"Laminopathies": a wide spectrum of human diseases

Most myopathic lamin variants aggregate: a functional genomics approach for assessing variants of uncertain significance

Hundreds of LMNA variants have been associated with several distinct disease phenotypes. However, genotype-phenotype relationships remain largely undefined and the impact for most variants remains unknown. We performed a functional analysis for 178 variants across five structural domains using two different overexpression models. We found that lamin A aggregation is a major determinant for skeletal and cardiac laminopathies. An in vitro solubility assay shows that aggregation-prone variants in the immunoglobulin-like domain correlate with domain destabilization. Finally, we demonstrate that myopathic-associated LMNA variants show aggregation patterns in induced pluripotent stem cell derived-cardiomyocytes (iPSC-CMs) in contrast to non-myopathic LMNA variants. Our data-driven approach (1) reveals that striated muscle laminopathies are predominantly protein misfolding diseases, (2) demonstrates an iPSC-CM experimental platform for characterizing laminopathic variants in human cardiomyocytes, and (3) supports a functional assay to aid in assessing pathogenicity for myopathic variants of uncertain significance.

Myocardial and Arrhythmic Spectrum of Neuromuscular Disorders in Children

Neuromuscular disorders (NMDs) are highly heterogenous from both an etiological and clinical point of view. Their signs and symptoms are often multisystemic, with frequent cardiac involvement. In fact, childhood onset forms can predispose a person to various progressive cardiac abnormalities including cardiomyopathies (CMPs), valvulopathies, atrioventricular conduction defects (AVCD), supraventricular tachycardia (SVT) and ventricular arrhythmias (VA). In this review, we selected and described five specific NMDs: Friedreich's Ataxia (FRDA), congenital and childhood forms of Myotonic Dystrophy type 1 (DM1), Kearns Sayre Syndrome (KSS), Ryanodine receptor type 1-related myopathies (RYR1-RM) and Laminopathies. These changes are widely investigated in adults but less researched in children. We focused on these specific topics due their relative frequency and their potential unexpected cardiac manifestations in children. Moreover these conditions present different inheritance patterns and mechanisms of action. We decided not to discuss Duchenne and Becker muscular dystrophies due to extensive work regarding the cardiac aspects in children. For each described NMD, we focused on the possible cardiac manifestations such as different types of CMPs (dilated-DCM, hypertrophic-HCM, restrictive-RCM or left ventricular non compaction-LVNC), structural heart abnormalities (including valvulopathies), and progressive heart rhythm changes (AVCD, SVT, VA). We describe the current management strategies for these conditions. We underline the importance, especially for children, of a serial multidisciplinary personalized approach and the need for periodic surveillance by a dedicated heart team. This is largely due to the fact that in children, the diagnosis of certain NMDs might be overlooked and the cardiac aspect can provide signs of their presence even prior to overt neurological diagnosis.

Lamin A/C Mechanosensor Drives Tumor Cell Aggressiveness and Adhesion on Substrates With Tissue-Specific Elasticity

Besides its structural properties in the nucleoskeleton, Lamin A/C is a mechanosensor protein involved in perceiving the elasticity of the extracellular matrix. In this study we provide evidence about Lamin A/C-mediated regulation of osteosarcoma cell adhesion and spreading on substrates with tissue-specific elasticities. Our working hypothesis is based on the observation that low-aggressive and bone-resident SaOS-2 osteosarcoma cells express high level of Lamin A/C in comparison to highly metastatic, preferentially to the lung, osteosarcoma 143B cells, thereby suggesting a role for Lamin A/C in tumor cell tropism. Specifically, LMNA gene over-expression in 143B cells induced a reduction in tumor cell aggressiveness in comparison to parental cells, with decreased proliferation rate and reduced migration capability. Furthermore, LMNA reintegration into 143B cells changed the adhesion properties of tumor cells, from a preferential tropism toward the 1.5 kPa PDMS substrate (resembling normal lung parenchyma) to the 28 kPa (resembling pre-mineralized bone osteoid matrix). Our study suggests that Lamin A/C expression could be involved in the organ tropism of tumor cells, thereby providing a rationale for further studies focused on the definition of cancer mechanism of metastatization.

Two Decades after Mandibuloacral Dysplasia Discovery: Additional Cases and Comprehensive View of Disease Characteristics

Pathogenic variants in the LMNA gene cause a group of heterogeneous genetic disorders, called laminopathies. In particular, homozygous or compound heterozygous variants in LMNA have been associated with “mandibuloacral dysplasia type A” (MADA), an autosomal recessive disorder, characterized by mandibular hypoplasia, growth retardation mainly postnatal, pigmentary skin changes, progressive osteolysis of the distal phalanges and/or clavicles, and partial lipodystrophy. The detailed characteristics of this multisystemic disease have yet to be specified due to its rarity and the limited number of cases described. Here, we report three unrelated Egyptian patients with variable severity of MAD features. Next-generation sequencing using a gene panel revealed a homozygous c.1580G>A-p.Arg527His missense variant in LMNA exon 9 in an affected individual with a typical MADA phenotype. Another homozygous c.1580G>T-p.Arg527Leu variant affecting the same amino acid was identified in two additional patients, who both presented with severe manifestations very early in life. We combined our observations together with data from all MADA cases reported in the literature to get a clearer picture of the phenotypic variability in this disease. This work raises the number of reported MADA families, argues for the presence of the founder effect in Egypt, and strengthens genotype–phenotype correlations.

Super-Resolution Imaging of the A- and B-Type Lamin Networks: A Comparative Study of Different Fluorescence Labeling Procedures

A- and B-type lamins are type V intermediate filament proteins. Mutations in the genes encoding these lamins cause rare diseases, collectively called laminopathies. A fraction of the cells obtained from laminopathy patients show aberrations in the localization of each lamin subtype, which may represent only the minority of the lamina disorganization. To get a better insight into more delicate and more abundant lamina abnormalities, the lamin network can be studied using super-resolution microscopy. We compared confocal scanning laser microscopy and stimulated emission depletion (STED) microscopy in combination with different fluorescence labeling approaches for the study of the lamin network. We demonstrate the suitability of an immunofluorescence staining approach when using STED microscopy, by determining the lamin layer thickness and the degree of lamin A and B1 colocalization as detected in fixed fibroblasts (co-)stained with lamin antibodies or (co-)transfected with EGFP/YFP lamin constructs. This revealed that immunofluorescence staining of cells does not lead to consequent changes in the detected lamin layer thickness, nor does it influence the degree of colocalization of lamin A and B1, when compared to the transfection approach. Studying laminopathy patient dermal fibroblasts (LMNA c.1130G>T (p.(Arg377Leu)) variant) confirmed the suitability of immunofluorescence protocols in STED microscopy, which circumvents the need for less convenient transfection steps. Furthermore, we found a significant decrease in lamin A/C and B1 colocalization in these patient fibroblasts, compared to normal human dermal fibroblasts. We conclude that super-resolution light microscopy combined with immunofluorescence protocols provides a potential tool to detect structural lamina differences between normal and laminopathy patient fibroblasts.

