Exome sequencing identifies a novel TTN mutation in a family with hereditary 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.

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.

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.

A novel deletion in the C-terminal region of HSPB8 in a family with rimmed vacuolar myopathy

Heat shock protein family B member 8, encoded by HSPB8, is an essential component of the chaperone-assisted selective autophagy complex, which maintains muscle function by degrading damaged proteins in the cells. Mutations in HSPB8 have been reported to cause Charcot-Marie-Tooth type 2L, distal hereditary motor neuropathy IIa, and rimmed vacuolar myopathies (RVM). In this study, we identified a novel heterozygous frameshift variant c.525_529del in HSPB8 in a large Japanese family with RVM, using whole exome sequencing. Three affected individuals had severe respiratory failure, which has not been addressed by previous studies. Muscle atrophy in the paraspinal muscles was also a clinical feature of the individuals affected with RVM in this study. The frameshift mutation was located in the last coding exon, and the mutated protein was predicted to harbor an isoleucine-leucine-valine (ILV) sequence, which corresponds to the IXI/V (isoleucine, X amino acids, and isoleucine or valine) motif. The IXI/V motif is essential for assembly into larger oligomers in other small heat shock proteins and all frameshift mutants of HSPB8 were predicted to share the ILV sequence in the C-terminal extension. The in silico prediction tools showed low protein solubility and increased aggregation propensity for the region around the ILV sequence. The IXI/V motif might be associated with the pathogenesis of HSPB8-related RVM.

Signs and Symptoms in Congenital Myopathies

Congenital myopathies (CM) represent a continuously growing group of disorders with a wide range of clinical and histopathologic presentations. The refinement and application of new technologies for genetic diagnosis have broadened our understanding of the genetic causes of CM. Our growing knowledge has revealed that there are no clear limits between each subgroup of CM, and thus the clinical overlap between genes has become more evident. The implementation of next generation sequencing has produced vast amounts of genomic data that may be difficult to interpret. With an increasing number of reports revealing variants of unknown significance, it is essential to support the genetic diagnosis with a well characterized clinical description of the patient. Phenotype-genotype correlation should be a priority at the moment of disclosing the genetic results. Thus, a detailed physical examination can provide us with subtle differences that are not only key in order to arrive at a correct diagnosis, but also in the characterization of new myopathies and candidate genes.

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.

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.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

[Titin-related muscle disorders: an expanding spectrum]

Titin-related diseases of the skeletal and cardiac muscles open a new, fruitful chapter of myology. Confined for a long time to a limited number of clinical entities, the phenotypic spectrum of titinopthies is nowadays expanding rapidly together with the discovery of many pathogenic mutations of the TTN gene. Like for many genes of large size, the fine tuning and use of high-throughput sequencing (NGS) constitutes a little revolution in the field. This powerful tool allows, although with real technical hurdles, the establishment of the definite diagnosis of titinopathy. A better knowledge of the natural history of each subtype of titinopathy enables as of now an optimized management of patients, notably when a cardiac or respiratory risk factor is identified. Research efforts in the titin-related conditions are gradually getting organized. Interactions between clinicians and geneticists are an absolute necessity. The still fragmentary knowledge of the pathogenesis of each titinopathy prevents to date to figure out any curative therapy in the very near future.

Lumbosacral stenosis in Labrador retriever military working dogs - an exomic exploratory study

Background

Canine lumbosacral stenosis is defined as narrowing of the caudal lumbar and/or sacral vertebral canal. A risk factor for neurologic problems in many large sized breeds, lumbosacral stenosis can also cause early retirement in Labrador retriever military working dogs. Though vital for conservative management of the condition, early detection is complicated by the ambiguous nature of clinical signs of lumbosacral stenosis in stoic and high-drive Labrador retriever military working dogs. Though clinical diagnoses of lumbosacral stenosis using CT imaging are standard, they are usually not performed unless dogs present with clinical symptoms. Understanding the underlying genomic mechanisms would be beneficial in developing early detection methods for lumbosacral stenosis, which could prevent premature retirement in working dogs. The exomes of 8 young Labrador retriever military working dogs (4 affected and 4 unaffected by lumbosacral stenosis, phenotypically selected by CT image analyses from 40 dogs with no reported clinical signs of the condition) were sequenced to identify and annotate exonic variants between dogs negative and positive for lumbosacral stenosis.

