Missense mutations in the rod domain of the lamin A/C gene as causes of dilated cardiomyopathy and conduction-system disease

Disruption of valosin-containing protein activity causes cardiomyopathy and reveals pleiotropic functions in cardiac homeostasis

Valosin-containing protein (VCP), also known as p97, is an ATPase with diverse cellular functions, although the most highly characterized is targeting of misfolded or aggregated proteins to degradation pathways, including the endoplasmic reticulum-associated degradation (ERAD) pathway. However, how VCP functions in the heart has not been carefully examined despite the fact that human mutations in VCP cause Paget disease of bone and frontotemporal dementia, an autosomal dominant multisystem proteinopathy that includes disease in the heart, skeletal muscle, brain, and bone. Here we generated heart-specific transgenic mice overexpressing WT VCP or a VCP^K524A mutant with deficient ATPase activity. Transgenic mice overexpressing WT VCP exhibit normal cardiac structure and function, whereas mutant VCP-overexpressing mice develop cardiomyopathy. Mechanistically, mutant VCP-overexpressing hearts up-regulate ERAD complex components and have elevated levels of ubiquitinated proteins prior to manifestation of cardiomyopathy, suggesting dysregulation of ERAD and inefficient clearance of proteins targeted for proteasomal degradation. The hearts of mutant VCP transgenic mice also exhibit profound defects in cardiomyocyte nuclear morphology with increased nuclear envelope proteins and nuclear lamins. Proteomics revealed overwhelming interactions of endogenous VCP with ribosomal, ribosome-associated, and RNA-binding proteins in the heart, and impairment of cardiac VCP activity resulted in aggregation of large ribosomal subunit proteins. These data identify multifactorial functions and diverse mechanisms whereby VCP regulates cardiomyocyte protein and RNA quality control that are critical for cardiac homeostasis, suggesting how human VCP mutations negatively affect the heart.

Cellular and Animal Models of Striated Muscle Laminopathies

The lamin A/C (LMNA) gene codes for nuclear intermediate filaments constitutive of the nuclear lamina. LMNA has 12 exons and alternative splicing of exon 10 results in two major isoforms—lamins A and C. Mutations found throughout the LMNA gene cause a group of diseases collectively known as laminopathies, of which the type, diversity, penetrance and severity of phenotypes can vary from one individual to the other, even between individuals carrying the same mutation. The majority of the laminopathies affect cardiac and/or skeletal muscles. The underlying molecular mechanisms contributing to such tissue-specific phenotypes caused by mutations in a ubiquitously expressed gene are not yet well elucidated. This review will explore the different phenotypes observed in established models of striated muscle laminopathies and their respective contributions to advancing our understanding of cardiac and skeletal muscle-related laminopathies. Potential future directions for developing effective treatments for patients with lamin A/C mutation-associated cardiac and/or skeletal muscle conditions will be discussed.

N-acetyl cysteine alleviates oxidative stress and protects mice from dilated cardiomyopathy caused by mutations in nuclear A-type lamins gene

Cardiomyopathy caused by lamin A/C gene (LMNA) mutations (hereafter referred as LMNA cardiomyopathy) is an anatomic and pathologic condition associated with muscular and electrical dysfunction of the heart, often leading to heart failure-related disability. There is currently no specific therapy available for patients that target the molecular pathophysiology of LMNA cardiomyopathy. We showed here an increase in oxidative stress levels in the hearts of mice carrying LMNA mutation, associated with a decrease of the key cellular antioxidant glutathione (GHS). Oral administration of N-acetyl cysteine, a GHS precursor, led to a marked improvement of GHS content, a decrease in oxidative stress markers including protein carbonyls and an improvement of left ventricular structure and function in a model of LMNA cardiomyopathy. Collectively, our novel results provide therapeutic insights into LMNA cardiomyopathy.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

Elevated dual specificity protein phosphatase 4 in cardiomyopathy caused by lamin A/C gene mutation is primarily ERK1/2-dependent and its depletion improves cardiac function and survival

Mutations in the lamin A/C gene (LMNA) encoding the nuclear intermediate filament proteins lamins A and C cause a group of tissue-selective diseases, the most common of which is dilated cardiomyopathy (herein referred to as LMNA cardiomyopathy) with variable skeletal muscle involvement. We previously showed that cardiomyocyte-specific overexpression of dual specificity protein phosphatase 4 (DUSP4) is involved in the pathogenesis of LMNA cardiomyopathy. However, how mutations in LMNA activate Dusp4 expression and whether it is necessary for the development of LMNA cardiomyopathy are currently unknown. We now show that female LmnaH222P/H222P mice, a model for LMNA cardiomyopathy, have increased Dusp4 expression and hyperactivation of extracellular signal-regulated kinase (ERK) 1/2 with delayed kinetics relative to male mice, consistent with the sex-dependent delay in the onset and progression of disease. Mechanistically, we show that the H222P amino acid substitution in lamin A enhances its binding to ERK1/2 and increases sequestration at the nuclear envelope. Finally, we show that genetic deletion of Dusp4 has beneficial effects on heart function and prolongs survival in LmnaH222P/H222P mice. These results further establish Dusp4 as a key contributor to the pathogenesis of LMNA cardiomyopathy and a potential target for drug therapy.

