Huntington's disease

Huntington's disease
Classification and external resources
A microscope image of Medium spiny neurons (yellow) with nuclear inclusions (orange), which occur as part of the disease process, image width 360 µm
ICD-10	G10, F02.2
ICD-9	333.4, 294.1
OMIM	143100
DiseasesDB	6060
MedlinePlus	000770
eMedicine	article/1150165 article/792600 article/289706
MeSH	D006816

Huntington's disease, chorea, or disorder (HD), is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called 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 on either of an individual's two copies of a gene called Huntingtin, which means any child of an affected parent has a 50% risk of inheriting the disease. In the rare situations where both parents have an affected copy, the risk increases to 75%, and when either parent has two affected copies, the risk is 100% (all children will be affected). 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. 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 normally provides the genetic information for a protein that is also called "Huntingtin". The mutation of the Huntingtin gene codes for a different form of the protein, whose presence results in gradual damage to specific areas of the brain. The exact way this happens is not fully understood. Genetic testing can be performed at any stage of development, even before the onset of symptoms. This fact raises several ethical debates: at what age is an individual considered mature enough to choose testing, do 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 among affected members of the same family, but the symptoms progress predictably for most individuals. The earliest symptoms are a general lack of coordination and an unsteady gait. 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. Emerging treatments can relieve some of its symptoms.

Self-help support organizations, first founded in the 1960s and increasing in number, have been working to increase public awareness, to provide support for individuals and their families, and to promote research. The Hereditary Disease Foundation, a research group born out of the first support organization, was instrumental in finding the gene in 1993. Since that time, every few years 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.

Signs and symptoms

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.^[1][2] In the early stages, there are subtle changes in personality, cognition, and physical skills.^[1] 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.^[1] 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.^[3][4]

The most characteristic initial physical symptoms are jerky, random, and uncontrollable movements called chorea.^[1] Chorea may be initially exhibited as general restlessness, small unintentionally initiated or uncompleted motions, lack of coordination, or slowed saccadic eye movements.^[1] These minor motor abnormalities usually precede more obvious signs of motor dysfunction by at least three years.^[3] The clear appearance of symptoms such as rigidity, writhing motions or abnormal posturing appear as the disorder progresses.^[5] These are signs that the system in the brain that is responsible for movement has been affected.^[6] 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.^[5] Eating difficulties commonly cause weight loss and may lead to malnutrition.^[7][8] Sleep disturbances are also associated symptoms.^[9] 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.^[5]

Reported prevalences of behavioral and psychiatric symptoms in Huntington's disease^[10]

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.^[6] 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.^[6] Cognitive problems tend to worsen over time, ultimately leading to dementia.^[6] 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.^[6]

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.^[10] Difficulties in recognizing other people's negative expressions have also been observed.^[6] The prevalence of these symptoms is highly variable between studies, with estimated rates for lifetime prevalence of psychiatric disorders between 33% and 76%.^[10] 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.^[10] Suicidal thoughts and suicide attempts are more common than in the general population.^[1]

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.^[11]

Genetics

All humans have the Huntingtin gene (HTT), which codes for the protein Huntingtin (Htt). 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.^[12]

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.^[13] 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.^[14]

Classification of the trinucleotide repeat, and resulting disease status, depends on the number of CAG repeats^[13]

Repeat count	Classification	Disease status
<28	Normal	Unaffected
28–35	Intermediate	Unaffected
36–40	Reduced Penetrance	+/- Affected
>40	Full Penetrance	Affected

Generally, people have fewer than 36 repeated glutamines in the polyQ region which results in production of the cytoplasmic protein Huntingtin.^[13] However, a sequence of 36 or more glutamines results in the production of a protein which has different characteristics.^[13] 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.^[5] 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.^[13] 36–40 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.^[15] 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.^[16]

Inheritance

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.

