Two recent studies demonstrate that the Huntington mutation changes its length over time

HUNTINGTON: The mutation modifies its length over time within neurons, and it is now possible to detect signs of biological changes many years before the onset of symptoms.

by Ferdinando Squitieri, MD, PhD

Two studies recently published reveal significant new insights into the understanding of Huntington's disease. The studies were conducted by different research groups, but share conclusions that point in the same direction. Therefore, 2025 begins with a strong acceleration of knowledge about Huntington's disease.

STUDY 1: Conducted by researchers from Harvard University (USA) on dozens of human brain samples stored at the Harvard Brain Tissue Resource Center – NIH NeuroBioBank. These samples were obtained through donations from families of deceased patients with Huntington's disease, ranging from early to advanced stages, all carrying the CAG repeat expansion mutation, commonly observed in the adult population affected by the disease. The study was conducted directly on human samples, rather than on animal or cell models.

The researchers, with an extensive effort, isolated single brain cells and analyzed how the CAG repeat expansion mutation behaves within each of these cells. The CAG expansion, as a reminder, becomes toxic and causes the disease when it reaches at least 36 repeats, and this is what is detected in blood cells when a genetic test on DNA is performed. In this case, the researchers analyzed not only blood samples but also individual brain cells from the deceased patients.

In this study it was observed that over the course of life, there is a significant change in the length of the mutation in striatal neurons, located in the part of the brain primarily affected by Huntington's disease, and that this change occurs at different rates. This change consists of subsequent expansion of the mutation in individual brain cells, reaching up to hundreds of CAG repeats. For example, if an individual is born with 44 CAG repeats, extensions of the repeat sequence will begin to occur over their lifetime in brain cells; as a result, some neurons will harbor 44 CAG repeats, while others will show much longer mutations. This phenomenon leads to what is known as somatic mosaicism. At first, the expansion may reach lengths of approximately 70-100 CAG repeats or more, without causing visible damage. However, once the mutation surpasses 150 CAG repeats, the first biological effects on the cell begin to emerge, including dysregulation of genes other than the HTT gene, which contributes to the poor functioning of the nerve cell. The expansion from 44 to 150 CAG repeats takes years within the cells, but once this threshold is reached, the process accelerates, and the mutation can grow from 150 repeats upwards in a matter of months rather than years, triggering the onset of more severe functional damage and neuronal loss due to cell death. At this point, the acceleration of repeat expansion in more brain cells of the patient leads to the presence of over 500 CAG repeats in some cells, resulting in increased dysregulation and cell death.

The study provides a real explanation for the devastating progression of the disease, which depends on a dynamic, asynchronous, and non-homogeneous phenomenon across nerve cells. It has been noted that more extensive somatic mosaicism occurs with longer initial CAG length—meaning the formation of expanded CAG repeats longer than those present at birth, in the different nerve cells. This somatic mosaicism phenomenon is influenced by the contribution of modifier genes, which are responsible for repairing the resulting DNA damage and could, therefore, represent a potential therapeutic target. The researchers are already evaluating how to use future therapies to modulate the function of these genes in the attempt to control the increase in somatic mosaicism and reduce the expansion of the toxic CAG repeats within striatal neurons (Publication 1).

STUDY 2: Directed by researchers from University College London (UK). The researchers focused the study on living patients; once again, this is a significant achievement derived from the observation of the disease in humans, rather than animal or laboratory models.

The objective was to detect clinical and biological changes much earlier than the onset of the disease—more than 20 years before, in fact. To achieve this, the study was conducted on young individuals, enrolled in the HD Young Adult Study (HD-YAS) project, who were carriers of the CAG repeat expansion mutation, but far from the clinical onset of the disease. It was found that, more than 20 years prior to the clinical manifestation of the disease—when there are no visible clinical changes in terms of movement, cognition, or psychiatry—biological changes are already present. These changes could represent crucial biomarkers for future therapeutic trials. It is important to have valid biomarkers, which serve as indicators of biological changes, if we aim to modify the disease course in the absence of symptoms. In other words, without reliable biomarkers, it is not feasible to begin experimental therapies in asymptomatic carriers of the mutation in order to test whether a preventive treatment truly works.

The researchers have highlighted that, more than 20 years before the onset of symptoms, changes occur in the striatum of the brain in individuals carrying the CAG mutation (a slight, imperceptible reduction in the putamen volume). Certain proteins, such as neurofilaments (NfL), increase in the cerebrospinal fluid, while others, like pro-enkephalins (PENK), decrease. 

Recently, Tabrizi's team has proposed a new staging system for the disease, called the Integrated Staging System (ISS). Thanks to the recent research, we now have more data to track disease phase changes, from the birth of an at-risk individual to the onset of the disease. The participation of young at-risk individuals has been crucial for this research, and their engagement in observational programs (such as Enroll-HD and HDClarity) will increasingly play a pivotal role (Publication 2).

IN SUMMARY: The two studies confirm that Huntington’s disease is a dynamic process: changes occur in the nervous system of individuals carrying a CAG repeat expansion mutation in the Huntingtin gene throughout their lifetime, and these changes are influenced by many factors, only some of which are known today. The effects observed in the brain are not visible during life, but fortunately, certain biomarkers can now be detected in at-risk individuals using new technologies, long before clinical manifestations appear. These tools cannot be used in daily clinical practice, but they are available in therapeutic research protocols to test new treatment strategies. Both studies agree in concluding that so-called somatic mosaicism plays a crucial role in the disease's onset. It is important to reiterate that somatic mosaicism means that individual nerve cells may harbor mutations of different lengths within a tissue (e.g., the striatum in the brain), and the most severe toxicity occurs when the CAG repeat length becomes hundreds of CAG triplets—a phenomenon not commonly detectable in blood cells, where mosaicism is less pronounced. Therefore, although genetic test result will not change whether done early or late in life, the length of the mutation varies significantly in brain cells, thereby driving neuronal death.

MY POINT OF VIEW: The mutation is truly toxic and leads to neuronal death in the striatum when it reaches a much, much higher length threshold than that observed in the same individual blood cells. This is why individuals who are born with a much more extensive mutation than the one observed in most patients, exhibit a more severe, devastating, and vastly different disease from what is commonly observed. This occurs in pediatric-onset Huntington's disease, as demonstrated by a research team I led in 2018 (Publication 3).

This phenomenon, as shown in Research 1, leads to dysregulation of many genes—up to 700 different genes—some of which increase their expression, while others decrease it. We are therefore pleased to note that our recent research is perfectly aligned with the aforementioned studies, as we have documented that in individuals with pediatric-onset Huntington's disease, glucose metabolism is reduced in many brain areas, while in others it is markedly increased compared to the brain of a patient with classic adult-onset Huntington's disease (Publication 4).

Finally, we are proud to note that we already had identified this phenomenon correctly back in 2005 in human lymphoblastoid cell cultures from children with Huntington’s disease, where the CAG tract expanded exponentially over time reaching hundreds of CAG triplets and could be pharmacologically reduced (Publication 5).

Therefore, although pediatric Huntington’s disease is fortunately extremely rare, this condition—which my team has been studying for a long time—offers valuable insights that enable us to detect phenomena that, as evident, are not observable in adults.