Huntington’s Disease

Definition of Huntington’s Disease

Huntington’s disease (HD) is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and psychiatric problems. It typically becomes noticeable in mid-adult life. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea, which is why the disease used to be called Huntington’s chorea.

It is much more common in people of Western European descent than in those of Asian or African ancestry. The disease is caused by an autosomal dominant mutation in either of an individual’s two copies of a gene called Huntingtin, which means any child of an affected person typically has a 50% chance of inheriting the disease. Physical symptoms of Huntington’s disease can begin at any age from infancy to old age, but usually begin between 35 and 44 years of age. Through genetic anticipation, the disease may develop earlier in life in each successive generation. About 6% of cases start before the age of 21 years with an akinetic-rigid syndrome; they progress faster and vary slightly. The variant is classified as juvenile, akinetic-rigid or Westphal variant HD.

The Huntingtin gene provides the genetic information for a protein that is also called “huntingtin”. Expansion of a CAG triplet repeat stretch within the Huntingtin gene results in a different (mutant) form of the protein, which gradually damages cells in the brain, through mechanisms that are not fully understood. The genetic basis of HD was discovered in 1993 by an international collaborative effort spearheaded by the Hereditary Disease Foundation.

Genetic testing can be performed at any stage of development, even before the onset of symptoms. This fact raises several ethical debates: the age at which an individual is considered mature enough to choose testing; whether parents have the right to have their children tested; and managing confidentiality and disclosure of test results. Genetic counseling has developed to inform and aid individuals considering genetic testing and has become a model for other genetically dominant diseases.

Symptoms of the disease can vary between individuals and even among affected members of the same family, but usually progress predictably. The earliest symptoms are often subtle problems with mood or cognition. A general lack of coordination and an unsteady gait often follows. As the disease advances, uncoordinated, jerky body movements become more apparent, along with a decline in mental abilities and behavioral and psychiatric problems. Physical abilities are gradually impeded until coordinated movement becomes very difficult. Mental abilities generally decline into dementia. Complications such as pneumonia, heart disease, and physical injury from falls reduce life expectancy to around twenty years after symptoms begin. There is no cure for HD, and full-time care is required in the later stages of the disease. Existing pharmaceutical and non-drug treatments can relieve many of its symptoms.

Research and support organizations, first founded in the 1960s and increasing in number, work to increase public awareness, to provide support for individuals and their families, and to promote and facilitate research. Many new research discoveries have been made and understanding of the disease is improving. Current research directions include determining the exact mechanism of the disease, improving animal models to expedite research, clinical trials of pharmaceuticals to treat symptoms or slow the progression of the disease, and studying procedures such as stem cell therapy with the goal of repairing damage caused by the disease.

Cause of Huntington’s Disease

Genetics

All humans have two copies of the Huntingtin gene (HTT), which codes for the protein Huntingtin (Htt). The gene is also called HD and IT15, which stands for ‘interesting transcript 15′. Part of this gene is a repeated section called a trinucleotide repeat, which varies in length between individuals and may change length between generations. When the length of this repeated section reaches a certain threshold, it produces an altered form of the protein, called mutant Huntingtin protein (mHtt). The differing functions of these proteins are the cause of pathological changes which in turn cause the disease symptoms. The Huntington’s disease mutation is genetically dominant and almost fully penetrant: mutation of either of a person’s HTT genes causes the disease. It is not inherited according to sex, but the length of the repeated section of the gene, and hence its severity can be influenced by the sex of the affected parent.

Genetic Mutation

HD is one of several trinucleotide repeat disorders which are caused by the length of a repeated section of a gene exceeding a normal range. The HTT gene is located on the short arm of chromosome 4[13] at 4p16.3. HTT contains a sequence of three DNA bases—cytosine-adenine-guanine (CAG)—repeated multiple times (i.e. … CAGCAGCAG …), known as a trinucleotide repeat.[13] CAG is the genetic code for the amino acid glutamine, so a series of them results in the production of a chain of glutamine known as a polyglutamine tract (or polyQ tract), and the repeated part of the gene, the PolyQ region.