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.^[1] 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.^[17]

Trinucleotide CAG repeats over 28 are unstable during replication and this instability increases with the number of repeats present.^[15] This usually leads to new expansions as generations pass (dynamic mutations) instead of reproducing an exact copy of the trinucleotide repeat.^[13] 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.^[13] 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.^[13] Instability is greater in spermatogenesis than oogenesis;^[13] maternally inherited alleles are usually of a similar repeat length, whereas paternally inherited ones have a higher chance of increasing in length.^[13][18] It is rare for Huntington's disease to be caused by a new mutation, where neither parent has over 36 CAG repeats.^[19]

Individuals with both genes affected are rare, except in large consanguineous families.^[20] 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,^[21] but it has since been found that it can affect the phenotype and the rate of progression.^[13][20] Offspring of an individual who has two affected genes will inherit one of them and therefore definitely inherit the disease. Offspring where both parents have one affected gene have a 75% risk of inheriting HD, including a 25% risk of inheriting two affected genes.^[17] Identical twins, who have inherited the same affected gene, typically have differing ages of onset and symptoms.^[20]

Mechanism

The Htt protein interacts with over 100 other proteins, and appears to have multiple biological functions.^[22] The behavior of mutated mHtt protein is not completely understood, but it is toxic to certain types of cells, particularly in the brain. Damage mainly occurs in the striatum, but as the disease progresses, other areas of the brain are also significantly affected. As the damage accumulates, symptoms associated with the functions of these brain areas appear. Planning and modulating movement are the main functions of the striatum, and difficulties with these are initial symptoms.^[12]

Htt function

See also: Huntingtin

Htt 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.^[12] The function of Htt in humans is unclear. It interacts with proteins which are involved in transcription, cell signaling and intracellular transporting.^[12][23] In animals genetically modified to exhibit HD, several functions of Htt have been found.^[24] In these animals, Htt is important for embryonic development, as its absence is related to embryonic death. 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.^[24] 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.^[24] In humans the disruption of the normal gene does not cause the disease.^[12] It is currently concluded that the disease is not caused by inadequate production of Htt, but by a gain of toxic function of mHtt.^[12]

Cellular changes due to mHtt

A microscope image of a neuron with inclusion (stained orange) caused by HD, image width 250 µm

There are multiple cellular changes through which the toxic function of mHtt may manifest and produce the HD pathology.^[25][26] 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.^[25] The polar nature of glutamine causes interactions with other proteins when it is overabundant in Htt proteins. Thus, the Htt molecule strands will form hydrogen bonds with one another, forming a protein aggregate rather than folding into functional proteins.^[27] 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.^[25][27] Neuronal inclusions run indirect interference. The excess protein aggregates clump together at axons and dendrites in neurons which mechanically stops the transmission of neurotransmitters because vesicles (filled with neurotransmitters) can no longer move through the cytoskeleton. Ultimately, over time, less and less neurotransmitters are available for release in signaling other neurons as the neuronal inclusions grow.^[27] Inclusion bodies have been found in both the cell nucleus and cytoplasm.^[25] 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.^[25]

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. The cytotoxic effects of mHtt are strongly enhanced by interactions with a protein called Rhes, which is expressed mainly in the striatum.^[28] Rhes was found to induce sumoylation of mHtt, which causes the protein clumps to disaggregate—studies in cell culture showed that the clumps were much less toxic than the disaggregated form.^[28]

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. Furthermore, the increase in sensitivity turn on capases are activated by the repeat expansion of polyglutamine and the increase in sensitivity. The huntingtin protein is cleaved into tiny pieces by capases; these nuclear aggregates disrupt transcription by interfering with the production of proteins by "slipping" into the nucleus of the neuron.^[29][30] Unfortunately, the cellular stress caused by the interference causes more huntingtin to be broken up until apoptosis occurs.^[29]

Macroscopic changes due to mHtt

Area of the brain damaged by Huntington's disease – striatum (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.^[12] 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.^[13] These areas are affected according to their structure and the types of neurons they contain, reducing in size as they lose cells.^[13] 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.^[13][31] HD also causes an abnormal increase in astrocytes.^[32]

The basal ganglia—the part of the brain most prominently affected by 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^[6] and the motor circuit.^[33] 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.^[33]

Transcriptional dysregulation

CREB-binding protein (CBP), a transcription factor, is essential for cell function because as a coactivator at a significant number of promoters, it activates the transcription of genes for survival pathways.^[30] Furthermore, the amino acids that form CBP include a strip 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.^[34] Specifically, CRB contains a acetyltransferase domain that, in an experiment performed by Steffan and colleagues, showed that a Htt exon 1 with 51 glutamines binded to this domain in CBP.^[30] Autopsied brains of those who had Huntington's disease also have been found to have incredibly reduced amounts of CBP.^[34] Plus, when CBP is overexpressed, polyglutamine-induced death diminished, further demonstrating that CBP plays an important role in Huntington's disease and neurons in general.^[30]

