Huntington’s disease is caused by a single genetic mutation: an abnormal expansion of a repeating DNA sequence in the HTT gene on chromosome 4. Everyone carries this gene, and everyone has some number of these repeats. But when the sequence repeats too many times, it produces a toxic form of a protein that gradually destroys neurons in the brain. The disease affects roughly 5 out of every 100,000 people worldwide.
The CAG Repeat Expansion
Deep inside the HTT gene sits a short stretch of DNA where three letters, C-A-G, repeat in a row. In most people, this sequence repeats 16 to 20 times. That’s normal, and the gene functions without problems. But in people with Huntington’s, this sequence has expanded far beyond the typical range.
The critical thresholds break down like this:
- 26 or fewer repeats: Normal. No risk of Huntington’s.
- 27 to 35 repeats: Intermediate. The person won’t develop symptoms, but the repeat count can expand when passed to the next generation.
- 36 to 39 repeats: Reduced penetrance. Some people in this range develop the disease, others don’t.
- 40 or more repeats: Full penetrance. Symptoms will develop at some point during the person’s lifetime.
The number of repeats also influences when symptoms first appear. People with repeat counts in the low 40s often develop symptoms in middle age, typically between 30 and 50. Higher repeat counts tend to push the onset earlier. Counts above 60 are associated with juvenile-onset Huntington’s, which can appear before age 20.
How the Mutation Damages the Brain
The CAG repeat sequence is essentially a set of instructions for building part of a protein called huntingtin. Each CAG triplet tells the cell to add one more copy of the amino acid glutamine to the protein chain. When the repeat is too long, the resulting protein has an abnormally long glutamine stretch that causes it to misfold and behave in toxic ways.
One key mechanism involves a section of the protein right next to the glutamine stretch. When the glutamine tract is expanded, it changes the protein’s shape and exposes a neighboring region that normally stays tucked away. This exposed region then latches onto other proteins inside neurons and triggers a chain of damaging signals. Research in both mouse models and post-mortem brain tissue from Huntington’s patients has confirmed that this process activates stress-signaling enzymes that interfere with axonal transport, the system neurons use to shuttle essential cargo along their length. When that internal transport system breaks down, neurons can’t maintain themselves and eventually die.
The neurons most vulnerable to this damage are in the striatum, a region deep in the brain involved in movement coordination, and in the cortex, which handles thinking and planning. This is why Huntington’s produces its characteristic combination of involuntary movements, cognitive decline, and psychiatric symptoms.
How Huntington’s Is Inherited
Huntington’s follows an autosomal dominant inheritance pattern, meaning only one copy of the mutated gene is enough to cause the disease. If one of your parents carries the mutation, you have a 50 percent chance of inheriting it. It doesn’t matter whether the affected parent is your mother or father, and it affects all sexes equally.
Most people with Huntington’s have a parent who also had the disease. Spontaneous new mutations are rare but possible, particularly when a parent carries a repeat count in the intermediate range (27 to 35). While that parent won’t develop symptoms, the repeat can expand during the process of passing DNA to the next generation, potentially crossing into the disease-causing range.
Why It Can Appear Earlier in Each Generation
One of the more unsettling features of Huntington’s is a phenomenon called genetic anticipation: the disease can show up earlier and with more severity in successive generations. This happens because the CAG repeat is unstable. Each time it’s copied and passed on, it can grow longer.
This expansion is especially pronounced when the mutation is inherited from the father. Sperm cells undergo many more rounds of DNA replication than egg cells, creating more opportunities for the repeat to lengthen. Research suggests that the tendency toward large expansions is inherited through the male line but may originate when a male inherits the Huntington’s allele from his mother, pointing to an epigenetic process tied to how cells mark DNA based on which parent it came from.
The practical consequence: a father with a repeat count of 42 might not show symptoms until his 50s, but his child could inherit a repeat count of 50 or higher and develop symptoms decades earlier.
Somatic Expansion After Birth
The mutation you’re born with isn’t necessarily the mutation your brain cells carry decades later. Throughout life, the CAG repeat continues to expand in certain tissues, particularly in the brain. This process, called somatic instability, means that neurons in the striatum may accumulate far longer repeats than what a blood test would show.
Growing evidence suggests that this ongoing expansion within brain cells is a major driver of when symptoms actually begin and how quickly the disease progresses. The original repeat length sets the trajectory, but somatic expansion in vulnerable neurons may be what ultimately pushes them past a toxicity threshold. This understanding has shifted how researchers think about the disease and has opened new lines of investigation into whether slowing somatic expansion could delay onset.
Environmental Factors That Influence Onset
While the CAG repeat length is the primary determinant of whether and roughly when someone develops Huntington’s, it doesn’t explain all the variation. Two people with identical repeat counts can develop symptoms years apart. Non-genetic factors play a role in that gap.
Animal studies have shown that enriched environments with more sensory, cognitive, and physical stimulation can delay the appearance of symptoms in mice carrying the Huntington’s mutation. The proposed mechanisms include increased production of growth factors in the brain, enhanced ability of synapses to adapt, and greater generation of new neurons. Stress, on the other hand, accelerates cognitive and sensory deficits in the same mouse models. Clinical investigations in humans support these findings, suggesting that lifetime activity levels can modestly influence when symptoms emerge and how quickly they progress.
None of this changes the underlying genetic cause. A person with 45 CAG repeats will develop Huntington’s regardless of lifestyle. But these factors appear to influence the timeline, which for a disease with no cure, carries real significance.