Antenatal diagnostic dilemma in a pseudodominant pedigree with lamin-B receptor (LBR)-related regressive spondylometaphyseal dysplasia

The lamin-B receptor (LBR) encodes a dual-functioning inner nuclear membrane protein essential for cholesterol biosynthesis and chromatin organization. LBR pathogenic variants cause distinct phenotypes due to the dual function of LBR, including Pelger-Huët anomaly (PHA), PHA with mild skeletal anomalies (PHASK; MIM# 618019), LBR-related regressive type of spondylometaphyseal dysplasia (LBR-R-SMD), Greenberg dysplasia (MIM# 215140). We here report the first case with radiological manifestations of LBR-R-SMD in the fetal period, and milder skeletal findings in the similarly affected father. Direct sequencing of LBR revealed homozygous c.1534C>T (p.Arg512Trp) in exon 12 in both affected individuals. Our report further refines the early phenotype in LBR-R-SMD, and demonstrates that the p.Arg512Trp mutation is associated with PHA. We propose that LBR-R-SMD should be considered as a differential diagnosis in pregnancies with sonographic evidence of short and bowed tubular bones with narrow thorax. Evaluating peripheral blood smears of expectant parents for the presence of PHA may lead to a clinical diagnosis, allowing for comprehensive prenatal genetic counseling.

Tyrosine phosphorylation of lamin A by Src promotes disassembly of nuclear lamina in interphase

Lamins form the nuclear lamina, which is important for nuclear structure and activity. Although posttranslational modifications, in particular serine phosphorylation, have been shown to be important for structural properties and functions of lamins, little is known about the role of tyrosine phosphorylation in this regard. In this study, we found that the constitutively active Src Y527F mutant caused the disassembly of lamin A/C. We demonstrate that Src directly phosphorylates lamin A mainly at Tyr45 both in vitro and in intact cells. The phosphomimetic Y45D mutant was diffusively distributed in the nucleoplasm and failed to assemble into the nuclear lamina. Depletion of lamin A/C in HeLa cells induced nuclear dysmorphia and genomic instability as well as increased nuclear plasticity for cell migration, all of which were partially restored by re-expression of lamin A, but further promoted by the Y45D mutant. Together, our results reveal a novel mechanism for regulating the assembly of nuclear lamina through Src and suggest that aberrant phosphorylation of lamin A by Src may contribute to nuclear dysmorphia, genomic instability, and nuclear plasticity.

Lamin regulates the dietary restriction response via the mTOR pathway in Caenorhabditis elegans

Animals subjected to dietary restriction (DR) have reduced body size, low fecundity, slower development, lower fat content and longer life span. We identified lamin as a regulator of multiple dietary restriction phenotypes. Downregulation of lmn-1, the single Caenorhabditis elegans lamin gene, increased animal size and fat content specifically in DR animals. The LMN-1 protein acts in the mTOR pathway, upstream of RAPTOR and S6 kinase β1 (S6K), a key component of and target of the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), respectively. DR excludes the mTORC1 activator RAGC-1 from the nucleus. Downregulation of lmn-1 restores RAGC-1 to the nucleus, a necessary step for the activation of the mTOR pathway. These findings further link lamin to metabolic regulation.

Summary: Downregulation of the single C. elegans lamin gene increases animal size and fat content specifically in dietary restricted animals. The lamin protein acts in the mTOR pathway to regulate these phenotypes.

Chromatin Conformation in Development and Disease

Chromatin domains and loops are important elements of chromatin structure and dynamics, but much remains to be learned about their exact biological role and nature. Topological associated domains and functional loops are key to gene expression and hold the answer to many questions regarding developmental decisions and diseases. Here, we discuss new findings, which have linked chromatin conformation with development, differentiation and diseases and hypothesized on various models while integrating all recent findings on how chromatin architecture affects gene expression during development, evolution and disease.

Nuclear Envelope Integrity in Health and Disease: Consequences on Genome Instability and Inflammation

The dynamic nature of the nuclear envelope (NE) is often underestimated. The NE protects, regulates, and organizes the eukaryote genome and adapts to epigenetic changes and to its environment. The NE morphology is characterized by a wide range of diversity and abnormality such as invagination and blebbing, and it is a diagnostic factor for pathologies such as cancer. Recently, the micronuclei, a small nucleus that contains a full chromosome or a fragment thereof, has gained much attention. The NE of micronuclei is prone to collapse, leading to DNA release into the cytoplasm with consequences ranging from the activation of the cGAS/STING pathway, an innate immune response, to the creation of chromosomal instability. The discovery of those mechanisms has revolutionized the understanding of some inflammation-related diseases and the origin of complex chromosomal rearrangements, as observed during the initiation of tumorigenesis. Herein, we will highlight the complexity of the NE biology and discuss the clinical symptoms observed in NE-related diseases. The interplay between innate immunity, genomic instability, and nuclear envelope leakage could be a major focus in future years to explain a wide range of diseases and could lead to new classes of therapeutics.

Small-Molecule Therapeutic Perspectives for the Treatment of Progeria

Hutchinson-Gilford progeria syndrome (HGPS), or progeria, is an extremely rare disorder that belongs to the class of laminopathies, diseases characterized by alterations in the genes that encode for the lamin proteins or for their associated interacting proteins. In particular, progeria is caused by a point mutation in the gene that codifies for the lamin A gene. This mutation ultimately leads to the biosynthesis of a mutated version of lamin A called progerin, which accumulates abnormally in the nuclear lamina. This accumulation elicits several alterations at the nuclear, cellular, and tissue levels that are phenotypically reflected in a systemic disorder with important alterations, mainly in the cardiovascular system, bones, skin, and overall growth, which results in premature death at an average age of 14.5 years. In 2020, lonafarnib became the first (and only) FDA approved drug for treating progeria. In this context, the present review focuses on the different therapeutic strategies currently under development, with special attention to the new small molecules described in recent years, which may represent the upcoming first-in-class drugs with new mechanisms of action endowed with effectiveness not only to treat but also to cure progeria.

Nuclear lamin phosphorylation: an emerging role in gene regulation and pathogenesis of laminopathies

Decades of studies have established that nuclear lamin polymers form the nuclear lamina, a protein meshwork that supports the nuclear envelope structure and tethers heterochromatin to the nuclear periphery. Much less is known about unpolymerized nuclear lamins in the nuclear interior, some of which are now known to undergo specific phosphorylation. A recent finding that phosphorylated lamins bind gene enhancer regions offers a new hypothesis that lamin phosphorylation may influence transcriptional regulation in the nuclear interior. In this review, we discuss the regulation, localization, and functions of phosphorylated lamins. We summarize kinases that phosphorylate lamins in a variety of biological contexts. Our discussion extends to laminopathies, a spectrum of degenerative disorders caused by lamin gene mutations, such as cardiomyopathies and progeria. We compare the prevailing hypothesis for laminopathy pathogenesis based on lamins’ function at the nuclear lamina with an emerging hypothesis based on phosphorylated lamins’ function in the nuclear interior.

