Identification of KLHL41 Mutations Implicates BTB-Kelch-Mediated Ubiquitination as an Alternate Pathway to Myofibrillar Disruption in Nemaline Myopathy

Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives

Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.

Tirasemtiv enhances submaximal muscle tension in an Acta1:p.Asp286Gly mouse model of nemaline myopathy

Nemaline myopathies are the most common form of congenital myopathies. Variants in ACTA1 (NEM3) comprise 15-25% of all nemaline myopathy cases. Patients harboring variants in ACTA1 present with a heterogeneous disease course characterized by stable or progressive muscle weakness and, in severe cases, respiratory failure and death. To date, no specific treatments are available. Since NEM3 is an actin-based thin filament disease, we tested the ability of tirasemtiv, a fast skeletal muscle troponin activator, to improve skeletal muscle function in a mouse model of NEM3, harboring the patient-based p.Asp286Gly variant in Acta1. Acute and long-term tirasemtiv treatment significantly increased muscle contractile capacity at submaximal stimulation frequencies in both fast-twitch extensor digitorum longus and gastrocnemius muscle, and intermediate-twitch diaphragm muscle in vitro and in vivo. Additionally, long-term tirasemtiv treatment in NEM3 mice resulted in a decreased respiratory rate with preserved minute volume, suggesting more efficient respiration. Altogether, our data support the therapeutic potential of fast skeletal muscle troponin activators in alleviating skeletal muscle weakness in a mouse model of NEM3 caused by the Acta1:p.Asp286Gly variant.

Clinico-pathological and gene features of 15 nemaline myopathy patients from a single Chinese neuromuscular center

BACKGROUND

Nemaline myopathy, the most common of the congenital myopathies, is caused by various genetic mutations. In this study, we attempted to investigate the clinical features, muscle pathology and genetic features of 15 patients with nemaline myopathy.

RESULTS

Among the 15 patients, there were 9 (60.00%) males and 6 (40.00%) females, and 9 (60.00%) of them came from three families respectively. The age of seeing a doctor ranged from 9 to 52 years old, the age of onset was from 5 to 23 years old, and the duration of disease ranged from 3 to 35 years. Ten out of the 15 patients had high arched palate and elongated face. Only one patient had mild respiratory muscle involvement and none had dysphagia. Muscle biopsies were performed in 9 out of the 15 patients. Pathologically, muscle fibers of different sizes, atrophic muscle fibers and compensatory hypertrophic fibers could be found, and occasionally degenerated and necrotic muscle fibers were observed. Different degrees of nemaline bodies aggregation could be seen in all 9 patients. The distribution of type I and type II muscle fibers were significantly abnormal in patients with nemaline myopathy caused by NEB gene, however, it was basically normal in patients with nemaline myopathy caused by TPM3 gene and ACTA1 gene. Electron microscopic analysis of 6 patients showed that nemaline bodies aggregated between myofibrils were found in 5(83.33%) cases, and most of them were located near the Z band, but no intranuclear rods were found. The gene analysis of 15 NM patients showed that three NM-related genes were harbored, including 11 (73.33%) patients with NEB, 3 (20.00%) patients with TPM3, and 1 (6.67%) patient with ACTA1, respectively. A total of 12 mutation sites were identified and included 10 (83.33%) mutations in exon and 2(16.67%) mutations in intron.

CONCLUSIONS

The clinical phenotype of nemaline myopathy is highly heterogeneous. Muscle pathology shows that nemaline bodies aggregation is an important feature for the diagnosis of NM. NEB is the most frequent causative gene in this cohort. The splicing mutation, c.21522 + 3A > G may be the hotspot mutation of the NEB gene in Chinese NM patients.

Emerging Roles of Cullin-RING Ubiquitin Ligases in Cardiac Development

Heart development is a spatiotemporally regulated process that extends from the embryonic phase to postnatal stages. Disruption of this highly orchestrated process can lead to congenital heart disease or predispose the heart to cardiomyopathy or heart failure. Consequently, gaining an in-depth understanding of the molecular mechanisms governing cardiac development holds considerable promise for the development of innovative therapies for various cardiac ailments. While significant progress in uncovering novel transcriptional and epigenetic regulators of heart development has been made, the exploration of post-translational mechanisms that influence this process has lagged. Culling-RING E3 ubiquitin ligases (CRLs), the largest family of ubiquitin ligases, control the ubiquitination and degradation of ~20% of intracellular proteins. Emerging evidence has uncovered the critical roles of CRLs in the regulation of a wide range of cellular, physiological, and pathological processes. In this review, we summarize current findings on the versatile regulation of cardiac morphogenesis and maturation by CRLs and present future perspectives to advance our comprehensive understanding of how CRLs govern cardiac developmental processes.

Cullin-3 intervenes in muscle atrophy in the elderly by mediating the degradation of nAchRs ubiquitination

Sarcopenia involves in the loss of muscle mass associated with aging, which is the major cause of progressive muscle weakness and deterioration in older adults. Muscle atrophy is a direct presentation of sarcopenia, and it greatly contributes to the decline in quality of life among older adults. Neuromuscular junction (NMJ) stability is the key link to maintain muscle function. Besides, the degenerative change of NMJ promotes the process of muscle atrophy in the elderly. Based on previous transcriptome sequencing and bioinformatics analyses of aged muscle, this study used the 18-month-old aged mouse model and the 6-month-old young mouse model to deliberate the role and underlying mechanisms of Cullin-3 (Cul3) in age-related muscle atrophy. The results of reverse transcriptase polymerase chain reaction (RT-PCR) and immunoblotting analysis showed that the expression of CUL3 increased in aged muscle tissue, while the expression level of postsynaptic membrane nicotinic acetylcholine receptors (nAChRs) decreased significantly, which manfested a negative correlation. Meanwhile, immunofluorescence demonstrated that Cul3 was highly expressed in senile muscle NMJ. The results of ubiquitin indicated that the ubiquitin level of aged muscle nAChRs was evidently increased. Co-immunoprecipitation furtherly verified the correlation between Cul3 and nAChRs. Taken together, Cul3 may mediate the ubiquitination degradation of nAChRs protein at the NMJ site in aged mice, leading to NMJ degeneration and accelerated atrophy of fast-twitch muscle fibers in aged muscle. As a prominent element to maintain the stability of NMJ, Cul3 is supposed to be one of candidate intervention targets in sarcopenia.

NRAP reduction rescues sarcomere defects in nebulin-related nemaline myopathy

Nemaline myopathy (NM) is a rare neuromuscular disorder associated with congenital or childhood-onset of skeletal muscle weakness and hypotonia, which results in limited motor function. NM is a genetic disorder and mutations in 12 genes are known to contribute to autosomal dominant or recessive forms of the disease. Recessive mutations in nebulin (NEB) are the most common cause of NM affecting about 50% of patients. Because of the large size of the NEB gene and lack of mutational hot spots, developing therapies that can benefit a wide group of patients is challenging. Although there are several promising therapies under investigation, there is no cure for NM. Therefore, targeting disease modifiers that can stabilize or improve skeletal muscle function may represent alternative therapeutic strategies. Our studies have identified Nrap upregulation in nebulin deficiency that contributes to structural and functional deficits in NM. We show that genetic ablation of nrap in nebulin deficiency restored sarcomeric disorganization, reduced protein aggregates and improved skeletal muscle function in zebrafish. Our findings suggest that Nrap is a disease modifier that affects skeletal muscle structure and function in NM; thus, therapeutic targeting of Nrap in nebulin-related NM and related diseases may be beneficial for patients.

