« How is the Presymptomatic Test Conducted? | Main | How is HD Diagnosed? »
November 02, 2003
What is Presymptomatic Testing?
Presymptomatic testing is used for people who have a family history of HD but have no symptoms themselves. If either parent had HD, the person's chance would be 50-50. In the past, no laboratory test could positively identify people carrying the HD gene—or those fated to develop HD—before the onset of symptoms. That situation changed in 1983, when a team of scientists supported by the NINDS located the first genetic marker for HD—the initial step in developing a laboratory test for the disease.
A marker is a piece of DNA that lies near a gene and is usually inherited with it. Discovery of the first HD marker allowed scientists to locate the HD gene on chromosome 4. The marker discovery quickly led to the development of a presymptomatic test for some individuals, but this test required blood or tissue samples from both affected and unaffected family members in order to identify markers unique to that particular family. For this reason, adopted individuals, orphans, and people who had few living family members were unable to use the test.
Discovery of the HD gene has led to a less expensive, scientifically simpler, and far more accurate presymptomatic test that is applicable to the majority of at-risk people. The new test uses CAG repeat length to detect the presence of the HD mutation in blood. This is discussed further in the next section.
There are many complicating factors that reflect the complexity of diagnosing HD. In a small number of individuals with HD—1 to 3 percent—no family history of HD can be found. Some individuals may not be aware of their genetic legacy, or a family member may conceal a genetic disorder from fear of social stigma. A parent may not want to worry children, scare them, or deter them from marrying. In other cases, a family member may die of another cause before he or she begins to show signs of HD. Sometimes, the cause of death for a relative may not be known, or the family is not aware of a relative's death. Adopted children may not know their genetic heritage, or early symptoms in an individual may be too slight to attract attention.
Prepared by:
Office of Communications and Public Liaison
National Institute of Neurological Disorders and Stroke
National Institutes of Health
Bethesda, MD 20892
NINDS health-related material is provided for information purposes only and does not necessarily represent endorsement by or an official position of the National Institute of Neurological Disorders and Stroke or any other Federal agency. Advice on the treatment or care of an individual patient should be obtained through consultation with a physician who has examined that patient or is familiar with that patient's medical history.
All NINDS-prepared information is in the public domain and may be freely copied. Credit to the NINDS or the NIH is appreciated.
Reviewed July 1, 2001
Posted by Dave at November 2, 2003 11:14 AM
Trackback Pings
TrackBack URL for this entry:
http://www.huntingtons.info/MT/mt-tb.cgi/164
Comments
Post a comment
Thanks for signing in, . Now you can comment. (sign out)
(If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)