While transfer of a protein encoded by a single nucleus to nearby nuclei in multinucleated cells has been known for almost 25 years, the biological consequences for gain-of-function diseases have not been considered. Here, the authors have investigated nuclear protein spreading and its potential consequences in two of the three most prevalent neuromuscular diseases. By performing co-cultures between diseased or control human myoblasts and murine C2C12 myoblasts, they demonstrated that in facioscapulohumeral dystrophy, although the transcription of the toxic protein DUX4 occurs in only a limited number of nuclei, the resulting protein diffuses into nearby nuclei within the myotubes, thus spreading aberrant gene expression. In myotonic dystrophy type 1, they observed that in human-mouse heterokaryons, the expression of a mutated DMPK from human nuclei titrates splicing factors produced by neighboring nuclei, inducing the mis-splicing of several pre-mRNAs in murine nuclei. In both cases, the spreading of the pathological phenotypes from one nucleus to another is observed, highlighting an additional mechanism that contributes to the dissemination and worsening of the muscle pathogenesis. These results indicate that nuclear protein spreading may be an important component in the pathophysiology of gain-of-function muscular diseases, which should be taken into consideration in the design of new therapeutic approaches.