A new homozygous missense variant in LMOD3 gene causing mild nemaline myopathy with prominent facial weakness

Nemaline myopathy (NEM) type 10, caused by biallelic mutations in LMOD3, is a severe congenital myopathy clinically characterized by generalized hypotonia and muscle weakness, respiratory insufficiency, joint contractures, and bulbar weakness. Here, we describe a family with two adult patients presenting mild nemaline myopathy due to a novel homozygous missense variant in LMOD3. Both patients presented mild delayed motor milestones, frequent falls during infancy, prominent facial weakness and mild muscle weakness in the four limbs. Muscle biopsy showed mild myopathic changes and small nemaline bodies in a few fibers. A neuromuscular gene panel revealed a homozygous missense variant in LMOD3 that co-segregated with the disease in the family (NM_198271.4: c.1030C>T; p.Arg344Trp). The patients described here provide evidence of the phenotype-genotype correlation, suggesting that non-truncating variants in LMOD3 lead to milder phenotypes of NEM type 10.

A novel loss-of-function mutation in NRAP is associated with left ventricular non-compaction cardiomyopathy

Background

The nebulin-related-anchoring protein (NRAP) gene encodes actin-associated ankyrin. Few studies reported the association of the NRAP gene with cardiomyopathy. Thus, the genetic role of this gene in cardiomyopathy remains to be investigated.

Methods

The clinical data of the rare case of left ventricular non-compaction (LVNC) were collected and analyzed. Whole-exome sequencing (WES) was performed on related family members. Western blot was used to detect the effect of mutation on the NRAP protein expression. The effect of the c.259delC variant on myocardial development was further evaluated in a zebrafish model.

Results

A novel homozygous frameshift mutation c.259delC of NRAP was found in the proband with LVNC. It was found that c.259delC decreased the expression of NRAP by Western blot. In the zebrafish model, the heart development was affected while knocking out the NRAP gene, which showed pericardial edema. The pathological manifestations were uneven hypertrophy, disordered arrangement of cardiomyocytes, enlarged intercellular space, and loose muscle fibers. RNA-sequencing (RNA-seq) showed that the expression of genes related to heart development decreased significantly, and the NRAP gene mutation could participate in biological processes (BPs) such as myocardial contraction, cell adhesion, myosin coarse filament assembly of striated muscle, myosin complex composition, and muscle α-actin binding.

Conclusion

We identified a rare case of LVNC associated with a novel homozygous NRAP frameshift variant. This study further strengthened the evidence linking mutations in the NRAP gene with LVNC, providing a new clue for further study of LVNC. NRAP may be one of the pathogenic genes of cardiomyopathy.

Modular UBE2H-CTLH E2-E3 complexes regulate erythroid maturation

The development of haematopoietic stem cells into mature erythrocytes - erythropoiesis - is a controlled process characterized by cellular reorganization and drastic reshaping of the proteome landscape. Failure of ordered erythropoiesis is associated with anaemias and haematological malignancies. Although the ubiquitin system is a known crucial post-translational regulator in erythropoiesis, how the erythrocyte is reshaped by the ubiquitin system is poorly understood. By measuring the proteomic landscape of in vitro human erythropoiesis models, we found dynamic differential expression of subunits of the CTLH E3 ubiquitin ligase complex that formed maturation stage-dependent assemblies of topologically homologous RANBP9- and RANBP10-CTLH complexes. Moreover, protein abundance of CTLH's cognate E2 ubiquitin conjugating enzyme UBE2H increased during terminal differentiation, and UBE2H expression depended on catalytically active CTLH E3 complexes. CRISPR-Cas9-mediated inactivation of CTLH E3 assemblies or UBE2H in erythroid progenitors revealed defects, including spontaneous and accelerated erythroid maturation as well as inefficient enucleation. Thus, we propose that dynamic maturation stage-specific changes of UBE2H-CTLH E2-E3 modules control the orderly progression of human erythropoiesis.

Nemaline Myopathy in Brazilian Patients: Molecular and Clinical Characterization

Nemaline myopathy (NM), a structural congenital myopathy, presents a significant clinical and genetic heterogeneity. Here, we compiled molecular and clinical data of 30 Brazilian patients from 25 unrelated families. Next-generation sequencing was able to genetically classify all patients: sixteen families (64%) with mutation in NEB, five (20%) in ACTA1, two (8%) in KLHL40, and one in TPM2 (4%) and TPM3 (4%). In the NEB-related families, 25 different variants, 11 of them novel, were identified; splice site (10/25) and frame shift (9/25) mutations were the most common. Mutation c.24579 G>C was recurrent in three unrelated patients from the same region, suggesting a common ancestor. Clinically, the “typical” form was the more frequent and caused by mutations in the different NM genes. Phenotypic heterogeneity was observed among patients with mutations in the same gene. Respiratory involvement was very common and often out of proportion with limb weakness. Muscle MRI patterns showed variability within the forms and genes, which was related to the severity of the weakness. Considering the high frequency of NEB mutations and the complexity of this gene, NGS tools should be combined with CNV identification, especially in patients with a likely non-identified second mutation.

Weighted Single-Step GWAS Identifies Genes Influencing Fillet Color in Rainbow Trout

The visual appearance of the fish fillet is a significant determinant of consumers' purchase decisions. Depending on the rainbow trout diet, a uniform bright white or reddish/pink fillet color is desirable. Factors affecting fillet color are complex, ranging from the ability of live fish to accumulate carotenoids in the muscle to preharvest environmental conditions, early postmortem muscle metabolism, and storage conditions. Identifying genetic markers of fillet color is a desirable goal but a challenging task for the aquaculture industry. This study used weighted, single-step GWAS to explore the genetic basis of fillet color variation in rainbow trout. We identified several SNP windows explaining up to 3.5%, 2.5%, and 1.6% of the additive genetic variance for fillet redness, yellowness, and whiteness, respectively. SNPs are located within genes implicated in carotenoid metabolism (β,β-carotene 15,15'-dioxygenase, retinol dehydrogenase) and myoglobin homeostasis (ATP synthase subunit β, mitochondrial (ATP5F1B)). These genes are involved in processes that influence muscle pigmentation and postmortem flesh coloration. Other identified genes are involved in the maintenance of muscle structural integrity (kelch protein 41b (klh41b), collagen α-1(XXVIII) chain (COL28A1), and cathepsin K (CTSK)) and protection against lipid oxidation (peroxiredoxin, superoxide dismutase 2 (SOD2), sestrin-1, Ubiquitin carboxyl-terminal hydrolase-10 (USP10)). A-to-G single-nucleotide polymorphism in β,β-carotene 15,15'-dioxygenase, and USP10 result in isoleucine-to-valine and proline-to-leucine non-synonymous amino acid substitutions, respectively. Our observation confirms that fillet color is a complex trait regulated by many genes involved in carotenoid metabolism, myoglobin homeostasis, protection against lipid oxidation, and maintenance of muscle structural integrity. The significant SNPs identified in this study could be prioritized via genomic selection in breeding programs to improve fillet color in rainbow trout.

