BAG3 mutation in a patient with atypical phenotypes of myofibrillar myopathy and Charcot-Marie-Tooth disease

Rare Variants Cause Charcot-Marie-Tooth Disease in Malian Families

INTRODUCTION/AIMS

Charcot-Marie-Tooth disease (CMT), the most common inherited peripheral neuropathy, is clinically and genetically heterogeneous with over 100 genes identified to date. Recently, next-generation sequencing (NGS) has enabled molecular diagnosis in previously unidentified CMT cases. However, less progress has been achieved in sub-Saharan African (SSA) populations. We report rare CMT variants found in four unrelated Malian families.

METHODS

Patients went through a thorough neurological examination and Nerve Conduction Studies (NCS) were performed. DNA was extracted for genetic analysis (CMT gene panel testing and whole-exome/genome sequencing). Putative variants were confirmed with Sanger sequencing and segregation was checked in all available family members. Deleteriousness was checked using several in silico prediction tools and protein modeling.

RESULTS

Nine patients (three males and six females) from four families were enrolled. The mean age at onset and diagnosis were 15 and 22.7 years, respectively (ranges: 3 to 55 years, and 12 to 58 years). Walking difficulty was the first symptom commonly reported. Neurological examination found distal muscle weakness and wasting with sensory loss, reduced tendon reflexes, and skeletal deformities. In addition, some patients presented with ataxic gait associated with incoordination that are not in the forefront of CMT features. NCS was consistent with the axonal pattern in three families. Genetic analysis revealed rare pathogenic variants in BSCL2, SH3TC2, and PEX10, and of unknown significance in BAG3.

DISCUSSION

This study reports rare variants in these CMT genes for the first time in SSA populations, expanding the global epidemiological, clinical, and genetic spectrum of these diseases.

[A case of myofibrillary myopathy due to Bcl2-Associated Athanogene 3 (BAG3) mutation complicated by peripheral neuropathy]

A 19-year-old female, normal at birth, grew up without neck movement when getting up. She needed a handrail to climb stairs since the age of 10 years old, and walked slowly since the age of 16 years old. Neurological examination revealed loss of deep tendon reflexes, decreased vibratory sensation, weakness of distal muscles of the lower extremities, and weakness of mainly cervical trunk muscles suspected to be due to myopathy. Nerve conduction studies suggested axonal polyneuropathy, and needle EMG showed short duration MUP, myotonic discharge, and rimmed vacuoles on muscle biopsy. Genetic analysis revealed a previously reported pathological mutation (p.P209L, heterozygous) in Bcl2-Associated Athanogene 3 (BAG3), and a diagnosis of MFM6 was made. P209L is a poor prognosis myopathy that develops in childhood and is associated with cardiomyopathy. P209L is a solitary myopathy associated with axonal neuropathy and characterized by apex foot contracture and weak neck to trunk flexion. This disease is suspected in young-onset neuromyopathy.

Expanding the genetic and clinical spectrum of SORD-related peripheral neuropathy by reporting a novel variant c.210T>G and evidence of subclinical muscle involvement

BACKGROUND AND AIMS

Biallelic variants in the sorbitol dehydrogenase (SORD) gene have been identified as the genetic cause of autosomal recessive (AR) peripheral neuropathy (PN) manifesting as Charcot-Marie-Tooth disease type 2 (CMT2) or distal hereditary motor neuropathy (dHMN). We aim to observe the genetic and clinical spectrum of a cohort of patients with SORD-related PN (SORD-PN).

METHODS

A total of 107 patients with AR or sporadic CMT2/dHMN underwent molecular diagnosis by whole-exome sequencing and subsequent Sanger sequencing validation. Available phenotypic data for SORD-PN were collected and analyzed.

