The genetic profile of dysferlinopathy in a cohort of 209 cases: Genotype-phenotype relationship and a hotspot on the inner DysF domain

Limb Girdle Muscular Dystrophy Type 2B (LGMD2B): Diagnosis and Therapeutic Possibilities

Dysferlin is a large transmembrane protein involved in critical cellular processes including membrane repair and vesicle fusion. Mutations in the dysferlin gene (DYSF) can result in rare forms of muscular dystrophy; Miyoshi myopathy; limb girdle muscular dystrophy type 2B (LGMD2B); and distal myopathy. These conditions are collectively known as dysferlinopathies and are caused by more than 600 mutations that have been identified across the DYSF gene to date. In this review, we discuss the key molecular and clinical features of LGMD2B, the causative gene DYSF, and the associated dysferlin protein structure. We also provide an update on current approaches to LGMD2B diagnosis and advances in drug development, including splice switching antisense oligonucleotides. We give a brief update on clinical trials involving adeno-associated viral gene therapy and the current progress on CRISPR/Cas9 mediated therapy for LGMD2B, and then conclude by discussing the prospects of antisense oligomer-based intervention to treat selected mutations causing dysferlinopathies.

Molecular Diagnosis of Limb-Girdle Muscular Dystrophy Using Next-Generation Sequencing Panels

Introduction

Limb-girdle muscular dystrophies (LGMDs) are clinically and genetically heterogeneous muscle disorders. We aimed to share the diagnostic yield of an NGS gene panel containing LGMD-related genes and our experience with LGMD.

Methods

Between February 2019 and October 2022, patients with a suspicion of LGMD and their relatives were reviewed in terms of demographic, clinical, and individual genetic data, age of symptom onset, sex, clinical features, LGMD types, cardiac involvement, muscle biopsy results, family history, and consanguinity. Our NGS gene panel consisted of ANO5, CAPN3, CAV3, DAG1, DES, DNAJB6, DYSF, FKTN, FLNC, FRKP, GAA, GMPPB, HNRNPDL, ISPD, LIMS2, LMNA, MYOT, PLEC, POMGNT1, POMK, POMT1, POMT2, SGCA, SGCB, SGCD, SGCG, TCAP, TNPO3, TRAPPC11, TRIM32, and TTN genes.

Results

The diagnosis rate was 61.1% (11/18). Twelve (80%) patients with LGMD were male and three (20%) were female. The median age was 15.9 (range, 1.5-39) years. Our patient collective was drawn up out of patients with the following variants: LGMDR1 (n = 6; 40%), LGMDR2 (n = 4; 26.6%), LGMDR3 (n = 4; 26.6%), and LGMDR12 (n = 1; 6.7%).

Conclusion

The present study showed that the NGS panel has a high success rate in the diagnosis of LGMD and contributes to early diagnosis.

Phenotypic and genotypic analysis of a patient with Miyoshi myopathy caused by truncated protein

Dysferlin protein deficiency can cause neuromuscular dysfunction, resulting in autosomal recessive dysferlinopathy, which is caused by DYSF gene mutation. Dysferlin proteins belongs to the Ferlin1-like protein family and are associated with muscle membrane repair and regeneration. In China, pathogenic mutations of the protein often result in two clinical phenotypes of Miyoshi muscular or limb band muscular dystrophy type 2B. It is clinically characterized by progressive muscle weakness and elevated serum creatine kinase. The data of the child were collected, blood samples of the child and his family members were collected, and whole exome sequencing (WES) was performed. The recombinant expression vector was constructed, the function of the mutation was verified by minigene, and the pathogenicity of the mutation was further analyzed by combining with biological information analysis. The patient initially presented with asymptomatic elevation of serum creatine kinase（CK）. Then progressive lower limb weakness, mainly distal limb weakness. Large amounts of scattered necrosis, myogenic lesions, and complete deletion of dysferlin protein were observed under muscle biopsy, which further improved genetic detection. Whole exome sequencing showed compound mutations (c.1397 + 1_1397 + 3del and c.1375dup p.M459Nfs*15) in DYSF gene. c.1375dup p.M459Nfs*15 have been reported. The other mutation is the deletion of c.1397 + 1_1397 + 3 in Intron15, which is an intron mutation that may affect splicing and the pathogenesis is still unknown. Minigene splicing assay verified that c.1397 + 1_1397 + 3del resulted in exon15 skipping and produced a premature termination codon. We report a novel pathogenic mutation in DYSF gene with Miyoshi myopathy and demonstrate this variant causes skipping of exon15 by minigene splicing assay. We point out the need of conducting functional analysis to verify the pathogenicity of intronic mutation. The finding enriches the mutation spectrum of DYSF gene and laid a foundation for future studies on the correlation between genotype and phenotype.

