Novel TCAP mutation c.32C>A causing limb girdle muscular dystrophy 2G

TCAP gene is not a common cause of cardiomyopathy in Iranian patients

BACKGROUND

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are the most frequent cardiomyopathies that cause acute heart failure and sudden cardiac death. Previous genetic reports have shown that pathogenic variants of genes encoding Z-disc components such as telethonin protein (TCAP) are the primary cause of DCM and HCM.

METHODS

This study was the first investigation on the TCAP gene among the Iranian cardiomyopathies population wherein the TCAP gene was analyzed in 40 unrelated patients (17 females and 23 males) who were clinically diagnosed with HCM and DCM. In addition, we conducted a thorough review of all published articles and the databases that were the first to report novel pathogenic or likely pathogenic variants the in TCAP gene.

RESULTS

In the cohort of this study, we identified only one intronic variant c.111-42G > A in one of the HCM patients that were predicted as polymorphism by in-silico analysis. Moreover, a total of 44 variants were reported for the TCAP gene in the literature where a majority of mutations were found to be missense. Pathogenic mutations in TCAP may cause diseases including limb-girdle muscular dystrophy 2G (LGMD-2G), DCM, HCM, intestinal pseudo-obstruction, and telethonin deficiency. However, a large number of affected patients were clinically diagnosed with limb-girdle 2G compared to other presenting phenotypes.

DISCUSSION

These findings suggest that the TCAP gene pathogenic mutations might not be a common cause of cardiomyopathies among Iranian patients. These gene disease-causing mutations may cause various manifestations, but it has a high prevalence among LGMD-2G, HCM, and DCM patients.

Muscle MRI patterns for limb girdle muscle dystrophies: systematic review

Limb girdle muscle dystrophies (LGMDs) are a group of inherited neuromuscular disorders comprising more than 20 genes. There have been increasing efforts to characterize this group with Muscle MRI. However, due to the complexity and similarities, the interpretation of the MRI patterns is usually done by experts in the field. Here, we proposed a step-by-step image interpretation of Muscle MRI in LGDM by evaluating the variability of muscle pattern involvement reported in the literature. A systematic review with an open start date to November 2022 was conducted to describe all LGMDs' muscle MRI patterns. Eighty-eight studies were included in the final review. Data were found to describe muscle MRI patterns for 15 out of 17 LGMDs types. Although the diagnosis of LGMDs is challenging despite the advanced genetic testing and other diagnostic modalities, muscle MRI is shown to help in the diagnosis of LGMDs. To further increase the yield for muscle MRI in the neuromuscular field, larger cohorts of patients need to be conducted.

Unique Clinical, Radiological and Histopathological Characteristics of a Southeast Asian Cohort of Patients with Limb-Girdle Muscular Dystrophy 2G/LGMD-R7-Telethonin-Related

AIM

We describe a cohort of five patients with limb-girdle muscular dystrophy (LGMD) 2G/LGMD-R7 in a South-east Asian cohort.

BACKGROUND

LGMD2G/LGMD-R7-telethonin-related is caused by mutations in the TCAP gene that encodes for telethonin.

METHODS

We identified consecutive patients with LGMD2G/LGMD-R7-telethonin-related, diagnosed at the National Neuroscience Institute (NNI) and National University Hospital (NUH) between January 2000 and June 2021.

RESULTS

At onset, three patients presented with proximal lower limb weakness, one patient presented with Achilles tendon contractures, and one patient presented with delayed gross motor milestones. At last follow up, three patients had a limb girdle pattern of muscle weakness and two had a facioscapular humeral pattern of weakness. Whole body muscle MRI performed for one patient with a facioscapular-humeral pattern of weakness showed a pattern of muscle atrophy similar to facioscapular-humeral dystrophy. One patient had histological features consistent with myofibrillar myopathy; electron microscopy confirmed the disruption of myofibrillar architecture. One patients also had reduced staining to telethonin antibody on immunohistochemistry.

CONCLUSION

We report the unique clinical and histological features of a Southeast Asian cohort of five patients with LGMD2G/LGMD-R7-telethonin-related muscular dystrophy and further expand its clinical and histopathological spectrum.

