HD is caused by a mutation of a single huntingtin (HTT) gene, found on chromosome four.¹ Everyone has the HTT gene, which contains a sequence of DNA building blocks known as a CAG repeat.^1,2 In people with HD, the CAG repeat series exceeds the normal range, triggering the formation of mutant huntingtin protein (mHTT) that causes the death of nerve cells in the brain.^2,3

HD causes problems with a person’s ability to think, behave and move.⁴ While every individual with HD is different, research has shown that the disease manifests in three different types of symptoms – motor (including jerky, involuntary movements known as chorea), cognitive (such as difficulty multi-tasking and managing time) and behavioral (such as mood swings and depression).^1,4

HD is frequently misunderstood as a motor disease because difficulty with movement is more obvious.^1,5 Yet research has shown that cognitive or behavioral issues such as poor planning and organization, impulsivity and memory loss can be early symptoms of HD.^1,5

HD is clinically diagnosed in the manifest stage of HD, which is defined by the onset of motor signs, such as chorea, become obvious.⁶ The manifest stage follows the prodromal stage, which is characterized by subtle motor, cognitive and behavioral changes such as depression, inability to identify emotion and impulsiveness.⁷ The prodromal stage can occur years before a clinical diagnosis in the manifest stage.⁷

While mHTT is thought to be present in the whole brain, the striatum and cortex appear to be particularly affected in patients with HD.⁸ These areas of the brain are important for controlling movement, thinking and personality, therefore leading to the classic symptoms associated with the disease.⁹

Individuals with 40 or more CAG repeats have HD onset during adult life.^2,3 Individuals with 27-35 CAG repeats do not develop HD, but have a potential risk of having HD appear in subsequent generations.^2,3 Individuals with 36-39 repeats may or may not develop HD.^2,3

CAG repeat lengths determine whether a person may develop HD.^1,10 The more CAG repeat lengths a person has in the gene for HD, the more likely earlier motor symptoms will start.^1,10 The average age of motor symptom onset is between 30 and 50 years old, when people are in their prime of life.¹

There are approximately 40,000 people living with manifest HD in the United States, with even more at risk for inheriting the disease.^{1, 15-16}

There are two types of tests for HD – predictive testing and confirmatory testing, the latter of which confirms an HD diagnosis in someone who shows HD symptoms.¹¹ Predictive testing is a complex and personal decision.¹² In fact, only 10 percent of the at-risk population in the U.S. decide to pursue predictive testing, often due to factors such as family, financial and long-term care planning.¹² Research has shown that individuals choose not to be tested for reasons such as there not being an effective cure or treatment, not being able to “undo” the knowledge, and that they are more comfortable being at risk.¹³

While there are no available medicines to stop or slow the progression of HD, current treatment strategies can provide symptom relief and maximize function.^1,14 Treatment goals and plans may continue to evolve, but currently include psychotherapy, speech therapy, physical therapy and occupational therapy.^1,14