Mutant lamins cause mechanically-induced nuclear envelope rupture, DNA damage, and DNA-PK activation in muscle

Abstract

Mutations in the human LMNA gene, which encodes the nuclear envelope proteins lamins A and C, cause Emery-Dreifuss muscular dystrophy (EDMD), congenital muscular dystrophy, limb-girdle muscular dystrophy, and a spectrum of other diseases. The molecular mechanisms responsible for these diseases remain incompletely understood, but the muscle-specific defects suggest that mutations may render nuclei more susceptible to mechanical stress. Using three mouse models of muscle laminopathies, we found that Lmna mutations reduced nuclear stability and caused widespread nuclear damage in skeletal muscle cells in vitro and in vivo, including the formation of chromatin protrusions, transient rupture of the nuclear envelope, DNA damage, and activation of DNA damage response pathways. Nuclear damage resulted from nuclear movement in maturing myotubes, and could be reversed by depletion of kinesin-1 or stabilization of microtubules surrounding the myonuclei. Inhibiting DNA-dependent protein kinase (DNA-PK), a major DNA damage response pathway, improved myofiber health, suggesting that DNA damage response signaling, rather than nuclear damage itself, causes the striated muscle defects. These findings point to the importance of DNA damage response pathways in post-mitotic muscle cells and open the door for novel therapeutic approaches for these currently untreatable diseases.