Clinico-pathological features and mutational spectrum of 16 nemaline myopathy patients from a Chinese neuromuscular center

Nemaline myopathy (NM) is a congenital myopathy of great heterogeneity, characterized by the presence of rods in the cytoplasm of muscle fibers. The samples of 16 nemaline myopathy patients diagnosed by characteristically pathological features went through whole exon sequencing. Clinico-pathological and genetic features of the cases were systematically analyzed. According to the classification of nemaline myopathy by ENMC, 8 cases are typical congenital subtype, 6 cases are childhood/juvenile onset subtype and 2 case are adult onset subtype. In histological findings, characteristic purple-colored rods are discovered under modified gömöri trichrome staining (MGT). Electron microscopy revealed the presence of high electron-dense nemaline bodies around the submucosa and the nucleus nine patients (9/16 56.3%) were detected pathogenic causative mutations, among whom mutations in the NEB gene were the most frequent (6 patients, 66.7%). KBTBD13 gene mutation was discovered in two patients and ACTA1 gene mutation was discovered in 1 patient. Nemaline myopathy is a congenital myopathy with highly clinico-pathological and genetic heterogeneity. NEB gene mutation is the most common mutation, in which splicing change c.21522 +3A > G is hotspot mutation in Chinese NM patients.

Myoblast deactivation within engineered human skeletal muscle creates a transcriptionally heterogeneous population of quiescent satellite-like cells

Satellite cells (SCs), the adult Pax7-expressing stem cells of skeletal muscle, are essential for muscle repair. However, in vitro investigations of SC function are challenging due to isolation-induced SC activation, loss of native quiescent state, and differentiation to myoblasts. In the present study, we optimized methods to deactivate in vitro expanded human myoblasts within a 3D culture environment of engineered human skeletal muscle tissues ("myobundles"). Immunostaining and gene expression analyses revealed that a fraction of myoblasts within myobundles adopted a quiescent phenotype (3D-SCs) characterized by increased Pax7 expression, cell cycle exit, and activation of Notch signaling. Similar to native SCs, 3D-SC quiescence is regulated by Notch and Wnt signaling while loss of quiescence and reactivation of 3D-SCs can be induced by growth factors including bFGF. Myobundle injury with a bee toxin, melittin, induces robust myofiber fragmentation, functional decline, and 3D-SC proliferation. By applying single cell RNA-sequencing (scRNA-seq), we discover the existence of two 3D-SC subpopulations (quiescent and activated), identify deactivation-associated gene signature using trajectory inference between 2D myoblasts and 3D-SCs, and characterize the transcriptomic changes within reactivated 3D-SCs in response to melittin-induced injury. These results demonstrate the ability of an in vitro engineered 3D human skeletal muscle environment to support the formation of a quiescent and heterogeneous SC population recapitulating several aspects of the native SC phenotype, and provide a platform for future studies of human muscle regeneration and disease-associated SC dysfunction.

PROTAC targeted protein degraders: the past is prologue

Targeted protein degradation (TPD) is an emerging therapeutic modality with the potential to tackle disease-causing proteins that have historically been highly challenging to target with conventional small molecules. In the 20 years since the concept of a proteolysis-targeting chimera (PROTAC) molecule harnessing the ubiquitin-proteasome system to degrade a target protein was reported, TPD has moved from academia to industry, where numerous companies have disclosed programmes in preclinical and early clinical development. With clinical proof-of-concept for PROTAC molecules against two well-established cancer targets provided in 2020, the field is poised to pursue targets that were previously considered 'undruggable'. In this Review, we summarize the first two decades of PROTAC discovery and assess the current landscape, with a focus on industry activity. We then discuss key areas for the future of TPD, including establishing the target classes for which TPD is most suitable, expanding the use of ubiquitin ligases to enable precision medicine and extending the modality beyond oncology.

Defective protein degradation in genetic disorders

Understanding the molecular mechanisms that underlie different human pathologies is necessary to develop novel therapeutic strategies. An emerging mechanism of pathogenesis in many genetic disorders is the dysregulation of protein degradation, which leads to the accumulation of proteins that are responsible for the disease phenotype. Among the different cellular pathways that regulate active proteolysis, the Cullin RING E3 ligases represent an important group of sophisticated enzymatic complexes that mediate substrate ubiquitination through the interaction with specific adaptors. However, pathogenic mutations in these adaptors affect the physiological ubiquitination of their substrates. This review discusses our current understanding of this emerging field.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Recent advances in nemaline myopathy

The nemaline myopathies constitute a large proportion of the congenital or structural myopathies. Common to all patients is muscle weakness and the presence in the muscle biopsy of nemaline rods. The causative genes are at least twelve, encoding structural or regulatory proteins of the thin filament, and the clinical picture as well as the histological appearance on muscle biopsy vary widely. Here, we suggest a renewed clinical classification to replace the original one, summarise what is known about the pathogenesis from mutations in each causative gene to the forms of nemaline myopathy described to date, and provide perspectives on pathogenetic mechanisms possibly open to therapeutic modalities.

Dysregulation of NRAP degradation by KLHL41 contributes to pathophysiology in nemaline myopathy

Nemaline myopathy (NM) is the most common form of congenital myopathy that results in hypotonia and muscle weakness. This disease is clinically and genetically heterogeneous, but three recently discovered genes in NM encode for members of the Kelch family of proteins. Kelch proteins act as substrate-specific adaptors for Cullin 3 (CUL3) E3 ubiquitin ligase to regulate protein turnover through the ubiquitin-proteasome machinery. Defects in thin filament formation and/or stability are key molecular processes that underlie the disease pathology in NM; however, the role of Kelch proteins in these processes in normal and diseases conditions remains elusive. Here, we describe a role of NM causing Kelch protein, KLHL41, in premyofibil-myofibil transition during skeletal muscle development through a regulation of the thin filament chaperone, nebulin-related anchoring protein (NRAP). KLHL41 binds to the thin filament chaperone NRAP and promotes ubiquitination and subsequent degradation of NRAP, a process that is critical for the formation of mature myofibrils. KLHL41 deficiency results in abnormal accumulation of NRAP in muscle cells. NRAP overexpression in transgenic zebrafish resulted in a severe myopathic phenotype and absence of mature myofibrils demonstrating a role in disease pathology. Reducing Nrap levels in KLHL41 deficient zebrafish rescues the structural and function defects associated with disease pathology. We conclude that defects in KLHL41-mediated ubiquitination of sarcomeric proteins contribute to structural and functional deficits in skeletal muscle. These findings further our understanding of how the sarcomere assembly is regulated by disease-causing factors in vivo, which will be imperative for developing mechanism-based specific therapeutic interventions.