Circadian regulation of cardiac muscle function and protein degradation

The circadian clock plays a fundamental role in physiology. In particular, the heart is a target organ where the clock orchestrates various aspects of cardiac function. At the molecular level, the clock machinery governs daily rhythms of gene expression. Such circadian regulation is in tune with the dynamic nature of heart structure and function, and provides the foundation for chronotherapeutic applications in cardiovascular diseases. In comparison, a regulatory role of the clock in cardiac protein degradation is poorly documented. Sarcomere is the structural and functional unit responsible for cardiac muscle contraction, and sarcomere components are closely regulated by protein folding and proteolysis. Emerging evidence supports a role of the circadian clock in governing sarcomere integrity and function. Particularly, recent studies uncovered a circadian regulation of a core sarcomere component TCAP. It is possible that circadian regulation of the cardiac muscle protein turnover is a key regulatory mechanism underlying cardiac remodeling in response to physiological and environmental stimuli. While the detailed regulatory mechanisms and the molecular links to cardiac (patho)physiology remain to be further studied, therapeutic strategies targeting circadian control in the heart may markedly enhance intervention outcomes against cardiovascular disease.

The circadian E3 ligase FBXL21 regulates myoblast differentiation and sarcomere architecture via MYOZ1 ubiquitination and NFAT signaling

Numerous molecular and physiological processes in the skeletal muscle undergo circadian time-dependent oscillations in accordance with daily activity/rest cycles. The circadian regulatory mechanisms underlying these cyclic processes, especially at the post-transcriptional level, are not well defined. Previously, we reported that the circadian E3 ligase FBXL21 mediates rhythmic degradation of the sarcomere protein TCAP in conjunction with GSK-3β, and Psttm mice harboring an Fbxl21 hypomorph allele show reduced muscle fiber diameter and impaired muscle function. To further elucidate the regulatory function of FBXL21 in skeletal muscle, we investigated another sarcomere protein, Myozenin1 (MYOZ1), that we identified as an FBXL21-binding protein from yeast 2-hybrid screening. We show that FBXL21 binding to MYOZ1 led to ubiquitination-mediated proteasomal degradation. GSK-3β co-expression and inhibition were found to accelerate and decelerate FBXL21-mediated MYOZ1 degradation, respectively. Previously, MYOZ1 has been shown to inhibit calcineurin/NFAT signaling important for muscle differentiation. In accordance, Fbxl21 KO and MyoZ1 KO in C2C12 cells impaired and enhanced myogenic differentiation respectively compared with control C2C12 cells, concomitant with distinct effects on NFAT nuclear localization and NFAT target gene expression. Importantly, in Psttm mice, both the levels and diurnal rhythm of NFAT2 nuclear localization were significantly diminished relative to wild-type mice, and circadian expression of NFAT target genes associated with muscle differentiation was also markedly dampened. Furthermore, Psttm mice exhibited significant disruption of sarcomere structure with a considerable excess of MYOZ1 accumulation in the Z-line. Taken together, our study illustrates a pivotal role of FBXL21 in sarcomere structure and muscle differentiation by regulating MYOZ1 degradation and NFAT2 signaling.

Single nuclei transcriptomics of muscle reveals intra-muscular cell dynamics linked to dystrophin loss and rescue

In Duchenne muscular dystrophy, dystrophin loss leads to chronic muscle damage, dysregulation of repair, fibro-fatty replacement, and weakness. We develop methodology to efficiently isolate individual nuclei from minute quantities of frozen skeletal muscle, allowing single nuclei sequencing of irreplaceable archival samples and from very small samples. We apply this method to identify cell and gene expression dynamics within human DMD and mdx mouse muscle, characterizing effects of dystrophin rescue by exon skipping therapy at single nuclei resolution. DMD exon 23 skipping events are directly observed and increased in myonuclei from treated mice. We describe partial rescue of type IIa and IIx myofibers, expansion of an MDSC-like myeloid population, recovery of repair/remodeling M2-macrophage, and repression of inflammatory POSTN1 + fibroblasts in response to exon skipping and partial dystrophin restoration. Use of this method enables exploration of cellular and transcriptomic mechanisms of dystrophin loss and repair within an intact muscle environment. Our initial findings will scaffold our future work to more directly examine muscular dystrophies and putative recovery pathways.

Findings of limb-girdle muscular dystrophy R7 telethonin-related patients from a Chinese neuromuscular center

