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Huntington Disease

Huntington disease

Definition

First described by Dr. George Huntington in 1872, Huntington disease (HD) is a relatively common hereditary neurological condition that most commonly affects people in their adult years. HD is a progressive disorder that often involves thinking and learning problems, psychological disturbances, and abnormal movements. HD has been well studied and documented in family histories across the world. This ultimately led to the discovery of the HD gene, now known to be responsible for the disorder.

Description

Huntington disease is also known by the name Huntington (or Huntington's) chorea ; "chorea" refers to neurological diseases that are characterized by spasmodic movements of the limbs and facial muscles. This is because about 90% of people with HD have chorea. These movements may be mild at first, but can worsen and become more involuntary with time.

About two-thirds of people with HD first present with neurological signs, while others first have psychiatric changes. Other neurological signs include various abnormal movements, changes in eye movements, difficulty speaking, difficulty swallowing, and increased reflexes.

A general decline in thinking skills occurs in essentially everyone with HD. This may begin as general forgetfulness and progress to difficulty gathering thoughts or keeping and using new knowledge. People with HD often also have psychiatric changes, including significant personality and behavior changes.

The majority of those with HD first develops symptoms between the ages of 35 and 50 years. Symptoms vary considerably between people and sometimes within families, so it is difficult to predict an individual's exact experience with HD if he or she is diagnosed with the condition. Disease progression occurs in everyone, with death usually seen 1030 years after its onset.

Demographics

HD is estimated to occur in the United States and most of Europe at a rate of about five cases per 100,000 people. Pockets of populations exist where the prevalence may be a bit higher, such as those with western European ancestors. Conversely, HD is estimated to have a much lower prevalence in Japan, China, Finland, and Africa. For example, the frequency of HD in Japan has been estimated at between 0.1 and 0.38 per 100,000 people.

Symptoms of HD typically begin after about age 35 years. However, in some families a juvenile form of HD has been seen with an onset of symptoms in the first or second decades of life. About a quarter of people with the condition are diagnosed past the age of 50 years. HD is a disease that affects males and females equally.

Currently, genetic testing is widely available to identify a well-documented mutation in the HD gene. Testing is available for confirmation of a clinical diagnosis, or for those at risk but who, as yet, have no symptoms. Predictive genetic testing (for those who are asymptomatic) typically involves a specialized protocol with pretest and post-test counseling, requiring coordinated care with various medical professionals.

Causes and symptoms

Some neurological changes have been seen in HD. However, the connection of many of these changes to the disease's symptoms is still not understood. Atrophy of the basal ganglia and corpus striatum are common neurological findings in HD, which may worsen over time. Cortical atrophy is often present, and this may be seen with magnetic resonance imaging (MRI) or computed tomography (CT ) scans. From pathology studies after death, brain atrophy is most prominent in the caudate, putamen, and cerebral cortex in people with HD. Total brain weight may be reduced by as much as 2530% in people who have advanced cases of HD.

A specific mutation in the HD gene called a triplet expansion causes symptoms of the condition to occur. The four different deoxyribonucleic acid (DNA) bases that make up genes are abbreviated as A, C, T, and G. Three DNA bases, CAG, are naturally repeated in the HD gene; a certain number of repeats is considered normal. People with symptoms of HD have a higher number of repeats than the usual range. Unfortunately, the number of CAG repeats can increase (or expand) from generation to generation, and this usually occurs in men. This genetic process is called anticipation; it cannot be predicted when and how the CAG repeats will expand in someone when they have children. A larger CAG repeat size is generally associated with developing symptoms at a younger age.

HD is inherited in an autosomal dominant manner, which means that an affected individual has a one in two chance to pass the disease-causing mutation to his or her children, regardless of the gender. Children who inherit a disease-causing mutation will develop signs of HD at some point in their lives. On the other side of that, children who do not inherit the mutation should not develop the disease. Strong family histories of HD have been well documented and studied across the globe.

HD is usually first suspected with the observation or progression of abnormal movements. The initial reasons for seeking medical attention are often clumsiness, tremor, balance trouble, or jerkiness. Chorea is a frequent symptom.

The areas of the body most commonly affected by chorea are the face, limbs, and trunk. As the chorea progresses, breathing, swallowing, and the mouth and nasal muscles may become involved. Muscles may become extremely rigid and gait may show signs of ataxia . Chorea may also be mixed with other movement disorders such as dystonia . Visual muscles may also be affected, and this can eventually lead to difficulties with vision, speech, swallowing, and breathing.

Weight loss is a common symptom in HD, which may occur despite a proper intake of calories and nutrients. Because people with HD are frequently moving, it is thought this continual activity increases metabolic rates and may explain the weight loss. However, the exact cause for weight loss in HD is still not well understood.

Mental impairment is an eventual sign of HD. This may begin at about the same time as movement abnormalities. If a diagnosis of HD is made, cognitive decline may have actually begun earlier, but might have gone unnoticed until other symptoms of the condition began to develop.

General forgetfulness, loss of mental flexibility, difficulty with mental planning, and organization of sequential activities may be early signs of HD. Reduced attention and concentration spans are common, and this may lead to one being quite distractible. Aphasia and agnosia are less evident than in Alzheimer's disease, but overall cognitive speed and efficiency are usually affected. The ability to speak is usually maintained, but people with HD may eventually have difficulty with complex words or finding the correct words to express their thoughts. Late-stage symptoms may include difficulty with visual and spatial relations.

The last category of symptoms in HD is that involving psychological disturbances. Irritability and depression are common early signs of HD. People may initially be incorrectly diagnosed with psychiatric diseases like schizophrenia and delusional disorder, particularly if they have no other symptoms of HD. This is probably because a large percentage of people with HD have significant personality changes or affective psychosis. Behavioral issues can include intermittent explosiveness, apathy, aggression, alcohol abuse, sexual problems and deviations, paranoid delusions, and an increased appetite.

Suicide occurs in 512% of people with HD. Late-stage disease is often quite significant and can be disabling. Weight loss, sleep problems, and incontinence are common signs of advanced HD.

Juvenile HD occurs when someone develops symptoms in the first two decades of life; this occurs in about 510% of all HD cases. Symptoms are distinct from those associated with adult-onset forms of HD. For example, chorea rarely occurs in people who develop HD in their first decade of life. However, dystonia and rigidity can be very significant for those individuals. Common characteristics of people with juvenile HD diagnosed before age 10 include declining performance in school, mouth muscle abnormalities, rigidity, and problems with their gait. Seizures are also a somewhat unique characteristic of juvenile HD.

Complications related to immobility are often the cause of death in people with HD. Abnormal muscular movements, particularly those related to swallowing and breathing, may cause someone to die from aspiration pneumonia and other infections; such a cause of death occurs years after the onset of the disease.

People with juvenile HD diagnosed between the age of 10 and 20 may have symptoms similar to adult-onset HD. Others may have more severe behavioral and psychiatric problems noticed before anything else. Common among people with juvenile HD is a father with adult-onset HD.

Diagnosis

Until the discovery of the HD gene on chromosome 4 in 1993, the diagnosis of the condition was made purely on a clinical basis. This can be somewhat challenging because of similarities with other hereditary and non-hereditary conditions involving chorea.

A careful neurological examination and documentation of abnormal movements are important to diagnose HD. Sydenham's chorea is a nonhereditary, infectious cause of chorea. It most often occurs in children and adolescents following a streptococcal infection, and the chorea associated is slightly different than that with HD. About 30% of people with rheumatic fever or polyarthritis develop Sydenham's chorea two to three months later. Symptoms may even come back in pregnancy, or in people taking oral contraceptives. The chorea in Sydenham's chorea is brisk and abrupt, but it is more flowing and somewhat slower in HD. Treatment for Sydenham's chorea usually involves bed rest, sedation, and antibiotic therapy with medications like penicillin.

