Soon after the HD gene was found in 1993, a test was devloped which enabled people to find out if they were carrying the mutated HD gene that causes HD. Earlier tests were based on a process of “linkage analysis” which required blood samples from several family members. The new “direct gene test” is much more accurate and requires blood only from the individual taking the test.
The HD gene was found to contain a specific section that was expanded in people with HD. In all people, this stretch of genetic material, or DNA, contains a pattern of so-called “trinucleotide repeats”. Nucleotides are the building blocks of DNA and are represented by the letters C,A,G and T ( Cytosine, Adenine, Guanide and Thymine). In most people, the repeated pattern “CAG” occurs 30 times or less. In HD, it occurs more than 36 times. There are people with a CAG repeat in the so called “grey zone”, between 30 and 36 repeats. If the person is a male, than it could be possible that his child(ren) has/have more than 36 repeats and will develop the disease. At the moment is not sure that somebody “high in the grey zone” may get the disease at a late age (above 60 or later) or not.
By analyzing a person’s DNA and counting the number of CAG’s, it is possible to tell if that person will develop HD. The test cannot predict the age of onset, but the juvenile form has normally more than 50 repeats upto more than 120.
The decision to undergo genetic testing is an intensely personal one and one that cannot be taken lightly. Everyone has their own circumstances to take into consideration, and there are no “right” or “wrong” answers. Testing should never be forced upon an at-risk individual. Children are generally not able to consider the full implications of testing and may be vulnerable to pressure from others. Therefore, the minimum age requirement is usually 18.
Various resources are or should be available to asssist you in making this decision. HD societies, social workers, genetic counselors at testing centers can help. Testing should only be done at (academic) testing centers where teams of professionals who are knowledgeable about HD are available.
The testing procedure involves sessions with various professionals. It typically includes one session devoted to each of the following: genetic counseling, a neurologic exam, a psychological interview, discussion of the results and follow-up. The genetic test itself is a blood test.
The purpose of the preliminary sessions is to ensure that the person understands the potential implications of his/her genetic status and is prepared to receive the results. The neurologic exam will determine if any early symptoms of HD are present. If the person is found to be symptomatic, he/she will be offered the option of discontinuing the testing procedure.
It is important to note that presymptomatic testing for HD cannot dertermine when the disease will begin or its course or severity. People who test positive for the gene may remain healthy for many years. HD can only be diagnosed by a neurological exam.
Recognizing that medical, ethical, legal and social issues arise from the availibility of predictive testing, recommendations concerning the use of a predictive test for the detection of HD were drawn up by a committee consisting of representatives of the International Huntington Asociation and the Research Group on Huntington’s Disease of the Worlf Federation of Neurology. These Guidelines, are approved and published in Neurology 1994; 44: 1533-1536 and J. Medical Genetics 1994; 31: 555-559