Unexpected extra exon skipping in the DYSF gene during restoring the reading frame by CRISPR/Cas9

The DYSF gene encoding dysferlin protein is one of the largest and has many transcripts. Pathogenic variants in the gene can lead to various types of myopathies, which makes it a good object for studying the events occurring in it during genome editing by the CRISPR/Cas method. In this study, we evaluated the possibility of permanent skipping of exons 3-4, and 26-27 which deletion does not violate the reading frame and allows to eliminate truncated variants within exons. Editing was performed with simultaneous transfection of two sgRNA- and sa/spCas9-containing plasmids on HEK293T cell cultures and healthy donor myoblasts. Skipping of exons 3-4 was performed by destroying the splicing acceptor sites, and exons 26-27 by cuts in the flanking exons with the corresponding deletion in the DNA. Some unexpected results were obtained, when exons 26-27 were skipped, exon 30 was also absent in the transcript, although it is not alternatively spliced and is normally present in all transcripts. This event indicates that DNA changes near splicing sites can affect adjacent exons and the whole gene. However, this fact requires further study.

Clinical features, imaging findings and molecular data of limb-girdle muscular dystrophies in a cohort of Chinese patients

BACKGROUND

Limb-girdle muscular dystrophies (LGMDs) are a group of heterogeneous inherited diseases predominantly characterized by limb-girdle muscle weakness and dystrophic changes on histological analysis. The frequency of LGMD subtypes varies among regions in China and ethnic populations worldwide. Here, we analyzed the prevalence of LGMD subtypes, their corresponding clinical manifestations, and molecular data in a cohort of LGMD patients in Southeast China.

METHODS

A total of 81 consecutive patients with clinically suspected LGMDs from 62 unrelated families across Southeast China were recruited for targeted next-generation sequencing and whole-exome sequencing from July 2017 to February 2020.

RESULTS

Among 50 patients (41 families) with LGMDs, the most common subtypes were LGMD-R2/LGMD2B (36.6%) and LGMD-R1/LGMD2A (29.3%). Dystroglycanopathies (including LGMD-R9/LGMD2I, LGMD-R11/LGMD2K, LGMD-R14/LGMD2N and LGMD-R20/LGMD2U) were the most common childhood-onset subtypes and were found in 12.2% of the families. A total of 14.6% of the families had the LGMD-R7/LGMD2G subtype, and the mutation c.26_33dupAGGTGTCG in TCAP was the most frequent (83.3%). The only patient with the rare subtype LGMD-R18/LGMD2S had TRAPPC11 mutations; had a later onset than those previously reported, and presented with proximal‒distal muscle weakness, walking aid dependency, fatty liver disease and diabetes at 33 years of age. A total of 22.0% of the patients had cardiac abnormalities, and one patient with LMNA-related muscular dystrophy/LGMD1B experienced sudden cardiac death at 37 years of age. A total of 15.4% of the patients had restrictive respiratory insufficiency. Muscle imaging in patients with LGMD-R1/LGMD2A and LGMD-R2/LGMD2B showed subtle differences, including more severe fatty infiltration of the posterior thigh muscles in those with LGMD-R1/LGMD2A and edema in the lower leg muscles in those with LGMD-R2/LGMD2B.