A Homozygous Deep Intronic Mutation Alters the Splicing of Nebulin Gene in a Patient With Nemaline Myopathy

Nemaline myopathy is a rare disorder affecting the muscle sarcomere. Mutations in nebulin gene (NEB) are known to be responsible for about 50% of nemaline myopathy cases. Nebulin is a giant protein which is formed integrally with the sarcomeric thin filament. This complex gene is under extensive alternative splicing giving rise to multiple isoforms. In this study, we report a 6-year-old boy presenting with general muscular weaknesses. Identification of rod-shaped structures in the patient' biopsy raised doubt about the presence of a nemaline myopathy. Next-generation sequencing was used to identify a causative mutation for the patient syndrome. A homozygous deep intronic substitution was found in the intron 144 of the NEB. The variant was predicted by in silico tools to create a new donor splice site. Molecular analysis has shown that the mutation could alter splicing events of the nebulin gene leading to a significant decrease of isoforms level. This change in the expression level of nebulin could give rise to functional consequences in the sarcomere. These results are consistent with the phenotypes observed in the patient. Such a discovery of variants in this gene will allow a better understanding of the involvement of nebulin in neuromuscular diseases and help find new treatments for the nemaline myopathy.

Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium

Troponin C (TnC) is a critical regulator of skeletal muscle contraction; it binds Ca2+ to activate muscle contraction. Surprisingly, the gene encoding fast skeletal TnC (TNNC2) has not yet been implicated in muscle disease. Here, we report 2 families with pathogenic variants in TNNC2. Patients present with a distinct, dominantly inherited congenital muscle disease. Molecular dynamics simulations suggested that the pathomechanisms by which the variants cause muscle disease include disruption of the binding sites for Ca2+ and for troponin I. In line with these findings, physiological studies in myofibers isolated from patients' biopsies revealed a markedly reduced force response of the sarcomeres to [Ca2+]. This pathomechanism was further confirmed in experiments in which contractile dysfunction was evoked by replacing TnC in myofibers from healthy control subjects with recombinant, mutant TnC. Conversely, the contractile dysfunction of myofibers from patients was repaired by replacing endogenous, mutant TnC with recombinant, wild-type TnC. Finally, we tested the therapeutic potential of the fast skeletal muscle troponin activator tirasemtiv in patients' myofibers and showed that the contractile dysfunction was repaired. Thus, our data reveal that pathogenic variants in TNNC2 cause congenital muscle disease, and they provide therapeutic angles to repair muscle contractility.

Transcriptome profile analysis reveals KLHL30 as an essential regulator for myoblast differentiation

Skeletal muscle development is a sophisticated multistep process orchestrated by diverse myogenic transcription factors. Recent studies have suggested that Kelch-like genes play vital roles in muscle disease and myogenesis. However, it is still unclear how Kelch-like genes impact myoblast physiology. Here, through integrative analysis of the mRNA expression profile during chicken primary myoblast and C2C12 differentiation, many differentially expressed genes were found and suggested to be enriched in myoblast differentiation and muscle development. Interestingly, a little-known Kelch-like gene KLHL30 was screened as skeletal muscle-specific gene with essential roles in myogenic differentiation. Transcriptomic data and quantitative PCR analysis indicated that the expression of KLHL30 is upregulated under myoblast differentiation state. KLHL30 overexpression upregulated the protein expression of myogenic transcription factors (MYOD, MYOG, MEF2C) and induced myoblast differentiation and myotube formation, while knockdown of KLHL30 caused the opposite effect. Furthermore, KLHL30 was found to significantly decrease the numbers of cells in the S stage and thereby depress myoblast proliferation. Collectively, this study highlights that KLHL30 as a muscle-specific regulator plays essential roles in myoblast proliferation and differentiation.

Acute and chronic tirasemtiv treatment improves in vivo and in vitro muscle performance in actin-based nemaline myopathy mice

Nemaline myopathy, a disease of the actin-based thin filament, is one of the most frequent congenital myopathies. To date, no specific therapy is available to treat muscle weakness in nemaline myopathy. We tested the ability of tirasemtiv, a fast skeletal troponin activator that targets the thin filament, to augment muscle force—both in vivo and in vitro—in a nemaline myopathy mouse model with a mutation (H40Y) in Acta1. In Acta1^H40Y mice, treatment with tirasemtiv increased the force response of muscles to submaximal stimulation frequencies. This resulted in a reduced energetic cost of force generation, which increases the force production during a fatigue protocol. The inotropic effects of tirasemtiv were present in locomotor muscles and, albeit to a lesser extent, in respiratory muscles, and they persisted during chronic treatment, an important finding as respiratory failure is the main cause of death in patients with congenital myopathy. Finally, translational studies on permeabilized muscle fibers isolated from a biopsy of a patient with the ACTA1^H40Y mutation revealed that at physiological Ca²⁺ concentrations, tirasemtiv increased force generation to values that were close to those generated in muscle fibers of healthy subjects. These findings indicate the therapeutic potential of fast skeletal muscle troponin activators to improve muscle function in nemaline myopathy due to the ACTA1^H40Y mutation, and future studies should assess their merit for other forms of nemaline myopathy and for other congenital myopathies.

NEM6, KBTBD13-Related Congenital Myopathy: Myopathological Analysis in 18 Dutch Patients Reveals Ring Rods Fibers, Cores, Nuclear Clumps, and Granulo-Filamentous Protein Material

Nemaline myopathy type 6 (NEM6), KBTBD13-related congenital myopathy is caused by mutated KBTBD13 protein that interacts improperly with thin filaments/actin, provoking impaired muscle-relaxation kinetics. We describe muscle morphology in 18 Dutch NEM6 patients and correlate it with clinical phenotype and pathophysiological mechanisms. Rods were found in in 85% of biopsies by light microscopy, and 89% by electron microscopy. A peculiar ring disposition of rods resulting in ring-rods fiber was observed. Cores were found in 79% of NEM6 biopsies by light microscopy, and 83% by electron microscopy. Electron microscopy also disclosed granulofilamentous protein material in 9 biopsies. Fiber type 1 predominance and prominent nuclear internalization were found. Rods were immunoreactive for α-actinin and myotilin. Areas surrounding the rods showed titin overexpression suggesting derangement of the surrounding sarcomeres. NEM6 myopathology hallmarks are prominent cores, rods including ring-rods fibers, nuclear clumps, and granulofilamentous protein material. This material might represent the histopathologic epiphenomenon of altered interaction between mutated KBTBD13 protein and thin filaments. We claim to classify KBTBD13-related congenital myopathy as rod-core myopathy.

A Cross-Sectional Study of Nemaline Myopathy

Objective

Nemaline myopathy (NM) is a rare neuromuscular condition with clinical and genetic heterogeneity. To establish disease natural history, we performed a cross-sectional study of NM, complemented by longitudinal assessment and exploration of pilot outcome measures.

Methods

Fifty-seven individuals with NM were recruited at 2 family workshops, including 16 examined at both time points. Participants were evaluated by clinical history and physical examination. Functional outcome measures included the Motor Function Measure (MFM), pulmonary function tests (PFTs), myometry, goniometry, and bulbar assessments.