Limb-girdle muscular dystrophy (LGMD) is a group of clinically and genetically heterogeneous neuromuscular disorders. LGMD-R7, which is caused by telethonin gene (TCAP) mutations, is one of the rarest forms of LGMD, and only a small number of LGMD-R7 cases have been described and mostly include patients from Brazil. A total of two LGMD-R7 patients were enrolled at a Chinese neuromuscular center. Demographic and clinical data were collected. Laboratory investigations and electromyography were performed. Routine and immunohistochemistry staining of muscle specimens was performed, and a next-generation sequencing panel array for genes associated with hereditary neuromuscular disorders was used for analysis. The patients exhibited predominant muscle weakness. Electromyography revealed myopathic changes. The muscle biopsy showed myopathic features, such as increased fiber size variation, muscle fiber atrophy and regeneration, slight hyperplasia of the connective tissue, and disarray of the myofibrillar network. Two patients were confirmed to have mutations in the open reading frame of TCAP by next-generation sequencing. One patient had compound heterozygous mutations, and the other patient harbored a novel homozygous mutation. Western blotting analysis of the skeletal muscle lysate confirmed the absence of telethonin in the patients. We described two LGMD-R7 patients presenting a classical LGMD phenotype and a novel homozygous TCAP mutation. Our research expands the spectrum of LGMD-R7 due to TCAP mutations based on patients from a Chinese neuromuscular center.

The GSK-3β-FBXL21 Axis Contributes to Circadian TCAP Degradation and Skeletal Muscle Function

FBXL21 is a clock-controlled E3 ligase modulating circadian periodicity via subcellular-specific CRYPTOCHROME degradation. How FBXL21 regulates tissue-specific circadian physiology and what mechanism operates upstream is poorly understood. Here we report the sarcomere component TCAP as a cytoplasmic substrate of FBXL21. FBXL21 interacts with TCAP in a circadian manner antiphasic to TCAP accumulation in skeletal muscle, and circadian TCAP oscillation is disrupted in Psttm mice with an Fbxl21 hypomorph mutation. GSK-3β phosphorylates FBXL21 and TCAP to activate FBXL21-mediated, phosphodegron-dependent TCAP degradation. GSK-3β inhibition or knockdown diminishes FBXL21-Cul1 complex formation and delays FBXL21-mediated TCAP degradation. Finally, Psttm mice show significant skeletal muscle defects, including impaired fiber size, exercise tolerance, grip strength, and response to glucocorticoid-induced atrophy, in conjunction with cardiac dysfunction. These data highlight a circadian regulatory pathway where a GSK-3β-FBXL21 functional axis controls TCAP degradation via SCF complex formation and regulates skeletal muscle function.

Wirianto et al. find that the circadian E3 ligase FBXL21 drives rhythmic degradation of the sarcomeric protein TCAP in skeletal muscle. GSK-3β co-phosphorylates FBXL21 and TCAP and promotes SCF complex formation and phosphodegron-dependent TCAP turnover. Psttm mice, expressing a hypomorphic Fbxl21 mutant, show dysregulated TCAP degradation and impaired muscle function.

The Role of Z-disc Proteins in Myopathy and Cardiomyopathy

The Z-disc acts as a protein-rich structure to tether thin filament in the contractile units, the sarcomeres, of striated muscle cells. Proteins found in the Z-disc are integral for maintaining the architecture of the sarcomere. They also enable it to function as a (bio-mechanical) signalling hub. Numerous proteins interact in the Z-disc to facilitate force transduction and intracellular signalling in both cardiac and skeletal muscle. This review will focus on six key Z-disc proteins: α-actinin 2, filamin C, myopalladin, myotilin, telethonin and Z-disc alternatively spliced PDZ-motif (ZASP), which have all been linked to myopathies and cardiomyopathies. We will summarise pathogenic variants identified in the six genes coding for these proteins and look at their involvement in myopathy and cardiomyopathy. Listing the Minor Allele Frequency (MAF) of these variants in the Genome Aggregation Database (GnomAD) version 3.1 will help to critically re-evaluate pathogenicity based on variant frequency in normal population cohorts.

Distal myopathy due to TCAP variants in four unrelated Chinese patients

Distal myopathies are a group of clinically and genetically heterogeneous hereditary muscle disorders characterized by progressive muscular weakness starting in the distal parts of the limbs. The most common subtype of distal myopathy is GNE myopathy, a rare muscle disease with autosomal recessive inheritance. Limb-girdle muscular dystrophy 2G (LGMD2G) is a rare autosomal recessive subtype of LGMDs caused by TCAP variant. Patients with LGMD2G can present with distal myopathy and rimmed vacuoles on muscle pathology. Thus far, the most reported TCAP mutations related to LGMD2G were recessive frameshift or nonsense variants. Here, we described four Chinese patients from unrelated families with LGMD2G due to TCAP mutations. The clinical symptoms of our patients were similar to those previously reported in LGMD2G patients. Three different pathogenic TCAP variants were identified in these patients, including two frameshift variants and one intronic variant. Autophagolysosomes have been observed in one patient by electron microscopy. Our research expands the genetic spectrum of TCAP mutations in China, indicating c.165-166insG is likely the common pathogenic variant. We also provide evidences that autophagy may be involved in the pathophysiology of LGMD2G.