CONCLUSION

We determined the prevalence of different LGMD subtypes in Southeast China, described the detailed clinical manifestations and distinct muscle MRI patterns of these LGMD subtypes and reported the frequent mutations and the cardiorespiratory involvement frequency in our cohort, all of which might facilitate the differential diagnosis of LGMDs, allowing more timely treatment and guiding future clinical trials.

Dysferlinopathy in Tunisia: clinical spectrum, genetic background and prognostic profile

Dysferlinopathy is a rare group of hereditary muscular dystrophy with an autosomal recessive mode of inheritance caused by a mutation in the DYSF gene. It encodes for the dysferlin protein, which has a crucial role in multiple cellular processes, including muscle fiber membrane repair. This deficit has heterogeneous clinical presentations. In this study, we collected 20 Tunisian patients with a sex ratio of 1 and a median age of 50.5 years old (Interquartile range (IQR) = [36,5-54,75]). They were followed for periods ranging from 5 to 48 years. The median age at onset was 17 years old (IQR = [16,8-28,4]). Five major phenotypes were identified: Limb-girdle muscular dystrophy (LGMDR2) (35%), a proximodistal phenotype (35%), Miyoshi myopathy (10%),  Distal myopathy with anterior tibial onset (DMAT) (10%), and asymptomatic HyperCKemia (10%). At the last evaluation, more than half of patients (55%) were on wheelchair. Loss of ambulation occurred generally during the fourth decade. After 20 years of disease progression, two patients with a proximodistal phenotype (10%) developed dilated cardiomyopathy and mitral valve regurgitation. Restrictive respiratory syndrome was observed in three patients (DMAT: 1 patient, proximodistal phenotype: 1 patient, LGMDR2: 1 patient). Genetic study disclosed five mutations. We observed clinical heterogeneity between families and even within the same family. Disease progression was mainly slow to intermediate regardless of the phenotype.

A female case report of LGMD2B with compound heterozygous mutations of the DYSF gene and asymptomatic mutation of the X-linked DMD gene

We report the case of a 31-year-old Chinese woman with a chief complaint of weakness in the lower limbs, which was diagnosed as limb-girdle muscular dystrophy 2B (LGMD2B) with compound heterozygous mutations of the DYSF gene. Meanwhile, this woman is an asymptomatic carrier with the mutation of the X-linked DMD gene. The electromyography, muscle MRI, and muscle biopsy indicated a chronic myogenic injury with dysferlin deletion. As a result of genetic testing, compound heterozygous G-to-T base substitution at position 5,497 in exon 49 of the DYSF gene, leading to a codon change from glutamic acid to termination codon at position 1,833, and a heterozygous C-to-G base change at position 4,638 + 8 in intron 42 of the DYSF gene with a consequence of splice, which has never been reported, were identified as candidate causative mutations. Unfortunately, DMD gene mutation c.3921+12A>G of the DMD gene on the X chromosome was also found in this patient. Finally, the patient was diagnosed as LGMD2B clinically and genetically. In the previous 2 years, the patient's lower limb weakness became slightly worse, resulting in even the total distance walked than before. Fortunately, during the follow-up, her son had not shown slowness or limitation of movement. Genetic testing by next-generation sequencing confirmed the final diagnosis of LGMD2B, and we identified the novel compound heterozygous variants in the DYSF gene, which is of great significance to the accurate diagnosis of genetically coded diseases. Much attention needs to be paid in clinics toward hereditary neuromuscular diseases with multiple pathogenic gene mutations. Genetic counseling and clinical follow-up should be the priorities in future, and promising treatments are also worth exploring.