Results

The most common clinical classification was typical congenital (54%), whereas 42% had more severe presentations. Fifty-eight percent of individuals needed mechanical support, with 26% requiring wheelchair, tracheostomy, and feeding tube. The MFM scale was performed in 44 of 57 participants and showed reduced scores in most with little floor/ceiling effect. Of the 27 individuals completing PFTs, abnormal values were observed in 65%. Last, bulbar function was abnormal in all patients examined, as determined with a novel outcome measure. Genotypes included mutations in ACTA1 (18), NEB (20), and TPM2 (2). Seventeen individuals were genetically unresolved. Patients with pathogenic ACTA1 and NEB variants were largely similar in clinical phenotype. Patients without genetic resolution had more severe disease.

Conclusion

We present a comprehensive cross-sectional study of NM. Our data identify significant disabilities and support a relatively stable disease course. We identify a need for further diagnostic investigation for the genetically unresolved group. MFM, PFTs, and the slurp test were identified as promising outcome measures for future clinical trials.

Identification of Potentially Relevant Genes for Excessive Exercise-Induced Pathological Cardiac Hypertrophy in Zebrafish

Exercise-induced cardiac remodeling has aroused public concern for some time, as sudden cardiac death is known to occur in athletes; however, little is known about the underlying mechanism of exercise-induced cardiac injury. In the present study, we established an excessive exercise-induced pathologic cardiac hypertrophy model in zebrafish with increased myocardial fibrosis, myofibril disassembly, mitochondrial degradation, upregulated expression of the pathological hypertrophy marker genes in the heart, contractile impairment, and cardiopulmonary function impairment. High-throughput RNA-seq analysis revealed that the differentially expressed genes were enriched in the regulation of autophagy, protein folding, and degradation, myofibril development, angiogenesis, metabolic reprogramming, and insulin and FoxO signaling pathways. FOXO proteins may be the core mediator of the regulatory network needed to promote the pathological response. Further, PPI network analysis showed that pik3c3, gapdh, fbox32, fzr1, ubox5, lmo7a, kctd7, fbxo9, lonrf1l, fbxl4, nhpb2l1b, nhp2, fbl, hsp90aa1.1, snrpd3l, dhx15, mrto4, ruvbl1, hspa8b, and faub are the hub genes that correlate with the pathogenesis of pathological cardiac hypertrophy. The underlying regulatory pathways and cardiac pressure-responsive molecules identified in the present study will provide valuable insights for the supervision and clinical treatment of pathological cardiac hypertrophy induced by excessive exercise.

Control of craniofacial and brain development by Cullin3-RING ubiquitin ligases: Lessons from human disease genetics

Metazoan development relies on intricate cell differentiation, communication, and migration pathways, which ensure proper formation of specialized cell types, tissues, and organs. These pathways are crucially controlled by ubiquitylation, a reversible post-translational modification that regulates the stability, activity, localization, or interaction landscape of substrate proteins. Specificity of ubiquitylation is ensured by E3 ligases, which bind substrates and co-operate with E1 and E2 enzymes to mediate ubiquitin transfer. Cullin3-RING ligases (CRL3s) are a large class of multi-subunit E3s that have emerged as important regulators of cell differentiation and development. In particular, recent evidence from human disease genetics, animal models, and mechanistic studies have established their involvement in the control of craniofacial and brain development. Here, we summarize regulatory principles of CRL3 assembly, substrate recruitment, and ubiquitylation that allow this class of E3s to fulfill their manifold functions in development. We further review our current mechanistic understanding of how specific CRL3 complexes orchestrate neuroectodermal differentiation and highlight diseases associated with their dysregulation. Based on evidence from human disease genetics, we propose that other unknown CRL3 complexes must help coordinate craniofacial and brain development and discuss how combining emerging strategies from the field of disease gene discovery with biochemical and human pluripotent stem cell approaches will likely facilitate their identification.

The Role of Cullin-RING Ligases in Striated Muscle Development, Function, and Disease

The well-orchestrated turnover of proteins in cross-striated muscles is one of the fundamental processes required for muscle cell function and survival. Dysfunction of the intricate protein degradation machinery is often associated with development of cardiac and skeletal muscle myopathies. Most muscle proteins are degraded by the ubiquitin-proteasome system (UPS). The UPS involves a number of enzymes, including E3-ligases, which tightly control which protein substrates are marked for degradation by the proteasome. Recent data reveal that E3-ligases of the cullin family play more diverse and crucial roles in cross striated muscles than previously anticipated. This review highlights some of the findings on the multifaceted functions of cullin-RING E3-ligases, their substrate adapters, muscle protein substrates, and regulatory proteins, such as the Cop9 signalosome, for the development of cross striated muscles, and their roles in the etiology of myopathies.

The ubiquitin-proteasome system in regulation of the skeletal muscle homeostasis and atrophy: from basic science to disorders

Skeletal muscle is one of the most abundant and highly plastic tissues. The ubiquitin-proteasome system (UPS) is recognised as a major intracellular protein degradation system, and its function is important for muscle homeostasis and health. Although UPS plays an essential role in protein degradation during muscle atrophy, leading to the loss of muscle mass and strength, its deficit negatively impacts muscle homeostasis and leads to the occurrence of several pathological phenotypes. A growing number of studies have linked UPS impairment not only to matured muscle fibre degeneration and weakness, but also to muscle stem cells and deficiency in regeneration. Emerging evidence suggests possible links between abnormal UPS regulation and several types of muscle diseases. Therefore, understanding of the role of UPS in skeletal muscle may provide novel therapeutic insights to counteract muscle wasting, and various muscle diseases. In this review, we focussed on the role of proteasomes in skeletal muscle and its regeneration, including a brief explanation of the structure of proteasomes. In addition, we summarised the recent findings on several diseases and elaborated on how the UPS is related to their pathological states.

Update on Congenital Myopathies in Adulthood

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

Mutational and clinical spectrum in a cohort of Chinese patients with hereditary nemaline myopathy

Hereditary nemaline myopathy (NM) is one of the most common congenital myopathies with the histopathological findings of nemaline bodies. We used targeted next-generation sequencing to identify causative mutations in 48 NM patients with confirmed myopathological diagnosis, analyze the mutational spectrum and phenotypic features. Furthermore, reverse transcription polymerase chain reaction (RT-PCR) was used to confirm the pathogenic effect of one nebulin (NEB) splicing variant. The results showed that variants were found in five NM-associated genes, including NEB, actin alpha 1 (ACTA1), troponin T1, Kelch repeat and BTB domain-containing 13, and cofilin-2, in 34 (73.9%), 7 (15.2%), 3 (6.5%), 1 (2.2%), and 1 (2.2%) patients, respectively, in a total of 46/48 (95.8%) NM patients. Of the total 64 variants identified, 51 were novel variants including 26 pathogenic, 1 probably pathogenic, and 24 variant of uncertain significance (VUS). Notably, one NEB splicing mutation, c.21417+3A>G causing exon 144 splicing (NM_001164508.1), as confirmed by RT-PCR, was found in 52.9% (18 patients) of NEB variant-carrying patients. Typical congenital NM, the most common clinical subtype (60.4%), was associated with five NM genes. We concluded that hereditary NM showed a highly variable genetic spectrum. NEB was the most frequent causative gene in this Chinese cohort, followed by ACTA1. We found a hotspot splicing mutation in NEB among Chinese cohort.

