A newborn girl is born profoundly hypotonic with respiratory distress, bilateral talipes equinovarus, and marked facial diplegia. Her mother has myotonic dystrophy with distal muscle weakness, myotonia, posterior subcapsular cataracts, and frontal balding. Which statement correctly describes the molecular basis of this presentation?

• A GAA repeat expansion in the frataxin gene inherited as an autosomal recessive trait
• B CAG repeat expansion in the DMPK gene, with the largest expansions transmitted through fathers
• C CGG repeat expansion in FMR1 with methylation of the promoter region
• D CTG repeat expansion in the DMPK gene, with the largest expansions transmitted through mothers, producing the congenital form ✓

Explanation

Myotonic dystrophy type 1 is caused by expansion of a CTG trinucleotide repeat in the DMPK gene on chromosome 19 and is autosomal dominant. Anticipation is dramatic, and very large expansions producing congenital myotonic dystrophy are almost always maternally transmitted, the opposite pattern to Huntington disease. GAA expansion in frataxin causes Friedreich ataxia, and CGG expansion in FMR1 causes fragile X syndrome.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.