The HTT Gene: Function, Mutation, and Huntington’s Disease

Everyone carries the HTT gene, and it does necessary work in the brain. But a small stutter in its code, repeated too many times, is the single cause of Huntington’s disease.1PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease That clarity has made HTT one of the first genes researchers are trying to silence with drugs in people.2PubMed. Targeting Huntingtin Expression in Patients with Huntington’s Disease

What the HTT Gene Normally Does

The HTT gene sits on chromosome 4, and you inherit one copy from each parent.3Wikipedia. Huntingtin It holds the instructions for a large protein called huntingtin, which is found in cells throughout the body.4PubMed Central. Skeletal muscle pathology in Huntington’s disease Near the start of the gene, the three DNA letters C-A-G repeat over and over. This stretch is called the CAG repeat, and each copy adds one building block to the protein.5Wikipedia. Huntingtin

Having a repeat is not a mistake in itself. Every person has a run of CAG letters in HTT, and this stretch has been preserved through evolution and seems to help nerve cells.6PubMed Central. The evolutionary history of the polyQ tract in huntingtin sheds light on its functional pro-neural activities Only the length matters for disease, so a test result reports a number rather than a yes or no.

A report usually lists two repeat numbers, one for each copy. Even in someone with Huntington’s, one copy is usually normal, and it keeps making working huntingtin.

Huntingtin is one of the larger proteins in the body, and it works more like a scaffold or meeting point than a single-task enzyme. It can touch many other proteins at once, so it takes part in several jobs inside the cell.

One of those jobs is supply delivery. Huntingtin helps move packets of BDNF, a growth signal nerve cells need, along the cell’s internal tracks.7PubMed. Huntingtin controls neurotrophic support and survival of neurons by enhancing BDNF vesicular transport along microtubules When cells have less normal huntingtin, these packets slow down and stall more often.8PubMed Central. Huntingtin as an Essential Integrator of Intracellular Vesicular Trafficking The normal protein also shields brain cells from triggers that would make them self-destruct.9PubMed Central. Wild-Type Huntingtin Protects from Apoptosis Upstream of Caspase-310PubMed. Region-specific pro-survival signaling and global neuronal protection by wild-type Huntingtin Losing some of this normal role may add to the harm the faulty version causes.11PubMed. Huntingtin controls neurotrophic support and survival of neurons by enhancing BDNF vesicular transport along microtubules

What Your CAG Repeat Number Means

A person with 40 or more repeats can feel entirely well for years, so “complete” describes the eventual outcome, not what is happening now. Neither category tells you when symptoms would start, only whether they eventually will.

The 36 to 39 range is the hardest to interpret, and it deserves a genetic counselor. Even if a person in this range never develops symptoms, their children can inherit a longer version, so family planning still matters.

The longer the repeat, the earlier symptoms tend to begin.15PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease But the number doesn’t set an exact age, because other genes and life factors also shift onset.16PubMed Central. Venezuelan kindreds reveal that genetic and environmental factors modulate Huntington’s disease age of onset Many of those other genes help repair DNA.17PubMed. Huntington disease: somatic expansion, pathobiology and therapeutics So two people with the same number can start years apart. If you have two different numbers, the higher one is the one that matters.

How Huntington’s Is Inherited

Huntington’s is dominant, so one expanded copy is enough to cause it.18PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease Each child of a carrier therefore has a 1 in 2 chance of inheriting it. The chance applies separately to every child, like a coin flip each time, so one child inheriting it says nothing about the next. A child who does not inherit the expanded copy generally cannot pass it to their own children.

The repeat is also unstable and can change length when passed to a child.19PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease From fathers it tends to grow longer, while from mothers it changes without a consistent increase.20PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease So children can develop symptoms younger than their parent, a pattern called anticipation.21PubMed Central. Anticipation and instability of IT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease Most childhood-onset cases inherit the gene from their father, about 80% in one large review.22PubMed. Clinical Presentation and Features of Juvenile-Onset Huntington’s Disease: A Systematic Review

Family history is not always clear. A parent may have died before symptoms showed, or their symptoms may have been labeled psychiatric illness or dementia. A parent in the 36 to 39 range may never have had symptoms at all, yet a child may carry a longer repeat than the parent.

