Obscurin deficiency leads to compensated dilated cardiomyopathy and increased arrhythmias

Obscurin is a large muscle protein whose multiple functions include providing mechanical strength to the M-band and linking the sarcomere to the sarcoplasmic reticulum. Mutations in obscurin are linked to various forms of muscle diseases. This study compares cardiac function in a murine model of obscurin deletion (KO) with wild-type (WT) in vivo and ex vivo. Echocardiography showed that KO hearts had larger (+20%) end-diastolic and end-systolic volumes, reduced fractional shortening, and impaired ejection fraction, consistent with dilated cardiomyopathy. However, stroke volume and cardiac output were preserved due to increased end-diastolic volume. Morphological analyses revealed reduced sarcoplasmic reticulum volume, with preserved T-tubule network. While myofilament function was preserved in isolated myofibrils and skinned trabeculae, experiments in intact trabeculae revealed that Obscn KO hearts compared with WT displayed (1) reduced active tension at high frequencies and during resting-state contractions, (2) impaired positive inotropic and lusitropic response to β-adrenergic stimulation (isoproterenol 0.1 μM), and (3) faster mechanical restitution, suggesting reduced sarcoplasmic reticulum refractoriness. Intracellular [Ca2+]i measurements showed reduced peak systolic and increased diastolic levels in KO versus WT cardiomyocytes. Western blot experiments revealed lower SERCA and phospholamban (PLB) expression and reduced PLB phosphorylation in KO mice. While action potential parameters and conduction velocity were unchanged, β-adrenergic stimulation induced more frequent spontaneous Ca2+ waves and increased arrhythmia susceptibility in KO compared with WT. Taken together, these findings suggest that obscurin deletion, in adult mice, is linked to compensated dilated cardiomyopathy, altered E-C coupling, impaired response to inotropic agents, and increased propensity to arrhythmias.

COL4A2 -Related Disorder Presenting in Adulthood With Rhabdomyolysis

The alpha 1 and 2 chains of type IV collagen, encoded by the COL4A1 (MIM 120130) and COL4A2 (MIM 120090) respectively, play essential roles in the vascular basement membranes. Pathogenic variants in COL4A1/ COL4A2 are associated with autosomal dominant cerebral angiopathies. The clinical manifestations of COL4A1/COL4A2-related disorders include: aneurysms, intracerebral hemorrhage, polymicrogyria, porencephaly, heterotopia, periventricular leukomalacia, epilepsy, and neurodevelopmental disorders. COL4A1 pathogenic variants that are in exons 24 and 25 have been associated with hereditary angiopathy, nephropathy, aneurysms, and cramps. The multisystemic phenotypes of COL4A1/COL4A2-related disorders are increasingly being studied. Animal models have suggested that COL4A2-related disorders may also manifest with a variable combination of multisystemic abnormalities affecting the eyes, muscles, and kidneys. Okano and colleagues recently reported a case of recurrent episodes of rhabdomyolysis in a 2-year-old with COL4A1-related disorder raising fundamental questions on mechanisms of COL4A1/COL4A2 variants in muscle homeostasis. To date, rhabdomyolysis has not been associated with COL4A2-related disorder in humans. Rhabdomyolysis is a medical emergency, where there is elevated creatine kinase (CK) level in the blood and increased excretion of myoglobin in urine, due to skeletal muscle damage and release of intracytoplasmic proteins into systemic circulation. Rhabdomyolysis is a serious medical condition. It require intensive care management due to an increased risk of some life-threatening complications [including disseminated intravascular coagulation, renal failure, and severe hyperkalemia]. Herein, we report a case of rhabdomyolysis in an adult with COL4A2-related structural brain malformations (including polymicrogyria and heterotopia).

Combined Loss of Obsc and Obsl1 in Murine Hearts Results in Diastolic Dysfunction, Altered Metabolism, and Deregulated Mitophagy

BACKGROUND

Muscle proteins of the obscurin protein family play important roles in sarcomere organization and sarcoplasmic reticulum and T-tubule architecture and function. However, their precise molecular functions and redundancies between protein family members as well as their involvement in cardiac diseases remain to be fully understood.

