Titin founder mutation is a common cause of myofibrillar myopathy with early respiratory failure

TTN-related hereditary myopathy with early respiratory failure presented with elevated hemoglobin initially: A case report and literature review

Background

As common abnormal conditions in clinical practice, hypoxemia and respiratory failure are mainly caused by various respiratory diseases. However, other causes are easily overlooked but deserve more attention from doctors.

Case presentation

A 44-year-old man presented with dyspnea for 10 years. In the early stage, his dyspnea was mild without hypoxemia, and he was misdiagnosed with polycythemia vera due to elevated hemoglobin level. He later developed to respiratory failure but he did not have weakness in his extremities. The positional difference in pulmonary function tests and arterial blood gas analysis led us to identify the respiratory muscle dysfunction. Fatty infiltration of the thigh muscle found by magnetic resonance imaging and muscle biopsies gave us more clues to the causes of diaphragmatic dysfunction. Finally, in combination with his family history and the results of whole exome sequencing, he was diagnosed with hereditary myopathy with early respiratory failure (HMERF, OMIM 603689) caused by a variant in the titin gene (TTN).

Conclusions

We have identified a Chinese family with HMERF due to genetic variants in TTN NM_001256850.1: c.90272C > T, p. Pro30091Leu, located at g.179410829A > G on chromosome 2 (GRCh37), which may be specifically associated with the diagrammatic dysfunction. And hyperhemoglobinemia could serve as a potential sign for the early identification of HMERF.

Walking with giants: The challenges of variant impact assessment in the giant sarcomeric protein titin

Titin, the so-called "third filament" of the sarcomere, represents a difficult challenge for the determination of damaging genetic variants. A single titin molecule extends across half the length of a sarcomere in striated muscle, fulfilling a variety of vital structural and signaling roles, and has been linked to an equally varied range of myopathies, resulting in a significant burden on individuals and healthcare systems alike. While the consequences of truncating variants of titin are well-documented, the ramifications of the missense variants prevalent in the general population are less so. We here present a compendium of titin missense variants-those that result in a single amino-acid substitution in coding regions-reported to be pathogenic and discuss these in light of the nature of titin and the variant position within the sarcomere and their domain, the structural, pathological, and biophysical characteristics that define them, and the methods used for characterization. Finally, we discuss the current knowledge and integration of the multiple fields that have contributed to our understanding of titin-related pathology and offer suggestions as to how these concurrent methodologies may aid the further development in our understanding of titin and hopefully extend to other, less well-studied giant proteins. This article is categorized under: Cardiovascular Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Molecular and Cellular Physiology.

Novel Filamin C Myofibrillar Myopathy Variants Cause Different Pathomechanisms and Alterations in Protein Quality Systems

Myofibrillar myopathies (MFM) are a group of chronic muscle diseases pathophysiologically characterized by accumulation of protein aggregates and structural failure of muscle fibers. A subtype of MFM is caused by heterozygous mutations in the filamin C (FLNC) gene, exhibiting progressive muscle weakness, muscle structural alterations and intracellular protein accumulations. Here, we characterize in depth the pathogenicity of two novel truncating FLNc variants (p.Q1662X and p.Y2704X) and assess their distinct effect on FLNc stability and distribution as well as their impact on protein quality system (PQS) pathways. Both variants cause a slowly progressive myopathy with disease onset in adulthood, chronic myopathic alterations in muscle biopsy including the presence of intracellular protein aggregates. Our analyses revealed that p.Q1662X results in FLNc haploinsufficiency and p.Y2704X in a dominant-negative FLNc accumulation. Moreover, both protein-truncating variants cause different PQS alterations: p.Q1662X leads to an increase in expression of several genes involved in the ubiquitin-proteasome system (UPS) and the chaperone-assisted selective autophagy (CASA) system, whereas p.Y2704X results in increased abundance of proteins involved in UPS activation and autophagic buildup. We conclude that truncating FLNC variants might have different pathogenetic consequences and impair PQS function by diverse mechanisms and to varying extents. Further studies on a larger number of patients are necessary to confirm our observations.

Fetal akinesia deformation sequence syndrome associated with recessive TTN variants

Arthrogryposis multiplex congenita (AMC) [also known as multiple joints contracture or Fetal Akinesia Deformation Sequence (FADS)] is etiologically a heterogeneous condition with an estimated incidence of approximately 1 in 3000 live births and much higher incidence when prenatally diagnosed cases are included. The condition can be acquired or secondary to fetal exposures and can also be caused by a variety of single-gene disorders affecting the brain, spinal cord, peripheral nerves, neuromuscular junction, muscle, and a variety of disorders affecting the connective tissues (Niles et al., Prenatal Diagnosis, 2019; 39:720-731). The introduction of next-generation gene sequencing uncovered many genes and causative variants of AMC but also identified genes that cause both dominant and recessive inherited conditions with the variability of clinical manifestations depending on the genes and variants. Molecular diagnosis in these cases is not only important for prognostication but also for the determination of recurrence risk and for providing reproductive options including preimplantation and prenatal diagnosis. TTN, the largest known gene in the human genome, has been known to be associated with autosomal dominant dilated cardiomyopathy. However, homozygote and compound heterozygote pathogenic variants with recessive inheritance have rarely been reported. We report the effect of recessive variants located within the fetal IC and/or N2BA isoforms in association with severe FADS in three families. All parents were healthy obligate carriers and none of them had cardiac or skeletal muscle abnormalities. This report solidifies FADS as an alternative phenotypic presentation associated with homozygote/compound heterozygous pathogenic variants in the TTN.

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.

Protein Quality Control at the Sarcomere: Titin Protection and Turnover and Implications for Disease Development

Sarcomeres are mainly composed of filament and signaling proteins and are the smallest molecular units of muscle contraction and relaxation. The sarcomere protein titin serves as a molecular spring whose stiffness mediates myofilament extensibility in skeletal and cardiac muscle. Due to the enormous size of titin and its tight integration into the sarcomere, the incorporation and degradation of the titin filament is a highly complex task. The details of the molecular processes involved in titin turnover are not fully understood, but the involvement of different intracellular degradation mechanisms has recently been described. This review summarizes the current state of research with particular emphasis on the relationship between titin and protein quality control. We highlight the involvement of the proteasome, autophagy, heat shock proteins, and proteases in the protection and degradation of titin in heart and skeletal muscle. Because the fine-tuned balance of degradation and protein expression can be disrupted under pathological conditions, the review also provides an overview of previously known perturbations in protein quality control and discusses how these affect sarcomeric proteins, and titin in particular, in various disease states.

