudate samples were obtained from 50 HD patients and 53 unaffected controls with a mean of 5,643 nuclei obtained from each donor.

The CAG-age-product (CAP) score, which is often used to estimate the onset and progression of HD, was also used in the analysis. To this end, HD patients with CAP scores of up to 300 had slightly lower proportions of striatal projection neurons (SPNs) as compared to healthy controls, whereas individuals with CAP scores exceeding 350 exhibited a significant reduction in the abundance of SPNs.

Notably, a CAP score of up to 300 is typically provided to HD patients without clinical motor symptoms, whereas those with CAP scores of 350 or more are experiencing HD symptoms. HD patients with CAP scores of 600 or more, which often reflects patients with advanced caudate atrophy, lost 80-99% of their SPNs, thus demonstrating the graduate degeneration of the caudate that is accompanied by increasingly severe HD symptom manifestation.

In addition to SPNs, the researchers also observed the differential expression of thousands of genes in every caudate cell type between individuals with and without HD. This finding exemplifies the profound destruction of the caudate that occurs in HD.

CAG repeats in the HTT gene expanded somatically from 40 to over 500 in SPNs. Somatic expansion from 40 to 150 CAGs did not significantly impact neuron health; however, expansion of 150 or more CAGs resulted in loss of positive and then negative features of neuronal identity, distorted gene expression, and ultimately degeneration of SPNs.

Computational extrapolation of the experimental data was applied to determine the rate and timing of expansion of CAG repeats in SPNs. To this end, a slow initial expansion that occurred less than once each year during the first two decades of life was observed.

When a neuron gains 80 CAG repeats, usually after several decades, the rate of expansion increases significantly and expands to 150 CAGs in only a few more years. Months thereafter, the neuron will die.  

These findings suggest that SPNs spend over 95% of their life with an innocuous HTT gene. Since different neurons reach the toxicity threshold of CAG repeats at different times, the neurons collectively and slowly disappear over an extended period that typically begins about 20 years before symptom appearance and more rapidly after symptom onset.

Study significance

The study findings provide important insights into the pathophysiology of HD and, potentially, other diseases that arise from abnormal numbers of DNA repeats, including fragile X syndrome and myotonic dystrophy. Encouraged by these findings, the researchers of the current study are currently investigating how over 150 CAG repeats leads to neuronal impairment and death, as well as the association between DNA repeat expansion and cell changes in other similar genetic disorders.

We’ve been able to piece together the full trajectory of the pathology as it unfolds over decades in individual neurons, and that gives us potentially many different time points at which we can intervene therapeutically.”

 

Journal reference:

Handsaker, R. E., Kashin, S., Reed, N. M., et al. (2025). Long somatic DNA-repeat expansion drives neurodegeneration in Huntington’s disease. Cell. doi:10.1016/j.cell.2024.11.038