WBR0310

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 25-year-old female presents to her primary care physician with complaints of generalized weakness, which is particularly pronounced in her hands, feet, and neck. On further questioning, she reports some difficulty swallowing the multivitamins she takes daily, and greater than normal daytime sleepiness. Medical history is significant only for occasional gastrointestinal distress over the past 5 years. You note that the patient is unable to release her grip after shaking hands. Which of the following techniques would allow a pathologist to confirm the diagnosis from a sample of the patient’s DNA?]]
Answer A	[[AnswerA::Northern blot; 5’(CAG)n3’ probe]]
Answer A Explanation	AnswerAExp::Northern blots are used to detect RNA, not DNA. The patient’s RNA is subjected to electrophoresis and probed with a labeled DNA probe.
Answer B	[[AnswerB::Northern blot; 3’(CTG)n5’ probe]]
Answer B Explanation	AnswerBExp::Northern blots are used to detect RNA, not DNA. The patient’s RNA is subjected to electrophoresis and probed with a labeled DNA probe.
Answer C	[[AnswerC::Southern blot; 5’(CAG)n3’ probe]]
Answer C Explanation	[[AnswerCExp::Southern blots are used to detect the presence of certain sequences of DNA. The probe 5’(CAG) 3’ is the reverse complement of the 5‘CTG 3’ repeat responsible for myotonic dystrophy. Coincidentally, 5’ CAG 3’ is the repeat responsible for Huntington’s disease.]]
Answer D	[[AnswerD::Southern blot; 5’(TCG)n3’ probe]]
Answer D Explanation	[[AnswerDExp::The repeat associated with myotonic dystrophy is 5‘ CTG 3’. The reverse complement of 5’ CTG 3’ is 5’ CAG 3’ either of these sequences could be used to detect the repeated region of DNA.]]
Answer E	[[AnswerE::Southern blot; 5’(CAA)n3’ probe]]
Answer E Explanation	[[AnswerEExp::While Sounthern blots are used to detect DNA, 5’ CAA 3’ is the repeat responsible for Friedrich’s ataxia.]]
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this scenario is most likely suffering from myotonic dystrophy. Myotonic dystrophy is an autosomal dominant disorder characterized by progressive muscle weakness and hypotonia, eventually leading to cardiopulmonary involvement and subsequent death in the majority of patients by the age of 65. Early signs of the disease include loss of grip strength and weakness in the neck, feet, and hands. Speech and swallowing may become difficult for patients due to loss of muscle tone in the tongue and the esophagus. Frequently, patients with the disease are unable to relax their hands following handshakes, due to myotonia (delayed relaxation of muscles after contraction) which accompanies muscle weakness. Myotonic dystrophy is caused by expansion of a CTG triplet repeat in the DMPK gene. A second form of myotonic dystrophy is caused by the expansion of a CCTG repeat in the ZNF9 gene. Like other triplet repeat diseases, such as Huntington’s disease, myotonic dystrophy can result in more severe symptoms with an earlier onset in successive generations (anticipation). There is currently no cure or treatment specific to myotonic dystrophy. Southern blots are used to detect the presence of a sequence of DNA. To assess the repeat length of an individual’s DNA, genomic DNA of the individual is subjected to polymerase chain reaction (PCR), in order to amplify the segment containing the repeat. The amplified DNA is then subjected to electrophoresis and probed with a complementary sequence of labeled DNA. Southern blotting is a method used to detect a specific fragment of a DNA sequesnce. Mnemonic: SNoW DRoP. Southern - DNA, Northern - RNA, Western - Protein. Educational Objective: The CTG repeat in the DMPK gene is associated with myotonic dystrophy. Southern blotting is used to detect the presence of certain sequences of DNA. References: First Aid 2014 page 82.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Molecular biology, WBRKeyword::Laboratory, WBRKeyword::Blot, WBRKeyword::Trinucleotide repeat disorders, WBRKeyword::Myotonic dystrophy
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