Myopathy With SQSTM1 and TIA1 Variants: Clinical and Pathological Features

Valosin-Containing Protein (VCP): A Review of Its Diverse Molecular Functions and Clinical Phenotypes

In this review we examine the functionally diverse ATPase associated with various cellular activities (AAA-ATPase), valosin-containing protein (VCP/p97), its molecular functions, the mutational landscape of VCP and the phenotypic manifestation of VCP disease. VCP is crucial to a multitude of cellular functions including protein quality control, endoplasmic reticulum-associated degradation (ERAD), autophagy, mitophagy, lysophagy, stress granule formation and clearance, DNA replication and mitosis, DNA damage response including nucleotide excision repair, ATM- and ATR-mediated damage response, homologous repair and non-homologous end joining. VCP variants cause multisystem proteinopathy, and pathology can arise in several tissue types such as skeletal muscle, bone, brain, motor neurons, sensory neurons and possibly cardiac muscle, with the disease course being challenging to predict.

Retrospective clinical and genetic analysis of COL6-RD patients with a long-term follow-up at a single French center

Collagen type VI-related dystrophies (COL6-RD) are rare diseases with a wide phenotypic spectrum ranging from severe Ullrich's congenital muscular dystrophy Ullrich congenital muscular dystrophy to much milder Bethlem myopathy Both dominant and recessive forms of COL6-RD are caused by pathogenic variants in three collagen VI genes (COL6A1, COL6A2 and COL6A3). The prognosis of these diseases is variable and difficult to predict during early disease stages, especially since the genotype-phenotype correlation is not always clear. For this reason, studies with long-term follow-up of patients with genetically confirmed COL6-RD are still needed. In this study, we present phenotypic and genetic data from 25 patients (22 families) diagnosed with COL6-RD and followed at a single French center, in both adult and pediatric neurology departments. We describe three novel pathogenic variants and identify COL6A2:c.1970-9G>A as the most frequent variant in our series (29%). We also observe an accelerated progression of the disease in a subgroup of patients. This large series of rare disease patients provides essential information on phenotypic variability of COL6-RD patients as well as on frequency of pathogenic COL6A gene variants in Southern France, thus contributing to the phenotypic and genetic description of Collagen type VI-related dystrophies.

Distal myopathy due to digenic inheritance of TIA1 and SQSTM1 variants in two unrelated Spanish patients

Welander distal myopathy typically manifests in late adulthood and is caused by the founder TIA1 c.1150G>A (p.Glu384Lys) variant in families of Swedish and Finnish descent. Recently, a similar phenotype has been attributed to the digenic inheritance of TIA1 c.1070A>G (p.Asn357Ser) and SQSTM1 c.1175C>T (p.Pro392Leu) variants. We describe two unrelated Spanish patients presenting with slowly progressive gait disturbance, distal-predominant weakness, and mildly elevated creatine kinase (CK) levels since their 6th decade. Electromyography revealed abnormal spontaneous activity and a myopathic pattern. Muscle magnetic resonance imaging (MRI) showed marked fatty replacement in distal leg muscles. A muscle biopsy, performed on one patient, revealed myopathic changes with rimmed vacuoles. Both patients carried the TIA1 p.Asn357Ser and SQSTM1 p.Pro392Leu variants. Digenic inheritance is supported by evidence from unrelated pedigrees and a plausible biological interaction between both proteins in protein quality control processes. Recent functional studies and additional case descriptions further support this. Clinical suspicion is necessary to seek both variants.

