DOCTOR'S VIEW ARCHIVE
Although Huntington disease (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 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, the U.S. 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 support for research, largely through the National Institutes of Health (NIH).
Research into Huntington's disease includes the following:
•Clinical research. Neurologists, psychologists, psychiatrists, and other investigators are improving our understanding of patients' symptoms and progression of the disease while attempting to develop new treatments.
•Imaging. Scientific investigations using specialized technologies are enabling scientists to visualize what the defective gene does to structures and chemicals in the brain.
•Animal models. Laboratory animals are used to study features of HD.
•Fetal tissue research. Investigators are implanting fetal tissue in rodents and nonhuman primates to understand and correct nerve cell degeneration.
For 10 years, scientists focused on a segment of chromosome 4 and, in 1993, finally isolated the HD gene. Scientists anticipate that identifying the location of the HD gene will be a major step toward finding a cure.
Investigators searching for the HD gene evaluated the largest known kindred with HD, 14,000 individuals who live on Lake Maracaibo in Venezuela. By studying these people, scientists can detect patterns of inheritance in interrelated families.
Although scientists know that certain brain cells die in HD, the cause of their death is still largely unknown. The HD gene produces an abnormal version of a protein--which has been (rather confusingly) named huntingtin. The abnormal huntingtin protein causes damage to certain parts of the brain.
The huntingtin protein is actually 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 are sensitive to injury by this abnormal protein.
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:
- Overstimulation of cells by natural chemicals found in the brain;
- A defect in the power plant of the cell, called mitochondria, where energy is produced;
- Normal metabolism in the brain that produces toxic compounds called free radicals; and
- 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 that transmit and receive information. Nerve cells in the inner brain area called the striatum are classified both by their size (large, medium, or small) and appearance (spiny or aspiny). The hallmark of HD, scientist are learning, is selective degeneration of medium-sized spiny nerve cells in the striatum. Studies also suggest that losses of certain types of nerve cells and receptors are responsible for the different symptoms and stages of HD.
Clinical studies of patients are in progress to develop new drugs or other treatments to halt the disease's progression. Examples of investigations, using both asymptomatic and symptomatic individuals, include:
•Genetic studies on the age of disease onset, inheritance patterns, and markers found within families. Further genetic studies may shed additional light on how HD is passed from generation to generation.
•Studies of thinking, intelligence, and movement. Studies of abnormal eye movements, 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. Classes of drugs being tested for the treatment of HD in patients include those that control symptoms, slow the rate of disease progression, and those that might correct or replace chemical imbalances in the brain that contribute to the development and progression of HD.
A specialized imaging technique, positron emission tomography (PET), is being used to learn how the gene affects the chemical systems of the body. PET visualizes metabolic or chemical abnormalities of tissue in the body. Investigators conducting HD research are also using PET to characterize nerve cells that have died and chemicals that are depleted in parts of the brain that are 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 nerve cells 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.
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 also provide a means to test the safety of new classes of drugs in nonhuman animals, including primates.
For example, the altered human HD gene is transferred into mouse embryos so that the animals will develop the anatomical and biological characteristics of HD.
Scientists are exploring the possibility of replacing tissue that has degenerated with implants of fresh, fetal tissue, taken at the very early stages of development. Extensive animal studies will be required to learn if this technique could be of value in humans with HD.
This report is based on information from the National Institutes of Health (NIH).