Generally, people have fewer than 36 repeated glutamines in the polyQ region which results in production of the cytoplasmic protein Huntingtin. However, a sequence of 36 or more glutamines results in the production of a protein which has different characteristics. This altered form, called mHtt (mutant Htt), increases the decay rate of certain types of neurons. Regions of the brain have differing amounts and reliance on these type of neurons, and are affected accordingly. Generally, the number of CAG repeats is related to how much this process is affected, and accounts for about 60% of the variation of the age of the onset of symptoms. The remaining variation is attributed to environment and other genes that modify the mechanism of HD. 36–39 repeats result in a reduced-penetrance form of the disease, with a much later onset and slower progression of symptoms. In some cases the onset may be so late that symptoms are never noticed. With very large repeat counts, HD has full penetrance and can occur under the age of 20, when it is then referred to as juvenile HD, akinetic-rigid, or Westphal variant HD. This accounts for about 7% of HD carriers.

Inheritance

Huntington’s disease has autosomal dominant inheritance, meaning that an affected individual typically inherits one copy of the gene with an expanded trinucleotide repeat (the mutant allele) from an affected parent. Since penetrance of the mutation is very high, those who have a mutated copy of the gene will have the disease. In this type of inheritance pattern, each offspring of an affected individual has a 50% risk of inheriting the mutant allele and therefore being affected with the disorder (see figure). This probability is sex-independent.

Trinucleotide CAG repeats over 28 are unstable during replication and this instability increases with the number of repeats present. This usually leads to new expansions as generations pass (dynamic mutations) instead of reproducing an exact copy of the trinucleotide repeat. This causes the number of repeats to change in successive generations, such that an unaffected parent with an “intermediate” number of repeats (28–35), or “reduced penetrance” (36–40), may pass on a copy of the gene with an increase in the number of repeats that produces fully penetrant HD. Such increases in the number of repeats (and hence earlier age of onset and severity of disease) in successive generations is known as genetic anticipation. Instability is greater in spermatogenesis than oogenesis; maternally inherited alleles are usually of a similar repeat length, whereas paternally inherited ones have a higher chance of increasing in length. It is rare for Huntington’s disease to be caused by a new mutation, where neither parent has over 36 CAG repeats.

In the rare situations where both parents have an expanded HD gene, the risk increases to 75%, and when either parent has two expanded copies, the risk is 100% (all children will be affected). Individuals with both genes affected are rare. For some time HD was thought to be the only disease for which possession of a second mutated gene did not affect symptoms and progression, but it has since been found that it can affect the phenotype and the rate of progression.

Signs and Symptoms of Huntington’s Disease

Symptoms of Huntington’s disease commonly become noticeable between the ages of 35 and 44 years, but they can begin at any age from infancy to old age. In the early stages, there are subtle changes in personality, cognition, and physical skills. The physical symptoms are usually the first to be noticed, as cognitive and psychiatric symptoms are generally not severe enough to be recognized on their own at the earlier stages. Almost everyone with Huntington’s disease eventually exhibits similar physical symptoms, but the onset, progression and extent of cognitive and psychiatric symptoms vary significantly between individuals.

The most characteristic initial physical symptoms are jerky, random, and uncontrollable movements called chorea. Chorea may be initially exhibited as general restlessness, small unintentionally initiated or uncompleted motions, lack of coordination, or slowed saccadic eye movements. These minor motor abnormalities usually precede more obvious signs of motor dysfunction by at least three years. The clear appearance of symptoms such as rigidity, writhing motions or abnormal posturing appear as the disorder progresses. These are signs that the system in the brain that is responsible for movement has been affected.Psychomotor functions become increasingly impaired, such that any action that requires muscle control is affected. Common consequences are physical instability, abnormal facial expression, and difficulties chewing, swallowing and speaking. Eating difficulties commonly cause weight loss and may lead to malnutrition. Sleep disturbances are also associated symptoms. Juvenile HD differs from these symptoms in that it generally progresses faster and chorea is exhibited briefly, if at all, with rigidity being the dominant symptom. Seizures are also a common symptom of this form of HD.