The Cutting Edge: The Role of mTOR Signaling in Laminopathies

The mechanistic target of rapamycin (mTOR) is a ubiquitous serine/threonine kinase that regulates anabolic and catabolic processes, in response to environmental inputs. The existence of mTOR in numerous cell compartments explains its specific ability to sense stress, execute growth signals, and regulate autophagy. mTOR signaling deregulation is closely related to aging and age-related disorders, among which progeroid laminopathies represent genetically characterized clinical entities with well-defined phenotypes. These diseases are caused by LMNA mutations and feature altered bone turnover, metabolic dysregulation, and mild to severe segmental progeria. Different LMNA mutations cause muscular, adipose tissue and nerve pathologies in the absence of major systemic involvement. This review explores recent advances on mTOR involvement in progeroid and tissue-specific laminopathies. Indeed, hyper-activation of protein kinase B (AKT)/mTOR signaling has been demonstrated in muscular laminopathies, and rescue of mTOR-regulated pathways increases lifespan in animal models of Emery-Dreifuss muscular dystrophy. Further, rapamycin, the best known mTOR inhibitor, has been used to elicit autophagy and degradation of mutated lamin A or progerin in progeroid cells. This review focuses on mTOR-dependent pathogenetic events identified in Emery-Dreifuss muscular dystrophy, LMNA-related cardiomyopathies, Hutchinson-Gilford Progeria, mandibuloacral dysplasia, and type 2 familial partial lipodystrophy. Pharmacological application of mTOR inhibitors in view of therapeutic strategies is also discussed.

LMNA-mutated Rabbits: A Model of Premature Aging Syndrome with Muscular Dystrophy and Dilated Cardiomyopathy

Premature aging syndromes are rare genetic disorders mimicking clinical and molecular features of aging. Products of the LMNA gene, primarily lamin A and C, are major components of the nuclear lamina. A recently identified group of premature aging syndromes was related to mutations of the LMNA gene. Although LMNA disorders have been identified in premature aging syndromes, affect specifically the skeletal muscles, cardiac muscles, and lipodystrophy, understanding the pathogenic mechanisms still need to be elucidated. Here, to establish a rabbit knockout (KO) model of premature aging syndromes, we performed precise LMNA targeting in rabbits via co-injection of Cas9/sgRNA mRNA into zygotes. The LMNA-KO rabbits exhibited reduced locomotion activity with abnormal stiff walking posture and a shortened stature, all of them died within 22 days. In addition, cardiomyopathy, muscular dystrophy, bone and joint abnormalities, as well as lipodystrophy were observed in LMNA-KO rabbits. In conclusion, the novel rabbit LMNA-KO model, displayed typical features of histopathological defects that are observed in premature aging syndromes, and may be utilized as a valuable resource for understanding the pathophysiological mechanisms of premature aging syndromes and elucidating mysteries of the normal process of aging in humans.

Dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a clinical diagnosis characterized by left ventricular or biventricular dilation and impaired contraction that is not explained by abnormal loading conditions (for example, hypertension and valvular heart disease) or coronary artery disease. Mutations in several genes can cause DCM, including genes encoding structural components of the sarcomere and desmosome. Nongenetic forms of DCM can result from different aetiologies, including inflammation of the myocardium due to an infection (mostly viral); exposure to drugs, toxins or allergens; and systemic endocrine or autoimmune diseases. The heterogeneous aetiology and clinical presentation of DCM make a correct and timely diagnosis challenging. Echocardiography and other imaging techniques are required to assess ventricular dysfunction and adverse myocardial remodelling, and immunological and histological analyses of an endomyocardial biopsy sample are indicated when inflammation or infection is suspected. As DCM eventually leads to impaired contractility, standard approaches to prevent or treat heart failure are the first-line treatment for patients with DCM. Cardiac resynchronization therapy and implantable cardioverter-defibrillators may be required to prevent life-threatening arrhythmias. In addition, identifying the probable cause of DCM helps tailor specific therapies to improve prognosis. An improved aetiology-driven personalized approach to clinical care will benefit patients with DCM, as will new diagnostic tools, such as serum biomarkers, that enable early diagnosis and treatment.

Lamin A mutation impairs interaction with nucleoporin NUP155 and disrupts nucleocytoplasmic transport in atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Here, we show the identification and functional characterization of one AF-associated mutation p.Arg399Cys in lamin A/C. Co-immunoprecipitation and GST pull-down assays demonstrate that lamin A/C interacts with NUP155, which is a nucleoporin and causes AF when mutated. Lamin A/C mutation p.Arg399Cys impairs the interaction between lamin A/C and NUP155, and increases extractability of NUP155 from the nuclear envelope (NE). Mutation p.Arg399Cys leads to aggregation of lamin A/C in the nucleus, although it does not impair the integrity of NE upon cellular stress. Mutation p.Arg399Cys inhibits the export of HSP70 mRNA and the nuclear import of HSP70 protein. Electrophysiological studies show that mutation p.Arg399Cys decreases the peak cardiac sodium current by decreasing the cell surface expression level of cardiac sodium channel Na_v 1.5, but does not affect I_Kr potassium current. In conclusion, our results indicate that lamin A/C mutation p.Arg399Cys weakens the interaction between nuclear lamina (lamin A/C) and the nuclear pore complex (NUP155), leading to the development of AF. The findings provide a novel molecular mechanism for the pathogenesis of AF.

Genetic and non-genetic determinants of clinical phenotypes in cardiomyopathy

Cardiomyopathy, a leading cause of death worldwide, is etiologically and phenotypically heterogeneous and is caused by a combination of genetic and non-genetic factors. Major genomic determinants of dilated cardiomyopathy (DCM) are titin truncating mutations and lamin A/C mutations. Patients with these two genotypes show critically different phenotypes, including penetrance, coexistence with a conduction system abnormality, cardiac prognosis, and treatment response. The transcriptomic and epigenomic characteristics of DCM include activation of the DNA damage response, metabolic reprogramming, and dedifferentiation. The proteomic and metabolomic signatures of the DCM heart include a rigorous dependency for free fatty acids, activation of the stress response, and metabolic reprogramming. Proteomic and metabolomic analyses of blood show a distinct immune response and an unexpected link with pathology-specific microbiota in DCM. The direct integration of multi-omics data will not only elucidate inter-omics associations but also enable omics-based patient stratification, which will lead to a deeper understanding of cardiomyopathy and the development of precision medicine in cardiology.

