Diverse Phenotypic Presentation of the Welander Distal Myopathy Founder Mutation, With Myopathy and Amyotrophic Lateral Sclerosis in the Same Family.

Welander distal myopathy is a rare myopathy with prominent and early involvement of distal upper extremity muscles, prevalent in individuals of Scandinavian origin, and caused by a founder mutation in the cytotoxic granule-associated RNA-binding protein (T-cell intracellular antigen-1; TIA1), E384K. Different pathogenic variants in the TIA1 gene, distinct from the founder 1, have recently been associated with frontotemporal dementia and amyotrophic lateral sclerosis (ALS), suggesting that TIA1-related disorders belong to the group of multisystem proteinopathies. We describe the first case of a two-generation family with the founder E384K TIA1 mutation demonstrating phenotypic variability; the mother manifested as Welander myopathy, whereas 2 daughters manifested as ALS. No other genetic cause of ALS was found in 1 of the affected daughters. We also discuss the possible mechanisms explaining this pleotropic presentation of the founder mutation.