Reported prevalences of behavioral and psychiatric symptoms in Huntington’s disease
Irritability 38–73%
Apathy 34–76%
Anxiety 34–61%
Depressed mood 33–69%
Obsessive and compulsive 10–52%
Psychotic 3–11%

Cognitive abilities are impaired progressively. Especially affected are executive functions which include planning, cognitive flexibility, abstract thinking, rule acquisition, initiating appropriate actions and inhibiting inappropriate actions.[6] As the disease progresses, memory deficits tend to appear. Reported impairments range from short-term memory deficits to long-term memory difficulties, including deficits in episodic (memory of one’s life), procedural (memory of the body of how to perform an activity) and working memory. Cognitive problems tend to worsen over time, ultimately leading to dementia. This pattern of deficits has been called a subcortical dementia syndrome to distinguish it from the typical effects of cortical dementias e.g. Alzheimer’s disease.

Reported neuropsychiatric manifestations are anxiety, depression, a reduced display of emotions (blunted affect), egocentrism, aggression, and compulsive behavior, the latter of which can cause or worsen addictions, including alcoholism, gambling, and hypersexuality. Difficulties in recognizing other people’s negative expressions have also been observed. The prevalence of these symptoms is highly variable between studies, with estimated rates for lifetime prevalence of psychiatric disorders between 33% and 76%. For many sufferers and their families, these symptoms are among the most distressing aspects of the disease, often affecting daily functioning and constituting reason for institutionalization. Suicidal thoughts and suicide attempts are more common than in the general population.
Mutant Huntingtin is expressed throughout the body and associated with abnormalities in peripheral tissues that are directly caused by such expression outside the brain. These abnormalities include muscle atrophy, cardiac failure, impaired glucose

Risk Factors for Huntington’s Disease

Huntington’s disease is inherited in an autosomal dominant fashion. The probability of each offspring inheriting an affected gene is 50%. Inheritance is independent of gender, and the phenotype does not skip generations. In rare cases, you may develop Huntington’s disease without having a family history of the condition. Such an occurrence may be the result of a genetic mutation that happened during your father’s sperm development.

Diagnosis of Huntington’s Disease

Medical diagnosis of the onset of HD can be made following the appearance of physical symptoms specific to the disease. Genetic testing can be used to confirm a physical diagnosis if there is no family history of HD. Even before the onset of symptoms, genetic testing can confirm if an individual or embryo carries an expanded copy of the trinucleotide repeat in the HTT gene that causes the disease. Genetic counseling is available to provide advice and guidance throughout the testing procedure, and on the implications of a confirmed diagnosis. These implications include the impact on an individual’s psychology, career, family planning decisions, relatives and relationships. Despite the availability of pre-symptomatic testing, only 5% of those at risk of inheriting HD choose to do so.

Clinical

A physical examination, sometimes combined with a psychological examination, can determine whether the onset of the disease has begun. Excessive unintentional movements of any part of the body are often the reason for seeking medical consultation. If these are abrupt and have random timing and distribution, they suggest a diagnosis of HD. Cognitive or psychiatric symptoms are rarely the first diagnosed; they are usually only recognized in hindsight or when they develop further. How far the disease has progressed can be measured using the unified Huntington’s disease rating scale which provides an overall rating system based on motor, behavioral, cognitive, and functional assessments. Medical imaging, such as computerized tomography (CT) and magnetic resonance imaging (MRI), can show atrophy of the caudate nuclei early in the disease, as seen in the illustration, but these changes are not diagnostic of HD. Cerebral atrophy can be seen in the advanced stages of the disease. Functional neuroimaging techniques such as fMRI and PET can show changes in brain activity before the onset of physical symptoms but are experimental tools, and not used clinically.

Predictive genetic testing

Because HD follows an autosomal dominant pattern of inheritance, there is a strong motivation for individuals who are at risk of inheriting it to seek a diagnosis. The genetic test for HD consists of a blood test which counts the numbers of CAG repeats in each of the HTT alleles. Cutoffs are given as follows:

40 or more CAG repeats: full penetrance allele (FPA). A “positive test” or “positive result” generally refers to this case. A positive result is not considered a diagnosis, since it may be obtained decades before the symptoms begin. However, a negative test means that the individual does not carry the expanded copy of the gene and will not develop HD. The test will tell a person who originally had a 50 percent chance of inheriting the disease if their risk goes up to 100 percent or is eliminated. A person who tests positive for the disease will develop HD sometime within their lifetime, provided he or she lives long enough for the disease to appear.
36 to 39 repeats: incomplete or reduced penetrance allele (RPA). It may cause symptoms, usually later in the adult life. There is a maximum risk of 60% that a person with an RPA will be symptomatic at the age of 65 years, and a 70% risk of being symptomatic at the age of 75 years.
27 to 35 repeats: intermediate allele (IA), or large normal allele. It is not associated with symptomatic disease in the tested individual, but may expand upon further inheritance to give symptoms in offspring.
26 or less repeats: Not associated with HD.