TECPR1 conjugates LC3 to damaged endomembranes upon detection of sphingomyelin exposure

Invasive bacteria enter the cytosol of host cells through initial uptake into bacteria-containing vacuoles (BCVs) and subsequent rupture of the BCV membrane, thereby exposing to the cytosol intraluminal, otherwise shielded danger signals such as glycans and sphingomyelin. The detection of glycans by galectin-8 triggers anti-bacterial autophagy, but how cells sense and respond to cytosolically exposed sphingomyelin remains unknown. Here, we identify TECPR1 (tectonin beta-propeller repeat containing 1) as a receptor for cytosolically exposed sphingomyelin, which recruits ATG5 into an E3 ligase complex that mediates lipid conjugation of LC3 independently of ATG16L1. TECPR1 binds sphingomyelin through its N-terminal DysF domain (N'DysF), a feature not shared by other mammalian DysF domains. Solving the crystal structure of N'DysF, we identified key residues required for the interaction, including a solvent-exposed tryptophan (W154) essential for binding to sphingomyelin-positive membranes and the conjugation of LC3 to lipids. Specificity of the ATG5/ATG12-E3 ligase responsible for the conjugation of LC3 is therefore conferred by interchangeable receptor subunits, that is, the canonical ATG16L1 and the sphingomyelin-specific TECPR1, in an arrangement reminiscent of certain multi-subunit ubiquitin E3 ligases.

Miyoshi Muscular Dystrophy Type 1 with Mutated DYSF Gene Misdiagnosed as Becker Muscular Dystrophy: A Case Report and Literature Review

Dysferlinopathy covers a spectrum of muscle disorder categorized by two major phenotypes, namely Miyoshi muscular dystrophy type 1 (MMD1, OMIM #254130) and limb-girdle muscular dystrophy autosomal recessive 2 (LGMDR2, OMIM #253601), and two minor symptoms, including asymptomatic hyperCKemia and distal myopathy with anterior tibial onset (DMAT, OMIM #606768). We report the first Korean MMD1 misdiagnosed as Becker muscular dystrophy (BMD), which was caused by a combination of compound heterozygous c.663 + 1G > C and p.Trp992Arg of the DYSF gene. A 70-year-old male previously diagnosed with BMD was admitted for genetic counseling. Since he was clinically suspected to have dysferlinopathy but not BMD, targeted panel sequencing was performed to discover the potential hereditary cause of the suspected muscular dystrophy in the proband. Consequently, two pathogenic single nucleotide variants of the DYSF gene, c.663 + 1G > C (rs398123800) and p.Trp992Arg (rs750028300), associated with dysferlinopathy were identified. These variants were previously reported with variant allele frequencies of 0.000455 (c.663 + 1G > C) and 0.000455 (c.2974T > C; p.Trp992Arg) in the Korean population. This report emphasizes the need for common variant screening in the diagnostic algorithms of certain muscle disorders or gene panels with potential pathogenic effects and high rates of recurrent variants.

Morpholino-Mediated Exons 28-29 Skipping of Dysferlin and Characterization of Multiexon-skipped Dysferlin using RT-PCR, Immunoblotting, and Membrane Wounding Assay

Dysferlinopathies are a group of disabling muscular dystrophies  that includes limb girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy, and distal myopathy with anterior tibial onset (DMAT) as the main phenotypes. They are associated with molecular defects in DYSF, which encodes dysferlin, a key player in sarcolemmal homeostasis. Previous investigations have suggested that exon skipping may be a promising therapy for many patients with dysferlinopathies. It was reported that exons 28-29 of DYSF are dispensable for dysferlin functions. Here, we present a method for multiexon skipping of DYSF exons 28-29 using a cocktail of two phosphorodiamidate morpholino oligomers (PMOs) on cells derived from a dystrophinopathy patient. Also, we describe assays to characterize the multiexon skipped dysferlin at several levels by using one-step RT-PCR, immunoblotting, and a membrane wounding assay.

Distal myopathy

Distal myopathies are a group of genetic, primary muscle diseases. Patients develop progressive weakness and atrophy of the muscles of forearm, hands, lower leg, or feet. Currently, over 20 different forms, presenting a variable age of onset, clinical presentation, disease progression, muscle involvement, and histological findings, are known. Some of them are dominant and some recessive. Different variants in the same gene are often associated with either dominant or recessive forms, although there is a lack of a comprehensive understanding of the genotype-phenotype correlations. This chapter provides a description of the clinicopathologic and genetic aspects of distal myopathies emphasizing known etiologic and pathophysiologic mechanisms.