Human herpesvirus 8 ORF57 protein is able to reduce TDP-43 pathology: network analysis identifies interacting pathways

Aggregation of TAR DNA-binding protein 43 kDa (TDP-43) is thought to drive the pathophysiology of amyotrophic lateral sclerosis and some frontotemporal dementias. TDP-43 is normally a nuclear protein that in neurons translocates to the cytoplasm and can form insoluble aggregates upon activation of the integrated stress response (ISR). Viruses evolved to control the ISR. In the case of Herpesvirus 8, the protein ORF57 acts to bind protein kinase R, inhibit phosphorylation of eIF2α and reduce activation of the ISR. We hypothesized that ORF57 might also possess the ability to inhibit aggregation of TDP-43. ORF57 was expressed in the neuronal SH-SY5Y line and its effects on TDP-43 aggregation characterized. We report that ORF57 inhibits TDP-43 aggregation by 55% and elicits a 2.45-fold increase in the rate of dispersion of existing TDP-43 granules. These changes were associated with a 50% decrease in cell death. Proteomic studies were carried out to identify the protein interaction network of ORF57. We observed that ORF57 directly binds to TDP-43 as well as interacts with many components of the ISR, including elements of the proteostasis machinery known to reduce TDP-43 aggregation. We propose that viral proteins designed to inhibit a chronic ISR can be engineered to remove aggregated proteins and dampen a chronic ISR.

Frameshift mutation in SQSTM1 causes proximal myopathy with rimmed vacuoles: A case report

p62/Sequestosome-1 (SQSTM1) is a stress-inducible scaffold protein involved in multiple cellular processes, including apoptosis, inflammation, cell survival, and selective autophagy. SQSTM1 mutations are associated with a spectrum of multisystem proteinopathy, including Paget disease of the bone, amyotrophic lateral sclerosis, frontotemporal dementia, and distal myopathy with rimmed vacuoles (MRV). Herein, we report a new phenotype of SQSTM1-associated proteinopathy, a novel frameshift mutation in SQSTM1 causing proximal MRV. A 44-year-old Chinese patient presented with progressive limb–girdle weakness. She had asymmetric proximal limb weakness and myopathic features on electromyography. The magnetic resonance images showed fatty infiltration into muscles, predominantly in the thighs and medial gastrocnemius, sparing the tibialis anterior. Muscle histopathology revealed abnormal protein deposition, p62/SQSTM1-positive inclusions, and rimmed vacuoles. Next-generation sequencing showed a novel pathogenic SQSTM1 frameshift mutation, c.542_549delACAGCCGC (p. H181Lfs*66). We expanded the pathogenic genotype of SQSTM1 to include a new, related phenotype: proximal MRV. We suggest that SQSTM1 variations should be screened in cases of proximal MRV.

Multisystem proteinopathies (MSPs) and MSP-like disorders: Clinical-pathological-molecular spectrum

OBJECTIVES

Mutations in VCP, HNRNPA2B1, HNRNPA1, and SQSTM1, encoding RNA-binding proteins or proteins in quality-control pathways, cause multisystem proteinopathies (MSP). They share pathological findings of protein aggregation and clinical combinations of inclusion body myopathy (IBM), neurodegeneration [motor neuron disorder (MND)/frontotemporal dementia (FTD)], and Paget disease of bone (PDB). Subsequently, additional genes were linked to similar but not full clinical-pathological spectrum (MSP-like disorders). We aimed to define the phenotypic-genotypic spectrum of MSP and MSP-like disorders at our institution, including long-term follow-up features.

METHODS

We searched the Mayo Clinic database (January 2010-June 2022) to identify patients with mutations in MSP and MSP-like disorders causative genes. Medical records were reviewed.

RESULTS

Thirty-one individuals (27 families) had pathogenic mutations in: VCP (n = 17), SQSTM1 + TIA1 (n = 5), TIA1 (n = 5), MATR3, HNRNPA1, HSPB8, and TFG (n = 1, each). Myopathy occurred in all but 2 VCP-MSP patients with disease onset at age 52 (median). Weakness pattern was limb-girdle in 12/15 VCP-MSP and HSPB8 patient, and distal-predominant in other MSP and MSP-like disorders. Twenty/24 muscle biopsies showed rimmed vacuolar myopathy. MND and FTD occurred in 5 (4 VCP, 1 TFG) and 4 (3 VCP, 1 SQSTM1 + TIA1) patients, respectively. PDB manifested in 4 VCP-MSP. Diastolic dysfunction occurred in 2 VCP-MSP. After 11.5 years (median) from symptom onset, 15 patients ambulated without gait-aids; loss of ambulation (n = 5) and death (n = 3) were recorded only in VCP-MSP.

