Mutation in lamin A/C sensitizes the myocardium to exercise-induced mechanical stress but has no effect on skeletal muscles in mouse

Palpitations in athletes: diagnosis, workup and treatment

Palpitations are a common reason for athletes to seek medical care. Although often benign, palpitations may serve as a harbinger for underling cardiac pathology. Given the unique challenges in this population, this review will serve to discuss the basic underlying pathophysiology, which may predispose athletes to palpitations. In addition, we will review the aetiologies, diagnostic evaluation, management and counselling strategies for some of the most common diagnoses seen in athletes.

Lamin A/C and PI(4,5)P2-A Novel Complex in the Cell Nucleus

Lamins, the nuclear intermediate filaments, are important regulators of nuclear structural integrity as well as nuclear functional processes such as DNA transcription, replication and repair, and epigenetic regulations. A portion of phosphorylated lamin A/C localizes to the nuclear interior in interphase, forming a lamin A/C pool with specific properties and distinct functions. Nucleoplasmic lamin A/C molecular functions are mainly dependent on its binding partners; therefore, revealing new interactions could give us new clues on the lamin A/C mechanism of action. In the present study, we show that lamin A/C interacts with nuclear phosphoinositides (PIPs), and with nuclear myosin I (NM1). Both NM1 and nuclear PIPs have been previously reported as important regulators of gene expression and DNA damage/repair. Furthermore, phosphorylated lamin A/C forms a complex with NM1 in a phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2)-dependent manner in the nuclear interior. Taken together, our study reveals a previously unidentified interaction between phosphorylated lamin A/C, NM1, and PI(4,5)P2 and suggests new possible ways of nucleoplasmic lamin A/C regulation, function, and importance for the formation of functional nuclear microdomains.

Sudden Cardiac Death in Young Athletes: JACC State-of-the-Art Review

Athletes epitomize the healthiest segment of society. Despite this premise, sudden cardiac death may occur in apparently healthy athletes, attracting significant attention not only in the medical community but also in laypersons and media. The incidence of sudden cardiac death is variably reported, and epidemiological burden differs among cohorts. Athletes appear to be at risk of developing fatal arrhythmias when harboring a quiescent cardiac disorder. Primary cardiomyopathies, ion channelopathies, and coronary artery anomalies are prevalent causes in young individuals. Cardiac assessment of athletes can be challenging because these individuals exhibit a plethora of electrical, structural, and functional physiological changes that overlap with cardiac pathology. A diagnosis of cardiac disease in a young athlete is not necessarily an indication to terminate competition and sports participation. International guidelines, traditionally focused on disqualification of individuals with cardiac disease, have recently adopted a more liberal attitude, based on a careful assessment of the risk and on a shared-decision making approach.

Epigenetics in LMNA-Related Cardiomyopathy

Mutations in the gene for lamin A/C (LMNA) cause a diverse range of diseases known as laminopathies. LMNA-related cardiomyopathy is a common inherited heart disease and is highly penetrant with a poor prognosis. In the past years, numerous investigations using mouse models, stem cell technologies, and patient samples have characterized the phenotypic diversity caused by specific LMNA variants and contributed to understanding the molecular mechanisms underlying the pathogenesis of heart disease. As a component of the nuclear envelope, LMNA regulates nuclear mechanostability and function, chromatin organization, and gene transcription. This review will focus on the different cardiomyopathies caused by LMNA mutations, address the role of LMNA in chromatin organization and gene regulation, and discuss how these processes go awry in heart disease.

The Nucleoskeleton: Crossroad of Mechanotransduction in Skeletal Muscle

Intermediate filaments (IFs) are a primary structural component of the cytoskeleton extending throughout the muscle cell (myofiber). Mechanotransduction, the process by which mechanical force is translated into a biochemical signal to activate downstream cellular responses, is crucial to myofiber function. Mechanical forces also act on the nuclear cytoskeleton, which is integrated with the myofiber cytoskeleton by the linker of the nucleoskeleton and cytoskeleton (LINC) complexes. Thus, the nucleus serves as the endpoint for the transmission of force through the cell. The nuclear lamina, a dense meshwork of lamin IFs between the nuclear envelope and underlying chromatin, plays a crucial role in responding to mechanical input; myofibers constantly respond to mechanical perturbation via signaling pathways by activation of specific genes. The nucleus is the largest organelle in cells and a master regulator of cell homeostasis, thus an understanding of how it responds to its mechanical environment is of great interest. The importance of the cell nucleus is magnified in skeletal muscle cells due to their syncytial nature and the extreme mechanical environment that muscle contraction creates. In this review, we summarize the bidirectional link between the organization of the nucleoskeleton and the contractile features of skeletal muscle as they relate to muscle function.