Pathogenic LMNA variants disrupt cardiac lamina-chromatin interactions and de-repress alternative fate genes

Pathogenic mutations in LAMIN A/C (LMNA) cause abnormal nuclear structure and laminopathies. These diseases have myriad tissue-specific phenotypes, including dilated cardiomyopathy (DCM), but how LMNA mutations result in tissue-restricted disease phenotypes remains unclear. We introduced LMNA mutations from individuals with DCM into human induced pluripotent stem cells (hiPSCs) and found that hiPSC-derived cardiomyocytes, in contrast to hepatocytes or adipocytes, exhibit aberrant nuclear morphology and specific disruptions in peripheral chromatin. Disrupted regions were enriched for transcriptionally active genes and regions with lower LAMIN B1 contact frequency. The lamina-chromatin interactions disrupted in mutant cardiomyocytes were enriched for genes associated with non-myocyte lineages and correlated with higher expression of those genes. Myocardium from individuals with LMNA variants similarly showed aberrant expression of non-myocyte pathways. We propose that the lamina network safeguards cellular identity and that pathogenic LMNA variants disrupt peripheral chromatin with specific epigenetic and molecular characteristics, causing misexpression of genes normally expressed in other cell types.

Floppy infant syndrome as a first manifestation of LMNA-related congenital muscular dystrophy

LMNA-related congenital muscular dystrophy (L-CMD) is the most severe phenotypic form of skeletal muscle laminopathies. This paper reports clinical presentation of the disease in 15 Polish patients from 13 families with genetically confirmed skeletal muscle laminopathy. In all these patients floppy infant syndrome was the first manifestation of the disease. The genetic diagnosis was established by next generation sequencing (targeted panel or exome; 11 patients) or classic Sanger sequencing (4 patients). In addition to known pathogenic LMNA variants: c.116A > G (p.Asn39Ser), c.745C > T (p.Arg249Trp), c.746G > A (p.Arg249Gln), c.1072G > A (p.Glu358Lys), c.1147G > A (p.Glu383Lys), c.1163G > C (p.Arg388Pro), c.1357C > T (p.Arg453Trp), c.1583C > G (p.Thr528Arg), we have identified three novel ones: c.121C > G (p.Arg41Gly), c.1127A > G (p.Tyr376Cys) and c.1160T > C (p.Leu387Pro). Eleven patients had de novo mutations, 4 - familial. In one family we observed intrafamilial variability of clinical course: severe L-CMD in the male proband, intermediate form in his sister and asymptomatic in their mother. One asymptomatic father had somatic mosaicism. L-CMD should be suspected in children with hypotonia in infancy and delayed motor development, who have poor head control, severe hyperlordosis and unstable and awkward gait. Serum creatine kinase may be high (~1000IU/l). Progression of muscle weakness is fast, leading to early immobilization. In some patients with L-CMD joint contractures can develop with time. MRI shows that the most frequently affected muscles are the serratus anterior, lumbar paraspinal, gluteus, vastus, adductor magnus, hamstrings, medial head of gastrocnemius and soleus. Ultra-rare laminopathies can be a relatively common cause of generalized hypotonia in children. Introduction of wide genome sequencing methods was a breakthrough in diagnostics of diseases with great clinical and genetic variability and allowed approach "from genotype do phenotype". However target sequencing of LMNA gene could be considered in selected patients with clinical picture suggestive for laminopathy.

Case Report: An Atypical Form of Familial Partial Lipodystrophy Type 2 Due to Mutation in the Rod Domain of Lamin A/C

Purpose

Familial partial lipodystrophy type 2 (FPLD2) patients generally develop a wide variety of severe metabolic complications. However, they are not usually affected by primary cardiomyopathy and conduction system disturbances, although a few cases of FPLD2 and cardiomyopathy have been reported in the literature. These were all due to amino-terminal heterozygous lamin A/C mutations, which are considered as new forms of overlapping syndromes.

Methods and Results

Here we report the identification of a female patient with FPLD2 due to a heterozygous missense variant c.604G>A in the exon 3 of the LMNA gene, leading to amino acid substitution (p.Glu202Lys) in the central alpha-helical rod domain of lamin A/C with a high propensity to form coiled-coil dimers. The patient's cardiac evaluations that followed the genetic diagnosis revealed cardiac rhythm disturbances which were promptly treated pharmacologically.

Conclusions

This report supports the idea that there are "atypical forms" of FPLD2 with cardiomyopathy, especially when a pathogenic variant affects the lamin A/C head or alpha-helical rod domain. It also highlights how increased understanding of the genotype-phenotype correlation could help clinicians to schedule personalized monitoring of the lipodystrophic patient, in order to prevent uncommon but possible devastating manifestations, including arrhythmias and sudden death.

Multinucleation associated DNA damage blocks proliferation in p53-compromised cells

Nuclear atypia is one of the hallmarks of cancers. Here, we perform single-cell tracking studies to determine the immediate and long-term impact of nuclear atypia. Tracking the fate of newborn cells exhibiting nuclear atypia shows that multinucleation, unlike other forms of nuclear atypia, blocks proliferation in p53-compromised cells. Because ~50% of cancers display compromised p53, we explored how multinucleation blocks proliferation. Multinucleation increases 53BP1-decorated nuclear bodies (DNA damage repair platforms), along with a heterogeneous reduction in transcription and protein accumulation across the multi-nucleated compartments. Multinucleation Associated DNA Damage associated with 53BP1-bodies remains unresolved for days, despite an intact NHEJ machinery that repairs laser-induced DNA damage within minutes. Persistent DNA damage, a DNA replication block, and reduced phospho-Rb, reveal a novel replication stress independent cell cycle arrest caused by mitotic lesions. These findings call for segregating protective and prohibitive nuclear atypia to inform therapeutic approaches aimed at limiting tumour heterogeneity.

Hart et al. track newborn single cells by live microscopy after inducing a variety of nuclear atypia by CENP-E inhibitor treatment. They find that that multinucleation, unlike other forms of nuclear atypia, blocks proliferation independently of p53 and is associated with persistent 53BP1 DNA damage foci, thus providing insights into the consequences of multinucleation, often observed in disease states.

De Novo 1q21.3q22 Duplication Revaluation in a "Cold" Complex Neuropsychiatric Case with Syndromic Intellectual Disability

Syndromic intellectual disability often obtains a genetic diagnosis due to the combination of first and next generation sequencing techniques, although their interpretation may require revaluation over the years. Here we report on a composite neuropsychiatric case whose phenotype includes moderate intellectual disability, spastic paraparesis, movement disorder, and bipolar disorder, harboring a 1.802 Mb de novo 1q21.3q22 duplication. The role of this duplication has been reconsidered in the light of negativity of many other genetic exams, and of the possible pathogenic role of many genes included in this duplication, potentially configuring a contiguous gene-duplication syndrome.

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

The LMNA 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 LMNA cause a wide variety of pathologies. In this study, we analyzed the nuclear lamina of embryonic fibroblasts from Lmna ^H222P/H222P 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 in situ and using a nuclear extraction protocol. Averaging of individual segments of filaments in Lmna ^H222P/H222P 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.