INTERPRETATION

VCP-MSP was the most common disorder; rimmed vacuolar myopathy was the most frequent manifestation; distal-predominant weakness occurred frequently in non-VCP-MSP; and cardiac involvement was observed only in VCP-MSP.

Clinical-pathological features and muscle imaging findings in 36 Chinese patients with rimmed vacuolar myopathies: case series study and review of literature

Introduction

Rimmed vacuolar myopathies (RVMs) are a group of genetically heterogeneous diseases that share histopathological characteristics on muscle biopsy, including the aberrant accumulation of autophagic vacuoles. However, the presence of non-coding sequences and structural mutations, some of which remain undetectable, confound the identification of pathogenic mutations responsible for RVMs. Therefore, we assessed the clinical profiles and muscle magnetic resonance imaging (MRI) changes in 36 Chinese patients with RVMs, emphasizing the role of muscle MRI in disease identification and differential diagnosis to propose a comprehensive literature-based imaging pattern to facilitate improved diagnostic workup.

Methods

All patients presented with rimmed vacuoles with varying degrees of muscular dystrophic changes and underwent a comprehensive evaluation using clinical, morphological, muscle MRI and molecular genetic analysis. We assessed muscle changes in the Chinese RVMs and provided an overview of the RVMs, focusing on the patterns of muscle involvement on MRI.

Results

A total of 36 patients, including 24 with confirmed distal myopathy and 12 with limb-girdle phenotype, had autophagic vacuoles with RVMs. Hierarchical clustering of patients according to the predominant effect of the distal or proximal lower limbs revealed that most patients with RVMs could be distinguished. GNE myopathy was the most prevalent form of RVMs observed in this study. Moreover, MRI helped identify the causative genes in some diseases (e.g., desminopathy and hereditary myopathy with early respiratory failure) and confirmed the pathogenicity of a novel mutation (e.g., adult-onset proximal rimmed vacuolar titinopathy) detected using next-generation sequencing.

Discussion

Collectively, our findings expand our knowledge of the genetic spectrum of RVMs in China and suggest that muscle imaging should be an integral part of assisting genetic testing and avoiding misdiagnosis in the diagnostic workup of RVM.

Comparison of multifidus degeneration between scoliosis and lumbar disc herniation

Objective

To assess and compare the pathological and radiological outcomes of multifidus degeneration in scoliosis and lumbar disc herniation patients.

Methods

We performed a retrospective review on 24 patients with scoliosis and 26 patients with lumbar disc herniation (LDH) in the Third Hospital of Hebei Medical University from January 2017 to March2021. The patients were divided into scoliosis group and LDH group according to the treatment. The MRI fatty infiltration rate (FIR) of multifidus and strength of back muscle were calculated to evaluate muscle condition. Multifidus biopsy samples were obtained during surgery in the affected side at L4 or L5 segment in LDH group and on the concavity side of apical vertebrae in scoliosis group. The biopsy fatty infiltration degree (FID) and FIR in two groups, the FIR of affected and unaffected side in LDH group, and the FIR of concavity and convexity side in scoliosis group were compared. The correlation between concavity-convexity FIR difference and cobb angle in scoliosis group, back muscle strength and FIR in LDH group, FID and FIR in both groups was calculated respectively.

Results

The FIR was higher in scoliosis group than in LDH group, higher in concavity side than convexity side in scoliosis group (both P < 0.05). The FID was higher in scoliosis group than in LDH group (P < 0.05). No significant difference was found between affected and unaffected side in LDH group (P > 0.05). There was a positive correlation between concavity-convexity FIR difference and cobb angle, FIR and FID (both P < 0.01). There was a negative correlation between back muscle strength and FIR (P < 0.01). The biopsy staining results showed that both two groups were found the existence of rimmed vacuoles, nuclear aggregation, and abnormal enzyme activity, indicating that the scoliosis and LDH may be associated with myogenic diseases.