A pre-symptomatic test is a life-changing event and a very personal decision. The main reason given for choosing testing for HD is to aid in career and family decisions. Before 1993 there was not an available test for individuals to learn if they carried the Huntington’s gene. At that time surveys indicated that 50–70% of at-risk individuals would have been interested in receiving testing, but since predictive testing has been offered far fewer choose to be tested. Over 95% of individuals at risk of inheriting HD do not proceed with testing, mostly because there is no treatment. A key issue is the anxiety an individual experiences about not knowing whether they will eventually develop HD, compared to the impact of a positive result. Irrespective of the result, stress levels have been found to be lower two years after being tested, but the risk of suicide is increased after a positive test result. Individuals found to have not inherited the disorder may experience survivor guilt with regard to family members who are affected. Other factors taken into account when considering testing include the possibility of discrimination and the implications of a positive result, which usually means a parent has an affected gene and that the individual’s siblings will be at risk of inheriting it. In one study genetic discrimination was found in 46% of individuals at risk for Huntington’s disease. It occurred at higher rates within personal relationships than health insurance or employment relations. Genetic counseling in HD can provide information, advice and support for initial decision-making, and then, if chosen, throughout all stages of the testing process. Because of the implications of this test, patients who wish to undergo testing must complete three counseling sessions which provide information about Huntington’s.

Counseling and guidelines on the use of genetic testing for HD have become models for other genetic disorders, such as autosomal dominant cerebellar ataxias. Presymptomatic testing for HD has also influenced testing for other illnesses with genetic variants such as polycystic kidney disease, familial Alzheimer’s disease and breast cancer. The European Molecular Genetics Quality Network have published yearly external quality assessment scheme for molecular genetic testing for this disease and have developed best practice guidelines for genetic testing for HD to assist in testing and reporting of results.

Preimplantation genetic diagnosis

Embryos produced using in vitro fertilization may be genetically tested for HD using preimplantation genetic diagnosis (PGD). This technique, where one or two cells are extracted from a typically 4 to 8 cell embryo and then tested for the genetic abnormality, can then be used to ensure embryos affected with HD genes are not implanted, and therefore any offspring will not inherit the disease. Some forms of preimplantation genetic diagnosis—non-disclosure or exclusion testing—allow at-risk people to have HD-free offspring without revealing their own parental genotype, giving no information about whether they themselves are destined to develop HD. In exclusion testing, the embryos’ DNA is compared with that of the parents and grandparents to avoid inheritance of the chromosomal region containing the HD gene from the affected grandparent. In non-disclosure testing, only disease-free embryos are replaced in the uterus while the parental genotype and hence parental risk for HD are never disclosed.

Prenatal testing

It is also possible to obtain a prenatal diagnosis for an embryo or fetus in the womb, using fetal genetic material acquired through chorionic villus sampling. An amniocentesis can be performed if the pregnancy is further along, within 14–18 weeks. This procedure looks at the amniotic fluid surrounding the baby for indicators of the HD mutation. This, too, can be paired with exclusion testing to avoid disclosure of parental genotype. Prenatal testing can be done when a parent has been diagnosed with HD, when they have had genetic testing showing the expansion of the HTT gene, or when they have a 50% chance of inheriting the disease. The parents can be counseled on their options, which include termination of pregnancy, and on the difficulties of a child with the identified gene.
In addition, in at-risk pregnancies due to an affected male partner, non-invasive prenatal diagnosis can be performed by analyzing cell-free fetal DNA taken by venipuncture on the mother between 6 and 12 weeks of pregnancy. It has no procedure-related risk of miscarriage.