Results

Two-hundred and fifty-two variants were detected to be homozygous for the wild allele and either homozygous or heterozygous for the variant allele. Seventeen non-disruptive variants were detected that could affect protein effectiveness in 7 annotated (SCN1B, RGS9BP, ASXL3, TTR, LRRC16B, PTPRO, ZBBX) and 3 predicted genes (EEF1A1, DNAJA1, ZFX). No exonic variants were detected in any of the canine orthologues for human lumbar spinal stenosis candidate genes.

Conclusions

TTR (transthyretin) gene could be a possible candidate for lumbosacral stenosis in Labrador retrievers based on previous human studies that have reported an association between human lumbar spinal stenosis and transthyretin protein amyloidosis. Other genes identified with exonic variants in this study but with no known published association with lumbosacral stenosis and/or lumbar spinal stenosis could also be candidate genes for future canine lumbosacral stenosis studies but their roles remain currently unknown. Human lumbar spinal stenosis candidate genes also cannot be ruled out as lumbosacral stenosis candidate genes. More definitive genetic investigations of this condition are needed before any genetic test for lumbosacral stenosis in Labrador retriever can be developed.

Electronic supplementary material

The online version of this article (10.1186/s40575-017-0052-6) contains supplementary material, which is available to authorized users.

Performance comparison of two commercial human whole-exome capture systems on formalin-fixed paraffin-embedded lung adenocarcinoma samples

Background

Next Generation Sequencing (NGS) has become a valuable tool for molecular landscape characterization of cancer genomes, leading to a better understanding of tumor onset and progression, and opening new avenues in translational oncology. Formalin-fixed paraffin-embedded (FFPE) tissue is the method of choice for storage of clinical samples, however low quality of FFPE genomic DNA (gDNA) can limit its use for downstream applications.

Methods

To investigate the FFPE specimen suitability for NGS analysis and to establish the performance of two solution-based exome capture technologies, we compared the whole-exome sequencing (WES) data of gDNA extracted from 5 fresh frozen (FF) and 5 matched FFPE lung adenocarcinoma tissues using: SeqCap EZ Human Exome v.3.0 (Roche NimbleGen) and SureSelect XT Human All Exon v.5 (Agilent Technologies).

Results

Sequencing metrics on Illumina HiSeq were optimal for both exome systems and comparable among FFPE and FF samples, with a slight increase of PCR duplicates in FFPE, mainly in Roche NimbleGen libraries. Comparison of single nucleotide variants (SNVs) between FFPE-FF pairs reached overlapping values >90 % in both systems. Both WES showed high concordance with target re-sequencing data by Ion PGM™ in 22 lung-cancer genes, regardless the source of samples. Exon coverage of 623 cancer-related genes revealed high coverage efficiency of both kits, proposing WES as a valid alternative to target re-sequencing.

Conclusions

High-quality and reliable data can be successfully obtained from WES of FFPE samples starting from a relatively low amount of input gDNA, suggesting the inclusion of NGS-based tests into clinical contest. In conclusion, our analysis suggests that the WES approach could be extended to a translational research context as well as to the clinic (e.g. to study rare malignancies), where the simultaneous analysis of the whole coding region of the genome may help in the detection of cancer-linked variants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2720-4) contains supplementary material, which is available to authorized users.

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.