Diagnosis

Medical diagnosis of the onset of HD can be made following the appearance of physical symptoms specific to the disease.^[1] 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.^[12]

Clinical

Coronal section from a 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.^[35]

A physical examination, sometimes combined with a psychological examination, can determine whether the onset of the disease has begun.^[1] 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.^[36][37] Medical imaging, such as computerized tomography (CT) and magnetic resonance imaging (MRI), only shows visible cerebral atrophy 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.^[13]

Genetic

See also: 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.^[38] 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.^[13]

A pre-symptomatic test is a life-changing event and a very personal decision.^[13] The main reason given for choosing testing for HD is to aid in career and family decisions.^[13] Over 95% of individuals at risk of inheriting HD do not proceed with testing, mostly because there is no treatment.^[13] 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.^[12] 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.^[12] Individuals found to have not inherited the disorder may experience survivor guilt with regard to family members who are affected.^[12] 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.^[12] 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.^[39] Counseling and guidelines on the use of genetic testing for HD have become models for other genetic disorders, such as autosomal dominant cerebellar ataxias.^[12][40][41] 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.^[40]

Embryonic

Embryos produced using in vitro fertilization may be genetically tested for HD using preimplantation genetic diagnosis. This technique, where a single cell is extracted from a 4 to 8 cell embryo and then tested for the genetic abnormality, can then be used to ensure embryos with affected HTT genes are not implanted, and therefore any offspring will not inherit the disease. It is also possible to obtain a prenatal diagnosis for an embryo or fetus in the womb.^[42]

Differential diagnosis

About 90% 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.^[5][43] Most of these other disorders are collectively labelled HD-like (HDL).^[43] 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).^[43] Other autosomal dominant diseases that can be misdiagnosed as HD are dentatorubral-pallidoluysian atrophy and neuroferritinopathy.^[43] 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.^[43]

Management

Chemical structure of tetrabenazine, an approved compound for the management of chorea in HD

There is no cure for HD, but there are treatments available to reduce the severity of some of its symptoms.^[44] For many of these treatments, comprehensive clinical trials to confirm their effectiveness in treating symptoms of HD specifically are incomplete.^[45][46] As the disease progresses and a person's ability to tend to his own needs reduces, carefully managed multidisciplinary caregiving becomes increasingly necessary.^[45]

Tetrabenazine was developed specifically to reduce the severity of chorea in HD,^[45] it was approved in 2008 for this use in the US.^[47] Other drugs that help to reduce chorea include neuroleptics and benzodiazepines.^[2] Compounds such as amantadine or remacemide are still under investigation but have shown preliminary positive results.^[48] Hypokinesia and rigidity can be treated with antiparkinsonian drugs, and myoclonic hyperkinesia can be treated with valproic acid.^[2]

Psychiatric symptoms can be treated with medications similar to those used in the general population.^[45][46] Selective serotonin reuptake inhibitors and mirtazapine have been recommended for depression, while atypical antipsychotic drugs are recommended for psychosis and behavioral problems.^[46]

Weight loss and eating difficulties due to dysphagia and other muscle discoordination are common, making nutrition management increasingly important as the disease advances.^[45] Thickening agents can be added to liquids as thicker fluids are easier and safer to swallow.^[45] Reminding the patient to eat slowly and to take smaller pieces of food into the mouth may also be of use to prevent choking.^[45] 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.^[49]

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. Patients with Huntington’s disease may see a physical therapist for non-invasive and non-medication ways of managing the physical symptoms of HD. Physical therapists may implement fall risk assessment and prevention, as well as strengthening, stretching, and cardiovascular exercises.^[50] The Tinetti Mobility Test has been found to be a valid outcome measure for determining whether a patient with HD is at risk for falling, as its scores correlated with the Unified Huntington Disease Rating Scale motor scores (r = –0.751, p<0.0001).^[51] Patients with TMT scores below 21 are at higher risk of falling.^[51] Other valid and responsive tools for assessing risk of falling in individuals with HD include the Functional Reach Test, Berg Balance Scale, and the Timed Up and Go Test.^[52] A Berg Balance Scale score of less than 40 or Timed Up and Go Test score of greater than 14 seconds indicate increased risk of falling in individuals with HD.^[53] However, more rigorous studies are needed for health authorities to endorse the usefulness of the rehabilitation professions.^[54] A multidisciplinary approach may be important to limit disability.^[55]