Couples who worry about passing the gene on can see a genetic counselor before pregnancy. Testing during pregnancy is available, and IVF with embryo testing can select embryos without the expansion. Some of these options avoid revealing a parent’s own status.

How the Faulty Gene Damages the Brain

In brain tissue, the inherited repeat keeps getting longer during life.23PubMed Central. Somatic expansion of the Huntington’s disease CAG repeat in the brain is associated with an earlier age of disease onset People whose brain repeats grow more tend to get symptoms earlier.24PubMed Central. Somatic expansion of the Huntington’s disease CAG repeat in the brain is associated with an earlier age of disease onset In certain movement-control neurons, the repeat may grow to about 150, a point at which those cells appear to break down.25PubMed. Somatic DNA-repeat expansions initiate distinct yet analogous changes in Huntington’s disease cortical and striatal neurons26PubMed. Huntington disease: somatic expansion, pathobiology and therapeutics

The growth is not equal everywhere. It is most marked in the same brain regions that are hit hardest, such as the striatum. Blood cells, which a genetic test reads, show far less of this change, so the blood number understates what is happening in vulnerable neurons. Neurons don’t divide, so the lengthening happens as the cell’s own repair machinery works on the DNA again and again over the years.

That slow build-up explains why symptoms wait until midlife: the gene is present from birth, but damage accumulates over decades. It is also why slowing repeat growth is a treatment goal.

The faulty protein harms cells directly too. Faulty huntingtin is cut into fragments that clump and gather in the cell’s nucleus,27PubMed Central. Modulation of Mutant Huntingtin N-Terminal Cleavage and Its Effect on Aggregation and Cell Death and specific fragments drive much of the toxicity.28PubMed Central. Mutant huntingtin N-terminal fragments of specific size mediate aggregation and toxicity in neuronal cells It also damages mitochondria, the cell’s energy units, and stops worn-out ones being cleared.29PubMed Central. Mutant HTT (huntingtin) impairs mitophagy in a cellular model of Huntington disease Damaged mitochondria pile up and stress the cell.30PubMed Central. VCP recruitment to mitochondria causes mitophagy impairment and neurodegeneration in models of Huntington’s disease

The striatum, a deep area controlling movement, is hit hardest.31PubMed Central. Huntington’s Disease and the Striatal Medium Spiny Neuron: Cell-Autonomous and Non-Cell-Autonomous Mechanisms of Disease Its main nerve cells are affected first.32PubMed Central. Differential vulnerability of neurons in Huntington’s disease: The role of cell type-specific features Large neurons in the outer brain that connect to the striatum are also lost.33PubMed Central. Differential vulnerability of neurons in Huntington’s disease: The role of cell type-specific features Meanwhile the brain’s immune cells become inflamed, sometimes before symptoms.34PubMed Central. The Role of Microglia and Astrocytes in Huntington’s Disease

Symptoms of Huntington’s Disease

Huntington’s causes movement, thinking, and psychiatric symptoms.35PubMed Central. Huntington’s disease: a clinical review Changes in thinking and personality are very common and can come before the movements.36PubMed Central. Neuropsychiatry of Huntington’s disease So depression or irritability can be the first sign, and these symptoms are often the most disabling.37PubMed Central. Timing and Impact of Psychiatric, Cognitive, and Motor Abnormalities in Huntington Disease

Early changes families notice include irritability, low mood, or apathy. A person may have trouble planning, multitasking, or keeping track of things. Restlessness, small fidgety movements, clumsiness, or more falls can follow.

Chorea means involuntary, dance-like jerky movements. It’s the best-known sign but not the only one, and the person cannot stop the movements by will. Medicines can calm chorea but do not slow the disease.