Novel KLHL26 variant associated with a familial case of Ebstein's anomaly and left ventricular noncompaction

BACKGROUND

Ebstein's anomaly (EA) is a rare congenital heart disease of the tricuspid valve and right ventricle. Patients with EA often manifest with left ventricular noncompaction (LVNC), a cardiomyopathy. Despite implication of cardiac sarcomere genes in some cases, very little is understood regarding the genetic etiology of EA/LVNC. Our study describes a multigenerational family with at least 10 of 17 members affected by EA/LVNC.

METHODS

We performed echocardiography on all family members and conducted exome sequencing of six individuals. After identifying candidate variants using two different bioinformatic strategies, we confirmed segregation with phenotype using Sanger sequencing. We investigated structural implications of candidate variants using protein prediction models.

RESULTS

Exome sequencing analysis of four affected and two unaffected members identified a novel, rare, and damaging coding variant in the Kelch-like family member 26 (KLHL26) gene located on chromosome 19 at position 237 of the protein (GRCh37). This variant region was confirmed by Sanger sequencing in the remaining family members. KLHL26 (c.709C > T p.R237C) segregates only with EA/LVNC-affected individuals (FBAT p < .05). Investigating structural implications of the candidate variant using protein prediction models suggested that the KLHL26 variant disrupts electrostatic interactions when binding to part of the ubiquitin proteasome, specifically Cullin3 (CUL3), a component of E3 ubiquitin ligase.

CONCLUSION

In this familial case of EA/LVNC, we have identified a candidate gene variant, KLHL26 (p.R237C), which may have an important role in ubiquitin-mediated protein degradation during cardiac development.

Nebulin nemaline myopathy recapitulated in a compound heterozygous mouse model with both a missense and a nonsense mutation in Neb

Nemaline myopathy (NM) caused by mutations in the gene encoding nebulin (NEB) accounts for at least 50% of all NM cases worldwide, representing a significant disease burden. Most NEB-NM patients have autosomal recessive disease due to a compound heterozygous genotype. Of the few murine models developed for NEB-NM, most are Neb knockout models rather than harbouring Neb mutations. Additionally, some models have a very severe phenotype that limits their application for evaluating disease progression and potential therapies. No existing murine models possess compound heterozygous Neb mutations that reflect the genotype and resulting phenotype present in most patients. We aimed to develop a murine model that more closely matched the underlying genetics of NEB-NM, which could assist elucidation of the pathogenetic mechanisms underlying the disease. Here, we have characterised a mouse strain with compound heterozygous Neb mutations; one missense (p.Tyr2303His), affecting a conserved actin-binding site and one nonsense mutation (p.Tyr935*), introducing a premature stop codon early in the protein. Our studies reveal that this compound heterozygous model, Neb^{Y2303H, Y935X}, has striking skeletal muscle pathology including nemaline bodies. In vitro whole muscle and single myofibre physiology studies also demonstrate functional perturbations. However, no reduction in lifespan was noted. Therefore, Neb^Y2303H,Y935X mice recapitulate human NEB-NM and are a much needed addition to the NEB-NM mouse model collection. The moderate phenotype also makes this an appropriate model for studying NEB-NM pathogenesis, and could potentially be suitable for testing therapeutic applications.

Discovery of Novel Therapeutics for Muscular Dystrophies using Zebrafish Phenotypic Screens

The recent availability and development of mutant and transgenic zebrafish strains that model human muscular dystrophies has created new research opportunities for therapeutic development. Not only do these models mimic many pathological aspects of human dystrophies, but their small size, large clutch sizes, rapid ex utero development, body transparency, and genetic tractability enable research approaches that would be inconceivable with mammalian model systems. Here we discuss the use of zebrafish models of muscular dystrophy to rapidly screen hundreds to thousands of bioactive compounds in order to identify novel therapeutic candidates that modulate pathologic phenotypes. We review the justification and rationale behind this unbiased approach, including how zebrafish screens have identified FDA-approved drugs that are candidates for treating Duchenne and limb girdle muscular dystrophies. Not only can these drugs be re-purposed for treating dystrophies in a fraction of the time and cost of new drug development, but their identification has revealed novel, unexpected directions for future therapy development. Phenotype-driven zebrafish drug screens are an important compliment to the more established mammalian, target-based approaches for rapidly developing and validating therapeutics for muscular dystrophies.

Novel Recessive TNNT1 Congenital Core-Rod Myopathy in French Canadians

OBJECTIVE

Recessive null variants of the slow skeletal muscle troponin T1 (TNNT1) gene are a rare cause of nemaline myopathy that is fatal in infancy due to respiratory insufficiency. Muscle biopsy shows rods and fiber type disproportion. We report on 4 French Canadians with a novel form of recessive congenital TNNT1 core-rod myopathy.

METHODS

Patients underwent full clinical characterization, lower limb magnetic resonance imaging (MRI), muscle biopsy, and genetic testing. A zebrafish loss-of-function model using morpholinos was created to assess the pathogenicity of the identified variant. Wild-type or mutated human TNNT1 mRNAs were coinjected with morpholinos to assess their abilities to rescue the morphant phenotype.

RESULTS

Three adults and 1 child shared a novel missense homozygous variant in the TNNT1 gene (NM_003283.6: c.287T > C; p.Leu96Pro). They developed from childhood very slowly progressive limb-girdle weakness with rigid spine and disabling contractures. They suffered from restrictive lung disease requiring noninvasive mechanical ventilation in 3 patients, as well as recurrent episodes of rhabdomyolysis triggered by infections, which were relieved by dantrolene in 1 patient. Older patients remained ambulatory into their 60s. MRI of the leg muscles showed fibrofatty infiltration predominating in the posterior thigh and the deep posterior leg compartments. Muscle biopsies showed multiminicores and lobulated fibers, rods in half the patients, and no fiber type disproportion. Wild-type TNNT1 mRNA rescued the zebrafish morphants, but mutant transcripts failed to do so.

INTERPRETATION

This study expands the phenotypic spectrum of TNNT1 myopathy and provides functional evidence for the pathogenicity of the newly identified missense mutation. ANN NEUROL 2020;87:568-583.

Deleting nebulin's C-terminus reveals its importance to sarcomeric structure and function and is sufficient to invoke nemaline myopathy

Nebulin is a large skeletal muscle protein wound around the thin filaments, with its C-terminus embedded within the Z-disk and its N-terminus extending out toward the thin filament pointed end. While nebulin's C-terminus has been implicated in both sarcomeric structure and function as well as the development of nemaline myopathy, the contributions of this region remain largely unknown. Additionally, the C-terminus is reported to contribute to muscle hypertrophy via the IGF-1 growth pathway. To study the functions of nebulin's C-terminus, we generated a mouse model deleting the final two unique C-terminal domains, the serine-rich region (SRR) and the SH3 domain (NebΔ163-165). Homozygous NebΔ163-165 mice that survive past the neonatal stage exhibit a mild weight deficit. Characterization of these mice revealed that the truncation caused a moderate myopathy phenotype reminiscent of nemaline myopathy despite the majority of nebulin being localized properly in the thin filaments. This phenotype included muscle weight loss, changes in sarcomere structure, as well as a decrease in force production. Glutathione S-transferase (GST) pull-down experiments found novel binding partners with the SRR, several of which are associated with myopathies. While the C-terminus does not appear to be a limiting step in muscle growth, the IGF-1 growth pathway remained functional despite the deleted domains being proposed to be essential for IGF-1 mediated hypertrophy. The NebΔ163-165 mouse model emphasizes that nebulin's C-terminus is necessary for proper sarcomeric development and shows that its loss is sufficient to induce myopathy.

