Huntington’s Disease
Huntington’s disease is an inherited neurodegenerative disorder that affects movement, thinking, and behavior. It is caused by a genetic mutation that is passed down in families, and unlike many other neurodegenerative diseases, the cause is known with certainty. The disease typically appears in mid-adulthood, often between ages 30 and 50, though earlier and later onset can occur. Early symptoms may include subtle changes in mood, coordination, or thinking, which gradually progress to more noticeable movement abnormalities, cognitive decline, and psychiatric symptoms. A defining feature of Huntington’s disease is the gradual loss of neurons in the striatum, a structure that plays a central role in coordinating movement and decision-making. As these neurons are lost, the brain’s ability to regulate movement and behavior becomes increasingly impaired.
Pathology
Huntington’s disease is caused by a mutation in the HTT gene, which encodes the huntingtin protein. This mutation involves an abnormal expansion of a repeating DNA sequence (CAG repeats), which leads to the production of a mutant huntingtin protein with an extended stretch of glutamine residues. Over time, the number of CAG repeats increases leading to production of increasingly toxic huntingtin.
This altered protein has a tendency to misfold and form aggregates inside of neurons. These aggregates can interfere with normal cellular processes, including gene expression, protein trafficking, and cellular stress responses. Importantly, the disease is not caused only by visible aggregates, but also by more subtle toxic effects of the mutant protein throughout the cell.
Neurons in the striatum are especially vulnerable, though over time other brain regions are affected as well. As these neurons degenerate, the brain undergoes progressive atrophy, particularly in regions responsible for movement, cognition, and behavior.
Biological Pathways
The central pathway in Huntington’s disease is protein misfolding and aggregation. The mutant huntingtin protein disrupts normal protein homeostasis, overwhelming the cell’s ability to properly fold and clear proteins.
Another key pathway is transcriptional dysregulation. Mutant huntingtin interferes with the normal control of gene expression, leading to widespread changes in which genes are turned on or off. This can impair neuronal survival programs and disrupt essential cellular functions.
Mitochondrial dysfunction and energy metabolism are also affected. Neurons require a lot of energy, and impairments in mitochondrial function can make them more vulnerable to stress and degeneration.
In addition, synaptic dysfunction and impaired communication between neurons occur early in the disease, even before significant cell death. Over time, these combined disruptions lead to progressive neuronal loss.
Causes
Huntington’s disease is caused by a single, well-defined genetic mutation in the HTT gene. It is inherited in an autosomal dominant pattern, meaning that a person only needs to inherit one copy of the mutated gene to develop the disease.
The number of CAG repeats in the gene strongly influences disease onset. Individuals with a higher number of repeats tend to develop symptoms earlier in life, a phenomenon known as anticipation, where the disease appears at a younger age in successive generations.
Unlike many other neurodegenerative diseases, environmental factors play a relatively minor role in determining whether someone develops Huntington’s disease. However, lifestyle and overall health may still influence the course and severity of symptoms.
Progression
Huntington’s disease progresses gradually over 10 to 25 years, though the rate can vary.
In the early stage, symptoms may include subtle coordination problems, mild involuntary movements (chorea), changes in mood such as irritability or depression, and difficulty with concentration or planning.
In the middle stage, movement abnormalities become more pronounced, including more obvious chorea, impaired balance, and difficulty with voluntary movements. Cognitive decline becomes more apparent, and daily activities often require assistance.
In the late stage, individuals typically lose the ability to walk, speak clearly, or care for themselves independently. Movement may become more rigid rather than hyperkinetic, and complications such as infections or swallowing difficulties can arise.
Treatment Landscape
There is currently no cure for Huntington’s disease, but treatments are available to help manage symptoms.
Medications can be used to control movement disorders, such as drugs that reduce chorea by modulating dopamine signaling. Psychiatric symptoms, including depression, anxiety, and irritability, can also be treated with standard therapies.
Supportive care is essential and includes physical therapy, speech therapy, nutritional support, and structured routines. Because the disease affects multiple aspects of function, care is typically multidisciplinary.
A major focus of current treatment development is lowering levels of mutant huntingtin protein. Approaches such as antisense oligonucleotides and gene-silencing therapies aim to reduce production of the toxic protein, representing a shift toward disease-modifying strategies.
Research Directions
Research in Huntington’s disease is highly active, in part because the genetic cause is clearly defined.
One major direction is gene-targeted therapy, particularly approaches that reduce or silence the mutant HTT gene. While some early clinical trials have faced challenges, the strategy remains one of the most promising avenues for altering disease progression.
Another area of research focuses on improving protein clearance systems, including enhancing autophagy to help cells remove toxic proteins more effectively.
Researchers are also exploring ways to support neuronal survival and function, including targeting mitochondrial health, synaptic stability, and cellular stress responses.
Because Huntington’s disease can be identified genetically long before symptoms appear, there is growing interest in early intervention, potentially treating individuals before significant neurodegeneration has occurred.
Sources
- MacDonald et al. “A Novel Gene Containing a Trinucleotide Repeat That Is Expanded and Unstable on Huntington’s Disease Chromosomes.” Cell. 1993.
- Christopher A. Ross and Sarah J. Tabrizi. “Huntington’s Disease: From Molecular Pathogenesis to Clinical Treatment.” The Lancet Neurology. 2011.
- Tabrizi et al. “Huntington Disease: New Insights into Molecular Pathogenesis and Therapeutic Opportunities.” Nature Reviews Disease Primers. 2020.
- Frédéric Saudou and Sandrine Humbert. “The Biology of Huntingtin.” Neuron. 2016.