A Japanese Patient with Hereditary Myopathy with Early Respiratory Failure Due to the p.P31732L Mutation of Titin

Hereditary myopathy with early respiratory failure (HMERF) is caused by titin A-band mutations in exon 344 and is considered quite rare. Respiratory insufficiency can be the sole symptom in the disease course. We herein report the first Japanese HMERF patient with a p.P31732L mutation in titin. The patient manifested respiratory failure and mild weakness of the neck flexor muscle at 69 years old and showed fatty replacement of the bilateral semitendinosus muscles on muscle imaging. Our case indicates that HMERF with a heterozygous p.P31732L mutation should be included in the differential diagnosis of muscular diseases presenting with early respiratory failure.

Genetic Insights into Primary Restrictive Cardiomyopathy

Restrictive cardiomyopathy is a rare cardiac disease causing severe diastolic dysfunction, ventricular stiffness and dilated atria. In consequence, it induces heart failure often with preserved ejection fraction and is associated with a high mortality. Since it is a poor clinical prognosis, patients with restrictive cardiomyopathy frequently require heart transplantation. Genetic as well as non-genetic factors contribute to restrictive cardiomyopathy and a significant portion of cases are of unknown etiology. However, the genetic forms of restrictive cardiomyopathy and the involved molecular pathomechanisms are only partially understood. In this review, we summarize the current knowledge about primary genetic restrictive cardiomyopathy and describe its genetic landscape, which might be of interest for geneticists as well as for cardiologists.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

In Vitro Fertilization Using Preimplantation Genetic Testing in a Romanian Couple Carrier of Mutations in the TTN Gene: A Case Report and Literature Review

Severe congenital myopathy with fatal cardiomyopathy (EOMFC) is a rare genetic neuromuscular disorder inherited in an autosomal recessive manner. Here we presented a successful pregnancy obtained by in vitro fertilization (IVF) using preimplantation genetic testing (PGT) in one young Romanian carrier couple that already lost mutation(s) within the TNN gene and whose first baby passed away due to multiple complications. It was delivered via emergency C-section at 36 weeks and fully dependent on artificial ventilation for a couple of months, weighing 2200 g and an APGAR score of 3. The aCGH + SNP analysis revealed an abnormal profile of the first newborn; three areas associated with loss of heterozygosity on chromosome 1 (q25.1–q25.3) of 6115 kb, 5 (p15.2–p15.1) of 2589 kb and 8 (q11.21–q11.23) of 4830 kb, a duplication of 1104 kb on chromosome 10 in the position q11.22, and duplication of 1193 kb on chromosome 16 in the position p11.2p11.1. Subsequently, we proceeded to test the parents and showed that both parents are carriers; confirmed by Sanger and NGS sequencing—father—on Chr2(GRCh37):g.179396832_179396833del—TTN variant c.104509_104510del p.(Leu34837Glufs*12)—exon 358 and mother—on Chr2(GRCh37):g.179479653G>C—TTN variant c.48681C>G p.(Tyr16227*)—exon 260. Their first child died shortly after birth due to multiple organ failures, possessing both parent’s mutations; weighing 2200 g at birth and received an APGAR score of 3 following premature delivery via emergency C-section at 36 weeks. Two embryos were obtained following the IVF protocol; one possessed the mother’s mutation, and the other had no mutations and was normal (WT). In contrast with the first birth, the second one was uneventful. A healthy female baby weighing 2990 g was delivered by C-section at 38 weeks, receiving an APGAR score of 9.

Clinical, pathological, and molecular genetic analysis of 7 Chinese patients with hereditary myopathy with early respiratory failure

Hereditary myopathy with early respiratory failure (HMERF) is a subtype of myofibrillar myopathy. Mutations located on exon 344 of the titin-A band, the 119th fibronectin-3 domain (FN₃ 119), are responsible for HMERF. In this article, we retrospectively analyzed the clinical features, findings of muscle imaging, muscle pathology, immunohistochemistry, and ultrastructural characteristics of seven patients diagnosed with HMERF at a single center in China. Muscle MRI showed the involvement of semitendinosus in four patients. The common pathological features were variability in fiber diameter, increased internal nuclei, endomysial fibrosis, and cytoplasmic bodies. On immunohistochemical examination, the cytoplasmic bodies stained positive for calpain-3, p53, and programmed death-ligand 1. Electron microscopy showed cytoplasmic bodies, distorted sarcomere architecture, glycogen pool, and subsarcolemmal accumulation of mitochondria and lysosomes. We retrospectively reviewed four reported HMERF patients in China. Among the 11 patients, the median age at onset was 34 years (range 14-54). Allelic frequency of mutation c.95195C > T was 36.36%. This study characterizes the phenotype and genotype spectrum of HMERF in China.

Whole-exome sequencing identified compound heterozygous variants in the TTN gene causing Salih myopathy with dilated cardiomyopathy in an Iranian family

BACKGROUND

Salih myopathy, characterised by both congenital myopathy and fatal dilated cardiomyopathy, is an inherited muscle disorder that affects skeletal and cardiac muscles. TTN has been identified as the main cause of this myopathy, the enormous size of this gene poses a formidable challenge to molecular genetic diagnostics.

METHOD

In the present study, whole-exome sequencing, cardiac MRI, and metabolic parameter assessment were performed to investigate the genetic causes of Salih myopathy in a consanguineous Iranian family who presented with titinopathy involving both skeletal and heart muscles in an autosomal recessive inheritance pattern.

RESULTS

Two missense variants of TTN gene (NM_001267550.2), namely c.61280A>C (p. Gln20427Pro) and c.54970G>A (p. Gly18324Ser), were detected and segregations were confirmed by polymerase chain reaction-based Sanger sequencing.

CONCLUSIONS

The compound heterozygous variants, c.61280A>C, (p. Gln20427Pro) and c.54970G>A, (p. Gly18324Ser) in the TTN gene appear to be the cause of Salih myopathy and dilated cardiomyopathy in the family presented. Whole-exome sequencing is an effective molecular diagnostic tool to identify the causative genetic variants of large genes such as TTN.

