Huntington Disease (cont.)
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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.
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