Conclusion

The scoliosis patients showed more serious fatty infiltration than LDH patients and rare pathological findings were found in both diseases.

Myogenesis defects in a patient-derived iPSC model of hereditary GNE myopathy

Hereditary muscle diseases are disabling disorders lacking effective treatments. UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) myopathy (GNEM) is an autosomal recessive distal myopathy with rimmed vacuoles typically manifesting in late adolescence/early adulthood. GNE encodes the rate-limiting enzyme in sialic acid biosynthesis, which is necessary for the proper function of numerous biological processes. Outside of the causative gene, very little is known about the mechanisms contributing to the development of GNE myopathy. In the present study, we aimed to address this knowledge gap by querying the underlying mechanisms of GNE myopathy using a patient-derived induced pluripotent stem-cell (iPSC) model. Control and patient-specific iPSCs were differentiated down a skeletal muscle lineage, whereby patient-derived GNEM iPSC clones were able to recapitulate key characteristics of the human pathology and further demonstrated defects in myogenic progression. Single-cell RNA sequencing time course studies revealed clear differences between control and GNEM iPSC-derived muscle precursor cells (iMPCs), while pathway studies implicated altered stress and autophagy signaling in GNEM iMPCs. Treatment of GNEM patient-derived iMPCs with an autophagy activator improved myogenic differentiation. In summary, we report an in vitro, iPSC-based model of GNE myopathy and implicate defective myogenesis as a contributing mechanism to the etiology of GNE myopathy.

Multisystem Proteinopathy Due to VCP Mutations: A Review of Clinical Heterogeneity and Genetic Diagnosis

In this work, we review clinical features and genetic diagnosis of diseases caused by mutations in the gene encoding valosin-containing protein (VCP/p97), the functionally diverse AAA-ATPase. VCP is crucial to a multitude of cellular functions including protein quality control, stress granule formation and clearance, and genomic integrity functions, among others. Pathogenic mutations in VCP cause multisystem proteinopathy (VCP-MSP), an autosomal dominant, adult-onset disorder causing dysfunction in several tissue types. It can result in complex neurodegenerative conditions including inclusion body myopathy, frontotemporal dementia, amyotrophic lateral sclerosis, or combinations of these. There is also an association with other neurodegenerative phenotypes such as Alzheimer-type dementia and Parkinsonism. Non-neurological presentations include Paget disease of bone and may also include cardiac dysfunction. We provide a detailed discussion of genotype-phenotype correlations, recommendations for genetic diagnosis, and genetic counselling implications of VCP-MSP.

Candidate gene expression and coding sequence variants in Warmblood horses with myofibrillar myopathy

BACKGROUND

Myofibrillar myopathy (MFM) of unknown aetiology has recently been identified in Warmblood (WB) horses. In humans, 16 genes have been implicated in various MFM-like disorders.

OBJECTIVES

To identify variants in 16 MFM candidate genes and compare allele frequencies of all variants between MFM WB and non-MFM WB and coding variants with moderate or severe predicted effects in MFM WB with publicly available data of other breeds. To compare differential gene expression and muscle fibre contractile force between MFM and non-MFM WB.

STUDY DESIGN

Case-control.

ANIMALS

8 MFM WB, 8 non-MFM WB, 33 other WB, 32 Thoroughbreds, 80 Quarter Horses and 77 horses of other breeds in public databases.

METHODS

Variants were called within transcripts of 16 candidate genes using gluteal muscle mRNA sequences aligned to EquCab3.0 and allele frequencies compared by Fisher's exact test among MFM WB, non-MFM WB and public sequences across breeds. Candidate gene differential expression was determined between MFM and non-MFM WB by fitting a negative binomial generalised log-linear model per gene (false discovery rate <0.05). The maximal isometric force/cross-sectional area generated by isolated membrane-permeabilised muscle fibres was determined.