METHODS

To investigate the functional roles of Obsc (obscurin) and its close homolog Obsl1 (obscurin-like 1) in the heart, we generated and analyzed knockout mice for Obsc, Obsl1, as well as Obsc/Obsl1 double knockouts.

RESULTS

We show that double-knockout mice are viable but show postnatal deficits in cardiac muscle sarcoplasmic reticulum and mitochondrial architecture and function at the microscopic, biochemical, and cellular levels. Altered sarcoplasmic reticulum structure resulted in perturbed calcium cycling, whereas mitochondrial ultrastructure deficits were linked to decreased levels of Chchd3 (coiled-coil-helix-coiled-coil-helix domain containing 3), a Micos (mitochondrial contact site and cristae organizing system) complex protein. Hearts of double-knockout mice also show altered levels of Atg4 proteins, novel Obsl1 interactors, resulting in abnormal mitophagy, and increased unfolded protein response. At the physiological level, loss of obscurin and Obsl1 resulted in a profound delay of cardiac relaxation, associated with metabolic signs of heart failure.

CONCLUSIONS

Taken together, our data suggest that Obsc and Obsl1 play crucial roles in cardiac sarcoplasmic reticulum structure, calcium cycling, mitochondrial function, turnover, and metabolism.

Identification of Compound Heterozygous Variants in OBSCN Gene Associated With Rhabdomyolysis: A Case Report

BACKGROUND

The obscurin protein encoded by the OBSCN gene is an important structural protein in the regulation of myocyte sarcoplasmic nodule stability and sarcoplasmic reticulum function and is particularly closely associated with calcium ion (Ca2+) signaling. With increasing genomic studies, pathogenic variants in the OBSCN gene have been shown to be associated with a variety of inherited diseases, such as cardiomyopathy. However, case reports of its variants causing rhabdomyolysis are more limited.

METHODS

We performed whole exome sequencing on a patient with exercise-induced rhabdomyolysis to identify possible causative gene variants. In addition, functional prediction of the pathogenicity of the variants was performed by combining multiple bioinformatics analysis tools and in-depth analyses with clinical phenotypes and family history.

RESULTS

The patient carried compound heterozygous variants, including c.21184C>T (nonsense variant) and c.15610+12C>T (intronic splicing variant). The c.21184C>T variant resulted in a premature termination of the protein, was not included in population-based databases, and was supported by multiple prediction tools as a potentially pathogenic variant. The c.15610+12C>T variant was also absent in the gnomAD_EAS database and predicted to disturb normal splicing, potentially creating a novel donor site. The pathogenicity of the variant is further supported by the fact that the patient's mother, with a homozygous OBSCN variant, also exhibited exercise-induced myalgia. Clinically, the patient presented with exercise-induced rhabdomyolysis accompanied by significant serum creatine kinase elevation, muscle pain, and MRI-demonstrated muscle edema of both lower limbs without significant muscle weakness or cardiac abnormalities.

CONCLUSION

We report the first case of rhabdomyolysis in China caused by OBSCN gene variants. This finding further extends the spectrum of the OBSCN gene variants. It also provides an important basis for genetic counseling and helps in the early diagnosis and management of similar cases.

276th ENMC workshop: recommendations on optimal diagnostic pathway and management strategy for patients with acute rhabdomyolysis worldwide. 15th-17th March 2024, Hoofddorp, The Netherlands

The 276th ENMC Workshop on rhabdomyolysis brought together 21 experts to address the compelling need for standardized guidelines on the clinical approach of rhabdomyolysis. There was a general agreement that a diagnosis of rhabdomyolysis require that 1) clinical symptoms include severe muscle swelling, weakness and/or myalgia; 2) serum CK-levels exceed 10,000 IU/L in case of exertional, and >5000 IU/L in non-exertional rhabdomyolysis; 3) CK-levels reaching a maximum 1-4 days after the event and normalizing to baseline within 1-2 weeks of rest. In case of an underlying neuromuscular condition, CK-levels should exceed 5-10 times the patient's baseline level. Treatment should be initiated only in case of high risk on acute kidney injury, which can be predicted by the McMahon score. Furthermore, recommendations on performing genetic testing were formulated and the use of the 'RHABDO'- acronym was generally agreed upon as a tool to aid clinicians in deciding which patients require genetic testing. Moreover, recommendations on follow-up were made, with a particular emphasis on evaluation of physical and psychological sequelae. Patient representatives present during the workshop emphasized the importance of the current recommendations for future clinical guidelines on rhabdomyolysis.