Genotype-phenotype correlation and natural history study of dysferlinopathy: a single-centre experience from India

Dysferlinopathies are a group of limb-girdle muscular dystrophies causing significant disability in the young population. There is a need for studies on large cohorts to describe the clinical, genotypic and natural history in our subcontinent. To describe and correlate the clinical, genetic profile and natural history of genetically confirmed dysferlinopathies. We analysed a retrospective cohort of patients with dysferlinopathy from a single quaternary care centre in India. A total of 124 patients with dysferlinopathy were included (40 females). Median age at onset and duration of illness were 21 years (range, 13-50) and 48 months (range, 8-288), respectively. The average follow-up period was 60 months (range, 12-288). Fifty-one percent had LGMD pattern of weakness at onset; 23.4% each had Miyoshi and proximo-distal type while isolated hyperCKemia was noted in 1.6%. About 60% were born to consanguineous parents and 26.6% had family history of similar illness. Twenty-three patients (18.6%) lost ambulation at follow-up; the median time to loss of independent ambulation was 120 months (range, 72-264). Single-nucleotide variants (SNVs) constituted 78.2% of patients; INDELs 14.5% and 7.3% had both SNVs and INDELs. Earlier age at onset was noted with SNVs. There was no correlation between the other clinical parameters and ambulatory status with the genotype. Thirty-seven (45.7%) novel pathogenic/likely pathogenic (P/LP) variants were identified out of a total of 81 variations. The c.3191G > A variant was the most recurrent mutation. Our cohort constitutes a clinically and genetically heterogeneous group of dysferlinopathies. There is no significant correlation between the clinico-genetic profile and the ambulatory status.

The clinical, myopathological, and molecular characteristics of 26 Chinese patients with dysferlinopathy: a high proportion of misdiagnosis and novel variants

BACKGROUND

Dysferlinopathy is an autosomal recessive muscular dystrophy caused by pathogenic variants in the dysferlin (DYSF) gene. This disease shows heterogeneous clinical phenotypes and genetic characteristics.

METHODS

We reviewed the clinical and pathological data as well as the molecular characteristics of 26 Chinese patients with dysferlinopathy screened by immunohistochemistry staining and pathogenic variants in DYSF genes.

RESULTS

Among 26 patients with dysferlinopathy, 18 patients (69.2%) presented as Limb-girdle Muscular Dystrophy Type R2 (LGMD R2), 4 (15.4%) had a phenotype of Miyoshi myopathy (MM), and 4 (15.4%) presented as asymptomatic hyperCKemia. Fifteen patients (57.7%) were originally misdiagnosed as inflammatory myopathy or other diseases. Fifteen novel variants were identified among the 40 variant sites identified in this cohort.

CONCLUSION

Dysferlinopathy is a clinically and genetically heterogeneous group of disorders with various phenotypes, a high proportion of novel variants, and a high rate of misdiagnosis before immunohistochemistry staining and genetic analysis.

Dysferlinopathies: clinical and genetic variability

Dysferlinopathies are a clinically heterogeneous group of diseases caused by mutations in the DYSF gene encoding the dysferlin protein. Dysferlin is mostly expressed in muscle tissues and is localized in the sarcolemma, where it performs its main function of resealing and maintaining of the integrity of the cell membrane. At least four forms of dysferlinopathies have been described: Miyoshi myopathy, limb-girdle muscular dystrophy type 2B, distal myopathy with anterior tibial onset, and isolated hyperCKemia. Here we review the clinical features of different forms of dysferlinopathies and attempt to identify genotype-phenotype correlations. Because of the great clinical variability and rarety of the disease and mutations little is known, how different phenotypes develop as a result of different mutations. However missense mutations seem to induce more severe disease than LoF, which is typical for many muscle dystrophies. The role of several specific mutations and possible gene modifiers is also discussed in the paper.