Making sense of missense variants in TTN-related congenital myopathies

Mutations in the sarcomeric protein titin, encoded by TTN, are emerging as a common cause of myopathies. The diagnosis of a TTN-related myopathy is, however, often not straightforward due to clinico-pathological overlap with other myopathies and the prevalence of TTN variants in control populations. Here, we present a combined clinico-pathological, genetic and biophysical approach to the diagnosis of TTN-related myopathies and the pathogenicity ascertainment of TTN missense variants. We identified 30 patients with a primary TTN-related congenital myopathy (CM) and two truncating variants, or one truncating and one missense TTN variant, or homozygous for one TTN missense variant. We found that TTN-related myopathies show considerable overlap with other myopathies but are strongly suggested by a combination of certain clinico-pathological features. Presentation was typically at birth with the clinical course characterized by variable progression of weakness, contractures, scoliosis and respiratory symptoms but sparing of extraocular muscles. Cardiac involvement depended on the variant position. Our biophysical analyses demonstrated that missense mutations associated with CMs are strongly destabilizing and exert their effect when expressed on a truncating background or in homozygosity. We hypothesise that destabilizing TTN missense mutations phenocopy truncating variants and are a key pathogenic feature of recessive titinopathies that might be amenable to therapeutic intervention.

Whole-exome sequencing in patients with protein aggregate myopathies reveals causative mutations associated with novel atypical phenotypes

Background

Myofibrillar myopathies (MFM) are a subgroup of protein aggregate myopathies (PAM) characterized by a common histological picture of myofibrillar dissolution, Z-disk disintegration, and accumulation of degradation products into inclusions. Mutations in genes encoding components of the Z-disk or Z-disk-associated proteins occur in some patients whereas in most of the cases, the causative gene defect is still unknown. We aimed to search for pathogenic mutations in genes not previously associated with MFM phenotype.

Methods

We performed whole-exome sequencing in four patients from three unrelated families who were diagnosed with PAM without aberrations in causative genes for MFM.

Results

In the first patient and her affected daughter, we identified a heterozygous p.(Arg89Cys) missense mutation in LMNA gene which has not been linked with PAM pathology before. In the second patient, a heterozygous p.(Asn4807Phe) mutation in RYR1 not previously described in PAM represents a novel, candidate gene with a possible causative role in the disease. Finally, in the third patient and his symptomatic daughter, we found a previously reported heterozygous p.(Cys30071Arg) mutation in TTN gene that was clinically associated with cardiac involvement.

Conclusions

Our study identifies a new genetic background in PAM pathology and expands the clinical phenotype of known pathogenic mutations.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10072-020-04876-7.

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.

Titinopathy, an atypical respiratory failure

Hereditary myopathy with early respiratory failure is a neuromuscular disease with an autosomal dominant inheritance pattern. Clinical presentation is characterised by proximal and distal muscle weakness, exertional dyspnoea and generalised fatigue. There is no disease-modifying therapy and the prognosis is unknown. Herein we present a case of a 40-year-old woman with long-standing asthenia and apathy and, more recently, daytime sleepiness, dyspnoea and difficulty in walking. A hypercapnic respiratory failure with severe acidemia was identified. The muscle biopsy showed the presence of cytoplasmatic bodies and rimmed vacuoles, suggestive of a hereditary myopathy with early respiratory failure disease. The genetic study confirmed this diagnosis identifying a heterozygous mutation on c.95134T>C (p.Cys31712Arg) in exon 343 in the titin gene. The patient was discharged home under supportive treatment with non-invasive ventilation.

The Axial Alignment of Titin on the Muscle Thick Filament Supports Its Role as a Molecular Ruler

The giant protein titin is expressed in vertebrate striated muscle where it spans half a sarcomere from the Z-disc to the M-band and is essential for muscle organisation, activity and health. The C-terminal portion of titin is closely associated with the thick, myosin-containing filament and exhibits a complex pattern of immunoglobulin and fibronectin domains. This pattern reflects features of the filament organisation suggesting that it acts as a molecular ruler and template, but the exact axial disposition of the molecule has not been determined. Here, we present data that allow us to precisely locate titin domains axially along the thick filament from its tip to the edge of the bare zone. We find that the domains are regularly distributed along the filament at 4-nm intervals and we can determine the domains that associate with features of the filament, such as the 11 stripes of accessory proteins. We confirm that the nine stripes ascribed to myosin binding protein-C are not related to the titin sequence previously assumed; rather, they relate to positions approximately 18 domains further towards the C terminus along titin. This disposition also allows a subgroup of titin domains comprising two or three fibronectin domains to associate with each of the 49 levels of myosin heads in each half filament. The results strongly support the role of titin as a blueprint for the thick filament and the arrangement of the myosin motor domains.

Differential diagnosis of vacuolar myopathies in the NGS era

Altered autophagy accompanied by abnormal autophagic (rimmed) vacuoles detectable by light and electron microscopy is a common denominator of many familial and sporadic non-inflammatory muscle diseases. Even in the era of next generation sequencing (NGS), late-onset vacuolar myopathies remain a diagnostic challenge. We identified 32 adult vacuolar myopathy patients from 30 unrelated families, studied their clinical, histopathological and ultrastructural characteristics and performed genetic testing in index patients and relatives using Sanger sequencing and NGS including whole exome sequencing (WES). We established a molecular genetic diagnosis in 17 patients. Pathogenic mutations were found in genes typically linked to vacuolar myopathy (GNE, LDB3/ZASP, MYOT, DES and GAA), but also in genes not regularly associated with severely altered autophagy (FKRP, DYSF, CAV3, COL6A2, GYG1 and TRIM32) and in the digenic facioscapulohumeral muscular dystrophy 2. Characteristic histopathological features including distinct patterns of myofibrillar disarray and evidence of exocytosis proved to be helpful to distinguish causes of vacuolar myopathies. Biopsy validated the pathogenicity of the novel mutations p.(Phe55*) and p.(Arg216*) in GYG1 and of the p.(Leu156Pro) TRIM32 mutation combined with compound heterozygous deletion of exon 2 of TRIM32 and expanded the phenotype of Ala93Thr-caveolinopathy and of limb-girdle muscular dystrophy 2i caused by FKRP mutation. In 15 patients no causal variants were detected by Sanger sequencing and NGS panel analysis. In 12 of these cases, WES was performed, but did not yield any definite mutation or likely candidate gene. In one of these patients with a family history of muscle weakness, the vacuolar myopathy was eventually linked to chloroquine therapy. Our study illustrates the wide phenotypic and genotypic heterogeneity of vacuolar myopathies and validates the role of histopathology in assessing the pathogenicity of novel mutations detected by NGS. In a sizable portion of vacuolar myopathy cases, it remains to be shown whether the cause is hereditary or degenerative.