Modelling inherited cardiac disease using human induced pluripotent stem cell-derived cardiomyocytes: progress, pitfalls, and potential

In the past few years, the use of specific cell types derived from induced pluripotent stem cells (iPSCs) has developed into a powerful approach to investigate the cellular pathophysiology of numerous diseases. Despite advances in therapy, heart disease continues to be one of the leading causes of death in the developed world. A major difficulty in unravelling the underlying cellular processes of heart disease is the extremely limited availability of viable human cardiac cells reflecting the pathological phenotype of the disease at various stages. Thus, the development of methods for directed differentiation of iPSCs to cardiomyocytes (iPSC-CMs) has provided an intriguing option for the generation of patient-specific cardiac cells. In this review, a comprehensive overview of the currently published iPSC-CM models for hereditary heart disease is compiled and analysed. Besides the major findings of individual studies, detailed methodological information on iPSC generation, iPSC-CM differentiation, characterization, and maturation is included. Both, current advances in the field and challenges yet to overcome emphasize the potential of using patient-derived cell models to mimic genetic cardiac diseases.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

Laminopathies; Mutations on single gene and various human genetic diseases

Lamin A and its alternative splicing product Lamin C are the key intermediate filaments (IFs) of the inner nuclear membrane intermediate filament. Lamin A/C forms the inner nuclear mesh with Lamin B and works as a frame with a nuclear shape. In addition to supporting the function of nucleus, nuclear lamins perform important roles such as holding the nuclear pore complex and chromatin. However, mutations on the Lamin A or Lamin B related proteins induce various types of human genetic disorders and diseases including premature aging syndromes, muscular dystrophy, lipodystrophy and neuropathy. In this review, we briefly overview the relevance of genetic mutations of Lamin A, human disorders and laminopathies. We also discuss a mouse model for genetic diseases. Finally, we describe the current treatment for laminopathies. [BMB Reports 2018; 51(7): 327-337].

"Primary" dilated hearts

Dilated cardiomyopathy is part of the spectrum of heart failure which is a syndrome with certain morphological and functional characteristics. Although significant progress in the management of those patients has been achieved, seems that risk stratification and future treatments will be related to the specific pathological substrate.

Role of Lamin A/C Gene Mutations in the Signaling Defects Leading to Cardiomyopathies

Nuclear lamin A/C are crucial components of the intricate protein mesh that underlies the inner nuclear membrane and confers mainly nuclear and cytosolic rigidity. However, throughout the years a number of other key physiological processes have been associated with lamins such as modulation of both genes expression and the activity of signaling mediators. To further solidify its importance in cell physiology, mutations in the lamin A/C gene (LMNA) have been associated to diverse pathological phenotypes with skeletal muscles and the heart being the most affected systems. When affected, the heart develops a wide array of phenotypes spanning from dilated cardiomyopathy with conduction defects to arrhythmogenic right ventricular cardiomyopathy. The surprising large number of cardiac phenotypes reflects the equally large number of specific mutations identified in the LMNA gene. In this review, we underlie how mutations in LMNA can impact the activity and the spatial/temporal organization of signaling mediators and transcription factors. We analyzed the ever-increasing amount of findings collected in Lmna^H222P/H222P mice whose cardiomyopathy resemble the most important features of the disease in humans and a number of key evidences from other experimental models. With this mini review, we attempt to combine the newest insights regarding both the pathogenic effects of LMNA mutations in terms of signaling abnormalities and cardiac laminopathies.

Mind the Gap: Genetic Variation and Personalized Therapies for Cardiomyopathies

Inherited cardiomyopathies are cardiovascular disorders that are one of the leading causes of death and are strongly associated with genetic mutations. These include hypertrophic, dilated, restrictive, as well as arrhythmogenic right ventricular cardiomyopathies. Among the patients presenting with these specific forms of cardiomyopathies, there is significant phenotypic, genotypic, and environmental heterogeneity. Over the years, the identification of the underlying mutations common to specific forms of cardiomyopathies have facilitated clinic diagnosis. However, the variation between patient genetics and phenotypes highlights the need for improved understanding of these diseases and the development of innovative treatments. To better understand the diseases, researchers are capitalizing on two innovative technologies: cardiac reprogramming and gene editing using CRISPR-Cas9. Deriving cardiomyocytes from patient blood samples and gene editing allows for the efficient generation of cellular and animal models that allow researchers to model the disease more accurately. In addition, the recent advances in high throughput drug screening allows for the efficient testing of patient-derived cardiomyocytes for patient-specific susceptibility to various drugs that are currently approved. In addition, this technology can facilitate the development of new pharmacological compounds for the treatment of specific cardiomyopathies. Overall, the recent technological advances in molecular medicine now presents an opportunity to gain unprecedented insight into solving the complex issue of inherited cardiomyopathies. These techniques pave the way for the new generation of personalized medicine in treating cardiovascular diseases.

Nesprins and Lamins in Health and Diseases of Cardiac and Skeletal Muscles

Since the discovery of the inner nuclear transmembrane protein emerin in the early 1990s, nuclear envelope (NE) components and related involvement in nuclei integrity and functionality have been highly investigated. The NE is composed of two distinct lipid bilayers described as the inner (INM) and outer (ONM) nuclear membrane. NE proteins can be specifically “integrated” in the INM (such as emerin and SUN proteins) or in the ONM such as nesprins. Additionally, flanked to the INM, the nuclear lamina, a proteinaceous meshwork mainly composed of lamins A and C completes NE composition. This network of proteins physically interplays to guarantee NE integrity and most importantly, shape the bridge between cytoplasmic cytoskeletons networks (such as microtubules and actin) and the genome, through the anchorage to the heterochromatin. The essential network driving the connection of nucleoskeleton with cytoskeleton takes place in the perinuclear space (the space between ONM and INM) with the contribution of the LINC complex (for Linker of Nucleoskeleton to Cytoskeleton), hosting KASH and SUN proteins interactions. This close interplay between compartments has been related to diverse functions from nuclear integrity, activity and positioning through mechanotransduction pathways. At the same time, mutations in NE components genes coding for proteins such as lamins or nesprins, had been associated with a wide range of congenital diseases including cardiac and muscular diseases. Although most of these NE associated proteins are ubiquitously expressed, a large number of tissue-specific disorders have been associated with diverse pathogenic mutations. Thus, diagnosis and molecular explanation of this group of diseases, commonly called “nuclear envelopathies,” is currently challenging. This review aims, first, to give a better understanding of diverse functions of the LINC complex components, from the point of view of lamins and nesprins. Second, to summarize human congenital diseases with a special focus on muscle and heart abnormalities, caused by mutations in genes coding for these two types of NE associated proteins.