Structural and ultrastructural changes in the skeletal muscles of dysferlin-deficient mice during postnatal ontogenesis

A number of sarcolemma proteins are responsible for muscle fiber repair. Dysferlin encoded by the DYSF gene is one of these proteins. Dysferlin promotes membrane repair in striated muscle fibers (MFs). Mutations in DYSF lead to loss of or decreased dysferlin expression, impaired membrane repair in MF, and its destruction, clinically manifesting as dysferlinopathy. Preclinical studies of cell and gene therapies aimed at restoring impaired muscle regeneration require well-characterized small animal models. Our investigation aimed to distinguish the histopathological features of a mouse strain lacking dysferlin expression (Bla/J strain). Ultrastructural changes in the sarcolemma, mitochondria and contractile apparatus were observed. It was shown that postnatal histogenesis of skeletal muscles in genetically determined dysferlin deficiency is characterized by a higher proportion of necrotic muscle fibers, compensatory hypertrophy of muscle fibers with their subsequent atrophy, and decreases in proliferative activity and the level of myogenic differentiation of myogenic progenitor cells compared to wild-type mice (C57Bl/6).

Genetic Appraisal of Hereditary Muscle Disorders In A Cohort From Mumbai, India

BACKGROUND

Hereditary muscle disorders are clinically and genetically heterogeneous. Limited information is available on their genetic makeup and their prevalence in India.

OBJECTIVE

To study the genetic basis of prevalent hereditary myopathies.

MATERIAL AND METHODS

This is a retrospective study conducted at a tertiary care center. The study was approved by the institutional ethics board. The point of the collection was the genetic database. The genetic data of myopathy patients for the period of two and half years (2019 to mid-2021) was evaluated. Those with genetic diagnoses of DMD, FSHD, myotonic dystrophies, mitochondriopathies, and acquired myopathies were excluded. The main outcome measures were diagnostic yield and the subtype prevalence with their gene variant spectrum.

RESULTS

The definitive diagnostic yield of the study was 39% (cases with two pathogenic variants in the disease-causing gene). The major contributing genes were GNE (15%), DYSF (13%), and CAPN3 (7%). Founder genes were documented in Calpainopathy and GNE myopathy. The uncommon myopathies identified were Laminopathy (0.9%), desminopathy (0.9%), and GMPPB-related myopathy (1.9%). Interestingly, a small number of patients showed pathogenic variants in more than one myopathy gene, the multigenic myopathies.

CONCLUSION

This cohort study gives hospital-based information on the prevalent genotypes of myopathies (GNE, Dysferlinopathy, and calpainopathy), founder mutations, and also newly documents the curious occurrence of multigenicity in a small number of myopathies.

Feeder-supported in vitro exercise model using human satellite cells from patients with sporadic inclusion body myositis

Contractile activity is a fundamental property of skeletal muscles. We describe the establishment of a “feeder-supported in vitro exercise model” using human-origin primary satellite cells, allowing highly-developed contractile myotubes to readily be generated by applying electrical pulse stimulation (EPS). The use of murine fibroblasts as the feeder cells allows biological responses to EPS in contractile human myotubes to be selectively evaluated with species-specific analyses such as RT-PCR. We successfully applied this feeder-supported co-culture system to myotubes derived from primary satellite cells obtained from sporadic inclusion body myositis (sIBM) patients who are incapable of strenuous exercise testing. Our results demonstrated that sIBM myotubes possess essentially normal muscle functions, including contractility development, de novo sarcomere formation, and contraction-dependent myokine upregulation, upon EPS treatment. However, we found that some of sIBM myotubes, but not healthy control myotubes, often exhibit abnormal cytoplasmic TDP-43 accumulation upon EPS-evoked contraction, suggesting potential pathogenic involvement of the contraction-inducible TDP-43 distribution peculiar to sIBM. Thus, our “feeder-supported in vitro exercise model” enables us to obtain contractile human-origin myotubes, potentially utilizable for evaluating exercise-dependent intrinsic and pathogenic properties of patient muscle cells. Our approach, using feeder layers, further expands the usefulness of the “in vitro exercise model”.

Miyoshi Muscular Dystrophy Due to Novel Splice Site Variants in DYSF Gene

Dysferlinopathies are a group of phenotypically heterogeneous disorders caused by pathogenic variants in the DYSF (DYStrophy-associated Fer-1-like) gene encoding dysferlin. The phenotypic spectrum includes Miyoshi muscular dystrophy (MMD), limb-girdle muscular dystrophy type R2, distal myopathy with anterior tibial onset, and isolated hyperCKemia. MMD is characterized by muscle weakness and atrophy predominantly affecting the calf muscles with symptoms onset between 14 and 40 years of age. There is no clear phenotype – genotype correlation for dysferlinopathy. We describe a 15-year-old girl who presented with a phenotype consistent with MMD. However, she was initially treated for presumed polymyositis without improvement. Subsequent genetic testing revealed two novel variants in DYSF: c.3225dup (p.Gly1076Trpfs*38) in exon 30 and c.3349-2A > G (Splice acceptor) in intron 30. No dysferlin was detected in a muscle biopsy using immunostains and western blots, a result consistent with dysferlinopathy that supports the pathogenicity of the DYSF variants.

Pex30-like proteins function as adaptors at distinct ER membrane contact sites

Membrane lipids and proteins synthesized in the ER are used for de novo assembly of organelles, such as lipid droplets and peroxisomes. After assembly, the growth of these organelles is supported by ER-derived lipids transferred at membrane contact sites (MCSs). How ER sites for organelle biogenesis and lipid transfer are established and regulated is unclear. Here, we investigate how the ER membrane protein Pex30 and its family members Pex28, Pex29, Pex31, and Pex32 target and function at multiple MCSs. We show that different Pex30 complexes function at distinct ER domains and MCSs. Pex30 targets ER–peroxisome MCSs when bound to Pex28 and Pex32, organizes the nuclear–vacuolar junction when bound to Pex29, and promotes the biogenesis of lipid droplets independently of other family members. Importantly, the reticulon homology domain (RHD) mediates the assembly of the various Pex30 complexes. Given the role of RHD in membrane shaping, our findings offer a mechanistic link between MCS and regulation of membrane curvature.

Molecular landscape of DYSF mutations in dysferlinopathy: From a Chinese multicenter analysis to a worldwide perspective

Dysferlinopathy is one of the most common subgroup of autosomal recessive limb-girdle muscular dystrophies that is caused by mutations in DYSF gene. However, there is currently no worldwide comprehensive genetic analysis of DYSF variants. Through a national multicenter collaborative effort in China, we identified 222 DYSF variants with 40 novel variants from 245 patients. We then integrated DYSF variants from disease-related genetic databases including LOVD (n = 1020) and Clinvar (n = 1179), to depict the global landscape of disease-related DYSF variants. Normal-population-derived DSYF variants from gnomAD (n = 4318) and ChinaMAP (n = 13,330) were also analyzed in comparison. In Chinese patients, gender instead of genotype showed influence on the onset age of dysferlinopathy, with males showing an earlier age of onset. After integrative analysis, we identified two hotspot DYSF mutations, c.2997G>T in world patients and c.1375dup in Chinese patients, respectively. Both the pathogenic and likely pathogenic variants scattered on the whole gene length of DYSF. However, three specific domains (C2F-C2G-TM, DysF, and C2B-Ferl-C2C) contained variants at higher frequencies than reported in both the databases and Chinese patients. This study comprehensively collected available DYSF variant data, which may pave way for genetic counselling and future clinical trial design for gene therapies in dysferlinopathy.

Deep phenotyping of an international series of patients with late-onset dysferlinopathy

BACKGROUND

To describe the clinical, pathological, and molecular characteristics of late-onset (LO) dysferlinopathy patients.

