Huntington Disease (cont.)
What research is being done on Huntington's disease?
Although HD attracted considerable attention from scientists in the early 20th century, there was little sustained research on the disease until the late 1960s when the Committee to Combat Huntington's Disease and the Huntington's Chorea Foundation, later called the Hereditary Disease Foundation, first began to fund research and to campaign for federal funding. In 1977, Congress established the Commission for the Control of Huntington's Disease and Its Consequences, which made a series of important recommendations. Since then, Congress has provided consistent support for federal research, primarily through the National Institute of Neurological Disorders and Stroke, the government's lead agency for biomedical research on disorders of the brain and nervous system. The effort to combat HD proceeds along the following lines of inquiry, each providing important information about the disease:
Basic neurobiology. Now that the HD gene has been located, investigators in the field of neurobiology-which encompasses the anatomy, physiology, and biochemistry of the nervous system-are continuing to study the HD gene with an eye toward understanding how it causes disease in the human body.
Clinical research. Neurologists, psychologists, psychiatrists, and other investigators are improving our understanding of the symptoms and progression of the disease in patients while attempting to develop new therapeutics.
Imaging. Scientific investigations using PET and other technologies are enabling scientists to see what the defective gene does to various structures in the brain and how it affects the body's chemistry and metabolism.
Animal models. Laboratory animals, such as mice, are being bred in the hope of duplicating the clinical features of HD and can soon be expected to help scientists learn more about the symptoms and progression of the disease.
Fetal tissue research. Investigators are implanting fetal tissue in rodents and nonhuman primates with the hope that success in this area will lead to understanding, restoring, or replacing functions typically lost by neuronal degeneration in individuals with HD.
These areas of research are slowly converging and, in the process, are yielding important clues about the gene's relentless destruction of mind and body. The NINDS supports much of this exciting work.
Molecular Genetics
For 10 years, scientists focused on a segment of
chromosome 4 and, in 1993, finally isolated the HD gene. The process of
isolating the responsible gene - motivated by the desire to find a cure - was more difficult than anticipated. Scientists now believe that identifying the location of the HD gene is the first step on the road to a cure.
Finding the HD gene involved an intense molecular
genetics research effort with cooperating investigators from around the globe.
In early 1993, the collaborating scientists announced they had isolated the
unstable triplet repeat DNA sequence that has the HD gene. Investigators relied on the NINDS-supported Research Roster for Huntington's Disease, based at Indiana University in Indianapolis, to accomplish this work. First started in 1979, the roster contains data on many American families with HD, provides statistical and demographic data to scientists, and serves as a liaison between investigators and specific families. It provided the DNA from many families affected by HD to investigators involved in the search for the gene and was an important component in the identification of HD markers.
For several years, NINDS-supported investigators involved in the search for the HD gene made yearly visits to the largest known kindred with HD
- 14,000 individuals - who live on Lake Maracaibo in Venezuela. The continuing trips enable scientists to study inheritance patterns of several interrelated families.
The HD Gene and Its Product
Although scientists know that certain brain cells die in
HD, the cause of their death is still unknown. Recessive diseases are usually
thought to result from a gene that fails to produce adequate amounts of a
substance essential to normal function. This is known as a loss-of-function
gene. Some dominantly inherited disorders, such as HD, are thought to involve a
gene that actively interferes with the normal function of the cell. This is known as a gain-of-function gene.
How does the defective HD gene cause harm? The HD gene
encodes a protein - which has been named huntingtin
- the function of which is as yet unknown. The repeated CAG sequence in the gene causes an abnormal form of huntingtin to be made, in which the amino acid glutamine is repeated. It is the presence of this abnormal form, and not the absence of the normal form, that causes harm in HD. This explains why the disease is dominant and why two copies of the defective gene
- one from both the mother and the father - do not cause a more serious case than inheritance from only one parent. With the HD gene isolated, NINDS-supported investigators are now turning their attention toward discovering the normal function of huntingtin and how the altered form causes harm. Scientists hope to reproduce, study, and correct these changes in animal models of the disease.
Huntingtin is found everywhere in the body but only outside the cell's nucleus. Mice called "knockout mice" are bred in the laboratory to produce no huntingtin; they fail to develop past a very early embryo stage and quickly die. Huntingtin, scientists now know, is necessary for life. Investigators hope to learn why the abnormal version of the protein damages only certain parts of the brain. One theory is that cells in these parts of the brain may be supersensitive to this abnormal protein.
Cell Death in HD
Although the precise cause of cell death in HD is not yet known, scientists are paying close attention to the process of genetically programmed cell death that occurs deep within the brains of individuals with HD. This process involves a complex series of interlinked events leading to cellular suicide. Related areas of investigation include:
Excitotoxicity. Overstimulation of cells by natural chemicals found in the brain.