Muscle Enriched Lamin Interacting Protein (Mlip) Binds Chromatin and Is Required for Myoblast Differentiation

Muscle-enriched A-type lamin-interacting protein (Mlip) is a recently discovered Amniota gene that encodes proteins of unknown biological function. Here we report Mlip’s direct interaction with chromatin, and it may function as a transcriptional co-factor. Chromatin immunoprecipitations with microarray analysis demonstrated a propensity for Mlip to associate with genomic regions in close proximity to genes that control tissue-specific differentiation. Gel mobility shift assays confirmed that Mlip protein complexes with genomic DNA. Blocking Mlip expression in C2C12 myoblasts down-regulates myogenic regulatory factors (MyoD and MyoG) and subsequently significantly inhibits myogenic differentiation and the formation of myotubes. Collectively our data demonstrate that Mlip is required for C2C12 myoblast differentiation into myotubes. Mlip may exert this role as a transcriptional regulator of a myogenic program that is unique to amniotes.

Ryanodine receptor remodeling in cardiomyopathy and muscular dystrophy caused by lamin A/C gene mutation

Mutations in the lamin A/C gene (LMNA), which encodes A-type lamins, cause several diseases called laminopathies, the most common of which is dilated cardiomyopathy with muscular dystrophy. The role of Ca2+ regulation in these diseases remain poorly understood. We now show biochemical remodeling of the ryanodine receptor (RyR)/intracellular Ca2+ release channel in heart samples from human subjects with LMNA mutations, including protein kinase A-catalyzed phosphorylation, oxidation and depletion of the stabilizing subunit calstabin. In the LmnaH222P/H222P murine model of Emery-Dreifuss muscular dystrophy caused by LMNA mutation, we demonstrate an age-dependent biochemical remodeling of RyR2 in the heart and RyR1 in skeletal muscle. This RyR remodeling is associated with heart and skeletal muscle dysfunction. Defective heart and muscle function are ameliorated by treatment with a novel Rycal small molecule drug (S107) that fixes 'leaky' RyRs. SMAD3 phosphorylation is increased in hearts and diaphragms of LmnaH222P/H222P mice, which enhances NADPH oxidase binding to RyR channels, contributing to their oxidation. There is also increased generalized protein oxidation, increased calcium/calmodulin-dependent protein kinase II-catalyzed phosphorylation of RyRs and increased protein kinase A activity in these tissues. Our data show that RyR remodeling plays a role in cardiomyopathy and skeletal muscle dysfunction caused by LMNA mutation and identify these Ca2+ channels as a potential therapeutic target.

Extracts Prepared from a Canadian Toxic Plant Induce Light-Dependent Perinuclear Vacuoles in Human Cells

Abstract

We are investigating plant species from the Canadian prairie ecological zone by phenotypic cell assays to discover toxins of biological interest. We provide the first report of the effects of extracts prepared from the shrub Symphoricarpos occidentalis in several human cell lines. S. occidentalis (Caprifoliaceae) extracts are cytotoxic, and, strikingly, treated cells undergo light-dependent vacuolation near the nucleus. The range of irradiation is present in standard ambient light and lies in the visible range (400-700 nm). Vacuolization in treated cells can be induced with specific wavelengths of 408 or 660 nm at 1 J/cm² energies. Vacuolated cells show a striking phenotype of a large perinuclear vacuole (nuclear associated vacuole, NAV) that is distinct from vesicles observed by treatment with an autophagy-inducing agent. Treatment with S. occidentalis extracts and light induces an intense lamin A/C signal at the junction of a nuclear vacuole and the nucleus. Further study of S. occidentalis extracts and vacuolation provide chemical tools that may contribute to the understanding of nuclear envelope organization and human cell biology.

Key Contribution

We provide the first description of the biological effects upon human cells of extracts from the toxic plant, Symphoricarpos occidentalis. Treated cells acquire striking nuclear associated vacuoles (NAVs), rarely observed in animal cell biology.

LINC complex regulation of genome organization and function

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

Induction of Senescence in Cancer Cells by a Novel Combination of Cucurbitacin B and Withanone: Molecular Mechanism and Therapeutic Potential

Cancer, an uncontrolled proliferation syndrome, is treated with synthetic chemotherapeutic drugs that are associated with severe adverse effects. Development and application of new natural compounds is warranted to deal with the exponentially increasing incidence of cancer worldwide. Keeping selective toxicity to cancer cells as a priority criterion, we developed a combination of Cucurbitacin B and Withanone, and analyzed its anticancer potential using non-small cell lung cancer cells. We demonstrate that the selective cytotoxicity of the combination, called CucWi-N, to cancer cells is mediated by induction of cellular senescence that was characterized by decrease in Lamin A/C, CDK2, CDK4, Cyclin D, Cyclin E, phosphorylated RB, mortalin and increase in p53 and CARF proteins. It compromised cancer cell migration that was mediated by decrease in mortalin, hnRNP-K, vascular endothelial growth factor, matrix metalloproteinase 2, and fibronectin. We provide in silico, molecular dynamics and experimental data to support that CucWi-N (i) possesses high capability to target mortalin-p53 interaction and hnRNP-K proteins, (ii) triggers replicative senescence and inhibits metastatic potential of the cancer cells, and (iii) inhibits tumor progression and metastasis in vivo. We propose that CucWi-N is a potential natural anticancer drug that warrants further mechanistic and clinical studies.

Tailoring Cellular Function: The Contribution of the Nucleus in Mechanotransduction

Cells sense a variety of different mechanochemical stimuli and promptly react to such signals by reshaping their morphology and adapting their structural organization and tensional state. Cell reactions to mechanical stimuli arising from the local microenvironment, mechanotransduction, play a crucial role in many cellular functions in both physiological and pathological conditions. To decipher this complex process, several studies have been undertaken to develop engineered materials and devices as tools to properly control cell mechanical state and evaluate cellular responses. Recent reports highlight how the nucleus serves as an important mechanosensor organelle and governs cell mechanoresponse. In this review, we will introduce the basic mechanisms linking cytoskeleton organization to the nucleus and how this reacts to mechanical properties of the cell microenvironment. We will also discuss how perturbations of nucleus-cytoskeleton connections, affecting mechanotransduction, influence health and disease. Moreover, we will present some of the main technological tools used to characterize and perturb the nuclear mechanical state.

Laminopathies' Treatments Systematic Review: A Contribution Towards a 'Treatabolome'

BACKGROUND

Variants in the LMNA gene, encoding lamins A/C, are responsible for a growing number of diseases, all of which complying with the definition of rare diseases. LMNA-related disorders have a varied phenotypic expression with more than 15 syndromes described, belonging to five phenotypic groups: Muscular Dystrophies, Neuropathies, Cardiomyopathies, Lipodystrophies and Progeroid Syndromes. Overlapping phenotypes are also reported. Linking gene and variants with phenotypic expression, disease mechanisms, and corresponding treatments is particularly challenging in laminopathies. Treatment recommendations are limited, and very few are variant-based.

OBJECTIVE

The Treatabolome initiative aims to provide a shareable dataset of existing variant-specific treatment for rare diseases within the Solve-RD EU project. As part of this project, we gathered evidence of specific treatments for laminopathies via a systematic literature review adopting the FAIR (Findable, Accessible, Interoperable, and Reusable) guidelines for scientific data production.

METHODS

Treatments for LMNA-related conditions were systematically collected from MEDLINE and Embase bibliographic databases and clinical trial registries (Cochrane Central Registry of Controlled Trials, clinicaltrial.gov and EudraCT). Two investigators extracted and analyzed the literature data independently. The included papers were assessed using the Oxford Centre for Evidence-Based Medicine 2011 Levels of Evidence.