METHODS

Retrospective series of patients with LO dysferlinopathy, defined by an age at onset of symptoms ≥30 years, from neuromuscular centers in France and the International Clinical Outcome Study for dysferlinopathy (COS). Patients with early-onset (EO) dysferlinopathy (<30 years) were randomly selected from the COS study as a control group, and the North Star Assessment for Dysferlinopathy (NSAD) and Activity Limitation (ACTIVLIM) scores were used to assess functionality. Muscle biopsies obtained from 11 LO and 11 EO patients were revisited.

RESULTS

Forty-eight patients with LO dysferlinopathy were included (28 females). Median age at onset of symptoms was 37 (range 30-57) years and most patients showed a limb-girdle (n = 26) or distal (n = 10) phenotype. However, compared with EO dysferlinopathy patients (n = 48), LO patients more frequently showed atypical phenotypes (7 vs. 1; p = 0.014), including camptocormia, lower creatine kinase levels (2855 vs. 4394 U/L; p = 0.01), and higher NSAD (p = 0.008) and ACTIVLIM scores (p = 0.016). Loss of ambulation in LO patients tended to occur later (23 ± 4.4 years after disease onset vs. 16.3 ± 6.8 years; p = 0.064). Muscle biopsy of LO patients more frequently showed an atypical pattern (unspecific myopathic changes) as well as significantly less necrosis regeneration and inflammation. Although LO patients more frequently showed missense variants (39.8% vs. 23.9%; p = 0.021), no differences in dysferlin protein expression were found on Western blot.

CONCLUSIONS

Late-onset dysferlinopathy patients show a higher frequency of atypical presentations, are less severely affected, and show milder dystrophic changes in muscle biopsy.

The Role of Autophagy in Skeletal Muscle Diseases

Skeletal muscle is the most abundant type of tissue in human body, being involved in diverse activities and maintaining a finely tuned metabolic balance. Autophagy, characterized by the autophagosome–lysosome system with the involvement of evolutionarily conserved autophagy-related genes, is an important catabolic process and plays an essential role in energy generation and consumption, as well as substance turnover processes in skeletal muscles. Autophagy in skeletal muscles is finely tuned under the tight regulation of diverse signaling pathways, and the autophagy pathway has cross-talk with other pathways to form feedback loops under physiological conditions and metabolic stress. Altered autophagy activity characterized by either increased formation of autophagosomes or inhibition of lysosome-autophagosome fusion can lead to pathological cascades, and mutations in autophagy genes and deregulation of autophagy pathways have been identified as one of the major causes for a variety of skeleton muscle disorders. The advancement of multi-omics techniques enables further understanding of the molecular and biochemical mechanisms underlying the role of autophagy in skeletal muscle disorders, which may yield novel therapeutic targets for these disorders.

Null variants in DYSF result in earlier symptom onset

We investigated the clinical, laboratory, and genetic spectra in Korean patients with dysferlinopathy to clarify its genotype-phenotype correlation. We retrospectively reviewed 101 patients from 96 unrelated families with pathogenic variants of DYSF. The most common initial phenotype was Miyoshi myopathy in 50 patients. Median ages at examination and symptom onset were 23 [interquartile range (IQR): 18-30] and 36 years [IQR: 27-48], respectively. We observed 38 variants, including nine novel variants. Four variants (c.2494C > T, c.1284 + 2 T > C, c.663 + 1G > C, and c.2997G > T) in DYSF accounted for 62% of total allele frequencies of pathogenic variants. To analyze the genotype-phenotype correlation, we compared the clinical phenotype between patients with null/null (N/N; n = 55) and null/missense variants (N/M; n = 35). The N/N group had an earlier symptom onset age (median: 20 years [IQR: 17-25]) than the N/M group (median: 29 years [IQR: 23-35], p < .001). Total manual muscle testing scores in lower extremities were lower in the N/N group (median: 80 [IQR: 56-92]) than in the N/M group (median: 89 [IQR: 78-98], p = .013). Our study is the first to report that null variants in DYSF result in an earlier symptom onset than missense variants.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.