[Selective muscular atrophy in a family with hereditary myopathy with early respiratory failure]

Hereditary myopathy with early respiratory failure (HMERF) with heterozygous mutations in the titin gene (TTN) is characterized by respiratory failure developing from the early phase of limb weakness or gait disturbance. Here, we describe a characteristic distribution of muscle involvement in three members of a HMERF family with a TTN mutation. Despite the differences in severity exhibited among the father, daughter and son, the systemic imaging studies showed a similar pattern among these individuals. The semitendinosus and fibularis longus muscles were selectively affected, as described previously. In addition, we found marked atrophy in the sternocleidomastoid and psoas major muscles, regardless of the disease severity. The atrophy in selective trunk muscles observed in routine CT scans can be useful for the differential diagnosis of hereditary myopathies with heart and respiratory failure.

Homozygous missense variant in the TTN gene causing autosomal recessive limb-girdle muscular dystrophy type 10

Background

Limb-girdle muscular dystrophies (LGMDs) are large group of heterogeneous genetic diseases, having a hallmark feature of muscle weakness. Pathogenic mutations in the gene encoding the giant skeletal muscle protein titin (TTN) are associated with several muscle disorders, including cardiomyopathy, recessive congenital myopathies and limb-girdle muscular dystrophy (LGMD) type10. The phenotypic spectrum of titinopathies is expanding, as next generation sequencing (NGS) technology makes screening of this large gene possible.

Aim

This study aimed to identify the pathogenic variant in a consanguineous Pakistani family with autosomal recessive LGMD type 10.

Methods

DNA from peripheral blood samples were obtained, whole exome sequencing (WES) was performed and several molecular and bioinformatics analysis were conducted to identify the pathogenic variant. TTN coding and near coding regions were further amplified using PCR and sequenced via Sanger sequencing.

Results

Whole exome sequencing analysis revealed a novel homozygous missense variant (c.98807G > A; p.Arg32936His) in the TTN gene in the index patients. No heterozygous individuals in the family presented LGMD features. The variant p.Arg32936His leads to a substitution of the arginine amino acid at position 32,936 into histidine possibly causing LGMD type 10.

Conclusion

We identified a homozygous missense variant in TTN, which likely explains LGMD type 10 in this family in line with similar previously reported data. Our study concludes that WES is a successful molecular diagnostic tool to identify pathogenic variants in large genes such as TTN in highly inbred population.

Two novel mutations in TTN of a patient with congenital myopathy: A case report

Background

Early‐onset myopathies show a wide spectrum of phenotypes and are composed of various types of inherited neuromuscular diseases, making it difficult to diagnose. TTN mutation‐related myopathy is a known cause of early‐onset myopathy. Since a next‐generation sequencing (NGS) has enabled sequencing of the vast amount of DNA, TTN, which is the longest coding sequence of any human gene, mutations can now be revealed. We report a 10‐year‐old female with severe congenital muscular weakness and delayed motor development since birth.

Methods

Next‐generation sequencing as well as electromyography and muscle biopsy were performed.

Results

To date, she is incapable of walking alone. Her younger sister had similar but more severe symptoms with respiratory failure. In electromyography, short‐duration, small‐amplitude motor unit action potential, and early recruitment patterns were observed in the involved proximal muscles, suggesting myopathy. Muscle histopathology showed a specific atrophy of increased fiber size variability, frequent nuclear internalization, as well as degeneration and regeneration of fibers with type I fiber predominance, consistent with the findings of a previous report about congenital titinopathy. A NGS study revealed two different possible pathogenic variants in TTN: (a) canonical splicing mutation in the intron 105 (c. 29963‐1G>C) and (b) frameshift and truncating mutation in the exon 339 (c.92812dup/p.Arg30938LysfsTer15). All variants were confirmed by conventional Sanger sequencing.

Conclusion

We propose that unbiased genomic sequencing can be helpful in screening patients with early‐onset myopathy.

Expanding the importance of HMERF titinopathy: new mutations and clinical aspects

Objective

Hereditary myopathy with early respiratory failure (HMERF) is caused by titin A-band mutations in exon 344 and considered quite rare. Respiratory insufficiency is an early symptom. A collection of families and patients with muscle disease suggestive of HMERF was clinically and genetically studied.

Methods

Altogether 12 new families with 19 affected patients and diverse nationalities were studied. Most of the patients were investigated using targeted next-generation sequencing; Sanger sequencing was applied in some of the patients and available family members. Histological data and muscle MRI findings were evaluated.

Results

Three families had several family members studied while the rest were single patients. Most patients had distal and proximal muscle weakness together with respiratory insufficiency. Five heterozygous TTN A-band mutations were identified of which two were novel. Also with the novel mutations the muscle pathology and imaging findings were compatible with the previous reports of HMERF.

Conclusions

Our collection of 12 new families expands mutational spectrum with two new mutations identified. HMERF is not that rare and can be found worldwide, but maybe underdiagnosed. Diagnostic process seems to be complex as this study shows with mostly single patients without clear dominant family history.

GNE Myopathy With Novel Mutations and Pronounced Paraspinal Muscle Atrophy

GNE myopathy is characterized by distal muscle weakness, and caused by recessive mutations in GNE. Its onset is characteristically in young adulthood, although a broad spectrum of onset age is known to exist. A large number of mutations in GNE are pathogenic and this clinical phenotype can be difficult to differentiate clinically from other late-onset myopathies. We describe two families with novel mutations in GNE, and describe their clinical and MRI features. We also describe the presence of striking paraspinal muscle involvement on MRI of the lumbar spine, which is an under-recognized feature of GNE myopathy.