Movements with characteristics of dystonia and athetosis, called choreoathetosis, are also common in HD. People with HD may be able to more easily mask their movements at first, because they are not that intrusive in the early stages. Tardive dyskinesia is a nonhereditary cause of chorea that may be mistaken for HD in an individual on antipsychotic medications.

Chorea occurs in 17% of people with lupus , and in a proportion of people with drug-related problems. It is important to rule out nonhereditary causes of chorea because treatments may exist for them, which may increase quality of life for the affected person.

Although very useful for many other neurological conditions, looking at the brain with techniques like magnetic resonance imaging (MRI ) or computed tomography (CT) scans currently are not as helpful in diagnosing HD. These techniques may help find some typical brain changes in HD. For example, caudate atrophy is typically associated with advanced HD. Studies have shown that serial CT scans of the basal ganglia in at-risk individuals without symptoms may show signs of caudate atrophy before the disease even shows symptoms. These types of imaging studies can be useful to rule out other diagnoses that may mimic HD, because those may involve other specific brain changes.

An important step in diagnosing HD is to take a careful family history. Strong family histories with multiple generations affected, with roughly equal males and females affected, are common in HD.

Many hereditary conditions mimic HD. People who are diagnosed with HD much later in life may seem similar to people with Parkinson's disease , because abnormal movements may be the primary symptom. Neuroacanthocytosis is a hereditary condition with chorea, but it should be considered if muscle loss, absent lower limb tendon reflexes, neuropathy, and specific results on a blood test are present. Benign hereditary chorea is an autosomal dominant condition in which the chorea is not progressive, and does not involve any cognitive decline. Dentatorubropallidoluysian atrophy (DRPLA) is another hereditary condition that mimics HD; it typically affects adults and involves dementia , ataxia, and seizures, along with chorea. As a group, the hereditary spinocerebellar ataxias (SCAs) may mimic some of the movement abnormalities seen in HD. However, the psychological and cognitive components may not be present in the SCAs.

Often, diagnosis is most clearly made with genetic testing, which is done to confirm a suspected clinical diagnosis. Genetic testing identifies the exact number of CAG repeats in each copy of a person's HD gene.

There are several CAG repeat ranges that may be found through testing. Each genetic laboratory may use slightly different ranges, so test results should be interpreted carefully. Generally, a range of 1027 CAG repeats is considered to be normal. If someone has results in these ranges, this person does not have HD, and will not develop signs of it.

A range of 2735 CAG repeats will not cause symptoms of HD in the person. In this range, the repeat size may rarely increase when passed on to children. In other words, the person with this test result will not develop symptoms of HD, but he or she may have a child who develops symptoms. This would particularly be the case if the person were a man, because of the anticipation phenomenon.

A range of 3639 CAG repeats is considered a range where the person may or may not develop HD symptoms at some point in his or her life. Additionally, the repeat may or may not expand to his or her children.

People with an HD gene that has greater than 39 CAG repeats will develop symptoms of HD at some point in their lives. They would have a 50% chance of passing this gene on to future children.

People with juvenile HD usually have much larger CAG repeat sizes than those who have the typical form of HD. Despite this, it is still impossible to predict exactly when someone may develop symptoms, or to predict the exact symptoms they will experience.

Genetic testing for those who have symptoms is fairly straightforward, and often ordered with the aid of a neurologist . Predictive testing for HD, as it is called when the person does not have symptoms, is a bit more complicated. This is because there are many complex factors in the testing process.

Ideally, at-risk asymptomatic individuals have several appointments before genetic testing is performed. They should see a neurologist for a thorough examination to identify any subtle signs of HD. They should also see a neuropsychologist for an evaluation. The neuropsychologist can help assess whether a person is a good candidate for genetic testing, potentially reducing the risk for poor outcomes, like suicide, following positive results. Individuals should also see a medical geneticist and genetic counselor to receive thorough information about the risks, benefits, and limitations of genetic testing.

Much has been studied about the myriad of issues with genetic testing in HD. Risks from any outcome can be considerable, and these may include a sudden change in family dynamics, self-image, or serious emotional and psychological harms.

Health, life, or disability insurance discrimination from HD testing may be a possibility, especially related to positive results. Employment may also be an issue. In October 2003, a young teacher in Germany was refused a permanent job because members of her family have HD; she was found to be at risk for the condition during a required governmental medical examination. Currently, there is not enough documentation in the medical literature to know what the actual risks are related to these issues. Awareness and discussion of these issues are important in pretest counseling.

Limitations and benefits from genetic testing should be given equal weight as well. Results may not be easily understood, simply identifying one and one's children to be potentially at risk. These types of vague results can cause great angst to an at-risk individual. However, benefits from testing may include relief from years of worry, empowerment from medical knowledge, and the ability to make life plans or tailor medical care based upon more accurate information.

Generally, at-risk asymptomatic children under age 18 are not tested for HD. The decision to learn their genetic status should be theirs, and at a time they feel is appropriate. Along the same lines, prenatal genetic testing for HD is not done, except in cases involving special circumstances or assistive reproductive techniques.

Treatment team

Treatment for people with HD is highly dependent on their symptoms. A multidisciplinary team and approach can be very helpful. A treatment team may include a neurologist, neuropsychologist, medical geneticist, genetic counselor, physical therapist, occupational therapist, speech therapist, registered dietitian, social worker, psychotherapist, psychiatrist, ophthalmologist, and a primary care provider. Some hospitals offer day clinics devoted to people with HD, which makes things much easier in terms of coordinating appointments. Pediatric specialists in these fields may help in the care for children.

Treatment

Currently, there is no known cure for Huntington disease. No specific treatment is known to slow, stop, or reverse the progressive nature of the disease. Current treatment for HD is mainly focused on relieving symptoms and reducing the impact of physical and mental complications related to the disease.

Medications are available to help treat chorea in HD, including therapies for blocking dopamine receptors, or those that deplete dopamine from its natural storage sites in the brain. Medications like these are tetrabenazine, pimozide, and haloperidol. They can have side effects, like drowsiness and a lessened ability to make voluntary movements. Some find the side effects to be more troublesome than the chorea, so medications should be prescribed under careful supervision.

Psychiatric problems in HD are often treated with medications as well. Some selective serotonin reuptake inhibitors (SSRIs) with trade names like Celexa, Paxil, Prozac, and others have been effective. Some tricyclic antidepressants like Nordil, Marplan, and Eldepryl have been effective. Lastly, some monoamine oxidase inhibitors (MAOIs) like Elavil, Tofranil, and Anafranil have been useful in treating depression.

Benzodiazepine and antipsychotic drugs can be used to treat anxiety, irritability, and agitation in HD. It is rare to find a medication without side effects, and drug interactions are also important to consider. As yet, no medications have been found helpful to treat the cognitive problems in HD.

Other therapies have been tested through clinical trials to see whether the disease progression may be slowed in any way. A combination of coenzyme Q10 and remacemide has been tested in mice, showing it to be helpful in reducing weight loss and brain loss. In a study by The Huntington Study Group in 2001, people with early-stage HD were given coenzyme Q10 or remacemide, but neither had significant effects. A 2000 study found that minocycline, an antibiotic, delayed motor decline in mice by 14%.

Riluzole is a drug currently used to treat people with amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease). In clinical trials with HD patients in 1999, the drug reduced chorea in about a third of people over six weeks. Behavior was improved by about 61% after 12 months.