Loss of OBSCN expression promotes bladder cancer progression but enhances the efficacy of PD-L1 inhibitors

BACKGROUND

As the objective overall response rate to immune checkpoint inhibitors (ICIs) is less than 30% in late stage or metastatic bladder cancer (BLCA), elucidating the intrinsic mechanisms of immune evasion is of great importance for the discovery of predictive and prognostic biomarkers and the exploration of novel targets for intervention. Recent studies have shown that OBSCN and the cytoskeletal protein it encodes, obscurin, play an important role in tumour progression. However, no studies have reported the role of OBSCN in BLCA.

METHODS

RNA sequencing and clinical data were downloaded from multiple public databases including The Cancer Genome Atlas and the Gene Expression Omnibus. Immunohistochemistry (IHC) was performed on tissue microarrays including 80 BLCA patients from Shuguang Hospital. Kaplan-Meier curves with log-rank test, univariate and multivariate COX regression were performed to evaluate the prognostic efficacy of OBSCN expression. In vitro experiments were conducted to determine the role of OBSCN deficiency in promoting BLCA progression. Pan-cancer tumour immune microenvironment (TIME) analysis was performed to explore the potential correlation between OBSCN deficiency and immune evasion.

RESULTS

Pan-cancers and single-cell sequencing analysis revealed that the expression level and proportion of OBSCN was significantly decreased in BLCA cells compared to normal urothelium. Survival curves showed that BLCA patients with low OBSCN expression had a worse prognosis, yet a better clinical response to PD-L1 ICIs. Gene set variation analysis and Gene set enrichment analysis revealed that epithelial-mesenchymal transition (EMT) and immune-related processes were significantly enriched in BLCA samples with low OBSCN expression. In vitro experiments identified that OBSCN-deficient BLCA cells enhanced invasion, migration and EMT. Pan-cancer analysis of TIME revealed that neoantigen, tumor mutation burden, CD8+T cells and immune checkpoints were significantly negatively associated with OBSCN expression. IHC and Western blot assay identified that BLCA samples with low OBSCN expression had more CD8+ T-cell infiltration and higher PD-L1 expression.

CONCLUSIONS

This study confirmed that BLCA patients with low OBSCN expression had a worse prognosis but a superior response to ICIs, providing a reference for individualised treatment of BLCA patients.

OBSCN undergoes extensive alternative splicing during human cardiac and skeletal muscle development

BACKGROUND

Highly expressed in skeletal muscles, the gene Obscurin (i.e. OBSCN) has 121 non-overlapping exons and codes for some of the largest known mRNAs in the human genome. Furthermore, it plays an essential role in muscle development and function. Mutations in OBSCN are associated with several hypertrophic cardiomyopathies and muscular disorders. OBSCN undergoes extensive and complex alternative splicing, which is the main reason that its splicing regulation associated with skeletal and cardiac muscle development has not previously been thoroughly studied.

METHODS

We analyzed RNA-Seq data from skeletal and cardiac muscles extracted from 44 postnatal individuals and six fetuses. We applied the intron/exon level splicing analysis software IntEREst to study the splicing of OBSCN in the studied samples. The differential splicing analysis was adjusted for batch effects. Our comparisons revealed the splicing variations in OBSCN between the human skeletal and cardiac muscle, as well as between post-natal muscle (skeletal and cardiac) and the pre-natal equivalent muscle.