Cofilin-1 phosphorylation catalyzed by ERK1/2 alters cardiac actin dynamics in dilated cardiomyopathy caused by lamin A/C gene mutation

Hyper-activation of extracellular signal-regulated kinase (ERK) 1/2 contributes to heart dysfunction in cardiomyopathy caused by mutations in the lamin A/C gene (LMNA cardiomyopathy). The mechanism of how this affects cardiac function is unknown. We show that active phosphorylated ERK1/2 directly binds to and catalyzes the phosphorylation of the actin depolymerizing factor cofilin-1 on Thr25. Cofilin-1 becomes active and disassembles actin filaments in a large array of cellular and animal models of LMNA cardiomyopathy. In vivo expression of cofilin-1, phosphorylated on Thr25 by endogenous ERK1/2 signaling, leads to alterations in left ventricular function and cardiac actin. These results demonstrate a novel role for cofilin-1 on actin dynamics in cardiac muscle and provide a rationale on how increased ERK1/2 signaling leads to LMNA cardiomyopathy.

Translating emerging molecular genetic insights into clinical practice in inherited cardiomyopathies

Cardiomyopathies are primarily genetic disorders of the myocardium associated with higher risk of life-threatening cardiac arrhythmias, heart failure, and sudden cardiac death. The evolving knowledge in genomic medicine during the last decade has reshaped our understanding of cardiomyopathies as diseases of multifactorial nature and complex pathophysiology. Genetic testing in cardiomyopathies has subsequently grown from primarily a research tool into an essential clinical evaluation piece with important clinical implications for patients and their families. The purpose of this review is to provide with a contemporary insight into the implications of genetic testing in diagnosis, therapy, and prognosis of patients with inherited cardiomyopathies. Here, we summarize the contemporary knowledge on genotype-phenotype correlations in inherited cardiomyopathies and highlight the recent significant achievements in the field of translational cardiovascular genetics.

MicroRNAs in hereditary and sporadic premature aging syndromes and other laminopathies

Hereditary and sporadic laminopathies are caused by mutations in genes encoding lamins, their partners, or the metalloprotease ZMPSTE24/FACE1. Depending on the clinical phenotype, they are classified as tissue‐specific or systemic diseases. The latter mostly manifest with several accelerated aging features, as in Hutchinson–Gilford progeria syndrome (HGPS) and other progeroid syndromes. MicroRNAs are small noncoding RNAs described as powerful regulators of gene expression, mainly by degrading target mRNAs or by inhibiting their translation. In recent years, the role of these small RNAs has become an object of study in laminopathies using in vitro or in vivo murine models as well as cells/tissues of patients. To date, few miRNAs have been reported to exert protective effects in laminopathies, including miR‐9, which prevents progerin accumulation in HGPS neurons. The recent literature has described the potential implication of several other miRNAs in the pathophysiology of laminopathies, mostly by exerting deleterious effects. This review provides an overview of the current knowledge of the functional relevance and molecular insights of miRNAs in laminopathies. Furthermore, we discuss how these discoveries could help to better understand these diseases at the molecular level and could pave the way toward identifying new potential therapeutic targets and strategies based on miRNA modulation.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

Exploring the Crosstalk Between LMNA and Splicing Machinery Gene Mutations in Dilated Cardiomyopathy

Mutations in the LMNA gene, which encodes for the nuclear lamina proteins lamins A and C, are responsible for a diverse group of diseases known as laminopathies. One type of laminopathy is Dilated Cardiomyopathy (DCM), a heart muscle disease characterized by dilation of the left ventricle and impaired systolic function, often leading to heart failure and sudden cardiac death. LMNA is the second most commonly mutated gene in DCM. In addition to LMNA, mutations in more than 60 genes have been associated with DCM. The DCM-associated genes encode a variety of proteins including transcription factors, cytoskeletal, Ca2+-regulating, ion-channel, desmosomal, sarcomeric, and nuclear-membrane proteins. Another important category among DCM-causing genes emerged upon the identification of DCM-causing mutations in RNA binding motif protein 20 (RBM20), an alternative splicing factor that is chiefly expressed in the heart. In addition to RBM20, several essential splicing factors were validated, by employing mouse knock out models, to be embryonically lethal due to aberrant cardiogenesis. Furthermore, heart-specific deletion of some of these splicing factors was found to result in aberrant splicing of their targets and DCM development. In addition to splicing alterations, advances in next generation sequencing highlighted the association between splice-site mutations in several genes and DCM. This review summarizes LMNA mutations and splicing alterations in DCM and discusses how the interaction between LMNA and splicing regulators could possibly explain DCM disease mechanisms.

The clinical outcome of LMNA missense mutations can be associated with the amount of mutated protein in the nuclear envelope

AIMS

Lamin A/C mutations are generally believed to be associated with a severe prognosis. The aim of this study was to investigate disease expression in three affected families carrying different LMNA missense mutations. Furthermore, the potential molecular disease mechanisms of the mutations were investigated in fibroblasts obtained from mutation carriers.