Studies are under way to see whether transplanting fetal cells from the corpus striatum will be helpful to treat people with HD. This follows closely on the heels of similar trials with people who have Parkinson's disease. As of early 2004, preliminary results seem promising but much more time is needed to fully study and interpret them.

Recovery and rehabilitation

Supportive therapy for people with HD is very helpful, and often greatly needed as time goes on. It may begin shortly after diagnosis and continue for years, until the disease becomes advanced and supportive care is needed.

Physical therapy, speech therapy, and dietary advice can be extremely important and most effective when in tandem. Special consideration should be given to nursing and supportive care, home health care options, diet, special adaptive equipment, and eligibility for governmental benefits. A practical approach with common sense, emotional support, and careful attention to a family's needs is effective for many people with HD.

Clinical trials

As of early 2004, many clinical trials were under way to study Huntington disease:

  • Family Health after Predictive Huntington Disease (HD) Testing, sponsored by National Institute of Nursing Research (NINR).
  • Minocycline in Patients with Huntington's Disease, sponsored by FDA Office of Orphan Products Development.
  • Prospective Huntington At-Risk Observational Study (PHAROS), sponsored by National Institute of Neurological Disorders and Stroke (NINDS) and National Human Genome Research Institute (NHGRI).
  • Neurobiological Predictors of Huntington's Disease (PREDICT-HD), sponsored by NINDS.
  • Brain Tissue Collection for Neuropathological Studies, sponsored by National Institute of Mental Health (NIMH).

Prognosis

Prognosis has historically been somewhat bleak for people with HD. Complications related to movement abnormalities and immobility, such as pneumonia and respiratory complications, are a common cause of death in HD. Though no cure is currently available, treatments or therapies may be available in the future to maintain a better quality of life, and these continue to offer hope.

Resources

BOOKS

Parker, James N., and Philip M. Parker. The Official Patient's Sourcebook on Huntington's Disease: A Revised and Updated Directory for the Internet Age. San Diego: Icon Health Publishers, 2002.

Quarrell, Oliver. Huntington's Disease: The Facts. Oxford: Oxford University Press, 1999.

PERIODICALS

Burgermeister, Jane. "Teacher Was Refused Job because Relatives Have Huntington's Disease." British Medical Journal (October 11, 2003) 327 (7419): 827.

Grimbergen, Yvette A. M., and Raymond A. C. Roos. "Therapeutic Options for Huntington's Disease." Current Opinion in Investigational Drugs (2003) 4(1): 5154.

Margolis, Russell L., and Christopher A. Ross. "Diagnosis of Huntington Disease." Clinical Chemistry (2003) 49(10): 17261732.

Sutton Brown, M., and O. Suchowersky. "Clinical and Research Advances in Huntington's Disease." The Canadian Journal of Neurological Sciences (2003) 30 (Suppl. 1): S45S52.

WEBSITES

Caring for People with Huntington's Disease. (June 2, 2004). <http://www.kumc.edu/hospital/huntingtons/index.html>.

GeneTests/GeneReviews. (June 2, 2004). <http://www.genetests.org>.

National Institute of Neurological Disorders and Stroke. (June 2, 2004). <http://www.ninds.nih.gov/index.htm>.

Testing for Huntington Disease: Making an Informed Choice. (June 2, 2004). <http://depts.washington.edu/neurogen/HuntingtonDis.pdf>.

Testing Guidelines in Huntington's Disease. (June 2, 2004). <http://www.hdfoundation.org/testread/hdsatest.htm>.

ORGANIZATIONS

Huntington's Disease Society of America. 158 West 29th Street, 7th Floor, New York, NY 10001-5300. (212) 242-1968 or (800) 345-HDSA (4372); Fax: (212) 239-3430. [email protected] <http://www.hdsa.org>.

Huntington Society of Canada. 151 Frederick Street, Suite 400, Kitchener, Ontario N2H 2M2, Canada. (519) 749-7063 or (800) 998-7398; Fax: (519) 749-8965. [email protected] <http://www.hsc-ca.org>.

International Huntington Association. Callunahof 8, 7217 St Harfsen, The Netherlands. + 31-573-431595. [email protected] <http://www.huntingtonassoc.com>.

Deepti Babu, MS, CGC

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Huntington Disease

Huntington Disease

Definition

Huntington disease (HD) is a progressive neuro-degenerative disease causing uncontrolled physical movements and mental deterioration. The disease was discovered by George Sumner Huntington (18501916), an Ohio doctor who first described the hereditary movement disorder in 1872.

Description

Huntington disease is also called Huntington chorea or hereditary chorea. The word chorea comes from the Greek word for "dance" and refers to the involuntary movements of the patient's feet, lower arms, and face that develop as the disease progresses. It is occasionally referred to as "Woody Guthrie's disease" for the American folk singer who died from it. Huntington disease (HD) causes progressive loss of cells in areas of the brain responsible for certain aspects of movement control and mental abilities. A person with HD gradually develops abnormal movements and changes in cognition (thinking), behavior and personality.

The onset of symptoms of HD usually occurs between the ages of 30 and 50; although in 10% of cases, onset is in late childhood or early adolescence. Approximately 30,000 people in the United States are affected by HD, with another 150,000 at risk for developing this disorder. The frequency of HD is 4-7 cases per 100,000 persons.

Causes and symptoms

Huntington disease is caused by a defect in the HD gene (an inherited unit which contains a code for a protein), which is located on the short arm of chromosome 4. The gene codes for a protein called huntingtin, whose function is not known as of early 2005. The nucleotide codes (building blocks of genes arranged in a specific code which chemically forms into proteins), contain CAG repeats (40 or more of these repeat sequences). The extra building blocks in the huntingtin gene cause the protein that is made from it to contain an extra section as well. It is currently thought that this extra protein section, or portion, interacts with other proteins in brain cells where it occurs, and that this interaction ultimately leads to cell death.

The HD gene is a dominant gene, meaning that only one copy of it is needed to develop the disease. HD affects both males and females. The gene may be inherited from either parent, who will also be affected by the disease. A parent with the HD gene has a 50% chance of passing it on to each offspring. The chances of passing on the HD gene are not affected by the results of previous pregnancies.

The symptoms of HD fall into three categories: motor or movement symptoms; personality and behavioral changes; and cognitive decline. The severity and rate of progression of each type of symptom can vary from person to person.

Early motor symptoms include restlessness, twitching and a desire to move about. Handwriting may become less controlled, and coordination may decline. Later symptoms include:

  • dystonia, or sustained abnormal postures, including facial grimaces, a twisted neck, or an arched back
  • chorea, in which involuntary jerking, twisting or writhing motions become pronounced
  • slowness of voluntary movements, inability to regulate the speed or force of movements, inability to initiate movement and slowed reactions
  • difficulty speaking and swallowing due to involvement of the throat muscles
  • localized or generalized weakness and impaired balance ability
  • rigidity, especially in late-stage disease

Personality and behavioral changes include depression, irritability, anxiety and apathy. The person with HD may become impulsive, aggressive or socially withdrawn.

Cognitive changes include loss of ability to plan and execute routine tasks, slowed thought, and impaired or inappropriate judgment. Short-term memory loss usually occurs, although long-term memory is usually not affected. The person with late-stage HD usually retains knowledge of his environment and recognizes family members or other loved ones, despite severe cognitive decline.

Diagnosis

Diagnosis of HD begins with a detailed medical history, and a thorough physical and neurological examination. The family's medical history is very important. Magnetic resonance imaging (MRI) or computed tomography scan (CT scan) imaging may be performed to look for degeneration in the basal ganglia and cortex, the brain regions most affected in HD.