RESULTS

From the 4783 selected articles by a systematic approach, we identified 78 papers for our final analysis that corresponded to the profile of data defined in the inclusion and exclusion criteria. These papers include 2 guidelines/consensus papers, 4 meta-analyses, 14 single-arm trials, 15 case series, 13 cohort studies, 21 case reports, 8 expert reviews and 1 expert opinion. The treatments were summarized electronically according to significant phenome-genome associations. The specificity of treatments according to the different laminopathic phenotypical presentations is variable.

CONCLUSIONS

We have extracted Treatabolome-worthy treatment recommendations for patients with different forms of laminopathies based on significant phenome-genome parings. This dataset will be available on the Treatabolome website and, through interoperability, on genetic diagnosis and treatment support tools like the RD-Connect's Genome Phenome Analysis Platform.

Nuclear filaments: role in chromosomal positioning and gene expression

Nuclear lamins form an elastic meshwork underlying the inner nuclear membrane and provide mechanical rigidity to the nucleus and maintain shape. Lamins also maintain chromosome positioning and play important roles in several nuclear processes like replication, DNA damage repair, transcription, and epigenetic modifications. LMNA mutations affect cardiac tissue, muscle tissues, adipose tissues to precipitate several diseases collectively termed as laminopathies. However, the rationale behind LMNA mutations and laminopathies continues to elude scientists. During interphase, several chromosomes form inter/intrachromosomal contacts inside nucleoplasm and several chromosomal loops also stretch out to make a ‘loop-cluster’ which are key players to regulate gene expressions. In this perspective, we have proposed that the lamin network in tandem with nuclear actin and myosin provide mechanical rigidity to the chromosomal contacts and facilitate loop-clusters movements. LMNA mutations thus might perturb the landscape of chromosomal contacts or loop-clusters positioning which can impair gene expression profile.

Lamin-Related Congenital Muscular Dystrophy Alters Mechanical Signaling and Skeletal Muscle Growth

Laminopathies are a clinically heterogeneous group of disorders caused by mutations in the LMNA gene, which encodes the nuclear envelope proteins lamins A and C. The most frequent diseases associated with LMNA mutations are characterized by skeletal and cardiac involvement, and include autosomal dominant Emery–Dreifuss muscular dystrophy (EDMD), limb-girdle muscular dystrophy type 1B, and LMNA-related congenital muscular dystrophy (LMNA-CMD). Although the exact pathophysiological mechanisms responsible for LMNA-CMD are not yet understood, severe contracture and muscle atrophy suggest that mutations may impair skeletal muscle growth. Using human muscle stem cells (MuSCs) carrying LMNA-CMD mutations, we observe impaired myogenic fusion with disorganized cadherin/β catenin adhesion complexes. We show that skeletal muscle from Lmna-CMD mice is unable to hypertrophy in response to functional overload, due to defective fusion of activated MuSCs, defective protein synthesis and defective remodeling of the neuromuscular junction. Moreover, stretched myotubes and overloaded muscle fibers with LMNA-CMD mutations display aberrant mechanical regulation of the yes-associated protein (YAP). We also observe defects in MuSC activation and YAP signaling in muscle biopsies from LMNA-CMD patients. These phenotypes are not recapitulated in closely related but less severe EDMD models. In conclusion, combining studies in vitro, in vivo, and patient samples, we find that LMNA-CMD mutations interfere with mechanosignaling pathways in skeletal muscle, implicating A-type lamins in the regulation of skeletal muscle growth.

A novel mutation in NEB causing foetal nemaline myopathy with arthrogryposis during early gestation

Nemaline myopathies are a clinically and genetically heterogeneous group of congenital myopathies, mainly characterized by muscle weakness, hypotonia and respiratory insufficiency. Here, we report a male foetus of consanguineous parents with a severe congenital syndrome characterized by arthrogryposis detected at 13 weeks of gestation. We describe severe complex dysmorphic facial and musculoskeletal features by post mortem fetal examination confirming the prenatal diagnosis. Histomorphological and ultrastructural studies of skeletal muscle reveal mini-rods in myotubes caused by a novel homozygous splice-site mutation in NEB (NM_001164508, chr2:g.152,417,623C>A GRCh37.p11 | c.19,102-1G>T ENST00000397345.3). No rods were seen in the myocardium. We discuss the relevance of this mutation in the context of nemaline myopathies associated with early developmental musculoskeletal disorders.

Cell-ECM contact-guided intracellular polarization is mediated via lamin A/C dependent nucleus-cytoskeletal connection

Cell polarization plays a crucial role in dynamic cellular events, such as cell proliferation, differentiation, and directional migration in response to diverse extracellular and intracellular signals. Although it is well known that cell polarization entails highly orchestrated intracellular molecular reorganization, the underlying mechanism of repositioning by intracellular organelles in the presence of multiple stimuli is still unclear. Here, we show that front-rear cell polarization based on the relative positions of nucleus and microtubule organizing center is precisely controlled by mechanical interactions including cellular adhesion to extracellular matrix and nucleus-cytoskeletal connections. By modulating the size and distribution of fibronectin-coated adhesive spots located in the polarized cell shape mimicking micropatterns, we monitored the alterations in cell polarity. We found that the localization of individual adhesive spots is more dominant than the cell shape itself to induce intracellular polarization. Further, the degree of cell polarization was diminished significantly by disrupting nuclear lamin A/C. We further confirm that geometrical cue-guided intracellular polarization determines directional cell migration via local activation of Cdc42. These findings provide novel insights into the role of nucleus-cytoskeletal connections in single cell polarization under a combination of physical, molecular, and genetic cues, where lamin A/C acts as a critical molecular mediator in ECM sensing and signal transduction via nucleus-cytoskeletal connection.

Need for NAD+: Focus on Striated Muscle Laminopathies

Laminopathies are a heterogeneous group of rare diseases caused by genetic mutations in the LMNA gene, encoding A-type lamins. A-type lamins are nuclear envelope proteins which associate with B-type lamins to form the nuclear lamina, a meshwork underlying the inner nuclear envelope of differentiated cells. The laminopathies include lipodystrophies, progeroid phenotypes and striated muscle diseases. Research on striated muscle laminopathies in the recent years has provided novel perspectives on the role of the nuclear lamina and has shed light on the pathological consequences of altered nuclear lamina. The role of altered nicotinamide adenine dinucleotide (NAD⁺) in the physiopathology of striated muscle laminopathies has been recently highlighted. Here, we have summarized these findings and reviewed the current knowledge about NAD⁺ alteration in striated muscle laminopathies, providing potential therapeutic approaches.

Familial dilated cardiomyopathy with a novel LMNA mutation (p.R429C): a case report

LMNA mutations cause a variety of inherited diseases referred to as laminopathies which are associated with a wide spectrum of disease phenotypes, ranging from skeletal muscle disease, pre-mature ageing, metabolic disorders, and cardiac abnormalities. We present a case of a 14-year-old boy with dilated cardiomyopathy induced by the LMNA mutation (p. R429C) and described its electrocardiogram and imaging features.

