Huntington's disease (HD), also known as Huntington's chorea, is an inherited disorder that results in death of brain cells. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unsteady gait often follow. As the disease advances, uncoordinated, jerky body movements become more apparent. Physical abilities gradually worsen until coordinated movement becomes difficult and the person is unable to talk. Mental abilities generally decline into dementia. The specific symptoms vary somewhat between people. Symptoms usually begin between 30 and 50 years of age, but can start at any age. The disease may develop earlier in life in each successive generation. About 8% of cases start before the age of 20 years and typically present with symptoms more similar to Parkinson's disease. People with HD often underestimate the degree of their problems. HD is typically inherited from a person's parents, with 10% of cases due to a new mutation. The disease is caused by an autosomal dominant mutation in either of an individual's two copies of a gene called Huntingtin. This means a child of an affected person typically has a 50% chance of inheriting the disease. The Huntingtin gene provides the genetic information for a protein that is also called "huntingtin". Expansion of CAG ( cytosine- adenine- guanine) triplet repeats in the gene coding for the Huntingtin protein results in an abnormal protein, which gradually damages cells in the brain, through mechanisms that are not fully understood. Diagnosis is by genetic testing, which can be carried out at any time, regardless of whether or not symptoms are present. 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. There is no cure for HD. Full-time care is required in the later stages of the disease. Treatments can relieve some symptoms and in some improve quality of life. The best evidence for treatment of the movement problems is with tetrabenazine. HD affects about 4 to 15 in 100,000 people of European descent. It is rare among Japanese, while the occurrence rate in Africa is unknown. The disease affects men and women equally. Complications such as pneumonia, heart disease, and physical injury from falls reduce life expectancy. Suicide is the cause of death in about 9% of cases. Death typically occurs fifteen to twenty years from when the disease was first detected. The first likely description of the disease was in 1841 by Charles Oscar Waters. The condition was described in further detail in 1872 by George Huntington, after whom it is named. The genetic basis was discovered in 1993 by an international collaborative effort led by the Hereditary Disease Foundation. Research and support organizations began forming in the late 1960s to increase public awareness, to provide support for individuals and their families, and to promote research. Current research directions include determining the exact mechanism of the disease, improving animal models to aid with research, testing of medications 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.
Signs and symptomsSymptoms of Huntington's disease most 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 behavioral 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 behavioral 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. Cognitive abilities are progressively impaired. Especially affected are executive functions, which include planning, cognitive flexibility, abstract thinking, rule acquisition, initiation of appropriate actions, and inhibition of inappropriate actions. 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. Often individuals have reduced awareness of chorea, cognitive and emotional impairments. 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 tolerance, weight loss, osteoporosis, and testicular atrophy.
GeneticsAll 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. If the repeat is present in a healthy gene, a dynamic mutation may increase the repeat count and result in a defective gene. 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 alleles 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 mutationHD 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 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. CAG is the 3-letter genetic code ( codon) 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 mutant huntingtin (mHTT), increases the decay rate of certain types of neurons. Regions of the brain have differing amounts and reliance on these types 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.
Inheritancefashion. The probability of each offspring inheriting an affected gene is 50%. Inheritance is independent of gender, and the phenotype does not skip generations.]] 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.
MechanismThe huntingtin protein interacts with over 100 other proteins, and appears to have multiple biological functions. The behavior of this mutated protein is not completely understood, but it is toxic to certain cell types, particularly in the brain. Early damage is most evident in the striatum, but as the disease progresses, other areas of the brain are also more conspicuously affected. Early symptoms are attributable to functions of the striatum and its cortical connections—namely control over movement, mood and higher cognitive function. DNA methylation also appears to be changed in HD.
Huntingtin functionHTT is expressed in all mammalian cells. The highest concentrations are found in the brain and testes, with moderate amounts in the liver, heart, and lungs. The function of HTT in humans is unclear. It interacts with proteins which are involved in transcription, cell signaling, and intracellular transporting. In animals genetically modified to exhibit HD, several functions of HTT have been found. In these animals, HTT is important for embryonic development, as its absence is related to embryonic death. Caspase, an enzyme which plays a role in catalyzing apoptosis, is thought to be activated by the mutated gene through damaging the ubiquitin-protease system. It also acts as an anti-apoptotic agent preventing programmed cell death and controls the production of brain-derived neurotrophic factor, a protein which protects neurons and regulates their creation during neurogenesis. HTT also facilitates vesicular transport and synaptic transmission and controls neuronal gene transcription. If the expression of HTT is increased and more HTT produced, brain cell survival is improved and the effects of mHTT are reduced, whereas when the expression of HTT is reduced, the resulting characteristics are more typical of the presence of mHTT. In humans the disruption of the normal gene does not cause the disease. It is thought that the disease is not caused by inadequate production of HTT, but by a gain of toxic function of mHTT.
