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CTG on chromosome 19
8%
5/62
CAG on chromosome 4
2%
1/62
GAA on chromomsome 9
5%
3/62
CGG on the sex chromosome X
79%
49/62
CTG on chromosome 8
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This patient's clinical presentation is consistent with Fragile X syndrome, which is caused by expansion of CGG repeats on the X-chromosome. Fragile X syndrome is an X-linked trinucleotide repeat expansion disorder of CGG trinucleotides. Although the syndrome may show anticipation, there may not always be a positive family history of the condition. Fragile X syndrome is the second most common cause of intellectual disability overall and the most common cause in boys. Behavioral problems may begin at birth or become apparent later. On physical exam, the disease presents with delay in motor milestones, hyperactive or impulsive behavior, mental retardation, speech and language delay, avoidance of eye contact, and autism spectrum disorder. Physical signs include flattened feet, flexible joints, low muscle tone, enlarged body habitus, enlarged/elongated forehead, long face, low-set or large protruding ears, pectus excavatum, soft skin, and macroorchidism. A common cardiac abnormality is mitral regurgitation which can be heard as a high-pitched holosystolic murmur at the apex of the heart that radiates to the axilla. Wattendorf and Muenke reviewed the management of Fragile X syndrome, which is typically supportive. Management includes measures to reduce stress/anxiety and focuses on behavioral therapy. For treatment of the psychiatric features, medications are often sufficient. Penagarikano et al. reviewed the pathophysiology of Fragile X syndrome. The expansion of the repeat CGG in the 5' UTR region of the FMR1 gene results in hypermethylation and silencing of the gene. Therefore, the disease is a result of the lack of expression of the FMR1 gene. Illustration A shows a picture of a young boy with Fragile X syndrome. Note the long thin face, large protruding ears, and broad forehead. Illustration B shows the characteristic physical features of someone with Fragile X syndrome. Incorrect answers: Answer 1: Myotonic dystrophy type 1 is due to a trinucleotide repeat expansion of CTG on chromosome 19. Answer 2: Huntington disease is due to a trinucleotide repeat expansion of CAG on chromosome 4. Answer 3: Freidrich ataxia is due to a trinucleotide repeat expansion of GAA on chromosome 9. Answer 5: Spinocerebellar ataxia type 8 is due to a trinucleotide repeat expansion of CTG on chromosome 8
4.1
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