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.

Increasing Role of Titin Mutations in Neuromuscular Disorders

The TTN gene with 363 coding exons encodes titin, a giant muscle protein spanning from the Z-disk to the M-band within the sarcomere. Mutations in the TTN gene have been associated with different genetic disorders, including hypertrophic and dilated cardiomyopathy and several skeletal muscle diseases.

Before the introduction of next generation sequencing (NGS) methods, the molecular analysis of TTN has been laborious, expensive and not widely used, resulting in a limited number of mutations identified. Recent studies however, based on the use of NGS strategies, give evidence of an increasing number of rare and unique TTN variants. The interpretation of these rare variants of uncertain significance (VOUS) represents a challenge for clinicians and researchers.

The main aim of this review is to describe the wide spectrum of muscle diseases caused by TTN mutations so far determined, summarizing the molecular findings as well as the clinical data, and to highlight the importance of joint efforts to respond to the challenges arising from the use of NGS. An international collaboration through a clinical and research consortium and the development of a single accessible database listing variants in the TTN gene, identified by high throughput approaches, may be the key to a better assessment of titinopathies and to systematic genotype– phenotype correlation studies.

Targeted Re-Sequencing Emulsion PCR Panel for Myopathies: Results in 94 Cases

BACKGROUND

Molecular diagnostics in the genetic myopathies often requires testing of the largest and most complex transcript units in the human genome (DMD, TTN, NEB). Iteratively targeting single genes for sequencing has traditionally entailed high costs and long turnaround times. Exome sequencing has begun to supplant single targeted genes, but there are concerns regarding coverage and needed depth of the very large and complex genes that frequently cause myopathies.

OBJECTIVE

To evaluate efficiency of next-generation sequencing technologies to provide molecular diagnostics for patients with previously undiagnosed myopathies.

METHODS

We tested a targeted re-sequencing approach, using a 45 gene emulsion PCR myopathy panel, with subsequent sequencing on the Illumina platform in 94 undiagnosed patients. We compared the targeted re-sequencing approach to exome sequencing for 10 of these patients studied.

RESULTS

We detected likely pathogenic mutations in 33 out of 94 patients with a molecular diagnostic rate of approximately 35%. The remaining patients showed variants of unknown significance (35/94 patients) or no mutations detected in the 45 genes tested (26/94 patients). Mutation detection rates for targeted re-sequencing vs. whole exome were similar in both methods; however exome sequencing showed better distribution of reads and fewer exon dropouts.

CONCLUSIONS

Given that costs of highly parallel re-sequencing and whole exome sequencing are similar, and that exome sequencing now takes considerably less laboratory processing time than targeted re-sequencing, we recommend exome sequencing as the standard approach for molecular diagnostics of myopathies.

Dysregulated autophagy in restrictive cardiomyopathy due to Pro209Leu mutation in BAG3

Myofibrillary myopathies (MFM) are hereditary myopathies histologically characterized by degeneration of myofibrils and aggregation of proteins in striated muscle. Cardiomyopathy is common in MFM but the pathophysiological mechanisms are not well understood. The BAG3-Pro209Leu mutation is associated with early onset MFM and severe restrictive cardiomyopathy (RCM), often necessitating heart transplantation during childhood. We report on a young male patient with a BAG3-Pro209Leu mutation who underwent heart transplantation at eight years of age. Detailed morphological analyses of the explanted heart tissue showed intracytoplasmic inclusions, aggregation of BAG3 and desmin, disintegration of myofibers and Z-disk alterations. The presence of undegraded autophagosomes, seen by electron microscopy, as well as increased levels of p62, LC3-I and WIPI1, detected by immunohistochemistry and western blot analyses, indicated a dysregulation of autophagy. Parkin and PINK1, proteins involved in mitophagy, were slightly increased whereas mitochondrial OXPHOS activities were not altered. These findings indicate that altered autophagy plays a role in the pathogenesis and rapid progression of RCM in MFM caused by the BAG3-Pro209Leu mutation, which could have implications for future therapeutic strategies.

Untangling the complexity of limb-girdle muscular dystrophies

The limb-girdle muscular dystrophies (LGMDs) are a group of genetically heterogeneous, autosomal inherited muscular dystrophies with a childhood to adult onset, manifesting with hip- and shoulder-girdle muscle weakness. When the term LGMD was first conceptualized in 1954, it was thought to be a single entity. Currently, there are 8 autosomal dominant (LGMD1A-1H) and 26 autosomal recessive (LGMD2A-2Z) variants according to the Online Mendelian Inheritance in Man database. In addition, there are other genetically identified muscular dystrophies with an LGMD phenotype not yet classified as LGMD. This highlights the entanglement of LGMDs, which represents an area in continuous expansion. Herein we aim to simplify the complexity of LGMDs by subgrouping them on the basis of the underlying defective protein and impaired function. Muscle Nerve 58: 167-177, 2018.

Role of titin in cardiomyopathy: from DNA variants to patient stratification

Dilated cardiomyopathy (DCM) affects approximately 1 in 250 individuals and is the leading indication for heart transplantation. DCM is often familial, and the most common genetic predisposition is a truncating variation in the giant sarcomeric protein, titin, which occurs in up to 15% of ambulant patients with DCM and 25% of end-stage or familial cases. In this article, we review the evidence for the role of titin truncation in the pathogenesis of DCM and our understanding of the molecular mechanisms and pathophysiological consequences of variation in the gene encoding titin (TTN). Such variation is common in the general population (up to 1% of individuals), and we consider key features that discriminate variants with disease-causing potential from those that are benign. We summarize strategies for clinical interpretation of genetic variants for use in the diagnosis of patients and the evaluation of their relatives. Finally, we consider the contemporary and potential future role for genetic stratification in cardiomyopathy and in the general population, evaluating titin variation as a predictor of outcome and treatment response for precision medicine.