RESULTS

We detected several splicing regulations located in the 5'end, 3' end, and the middle of OBSCN that are associated with human cardiac or skeletal muscle development. Many of these alternative splicing events have not previously been reported. Our results also suggest that many of these muscle-development associated splicing events may be regulated by BUB3.

CONCLUSIONS

We conclude that the splicing of OBSCN is extensively regulated during the human skeletal/cardiac muscle development. We developed an interactive visualization tool that can be used by clinicians and researchers to study the inclusion of specific OBSCN exons in pre- and postnatal cardiac and skeletal muscles and access the statistics for the differential inclusion of the exons across the studied sample groups. The OBSCN exon inclusion map related to the human cardiac and skeletal muscle development is available at http://psivis.it.helsinki.fi:3838/OBSCN_PSIVIS/ . These findings are essential for an accurate pre- and postnatal clinical interpretation of the OBSCN exonic variants.

Cellular mechanisms of acute rhabdomyolysis in inherited metabolic diseases

Acute rhabdomyolysis (RM) constitutes a life-threatening emergency resulting from the (acute) breakdown of skeletal myofibers, characterized by a plasma creatine kinase (CK) level exceeding 1000 IU/L in response to a precipitating factor. Genetic predisposition, particularly inherited metabolic diseases, often underlie RM, contributing to recurrent episodes. Both sporadic and congenital forms of RM share common triggers. Considering the skeletal muscle's urgent need to rapidly adjust to environmental cues, sustaining sufficient energy levels and functional autophagy and mitophagy processes are vital for its preservation and response to stressors. Crucially, the composition of membrane lipids, along with lipid and calcium transport, and the availability of adenosine triphosphate (ATP), influence membrane biophysical properties, membrane curvature in skeletal muscle, calcium channel signaling regulation, and determine the characteristics of autophagic organelles. Consequently, a genetic defect involving ATP depletion, aberrant calcium release, abnormal lipid metabolism and/or lipid or calcium transport, and/or impaired anterograde trafficking may disrupt autophagy resulting in RM. The complex composition of lipid membranes also alters Toll-like receptor signaling and viral replication. In response, infections, recognized triggers of RM, stimulate increased levels of inflammatory cytokines, affecting skeletal muscle integrity, energy metabolism, and cellular trafficking, while elevated temperatures can reduce the activity of thermolabile enzymes. Overall, several mechanisms can account for RMs and may be associated in the same disease-causing RM.

Discovering Novel Loci of Chronic Kidney Disease via Principal Component Analysis-Based Multiple-Trait Genome-Wide Association Study

INTRODUCTION

Chronic kidney diseases (CKD) encompass a spectrum of complex pathophysiological processes. While numerous genome-wide association studies (GWASs) have focused on individual traits such as albuminuria, estimated glomerular filtration rate (eGFR), and eGFR change, there remains a paucity of genetic studies integrating these traits collectively for comprehensive evaluation.

METHODS

In this study, we performed individual GWASs for albuminuria, baseline eGFR, and eGFR slope utilizing data from non-diabetic individuals enrolled from the Taiwan Biobank (TWB). Subsequently, we employed principal component analysis to transform these three quantitative traits into principal components (PCs) and performed GWAS based on these principal components (PC-based GWAS).

RESULTS

The individual GWAS analyses of albuminuria, baseline eGFR, and eGFR slope identified 10, 13, and 210 candidate loci respectively, with 2, 3, and 99 of them representing previously reported loci. PC-based GWAS identified additional 20 novel candidate loci linked to CKD (p values ranging from 5.8 × 10-7 to 9.1 × 10-6). Notably, 4 of these 20 single nucleotide polymorphisms (rs9332641, rs10737429, rs117231653, and rs73360624) exhibited significant associations with kidney expression quantitative trait loci.

CONCLUSION

To our knowledge, this study represents the first PC-based GWAS integrating albuminuria, baseline eGFR, and eGFR slope. Our approach found 20 novel candidate loci suggestively associated with CKD, underscoring the value of integrating multiple kidney traits in unraveling the pathophysiology of this complex disorder.