METHODS AND RESULTS

A LMNA-p.Arg216Cys missense mutation was identified in a large family with 36 mutation carriers. Disease expression was unusual with a late onset and a favourable prognosis. Two smaller families with severe disease expression were shown to carry a LMNA-p.Arg471Cys and LMNA-p.Arg471His mutation, respectively. LMNA gene and protein expression was investigated in eight different mutation carriers by quantitative reverse transcriptase polymerase chain reaction, Western blotting, immunohistochemistry, and protein mass spectrometry. The results showed that all mutation carriers incorporated mutated lamin protein into the nuclear envelope. Interestingly, the ratio of mutated to wild-type protein was only 30:70 in LMNA-p.Arg216Cys carriers with a favourable prognosis while LMNA-p.Arg471Cys and LMNA-p.Arg471His carriers with a more severe outcome expressed significantly more of the mutated protein by a ratio of 50:50.

CONCLUSION

The clinical findings indicated that some LMNA mutations may be associated with a favourable prognosis and a low risk of sudden death. Protein expression studies suggested that a severe outcome was associated with the expression of high amounts of mutated protein. These findings may prove to be helpful in counselling and risk assessment of LMNA families.

A gene-centric strategy for identifying disease-causing rare variants in dilated cardiomyopathy

PURPOSE

We evaluated strategies for identifying disease-causing variants in genetic testing for dilated cardiomyopathy (DCM).

METHODS

Cardiomyopathy gene panel testing was performed in 532 DCM patients and 527 healthy control subjects. Rare variants in 41 genes were stratified using variant-level and gene-level characteristics.

RESULTS

A majority of DCM cases and controls carried rare protein-altering cardiomyopathy gene variants. Variant-level characteristics alone had limited discriminative value. Differentiation between groups was substantially improved by addition of gene-level information that incorporated ranking of genes based on literature evidence for disease association. The odds of DCM were increased to nearly 9-fold for truncating variants or high-impact missense variants in the subset of 14 genes that had the strongest biological links to DCM (P <0.0001). For some of these genes, DCM-associated variants appeared to be clustered in key protein functional domains. Multiple rare variants were present in many family probands, however, there was generally only one "driver" pathogenic variant that cosegregated with disease.

CONCLUSION

Rare variants in cardiomyopathy genes can be effectively stratified by combining variant-level and gene-level information. Prioritization of genes based on their a priori likelihood of disease causation is a key factor in identifying clinically actionable variants in cardiac genetic testing.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

Prioritization of Variants Detected by Next Generation Sequencing According to the Mutation Tolerance and Mutational Architecture of the Corresponding Genes

The biggest challenge geneticists face when applying next-generation sequencing technology to the diagnosis of rare diseases is determining which rare variants, from the dozens or hundreds detected, are potentially implicated in the patient’s phenotype. Thus, variant prioritization is an essential step in the process of rare disease diagnosis. In addition to conducting the usual in-silico analyses to predict variant pathogenicity (based on nucleotide/amino-acid conservation and the differences between the physicochemical features of the amino-acid change), three important concepts should be borne in mind. The first is the “mutation tolerance” of the genes in which variants are located. This describes the susceptibility of a given gene to any functional mutation and depends on the strength of purifying selection acting against it. The second is the “mutational architecture” of each gene. This describes the type and location of mutations previously identified in the gene, and their association with different phenotypes or degrees of severity. The third is the mode of inheritance (inherited vs. de novo) of the variants detected. Here, we discuss the importance of each of these concepts for variant prioritization in the diagnosis of rare diseases. Using real data, we show how genes, rather than variants, can be prioritized by calculating a gene-specific mutation tolerance score. We also illustrate the influence of mutational architecture on variant prioritization using five paradigmatic examples. Finally, we discuss the importance of familial variant analysis as final step in variant prioritization.

Human pluripotent stem cell models of cardiac disease: from mechanisms to therapies

It is now a decade since human induced pluripotent stem cells (hiPSCs) were first described. The reprogramming of adult somatic cells to a pluripotent state has become a robust technology that has revolutionised our ability to study human diseases. Crucially, these cells capture all the genetic aspects of the patient from which they were derived. Combined with advances in generating the different cell types present in the human heart, this has opened up new avenues to study cardiac disease in humans and investigate novel therapeutic approaches to treat these pathologies. Here, we provide an overview of the current state of the field regarding the generation of cardiomyocytes from human pluripotent stem cells and methods to assess them functionally, an essential requirement when investigating disease and therapeutic outcomes. We critically evaluate whether treatments suggested by these in vitro models could be translated to clinical practice. Finally, we consider current shortcomings of these models and propose methods by which they could be further improved.

Mechanisms of allelic and clinical heterogeneity of lamin A/C phenotypes

Mutations in the lamin A/C (LMNA) gene cause a broad range of clinical syndromes that show tissue-restricted abnormalities of post mitotic tissues, such as muscle, nerve, heart, and adipose tissue. Mutations in other nuclear envelope proteins cause clinically overlapping disorders. The majority of mutations are dominant single amino acid changes (toxic protein produced by the single mutant gene), and patients are heterozygous with both normal and abnormal proteins. Experimental support has been provided for different models of cellular pathogenesis in nuclear envelope diseases, including changes in heterochromatin formation at the nuclear membrane (epigenomics), changes in the timing of steps during terminal differentiation of cells, and structural abnormalities of the nuclear membrane. These models are not mutually exclusive and may be important in different cells at different times of development. Recent experiments using fusion proteins of normal and mutant lamin A/C proteins fused to a bacterial adenine methyltransferase (DamID) provided compelling evidence of mutation-specific perturbation of epigenomic imprinting during terminal differentiation. These gain-of-function properties include lineage-specific ineffective genomic silencing during exit from the cell cycle (heterochromatinization), as well as promiscuous initiation of silencing at incorrect places in the genome. To date, these findings have been limited to a few muscular dystrophy and lipodystrophy LMNA mutations but seem shared with a distinct nuclear envelope disease, emerin-deficient muscular dystrophy. The dominant-negative structural model and gain-of-function epigenomic models for distinct LMNA mutations are not mutually exclusive, and it is likely that both models contribute to aspects of the many complex clinical phenotypes observed.