Clinical, pathological, imaging, and genetic characterization in a Taiwanese cohort with limb-girdle muscular dystrophy

Background

Limb-girdle muscular dystrophy (LGMD) is a genetically heterogeneous, hereditary disease characterized by limb-girdle weakness and histologically dystrophic changes. The prevalence of each subtype of LGMD varies among different ethnic populations. This study for the first time analyzed the phenotypes and genotypes in Taiwanese patients with LGMD in a referral center for neuromuscular diseases (NMDs).

Results

We enrolled 102 patients clinically suspected of having LGMD who underwent muscle biopsy with subsequent genetic analysis in the previous 10 years. On the basis of different pathological categories, we performed sequencing of target genes or panel for NMDs and then identified patients with type 1B, 1E, 2A, 2B, 2D, 2I, 2G, 2 N, and 2Q. The 1B patients with LMNA mutation presented with mild limb-girdle weakness but no conduction defect at the time. All 1E patients with DES mutation exhibited predominantly proximal weakness along with distal weakness. In our cohort, 2B and 2I were the most frequent forms of LGMD; several common or founder mutations were identified, including c.1097_1099delACA (p.Asn366del) in DES, homozygous c.101G > T (p.Arg34Leu) in SGCA, homozygous c.26_33dup (p.Glu12Argfs*20) in TCAP, c.545A > G (p.Tyr182Cys), and c.948delC (p.Cys317Alafs*111) in FKRP. Clinically, the prevalence of dilated cardiomyopathy in our patients with LGMD2I aged > 18 years was 100%, much higher than that in European cohorts. The only patient with LGMD2Q with PLEC mutation did not exhibit skin lesions or gastrointestinal abnormalities but had mild facial weakness. Muscle imaging of LGMD1E and 2G revealed a more uniform involvement than did other LGMD types.

Conclusion

Our study revealed that detailed clinical manifestation together with muscle pathology and imaging remain critical in guiding further molecular analyses and are crucial for establishing genotype–phenotype correlations. We also determined the common mutations and prevalence for different subtypes of LGMD in our cohort, which could be useful when providing specific care and personalized therapy to patients with LGMD.

Clinical and genetic characterization of limb girdle muscular dystrophy R7 telethonin-related patients from three unrelated Chinese families

Limb girdle muscular dystrophy LGMD R7 telethonin-related is a rare autosomal recessive muscle disorder characterized by proximal muscle weakness of pelvic and shoulder girdles. Mutation in TCAP is responsible for LGMD R7, and the disease has a wide geographic distribution in diverse populations, but genotype-phenotype relationships remain unclear. We collected 5 LGMD R7 patients from three unrelated Chinese families. The average onset age was 16 ± 1.41; the initial symptoms included progressive proximal muscle weakness in limbs, difficulty in fast running, and asymmetric muscle atrophy in calves. Muscle MR imaging showed varying severity of fatty infiltration in the pelvic girdle, thigh, and calf muscles, and the severity of muscle infiltration was related to the length of the disease course. Muscle histopathology revealed aberrantly sized muscle fibers, internal nuclei, split fibers, rimmed vacuoles, monocyte invasion, and necrotic fibers. Sequencing identified one novel and one previously reported TCAP mutation. Our study extends the known distribution of this rare muscular dystrophy and presents the first detailed clinical and genetic characterizations of LGMD R7 cases from the Chinese population. Our work expands the mutation spectrum known for LGMD R7 and emphasizes the need for clinicians to consider TCAP mutations when evaluating patients with symptoms of limb girdle muscular dystrophy.

Limb girdle muscular dystrophy 2G in a religious minority of Bulgarian Muslims homozygous for the c.75G>A, p.Trp25X mutation

Mutations in TCAP gene cause autosomal recessive limb-girdle muscular dystrophy type 2G (LGMD2G), congenital muscular dystrophy and autosomal dominant dilated and hypertrophic cardiomyopathy. We studied 18 affected individuals from 12 pedigrees, belonging to a Bulgarian Muslim minority from the South-West of Bulgaria, homozygous for the c.75G>A, p.Trp25X mutation in TCAP gene. The heterozygous carrier rate of p.Trp25X among 100 newborns in this region was found to be 2%. The clinical features in the Bulgarian TCAP group include disease onset in the first to the third decade of life, proximal muscle weakness in the lower limbs, followed or accompanied by difficulties in ankle dorsiflexion and involvement of the proximal muscles of the upper limbs 5-9 years after the disease onset. Asymmetry between left and right was present in more than 20% of the affected. Respiratory and cardiac functions were not affected. On the MRI the muscles of the posterior pelvic area, thigh and anterior leg were predominantly affected, while sartorius, gracilis and biceps femoris muscles remained relatively spared. In conclusion, LGMD2G appears to be a common form among Bulgarian Muslims. Homozygosity for c.75G>A, p.Trp25X is associated with a homogeneous clinical presentation, but the clinical course and severity of the disease show inter- and intra-familial variation.