Identification of Genes and Pathways Regulated by Lamin A in Heart

Background Mutations in the LMNA gene, encoding LMNA (lamin A/C), causes distinct disorders, including dilated cardiomyopathies, collectively referred to as laminopathies. The genes (coding and noncoding) and regulatory pathways controlled by LMNA in the heart are not completely defined. Methods and Results We analyzed cardiac transcriptome from wild-type, loss-of-function (Lmna-/-), and gain-of-function (Lmna-/- injected with adeno-associated virus serotype 9 expressing LMNA) mice with normal cardiac function. Deletion of Lmna (Lmna-/-) led to differential expression of 2193 coding and 629 long noncoding RNA genes in the heart (q<0.05). Re-expression of LMNA in the Lmna-/- mouse heart, completely rescued 501 coding and 208 non-coding and partially rescued 1862 coding and 607 lncRNA genes. Pathway analysis of differentially expressed genes predicted activation of transcriptional regulators lysine-specific demethylase 5A, lysine-specific demethylase 5B, tumor protein 53, and suppression of retinoblastoma 1, paired-like homeodomain 2, and melanocyte-inducing transcription factor, which were completely or partially rescued upon reexpression of LMNA. Furthermore, lysine-specific demethylase 5A and 5B protein levels were increased in the Lmna-/- hearts and were partially rescued upon LMNA reexpression. Analysis of biological function for rescued genes identified activation of tumor necrosis factor-α, epithelial to mesenchymal transition, and suppression of the oxidative phosphorylation pathway upon Lmna deletion and their restoration upon LMNA reintroduction in the heart. Restoration of the gene expression and transcriptional regulators in the heart was associated with improved cardiac function and increased survival of the Lmna-/- mice. Conclusions The findings identify LMNA-regulated cardiac genes and their upstream transcriptional regulators in the heart and implicate lysine-specific demethylase 5A and B as epigenetic regulators of a subset of the dysregulated genes in laminopathies.

Structural and Mechanical Aberrations of the Nuclear Lamina in Disease

The nuclear lamins are the major components of the nuclear lamina in the nuclear envelope. Lamins are involved in numerous functions, including a role in providing structural support to the cell and the mechanosensing of the cell. Mutations in the genes encoding for lamins lead to the rare diseases termed laminopathies. However, not only laminopathies show alterations in the nuclear lamina. Deregulation of lamin expression is reported in multiple cancers and several viral infections lead to a disrupted nuclear lamina. The structural and mechanical effects of alterations in the nuclear lamina can partly explain the phenotypes seen in disease, such as muscular weakness in certain laminopathies and transmigration of cancer cells. However, a lot of answers to questions about the relation between changes in the nuclear lamina and disease development remain elusive. Here, we review the current understandings of the contribution of the nuclear lamina in the structural support and mechanosensing of healthy and diseased cells.

The nuclear envelope: LINCing tissue mechanics to genome regulation in cardiac and skeletal muscle

Regulation of the genome is viewed through the prism of gene expression, DNA replication and DNA repair as controlled through transcription, chromatin compartmentalisation and recruitment of repair factors by enzymes such as DNA polymerases, ligases, acetylases, methylases and cyclin-dependent kinases. However, recent advances in the field of muscle cell physiology have also shown a compelling role for 'outside-in' biophysical control of genomic material through mechanotransduction. The crucial hub that transduces these biophysical signals is called the Linker of Nucleoskeleton and Cytoskeleton (LINC). This complex is embedded across the nuclear envelope, which separates the nucleus from the cytoplasm. How the LINC complex operates to mechanically regulate the many functions of DNA is becoming increasingly clear, and recent advances have provided exciting insight into how this occurs in cells from mechanically activated tissues such as skeletal and cardiac muscle. Nevertheless, there are still some notable shortcomings in our understanding of these processes and resolving these will likely help us understand how muscle diseases manifest at the level of the genome.

The Broad Spectrum of LMNA Cardiac Diseases: From Molecular Mechanisms to Clinical Phenotype

Mutations of Lamin A/C gene (LMNA) cause laminopathies, a group of disorders associated with a wide spectrum of clinically distinct phenotypes, affecting different tissues and organs. Heart involvement is frequent and leads to cardiolaminopathy LMNA-dependent cardiomyopathy (LMNA-CMP), a form of dilated cardiomyopathy (DCM) typically associated with conduction disorders and arrhythmias, that can manifest either as an isolated event or as part of a multisystem phenotype. Despite the recent clinical and molecular developments in the field, there is still lack of knowledge linking specific LMNA gene mutations to the distinct clinical manifestations. Indeed, the severity and progression of the disease have marked interindividual variability, even amongst members of the same family. Studies conducted so far have described Lamin A/C proteins involved in diverse biological processes, that span from a structural role in the nucleus to the regulation of response to mechanical stress and gene expression, proposing various mechanistic hypotheses. However, none of those is per se able to fully justify functional and clinical phenotypes of LMNA-CMP; therefore, the role of Lamin A/C in cardiac pathophysiology still represents an open question. In this review we provide an update on the state-of-the-art studies on cardiolaminopathy, in the attempt to draw a line connecting molecular mechanisms to clinical manifestations. While investigators in this field still wonder about a clear genotype/phenotype correlation in LMNA-CMP, our intent here is to recapitulate common mechanistic hypotheses that link different mutations to similar clinical presentations.

Novel LMNA mutations in Greek and Myanmar Patients with Progeroid Features and Cardiac Manifestations

Segmental progeroid syndromes are groups of genetic disorders with multiple features resembling accelerated aging. The International Registry of Werner Syndrome (Seattle, WA) recruits pedigrees of progeroid syndromes from all over the world. We identified two novel LMNA mutations, p.Asp300Gly in a patient from Myanmar, and p.Asn466Lys, in a patient from Greece. Both were referred to our Registry for the genetic diagnosis because of the accelerated aged-appearance and cardiac complications. LMNA mutations are the second most common genetic cause of progeroid syndromes after WRN mutations in our Registry. As the next generation sequencing becomes readily available, we expect to identify more cases of rare genetic diseases in the developing countries.

Withaferin-A Can Be Used to Modulate the Keratin Network of Intermediate Filaments in Human Epidermal Keratinocytes

The mechanical state of cells is a critical part of their healthy functioning and it is controlled primarily by cytoskeletal networks (actin, microtubules and intermediate filaments). Drug-based strategies targeting the assembly of a given cytoskeletal network are often used to pinpoint their role in cellular function. Unlike actin and microtubules, there has been limited interest in the role of intermediate filaments, and fewer drugs have thus been identified and characterised as modulators of its assembly. Here, we evaluate whether Withaferin-A (WFA), an established disruptor of vimentin filaments, can also be used to modulate keratin filament assembly. Our results show that in keratinocytes, which are keratin-rich but vimentin-absent, Withaferin-A disrupts keratin filaments. Importantly, the dosages required are similar to those previously reported to disrupt vimentin in other cell types. Furthermore, Withaferin-A-induced keratin disassembly is accompanied by changes in cell stiffness and migration. Therefore, we propose that WFA can be repurposed as a useful drug to disrupt the keratin cytoskeleton in epithelial cells.