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.^[12][56]

Prognosis

The length of the trinucleotide repeat accounts for 60% of the variation in the age of onset and the rate of progression of symptoms. A longer repeat results in an earlier age of onset and a faster progression of symptoms.^[13][57] For example, individuals with a trinucleotide repeat greater than sixty repeats often develop the disease before twenty years of age, and those with less than forty repeats may not develop noticeable symptoms.^[58] The remaining variation is due to environmental factors and other genes that influence the mechanism of the disease.^[13]

Life expectancy in HD is generally around 20 years following the onset of visible symptoms.^[5] Most of the complications that are life-threatening result from muscle coordination issues, or to a lesser extent from behavioral changes resulting from the decline in cognitive function. The largest risk is pneumonia, which is the cause of death of one-third of those with HD. As the ability to synchronize movements deteriorates, difficulty clearing the lungs and an increased risk of aspirating food or drink both increase the risk of contracting pneumonia. The second greatest risk is heart disease, which causes almost a quarter of fatalities of those with HD.^[5] Suicide is the next greatest cause of fatalities, with 7.3% of those with HD taking their own lives and up to 27% attempting to do so. It is unclear to what extent suicidal thoughts are influenced by psychiatric symptoms, as they may be considered to be a response of an individual to retain a sense of control of their life or to avoid the later stages of the disease.^[59][60][61] Other associated risks include choking, physical injury from falls, and malnutrition.^[5]

Epidemiology

The late onset of Huntington's disease means it does not usually affect reproduction.^[12] The worldwide prevalence of HD is 5-10 cases per 100,000 persons,^[62][63] but varies greatly geographically as a result of ethnicity, local migration and past immigration patterns.^[12] Prevalence is similar for men and women. The rate of occurrence is highest in peoples of Western European descent, averaging around seventy per million people, and is lower in the rest of the world, e.g. one per million people of Asian and African descent.^[12] Additionally, some localized areas have a much higher prevalence than their regional average.^[12] One of the highest prevalences is in the isolated populations of the Lake Maracaibo region of Venezuela, where HD affects up to seven thousand per million people.^[12][64] Other areas of high localization have been found in Tasmania and specific regions of Scotland, Wales and Sweden.^[61] Increased prevalence in some cases occurs due to a local founder effect, a historical migration of carriers into an area of geographic isolation.^[61][65] Some of these carriers have been traced back hundreds of years using genealogical studies.^[61] Genetic haplotypes can also give clues for the geographic variations of prevalence.^[61][66]

Until the discovery of a genetic test, statistics could only include clinical diagnosis based on physical symptoms and a family history of HD, excluding those who died of other causes before diagnosis. These cases can now be included in statistics and as the test becomes more widely available, estimates of the prevalence and incidence of the disorder are likely to increase.^[61][67]

History

In 1872 George Huntington described the disorder in his first paper "On Chorea" at the age of 22.^[68]

The first definite mention of HD was in a letter by Charles Oscar Waters, published in the first edition of Robley Dunglison's Practice of Medicine in 1842. Waters described 'a form of chorea, vulgarly called magrums', including accurate descriptions of the chorea, its progression, and the strong heredity of the disease.^[69] In 1846 Charles Gorman observed how higher prevalence seemed to occur in localized regions.^[69] Independently of Gorman and Waters, both students of Dunglison at Jefferson Medical College in Philadelphia^[70], Johan Christian Lund also produced an early description in 1860.^[69] He specifically noted that in Setesdalen, a secluded area in Norway, there was a high prevalence of dementia associated with a pattern of jerking movement disorders that ran in families.^[71]

The first thorough description of the disease was by George Huntington in 1872. Examining the combined medical history of several generations of a family exhibiting similar symptoms, he realized their conditions must be linked; he presented his detailed and accurate definition of the disease as his first paper. Huntington described the exact pattern of inheritance of autosomal dominant disease years before the rediscovery by scientists of Mendelian inheritance.

