A lamin A/C variant causing striated muscle disease provides insights into filament organization.

The gene encodes the A-type lamins, which polymerize into ∼3.5-nm-thick filaments and, together with B-type lamins and associated proteins, form the nuclear lamina. Mutations in cause a wide variety of pathologies. In this study, we analyzed the nuclear lamina of embryonic fibroblasts from mice, which develop cardiomyopathy and muscular dystrophy. Although the organization of the lamina appeared unaltered, there were changes in chromatin and B-type lamin expression. An increase in nuclear size and consequently a relative reduction in heterochromatin near the lamina allowed for a higher resolution structural analysis of lamin filaments using cryo-electron tomography. This was most apparent when visualizing lamin filaments and using a nuclear extraction protocol. Averaging of individual segments of filaments in mouse fibroblasts resolved two polymers that constitute the mature filaments. Our findings provide better views of the organization of lamin filaments and the effect of a striated muscle disease-causing mutation on nuclear structure.