Physicians have recently developed a Uniform Huntington's Disease Rating Scale, or UHDRS, to assess a patient's symptoms and the speed of progression of the disease.

A genetic test is available for confirmation of the clinical diagnosis. In this test, a small blood sample is taken, and DNA from it is analyzed to determine the CAG repeat number. A person with a repeat number of 30 or below will not develop HD. A person with a repeat number between 35 and 40 may not develop the disease within their normal lifespan. A person with a very high number of repeats (70 or above) is likely to develop the juvenile-onset form. An important part of genetic testing is extensive genetic counseling.

Prenatal testing is available. A person at risk for HD (a child of an affected person) may obtain fetal testing without determining whether she herself carries the gene. This test, also called a linkage test, examines the pattern of DNA near the gene in both parent and fetus, but does not analyze for the triple nucleotide repeat (CAG). If the DNA patterns do not match, the fetus can be assumed not to have inherited the HD gene, even if present in the parent. A pattern match indicates the fetus probably has the same genetic makeup of the at-risk parent.

Treatment

There is no cure for HD, nor any treatment that can slow the rate of progression. Treatment is aimed at reducing the disability caused by the motor impairments, and treating behavioral and emotional symptoms.

Physical therapy is used to maintain strength and compensate for lost strength and balance. Stretching and range of motion exercises help minimize contracture, or muscle shortening, a result of weakness and disuse. The physical therapist also advises on the use of mobility aids such as walkers or wheelchairs.

Motor symptoms may be treated with drugs, although some studies suggest that anti-chorea treatment rarely improves function. Chorea (movements caused by abnormal muscle contractions) can be suppressed with drugs that deplete dopamine, an important brain chemical regulating movement. As HD progresses, natural dopamine levels fall, leading to loss of chorea and an increase in rigidity and movement slowness. Treatment with L-dopa (which resupplies dopamine) may be of some value. Frequent reassessment of the effectiveness and appropriateness of any drug therapy is necessary.

Occupational therapy is used to design compensatory strategies for lost abilities in the activities of daily living, such as eating, dressing, and grooming. The occupational therapist advises on modifications to the home that improve safety, accessibility, and comfort.

Difficulty swallowing may be lessened by preparation of softer foods, blending food in an electric blender, and taking care to eat slowly and carefully. Use of a straw for all liquids can help. The potential for choking on food is a concern, especially late in the disease progression. Caregivers should learn the use of the Heimlich maneuver. In addition, passage of food into the airways increases the risk for pneumonia. A gastric feeding tube may be needed, if swallowing becomes too difficult or dangerous.

Speech difficulties may be partially compensated by using picture boards or other augmentative communication devices. Loss of cognitive ability affects both speech production and understanding. A speech-language pathologist can work with the family to develop simplified and more directed communication strategies, including speaking slowly, using simple words, and repeating sentences exactly.

Early behavioral changes, including depression and anxiety, may respond to drug therapy. Maintaining a calm, familiar, and secure environment is useful as the disease progresses. Support groups for both patients and caregivers form an important part of treatment.

Experimental transplant of fetal brain tissue has been attempted in a few HD patients. Early results show some promise, but further trials are needed to establish the effectiveness of this treatment.

Tetrabenazine (Nitoman), a drug that has been considered investigational in the United States, appears to benefit some patients with HD by controlling the involuntary movements of chorea. It works by lowering the levels of dopamine and other neurotransmitters in the brain. The Food and Drug Administration (FDA) granted tetrabenazine fast-track and orphan drug status in the United States as of 2004. It is not yet manufactured in the United States but can be obtained from the United Kingdom.

In 2004 the Food and Drug Administration (FDA) also approved deep brain stimulation (DBS) as an acceptable treatment for HD and other movement disorders. In DBS, the surgeon implants a battery-operated medical device called a neurostimulator, which delivers electrical impulses to the areas of the brain that govern movement. In the case of Huntington's patients, the part of the brain that is targeted is a structure called the globus pallidus. It is thought that DBS works by increasing the flow of blood to this area. A group of Canadian researchers reported in 2004 that some patients with HD are better able to control their movements after DBS of the globus pallidus.

Psychotherapy is often recommended for individuals who know themselves to be at risk for the disease. Some persons want to know their risk status while others prefer not to be tested. Psychotherapy may be useful in helping at-risk persons decide about testing as well as coping with the results of the test.

Prognosis

The person with Huntington disease may be able to maintain a job for several years after diagnosis, despite the increase in disability. Loss of cognitive functions and increase in motor and behavioral symptoms eventually prevent the person with HD from continuing employment. Ultimately, severe motor symptoms prevent mobility. Death usually occurs between 10 and 30 years after disease onset, typically as the result of pneumonia or a fall. Progressive weakness of respiratory and swallowing muscles leads to increased risk of respiratory infection and choking, the most common causes of death. Future research in this area is currently focusing on nerve cell transplantation.

KEY TERMS

Basal ganglia A structure at the base of the brain composed of four groups of nerve cells, responsible for body movements and coordination.

Chorea Brief and purposeless involuntary movements of the lower arms, feet, and face.

Chromosome The structures that carry genetic information. Chromosomes are located within every cell, and are responsible for directing the development and functioning of all the cells in the body. The normal number is 46 (23 pairs).

Cortex The part of the brain responsible for thought, memory, and sensory perception.

Dystonia A movement disorder characterized by sustained muscle contractions that result in writhing or twisting movements and unsual body postures.

Globus pallidus A pale-colored spherical structure within the basal ganglia. Deep brain stimulation of this area is helpful in controlling the chorea of some patients with HD.

Huntingtin A protein of unknown function encoded by the HD gene. The repeated CAG sequence in a defective HD gene causes the body to produce an abnormal form of huntingtin. It is not yet known why the abnormal form of huntingtin affects only certain regions of the brain.

Orphan drug A term for a drug that treats a rare disease, defined by the Food and Drug Administration (FDA) as one that affects fewer than 200,000 Americans. The FDA has an Office of Orphan Products Development (OOPD), which offers grants to researchers to develop these products.

Tetrabenazine A drug given to control chorea that appears to benefit HD patients.

Resources

BOOKS

Beers, Mark H., MD, and Robert Berkow, MD., editors. "Disorders of Movement." Section 14, Chapter 179 In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

PERIODICALS

Montgomery, E. B., Jr. "Deep Brain Stimulation for Hyperkinetic Disorders." Neurosurgical Focus 17 (July 15, 2004): E1.

Moro, E., A. E. Lang, A. P. Strafella, et al. "Bilateral Globus Pallidus Stimulation for Huntington's Disease." Annals of Neurology 56 (August 2004): 290-294.

Revilla, Fredy J., MD, and Jaime Grutzendler, MD. "Huntington Disease." eMedicine November 3, 2004. http://www.emedicine.com/NEURO/topic81.htm.

Richartz, E. R., and C. Frank. "A Psychodynamic Approach in Counselling Vulnerable Persons for Chorea HuntingtonA Case Report." [in German] Psychiatrische Praxis 31 (July 2004): 255-258.

Seneca, S., D. Fagnart, K. Keymolen, et al. "Early-Onset Huntington Disease: A Neuronal Degeneration Syndrome." European Journal of Pediatrics 26 (August 2004): e-pub.

ORGANIZATIONS

Huntington Disease Society of America. 140 W. 22nd St. New York, NY 10011. (800) 345-HDSA.

National Institute of Neurological Disorders and Stroke (NINDS). NIH Neurological Institute, P. O. Box 5801, Bethesda, MD 20824. (800) 352-9424 or (301) 496-5751. http://www.ninds.nih.gov.