Weight loss and muscle wasting are common.38PubMed Central. Molecular Mechanisms Underlying Muscle Wasting in Huntington’s Disease The heart and metabolism can be affected.39PubMed Central. Skeletal muscle pathology in Huntington’s disease Childhood onset usually involves stiffness, falls, behavior problems, and learning decline rather than jerky movements.40PubMed. Clinical Presentation and Features of Juvenile-Onset Huntington’s Disease: A Systematic Review Childhood cases usually involve very long repeats, with a median of about 64 in one review.41PubMed. Clinical Presentation and Features of Juvenile-Onset Huntington’s Disease: A Systematic Review

Symptoms usually arrive gradually and worsen over about a decade or two, and the order and mix differ from person to person. Early on a person may still work and manage daily life, while later stages need more help with walking, swallowing, and speech. Because the course is long, care needs change over time, and a team including a neurologist, therapists, and a social worker often shares the work.

Testing for the HTT Expansion

A person with symptoms is tested to confirm a diagnosis. Someone without symptoms may be tested to learn their future risk, which is called predictive testing. It follows a long-established protocol,42PubMed. 40+ years of the Huntington disease predictive genetic testing protocol: Recommendations for an update and it includes more counseling and waiting. Results from either route give the same repeat numbers.

The process typically includes an information call, psychological assessment, family history review, counseling, a results visit, and follow-up.43PubMed. Experience over fifteen years with a protocol for predictive testing for Huntington disease Blood is drawn only after the early talks, and then you wait. Follow-up matters because the days and weeks after the news can be hard.44PubMed. Experience over fifteen years with a protocol for predictive testing for Huntington disease Genome sequencing or consumer tests can now reveal Huntington’s status without that preparation.45PubMed. 40+ years of the Huntington disease predictive genetic testing protocol: Recommendations for an update

Testing is a personal choice, and many at-risk people choose not to. Reasons to test include family planning and career decisions. If you do, bring a support person to the results appointment, and ask about insurance protections in your country beforehand.

Treatments Targeting the HTT Gene

No approved treatment fixes the gene yet, but some drugs block the gene’s message so cells make less huntingtin.46PubMed. Oligonucleotide therapeutic approaches for Huntington disease An injected spinal drug lowered faulty huntingtin in spinal fluid by up to about 40%.47PubMed. Targeting Huntingtin Expression in Patients with Huntington’s Disease An oral drug lowered it by about 25% compared with placebo.48PubMed Central. Oral splicing modulator branaplam in Huntington’s disease: a phase 2 randomized controlled trial Lowering the protein in fluid proves the drug is hitting its target, but not that people feel or function better. Because the disease progresses slowly, a trial may need to run for years before a difference appears.

Lowering huntingtin is tricky because normal huntingtin helps neurons survive. Some drugs lower both normal and faulty versions, so researchers watch whether lowering the normal protein causes harm. Targeting only the faulty copy is a goal.

Other approaches are earlier. In pigs, replacing the long repeat with a normal length reduced damage and symptoms.49PubMed. Cas9-mediated replacement of expanded CAG repeats in a pig model of Huntington’s disease Researchers are also seeking ways to slow repeat growth in the brain.50PubMed Central. In vivo CRISPR–Cas9 genome editing in mice identifies genetic modifiers of somatic CAG repeat instability in Huntington’s disease

Measuring progress is getting easier too. A blood protein called neurofilament light can track brain damage and could monitor treatment,51PubMed. Evaluation of mutant huntingtin and neurofilament proteins as potential markers in Huntington’s disease which means trials can measure effects before symptoms appear.52PubMed. Evaluation of mutant huntingtin and neurofilament proteins as potential markers in Huntington’s disease

Trials need both people with symptoms and gene carriers without them. A neurologist or HD clinic can tell you about current trials. Some require spinal fluid samples, and trials carry risks: some drugs have been stopped.