An Omics View of Emery-Dreifuss Muscular Dystrophy

Recent progress in Omics technologies has started to empower personalized healthcare development at a thorough biomolecular level. Omics have subsidized medical breakthroughs that have started to enter clinical proceedings. The use of this scientific know-how has surfaced as a way to provide a more far-reaching view of the biological mechanisms behind diseases. This review will focus on the discoveries made using Omics and the utility of these approaches for Emery–Dreifuss muscular dystrophy.

Mutated lamin A modulates stiffness in muscle cells

The cytoskeleton is a complex network interlinking filaments that extend throughout the cytoplasm from the nucleus to the plasma membrane. Three major types of filaments are found in the cytoskeleton: actin filaments, microtubules, and intermediate filaments. They play a key role in the ability of cells to both resist mechanical stress and generate force. However, the precise involvement of intermediate filament proteins in these processes remains unclear. Here, we focused on nuclear A-type lamins, which are connected to the cytoskeleton via the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. Using micro-constriction rheology, we investigated the impact of A-type lamins (p.H222P) mutation on the mechanical properties of muscle cells. We demonstrate that the expression of point mutation of lamin A in muscle cells increases cellular stiffness compared with cells expressing wild type lamin A and that the chemical agent selumetinib, an inhibitor of the ERK1/2 signaling, reversed the mechanical alterations in mutated cells. These results highlight the interplay between A-type lamins and mechano-signaling, which are supported by cell biology measurements.

Differentiating Luminal and Membrane-Associated Nuclear Envelope Proteins

The nuclear envelope (NE) consists of two concentric nuclear membranes separated by the lumen, an ∼40-nm-wide fluid layer. NE proteins are implicated in important cellular processes ranging from gene expression to nuclear positioning. Although recent progress has been achieved in quantifying the assembly states of NE proteins in their native environment with fluorescence fluctuation spectroscopy, these studies raised questions regarding the association of NE proteins with nuclear membranes during the assembly process. Monitoring the interaction of proteins with membranes is important because the binding event is often associated with conformational changes that are critical to cellular signaling pathways. Unfortunately, the close physical proximity of both membranes poses a severe experimental challenge in distinguishing luminal and membrane-associated NE proteins. This study seeks to address this problem by introducing new, to our knowledge, fluorescence-based assays that overcome the restrictions imposed by the NE environment. We found that luminal proteins violate the Stokes-Einstein relation, which eliminates a straightforward use of protein mobility as a marker of membrane association within the NE. However, a surprising anomaly in the temperature-dependent mobility of luminal proteins was observed, which was developed into an assay for distinguishing between soluble and membrane-bound NE proteins. We further introduced a second independent tool for distinguishing both protein populations by harnessing the previously reported undulations of the nuclear membranes. These membrane undulations introduce local volume changes that produce an additional fluorescence fluctuation signal for luminal, but not for membrane-bound, proteins. After testing both methods using simple model systems, we apply the two assays to investigate a previously proposed model of membrane association for the luminal domain of SUN2, a constituent protein of the linker of nucleoskeleton and cytoskeleton complex. Finally, we investigate the effect of C- and N-terminal tagging of the luminal ATPase torsinA on its ability to associate with nuclear membranes.

The Laminopathies and the Insights They Provide into the Structural and Functional Organization of the Nucleus

In recent years, our perspective on the cell nucleus has evolved from the view that it is a passive but permeable storage organelle housing the cell's genetic material to an understanding that it is in fact a highly organized, integrative, and dynamic regulatory hub. In particular, the subcompartment at the nuclear periphery, comprising the nuclear envelope and the underlying lamina, is now known to be a critical nexus in the regulation of chromatin organization, transcriptional output, biochemical and mechanosignaling pathways, and, more recently, cytoskeletal organization. We review the various functional roles of the nuclear periphery and their deregulation in diseases of the nuclear envelope, specifically the laminopathies, which, despite their rarity, provide insights into contemporary health-care issues.

K6-linked SUMOylation of BAF regulates nuclear integrity and DNA replication in mammalian cells

Barrier-to-autointegration factor (BAF) is a highly conserved protein in metazoans that has multiple functions during the cell cycle. We found that BAF is SUMOylated at K6, and that this modification is essential for its nuclear localization and function, including nuclear integrity maintenance and DNA replication. K6-linked SUMOylation of BAF promotes binding and interaction with lamin A/C to regulate nuclear integrity. K6-linked SUMOylation of BAF also supports BAF binding to DNA and proliferating cell nuclear antigen and regulates DNA replication. SENP1 and SENP2 catalyze the de-SUMOylation of BAF at K6. Disrupting the SUMOylation and de-SUMOylation cycle of BAF at K6 not only disturbs nuclear integrity, but also induces DNA replication failure. Taken together, our findings demonstrate that SUMOylation at K6 is an important regulatory mechanism that governs the nuclear functions of BAF in mammalian cells.

Lamin Mutations Cause Increased YAP Nuclear Entry in Muscle Stem Cells

Mutations in the LMNA gene, encoding the nuclear envelope A-type lamins, are responsible for muscular dystrophies, the most severe form being the LMNA-related congenital muscular dystrophy (L-CMD), with severe defects in myonucleus integrity. We previously reported that L-CMD mutations compromise the ability of muscle stem cells to modulate the yes-associated protein (YAP), a pivotal factor in mechanotransduction and myogenesis. Here, we investigated the intrinsic mechanisms by which lamins influence YAP subcellular distribution, by analyzing different conditions affecting the balance between nuclear import and export of YAP. In contrast to wild type (WT) cells, LMNA^DK32 mutations failed to exclude YAP from the nucleus and to inactivate its transcriptional activity at high cell density, despite activation of the Hippo pathway. Inhibiting nuclear pore import abolished YAP nuclear accumulation in confluent mutant cells, thus showing persistent nuclear import of YAP at cell confluence. YAP deregulation was also present in congenital myopathy related to nesprin-1 ^KASH mutation, but not in cells expressing the LMNA^H222P mutation, the adult form of lamin-related muscle dystrophy with reduced nuclear deformability. In conclusion, our data showed that L-CMD mutations increased YAP nuclear localization via an increased nuclear import and implicated YAP as a pathogenic contributor in muscle dystrophies caused by nuclear envelop defects.

Silencing of Euchromatic Transposable Elements as a Consequence of Nuclear Lamina Dysfunction

Transposable elements (TEs) are mobile genomic sequences that are normally repressed to avoid proliferation and genome instability. Gene silencing mechanisms repress TEs by RNA degradation or heterochromatin formation. Heterochromatin maintenance is therefore important to keep TEs silent. Loss of heterochromatic domains has been linked to lamin mutations, which have also been associated with derepression of TEs. In fact, lamins are structural components of the nuclear lamina (NL), which is considered a pivotal structure in the maintenance of heterochromatin domains at the nuclear periphery in a silent state. Here, we show that a lethal phenotype associated with Lamin loss-of-function mutations is influenced by Drosophila gypsy retrotransposons located in euchromatic regions, suggesting that NL dysfunction has also effects on active TEs located in euchromatic loci. In fact, expression analysis of different long terminal repeat (LTR) retrotransposons and of one non-LTR retrotransposon located near active genes shows that Lamin inactivation determines the silencing of euchromatic TEs. Furthermore, we show that the silencing effect on euchromatic TEs spreads to the neighboring genomic regions, with a repressive effect on nearby genes. We propose that NL dysfunction may have opposed regulatory effects on TEs that depend on their localization in active or repressed regions of the genome.