RESULTS

Eleven (10.28%) of 107 patients were identified as SORD-PN, including four with CMT2 and seven with dHMN. The SORD variant c.210 T > G;p.His70Gln in F-d3 was firstly reported and subsequent analysis showed that it resulted in loss of SORD enzyme function. Evidence of subclinical muscle involvement was frequently detected in patients with SORD-PN, including mildly to moderately elevated serum creatine kinase (CK) levels in 10 patients, myogenic electrophysiological changes in one patient, and muscle edema in five patients undergoing lower extremity MRI. Fasting serum sorbitol level was 88-fold higher in SORD-PN patients (9.69 ± 1.07 mg/L) than in healthy heterozygous subjects (0.11 ± 0.01 mg/L) and 138-fold higher than in healthy controls (0.07 ± 0.02 mg/L).

INTERPRETATION

The novel SORD variant c.210 T > G;p.His70Gln and evidence of subclinical muscle involvement were identified, which expanded the genetic and clinical spectrum of SORD-PN. Subclinical muscle involvement might be a common but easily overlooked clinical feature. The serum CK and fasting serum sorbitol levels were expected to be sensitive biomarkers confirmed by follow-up cohort study.

Wide Spectrum of Cardiac Phenotype in Myofibrillar Myopathy Associated With a Bcl-2-Associated Athanogene 3 Mutation: A Case Report and Literature Review

ABSTRACT

Myofibrillar myopathy is a clinically and genetically heterogeneous group of muscle disorders characterized by myofibrillar degeneration. Bcl-2-associated athanogene 3 (BAG3)-related myopathy is the rarest form of myofibrillar myopathy. Patients with BAG3-related myopathy present with early-onset and progressive muscle weakness, rigid spine, respiratory insufficiency, and cardiomyopathy. Notably, the heterozygous mutation (Pro209Leu) in BAG3 is commonly associated with rapidly progressive cardiomyopathy in childhood. We describe a male patient with the BAG3 (Pro209Leu) mutation. The patient presented at age 7 years with muscle weakness predominantly in the proximal lower limbs. Histologic findings revealed a mixture of severe neurogenic and myogenic changes. His motor symptoms progressed rapidly in the next decade, becoming wheelchair-dependent by age 17 years; however, at the age of 19 years, cardiomyopathy was not evident. This study reports a case of BAG3-related myopathy without cardiac involvement and further confirmed the wide phenotypic spectrum of BAG3-related myopathy.

Long non-coding RNA CYTOR modulates cancer progression through miR-136-5p/MAT2B axis in renal cell carcinoma

BACKGROUND

To explore the role of long noncoding RNAs (lncRNAs) cytoskeleton regulator RNA (CYTOR) in renal cell carcinoma (RCC).

METHODS

The levels of CYTOR in RCC tissues and cell lines were detected by RT-qPCR. 786-O and Caki-1 cells were transfected with CYTOR-shRNA or pcDNA-CYTOR respectively, or co-transfected with CYTOR-shRNA and miR-136-5p inhibitor, or co-transfected with miR-136-5p mimic and pcDNA-MAT2B. MTT assay, Transwell assay and flow cytometry were used to evaluate cell proliferation, invasion and apoptosis. The relationship between lncRNA CYTOR and miRNA-136-5p was detected by dual luciferase reporter gene and RNA pull down assays, and the targeted relationship between miRNA-136-5p and MAT2B was verified by dual luciferase reporter gene assay. The interaction between MAT2B and BAG3 protein was verified by co-IP experiment. The role of lncRNA CYTOR in vivo was also examined.

RESULTS

LncRNA CYTOR was up-regulated in RCC tissues and cell lines, and miR-136-5p was down-regulated in renal carcinoma cell lines and tissues. Downregulation of CYTOR inhibited cell proliferation and invasion and promoted apoptosis. miR-136-5p was sponged by lncRNA CYTOR, which negatively regulated the development of RCC. MAT2B was a target gene of miR-136-5p. MAT2B protein interacted directly with BAG3 protein to affect the proliferation, invasion and apoptosis of RCC cells. In vivo experiments showed that the expression level of miR-136-5p was increased, and MAT2B expression was decreased after CYTOR knockdown, thereby inhibiting the development of RCC.

CONCLUSIONS

LncRNA CYTOR promoted the progression of RCC by targeting miR-136-5p to regulate the target gene MAT2B, which interacted with BAG3 protein.

Role of the Ubiquitin System in Stress Granule Metabolism

Eukaryotic cells react to various stress conditions with the rapid formation of membrane-less organelles called stress granules (SGs). SGs form by multivalent interactions between RNAs and RNA-binding proteins and are believed to protect stalled translation initiation complexes from stress-induced degradation. SGs contain hundreds of different mRNAs and proteins, and their assembly and disassembly are tightly controlled by post-translational modifications. The ubiquitin system, which mediates the covalent modification of target proteins with the small protein ubiquitin (‘ubiquitylation’), has been implicated in different aspects of SG metabolism, but specific functions in SG turnover have only recently emerged. Here, we summarize the evidence for the presence of ubiquitylated proteins at SGs, review the functions of different components of the ubiquitin system in SG formation and clearance, and discuss the link between perturbed SG clearance and the pathogenesis of neurodegenerative disorders. We conclude that the ubiquitin system plays an important, medically relevant role in SG biology.

Ultrasound evaluation of diaphragm motion in BAG-3 myofibrillar myopathy: A case report

RATIONALE

Mutations in Bcl-2-associated athanogene-3 (BAG-3) can cause a rare subtype of myofibrillar myopathies (MFMs), characterized by progressive muscle weakness, cardiomyopathy, and severe respiratory insufficiency in childhood. Little is known about diaphragmatic function in BAG-3 MFM. To our knowledge, this is the first case report of detailed evaluation of diaphragmatic function with ultrasound in BAG-3 MFM.

PATIENT CONCERN

We describe the case of a 15-year-old girl who complained of fever and shortness of breath. Diaphragmatic sonography revealed bilateral diaphragmatic paralysis. Shortness of breath progressed to respiratory failure approximately 3 months later.

DIAGNOSIS

A neurologist was consulted and genetic sequencing identified a p.Pro209Leu mutation in BAG-3, yielding diagnosis of BAG-3 MFM leading to bilateral diaphragmatic paralysis.

INTERVENTIONS

Respiratory muscle training and long-term mechanical ventilation.

OUTCOMES

It is quite unfortunate for this patient to have a poor prognosis due to the lack of effective treatment for this genetic disorder.

LESSONS

This case provides more clinical information for this rare disease which may cause severe diaphragm pathological damage leading to respiratory failure in BAG3 MFM and a future study with a systematic evaluation of a greater number of patients will be necessary to characterize this population.

BAG3 Myopathy Presenting With Prominent Neuropathic Phenotype and No Cardiac or Respiratory Involvement: A Case Report and Literature Review

Bcl-2-associated athanogene 3 (BAG3) myopathy is a rare myofibrillar myopathy characterized by toe walking and clumsiness in the first decade with rapid progression to cardiomyopathy and restrictive lung disease in the second decade. Most patients (18 patients) have the c.626C >T (p.Pro209Leu) mutation. We describe BAG3 myopathy due to p.Pro209Leu in a 13-year-old girl with initial prominent neuropathic phenotype and no cardiac or respiratory involvement. Parents reported toe walking and clumsiness since 3 years old. Examination at the age of 13 years showed findings suggestive of Charcot-Marie-Tooth disease. Nerve conduction studies revealed demyelinating polyneuropathy. Next-generation sequencing panel for inherited neuropathies was unrevealing. Whole exome sequencing identified a de novo mutation in BAG3. Muscle biopsy confirmed myofibrillar myopathy. No cardiac involvement or symptoms of respiratory involvement at the age of 14 years. This case emphasizes the phenotypic variability of BAG3 myopathy and the importance of thorough electrophysiological examination and muscle pathology for establishing a precise diagnosis.