Exome sequences versus sequential gene testing in the UK highly specialised Service for Limb Girdle Muscular Dystrophy

BACKGROUND

Limb girdle muscular dystrophies are a group of rare and genetically heterogeneous diseases that share proximal weakness as a common feature; however they are often lacking very specific phenotypic features to allow an accurate differential diagnosis based on the clinical signs only, limiting the diagnostic rate using phenotype driven genetic testing. Next generation sequencing provides an opportunity to obtain molecular diagnoses for undiagnosed patients, as well as identifying novel genetic causes of muscle diseases. We performed whole exome sequencing (WES) on 104 affected individuals from 75 families in who standard gene by gene testing had not yielded a diagnosis. For comparison we also evaluated the diagnostic rate using sequential gene by gene testing for 91 affected individuals from 84 families over a 2 year period.

RESULTS

Patients selected for WES had undergone more extensive prior testing than those undergoing standard genetic testing and on average had had 8 genes screened already. In this extensively investigated cohort WES identified the genetic diagnosis in 28 families (28/75, 37%), including the identification of the novel gene ZAK and two unpublished genes. WES of a single affected individual with sporadic disease yielded a diagnosis in 13/38 (34%) of cases. In comparison, conventional gene by gene testing provided a genetic diagnosis in 28/84 (33%) families. Titinopathies and collagen VI related dystrophy were the most frequent diagnoses made by WES. Reasons why mutations in known genes were not identified previously included atypical phenotypes, reassignment of pathogenicity of variants, and in one individual mosaicism for a COL6A1 mutation which was undetected by prior direct sequencing.

CONCLUSION

WES was able to overcome many limitations of standard testing and achieved a higher rate of diagnosis than standard testing even in this cohort of extensively investigated patients. Earlier application of WES is therefore likely to yield an even higher diagnostic rate. We obtained a high diagnosis rate in simplex cases and therefore such individuals should be included in exome or genome sequencing projects. Disease due to somatic mosaicism may be increasingly recognised due to the increased sensitivity of next generation sequencing techniques to detect low level mosaicism.

A 'second truncation' in TTN causes early onset recessive muscular dystrophy

Mutations in the gene encoding the giant skeletal muscle protein titin are associated with a variety of muscle disorders, including recessive congenital myopathies ±cardiomyopathy, limb girdle muscular dystrophy (LGMD) and late onset dominant distal myopathy. Heterozygous truncating mutations have also been linked to dilated cardiomyopathy. The phenotypic spectrum of titinopathies is emerging and expanding, as next generation sequencing techniques make this large gene amenable to sequencing. We undertook whole exome sequencing in four individuals with LGMD. An essential splice site mutation, previously reported in dilated cardiomyopathy, was identified in all families in combination with a second truncating mutation. Affected individuals presented with childhood onset proximal weakness associated with joint contractures and elevated CK. Cardiac dysfunction was present in two individuals. Muscle biopsy showed increased internal nuclei and immunoblotting identified reduction or absence of calpain-3 and demonstrated a marked reduction of C-terminal titin fragments. We confirm the co-occurrence of cardiac and skeletal myopathies associated with recessive truncating titin mutations. Compound heterozygosity of a truncating mutation previously associated with dilated cardiomyopathy and a 'second truncation' in TTN was identified as causative in our skeletal myopathy patients. These findings add to the complexity of interpretation and genetic counselling for titin mutations.

Genetic epidemiology of titin-truncating variants in the etiology of dilated cardiomyopathy

Heart failure (HF) is a complex clinical syndrome defined by the inability of the heart to pump enough blood to meet the body's metabolic demands. Major causes of HF are cardiomyopathies (diseases of the myocardium associated with mechanical and/or electrical dysfunction), among which the most common form is dilated cardiomyopathy (DCM). DCM is defined by ventricular chamber enlargement and systolic dysfunction with normal left ventricular wall thickness, which leads to progressive HF. Over 60 genes are linked to the etiology of DCM. Titin (TTN) is the largest known protein in biology, spanning half the cardiac sarcomere and, as such, is a basic structural and functional unit of striated muscles. It is essential for heart development as well as mechanical and regulatory functions of the sarcomere. Next-generation sequencing (NGS) in clinical DCM cohorts implicated truncating variants in titin (TTNtv) as major disease alleles, accounting for more than 25% of familial DCM cases, but these variants have also been identified in 2-3% of the general population, where these TTNtv blur diagnostic and clinical utility. Taking into account the published TTNtv and their association to DCM, it becomes clear that TTNtv harm the heart with position-dependent occurrence, being more harmful when present in the A-band TTN, presumably with dominant negative/gain-of-function mechanisms. However, these insights are challenged by the depiction of position-independent toxicity of TTNtv acting via haploinsufficient alleles, which are sufficient to induce cardiac pathology upon stress. In the current review, we provide an overview of TTN and discuss studies investigating various TTN mutations. We also present an overview of different mechanisms postulated or experimentally validated in the pathogenicity of TTNtv. DCM-causing genes are also discussed with respect to non-truncating mutations in the etiology of DCM. One way of understanding pathogenic variants is probably to understand the context in which they may or may not affect protein-protein interactions, changes in cell signaling, and substrate specificity. In this regard, we also provide a brief overview of TTN interactions in situ. Quantitative models in the risk assessment of TTNtv are also discussed. In summary, we highlight the importance of gene-environment interactions in the etiology of DCM and further mechanistic studies used to delineate the pathways which could be targeted in the management of DCM.

Hereditary myopathies with early respiratory insufficiency in adults

INTRODUCTION

Hereditary myopathies with early respiratory insufficiency as a predominant feature of the clinical phenotype are uncommon and underestimated in adults.

METHODS

We reviewed the clinical and laboratory data of patients with hereditary myopathies who demonstrated early respiratory insufficiency before the need for ambulatory assistance. Only patients with disease-causing mutations or a specific histopathological diagnosis were included. Patients with cardiomyopathy were excluded.

RESULTS

We identified 22 patients; half had isolated respiratory symptoms at onset. The diagnosis of the myopathy was often delayed, resulting in delayed ventilatory support. The most common myopathies were adult-onset Pompe disease, myofibrillar myopathy, multi-minicore disease, and myotonic dystrophy type 1. Single cases of laminopathy, MELAS (mitochondrial encephalomyopathy with lactic acidosis and strokelike events), centronuclear myopathy, and cytoplasmic body myopathy were identified.

CONCLUSION

We highlighted the most common hereditary myopathies associated with early respiratory insufficiency as the predominant clinical feature, and underscored the importance of a timely diagnosis for patient care. Muscle Nerve 56: 881-886, 2017.

