WBR0098
| Author | PageAuthor::William J Gibson |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Genetics |
| Sub Category | SubCategory::Neurology |
| Prompt | [[Prompt::A 36 year old woman presents to her physician for the gradual onset of uncontrolled spastic movements. She reports having begun experiencing mood disturbances and difficulty performing her job at work 3 months prior to the onset of her movement disorder began. Further questioning reveals that her father died of a neurodegenerative condition at the age of 50. The patient has researched her likely condition on the internet and is most concerned about having passed the trait onto her kids. She inquires about genetic testing. Which of the following would one expect to find in this patient?]] |
| Answer A | AnswerA::Loss of function mutation |
| Answer A Explanation | AnswerAExp::Huntington’s disease is caused by a trinucleotide expansion |
| Answer B | AnswerB::Expansion of CAA repeats |
| Answer B Explanation | AnswerBExp::This repeat is expanded in Friedreich’s ataxia |
| Answer C | AnswerC::Expansion of CAG repeats |
| Answer C Explanation | AnswerCExp:: |
| Answer D | AnswerD::Expansion of CGG repeats |
| Answer D Explanation | AnswerDExp::This repeat is expanded in Fragile X Syndrome |
| Answer E | AnswerE::Expansion of CTG repeats |
| Answer E Explanation | AnswerEExp::This repeat is expanded in Myotonic dystrophy |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::The patient in this vignette has Huntington’s disease, a uniformly fatal neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline. Symptoms manifest in affected individuals between the ages of 20 and 50. Clinical findings include depression, progressive dementia, choreiform movements, caudate atrophy and decreased levels of both GABA and acetylcholine in the brain. It is a trinucleotide repeat disorder caused by (CAG)n repeats in the Huntingtin gene located on chromosome 4.
Expansion of this CAG triplet repeat stretch within the Huntingtin gene results in a different (mutant) form of the protein that gradually damages cells in the brain through mechanisms that are not fully understood. The disease is inherited in an autosomal dominant manner and displays genetic anticipation. This anticipation is thought to be caused by polymerase "slippage" during DNA replication in germ cells, thereby increasing the length of the repeat segment. The common neuropathology in Huntington's disease occurs within the neostriatum, in which gross atrophy of the caudate and putamen nuclei occurs and accompanied by selective neuronal loss and astrogliosis. Wiki-mnemomic: You hunt animals and put them in the CAGe: Huntington Disease is a CAG repeat disorder. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Huntington's disease, WBRKeyword::Huntington disease, WBRKeyword::Genetics, WBRKeyword::Inherited, WBRKeyword::Anticipation, WBRKeyword::trinucleotide repeat disorders, WBRKeyword::Inheritance, WBRKeyword::Neurodegenerative |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |