Fragile X-Associated Tremor/Ataxia Syndrome — SCE Neurology MCQ

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HardMovement DisordersFragile X-Associated Tremor/Ataxia SyndromeSCE Neurology

A 68-year-old man develops progressive intention tremor, gait ataxia and executive dysfunction. MRI shows symmetrical T2 hyperintensity of the middle cerebellar peduncles. His grandson has fragile X syndrome. Which genetic result would establish the suspected late-onset disorder?

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Correct answer: C — An FMR1 premutation-range CGG expansion

The best answer is “An FMR1 premutation-range CGG expansion”. FXTAS occurs mainly in older male carriers of an FMR1 premutation and combines action tremor, cerebellar ataxia, cognitive change and the middle-cerebellar-peduncle MRI sign. “Biallelic RFC1 expansions in the pathogenic repeat range” is less appropriate because biallelic RFC1 expansions cause CANVAS or a related late-onset ataxia rather than the fragile-X pedigree “An FMR1 full mutation above 200 methylated CGG repeats” is less appropriate because a methylated full mutation causes fragile X syndrome and is not the usual molecular basis of FXTAS “A pathogenic heteroplasmic MT-ATP6 mitochondrial variant” is less appropriate because HTT expansion causes Huntington disease with a different movement and cognitive phenotype “A heterozygous pathogenic HTT CAG-repeat expansion” is less appropriate because MT-ATP6 disease produces a mitochondrial syndrome not linked to this X-linked pedigree

Reference: GeneReviews: FMR1 Disorders. https://www.ncbi.nlm.nih.gov/books/NBK1384/