Respiratory management of patients with neuromuscular disease: current perspectives

Neuromuscular ventilatory weakness can be difficult to recognize because the symptoms can be nocturnal, nonspecific, or attributed to other conditions. The presence of respiratory muscle weakness suggests a number of possible heterogeneous conditions, including neurodegenerative, autoimmune, and genetic neuromuscular diseases. In some conditions, disease-modifying management exists, but in the absence of such intervention, supportive respiratory therapy can improve quality of life and survival. In this review, we discuss the differential diagnosis and diagnostic approach to chronic neuromuscular respiratory weakness. We also review the clinical assessment and management of respiratory failure in these conditions.

Cardiac involvement in hereditary myopathy with early respiratory failure: A cohort study

Objective:

To assess whether hereditary myopathy with early respiratory failure (HMERF) due to the c.951434T>C; (p.Cys31712Arg) TTN missense mutation also includes a cardiac phenotype.

Method:

Clinical cohort study of our HMERF cohort using ECG, 2D echocardiogram, and cross-sectional cardiac imaging with MRI or CT.

Results:

We studied 22 participants with the c.951434T>C; (p.Cys31712Arg) TTN missense mutation. Three were deceased. Cardiac conduction abnormalities were identified in 7/22 (32%): sustained atrioventricular tachycardia (n = 2), atrial fibrillation (n = 2), nonsustained atrial tachycardia (n = 1), premature supraventricular complexes (n = 1), and unexplained sinus bradycardia (n = 1). In addition, 4/22 (18%) had imaging evidence of otherwise unexplained cardiomyopathy. These findings are supported by histopathologic correlation suggestive of myocardial cytoskeletal remodeling.

Conclusions:

Coexisting cardiac and skeletal muscle involvement is not uncommon in patients with HMERF arising due to the c.951434T>C; (p.Cys31712Arg) TTN mutation. All patients with pathogenic or putative pathogenic TTN mutations should be offered periodic cardiac surveillance.

New era in genetics of early-onset muscle disease: Breakthroughs and challenges

Early-onset muscle disease includes three major entities that present generally at or before birth: congenital myopathies, congenital muscular dystrophies and congenital myasthenic syndromes. Almost exclusively there is weakness and hypotonia, although cases manifesting hypertonia are increasingly being recognised. These diseases display a wide phenotypic and genetic heterogeneity, with the uptake of next generation sequencing resulting in an unparalleled extension of the phenotype-genotype correlations and "diagnosis by sequencing" due to unbiased sequencing. Perhaps now more than ever, detailed clinical evaluations are necessary to guide the genetic diagnosis; with arrival at a molecular diagnosis frequently occurring following dialogue between the molecular geneticist, the referring clinician and the pathologist. There is an ever-increasing blurring of the boundaries between the congenital myopathies, dystrophies and myasthenic syndromes. In addition, many novel disease genes have been described and new insights have been gained into skeletal muscle development and function. Despite the advances made, a significant percentage of patients remain without a molecular diagnosis, suggesting that there are many more human disease genes and mechanisms to identify. It is now technically- and clinically-feasible to perform next generation sequencing for severe diseases on a population-wide scale, such that preconception-carrier screening can occur. Newborn screening for selected early-onset muscle diseases is also technically and ethically-achievable, with benefits to the patient and family from early management of these diseases and should also be implemented. The need for world-wide Reference Centres to meticulously curate polymorphisms and mutations within a particular gene is becoming increasingly apparent, particularly for interpretation of variants in the large genes which cause early-onset myopathies: NEB, RYR1 and TTN. Functional validation of candidate disease variants is crucial for accurate interpretation of next generation sequencing and appropriate genetic counseling. Many published "pathogenic" variants are too frequent in control populations and are thus likely rare polymorphisms. Mechanisms need to be put in place to systematically update the classification of variants such that accurate interpretation of variants occurs. In this review, we highlight the recent advances made and the challenges ahead for the molecular diagnosis of early-onset muscle diseases.

Exome sequencing identifies variants in two genes encoding the LIM-proteins NRAP and FHL1 in an Italian patient with BAG3 myofibrillar myopathy

Myofibrillar myopathies (MFMs) are genetically heterogeneous dystrophies characterized by the disintegration of Z-disks and myofibrils and are associated with mutations in genes encoding Z-disk or Z-disk-related proteins. The c.626 C > T (p.P209L) mutation in the BAG3 gene has been described as causative of a subtype of MFM. We report a sporadic case of a 26-year-old Italian woman, affected by MFM with axonal neuropathy, cardiomyopathy, rigid spine, who carries the c.626 C > T mutation in the BAG3 gene. The patient and her non-consanguineous healthy parents and brother were studied with whole exome sequencing (WES) to further investigate the genetic basis of this complex phenotype. In the patient, we found that the BAG3 mutation is associated with variants in the NRAP and FHL1 genes that encode muscle-specific, LIM domain containing proteins. Quantitative real time PCR, immunohistochemistry and Western blot analysis of the patient’s muscular biopsy showed the absence of NRAP expression and FHL1 accumulation in aggregates in the affected skeletal muscle tissue. Molecular dynamic analysis of the mutated FHL1 domain showed a modification in its surface charge, which could affect its capability to bind its target proteins. To our knowledge this is the first study reporting, in a BAG3 MFM, the simultaneous presence of genetic variants in the BAG3 and FHL1 genes (previously described as independently associated with MFMs) and linking the NRAP gene to MFM for the first time.