National Organization for Rare Disorders (NORD). 55 Kenosia Avenue, P. O. Box 1968, Danbury, CT 06813-1968. (203) 744-0100. Fax: (203) 798-2291. http://www.rarediseases.org.

United States Food and Drug Administration (FDA). 5600 Fishers Lane, Rockville, MD 20857-0001. (888) INFOFDA. http://www.fda.gov.

OTHER

Food and Drug Administration (FDA). "Grants Awarded by the OOPD Program." http://www.fda.gov/orphan/grants/previous.htm.

National Institute of Neurological Disorders and Stroke (NINDS). "Huntington's Disease: Hope Through Research." NIH Publication No. 98-49. Bethesda, MD: NINDS, 2005. http://www.ninds.nih.gov/disorders/huntington/detail_huntington.htm.

National Institute of Neurological Disorders and Stroke (NINDS). "NINDS Deep Brain Stimulation for Parkinson's Disease Information Page." Bethesda, MD: NINDS, 2004. http://www.ninds.nih.gov/disorders/deep_brain_stimulation/deep_brain_stimulation.htm.

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Huntington's disease

Huntington's disease, hereditary, acute disturbance of the central nervous system usually beginning in middle age and characterized by involuntary muscular movements and progressive intellectual deterioration; formerly called Huntington's chorea. The disease is sometimes confused with chorea or St. Vitus's dance, which is not hereditary. A faulty gene produces a defective protein attacks neurons in the basal ganglia, clusters of nerve tissue deep within the brain that govern coordination.

The onset is insidious and inexorably progressive; no treatment is known. Psychiatric disturbances range from personality changes involving apathy and irritability to bipolar or schizophreniform illness. Motor manifestations include flicking movements of the extremities, a lilting gait, and motor impersistence (inability to sustain a motor act such as tongue protrusion).

In 1993 the gene responsible for the disease was located; within that gene a small segment of code is, for some reason, copied over and over. Genetic counseling is extremely important, since 50% of the offspring of an affected parent inherit the gene, which inevitably leads to the disease.

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Huntington's disease

Huntington's disease (HD, Huntington's chorea) (hunt-ing-tŏnz) n. a hereditary disease caused by a defect in a single gene that is inherited as an autosomal dominant characteristic, tending to appear in half of the children of the parents with this condition. Symptoms, which begin to appear in early middle age, include unsteady gait and jerky involuntary movements (see chorea), accompanied later by behavioural changes and progressive dementia. [ G. Huntington (1850–1916), US physician]

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Huntingtons disease

Huntington's disease (Huntington's chorea) Acute degenerative disorder. It is genetically transmitted and usually occurs in early-middle life. It is caused by the presence of abnormally large amounts of glutamate and aspartate. Physical symptoms include loss of motor coordination. Mental deterioration can take various forms.

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Huntington Disease

Huntington Disease

History

Symptoms

Genetic defect responsible for disease

The quest for the Huntington disease gene

Lake Maracaibo

Ethical questions

Resources

Huntington disease (HD), also called Huntingtons disease, is a rare, incurable genetic disease that results in the progressive degeneration of both physical and mental abilities. Huntington disease was formerly known as Huntington chorea since the most obvious symptoms involve uncontrollable body movements known as chorea. The term chorea comes from the Greek word choreia, which means to dance. The term aptly describes the fitful, jerking movements associated with the condition, and the more advanced symptoms such as abnormal posture, dementia, and involuntary movements. As the disease progresses, its symptoms worsen and patients eventually die of respiratory failure or complications related to the neurodegenerative progression of the disease. Huntington disease is a late onset disorder, where affected individuals usually become symptomatic after 40 years of age. When someone is diagnosed with Huntington disease at age 20 years or younger, it is called juvenile Huntington disease. A genetic test for the disease is available, and its use brings to the forefront ethical and social issues related to the clinical diagnosis, particularly in the absence of a cure.

History

Huntington disease is named for American physician George Sumner Huntington (1850-1916), who described the illness in an 1872 paper titled On Chorea. Huntington practiced medicine on the eastern tip of Long Island, New York. His description of the disease was drawn from his familiarity with several affected families in his community. Both Huntingtons father and grandfather had practiced medicine in the same area. Their encounters with the disease gave Huntington an appreciation of the heredity aspect of the illness.

Historically, the mental and emotional deterioration that marks the illness has frequently led to the confinement of Huntington disease patients to psychiatric hospitals. Some historians speculate that a few of the women accused of witchcraft in Salem, Massachusetts, may have exhibited the involuntary twitches and turns that are hallmarks of the disease.

Symptoms

The symptoms in Huntington disease begin with noticeable behavioral changes including aggression, paranoia, and irritability. Affected individuals may seem restless, with tapping feet or odd twitches. Patients begin to suffer from impaired judgment and an inability to be organized. They become forgetful and their I.Q. (intelligence quotient) declines, coinciding with the deterioration of the brain. Emotionally, they may suffer from psychiatric disorders and even suicidal thoughts or actions. They may drop things and become less efficient in their usual activities. Depression, anxiety, and apathy are also common experiences in the beginning stages of the disease. As the illness progresses, the chorea worsens. The entire body moves in uncoordinated, jerky movements.

Although wide variations in clinical manifestations exist, the illness typically lasts 13 to 16 years. In later stages of the illness, patients cannot walk or care for themselves. They may barely speak and may fail to recognize friends and family. They eventually require full time nursing care. Eating is quite difficult and death is very frequently caused by choking or by the pneumonia that results after accidentally inhaling bits of food. Although some symptoms can be treated with medication, currently no cure exists to delay the onset of Huntington disease or to slow its course.

Huntington disease is a hereditary disease caused by a dominant gene and, therefore, follows an autosomal dominant pattern of inheritance. This means that only one copy of the gene is necessary to cause the disease. It is transmitted from one generation to the next. The child of a mother or father suffering from Huntington disease has a 50% chance of inheriting the disease gene and, thus, of contracting the disease later in life. About 30,000 Americans suffer from Huntington disease and another 150,000 are at risk for developing it. One of the best-known disease victims was American folk singer Woody Guthrie (1912-1967), who died of the disease.

Genetic defect responsible for disease

In 1993, scientists discovered the genetic defect that causes Huntington disease. A gene located on the chromosome 4 normally contains a sequence of three nucleotide bases (the alphabet of the genetic code) that repeats several times. The sequence is cytosine, adenine, and guanine (or CAG), which codes for the amino acid glutamine that is a building block for protein synthesis. In Huntington disease, patients have too many repeats. While unaffected individuals normally have 11 to 24 repeats, a person with Huntington disease may have anywhere from 36 to 100 or more repeats. Clinical research studies have demonstrated that the greater the number of repeats, the earlier the disease will develop and the clinical manifestations will be more severe. If the expanded trinucleotide sequence is passed from the father to the offspring, the offspring that inherit this expansion can have an earlier age of onset of the disease. This phenomenon is called anticipation and is paternal in origin if the father inherited the disease gene from his mother. There are also other diseases characterized by expansion of a repetitive sequence in the DNA (deoxyribonucleic acid) and developmental delay, such as fragile X.

Despite the discovery of the Huntington disease gene, scientists were baffled by how this genetic defect produces such a devastating disease course. The Huntington disease gene codes for a large protein with no similarities to known proteins. It has been named the huntingtin (htt) protein. It is important for normal development of the nervous system and interacts with many other proteins. Through autopsy, it was shown that an abnormality in the huntingtin protein caused the destruction of brain cells in the basal ganglia, a region of the brain with unknown functions. The mutated form of Htt, which causes the disease, is named mHtt. Using genetic engineering, scientists have developed strains of mice that express the Huntington disease gene. These mice display the symptoms of the disease. It has been found that the huntingtin protein, normally present in the cytoplasm (internal fluid-like content) of cells, collects in the brain cell nuclei, forming masses that kill the cell. This dominant-negative effect explains why clinically asymptomatic patients develop progressive neurodegeration of the brain in the fourth decade of life.

