Other Free Encyclopedias » Medicine Encyclopedia » Genetics in Medicine - Part 2 » Genetic Testing: Ethical Issues - Advances In Genetic Science, A Range Of Ethical Issues, Prenatal And Childhood Testing, Presymptomatic Testing: Huntington's Disease

Genetic Testing: Ethical Issues - Presymptomatic Testing: Huntington's Disease

clinical family diagnosis proband mother

Presymptomatic genetic testing when no treatment is available stands in contrast to genetic testing done for the diagnosis of a current disorder. Huntington's disease (HD) is an example of a genetic disorder in which clinical manifestations begin in adulthood. Although some symptoms can be managed with medications, no treatment is yet available to alter the gradual loss of muscle control, psychiatric changes, and progressive dementia.

HD is an autosomal dominant disorder, which means that an individual with only one abnormal copy of the HD gene will develop the disease, and the children of affected individuals have a 50 percent chance of inheriting the genetic mutation. The gene abnormality occurs when a polymorphic CAG repeat sequence is expanded beyond the normal number of 10 to 29 copies. Diagnosis of HD is almost 100 percent sure when the number of CAG repeats is in the range of 36 to 121.

Individuals at risk for HD may want to undergo genetic testing to end the uncertainty of not knowing whether they will be affected. This knowledge may allow for career and life planning. However, concerns that presymptomatic diagnosis of HD would lead to serious psychological distress or suicide led to the development of testing guidelines. The standard was established that all individuals who sought HD testing should receive pretest counseling to explain the test, assess their psychological status, and Figure 1. The person desiring the test, known as the proband (arrow; III,2), knows that her grandfather (solid colored square; I,1) had Huntington's Disease (HD). Her mother (II,2) is therefore at fifty percent risk for HD (shaded). If the proband desires to know whether she will develop HD, a direct genetic test can be performed. If the proband is tested and her HD gene is abnormal, then automatically she knows her mother's diagnosis as well, even if the mother does not want to know this information about herself. Similarly, if the proband's sister-in-law (III,4) desires to know the HD status of her fetus (IV), and the child tests positive, a diagnosis of HD could be obtained about her husband (III,3) as well as his mother (II,2) regardless of their preferences for knowing or not knowing their status. prepare them to consider their possible reactions to the results, whether positive or negative. In addition, results should be given in person, and counseling and support should be available following the test.

Before the identification of a specific gene for HD, genetic testing was done by linkage studies within a family. Linkage analysis required the voluntary participation of affected and unaffected family members across several generations. Identification of the specific gene defect now allows direct genetic analysis, without examining the DNA of any other family member. However, because of the inheritance patterns of HD, information about one family member can give diagnostic information about another. Figure 1 diagrams the ethical complexities arising from situations in which one family member's genetic test results in necessary implications for other family members. Note that it is not clear whether the individual desiring testing has decisive rights, or if those rights belong to those family members who do not desire such testing.

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