Exercise is Associated With Impaired Left Ventricular Systolic Function in Patients With Lamin A/C Genotype

Background

Lamin A/C cardiomyopathy is a malignant and highly penetrant inheritable cardiomyopathy. Competitive sports have been associated with adverse events in these patients, but data on recreational exercise are lacking. We aimed to explore associations between exercise exposure and disease severity in patients with lamin A/C genotype.

Methods and Results

Lamin A/C genotype positive patients answered a questionnaire on exercise habits from age 7 years until genetic diagnosis. We recorded exercise hours >3 metabolic equivalents and calculated cumulative lifetime exercise. Patients were grouped in active or sedate based on lifetime exercise hours above or below median. We performed echocardiography, 12‐lead ECG, Holter monitoring, and biomarkers including NT‐proBNP (N‐terminal pro‐B‐type natriuretic peptide). We defined left ventricular ejection fraction <45% as a clinically significant impairment of left ventricular function. We included 69 patients (age 42±14 years, 41% probands, 46% women) with median lifetime exercise 4160 (interquartile range 1041–6924) hours. Active patients were more frequently probands (53% versus 29%, P=0.04), had lower left ventricular ejection fraction (43±13% versus 51±11%, P=0.006), and higher NT‐proBNP (78 [interquartile range 32–219] pmol/L versus 30 [interquartile range 13–64] pmol/L, P=0.03) compared with sedate, while age did not differ (45±13 years versus 40±16 years, P=0.16). The decrease in left ventricular ejection fraction per tertile increment in lifetime exercise was 4% (95% CI −7% to −0.4%, P=0.03), adjusted for age and sex and accounting for dependence within families. Left ventricular ejection fraction <45% was observed at a younger age in active patients (log rank P=0.007).

Conclusions

Active lamin A/C patients had worse systolic function compared with sedate which occurred at younger age. Our findings may improve exercise recommendations in patients with lamin A/C.

Definition and treatment of arrhythmogenic cardiomyopathy: an updated expert panel report

It is 35 years since the first description of arrhythmogenic right ventricular cardiomyopathy (ARVC) and more than 20 years since the first reports establishing desmosomal gene mutations as a major cause of the disease. Early advances in the understanding of the clinical, pathological and genetic architecture of ARVC resulted in consensus diagnostic criteria, which proved to be sensitive but not entirely specific for the disease. In more recent years, clinical and genetic data from families and the recognition of a much broader spectrum of structural disorders affecting both ventricles and associated with a propensity to ventricular arrhythmia have raised many questions about pathogenesis, disease terminology and clinical management. In this paper, we present the conclusions of an expert round table that aimed to summarise the current state of the art in arrhythmogenic cardiomyopathies and to define future research priorities.

Modeling Skeletal Muscle Laminopathies Using Human Induced Pluripotent Stem Cells Carrying Pathogenic LMNA Mutations

Laminopathies are a clinically heterogeneous group of disorders caused by mutations in LMNA. The main proteins encoded by LMNA are Lamin A and C, which together with Lamin B1 and B2, form the nuclear lamina: a mesh-like structure located underneath the inner nuclear membrane. Laminopathies show striking tissue specificity, with subtypes affecting striated muscle, peripheral nerve, and adipose tissue, while others cause multisystem disease with accelerated aging. Although several pathogenic mechanisms have been proposed, the exact pathophysiology of laminopathies remains unclear, compounded by the rarity of these disorders and lack of easily accessible cell types to study. To overcome this limitation, we used induced pluripotent stem cells (iPSCs) from patients with skeletal muscle laminopathies such as LMNA-related congenital muscular dystrophy and limb-girdle muscular dystrophy 1B, to model disease phenotypes in vitro. iPSCs can be derived from readily accessible cell types, have unlimited proliferation potential and can be differentiated into cell types that would otherwise be difficult and invasive to obtain. iPSC lines from three skeletal muscle laminopathy patients were differentiated into inducible myogenic cells and myotubes. Disease-associated phenotypes were observed in these cells, including abnormal nuclear shape and mislocalization of nuclear lamina proteins. Nuclear abnormalities were less pronounced in monolayer cultures of terminally differentiated skeletal myotubes than in proliferating myogenic cells. Notably, skeletal myogenic differentiation of LMNA-mutant iPSCs in artificial muscle constructs improved detection of myonuclear abnormalities compared to conventional monolayer cultures across multiple pathogenic genotypes, providing a high-fidelity modeling platform for skeletal muscle laminopathies. Our results lay the foundation for future iPSC-based therapy development and screening platforms for skeletal muscle laminopathies.