Loss of an H3K9me anchor rescues laminopathy-linked changes in nuclear organization and muscle function in an Emery-Dreifuss muscular dystrophy model

In this study, Harr et al. use C. elegans to investigate the consequences of a missense mutation (Y45C) in lamin A (encoded by LMNA) found in the human Emery-Dreifuss muscular dystrophy (EDMD) syndrome. Using muscle-specific emerin Dam-ID and other in vivo approaches, the authors report that they were able to counteract the dominant muscle-specific defects provoked by LMNA mutation by the ablation of a lamin-associated H3K9me anchor, suggesting a novel therapeutic pathway for treating EDMD.

Mutations in the nuclear structural protein lamin A produce rare, tissue-specific diseases called laminopathies. The introduction of a human Emery-Dreifuss muscular dystrophy (EDMD)-inducing mutation into the C. elegans lamin (LMN-Y59C), recapitulates many muscular dystrophy phenotypes, and correlates with hyper-sequestration of a heterochromatic array at the nuclear periphery in muscle cells. Using muscle-specific emerin Dam-ID in worms, we monitored the effects of the mutation on endogenous chromatin. An increased contact with the nuclear periphery along chromosome arms, and an enhanced release of chromosomal centers, coincided with the disease phenotypes of reduced locomotion and compromised sarcomere integrity. The coupling of the LMN-Y59C mutation with the ablation of CEC-4, a chromodomain protein that anchors H3K9-methylated chromatin at the nuclear envelope (NE), suppressed the muscle-associated disease phenotypes. Deletion of cec-4 also rescued LMN-Y59C-linked alterations in chromatin organization and some changes in transcription. Sequences that changed position in the LMN-Y59C mutant, are enriched for E2F (EFL-2)-binding sites, consistent with previous studies suggesting that altered Rb-E2F interaction with lamin A may contribute to muscle dysfunction. In summary, we were able to counteract the dominant muscle-specific defects provoked by LMNA mutation by the ablation of a lamin-associated H3K9me anchor, suggesting a novel therapeutic pathway for EDMD.

Dynamic Lamin B1-Gene Association During Oligodendrocyte Progenitor Differentiation

Differentiation of oligodendrocytes (OL) from progenitor cells (OPC) is the result of a unique program of gene expression, which is further regulated by the formation of topological domains of association with the nuclear lamina. In this study, we show that cultured OPC were characterized by progressively declining levels of endogenous Lamin B1 (LMNB1) during differentiation into OL. We then identify the genes dynamically associated to the nuclear lamina component LMNB1 during this transition, using a well established technique called DamID, which is based on the ability of a bacterially-derived deoxyadenosine methylase (Dam), to modify genomic regions in close proximity. We expressed a fusion protein containing Dam and LMNB1 in OPC (OPCLMNB1-Dam) and either kept them proliferating or differentiated them into OL (OLLMNB1-Dam) and identified genes that were dynamically associated to LMNB1 with differentiation. Importantly, we identified Lss, the gene encoding for lanosterol synthase, a key enzyme in cholesterol synthesis, as associated to the nuclear lamina in OLLMNB1-Dam. This finding could at least in part explain the lipid dysregulation previously reported for mouse models of ADLD characterized by persistent LMNB1 expression in oligodendrocytes.

Cardiovascular Involvement in Pediatric Laminopathies. Report of Six Patients and Literature Revision

Lamin A/C (LMNA) encodes for two nuclear intermediate filament proteins. Mutations in LMNA cause a highly heterogeneous group of diseases predominantly leading to muscular or cardiac disease, lipodystrophy syndromes, peripheral neuropathy, and accelerated aging disorders. Cardiac involvement includes progressive arrhythmias (brady/tachyarrhythmias, sudden cardiac death). Furthermore, cardiomyocyte damage often progresses into dilated cardiomyopathy (DCM), rarely described in the pediatric age group. Neuromuscular manifestations are even rarer in children. We report on six pediatric patients with LMNA mutations: patient 1 was operated on for aortic coarctation, non-compact left ventricle, atrial fibrillation (AF) preceding the diagnosis of DCM; patient 2 was operated on for ventricular septal defect (VSD), developed after years malignant arrhythmias preceding the progression to DCM (left ventricular non-compaction with LV dysfunction); patient 3 had ectopic atrial tachycardia as first manifestation of a DCM; patients 4 and 5 had no major arrhythmic events but only dilated ascending aorta, mildly dilated LV with mild hypertrabeculation of the lateral wall and a normally functioning but dilated left ventricle, respectively; patient 6 showed aortic coarctation, supraventricular tachycardia. Paroxysmal AF occurred in patients 1, 2, and 3 (50% of cases). Our series highlight the coexistence of congenital heart defects (CHDs) and aortic involvement with laminopathies in four of our patients: consisting of aortic coarctation (two patients), aortic root dilatation (one patient), and VSD (one patient). Aortic changes in laminopathies have been reported only once in an adult patient. This is the first report in the pediatric setting, and no associations with CHD have been previously described.

The role of transposable elements activity in aging and their possible involvement in laminopathic diseases

Eukaryotic genomes contain a large number of transposable elements, part of which are still active and able to transpose in the host genome. Mobile element activation is repressed to avoid deleterious effects, such as gene mutations or chromosome rearrangements. Control of transposable elements includes a variety of mechanisms comprising silencing pathways, which are based on the production of small non-coding RNAs. Silencing can occur either through transposable element RNA degradation or through the targeting of DNA sequences by heterochromatin formation and consequent transcriptional inhibition. Since the important role of the heterochromatin silencing, the gradual loss of heterochromatin marks in constitutive heterochromatin regions during the aging process promotes derepression of transposable elements, which is considered a cause of the progressive increase in genomic instability and of the activation of inflammatory responses. This review provides an overview of the effects of heterochromatin loss on the activity of transposable elements during the aging process and the possible impact on genome function. In this context, we discuss the possible role of the nuclear lamina, a major player in heterochromatin dynamics, in the regulation of transposable element activity and potential implications in laminopathic diseases.

The role of extracellular matrix in biomechanics and its impact on bioengineering of cells and 3D tissues

The cells and tissues of the human body are constantly exposed to exogenous and endogenous forces that are referred to as biomechanical cues. They guide and impact cellular processes and cell fate decisions on the nano-, micro- and macro-scale, and are therefore critical for normal tissue development and maintaining tissue homeostasis. Alterations in the extracellular matrix composition of a tissue combined with abnormal mechanosensing and mechanotransduction can aberrantly activate signaling pathways that promote disease development. Such processes are therefore highly relevant for disease modelling or when aiming for the development of novel therapies. In this mini review, we describe the main biomechanical cues that impact cellular fates. We highlight their role during development, homeostasis and in disease. We also discuss current techniques and tools that allow us to study the impact of biomechanical cues on cell and tissue development under physiological conditions, and we point out directions, in which in vitro biomechanics can be of use in the future.