RESULTS

None of the 426 variants identified in 16 candidate genes were associated with MFM including 26 missense variants. Breed-specific differences existed in allele frequencies. Candidate gene differential expression and muscle fibre-specific force did not differ between MFM WB (143.1 ± 34.7 kPa) and non-MFM WB (140.2 ± 43.7 kPa) (P = .8).

MAIN LIMITATIONS

RNA-seq-only assays transcripts expressed in skeletal muscle. Other possible candidate genes were not evaluated.

CONCLUSIONS

Evidence for association of variants with a disease is essential because coding sequence variants are common in the equine genome. Variants identified in MFM candidate genes, including two coding variants offered as commercial MFM equine genetic tests, did not associate with the WB MFM phenotype.

Pathogenic Mutations and Putative Phenotype-Affecting Variants in Polish Myofibrillar Myopathy Patients

Myofibrillar myopathies (MFM) are heterogeneous hereditary muscle diseases with characteristic myopathological features of Z-disk dissolution and aggregates of its degradation products. The onset and progression of the disease are variable, with an elusive genetic background, and around half of the cases lacking molecular diagnosis. Here, we attempted to establish possible genetic foundations of MFM by performing whole exome sequencing (WES) in eleven unrelated families of 13 patients clinically diagnosed as MFM spectrum. A filtering strategy aimed at identification of variants related to the disease was used and included integrative analysis of WES data and human phenotype ontology (HPO) terms, analysis of muscle-expressed genes, and analysis of the disease-associated interactome. Genetic diagnosis was possible in eight out of eleven cases. Putative causative mutations were found in the DES (two cases), CRYAB, TPM3, and SELENON (four cases) genes, the latter typically presenting with a rigid spine syndrome. Moreover, a variety of additional, possibly phenotype-affecting variants were found. These findings indicate a markedly heterogeneous genetic background of MFM and show the usefulness of next generation sequencing in the identification of disease-associated mutations. Finally, we discuss the emerging concept of variant load as the basis of phenotypic heterogeneity.

Case Report: Calpainopathy Presenting After Bone Marrow Transplantation, With Studies of Donor Genetic Content in Various Tissue Types

We present a patient who had two allogeneic bone marrow transplantations for acute lymphocytic leukemia. She developed slowly progressive limb-girdle weakness in the context of other symptoms of graft-vs.-host disease (GVHD). Her myopathy symptoms had been initially attributed to GVHD, but when she progressed despite immunotherapy, genetic testing was requested. Initial testing was performed on a blood sample, identifying a variant of unknown significance in DMD. Subsequent testing of DNA from the patient's muscle tissue identified two pathogenic variants in CAPN3, with absence of the DMD variant (this latter variant presumed to have been received from the donor). Allele-specific digital droplet qPCR permitted the quantification of the donor variant in various tissues from the patient (whole skin, isolated fibroblasts, whole blood, saliva, buccal cells, urine sediment, and two muscle biopsies taken at a 2 year interval). This report emphasizes that genetic disease should still be considered in the context of presumably acquired disease, and also demonstrates the extent of transdifferentiation of donor cells into other tissues.

Whole-body muscle MRI of patients with MATR3-associated distal myopathy reveals a distinct pattern of muscular involvement and highlights the value of whole-body examination

OBJECTIVE

MATR3-associated distal myopathy is a rare distal myopathy predominantly affecting lower legs as well as wrist- and finger extensors. Whilst most distal myopathies are clinically and genetically well characterized, diagnosis often remains challenging. Pattern-based magnetic resonance imaging (MRI) approaches offer valuable additional information. However, a consistent pattern of muscular affection is missing for most distal myopathies. Thus, the aim of the present study was to establish a disease-specific pattern of muscular involvement in MATR3-associated distal myopathy using whole-body MRI.

METHODS

15 patients (25-79 years of age, 7 female) with MATR3-associated distal myopathy were subjected to whole-body MRI. The grade of fatty involution for individual muscles was determined using Fischer-Grading. Results were compared to established MRI-patterns of other distal myopathies.

RESULTS

There was a predominant affection of the distal lower extremities. Lower legs showed a severe fatty infiltration, prominently affecting gastrocnemius and soleus muscle. In thighs, a preferential involvement of semimembranous and biceps femoris muscle was observed. Severe affection of gluteus minimus muscle as well as axial musculature, mainly affecting the thoracic segments, was seen. A sufficient discrimination to other forms of distal myopathy based solely on MRI-findings of the lower extremities was not possible. However, the inclusion of additional body parts seemed to yield specificity.

INTERPRETATION

Muscle MRI of patients with MATR3-associated distal myopathy revealed a distinct pattern of muscular involvement. The usage of whole-body muscle MRI provided valuable additional findings as compared to regular MRI of the lower extremities to improve distinction from other disease entities.

Congenital myopathies in the adult neuromuscular clinic: Diagnostic challenges and pitfalls

Objective

To investigate the spectrum of undiagnosed congenital myopathies (CMs) in adults presenting to our neuromuscular clinic and to identify the pitfalls responsible for diagnostic delays.

Methods

We conducted a retrospective review of patients diagnosed with CM in adulthood in our neuromuscular clinic between 2008 and 2018. Patients with an established diagnosis of CM before age 18 years were excluded.

Results

We identified 26 patients with adult-onset CM and 18 patients with pediatric-onset CM who were only diagnosed in adulthood. Among patients with adult onset, the median age at onset was 47 years, and the causative genes were RYR1 (11 families), MYH7 (3 families) and ACTA1 (2 families), and SELENON, MYH2, DNM2, and CACNA1S (1 family each). Of 33 patients who underwent muscle biopsy, only 18 demonstrated histologic abnormalities characteristic of CM. Before their diagnosis of CM, 23 patients had received other diagnoses, most commonly non-neurologic disorders. The main causes of diagnostic delays were mildness of the symptoms delaying neurologic evaluation and attribution of the symptoms to coexisting comorbidities, particularly among pediatric-onset patients.

Conclusions

CMs in adulthood represent a diagnostic challenge, as they may lack the clinical and myopathologic features classically associated with CM. Our findings underscore the need for a revision of the terminology and current classification of these disorders.

Colchicine Myopathy: A Case Series Including Muscle MRI and ABCB1 Polymorphism Data

Colchicine is a medication most commonly used in the treatment of gout and familial mediterannean fever. A rare complication of therapy is toxicity causing proximal myopathy and polyneuropathy. Colchicine myopathy has been associated with the coadministration of other medications with colchicine, such as statins or tacrolimus, and is more common in patients with renal impairment. Otherwise, it is unclear which patients are at greatest risk of developing this adverse drug reaction. ABCB1 is important to the metabolism of colchicine, so we speculated that it was possible that colchicine myopathy patients may have a particular genotype that is associated with this side effect. We describe two cases of colchicine myopathy which occurred with co-administration of rosuvastatin. From one case, we present the first published data on muscle MRI in this condition. We additionally present an analysis of four genetic polymorphisms in ABCB1 and transcript levels in muscle tissue, and demonstrate the descriptive finding of reduced ABCB1 transcript levels in the colchicine myopathy patients.

High diagnostic yield and novel variants in very late-onset spasticity

Hereditary spastic paraplegias (HSPs) are a diverse group of genetic conditions with variable severity and onset age. From a neurogenetic clinic, we identified 14 patients with very late-onset HSP, with symptoms starting after the age of 35. In this cohort, sequencing of known genetic causes was performed using clinically available HSP sequencing panels. We identified 4 patients with mutations in SPG7 and 3 patients with SPAST mutations, representing 50% of the cohort and indicating a very high diagnostic yield. In the SPG7 group, we identified novel variants in two patients. We have also identified two novel mutations in the SPAST group. We present sequencing data from cDNA and RT-qPCR to support the pathogenicity of these variants, and provide observations regarding the poor genotype-phenotype correlation in these conditions that should be the subject of future study.

