An increasing number of diseases, collectively named laminopathies, are caused by mutations in LMNA, the gene encoding alternatively spliced lamins A and C. Each disease affects specific combination of tissues, such as skeletal muscles, heart and tendons in the Emery-Dreifuss muscular dystrophy, heart and skeletal muscles in the limb-girdle muscular dystrophy 1B, heart in the dilated cardiomyopathy and conduction system disease, adipocytes in the Dunningan-type FPLD, neurons and muscles in the Charcot-Marie-Tooth disorder type 2B1, bone and adipocytes in the MAD, and multiple tissues that undergo premature aging in the Hutchison-Gilford progeria syndrome. The heterogeneity of LMNA-related diseases suggests that lamin A/C is involved in a number of tissue-specific functions, related to the well-known up-regulation of lamin A/C in differentiated cells. The proposed disease mechanisms in laminopathies are thus based on the multiple functions of lamins, that are involved in nuclear stability and growth, DNA replication, apoptosis and RNA transcription. Some pathogenetic models emphasize disruption of the lamin filaments that control the structural integrity of the nucleus and the arrangement of the chromatin. Other models consider lamins as scaffolds for the assembly of nuclear factors that regulate template availability and gene transcription. Both models are possibly correct and we previously presented evidence that characteristic alterations in nuclear shape and chromatin organization occurring in Emery-Dreifuss muscular dystrophy are possibly due to impaired interactions of mutated lamin A/C with structural nuclear proteins such as emerin and actin. In this study, we analyzed the molecular interactions altered in the nucleus as a consequence of a specific LMNA missense mutation at position 482 in the C-terminal globular domain of lamin A/C detected in Dunningan-type FPLD. The clinical phenotype of FPLD is characterized by loss of fat in the trunk and limbs and accumulation in neck and face, as well as by resistance to insulin action and hyper triglyceridemia. We analyzed lamin A/C interactions with emerin, the nuclear envelope protein whose anchorage at the nuclear rim is dependent upon lamin A/C; co-immunoprecipitation experiments in cultured fibroblasts from a FPLD patient carrying an R482L mutation demonstrated a lack of interaction between lamin A and emerin in vivo, while the interaction with lamin C was preserved in vitro (Capanni et al., 2003). Altered localization of lamin A/C in the nucleus, consisting of aggregates localized close to the nuclear lamina, was also observed; emerin did not co-localize with these nuclear lamin A/C aggregates. Typical changes in the chromatin distribution, similar to those previously found in Emery-Dreifuss muscular dystrophy, were present in a significant percentage of FPLD cells, confirming that an altered organization of the nuclear lamina can affect the chromatin arrangement and eventually, transcription. Interestingly, the presence of lamin A/C aggregates was restricted to actively transcribing cells. Moreover, the transcription rate of FPLD fibroblasts showing abnormal lamin A/C aggregates, monitored by bromouridine uptake, was reduced, demonstrating that mutated lamin A/C in FPLD cells interferes with RNA transcription. Finally, we found unprocessed lamin A precursors in the abnormal intranuclear aggregates, a finding that could contribute to the understanding of the pathogenetic mechanism of laminopathies.
ASJC Scopus subject areas
- Molecular Biology