KBTBD13 is an actin-binding protein that modulates muscle kinetics

The mechanisms that modulate the kinetics of muscle relaxation are critically important for muscle function. A prime example of the impact of impaired relaxation kinetics is nemaline myopathy caused by mutations in KBTBD13 (NEM6). In addition to weakness, NEM6 patients have slow muscle relaxation, compromising contractility and daily life activities. The role of KBTBD13 in muscle is unknown, and the pathomechanism underlying NEM6 is undetermined. A combination of transcranial magnetic stimulation-induced muscle relaxation, muscle fiber- and sarcomere-contractility assays, low-angle x-ray diffraction, and superresolution microscopy revealed that the impaired muscle-relaxation kinetics in NEM6 patients are caused by structural changes in the thin filament, a sarcomeric microstructure. Using homology modeling and binding and contractility assays with recombinant KBTBD13, Kbtbd13-knockout and Kbtbd13R408C-knockin mouse models, and a GFP-labeled Kbtbd13-transgenic zebrafish model, we discovered that KBTBD13 binds to actin - a major constituent of the thin filament - and that mutations in KBTBD13 cause structural changes impairing muscle-relaxation kinetics. We propose that this actin-based impaired relaxation is central to NEM6 pathology.

Expressing a Z-disk nebulin fragment in nebulin-deficient mouse muscle: effects on muscle structure and function

BACKGROUND

Nebulin is a critical thin filament-binding protein that spans from the Z-disk of the skeletal muscle sarcomere to near the pointed end of the thin filament. Its massive size and actin-binding property allows it to provide the thin filaments with structural and regulatory support. When this protein is lost, nemaline myopathy occurs. Nemaline myopathy causes severe muscle weakness as well as structural defects on a sarcomeric level. There is no known cure for this disease.

METHODS

We studied whether sarcomeric structure and function can be improved by introducing nebulin's Z-disk region into a nebulin-deficient mouse model (Neb cKO) through adeno-associated viral (AAV) vector therapy. Following this treatment, the structural and functional characteristics of both vehicle-treated and AAV-treated Neb cKO and control muscles were studied.

RESULTS

Intramuscular injection of this AAV construct resulted in a successful expression of the Z-disk fragment within the target muscles. This expression was significantly higher in Neb cKO mice than control mice. Analysis of protein expression revealed that the nebulin fragment was localized exclusively to the Z-disks and that Neb cKO expressed the nebulin fragment at levels comparable to the level of full-length nebulin in control mice. Additionally, the Z-disk fragment displaced full-length nebulin in control mice, resulting in nemaline rod body formation and a worsening of muscle function. Neb cKO mice experienced a slight functional benefit from the AAV treatment, with a small increase in force and fatigue resistance. Disease progression was also slowed as indicated by improved muscle structure and myosin isoform expression.

CONCLUSIONS

This study reveals that nebulin fragments are well-received by nebulin-deficient mouse muscles and that limited functional benefits are achievable.

Myostatin Inhibition Using ActRIIB-mFc Does Not Produce Weight Gain or Strength in the Nebulin Conditional KO Mouse

Mutations in at least 12 genes are responsible for a group of congenital skeletal muscle diseases known as nemaline myopathies (NMs). NMs are associated with a range of clinical symptoms and pathological changes often including the presence of cytoplasmic rod-like structures (nemaline bodies) and myofiber hypotrophy. Our recent work has identified a variable degree of behavioral benefit when treating 2 NM mouse models due to mutations in Acta1 with myostatin inhibition. This study is focused on the effects of delivering ActRIIB-mFc (Acceleron; a myostatin inhibitor) to the nebulin conditional knockout KO (Neb cKO) mouse model of NM. Treatment of Neb cKO mice with ActRIIB-mFc did not produce increases in weight gain, strength, myofiber size, or hypertrophic pathway signaling. Overall, our studies demonstrate a lack of response in Neb cKO mice to myostatin inhibition, which differs from the response observed when treating other NM models.

Failure to identify modifiers of NEBULIN-related nemaline myopathy in two pre-clinical models of the disease

Nemaline myopathy is a rare neuromuscular disorder that affects 1 in 50,000 live births, with prevalence as high as 1 in 20,000 in certain populations. 13 genes have been linked to nemaline myopathy (NM), all of which are associated with the thin filament of the muscle sarcomere. Of the 13 associated genes, mutations in NEBULIN (NEB) accounts for up to 50% of all cases. Currently, the disease is incompletely understood and there are no available therapeutics for patients. To address this urgent need for effective treatments for patients affected by NM, we conducted a large scale chemical screen in a zebrafish model of NEB-related NM and an N-ethyl-N-nitrosourea (ENU)-based genetic screen in a mouse model of NEB exon 55 deletion, the most common NEB mutation in NM patients. Neither screen was able to identify a candidate for therapy development, highlighting the need to transition from conventional chemical therapeutics to gene-based therapies for the treatment of NM.

Summary: NEBULIN-related nemaline myopathy currently has no treatment. We attempted to uncover new avenues for therapy by performing modifier screens, which unfortunately failed to identify modifiers that improved disease relevant phenotypes.

'Amish Nemaline Myopathy' in 2 Italian siblings harbouring a novel homozygous mutation in Troponin-I gene

Amish Nemaline Myopathy is a severe form of nemaline myopathy associated to mutation in TNNT1 gene, firstly reported among the Old Order Amish. Here we report two Italian siblings who manifested, by the age of 7 months, progressive and severe muscle weakness and wasting, respiratory insufficiency, pectus carinatum deformity and failure to thrive. Muscle biopsy was consistent with nemaline myopathy and novel homozygous missense mutation in TNNT1 was found. Our cases expand the mutational spectrum of TNNT1, confirm the invariable peculiar clinical phenotype also outside the Amish population, and suggest that TNNT1 should be considered for molecular analysis in NM patients with chest deformities and progressive contractures.