A Novel Truncating LMNA Mutation in Patients with Cardiac Conduction Disorders and Dilated Cardiomyopathy

The cardiac phenotype of laminopathies is characterized by cardiac conduction disorders (CCDs) and dilated cardiomyopathy (DCM). Although laminopathies have been considered monogenic, they exhibit a remarkable degree of clinical variability. This case series aimed to detect the causal mutation and to investigate the causes of clinical variability in a Japanese family with inherited CCD and DCM.Of the five family members investigated, four had either CCD/DCM or CCD alone, while one subject had no cardiovascular disease and acted as a normal control. We performed targeted resequencing of 174 inherited cardiovascular disease-associated genes in this family and pathological mutations were confirmed using Sanger sequencing. The degree of clinical severity and variability were also evaluated using long-term medical records. We discovered a novel heterozygous truncating lamin A/C (LMNA) mutation (c.774delG) in all four subjects with CCD. Because this mutation was predicted to cause a frameshift mutation and premature termination (p.Gln258HisfsTer222) in LMNA, we believe that this LMNA mutation was the causal mutation in this family with CCD and laminopathies. In addition, gender-specific intra-familiar clinical variability was observed in this Japanese family where affected males exhibited an earlier onset of CCD and more severe DCM compared to affected females. Using targeted resequencing, we discovered a novel truncating LMNA mutation associated with CCD and DCM in this family characterized by gender differences in clinical severity in LMNA carriers. Our results suggest that in patients with laminopathy, clinical severity may be the result of multiple factors.

Biomechanical defects and rescue of cardiomyocytes expressing pathologic nuclear lamins

Aims

Given the clinical impact of LMNA cardiomyopathies, understanding lamin function will fulfill a clinical need and will lead to advancement in the treatment of heart failure. A multidisciplinary approach combining cell biology, atomic force microscopy (AFM), and molecular modeling was used to analyse the biomechanical properties of human lamin A/C gene (LMNA) mutations (E161K, D192G, N195K) using an in vitro neonatal rat ventricular myocyte model.

Methods and results

The severity of biomechanical defects due to the three LMNA mutations correlated with the severity of the clinical phenotype. AFM and molecular modeling identified distinctive biomechanical and structural changes, with increasing severity from E161K to N195K and D192G, respectively. Additionally, the biomechanical defects were rescued with a p38 MAPK inhibitor.

Conclusions

AFM and molecular modeling were able to quantify distinct biomechanical and structural defects in LMNA mutations E161K, D192G, and N195K and correlate the defects with clinical phenotypic severity. Improvements in cellular biomechanical phenotype was demonstrated and may represent a mechanism of action for p38 MAPK inhibition therapy that is now being used in human clinical trials to treat laminopathies.

Lamins and Lamin-Associated Proteins in Gastrointestinal Health and Disease

The nuclear lamina is a multi-protein lattice composed of A- and B-type lamins and their associated proteins. This protein lattice associates with heterochromatin and integral inner nuclear membrane proteins, providing links among the genome, nucleoskeleton, and cytoskeleton. In the 1990s, mutations in EMD and LMNA were linked to Emery-Dreifuss muscular dystrophy. Since then, the number of diseases attributed to nuclear lamina defects, including laminopathies and other disorders, has increased to include more than 20 distinct genetic syndromes. Studies of patients and mouse genetic models have pointed to important roles for lamins and their associated proteins in the function of gastrointestinal organs, including liver and pancreas. We review the interactions and functions of the lamina in relation to the nuclear envelope and genome, the ways in which its dysfunction is thought to contribute to human disease, and possible avenues for targeted therapies.

Upregulation of the aging related LMNA splice variant progerin in dilated cardiomyopathy

Background

Mutations in the LMNA gene are a common cause (6–8%) of dilated cardiomyopathy (DCM) leading to heart failure, a growing health care problem worldwide. The premature aging disease Hutchinson-Gilford syndrome (HGPS) is also caused by defined mutations in the LMNA gene resulting in activation of a cryptic splice donor site leading to a defective truncated prelamin A protein called progerin. Low levels of progerin are expressed in healthy individuals associated with ageing. Here, we aimed to address the role of progerin in dilated cardiomyopathy.

Methods and results

mRNA expression of progerin was analyzed in heart tissue of DCM (n = 15) and non-failing hearts (n = 10) as control and in blood samples from patients with DCM (n = 56) and healthy controls (n = 10). Sequencing confirmed the expression of progerin mRNA in the human heart. Progerin mRNA levels derived from DCM hearts were significantly upregulated compared to controls (1.27 ± 0.42 vs. 0.81 ± 0.24; p = 0.005). In contrast, progerin mRNA levels in whole blood cells were not significantly different in DCM patients compared to controls. Linear regression analyses revealed that progerin mRNA in the heart is significantly negatively correlated to ejection fraction (r = -0.567, p = 0.003) and positively correlated to left ventricular enddiastolic diameter (r = 0.551, p = 0.004) but not with age of the heart per se. Progerin mRNA levels were not influenced by inflammation in DCM hearts. Immunohistochemistry and Immunofluorescence analysis confirmed increased expression of progerin protein in cell nuclei of DCM hearts associated with increased TUNEL+ apoptotic cells.

Conclusion

Our data suggest that progerin is upregulated in human DCM hearts and strongly correlates with left ventricular remodeling. Progerin might be involved in progression of heart failure and myocardial aging.

Dynein pulling forces counteract lamin-mediated nuclear stability during nuclear envelope repair

Transient nuclear envelope (NE) ruptures in the Caenorhabditis elegans zygote are caused by a weakened nuclear lamina during nuclear positioning. Dynein-pulling forces enhance the severity of ruptures, while lamin restricts nucleocytoplasmic mixing and allows stable NE repair. This work is the first mechanistic analysis of NE rupture and repair in an organism.