Cellular changes]] There are multiple cellular changes through which the toxic function of mHTT may manifest and produce the HD pathology. During the biological process of posttranslational modification of mHTT, cleavage of the protein can leave behind shorter fragments constituted of parts of the polyglutamine expansion. The polar nature of glutamine causes interactions with other proteins when it is overabundant in unmodified HTT proteins or the HTT fragments created from HTT cleavage. Thus, the mHTT molecule strands will form hydrogen bonds with one another, forming a protein aggregate rather than folding into functional proteins. Over time, the aggregates accumulate, ultimately interfering with neuron function because these fragments can then misfold and coalesce, in a process called protein aggregation, to form inclusion bodies within cells. Neuronal inclusions run indirect interference. Inclusion bodies have been found in both the cell nucleus and cytoplasm. Inclusion bodies in cells of the brain are one of the earliest pathological changes, and some experiments have found that they can be toxic for the cell, but other experiments have shown that they may form as part of the body's defense mechanism and help protect cells. Several pathways by which mHTT may cause cell death have been identified. These include: effects on chaperone proteins, which help fold proteins and remove misfolded ones; interactions with caspases, which play a role in the process of removing cells; the toxic effects of glutamine on nerve cells; impairment of energy production within cells; and effects on the expression of genes. An additional theory that explains another way cell function may be disrupted by HD proposes that damage to mitochondria in striatal cells (numerous accounts of mitochondrial metabolism deficiency have been found) and the interactions of the altered huntingtin protein with numerous proteins in neurons leads to an increased vulnerability of glutamine, which, in large amounts, has been found to be an excitotoxin. Excitotoxins may cause damage to numerous cellular structures. Although glutamine is not found in excessively high amounts, it has been postulated that because of the increased vulnerability, even normal amounts glutamine can cause excitotoxins to be expressed.
Macroscopic changes(shown in purple)]] HD affects the whole brain, but certain areas are more vulnerable than others. The most prominent early effects are in a part of the basal ganglia called the neostriatum, which is composed of the caudate nucleus and putamen. Other areas affected include the substantia nigra, layers 3, 5 and 6 of the cerebral cortex, the hippocampus, purkinje cells in the cerebellum, lateral tuberal nuclei of the hypothalamus and parts of the thalamus. These areas are affected according to their structure and the types of neurons they contain, reducing in size as they lose cells. Striatal spiny neurons are the most vulnerable, particularly ones with projections towards the external globus pallidus, with interneurons and spiny cells projecting to the internal pallidum being less affected. HD also causes an abnormal increase in astrocytes and activation of the brain's immune cells, microglia. The basal ganglia—the part of the brain most prominently affected in early HD—play a key role in movement and behavior control. Their functions are not fully understood, but current theories propose that they are part of the cognitive executive system and the motor circuit. The basal ganglia ordinarily inhibit a large number of circuits that generate specific movements. To initiate a particular movement, the cerebral cortex sends a signal to the basal ganglia that causes the inhibition to be released. Damage to the basal ganglia can cause the release or reinstatement of the inhibitions to be erratic and uncontrolled, which results in an awkward start to motion or motions to be unintentionally initiated, or a motion to be halted before, or beyond, its intended completion. The accumulating damage to this area causes the characteristic erratic movements associated with HD. The spontaneous and erratic physical movements associated with HD are classified as a type of hyperkinetic dysarthria. Because of the basal ganglia's inability to inhibit movements, individuals affected by it will inevitably experience a reduced ability to produce speech and swallow foods and liquids (dysphagia).
Transcriptional dysregulationCREB-binding protein (CBP), a transcriptional coregulator, is essential for cell function because as a coactivator at a significant number of promoters, it activates the transcription of genes for survival pathways. Furthermore, the amino acids that form CBP include a strip of 18 glutamines. Thus, the glutamines on CBP interact directly with the increased numbers of glutamine on the HTT chain and CBP gets pulled away from its typical location next to the nucleus. Specifically, CBP contains an acetyltransferase domain to which HTT binds through its polyglutamine-containing domain. Autopsied brains of those who had Huntington's disease also have been found to have incredibly reduced amounts of CBP. In addition, when CBP is overexpressed, polyglutamine-induced death is diminished, further demonstrating that CBP plays an important role in Huntington's disease and neurons in general.
DiagnosisMedical 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.
Clinicalsection from an MR brain scan of a patient with HD, showing atrophy of the heads of the caudate nuclei, enlargement of the frontal horns of the lateral ventricles (hydrocephalus ex vacuo), and generalized cortical atrophy]] 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 behavioral symptoms are rarely the first symptoms 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 to the right, but these changes are not, by themselves, diagnostic of HD. Cerebral atrophy can be seen in the advanced stages of the disease. Functional neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), can show changes in brain activity before the onset of physical symptoms, but they are experimental tools, and are not used clinically.
Predictive genetic testingBecause 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 fewer repeats: Not associated with HD.