GNE myopathy associated with congenital thrombocytopenia: a report of two siblings

GNE myopathy is an autosomal recessive muscular disorder caused by mutations in the gene encoding the key enzyme in sialic acid biosynthesis, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE/MNK). Here, we report two siblings with myopathy with rimmed vacuoles and congenital thrombocytopenia who harbored two compound heterozygous GNE mutations, p.V603L and p.G739S. Thrombocytopenia, which is characterized by shortened platelet lifetime rather than ineffective thrombopoiesis, has been observed since infancy. We performed exome sequencing and array CGH to identify the underlying genetic etiology of thrombocytopenia. No pathogenic variants were detected among the known causative genes of recessively inherited thrombocytopenia; yet, candidate variants in two genes that followed an autosomal recessive mode of inheritance, including previously identified GNE mutations, were detected. Alternatively, it is possible that the decreased activity of GNE/MNK itself, which would lead to decreased sialic content in platelets, is associated with thrombocytopenia in these patients. Further investigations are required to clarify the association between GNE myopathy and the pathogenesis of thrombocytopenia.

Unusual multisystemic involvement and a novel BAG3 mutation revealed by NGS screening in a large cohort of myofibrillar myopathies

Background

Myofibrillar myopathies (MFM) are a group of phenotypically and genetically heterogeneous neuromuscular disorders, which are characterized by protein aggregations in muscle fibres and can be associated with multisystemic involvement.

Methods

We screened a large cohort of 38 index patients with MFM for mutations in the nine thus far known causative genes using Sanger and next generation sequencing (NGS). We studied the clinical and histopathological characteristics in 38 index patients and five additional relatives (n = 43) and particularly focused on the associated multisystemic symptoms.

Results

We identified 14 heterozygous mutations (diagnostic yield of 37%), among them the novel p.Pro209Gln mutation in the BAG3 gene, which was associated with onset in adulthood, a mild phenotype and an axonal sensorimotor polyneuropathy, in the absence of giant axons at the nerve biopsy. We revealed several novel clinical phenotypes and unusual multisystemic presentations with previously described mutations: hearing impairment with a FLNC mutation, dysphonia with a mutation in DES and the first patient with a FLNC mutation presenting respiratory insufficiency as the initial symptom. Moreover, we described for the first time respiratory insufficiency occurring in a patient with the p.Gly154Ser mutation in CRYAB. Interestingly, we detected a polyneuropathy in 28% of the MFM patients, including a BAG3 and a MYOT case, and hearing impairment in 13%, including one patient with a FLNC mutation and two with mutations in the DES gene. In four index patients with a mutation in one of the MFM genes, typical histological findings were only identified at the ultrastructural level (29%).

Conclusions

We conclude that extraskeletal symptoms frequently occur in MFM, particularly cardiac and respiratory involvement, polyneuropathy and/or deafness. BAG3 mutations should be considered even in cases with a mild phenotype or an adult onset. We identified a genetic defect in one of the known genes in less than half of the MFM patients, indicating that more causative genes are still to be found. Next generation sequencing techniques should be helpful in achieving this aim.

A rising titan: TTN review and mutation update

The 364 exon TTN gene encodes titin (TTN), the largest known protein, which plays key structural, developmental, mechanical, and regulatory roles in cardiac and skeletal muscles. Prior to next-generation sequencing (NGS), routine analysis of the whole TTN gene was impossible due to its giant size and complexity. Thus, only a few TTN mutations had been reported and the general incidence and spectrum of titinopathies was significantly underestimated. In the last months, due to the widespread use of NGS, TTN is emerging as a major gene in human-inherited disease. So far, 127 TTN disease-causing mutations have been reported in patients with at least 10 different conditions, including isolated cardiomyopathies, purely skeletal muscle phenotypes, or infantile diseases affecting both types of striated muscles. However, the identification of TTN variants in virtually every individual from control populations, as well as the multiplicity of TTN isoforms and reference sequences used, stress the difficulties in assessing the relevance, inheritance, and correlation with the phenotype of TTN sequence changes. In this review, we provide the first comprehensive update of the TTN mutations reported and discuss their distribution, molecular mechanisms, associated phenotypes, transmission pattern, and phenotype-genotype correlations, alongside with their implications for basic research and for human health.