Large-scale exome sequence analysis identifies sex- and age-specific determinants of obesity

Obesity contributes substantially to the global burden of disease and has a significant heritable component. Recent large-scale exome sequencing studies identified several genes in which rare, protein-coding variants have large effects on adult body mass index (BMI). Here we extended such work by performing sex-stratified associations in the UK Biobank study (N∼420,000). We identified genes in which rare heterozygous loss-of-function increases adult BMI in women (DIDO1, PTPRG, and SLC12A5) and in men (SLTM), with effect sizes up to ∼8 kg/m2. This is complemented by analyses implicating rare variants in OBSCN and MADD for recalled childhood adiposity. The known functions of these genes, as well as findings of common variant genome-wide pathway enrichment analyses, suggest a role for neuron death, apoptosis, and DNA damage response mechanisms in the susceptibility to obesity across the life-course. These findings highlight the importance of considering sex-specific and life-course effects in the genetic regulation of obesity.

A novel IBA57 variant is associated with mitochondrial iron-sulfur protein deficiency and necrotizing myelopathy in dogs

Introduction: Hereditary necrotizing myelopathy (HNM) in young Kooiker dogs is characterized by progressive ataxia and paralysis with autosomal recessive inheritance. The basic genetic defect is unknown. We investigated the possible cause by a genome-wide analysis using six affected and 17 unrelated unaffected Kooiker dogs and by functional follow-up studies. Method: The HNM locus was mapped by a case-control study using a dense SNP array and confirmed by linkage analysis of two pedigrees. The gene exons in the critical region were analyzed by next-generation sequencing. The functional effect of the candidate canine IBA57 pathogenic variant was biochemically examined in an established HeLa cell culture model in which the endogenous IBA75 gene product was depleted by RNAi. Results: The basic defect was localized in the centromeric 5 Mb region of canine chromosome 14. The most associated SNP co-segregated fully with HNM and reached an LOD score of 6.1. A candidate pathogenic mutation was found in the iron-sulfur cluster assembly gene IBA57 and led to the amino acid substitution R147W. The expression of human IBA57 harboring the canine R147W exchange could only partially restore the biochemical defects of several mitochondrial [4Fe-4S] proteins upon IBA57 depletion, showing that the mutant protein is functionally impaired. Discussion: Pathogenic variants in human IBA57 cause multiple mitochondrial dysfunction syndrome 3 (MMDS3), a neurodegenerative disorder with distant similarities to HNM. The incomplete functional complementation of IBA57-depleted human cells by IBA57-R147W identifies the DNA mutation in affected Kooiker dogs as the genetic cause of HNM. Our findings further expand the phenotypic spectrum of pathogenic IBA57 variants.

Recent advances in our understanding of genetic rhabdomyolysis

PURPOSE OF REVIEW

This review summarizes recent advances in our understanding of the genetics of rhabdomyolysis.

RECENT FINDINGS

Rhabdomyolysis is the acute breakdown of myofibres resulting in systemic changes that can be life-threatening. Environmental triggers, including trauma, exercise, toxins and infections, and/or gene defects can precipitate rhabdomyolysis. A schema (aptly titled RHABDO) has been suggested for evaluating whether a patient with rhabdomyolysis is likely to harbour an underlying genetic defect. It is becoming increasingly recognized that defects in muscular dystrophy and myopathy genes can trigger rhabdomyolysis, even as the sole or presenting feature. Variants in genes not previously associated with human disease have been identified recently as causative of rhabdomyolysis, MLIP , MYH1 and OBSCN . Our understanding of the pathomechanisms contributing to rhabdomyolysis have also improved with an increased awareness of the role of mitochondrial dysfunction in LPIN1 , FDX2 , ISCU and TANGO2 -mediated disease.

SUMMARY

An accurate genetic diagnosis is important for optimal clinical management of the patient, avoiding associated triggers and genetic counselling and cascade screening. Despite recent advances in our understanding of the genetics contributing to rhabdomyolysis, many patients remain without an accurate genetic diagnosis, suggesting there are many more causative genes, variants and disease mechanisms to uncover.