FLNC myofibrillar myopathy results from impaired autophagy and protein insufficiency

Myofibrillar myopathy is a progressive muscle disease characterized by the disintegration of muscle fibers and formation of protein aggregates. Causative mutations have been identified in nine genes encoding Z-disk proteins, including the actin binding protein filamin C (FLNC). To investigate the mechanism of disease in FLNC^W2710X myopathy we overexpressed fluorescently tagged FLNC or FLNC^W2710X in zebrafish. Expression of FLNC^W2710X causes formation of protein aggregates but surprisingly, our studies reveal that the mutant protein localizes correctly to the Z-disk and is capable of rescuing the fiber disintegration phenotype that results from FLNC knockdown. This demonstrates that the functions necessary for muscle integrity are not impaired, and suggests that it is the formation of protein aggregates and subsequent sequestration of FLNC away from the Z-disk that results in myofibrillar disintegration. Similar to those found in patients, the aggregates in FLNC^W2710X expressing fish contain the co-chaperone BAG3. FLNC is a target of the BAG3-mediated chaperone assisted selective autophagy (CASA) pathway and therefore we investigated its role, and the role of autophagy in general, in clearing protein aggregates. We reveal that despite BAG3 recruitment to the aggregates they are not degraded via CASA. Additionally, recruitment of BAG3 is sufficient to block alternative autophagy pathways which would otherwise clear the aggregates. This blockage can be relieved by reducing BAG3 levels or by stimulating autophagy. This study therefore identifies both BAG3 reduction and autophagy promotion as potential therapies for FLNC^W2710X myofibrillar myopathy, and identifies protein insufficiency due to sequestration, compounded by impaired autophagy, as the cause.

Broad-based molecular autopsy: a potential tool to investigate the involvement of subtle cardiac conditions in sudden unexpected death in infancy and early childhood

OBJECTIVES

Sudden unexplained death in children is a tragic and traumatic event, often worsened when the cause of death cannot be determined. This work aimed to investigate the presence of putative pathogenic genetic variants in a broad spectrum of cardiomyopathy, channelopathy and aortic disease associated genes that may have increased these children's vulnerability to sudden cardiac death.

DESIGN

We performed molecular autopsy of 41 cases of sudden unexplained death in infants and children through massive parallel sequencing of up to 86 sudden cardiac death-related genes. Multiple in silico analyses were conducted together with a thorough review of the literature in order to prioritise the putative pathogenic variants.

RESULTS

A total of 63 variants in 35 cases were validated. The largest proportion of these variants is located within cardiomyopathy genes although this would have been more expected of channelopathy gene variants. Subtle microscopic features of heart tissue may indicate the presence of an early onset cardiomyopathy as a predisposing condition to sudden unexpected death in some individuals.

CONCLUSIONS

Next-generation sequencing technologies reveal the existence of a wide spectrum of rare and novel genetic variants in sarcomere genes, compared with that of cardiac ion channels, in sudden unexplained death in infants and children. Our findings encourage further investigation of the role of early onset inherited cardiomyopathies and other diseases involving myocardial dysfunction in these deaths. Early detection of variants in these individuals could help to unmask subtle forms of disease within their relatives, who would eventually benefit from better counselling about their genetic history.

Phosphorylation of NBR1 by GSK3 modulates protein aggregation

The autophagy receptor NBR1 (neighbor of BRCA1 gene 1) binds UB/ubiquitin and the autophagosome-conjugated MAP1LC3/LC3 (microtubule-associated protein 1 light chain 3) proteins, thereby ensuring ubiquitinated protein degradation. Numerous neurodegenerative and neuromuscular diseases are associated with inappropriate aggregation of ubiquitinated proteins and GSK3 (glycogen synthase kinase 3) activity is involved in several of these proteinopathies. Here we show that NBR1 is a substrate of GSK3. NBR1 phosphorylation by GSK3 at Thr586 prevents the aggregation of ubiquitinated proteins and their selective autophagic degradation. Indeed, NBR1 phosphorylation decreases protein aggregation induced by puromycin or by the DES/desmin N342D mutant found in desminopathy patients and stabilizes ubiquitinated proteins. Importantly, decrease of protein aggregates is due to an inhibition of their formation and not to their autophagic degradation as confirmed by data on Atg7 knockout mice. The relevance of NBR1 phosphorylation in human pathology was investigated. Analysis of muscle biopsies of sporadic inclusion body myositis (sIBM) patients revealed a strong decrease of NBR1 phosphorylation in muscles of sIBM patients that directly correlated with the severity of protein aggregation. We propose that phosphorylation of NBR1 by GSK3 modulates the formation of protein aggregates and that this regulation mechanism is defective in a human muscle proteinopathy.

Necklace cytoplasmic bodies in hereditary myopathy with early respiratory failure

BACKGROUND

In hereditary myopathy with early respiratory failure (HMERF), cytoplasmic bodies (CBs) are often localised in subsarcolemmal regions, with necklace-like alignment (necklace CBs), in muscle fibres although their sensitivity and specificity are unknown.

OBJECTIVE

To elucidate the diagnostic value of the necklace CBs in the pathological diagnosis of HMERF among myofibrillar myopathies (MFMs).

METHODS

We sequenced the exon 343 of TTN gene (based on ENST00000589042), which encodes the fibronectin-3 (FN3) 119 domain of the A-band and is a mutational hot spot for HMERF, in genomic DNA from 187 patients from 175 unrelated families who were pathologically diagnosed as MFM. We assessed the sensitivity and specificity of the necklace CBs for HMERF by re-evaluating the muscle pathology of our patients with MFM.

RESULTS

TTN mutations were identified in 17 patients from 14 families, whose phenotypes were consistent with HMERF. Among them, 14 patients had necklace CBs. In contrast, none of other patients with MFM had necklace CBs except for one patient with reducing body myopathy. The sensitivity and specificity were 82% and 99%, respectively. Positive predictive value was 93% in the MFM cohort.

CONCLUSIONS

The necklace CB is a useful diagnostic marker for HMERF. When muscle pathology shows necklace CBs, sequencing the FN3 119 domain of A-band in TTN should be considered.

Anesthetic considerations in myofibrillar myopathy

Myofibrillar myopathy (MFM) is a relatively newly recognized genetic disease that leads to progressive muscle deterioration. MFM has a varied phenotypic presentation and impacts cardiac, skeletal, and smooth muscles. Affected individuals are at increased risk of respiratory failure, significant cardiac conduction abnormalities, cardiomyopathy, and sudden cardiac death. In addition, significant skeletal muscle involvement is common, which may lead to contractures, respiratory insufficiency, and airway compromise as the disease progresses. This study is the first report of anesthetic management of a patient with MFM. We report multiple anesthetic encounters of a child with genetically confirmed BAG3-myopathy, a subtype of MFM with severe childhood disease onset. A review of the anesthetic implications of the disease is provided, with specific exploration of possible susceptibility to malignant hyperthermia, rhabdomyolysis, and sensitivity to other anesthetic agents.