A genetic-phenotypic classification for syndromic micrognathia

Micrognathia is a common craniofacial deformity which represents hypoplastic development of the mandible, accompanied by retrognathia and consequent airway problems. Usually, micrognathia is accompanied by multiple systematic defects, known as syndromic micrognathia, and is in close association with genetic factors. Now, large quantities of pathogenic genes of syndromic micrognathia have been revealed. However, how these different pathogenic genes could lead to similar phenotypes, and whether there are some common characteristics among these pathogenic genes are still unknown. In this study, we proposed a genetic-phenotypic classification of syndromic micrognathia based on pathogenic genes information obtained from Phenolyzer, DAVID, OMIM, and PubMed database. Pathogenic genes of syndromic micrognathia could be divided into four groups based on gene function, including cellular processes and structures, cell metabolism, cartilage and bone development, and neuromuscular function. In addition, these four groups exhibited various clinical characteristics, and the affected systems, such as central nervous system, skeletal system, cardiovascular system, oral and dental system, respiratory system and muscle, were different in these four groups. This classification could provide meaningful insights into the pathogenesis of syndromic micrognathia, and offer some clues for understanding the molecular mechanism, as well as guiding precise clinical diagnosis and treatment for syndromic micrognathia.

Sarcomere Dysfunction in Nemaline Myopathy

Nemaline myopathy (NM) is among the most common non-dystrophic congenital myopathies (incidence 1:50.000). Hallmark features of NM are skeletal muscle weakness and the presence of nemaline bodies in the muscle fiber. The clinical phenotype of NM patients is quite diverse, ranging from neonatal death to normal lifespan with almost normal motor function. As the respiratory muscles are involved as well, severely affected patients are ventilator-dependent. The mechanisms underlying muscle weakness in NM are currently poorly understood. Therefore, no therapeutic treatment is available yet.

Eleven implicated genes have been identified: ten genes encode proteins that are either components of thin filament, or are thought to contribute to stability or turnover of thin filament proteins. The thin filament is a major constituent of the sarcomere, the smallest contractile unit in muscle. It is at this level of contraction – thin-thick filament interaction – where muscle weakness originates in NM patients.

This review focusses on how sarcomeric gene mutations directly compromise sarcomere function in NM. Insight into the contribution of sarcomeric dysfunction to muscle weakness in NM, across the genes involved, will direct towards the development of targeted therapeutic strategies.

Shaping Striated Muscles with Ubiquitin Proteasome System in Health and Disease

For long-lived contractile cells, such as striated muscle cells, maintaining proteome integrity is a challenging task. These cells require hundreds of components that must be properly synthesized, folded, and incorporated into the basic contractile unit, the sarcomere. Muscle protein quality control in cells is mainly guaranteed by the ubiquitin-proteasome system (UPS), the lysosome-autophagy system, and various molecular chaperones. Recent studies establish the concept of dedicated UPS in the regulation of sarcomere assembly during development and in adult life to maintain the intricate and interwoven organization of protein complexes in muscle. Failure of sarcomere protein quality control often represents the basis of severe myopathies and cardiomyopathies in human, further highlighting its importance in producing and maintaining the contractile machinery of muscle cells in shape.

Cullin-3 dependent deregulation of ACTN1 represents a new pathogenic mechanism in nemaline myopathy

Nemaline myopathy is a congenital neuromuscular disorder characterized by muscle weakness, fiber atrophy and presence of nemaline bodies within myofibers. However, the understanding of underlying pathomechanisms is lacking. Recently, mutations in KBTBD13, KLHL40 and KLHL41, three substrate adaptors for the E3-ubiquitin ligase Cullin-3, have been associated with early-onset nemaline myopathies. We hypothesized that deregulation of Cullin-3 and its muscle protein substrates may be responsible for the disease development. Using Cullin-3 knockout mice, we identified accumulation of non-muscle alpha-Actinins (ACTN1 and ACTN4) in muscles of these mice, which we also observed in KBTBD13 patients. Our data reveal that proper regulation of Cullin-3 activity and ACTN1 levels is essential for normal muscle and neuromuscular junction development. While ACTN1 is naturally downregulated during myogenesis, its overexpression in C2C12 myoblasts triggered defects in fusion, myogenesis and acetylcholine receptor clustering; features that we characterized in Cullin-3 deficient mice. Taken together, our data highlight the importance for Cullin-3 mediated degradation of ACTN1 for muscle development, and indicate a new pathomechanism for the etiology of myopathies seen in Cullin-3 knockout mice and nemaline myopathy patients.

Therapeutic Aspects in Congenital Myopathies

The congenital myopathies are a genetically heterogeneous and diverse group of early-onset, nondystrophic neuromuscular disorders. While the originally reported "classical" entities within this group - Central Core Disease, Multiminicore Disease, Nemaline Myopathy, and Centronuclear Myopathy - were defined by the predominant finding on muscle biopsy, "novel" forms with multiple, subtle, and unusual histopathologic features have been described more recently, reflective of an expanding phenotypical spectrum. The main disease mechanisms concern excitation-contraction coupling, intracellular calcium homeostasis, and thin/thick filament interactions. Management to date has been mainly supportive. Therapeutic strategies currently at various stages of exploration include genetic interventions aimed at direct correction of the underlying genetic defect, enzyme replacement therapy, and pharmacologic approaches, either specifically targeting the principal effect of the underlying gene mutation, or addressing its downstream consequences more generally. Clinical trial development is accelerating but will require more robust natural history data and tailored outcome measures.

Update on the Genetics of Congenital Myopathies

The congenital myopathies form a large clinically and genetically heterogeneous group of disorders. Currently mutations in at least 27 different genes have been reported to cause a congenital myopathy, but the number is expected to increase due to the accelerated use of next-generation sequencing methods. There is substantial overlap between the causative genes and the clinical and histopathologic features of the congenital myopathies. The mode of inheritance can be autosomal recessive, autosomal dominant or X-linked. Both dominant and recessive mutations in the same gene can cause a similar disease phenotype, and the same clinical phenotype can also be caused by mutations in different genes. Clear genotype-phenotype correlations are few and far between.

KBTBD11, a novel BTB-Kelch protein, is a negative regulator of osteoclastogenesis through controlling Cullin3-mediated ubiquitination of NFATc1

Kelch repeat and BTB domain-containing protein 11 (KBTBD11) is a member of the KBTBD subfamily of proteins that possess a BTB domain and Kelch repeats. Despite the presence of the Kbtbd11 gene in mammalian genomes, there are few reports about KBTBD11 at present. In this study, we identified the novel protein KBTBD11 as a negative regulator of osteoclast differentiation. We found that expression of KBTBD11 increased during osteoclastogenesis. Small-interfering-RNA-mediated knockdown of KBTBD11 enhanced osteoclast formation, and markedly increased the expression of several osteoclast marker genes compared with control cells. Conversely, KBTBD11 overexpression impaired osteoclast differentiation, and decreased the expression of osteoclast marker genes. Among six major signaling pathways regulating osteoclast differentiation, KBTBD11 predominantly influenced the nuclear factor of activated T cell cytoplasmic-1 (NFATc1) pathway. Mechanistically, KBTBD11 was found to interact with an E3 ubiquitin ligase, Cullin3. Further experiments involving immunoprecipitation and treatment with MG132, a proteasome inhibitor, showed that the KBTBD11–Cullin3 promotes ubiquitination and degradation of NFATc1 by the proteasome. Considering that NFATc1 is an essential factor for osteoclast differentiation, the KBTBD11 and Cullin3 probably regulate the levels of NFATc1 through the ubiquitin-proteasome degradation system. Thus, KBTBD11 negatively modulates osteoclast differentiation by controlling Cullin3-mediated ubiquitination of NFATc1.