Next-Generation Sequencing Technologies and Neurogenetic Diseases

Next-generation sequencing (NGS) technology has led to great advances in understanding the causes of Mendelian and complex neurological diseases. Owing to the complexity of genetic diseases, the genetic factors contributing to many rare and common neurological diseases remain poorly understood. Selecting the correct genetic test based on cost-effectiveness, coverage area, and sequencing range can improve diagnosis, treatments, and prevention. Whole-exome sequencing and whole-genome sequencing are suitable methods for finding new mutations, and gene panels are suitable for exploring the roles of specific genes in neurogenetic diseases. Here, we provide an overview of the classifications, applications, advantages, and limitations of NGS in research on neurological diseases. We further provide examples of NGS-based explorations and insights of the genetic causes of neurogenetic diseases, including Charcot-Marie-Tooth disease, spinocerebellar ataxias, epilepsy, and multiple sclerosis. In addition, we focus on issues related to NGS-based analyses, including interpretations of variants of uncertain significance, de novo mutations, congenital genetic diseases with complex phenotypes, and single-molecule real-time approaches.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

On the differential diagnosis of neuropathy in neurogenetic disorders

Neuropathy might be the presenting or accompanying sign in many neurogenetic and metabolic disorders apart from the classical-peripheral neuropathies or motor-neuron diseases. This causes a diagnostic challenge which is of particular relevance since a number of the underlying diseases could be treated. Thus, we attempt to give a clinical overview on the most common genetic diseases with clinically manifesting neuropathy.

Expanding the Clinico-Genetic Spectrum of Myofibrillar Myopathy: Experience From a Chinese Neuromuscular Center

Background: Myofibrillar myopathy is a group of hereditary neuromuscular disorders characterized by dissolution of myofibrils and abnormal intracellular accumulation of Z disc-related proteins. We aimed to characterize the clinical, physiological, pathohistological, and genetic features of Chinese myofibrillar myopathy patients from a single neuromuscular center.

Methods: A total of 18 patients were enrolled. Demographic and clinical data were collected. Laboratory investigations, electromyography, and cardiac evaluation was performed. Routine and immunohistochemistry stainings against desmin, αB-crystallin, and BAG3 of muscle specimen were carried out. Finally, next-generation sequencing panel array for genes associated with hereditary neuromuscular disorders were performed.

Results: Twelve pathogenic variants in DES, BAG3, FLNC, FHL1, and TTN were identified, of which seven were novel mutations. The novel DES c.1256C>T substitution is a high frequency mutation. The combined recessively/dominantly transmitted c.19993G>T and c.107545delG mutations in TTN gene cause a limb girdle muscular dystrophy phenotype with the classical myofibrillar myopathy histological changes.

Conclusions: We report for the first time that hereditary myopathy with early respiratory failure patient can have peripheral nerve and severe spine involvement. The mutation in Ig-like domain 16 of FLNC is associated with the limb girdle type of filaminopathy, and the mutation in Ig-like domain 18 with distal myopathy type. These findings expand the phenotypic and genotypic correlation spectrum of myofibrillar myopathy.

A family with adult-onset myofibrillar myopathy with BAG3 mutation (P470S) presenting with axonal polyneuropathy

We report a family with adult-onset myofibrillar myopathy with BAG3 mutation who presented peroneal weakness and axonal polyneuropathy, mimicking axonal Charcot-Marie-Tooth disease. The male proband noticed difficulty in tiptoeing at age 34. At age 42, the examination showed muscle weakness and atrophy in distal lower extremities with diminished patellar and Achilles tendon reflexes. Thermal and vibration sensations were also impaired in both feet. The serum CK level was 659 U/L. On muscle imaging, predominant semitendinosus muscle atrophy coexisted with atrophies in the quadriceps, gastrocnemius and lumbar paraspinal muscles. The muscle biopsy showed myofibrillar myopathy with fiber type grouping. His 68-year-old mother also had suffered from distal leg weakness and sensory impairment since her forties. A heterozygous mutation in BAG3 (P470S) was identified in both patients. Clinical features of myofibrillar myopathy with axonal polyneuropathy were consistent with BAG3-related myopathy. Our patients showed remarkably mild presentations without cardiomyopathy, unlike the majorities of previously reported cases.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

