Pathogenesis in facioscapulohumeral muscular dystrophy (FSHD) appears to be due to aberrant expression, particularly in skeletal muscle nuclei, of the full‐length isoform of DUX4 (DUX4‐FL). Expression of DUX4‐FL is known to alter gene expression and to be cytotoxic, but cell responses to DUX4‐FL are not fully understood. Our study was designed to identify cellular mechanisms of pathogenesis caused by DUX4‐FL expression.

We used human myogenic cell cultures to analyze the effects of DUX4‐FL when it was expressed either from its endogenous promoter in FSHD cells or by exogenous expression using BacMam vectors. We focused on determining the effects of DUX4‐FL on protein ubiquitination and turnover and on aggregation of TDP‐43.

Human FSHD myotubes with endogenous DUX4‐FL expression showed both altered nuclear and cytoplasmic distributions of ubiquitinated proteins and aggregation of TDP‐43 in DUX4‐FL‐expressing nuclei. Similar changes were found upon exogenous expression of DUX4‐FL, but were not seen upon expression of the non‐toxic short isoform DUX4‐S. DUX4‐FL expression also inhibited protein turnover in a model system and increased the amounts of insoluble ubiquitinated proteins and insoluble TDP‐43. Finally, inhibition of the ubiquitin–proteasome system with MG132 produced TDP‐43 aggregation similar to DUX4‐FL expression.

Our results identify DUX4‐FL‐induced inhibition of protein turnover and aggregation of TDP‐43, which are pathological changes also found in diseases such as amyotrophic lateral sclerosis and inclusion body myopathy, as potential pathological mechanisms in FSHD.