The quest for the Huntington disease gene

The quest for the Huntington disease gene was made possible by a new era in medicine and biology. The researcher who found the first genetic marker for the disease used a novel scientific approach. Much of the credit for the discovery of the gene belongs to Nancy S. Wexler (1945-), a American clinical psychologist who organized and championed the gene hunt with unflagging enthusiasm, in part due to the fact that she had a positive family history of the disease.

In 1968, at the age of 23 years, Nancy and her sister, Alice, learned from their father, Milton Wexler, that their mother had been diagnosed with Huntington disease. With their mothers diagnosis, Nancy and Alice had a 50% risk of developing the disease themselves. Milton Wexler, a lawyer and psychoanalyst, later founded the Hereditary Disease Foundation. The foundation worked to attract scientists to the study Huntington disease. It formed a board of scientific advisors, held conferences, and funded workshops particularly for younger scientists. It successfully urged the U.S. Congress to appropriate money for the study of the disease. Nancy Wexler, a graduate student, became increasingly involved in her fathers foundation and eventually became president.

Lake Maracaibo

In 1972, Wexler learned of several large, interrelated families affected with Huntington disease who lived in small villages along Lake Maracaibo in Venezuela. Wexler realized that this was a unique and valuable resource due to the large family pedigree. The larger the family tree, the easier it is to find genes by linking their location on the chromosome to specific DNA markers within the genome. In 1979, she began making annual trips to Lake Maracaibo. With the help of a team of investigators, she created a genealogy of the families and, beginning in 1981, took blood samples from both sick and the healthy family members. Wexler was convinced that the key to Huntington disease lay locked in the DNA of these families.

In 1983, James Gusella, a young scientist at Massachusetts General Hospital, began applying a new technique of molecular biology to the blood samples from Venezuela. He was looking for patterns that were present in the DNA sequences of people with Huntington disease but absent in the DNA of people without the disease. If one particular pattern of DNA was always associated with the illness in a given family, then it could be used as a marker for the disease gene. For instance, if it were true that people who developed the disease always had green eyes, then scientists could say that the gene that is responsible for green eyes located in a position along the DNA strand that is close to the gene that causes the disease. This genetic evaluation is called linkage analysis.

Eventually, Gusella found a genetic marker for Huntington disease, and remarkably it was almost immediate. Although the gene itself was still unknown, the discovery of this genetic marker made it possible to create a genetic test for the disease (linkage analysis), in the following year. By studying blood samples from several family members, persons who had a parent die of Huntington disease could be told whether or not they had inherited the genetic marker linked to the disease in their family. Other scientific teams also began using linkage analysis to search for disease genes as a result of these studies.

Even though the discovery of the marker indicated to scientists the general location of the gene itself, that gene hunt proceeded slowly. At Wexlers urging, and in a break from usual scientific practice, a consortium of six scientific teams worked together to find the gene. Finally, on March 26, 1993, in the scientific journal Cell, the 58 members of the Huntingtons Collaborative Research Group announced to the world the discovery of the gene that causes Huntington disease. In June 2006, researchers from Merck Labs and the University of British Columbia announced that the neurological degeneration caused by mHtt is associated with the caspase-6 enzyme, which cleaves the Htt protein.

Ethical questions

The genetic test for Huntington disease raises profound ethical questions. It offers people who are at risk the opportunity to know whether they inherited the

KEY TERMS

Dominant gene An allele of a gene that results in a visible phenotype if expressed in a heterozygote.

Genetic marker DNA segment that can be linked to an identifiable trait, although it is not the gene for that trait.

Nucleotides Building blocks of DNA: a phosphate and a sugar attached to one of the bases, adenine (A), cytosine (C), guanine (G), or thymine (T).

RFLP (restriction fragment length polymorphism) A variation in the DNA sequence, identifiable by restriction enzymes.

gene. Yet many people at risk choose not to be tested. Currently, no treatment existed to cure Huntington disease or even to delay the onset of the disease. Given this reality, many people would rather live with uncertainty than take the chance of learning that they will develop an incurable, fatal illness. Additionally, an ethical dilemma arises in cases where a grandson or granddaughter desires testing but their parent does not. If a grandparent is affected and the grandchild is affected, then by default the parent that is biological related to both is affected. Other concerns related to genetic testing of Huntington disease involves guilt associated with not having the disease gene when a sibling is a carrier. The parent that is responsible for passing the disease gene to their offspring also commonly experiences guilt. These emotional experiences can have a profound effect on the family dynamics.

Prenatal testing, now offered for several genetic diseases, is also available to parents whose fetus is at risk for Huntington disease. Genetic testing also raises the right to privacy. Do employers, health insurers, or the government have the right to know whether a person at risk has been tested, or the right to know the results of the test? Most researchers and ethicists, including Wexler, promote the need for privacy. These ethical questions are not unique to Huntington disease.

As the genetic components of other illnesses are discovered, especially for late-onset illnesses like Alzheimer disease and certain cancers, these questions will become more relevant and pressing. In many ways, the implementation of the genetic test for Huntington disease may serve as a model for how genetic testing is used in medicine and impacts society.

See also Genetics; Gene therapy.

Resources

BOOKS

Bates, Gillian, Peter S. Harper, and Lesley Jones, eds. Huntingtons Disease. Oxford, UK, and New York: Oxford University Press, 2002.

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia, PA: Saunders, 2001.

Rimoin, David L. Emery and Rimoins Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

OTHER

Online Mendelian Inheritance in Man. 143100 HUNTINGTON DISEASE; HD. December 17, 2002. <http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=143100> (accessed November 15, 2006).

Liz Marshall

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Huntington Disease

Huntington disease

Huntington disease is a rare, incurable genetic disease that results in the progressive degeneration of both physical and mental abilities. Huntington disease was formly known as Huntington chorea since the most obvious symptoms involve uncontrollable body movements known as chorea. As the disease progresses, its symptoms worsen and patients eventually die of respiratory failure or complications related to the neurodegenerative progression of the disease. Huntington disease is a late onset disorder, where affected individuals usually become symptomatic after 40 years of age. A genetic test for the disease is available, and its use brings to the forefront ethical and social issues related to the clinical diagnosis , particularly in the absence of a cure.


History

Huntington disease is named for physician George Huntington, who described the illness in an 1872 paper titled "On Chorea." Huntington practiced medicine on the eastern tip of Long Island, New York. His description of the disease was drawn from his familiarity with several affected families in his community. Both Huntington's father and grandfather had practiced medicine in the same area. Their encounters with the disease gave Huntington an appreciation of the heredity aspect of the illness.

Historically, the mental and emotional deterioration that marks the illness has frequently led to the confinement of Huntington disease patients to psychiatric hospitals. Some historians speculate that a few of the women accused of witchcraft in Salem, Massachusetts may have exhibited the involuntary twitches and turns that are hallmarks of the disease.


Symptoms

The symptoms in Huntington disease begin with noticeable behavioral changes including aggression, paranoia, and irritability. Affected individuals may seem restless, with tapping feet or odd twitches. Patients begin to suffer from impaired judgment and an inability to be organized. They become forgetful and their I.Q. declines, coinciding with the deterioration of the brain . Emotionally, they may suffer from psychiatric disorders and even suicidal thoughts or actions. They may drop things and become less efficient in their usual activities. Depression , anxiety , and apathy are also common experiences in the beginning stages of the disease. As the illness progresses, the chorea worsens. The entire body moves in uncoordinated, jerky movements.

