Insights into the heterogeneity of oculopharyngeal muscular dystrophy.

Oculopharyngeal muscular dystrophy (OPMD) is a rare, adult-onset, autosomal dominant myopathy characterized by variability in the age of onset and disease progression. However, its pathogenesis and phenotypic variability remain poorly understood. The disorder is caused by an expansion of a short polyalanine tract in the poly(A) binding protein nuclear 1 (PABPN1) gene. This study presents data from 23 patients across 19 Greek families with pathogenic PABPN1 expansions, including demographic and laboratory data, as well as molecular and electron microscopy findings. Eight distinct trinucleotide expansion genotypes were identified. Electron microscopy consistently demonstrated mitochondrial abnormalities, including swelling, disrupted cristae and atypical lipid inclusions. Clinical heterogeneity was observed at both inter- and intrafamilial levels, and milder phenotypes were generally linked to smaller alleles. Notably, maternally inherited expansions were associated with an earlier disease onset and more severe progression in affected offspring. Given the genetic variability observed in the cohort, the presence of a founder effect could not be supported. A significant degree of underdiagnosis or diagnostic delay was noted, largely attributable to the rarity and clinical heterogeneity of the disease. The observed intrafamilial heterogeneity - particularly in maternally inherited expansions - supports previous reports suggesting that mitochondrial dysfunction may contribute to transgenerational disease progression in the context of a dominant, causative nuclear variant.