Atypical Phenotype of Myotonic Dystrophy Type 1 with Variant Repeats at the Age of Diagnosis

Myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats in the DMPK gene. In a proportion of patients, the expanded allele contains variant repeats, which have been associated with later disease onset and different clinical presentation, although their full impact remains incompletely defined. We compared sociodemographic, neuromuscular, and multisystem clinical features between DM1 patients with pure CTG expansions (n = 66) and those with variant repeats (n = 9), who formed a consecutive cohort of unrelated index cases evaluated at the age of diagnosis in routine clinical practice. Patients with variant repeats were nine years older at diagnosis than patients with pure repeat expansions (p = 0.025), had more years of formal education (p = 0.024), and showed reduced muscle strength in proximal lower limbs (p = 0.049). No childhood or juvenile forms were observed among patients with variant repeats. Sex, disease duration, and most other clinical parameters, including multisystem involvement, did not differ between groups. The results of our exploratory study support variant repeats as disease modifiers in both age at onset and pattern of muscle involvement, and imply a two-sequential-component hypothesis in DM1 pathogenesis.