A new case of limb girdle muscular dystrophy 2G in a Greek patient, founder effect and review of the literature

Limb girdle muscular dystrophy (LGMD) type 2G is a rare form of muscle disease, described only in a few patients worldwide, caused by mutations in TCAP gene, encoding the protein telethonin. It is characterised by proximal limb muscle weakness associated with distal involvement of lower limbs, starting in the first or second decade of life. We describe the case of a 37-year-old woman of Greek origin, affected by disto-proximal lower limb weakness. No cardiac or respiratory involvement was detected. Muscle biopsy showed myopathic changes with type I fibre hypotrophy, cytoplasmic vacuoles, lipid overload, multiple central nuclei and fibre splittings; ultrastructural examination showed metabolic abnormalities. Next generation sequencing analysis detected a homozygous frameshift mutation in the TCAP gene (c.90_91del), previously described in one Turkish family. Immunostaining and Western blot analysis showed complete absence of telethonin. Interestingly, Single Nucleotide Polymorphism analysis of the 10 Mb genomic region containing the TCAP gene showed a shared homozygous haplotype of both the Greek and the Turkish patients, thus suggesting a possible founder effect of TCAP gene c.90_91del mutation in this part of the Mediterranean area.

Rare diagnosis of telethoninopathy (LGMD2G) in a Turkish patient

Telethoninopathy is one of the rarest forms of Limb-girdle muscular dystrophy (LGMD). So far, only a small number of LGMD type 2 G (LGMD2G) patients have been described, mostly patients from Brazil. Here we present a 35-year-old female patient of Turkish ethnicity with LGMD2G due to a novel homozygous frame-shift mutation c.90_91del (p.Ser31Hisfs*11) in the telethonin gene, probably leading to truncated protein or nonsense mediated decay. Myalgia and walking on tiptoes were the first symptoms starting in early childhood, around age 22 proximal, later distal leg muscles became affected. Muscle biopsy showed a degenerative myopathy with lobulated fibers, creatine kinase levels were elevated to 1200 U/l. No cardiomyopathy has been detected but ventricular extrasystoles were treated with verapamil. Even though telethoninopathy represents a rare condition, testing for LGMD2G should be included into the diagnostic work-up of mild myopathies with early toe walking and distal and proximal involvement.

Novel mutation in TCAP manifesting with asymmetric calves and early-onset joint retractions

A 29-year-old man, born from consanguineous parents, started with toe walking and frequent falls during his second year of life. He developed weakness in lower limbs during the first decade that subsequently extended to upper limbs. On examination, the patient had weakness in proximal muscles of all four limbs and in the tibialis anterior muscle. In addition, he had bilateral Achilles and patellar contractures, bilateral scapular winging, asymmetric calves and a positive Beevor sign, an upward movement of the umbilicus on contraction of rectus femoris due to weakness in the lower part. The muscle biopsy showed dystrophic changes and lobulated fibers. Genetic analysis through a next-generation sequencing panel of genes related to neuromuscular disorders revealed a novel homozygous nonsense mutation (p.Tyr85*) in the TCAP gene. Subsequent western blot assay showed a complete telethonin deficiency. Our observation expands the phenotypic spectrum of TCAP mutations and indicates that telethonin deficiency should be considered in the differential diagnosis of patients presenting with asymmetric calves and early joint retractions.

Conserved expression of truncated telethonin in a patient with limb-girdle muscular dystrophy 2G

Limb-girdle muscular dystrophy 2G is caused by mutations in the TCAP gene that encodes for telethonin. Here we describe a 49 year-old male patient of Indian descent presenting a classical LGMD phenotype. He had normal motor milestones but became noticeably slower in his early teens and was wheelchair bound by age 44. The muscle biopsy showed myopathic features and absence of labeling with an antibody to the C-terminal portion of telethonin. Sequence analysis of the TCAP gene revealed a novel homozygous mutation in exon 2, predicted to generate a truncated protein of 81 amino acids. Interestingly, an antibody for the full-length protein showed labeling on sections and a single band of ~10 kDa on Western blot. The truncated protein co-localized with filamin C at the Z-line. Our findings indicate that mutant telethonin can be incorporated into the sarcomere and that other LGMD2G patients with retention of telethonin expression may exist.