Lmna knockout mouse embryonic fibroblasts are less contractile than their wild-type counterparts

In order to maintain tissue homeostasis and functionality, adherent cells need to sense and respond to environmental mechanical stimuli. An important ability that adherent cells need in order to properly sense and respond to mechanical stimuli is the ability to exert contractile stress onto the environment via actin stress fibers. The actin stress fibers form a structural chain between the cells' environment via focal adhesions and the nucleus via the nuclear lamina. In case one of the links in this chain is missing or aberrant, contractile stress generation will be affected. This is especially the case in laminopathic cells, which have a missing or mutated form of the LMNA gene encoding for part of the nuclear lamina. Using the thin film method combined with sample specific finite element modeling, we quantitatively showed a fivefold lower contractile stress generation of Lmna knockout mouse embryonic fibroblasts (MEFs) as compared to wild-type MEFs. Via fluorescence microscopy it was demonstrated that the lower contractile stress generation was associated with an impaired actin stress fiber organization with thinner actin fibers and smaller focal adhesions. Similar experiments with wild-type MEFs with chemically disrupted actin stress fibers verified these findings. These data illustrate the importance of an organized actin stress fiber network for contractile stress generation and demonstrate the devastating effect of an impaired stress fiber organization in laminopathic fibroblasts. Next to this, the thin film method is expected to be a promising tool in unraveling contractility differences between fibroblasts with different types of laminopathic mutations.

Gene Therapy via Trans-Splicing for LMNA-Related Congenital Muscular Dystrophy

We assessed the potential of Lmna-mRNA repair by spliceosome-mediated RNA trans-splicing as a therapeutic approach for LMNA-related congenital muscular dystrophy. This gene therapy strategy leads to reduction of mutated transcript expression for the benefit of corresponding wild-type (WT) transcripts. We developed 5′-RNA pre-trans-splicing molecules containing the first five exons of Lmna and targeting intron 5 of Lmna pre-mRNA. Among nine pre-trans-splicing molecules, differing in the targeted sequence in intron 5 and tested in C2C12 myoblasts, three induced trans-splicing events on endogenous Lmna mRNA and confirmed at protein level. Further analyses performed in primary myotubes derived from an LMNA-related congenital muscular dystrophy (L-CMD) mouse model led to a partial rescue of the mutant phenotype. Finally, we tested this approach in vivo using adeno-associated virus (AAV) delivery in newborn mice and showed that trans-splicing events occurred in WT mice 50 days after AAV delivery, although at a low rate. Altogether, while these results provide the first evidence for reprogramming LMNA mRNA in vitro, strategies to improve the rate of trans-splicing events still need to be developed for efficient application of this therapeutic approach in vivo.

Current insights into LMNA cardiomyopathies: Existing models and missing LINCs

The nuclear lamina is a critical structural domain for the maintenance of genomic stability and whole-cell mechanics. Mutations in the LMNA gene, which encodes nuclear A-type lamins lead to the disruption of these key cellular functions, resulting in a number of devastating diseases known as laminopathies. Cardiomyopathy is a common laminopathy and is highly penetrant with poor prognosis. To date, cell mechanical instability and dysregulation of gene expression have been proposed as the main mechanisms driving cardiac dysfunction, and indeed discoveries in these areas have provided some promising leads in terms of therapeutics. However, important questions remain unanswered regarding the role of lamin A dysfunction in the heart, including a potential role for the toxicity of lamin A precursors in LMNA cardiomyopathy, which has yet to be rigorously investigated.