Although wide variations in clinical manifestations exist, the illness typically lasts 13–16 years. In later stages of the illness, patients cannot walk or care for themselves. They may barely speak and may fail to recognize friends and family. They eventually require full time nursing care. Eating is quite difficult and death is very frequently caused by choking or by the pneumonia that results after accidentally inhaling bits of food. Although some symptoms can be treated with medication, currently no cure exists to delay the onset of Huntington disease or to slow its course.

Huntington disease is a hereditary disease caused by a dominant gene and, therefore, follows an autosomal dominant pattern of inheritance. This means that only one copy of the gene is necessary to cause the disease. It is transmitted from one generation to the next. The child of a mother or father suffering from Huntington disease has a 50% chance of inheriting the disease gene and, thus, of contracting the disease later in life. About 30,000 Americans suffer from Huntington disease and another 150,000 are at risk for developing it. One of the best-known disease victims was an American folk singer Woody Guthrie (1912–1967), who died of the disease.


Genetic defect responsible for disease

In 1993, scientists discovered the genetic defect that causes Huntington disease. A gene located on the chromosome 4 normally contains a sequence of three nucleotide bases (the alphabet of the genetic code) that repeats several times. The sequence is cytosine, adenine, and guanine, or CAG, which codes for the amino acid glutamine that is a building block for protein synthesis. In Huntington disease, patients have too many repeats. While unaffected individuals normally have 11-24 repeats, a person with Huntington disease may have anywhere from 36-100 or more repeats. Clinical research studies have demonstrated that the greater the number of repeats, the earlier the disease will develop and the clinical manifestations will be more severe. If the expanded trinucleotide sequence is passed from the father to the offspring, the offspring that inherit this expansion can have an earlier age of onset of the disease. This phenomenon is called anticipation and is paternal in origin if the father inherited the disease gene from his mother. There are also other diseases characterized by expansion of a repetitive sequence in the DNA and developmental delay, such as fragile X.

Despite the discovery of the Huntington disease gene, scientists were baffled by how this genetic defect produces such a devastating disease course. The Huntington disease gene codes for a large protein with no similarities to known proteins . It has been named the huntintin protein. It is important for normal development of the nervous system and interacts with many other proteins. Through autopsy, it was shown that an abnormality in the huntingtin protein caused the destruction of brain cells in the basal ganglia, a region of the brain with unknown functions. Using genetic engineering , scientists have developed strains of mice that express the Huntington disease gene. These mice display the symptoms of the disease. It has been found that the huntington protein, normally present in the cytoplasm (internal fluid-like content) of cells, collects in the brain cell nuclei, forming masses that kill the cell. This dominant-negative effect explains why clinically asymptomatic patients develop progressive neurodegeration of the brain in the fourth decade of life.

The quest for the Huntington disease gene

The quest for the Huntington disease gene was made possible by a new era in medicine and biology . The researcher who found the first genetic marker for the disease used a novel scientific approach. Much of the credit for the discovery of the gene belongs to Nancy Wexler, a American clinical psychologist who organized and championed the gene hunt with unflagging enthusiasm, in part due to the fact that she had a positive family history of the disease.

In 1968, at the age of 23, Nancy and her sister, Alice, learned from their father, Milton Wexler, that their mother had been diagnosed with Huntington disease. With their mother's diagnosis, Nancy and Alice had a 50% risk of developing the disease themselves. Milton Wexler, a lawyer and psychoanalyst, later founded the Hereditary Disease Foundation. The foundation worked to attract scientists to the study Huntington disease. It formed a board of scientific advisors, held conferences, and funded workshops particularly for younger scientists. It successfully urged Congress to appropriate money for the study of the disease. Nancy Wexler, a graduate student, became increasingly involved in her father's foundation and eventually became president.


Lake Maracaibo

In 1972, Wexler learned of several large, interrelated families affected with Huntington disease who lived in small villages along Lake Maracaibo in Venezuela. Wexler realized that this was a unique and valuable resource due to the large family pedigree. The larger the family tree , the easier it is to find genes by linking their location on the chromosome to specific DNA markers within the genome . In 1979, she began making annual trips to Lake Maracaibo. With the help of a team of investigators, she created a genealogy of the families and, beginning in 1981, took blood samples from both sick and the healthy family members. Wexler was convinced that the key to Huntington disease lay locked in the DNA of these families.

In 1983, James Gusella, a young scientist at Massachusetts General Hospital, began applying a new technique of molecular biology to the blood samples from Venezuela. He was looking for patterns that were present in the DNA sequences of people with Huntington disease but absent in the DNA of people without the disease. If one particular pattern of DNA was always associated with the illness in a given family, then it could be used as a marker for the disease gene. For instance, if it were true that people who developed the disease always had green eyes, then scientists could say that the gene that is responsible for green eyes located in a position along the DNA strand that is close to the gene that causes the disease. This genetic evaluation is called linkage analysis.

Eventually, Gusella found a genetic marker for Huntington disease, and remarkably it was almost immediate. Although the gene itself was still unknown, the discovery of this genetic marker made it possible to create a genetic test for the disease (linkage analysis), in the following year. By studying blood samples from several family members, persons who had a parent die of Huntington disease could be told whether or not they had inherited the genetic marker linked to the disease in their family. Other scientific teams also began using linkage analysis to search for disease genes as a result of these studies.

Even though the discovery of the marker indicated to scientists the general location of the gene itself, that gene hunt proceeded slowly. At Wexler's urging, and in a break from usual scientific practice, a consortium of six scientific teams worked together to find the gene. Finally, on March 26, 1993, in the scientific journal Cell, the 58 members of the Huntington's Collaborative Research Group announced to the world the discovery of the gene that causes Huntington disease.


Ethical questions

The genetic test for Huntington disease raises profound ethical questions. It offers people who are at risk the opportunity to know whether they inherited the gene. Yet many people at risk choose not to be tested. Currently, no treatment existed to cure Huntington disease or even to delay the onset of the disease. Given this reality, many people would rather live with uncertainty than take the chance of learning that they will develop an incurable, fatal illness. Additionally, an ethical dilemma arises in cases where a grandson or granddaughter desires testing but their parent does not. If a grandparent is affected and the grandchild is affected, then by default the parent that is biological related to both is affected. Other concerns related to genetic testing of Huntington disease involves guilt associated with not having the disease gene when a sibling is a carrier. Guilt is also commonly experienced by the parent that is responsible for passing the disease gene to their offspring. These emotional experiences can have a profound effect on the family dynamics.

Prenatal testing, now offered for several genetic diseases, is also available to parents whose fetus is at risk for Huntington disease. Genetic testing also raises the right to privacy. Do employers, health insurers, or the government have the right to know whether a person at risk has been tested, or the right to know the results of the test? Most researchers and ethicists, including Wexler, promote the need for privacy. These ethical questions are not unique to Huntington disease.

As the genetic components of other illnesses are discovered, especially for late-onset illnesses like Alzheimer disease and certain cancers, these questions will become more relevant and pressing. In many ways, the implementation of the genetic test for Huntington disease may serve as a model for how genetic testing is used in medicine and impacts society.

See also Genetics; Gene therapy.


Resources

books

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

periodicals

Revkin, Andrew. "Hunting Down Huntington's." Discover (December 1993).

other

Online Mendelian Inheritance in Man. "143100 HUNTING TON DISEASE; HD." December 17, 2002 [cited January 10, 2003]. <http://www. ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?143100>.