BAG3P215L/KO Mice as a Model of BAG3P209L Myofibrillar Myopathy

BCL-2-associated athanogene 3 (BAG3) is a co-chaperone to heat shock proteins important in degrading misfolded proteins through chaperone-assisted selective autophagy. The recurrent dominant BAG3-P209L mutation results in a severe childhood-onset myofibrillar myopathy (MFM) associated with progressive muscle weakness, cardiomyopathy, and respiratory failure. Because a homozygous knock-in (KI) strain for the mP215L mutation homologous to the human P209L mutation did not have a gross phenotype, compound heterozygote knockout (KO) and KI mP215L mice were generated to establish whether further reduction in BAG3 expression would lead to a phenotype. The KI/KO mice have a significant decrease in voluntary movement compared with wild-type and KI/KI mice in the open field starting at 7 months. The KI/KI and KI/KO mice both have significantly smaller muscle fiber cross-sectional area. However, only the KI/KO mice have clear skeletal muscle histologic changes in MFM. As in patient muscle, there are increased levels of BAG3-interacting proteins, such as p62, heat shock protein B8, and αB-crystallin. The KI/KO mP215L strain is the first murine model of BAG3 myopathy that resembles the human skeletal muscle pathologic features. The results support the hypothesis that the pathologic development of MFM requires a significant decrease in BAG3 protein level and not only a gain of function caused by the dominant missense mutation.

BAG3 p.Pro209Ser mutation identified in a Chinese family with Charcot-Marie-Tooth disease

Bcl2-associated athanogene 3 (BAG3) gene mutations cause dilated cardiomyopathy and myofibrillar myopathy. Recently, a novel c.625C>T (p.Pro209Ser) mutation in BAG3 was reported to cause axonal Charcot-Marie-Tooth (CMT) disease in three families. Here, we describe two patients with adult-onset and moderate CMT in a Chinese family. Nerve conduction velocity studies revealed an axonal sensorimotor neuropathy, which was supported by sural nerve biopsy. Lower limb magnetic resonance imaging (MRI) revealed fatty infiltration more severe in the soleus and deep posterior compartment muscles than in the medial gastrocnemius and anterior compartment muscles. Whole exome sequencing identified the same c.625C>T (p.Pro209Ser) mutation in BAG3, which co-segregated with the CMT disease in this family. This study further enforces the association between BAG3 gene and CMT disease, indicating that BAG3 should be considered in the genetic testing for CMT. The p.Pro209Ser mutation with different ethnic origins might be another hotspot mutation of BAG3. MRI is helpful to detect accurate extent of muscle involvement.

Compound heterozygous mutations of SH3TC2 in Charcot-Marie-Tooth disease type 4C patients

Charcot-Marie-Tooth disease type 4C (CMT4C) is an autosomal recessive neuropathy caused by SH3TC2 mutations, characterized by spine deformities and cranial nerve involvement. This study identified four CMT4C families with compound heterozygous SH3TC2 mutations from 504 Korean demyelinating or intermediate CMT patients. The frequency of the CMT4C was calculated as 0.79% in demyelinating and intermediate patients (n = 504), but it was calculated as 2.02% in patients without PMP22 duplication (n = 198). The CMT4C frequency was similar to patients in Japan, but it was relatively low compared to those patients in other populations. The symptom was less severe and slowly progressed compared to the other AR-CMT. A patient harboring an intermediate neuropathy showed cranial nerve involvement but did not have scoliosis. This study will be helpful in making molecular diagnoses of demyelinating or intermediate CMT due to SH3TC2 mutations.