"Of its hereditary nature. When either or both the parents have shown manifestations of the disease ..., one or more of the offspring almost invariably suffer from the disease ... But if by any chance these children go through life without it, the thread is broken and the grandchildren and great-grandchildren of the original shakers may rest assured that they are free from the disease.".^[68][72]

Sir William Osler was interested in the disorder and chorea in general, and was impressed with Huntington's paper, stating that "In the history of medicine, there are few instances in which a disease has been more accurately, more graphically or more briefly described."^[69][73] Osler's continued interest in HD, combined with his influence in the field of medicine, helped to rapidly spread awareness and knowledge of the disorder throughout the medical community.^[69] Great interest was shown by scientists in Europe, including Louis Théophile Joseph Landouzy, Désiré-Magloire Bourneville, Camillo Golgi, and Joseph Jules Dejerine, and until the end of the century, much of the research into HD was European in origin.^[69] By the end of the 19th century, research and reports on HD had been published in many countries and the disease was recognized as a worldwide condition.^[69]

During the rediscovery of Mendelian inheritance at the turn of the 20th century, HD was used tentatively as an example of autosomal dominant inheritance.^[69] The English biologist William Bateson used the pedigrees of affected families to establish that HD had an autosomal dominant inheritance pattern.^[70] The strong inheritance pattern prompted several researchers, including Smith Ely Jelliffe, to attempt to trace and connect family members of previous studies.^[69] Jelliffe collected information from across New York State and published several articles regarding the genealogy of HD in New England.^[74] Jelliffe's research roused the interest of his college friend, Charles Davenport, who commissioned Elizabeth Muncey to produce the first field study on the East Coast of the United States of families with HD and to construct their pedigrees.^[75] Davenport used this information to document the variable age of onset and range of symptoms of HD; he claimed that most cases of HD in the USA could be traced back to a handful of individuals.^[75] This research was further embellished in 1932 by P. R. Vessie, who popularized the idea that three brothers who left England in 1630 bound for Boston were the progenitors of HD in the USA.^[76] The claim that the earliest progenators had been established and eugenic bias of Muncey's, Davenport, and Vessie's work contributed to misunderstandings and prejudice about HD.^[70] Muncey and Davenport also popularized the idea that in the past some HD sufferers may have been thought to be possessed by spirits or victims of witchcraft, and were sometimes shunned or exiled by society.^[77][78] This idea has not been proven. Researchers have found contrary evidence; for instance, the community of the family studied by George Huntington openly accommodated those who exhibited symptoms of HD.^[70][77]

Research into the disorder continued steadily through the 20th century, reaching a major breakthrough in 1983 when the US–Venezuela Huntington's Disease Collaborative Research Project discovered the approximate location of a causal gene.^[65] This was the result of an extensive study begun in 1979, focusing on the populations of two isolated Venezuelan villages, Barranquitas and Lagunetas, where there was an unusually high prevalence of the disease. Among other innovations, the project developed DNA-marking methods which were an important step in making the Human Genome Project possible.^[79] In 1993 the research group isolated the precise causal gene at 4p16.3^[80], making this the first autosomal disease locus found using genetic linkage analysis.^[80][81] In the same time frame, key discoveries concerning the mechanisms of the disorder were being made, including the findings by Anita Harding's research group on the effects of the gene's length.^[82]

Modelling the disease in various types of animals, such as the transgenic mouse developed in 1996, enabled larger scale experiments. As these animals have faster metabolisms and much shorter lifespans than humans, results from experiments are received sooner, speeding research.^[83][84] The 1997 discovery that mHtt fragments misfold led to the discovery of the nuclear inclusions they cause.^[85] These advances have led to increasingly extensive research into the proteins involved with the disease, potential drug treatments, care methods, and the gene itself.^[69][86][87]

Society and culture

See also: List of Huntington's disease media depictions

Ethics

See also: In vitro fertilisation#Ethics and Stem cell controversy

Huntington's disease, particularly the application of the genetic test for the disease, has raised several ethical issues. The issues for genetic testing include defining how mature an individual should be before being considered eligible for testing, ensuring the confidentiality of results, and whether companies should be allowed to use test results for decisions on employment, life insurance or other financial matters. There was controversy when Charles Davenport proposed in 1910 that compulsory sterilization and immigration control be used for people with certain diseases, including HD, as part of the eugenics movement.^[88] In vitro fertilization has some issues regarding its use of embryos. Some HD research has ethical issues due to its use of animal testing and embryonic stem cells.^[89][90]