Differential diagnosis

About 99% of HD diagnoses based on the typical symptoms and a family history of the disease are confirmed by genetic testing to have the expanded trinucleotide repeat that causes HD. Most of the remaining are called HD-like disorders. Most of these other disorders are collectively labelled HD-like (HDL). The cause of most HDL diseases is unknown, but those with known causes are due to mutations in the prion protein gene (HDL1), the junctophilin 3 gene (HDL2), a recessively inherited HTT gene (HDL3—only found in one family and poorly understood), and the gene encoding the TATA box-binding protein (HDL4/SCA17). Other autosomal dominant diseases that can be misdiagnosed as HD are dentatorubral-pallidoluysian atrophy and neuroferritinopathy. There are also autosomal recessive disorders that resemble sporadic cases of HD. Main examples are chorea acanthocytosis, pantothenate kinase-associated neurodegeneration and X-linked McLeod syndrome.

Prevention of Huntington’s Disease

There is no way to prevent the onset of HD if a person has inherited the gene for the disorder. A prospective parent with HD or a family history of HD can seek genetic counseling when deciding whether or not to have children.

A genetic counselor will discuss the potential risks of a positive test result, which would indicate the parent will develop the disease. Also, couples will need to make additional choices about whether to have children or to consider alternatives, such as prenatal testing for the gene or in vitro fertilization with donor sperm or eggs. Another option for couples is in vitro fertilization and preimplantation genetic diagnosis.

Treatment of Huntington’s Disease

There is no cure for HD, but there are treatments available to reduce the severity of some of its symptoms. For many of these treatments, comprehensive clinical trials to confirm their effectiveness in treating symptoms of HD specifically are incomplete. As the disease progresses the ability to care for oneself declines and carefully managed multidisciplinary caregiving becomes increasingly necessary. Although there have been relatively few studies of exercises and therapies that help rehabilitate cognitive symptoms of HD, there is some evidence for the usefulness of physical therapy, occupational therapy, and speech therapy.
Tetrabenazine was approved in 2008 for treatment of chorea in Huntington’s disease in the US. Other drugs that help to reduce chorea include neuroleptics and benzodiazepines. Compounds such as amantadine or remacemide are still under investigation but have shown preliminary positive results. Hypokinesia and rigidity, especially in juvenile cases, can be treated with antiparkinsonian drugs, and myoclonic hyperkinesia can be treated with valproic acid.

Psychiatric symptoms can be treated with medications similar to those used in the general population. Selective serotonin reuptake inhibitors and mirtazapine have been recommended for depression, while atypical antipsychotic drugs are recommended for psychosis and behavioral problems. Specialist neuropsychiatric input is recommended as patients may require long-term treatment with multiple medications in combination.

Weight loss and eating difficulties due to dysphagia and other muscle discoordination are common, making nutrition management increasingly important as the disease advances. Thickening agents can be added to liquids as thicker fluids are easier and safer to swallow. Reminding the patient to eat slowly and to take smaller pieces of food into the mouth may also be of use to prevent choking. If eating becomes too hazardous or uncomfortable, the option of using a percutaneous endoscopic gastrostomy is available. This is a feeding tube, permanently attached through the abdomen into the stomach, which reduces the risk of aspirating food and provides better nutritional management. Assessment and management by speech and language therapists with experience in Huntington’s disease is recommended.

Patients with Huntington’s disease may see a physical therapist for non-invasive and non-medication-based ways of managing the physical symptoms. Physical therapists may implement fall risk assessment and prevention, as well as strengthening, stretching, and cardiovascular exercises. Walking aids may be prescribed as appropriate. Physical therapists also prescribe breathing exercises and airway clearance techniques with the development of respiratory problems. Consensus guidelines on physiotherapy in Huntington’s disease have been produced by the European HD Network. Goals of early rehabilitation interventions are prevention of loss of function. Participation in rehabilitation programs during early to middle stage of the disease may be beneficial as it translates into long term maintenance of motor and functional performance. Rehabilitation during the late stage aims to compensate for motor and functional losses. For long-term independent management, the therapist may develop home exercise programs for appropriate patients.

The families of individuals who have inherited or are at risk of inheriting HD, have generations of experience of HD which may be outdated and lack knowledge of recent breakthroughs and improvements in genetic testing, family planning choices, care management, and other considerations. Genetic counseling benefits these individuals by updating their knowledge, dispelling any myths they may have and helping them consider their future options and plans.