Huntington disease (HD), an adult onset autosomal dominant disorder, is caused b
ID: 174221 • Letter: H
Question
Huntington disease (HD), an adult onset autosomal dominant disorder, is caused by the expansion of a CAG trinucleotide repeat. Individuals with HD have an allele with a repeat size of 36 or greater; however, alleles with 36-39 repeats demonstrate incomplete penetrance. Individuals in the general population with no family history of the disease, have alleles with 35 or fewer CAG repeats; however, alleles with 27-35 repeats are unstable and can expand into the pathogenic range during gametogenesis. Complete the following table indicating; If the individual with the indicated genotype will develop HD; If there is a risk that the repeat will expand during gametogenesis; If there is a risk that the individual will have a child that has/will develop HD. Can you predict the age of onset or disease severity based on genotype?Explanation / Answer
A) 23,49- intermediate allele (IA), or large normal allele in offsprings.
B)19,34- intermediate allele (IA), or large normal allele in offsprings.
c)18,25-26 or fewe- repeats: Not associated with HD.
d)21,38- incomplete or reduced penetrance allele (RPA). It may cause symptoms, usually later in the adult life. There is a maximum risk of 60% that a person with an RPA will be symptomatic at the age of 65 years, and a 70% risk of being symptomatic at the age of 75 years.
E)19,125- full penetrance allele (FPA)
40 or more CAG repeats: full penetrance allele (FPA). A "positive test" or "positive result" generally refers to this case. A positive result is not considered a diagnosis, since it may be obtained decades before the symptoms begin. However, a negative test means that the individual does not carry the expanded copy of the gene and will not develop HD. The test will tell a person who originally had a 50 percent chance of inheriting the disease if their risk goes up to 100 percent or is eliminated. A person who tests positive for the disease will develop HD sometime within their lifetime, provided he or she lives long enough for the disease to appear.
36 to 39 repeats: incomplete or reduced penetrance allele (RPA). It may cause symptoms, usually later in the adult life. There is a maximum risk of 60% that a person with an RPA will be symptomatic at the age of 65 years, and a 70% risk of being symptomatic at the age of 75 years.
27 to 35 repeats: intermediate allele (IA), or large normal allele. It is not associated with symptomatic disease in the tested individual, but may expand upon further inheritance to give symptoms in offspring.
26 or fewer repeats: Not associated with HD
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