The development of an accurate diagnostic test for Huntington's disease has caused social, legal, and ethical concerns over access to and use of a person's results.^[91][92] Many guidelines and testing procedures have strict procedures for disclosure and confidentiality to allow individuals to decide when and how to receive their results and also to whom the results are made available.^[12] Financial institutions and businesses are faced with the question of whether to use genetic test results when assessing an individual, such as for life insurance or employment. Although the United Kingdom's insurance companies have agreed that until 2014 they will not use genetic information when writing most insurance policies,^[93] Huntington's is explicitly excluded from this agreement.^[94] As with other untreatable genetic conditions with a later onset, it is ethically questionable to perform pre-symptomatic testing on a child or adolescent, as there would be no medical benefit for that individual.^[26][95][96] There is consensus for testing only individuals who are considered cognitively mature, although there is a counter-argument that parents have a right to make the decision on their child's behalf.^[26][95][96] With the lack of an effective treatment, testing a person under legal age who is not judged to be competent is considered unethical in most cases.^[26][95][96]

There are ethical concerns related to prenatal genetic testing or preimplantation genetic diagnosis to ensure a child is not born with a given disease.^[97] For example, prenatal testing raises the issue of selective abortion, a choice considered unacceptable by some.^[97] Using preimplantation testing for HD requires twice as many embryos to be used for in vitro fertilization, as half of them will be positive for HD. As it is a dominant disease, there are difficulties in situations in which a parent does not want to know his or her own diagnosis. This would require parts of the process to be kept secret from the parent.^[97]

Support organizations

The death of Woody Guthrie led to the foundation of the Committee to Combat Huntington's Disease.

Poster of Recent studies of Huntington's disease Marjorie Guthrie lecture in genetics; 1985

In 1968, after experiencing HD in his wife's family, Dr. Milton Wexler was inspired to start the Hereditary Disease Foundation (HDF), with the aim of curing genetic illnesses by coordinating and supporting research.^[98] The foundation and Dr. Wexler's daughter, Nancy Wexler, were key parts of the research team in Venezuela which discovered the HD gene.^[98] At roughly the same time as the HDF formed, Marjorie Guthrie helped to found the Committee to Combat Huntington's Disease (now the Huntington's Disease Society of America), after her husband Woody Guthrie died from complications of HD.^[99] Since then, support and research organizations have formed in many countries around the world and have helped to increase public awareness of HD. A number of these collaborate in umbrella organizations, like the International Huntington Association and the EuroHD network.^[100] Many support organizations hold an annual HD awareness event, some of which have been endorsed by their respective governments. For example, June 6 is designated "National Huntington's Disease Awareness Day" by the US Senate.^[101]

Research directions

Main article: Huntington's disease clinical research

Research into the mechanism of HD has focused on identifying the functioning of Htt, how mHtt differs or interferes with it, and the brain pathology that the disease produces.^[25] Most research is conducted in animals. Appropriate animal models are critical for understanding the fundamental mechanisms causing the disease and for supporting the early stages of drug development.^[102] Mice and monkeys, chemically induced to exhibit HD-like symptoms were initially used,^{[102][103][104]} but they did not mimic the progressive features of the disease. Since the Huntingtin gene was identified, transgenic animals (mice,^{[102][105][106]} Drosophila fruit flies,^[102][107] and more recently monkeys^[108]) exhibiting HD-like syndromes can be generated by inserting a CAG repeat expansion into the gene. Nematode worms also provide a valuable model when the gene is expressed.^[102][109]

Genetically engineered intracellular antibody fragments called intrabodies have been shown to prevent mortality during the development stages of Drosophila models. Their mechanism of action was an inhibition of mHtt aggregation.^{[102][110][111]} As HD has been conclusively linked to a single gene, gene silencing is potentially possible and by using gene knockdown in mouse models, researchers have shown that when the influence of mHtt is reduced, symptoms improve.^{[48][112][113]} Stem cell therapy is the replacement of damaged neurons by transplantation of stem cells into affected regions of the brain. Experiments have yielded some positive results using this technique in animal models and preliminary human clinical trials.^[114]

Numerous drugs have been reported to produce benefits in animals, including creatine, coenzyme Q10 and the antibiotic minocycline.^[48] Some of these have then been tested by humans in clinical trials, and as of 2009 several are at different stages of these trials.^[48] In 2010, minocycline was found to be ineffective for humans in a multi-center trial.^[115]

See also

• Proteopathy

References

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External links

• Huntington's Disease links directory at the Open Directory Project