Muscle phenotypic variability in limb girdle muscular dystrophy 2 G

Clinical and molecular characterization of limb-girdle muscular dystrophy 2G/R7 in a large cohort of Brazilian patients

Limb-girdle muscular dystrophy type 2G/R7 (LGMD2G/R7) is an ultra-rare condition initially identified within the Brazilian population. We aimed to expand clinical and genetic information about this disease, including its worldwide distribution. A multicenter historical cohort study was performed at 13 centers in Brazil in which data from index cases and their affected relatives from consecutive families with LGMD2G/R7 were reviewed from July 2017 to August 2023. Additionally, a systematic literature review was conducted to identify case reports and series of the disease worldwide. Forty-one LGMD2G/R7 cases were described in the Brazilian cohort, being all subjects homozygous for the c.157C>T/(p.Gln53*) variant in TCAP. Survival curves showed that the median disease duration before individuals required walking aids was 21 years. Notably, women exhibited a slower disease progression, requiring walking aids 13 years later than men. LGMD2G/R7 was frequently reported not only in Brazil but also in China and Bulgaria, with 119 cases identified globally, with possible founder effects in the Brazilian, Eastern European, and Asian populations. These findings are pivotal in raising awareness of LGMD2G/R7, understanding its progression, and identifying potential modifiers. This can significantly contribute to the development of future natural history studies and clinical trials for this disease.

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.

Tcap Deficiency in Zebrafish Leads to ROS Production and Mitophagy, and Idebenone Improves its Phenotypes

Limb-girdle muscular dystrophy 2G (LGMD2G) is a subtype of limb-girdle muscular dystrophy. However, the disease’s mechanisms are still not fully understood, and no established therapeutic targets have been found. Using a morpholino-based knockdown approach, we established an LGMD2G zebrafish model. In this study, we found that the ROS level increased in LGMD2G zebrafish. The expression of the mitophagy-related protein BNIP3L, LC3A-II/LC3A-I, and LAMP1 were increased in LGMD2G zebrafish. The oxygen consumption rate and citrate synthase expression was significantly decreased. Thus, mitophagy was presumed to be involved in the LGMD2G to reduce ROS levels. Then, we administered vitamin C, coenzyme Q10, idebenone, metformin, or dexamethasone to rescue LGMD2G in zebrafish. Idebenone reduced the curly tail phenotype and ROS level. Also, it reduced BNIP3L expression in LGMD2G zebrafish models and improved their motor function. In conclusion, mitophagy might be involved in the LGMD2G, and idebenone ameliorated LGMD2G by downregulating ROS level.

Muscle biopsy essential diagnostic advice for pathologists

Background

Muscle biopsies are important diagnostic procedures in neuromuscular practice. Recent advances in genetic analysis have profoundly modified Myopathology diagnosis.

Main body

The main goals of this review are: (1) to describe muscle biopsy techniques for non specialists; (2) to provide practical information for the team involved in the diagnosis of muscle diseases; (3) to report fundamental rules for muscle biopsy site choice and adequacy; (4) to highlight the importance of liquid nitrogen in diagnostic workup. Routine techniques include: (1) histochemical stains and reactions; (2) immunohistochemistry and immunofluorescence; (3) electron microscopy; (4) mitochondrial respiratory chain enzymatic studies; and (5) molecular studies. The diagnosis of muscle disease is a challenge, as it should integrate data from different techniques.

Conclusion

Formalin-fixed paraffin embedded muscle samples alone almost always lead to inconclusive or unspecific results. Liquid nitrogen frozen muscle sections are imperative for neuromuscular diagnosis. Muscle biopsy interpretation is possible in the context of detailed clinical, neurophysiological, and serum muscle enzymes data. Muscle imaging studies are strongly recommended in the diagnostic workup. Muscle biopsy is useful for the differential diagnosis of immune mediated myopathies, muscular dystrophies, congenital myopathies, and mitochondrial myopathies. Muscle biopsy may confirm the pathogenicity of new gene variants, guide cost-effective molecular studies, and provide phenotypic diagnosis in doubtful cases. For some patients with mitochondrial myopathies, a definite molecular diagnosis may be achieved only if performed in DNA extracted from muscle tissue due to organ specific mutation load.

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.

Identification of a novel titin-cap/telethonin mutation in a Portuguese family with hypertrophic cardiomyopathy

INTRODUCTION AND OBJECTIVES

Hypertrophic cardiomyopathy (HCM) is a genetically and phenotypically heterogeneous disease; there is still a large proportion of patients with no identified disease-causing mutation. Although the majority of mutations are found in the MYH7 and MYBPC3 genes, mutations in Z-disk-associated proteins have also been linked to HCM.

METHODS

We assessed a small family with HCM based on family history, physical examination, 12-lead ECG, echocardiogram and magnetic resonance imaging. After exclusion of mutations in eleven HCM disease genes, we performed direct sequencing of the TCAP gene encoding the Z-disk protein titin-cap (also known as telethonin).

