Abstract
Mutations in the human LMNA gene cause muscular dystrophy by mechanisms that are incompletely understood. The LMNA gene encodes A-type lamins, intermediate filaments that form a network underlying the inner nuclear membrane, providing structural support for the nucleus and organizing the genome. To better understand the pathogenesis caused by mutant lamins, we performed a structural and functional analysis on LMNA missense mutations identified in muscular dystrophy patients. These mutations perturb the tertiary structure of the conserved A-type lamin Ig-fold domain. To identify the effects of these structural perturbations on lamin function, we modeled these mutations in Drosophila Lamin C and expressed the mutant lamins in muscle. We found that the structural perturbations had minimal dominant effects on nuclear stiffness, suggesting that the muscle pathology was not accompanied by major structural disruption of the peripheral nuclear lamina. However, subtle alterations in the lamina network and subnuclear reorganization of lamins remain possible. Affected muscles had cytoplasmic aggregation of lamins and additional nuclear envelope proteins. Transcription profiling revealed upregulation of many Nrf2 target genes. Nrf2 is normally sequestered in the cytoplasm by Keap-1. Under oxidative stress Nrf2 dissociates from Keap-1, translocates into the nucleus, and activates gene expression. Unexpectedly, biochemical analyses revealed high levels of reducing agents, indicative of reductive stress. The accumulation of cytoplasmic lamin aggregates correlated with elevated levels of the autophagy adaptor p62/SQSTM1, which also binds Keap-1, abrogating Nrf2 cytoplasmic sequestration, allowing Nrf2 nuclear translocation and target gene activation. Elevated p62/SQSTM1 and nuclear enrichment of Nrf2 were identified in muscle biopsies from the corresponding muscular dystrophy patients, validating the disease relevance of our Drosophila model. Thus, novel connections were made between mutant lamins and the Nrf2 signaling pathway, suggesting new avenues of therapeutic intervention that include regulation of protein folding and metabolism, as well as maintenance of redox homoeostasis.
Highlights
The human LMNA gene exemplifies the rich source of genetic variation that exists in the human genome
Mutations in the human LMNA gene cause muscular dystrophy that is often accompanied by heart disease
The LMNA gene makes proteins that form a network on the inner side of the nuclear envelope, a structure that reinforces the cell nucleus
Summary
The human LMNA gene exemplifies the rich source of genetic variation that exists in the human genome. Over 283 sequence variants and 460 disease-causing mutations have been identified to date. These mutations cause at least 13 distinct clinical diseases, called laminopathies, which have mainly tissue-restricted phenotypes, despite the fact that A-type lamins are expressed in most cells [1]. Mutations are scattered throughout the LMNA gene [2]. Neighboring missense mutations can give rise to dramatically different disease phenotypes. These findings suggest that defined protein domains do not have tissue-specific functions
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have