Defective energy metabolism. A defect in the power plant of the cell, called mitochondria, where energy is produced.
Oxidative stress. Normal
metabolic activity in the brain that produces toxic compounds called free radicals.
Trophic factors. Natural chemical substances found in the human body that may protect against cell death.
Several HD studies are aimed at understanding losses of nerve cells and receptors in HD. Neurons in the striatum are classified both by their size (large, medium, or small) and appearance (spiny or aspiny). Each type of neuron contains combinations of neurotransmitters. Scientists know that the destructive process of HD affects different subsets of neurons to varying degrees. The hallmark of HD, they are learning, is selective degeneration of medium-sized spiny neurons in the striatum. NINDS-supported studies also suggest that losses of certain types of neurons and receptors are responsible for different symptoms and stages of HD.
What do these changes look like? In spiny neurons, investigators have observed two types of changes, each affecting the nerve cells' dendrites. Dendrites, found on every nerve cell, extend out
from the cell body and are responsible for receiving messages from other nerve
cells. In the intermediate stages of HD, dendrites grow out of control. New,
incomplete branches form and other branches become contorted. In advanced,
severe stages of HD, degenerative changes cause sections of dendrites to swell,
break off, or disappear altogether. Investigators believe that these alterations
may be an attempt by the cell to rebuild nerve cell contacts lost early in the
disease. As the new dendrites establish connections, however, they may in fact
contribute to nerve cell death. Such studies give compelling, visible evidence
of the progressive nature of HD and suggest that new experimental therapies must consider the state of cellular degeneration. Scientists do not yet know exactly how these changes affect subsets of nerve cells outside the striatum.
Animal Models of HD
As more is learned about cellular degeneration in HD, investigators hope to reproduce these changes in animal models and to find a way to correct or halt the process of nerve cell death. Such models serve the scientific community in general by providing a means to test the safety of new classes of drugs in nonhuman primates. NINDS-supported scientists are currently working to develop both nonhuman primate and mouse models to investigate nerve degeneration in HD and to study the effects of excitotoxicity on nerve cells in the brain.
Investigators are working to build genetic models of HD using transgenic mice. To do this, scientists transfer the altered human HD gene into mouse embryos so that the animals will develop the anatomical and biological characteristics of HD. This genetic model of mouse HD will enable in-depth study of the disease and testing of new therapeutic compounds.
Another idea is to insert into mice a section of DNA containing CAG repeats in the abnormal, disease gene range. This mouse equivalent of HD could allow scientists to explore the basis of CAG instability and its role in the disease process.
Fetal Tissue Research
A relatively new field in biomedical research involves the use of brain tissue grafts to study, and potentially treat, neurodegenerative disorders. In this technique, tissue that has degenerated is replaced with implants of fresh, fetal tissue, taken at the very early stages of development. Investigators are interested in applying brain tissue implants to HD research. Extensive animal studies will be required to learn if this technique could be of value in patients with HD.
Clinical Studies
Scientists are pursuing clinical studies that may one day lead to the development of new drugs or other treatments to halt the disease's progression. Examples of NINDS-supported investigations, using both asymptomatic and symptomatic individuals, include:
Genetic studies on age of onset, inheritance patterns, and markers found within families.
These studies may shed additional light on how HD is passed from generation to generation.
Studies of cognition, intelligence, and movement. Studies of abnormal eye movements, both horizontal and vertical, and tests of patients' skills in a number of learning, memory, neuropsychological, and motor tasks may serve to identify when the various symptoms of HD appear and to characterize their range and severity.
Clinical trials of drugs. Testing of various drugs may lead to new treatments and at the same time improve our understanding of the disease process in HD. Classes of drugs being tested include those that control symptoms, slow the rate of progression of HD, and block effects of excitotoxins, and those that might correct or replace other metabolic defects contributing to the development and progression of HD.
Imaging
NINDS-supported scientists are using positron emission
tomography (PET) to learn how the gene affects the chemical systems of the body. PET visualizes metabolic or chemical abnormalities in the body, and investigators hope to ascertain if PET scans can reveal any abnormalities that signal HD. Investigators conducting HD research are also using PET to characterize neurons that have died and chemicals that are depleted in parts of the brain affected by HD.
Like PET, a form of magnetic resonance imaging (MRI) called functional MRI can measure increases or decreases in certain brain chemicals thought to play a key role in HD. Functional MRI studies are also helping investigators understand how HD kills neurons in different regions of the brain.
Imaging technologies allow investigators to view changes
in the volume and structures of the brain and to pinpoint when these changes
occur in HD. Scientists know that in brains affected by HD, the basal ganglia,
cortex, and ventricles all show atrophy or other alterations.
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