Recent work done exclusively in tissue culture cells revealed that the nuclear envelope (NE) ruptures and repairs in interphase. The duration of NE ruptures depends on lamins; however, the underlying mechanisms and relevance to in vivo events are not known. Here, we use the Caenorhabditis elegans zygote to analyze lamin’s role in NE rupture and repair in vivo. Transient NE ruptures and subsequent NE collapse are induced by weaknesses in the nuclear lamina caused by expression of an engineered hypomorphic C. elegans lamin allele. Dynein-generated forces that position nuclei enhance the severity of transient NE ruptures and cause NE collapse. Reduction of dynein forces allows the weakened lamin network to restrict nucleo–cytoplasmic mixing and support stable NE recovery. Surprisingly, the high incidence of transient NE ruptures does not contribute to embryonic lethality, which is instead correlated with stochastic chromosome scattering resulting from premature NE collapse, suggesting that C. elegans tolerates transient losses of NE compartmentalization during early embryogenesis. In sum, we demonstrate that lamin counteracts dynein forces to promote stable NE repair and prevent catastrophic NE collapse, and thus provide the first mechanistic analysis of NE rupture and repair in an organismal context.

Risk stratification in laminopathies and Emery Dreifuss muscular dystrophy

Laminopathies are genetic disorders due to gene mutation encoding for proteins of the nuclear envelope. Patients are at risk of conduction defect, arrhythmia, sudden death and heart failure. The authors summarize predictive factors for cardiac events reported in the literature in this group of disease.

Clinical and Molecular Aspects of Cardiomyopathies: Emerging Therapies and Clinical Trials

Cardiomyopathies are diseases of the myocardium, often genetically determined, associated with heterogeneous phenotypes and clinical manifestations. Despite significant progress in the understanding of these conditions, available treatments mostly target late complications, whereas approaches that promise to interfere with the primary mechanisms and natural history are just beginning to surface. The last decade has witnessed the establishment of large international cardiomyopathy registries, paralleled by advances in cardiac imaging and genetic testing, deeper understanding of the pathophysiology and growing involvement by the pharmaceutical industry. As a result, the number of molecular interventions under scrutiny is increasing sharply.

Luma is not essential for murine cardiac development and function

AIMS

Luma is a recently discovered, evolutionarily conserved protein expressed in mammalian heart, which is associated with the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex. The LINC complex structurally integrates the nucleus and the cytoplasm and plays a critical role in mechanotransduction across the nuclear envelope. Mutations in several LINC components in both humans and mice result in various cardiomyopathies, implying they play essential, non-redundant roles. A single amino acid substitution of serine 358 to leucine (S358L) in Luma is the unequivocal cause of a distinct form of arrhythmogenic cardiomyopathy. However, the role of Luma in heart has remained obscure. In addition, it also remains to be determined how the S358L mutation in Luma leads to cardiomyopathy.

METHODS AND RESULTS

To determine the role of Luma in the heart, we first determined the expression pattern of Luma in mouse heart. Luma was sporadically expressed in cardiomyocytes throughout the heart, but was highly and uniformly expressed in cardiac fibroblasts and vascular smooth muscle cells. We also generated germline null Luma mice and discovered that germline null mutants were viable and exhibited normal cardiac function. Luma null mice also responded normally to pressure overload induced by transverse aortic constriction. In addition, localization and expression of other LINC complex components in both cardiac myocytes and fibroblasts was unaffected by global loss of Luma. Furthermore, we also generated and characterized Luma S358L knock-in mice, which displayed normal cardiac function and morphology.

CONCLUSION

Our data suggest that Luma is dispensable for murine cardiac development and function and that the Luma S358L mutation alone may not cause cardiomyopathy in mice.

Emerging views of the nucleus as a cellular mechanosensor

The ability of cells to respond to mechanical forces is critical for numerous biological processes. Emerging evidence indicates that external mechanical forces trigger changes in nuclear envelope structure and composition, chromatin organization and gene expression. However, it remains unclear if these processes originate in the nucleus or are downstream of cytoplasmic signals. Here we discuss recent findings that support a direct role of the nucleus in cellular mechanosensing and highlight novel tools to study nuclear mechanotransduction.

Whole Exome Sequencing Identifies Truncating Variants in Nuclear Envelope Genes in Patients With Cardiovascular Disease

BACKGROUND

The genetic variation underlying many heritable forms of cardiovascular disease is incompletely understood, even in patients with strong family history or early age at onset.

METHODS AND RESULTS

We used whole exome sequencing to detect pathogenic variants in 55 patients with suspected monogenic forms of cardiovascular disease. Diagnostic analysis of established disease genes identified pathogenic variants in 21.8% of cases and variants of uncertain significance in 34.5% of cases. Three patients harbored heterozygous nonsense or splice-site variants in the nucleoporin genes NUP37, NUP43, and NUP188, which have not been implicated previously in cardiac disease. We also identified a heterozygous splice site variant in the nuclear envelope gene SYNE1 in a child with severe dilated cardiomyopathy that underwent transplant, as well as in his affected father. To confirm a cardiovascular role for these candidate genes in vivo, we used morpholinos to reduce SYNE1, NUP37, and NUP43 gene expression in zebrafish. Morphant embryos displayed cardiac abnormalities, including pericardial edema and heart failure. Furthermore, lymphoblasts from the patient carrying a SYNE1 splice-site variant displayed changes in nuclear morphology and protein localization that are consistent with disruption of the nuclear envelope.

CONCLUSIONS

These data expand the repertoire of pathogenic variants associated with cardiovascular disease and validate the diagnostic and research use of whole exome sequencing. We identify NUP37, NUP43, and NUP188 as novel candidate genes for cardiovascular disease, and suggest that dysfunction of the nuclear envelope may be an under-recognized component of inherited cardiac disease in some cases.

