Proteins of the nuclear envelope (NE) are associated with a range of inherited disorders, most commonly involving muscular dystrophy and cardiomyopathy, as exemplified by Emery-Dreifuss muscular dystrophy (EDMD). EDMD is both genetically and phenotypically variable, and some evidence of modifier genes has been reported. Six genes have so far been linked to EDMD, four encoding proteins associated with the LINC complex that connects the nucleus to the cytoskeleton. However, 50% of patients have no identifiable mutations in these genes. Using a candidate approach, we have identified putative disease-causing variants in the SUN1 and SUN2 genes, also encoding LINC complex components, in patients with EDMD and related myopathies. Our data also suggest that SUN1 and SUN2 can act as disease modifier genes in individuals with co-segregating mutations in other EDMD genes. Five SUN1/SUN2 variants examined impaired rearward nuclear repositioning in fibroblasts, confirming defective LINC complex function in nuclear-cytoskeletal coupling. Furthermore, myotubes from a patient carrying compound heterozygous SUN1 mutations displayed gross defects in myonuclear organization. This was accompanied by loss of recruitment of centrosomal marker, pericentrin, to the NE and impaired microtubule nucleation at the NE, events that are required for correct myonuclear arrangement. These defects were recapitulated in C2C12 myotubes expressing exogenous SUN1 variants, demonstrating a direct link between SUN1 mutation and impairment of nuclear-microtubule coupling and myonuclear positioning. Our findings strongly support an important role for SUN1 and SUN2 in muscle disease pathogenesis and support the hypothesis that defects in the LINC complex contribute to disease pathology through disruption of nuclear-microtubule association, resulting in defective myonuclear positioning. Emery-Dreifuss muscular dystrophy (EDMD) is an inherited disorder involving muscle wasting and weakness, accompanied by cardiac defects. The disease is variable in its severity and also in its genetic cause. So far, 6 genes have been linked to EDMD, most encoding proteins that form a structural network that supports the nucleus of the cell and connects it to structural elements of the cytoplasm. This network is particularly important in muscle cells, providing resistance to mechanical strain. Weakening of this network is thought to contribute to development of muscle disease in these patients. Despite rigorous screening, at least 50% of patients with EDMD have no detectable mutation in the 6 known genes. We therefore undertook screening and identified mutations in two additional genes that encode other components of the nuclear structural network, SUN1 and SUN2. Our findings add to the genetic complexity of this disease since some individuals carry mutations in more than one gene. We also show that the mutations disrupt connections between the nucleus and the structural elements of cytoplasm, leading to mis-positioning and clustering of nuclei in muscle cells. This nuclear mis-positioning is likely to be another factor contributing to pathogenesis of EDMD.

Nuclear envelope links to inherited disease gave the conundrum of how mutations in near-ubiquitous proteins can yield many distinct pathologies, each focused in different tissues. One conundrum-resolving hypothesis is that tissue-specific partner proteins mediate these pathologies. Such partner proteins may have now been identified with recent proteome studies determining nuclear envelope composition in different tissues. These studies revealed that the majority of the total nuclear envelope proteins are tissue restricted in their expression. Moreover, functions have been found for a number these tissue-restricted nuclear envelope proteins that fit with mechanisms proposed to explain how the nuclear envelope could mediate disease, including defects in mechanical stability, cell cycle regulation, signaling, genome organization, gene expression, nucleocytoplasmic transport, and differentiation. The wide range of functions to which these proteins contribute is consistent with not only their involvement in ti...

The LINC (linker of nucleoskeleton and cytoskeleton) complex is a proposed mechanical link tethering the nucleo- and cyto-skeleton via the NE (nuclear envelope). The LINC components emerin, lamin A/C, SUN1, SUN2, nesprin-1 and nesprin-2 interact with each other at the NE and also with other binding partners including actin filaments and B-type lamins. Besides the mechanostructural functions, the LINC complex is also involved in signalling pathways and gene regulation. Emerin was the first LINC component associated with a human disease, namely EDMD (Emery–Dreifuss muscular dystrophy). Later on, other components of the LINC complex, such as lamins A/C and small isoforms of nesprin-1 and nesprin-2, were found to be associated with EDMD, reflecting a genetic heterogeneity that has not been resolved so far. Only approximately 46% of the EDMD patients can be linked to genes of LINC and non-LINC components, pointing to further genes involved in the pathology of EDMD. Obvious candidates are the LINC proteins SUN1 and SUN2. Recently, screening of binding partners of LINC components as candidates identified LUMA (TMEM43), encoding a binding partner of emerin and lamins, as a gene involved in atypical EDMD. Nevertheless, such mutations contribute only to a very small fraction of EDMD patients. EDMD-causing mutations in STA/EMD (encoding emerin) that disrupt emerin binding to Btf (Bcl-2-associated transcription factor), GCL (germ cell-less) and BAF (barrier to autointegration factor) provide the first glimpses into LINC being involved in gene regulation and thus opening new avenues for functional studies. Thus the association of LINC with human disease provides tools for understanding its functions within the cell.