RESULTS

We present a novel TCAP mutation in a small family affected by HCM. The identified p.C57W mutation showed a very low population frequency, as well as high conservation across species. All of the bioinformatic prediction tools used considered this mutation to be damaging/deleterious. Family members were screened for this new mutation and a co-segregation pattern was detected. Both affected members of this family presented with late-onset HCM, moderate asymmetric left ventricular hypertrophy, atrial fibrillation and heart failure with preserved ejection fraction and low risk of sudden cardiac death.

CONCLUSIONS

We present evidence supporting the classification of the TCAP p.C57W mutation, encoding the Z-disk protein titin-cap/telethonin as a new likely pathogenic variant of hypertrophic cardiomyopathy, with a specific phenotype in the family under analysis.

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.

Upregulation of MuRF1 and MAFbx participates to muscle wasting upon gentamicin-induced acute kidney injury

Acute Kidney Injury (AKI) is frequently encountered in hospitalized patients where it is associated with increased mortality and morbidity notably affecting muscle wasting. Increased protein degradation has been shown to be the main actor of AKI-induced muscle atrophy, but the proteolytic pathways involved are poorly known. The Ubiquitin Proteasome System (UPS) is almost systematically activated in various catabolic situations, and the E3 ligases MuRF1 and MAFbx are generally up regulated in atrophying muscles. We hypothesized that the UPS may be one of the main actors in catabolic skeletal muscles from AKI animals. We used gentamicin-induced acute kidney disease (G-AKI) in rats fed a high protein diet to promote acidosis. We first addressed the impact of G-AKI in the development of mild catabolic conditions. We found that both muscle atrophy and UPS activation were induced with the development of G-AKI. In addition, the phasic muscles were more sensitive to 7-days G-AKI (-11 to -17%, P<0.05) than the antigravity soleus muscle (-11%, NS), indicating a differential impact of AKI in the musculature. We observed an increased expression of the muscle-specific E3 ligases MuRF1 and MAFbx in phasic muscles that was highly correlated to the G-AKI severity (R²=0.64, P<0.01 and R²=0.71, P<0.005 respectively). Conversely, we observed no variation in the expression of three other E3 ligases (Nedd4, Trim32 and Fbxo30/MUSA1). Altogether, our data indicate that MuRF1 and MAFbx are sensitive markers and potential targets to prevent muscle atrophy during G-AKI.

Common recessive limb girdle muscular dystrophies differential diagnosis: why and how?

Limb girdle muscular dystrophies are heterogeneous autosomal hereditary neuromuscular disorders. They produce dystrophic changes on muscle biopsy and they are associated with mutations in several genes involved in muscular structure and function. Detailed clinical, laboratorial, imaging, diagnostic flowchart, photographs, tables, and illustrated diagrams are presented for the differential diagnosis of common autosomal recessive limb girdle muscular dystrophy subtypes diagnosed nowadays at one reference center in Brazil. Preoperative image studies guide muscle biopsy site selection. Muscle involvement image pattern differs depending on the limb girdle muscular dystrophy subtype. Muscle involvement is conspicuous at the posterior thigh in calpainopathy and fukutin-related proteinopathy; anterior thigh in sarcoglycanopathy; whole thigh in dysferlinopathy, and telethoninopathy. The precise differential diagnosis of limb girdle muscular dystrophies is important for genetic counseling, prognostic orientation, cardiac and respiratory management. Besides that, it may probably, in the future, provide specific genetic therapies for each subtype.

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.

Limb girdle muscular dystrophy type 2G with myopathic-neurogenic motor unit potentials and a novel muscle image pattern

Background

Limb girdle muscular dystrophy type 2G (LGMD2G) is a subtype of autosomal recessive muscular dystrophy caused by mutations in the telethonin gene. There are few LGMD2G patients worldwide reported, and this is the first description associated with early tibialis anterior sparing on muscle image and myopathic-neurogenic motor unit potentials.

Case presentation

Here we report a 31 years old caucasian male patient with progressive gait disturbance, and severe lower limb proximal weakness since the age of 20 years, associated with subtle facial muscle weakness. Computed tomography demonstrated soleus, medial gastrocnemius, and diffuse thigh muscles involvement with tibialis anterior sparing. Electromyography disclosed both neurogenic and myopathic motor unit potentials. Muscle biopsy demonstrated large groups of atrophic and hypertrophic fibers, frequent fibers with intracytoplasmic rimmed vacuoles full of autophagic membrane and sarcoplasmic debris, and a total deficiency of telethonin. Molecular investigation identified the common homozygous c.157C > T in the TCAP gene.

Conclusion

This report expands the phenotypic variability of telethoninopathy/ LGMD2G, including: 1) mixed neurogenic and myopathic motor unit potentials, 2) facial weakness, and 3) tibialis anterior sparing. Appropriate diagnosis in these cases is important for genetic counseling and prognosis.