Lamin A/C-Related Cardiac Disease: Late Onset With a Variable and Mild Phenotype in a Large Cohort of Patients With the Lamin A/C p.(Arg331Gln) Founder Mutation

BACKGROUND

Interpretation of missense variants can be especially difficult when the variant is also found in control populations. This is what we encountered for the LMNA c.992G>A (p.(Arg331Gln)) variant. Therefore, to evaluate the effect of this variant, we combined an evaluation of clinical data with functional experiments and morphological studies.

METHODS AND RESULTS

Clinical data of 23 probands and 35 family members carrying this variant were retrospectively collected. A time-to-event analysis was performed to compare the course of the disease with carriers of other LMNA mutations. Myocardial biopsies were studied with electron microscopy and by measuring force development of the sarcomeres. Morphology of the nuclear envelope was assessed with immunofluorescence on cultured fibroblasts. The phenotype in probands and family members was characterized by atrioventricular conduction disturbances (61% and 44%, respectively), supraventricular arrhythmias (69% and 52%, respectively), and dilated cardiomyopathy (74% and 14%, respectively). LMNA p.(Arg331Gln) carriers had a significantly better outcome regarding the composite end point (malignant ventricular arrhythmias, end-stage heart failure, or death) compared with carriers of other pathogenic LMNA mutations. A shared haplotype of 1 Mb around LMNA suggested a common founder. The combined logarithm of the odds score was 3.46. Force development in membrane-permeabilized cardiomyocytes was reduced because of decreased myofibril density. Structural nuclear LMNA-associated envelope abnormalities, that is, blebs, were confirmed by electron microscopy and immunofluorescence microscopy.

CONCLUSIONS

Clinical, morphological, functional, haplotype, and segregation data all indicate that LMNA p.(Arg331Gln) is a pathogenic founder mutation with a phenotype reminiscent of other LMNA mutations but with a more benign course.

Suppression of Activated FOXO Transcription Factors in the Heart Prolongs Survival in a Mouse Model of Laminopathies

RATIONALE

Mutations in the LMNA gene, encoding nuclear inner membrane protein lamin A/C, cause distinct phenotypes, collectively referred to as laminopathies. Heart failure, conduction defects, and arrhythmias are the common causes of death in laminopathies.

OBJECTIVE

The objective of this study was to identify and therapeutically target the responsible mechanism(s) for cardiac phenotype in laminopathies.

METHODS AND RESULTS

Whole-heart RNA sequencing was performed before the onset of cardiac dysfunction in the Lmna^-/- and matched control mice. Differentially expressed transcripts and their upstream regulators were identified, validated, and targeted by adeno-associated virus serotype 9-short hairpin RNA constructs. A total of 576 transcripts were upregulated and 233 were downregulated in the Lmna^-/- mouse hearts (q<0.05). Forkhead box O (FOXO) transcription factors (TFs) were the most activated while E2 factors were the most suppressed transcriptional regulators. Transcript levels of FOXO targets were also upregulated in the isolated Lmna^-/- cardiac myocytes and in the myocardium of human heart failure patients. Nuclear localization of FOXO1 and 3 was increased, whereas phosphorylated (inactive) FOXO1 and 3 levels were reduced in the Lmna^-/- hearts. Gene set enrichment analysis and gene ontology showed activation of apoptosis and inflammation and suppression of cell cycle, adipogenesis, and oxidative phosphorylation in the Lmna^-/- hearts. Adeno-associated virus serotype 9-short hairpin RNA-mediated suppression of FOXO TFs rescued selected molecular signatures, improved apoptosis, and prolonged survival by ≈2-fold.

CONCLUSIONS

FOXO TFs are activated and contribute to the pathogenesis of cardiac phenotype in laminopathies. Suppression of the FOXO TFs in cardiac myocytes partially rescues the phenotype and prolongs survival. The findings identify FOXO TFs as potential therapeutic targets for cardiac phenotype in laminopathies.

The increasing relevance of nuclear envelope myopathies

PURPOSE OF REVIEW

Nuclear envelope links to a wide range of disorders, including several myopathies and neuropathies over the past 2 decades, has spurred research leading to a completely changed view of this important cellular structure and its functions. However, the many functions now assigned to the nuclear envelope make it increasingly hard to determine which functions underlie these disorders.

RECENT FINDINGS

New nuclear envelope functions in genome organization, regulation and repair, signaling, and nuclear and cellular mechanics have been added to its classical barrier function. Arguments can be made for any of these functions mediating abnormality in nuclear envelope disorders and data exist supporting many. Moreover, transient and/or distal nuclear envelope connections to other cellular proteins and structures may increase the complexity of these disorders.

SUMMARY

Although the increased understanding of nuclear envelope functions has made it harder to distinguish specific causes of nuclear envelope disorders, this is because it has greatly expanded the spectrum of possible mechanisms underlying them. This change in perspective applies well beyond the known nuclear envelope disorders, potentially implicating the nuclear envelope in a much wider range of myopathies and neuropathies.

Updated clinical overview on cardiac laminopathies: an electrical and mechanical disease

Cardiac laminopathies, associated with mutations in the LMNA gene, encompass a wide spectrum of clinical manifestations, involving electrical and mechanical alterations of cardiomyocytes. Thus, dilated cardiomyopathy, bradyarrhythmias and atrial or ventricular tachyarrhythmias may occur in a number of combined phenotypes. Nowadays, some attempt has been made to identify clinical predictors for the most life-threatening complications of LMNA-associated heart disease, i.e. sudden cardiac death and end-stage heart failure. The goal of this manuscript is to combine the most recent evidences in an updated review to show the state-of-the-art of such a complex disease group. This is supposed to be the starting point to collect more data and design new ad hoc studies to identify clinically useful predictors to stratify risk in mutation carriers, including probands and their asymptomatic relatives.