Liz Marshall

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dominant gene

—An allele of a gene that results in a visible phenotype if expressed in a heterozygote.

Genetic marker

—DNA segment that can be linked to an identifiable trait, although it is not the gene for that trait.

Nucleotides

—Building blocks of DNA: a phosphate and a sugar attached to one of the bases, adenine (A), cytosine (C), guanine (G), or thymine (T).

RFLP (restriction fragment length polymorphism)

—A variation in the DNA sequence, identifiable by restriction enzymes.

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Huntington Disease

Huntington disease

Definition

Huntington disease is a progressive, neurodegenerative disease causing uncontrolled physical movements and mental deterioration. The disease was discovered by George Huntington of Pomeroy, Ohio, who first described a hereditary movement disorder.

Description

Huntington disease is also called Huntington chorea, from the Greek word for "dance," referring to the involuntary movements that develop as the disease progresses. It is occasionally referred to as "Woody Guthrie disease" for the American folk singer who died from it. Huntington disease (HD) causes progressive loss of cells in areas of the brain responsible for some aspects of movement control and mental abilities. A person with HD gradually develops abnormal movements and changes in cognition (thinking), behavior and personality.

Demographics

The onset of symptoms of HD is usually between the ages of 30 and 50; although in 10% of cases, onset is in late childhood or early adolescence. Approximately

30,000 people in the United States are affected by HD, with another 150,000 at risk for developing this disorder. The frequency of HD is four to seven per 100,000 persons.

Genetic profile

Huntington disease is caused by a change in the gene (an inherited unit which contains a code for a protein) of unknown function called huntingtin. The nucleotide codes (building blocks of genes arranged in a specific code that chemically form proteins), contain CAG repeats (40 or more of these repeat sequences). The extra building blocks in the huntingtin gene cause the protein that is made from it to contain an extra section as well. It is currently thought that this extra protein section, or portion, interacts with other proteins in brain cells where it occurs, and that this interaction ultimately leads to cell death.

The HD gene is a dominant gene, meaning that only one copy of it is needed to develop the disease. HD affects both males and females. The gene may be inherited from either parent, who will also be affected by the disease. A parent with the HD gene has a 50% chance of passing it on to each offspring. The chances of passing on the HD gene are not affected by the results of previous pregnancies.

Signs and symptoms

The symptoms of HD fall into three categories: motor or movement symptoms, personality and behavioral changes, and cognitive decline. The severity and rate of progression of each type of symptom can vary from person to person.

Early motor symptoms include restlessness, twitching and a desire to move about. Handwriting may become less controlled, and coordination may decline. Later symptoms include:

  • Dystonia, or sustained abnormal postures, including facial grimaces, a twisted neck, or an arched back.
  • Chorea, in which involuntary jerking, twisting or writhing motions become pronounced.
  • Slowness of voluntary movements, inability to regulate the speed or force of movements, inability to initiate movement, and slowed reactions.
  • Difficulty speaking and swallowing due to involvement of the throat muscles.
  • Localized or generalized weakness and impaired balance ability.
  • Rigidity, especially in late-stage disease.

Personality and behavioral changes include depression , irritability, anxiety and apathy. The person with HD may become impulsive, aggressive, or socially withdrawn.

Cognitive changes include loss of ability to plan and execute routine tasks, slowed thought, and impaired or inappropriate judgment. Short-term memory loss usually occurs, although long-term memory is usually not affected. The person with late-stage HD usually retains knowledge of his environment and recognizes family members or other loved ones, despite severe cognitive decline.

Diagnosis

Diagnosis of HD begins with a detailed medical history, and a thorough physical and neurological exam. Family medical history is very important. Magnetic resonance imaging (MRI) or computed tomography scan (CT scan) imaging may be performed to look for degeneration in the basal ganglia and cortex, the brain regions most affected in HD.

A genetic test is available for confirmation of the clinical diagnosis. In this test, a small blood sample is taken, and DNA from it is analyzed to determine the CAG repeat number. A person with a repeat number of 30 or below will not develop HD. A person with a repeat number between 35 and 40 may not develop the disease within their normal life span. A person with a very high number of repeats (70 or above) is likely to develop the juvenile-onset form. An important part of genetic testing is extensive genetic counseling .

Prenatal testing is available. A person at risk for HD (a child of an affected person) may obtain fetal testing without determining whether she herself carries the gene. This test, also called a linkage test, examines the pattern of DNA near the gene in both parent and fetus, but does not analyze for the triple nucleotide repeat (CAG). If the DNA patterns do not match, the fetus can be assumed not to have inherited the HD gene, even if present in the parent. A pattern match indicates the fetus probably has the same genetic makeup of the at-risk parent.

Treatment and management

There is no cure for HD, nor any treatment that can slow the rate of progression. Treatment is aimed at reducing the disability caused by the motor impairments, and treating behavioral and emotional symptoms.

Physical therapy is used to maintain strength and compensate for lost strength and balance. Stretching and range of motion exercises help minimize contracture, or muscle shortening, a result of weakness and disuse. The physical therapist also advises on the use of mobility aids such as walkers or wheelchairs.

Motor symptoms may be treated with drugs, although some studies suggest that anti-chorea treatment rarely improves function. Chorea (movements caused by abnormal muscle contractions) can be suppressed with drugs that deplete dopamine, an important brain chemical regulating movement. As HD progresses, natural dopamine levels fall, leading to loss of chorea and an increase in rigidity and movement slowness. Treatment with L-dopa (which resupplies dopamine) may be of some value. Frequent reassessment of the effectiveness and appropriateness of any drug therapy is necessary.

Occupational therapy is used to design compensatory strategies for lost abilities in the activities of daily living, such as eating, dressing, and grooming. The occupational therapist advises on modifications to the home that improve safety, accessibility, and comfort.

Difficulty swallowing may be lessened by preparation of softer foods, blending food in an electric blender, and taking care to eat slowly and carefully. Use of a straw for all liquids can help. The potential for choking on food is a concern, especially late in the disease progression. Caregivers should learn the use of the Heimlich maneuver. In addition, passage of food into the airways increases the risk for pneumonia. A gastric feeding tube may be needed, if swallowing becomes too difficult or dangerous.

Speech difficulties may be partially compensated by using picture boards or other augmentative communication devices. Loss of cognitive ability affects both speech production and understanding. A speech-language pathologist can work with the family to develop simplified and more directed communication strategies, including speaking slowly, using simple words, and repeating sentences exactly.

Early behavioral changes, including depression and anxiety, may respond to drug therapy. Maintaining a calm, familiar, and secure environment is useful as the disease progresses. Support groups for both patients and caregivers form an important part of treatment.

Experimental transplant of fetal brain tissue has been attempted in a few HD patients. Early results show some promise, but further trials are needed to establish the effectiveness of this treatment.

Prognosis

The person with Huntington disease may be able to maintain a job for several years after diagnosis, despite the increase in disability. Loss of cognitive functions and increase in motor and behavioral symptoms eventually prevent the person with HD from continuing employment. Ultimately, severe motor symptoms prevent mobility. Death usually occurs 15–20 years after disease onset. Progressive weakness of respiratory and swallowing muscles leads to increased risk of respiratory infection and choking, the most common causes of death. Future research in this area is currently focusing on nerve cell transplantation.

Resources

BOOK

Watts R. L., and W. C. Koller, eds. Movement Disorders. New York: McGraw-Hill, 1997.

ORGANIZATION

Huntington Disease Society of America. 140 W. 22nd St. New York, NY 10011. (800) 345-HDSA.

Laith Gulli, MD

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