A Heterologous Cell Model for Studying the Role of T-Cell Intracellular Antigen 1 in Welander Distal Myopathy

Welander distal myopathy (WDM) is a muscle dystrophy characterized by adult-onset distal muscle weakness, prevalently impacting the distal long extensors of the hands and feet. WDM is an autosomal dominant disorder caused by a missense mutation (c.1362G>A; p.E384K) in the TIA1 (T-cell intracellular antigen 1) gene, which encodes an RNA-binding protein basically required for the posttranscriptional regulation of RNAs. We have developed a heterologous cell model of WDM to study the molecular and cellular events associated with mutated TIA1 expression. Specifically, we analyzed how this mutation affects three regulatory functions mediated by TIA1: (i) control of alternative SMN2 (survival motor neuron 2) splicing; (ii) formation, assembly, and disassembly of stress granules; and (iii) mitochondrial dynamics and its consequences for mitophagy, autophagy, and apoptosis. Our results show that whereas WDM-associated TIA1 expression had only a mild effect on SMN2 splicing, it led to suboptimal adaptation to environmental stress, with exacerbated stress granule formation that was accompanied by mitochondrial dysfunction and autophagy. Overall, our observations indicate that some aspects of the cell phenotype seen in muscle of patients with WDM can be recapitulated by ectopic expression of WDM-TIA1 in embryonic kidney cells, highlighting the potential of this model to investigate the pathogenesis of this degenerative disease and possible therapeutics.

The unfolding spectrum of inherited distal myopathies

Distal myopathies are a group of rare muscle diseases characterized by distal weakness at onset. Although acquired myopathies can occasionally present with distal weakness, the majority of distal myopathies have a genetic etiology. Their age of onset varies from early-childhood to late-adulthood while the predominant muscle weakness can affect calf, ankle dorsiflexor, or distal upper limb muscles. A spectrum of muscle pathological changes, varying from nonspecific myopathic changes to rimmed vacuoles to myofibrillar pathology to nuclei centralization, have been noted. Likewise, the underlying molecular defect is heterogeneous. In addition, there is emerging evidence that distal myopathies can result from defective proteins encoded by genes causative of neurogenic disorders, be manifestation of multisystem proteinopathies or the result of the altered interplay between different genes. In this review, we provide an overview on the clinical, electrophysiological, pathological, and molecular aspects of distal myopathies, focusing on the most recent developments in the field. Muscle Nerve 59:283-294, 2019.

Myofibrillar myopathy in the genomic context

Myofibrillar myopathy (MFM) is a group of inherited muscular disorders characterized by myofibril dissolution and abnormal accumulation of degradation products. The diagnosis of muscular disorders based on clinical presentation is difficult due to phenotypic heterogeneity and overlapping symptoms. In addition, precise diagnosis does not always explain the disease etiopathology or the highly variable clinical course even among patients diagnosed with the same type of myopathy. The advent of high-throughput next-generation sequencing (NGS) has provided a successful and cost-effective strategy for identification of novel causative genes in myopathies, including MFM. So far, pathogenic mutations associated with MFM phenotype, including atypical MFM-like cases, have been identified in 17 genes: DES, CRYAB, MYOT, ZASP, FLNC, BAG3, FHL1, TTN, DNAJB6, PLEC, LMNA, ACTA1, HSPB8, KY, PYROXD1, and SQSTM + TIA1 (digenic). Most of these genes are also associated with other forms of muscle diseases. In addition, in many MFM patients, numerous genomic variants in muscle-related genes have been identified. The various myopathies and muscular dystrophies seem to form a single disease continuum; therefore, gene identification in one disease impacts the genetic etiology of the others. In this review, we describe the heterogeneity of the MFM genetic background focusing on the role of rare variants, the importance of whole genome sequencing in the identification of novel disease-associated mutations, and the emerging concept of variant load as the basis of the phenotypic heterogeneity.