Performance comparison of four exome capture systems for deep sequencing

BACKGROUND

Recent developments in deep (next-generation) sequencing technologies are significantly impacting medical research. The global analysis of protein coding regions in genomes of interest by whole exome sequencing is a widely used application. Many technologies for exome capture are commercially available; here we compare the performance of four of them: NimbleGen's SeqCap EZ v3.0, Agilent's SureSelect v4.0, Illumina's TruSeq Exome, and Illumina's Nextera Exome, all applied to the same human tumor DNA sample.

RESULTS

Each capture technology was evaluated for its coverage of different exome databases, target coverage efficiency, GC bias, sensitivity in single nucleotide variant detection, sensitivity in small indel detection, and technical reproducibility. In general, all technologies performed well; however, our data demonstrated small, but consistent differences between the four capture technologies. Illumina technologies cover more bases in coding and untranslated regions. Furthermore, whereas most of the technologies provide reduced coverage in regions with low or high GC content, the Nextera technology tends to bias towards target regions with high GC content.

CONCLUSIONS

We show key differences in performance between the four technologies. Our data should help researchers who are planning exome sequencing to select appropriate exome capture technology for their particular application.

Myofibrillar myopathies: new developments

PURPOSE OF REVIEW

Myofibrillar myopathies (MFMs) are a heterogeneous group of skeletal and cardiac muscle diseases. In this review, we highlight recent discoveries of new genes and disease mechanisms involved in this group of disorders.

RECENT FINDINGS

The advent of next-generation sequencing technology, laser microdissection and mass spectrometry-based proteomics has facilitated the discovery of new MFM causative genes and pathomechanisms. New mutations have also been discovered in 'older' genes, helping to find a classification niche for MFM-linked disorders showing variant phenotypes. Cell transfection experiments using primary cultured myoblasts and newer animal models provide insights into the pathogenesis of MFMs.

SUMMARY

An increasing number of genes are involved in the causation of variant subtypes of MFM. The application of modern technologies in combination with classical histopathological and ultrastructural studies is helping to establish the molecular diagnosis and reach a better understanding of the pathogenic mechanisms of each MFM subtype, thus putting an emphasis on the development of specific means for prevention and therapy of these incapacitating and frequently fatal diseases.

Hereditary myopathy with early respiratory failure: occurrence in various populations

OBJECTIVE

Several families with characteristic features of hereditary myopathy with early respiratory failure (HMERF) have remained without genetic cause. This international study was initiated to clarify epidemiology and the genetic underlying cause in these families, and to characterise the phenotype in our large cohort.

METHODS

DNA samples of all currently known families with HMERF without molecular genetic cause were obtained from 12 families in seven different countries. Clinical, histopathological and muscle imaging data were collected and five biopsy samples made available for further immunohistochemical studies. Genotyping, exome sequencing and Sanger sequencing were used to identify and confirm sequence variations.

RESULTS

All patients with clinical diagnosis of HMERF were genetically solved by five different titin mutations identified. One mutation has been reported while four are novel, all located exclusively in the FN3 119 domain (A150) of A-band titin. One of the new mutations showed semirecessive inheritance pattern with subclinical myopathy in the heterozygous parents. Typical clinical features were respiratory failure at mid-adulthood in an ambulant patient with very variable degree of muscle weakness. Cytoplasmic bodies were retrospectively observed in all muscle biopsy samples and these were reactive for myofibrillar proteins but not for titin.

CONCLUSIONS

We report an extensive collection of families with HMERF with five different mutations in exon 343 of TTN, which establishes this exon as the primary target for molecular diagnosis of HMERF. Our relatively large number of new families and mutations directly implies that HMERF is not extremely rare, not restricted to Northern Europe and should be considered in undetermined myogenic respiratory failure.

Atypical phenotypes in titinopathies explained by second titin mutations

OBJECTIVE

Several patients with previously reported titin gene (TTN) mutations causing tibial muscular dystrophy (TMD) have more complex, severe, or unusual phenotypes. This study aimed to clarify the molecular cause of the variant phenotypes in 8 patients of 7 European families.

METHODS

Clinical, histopathological, and muscle imaging data of patients and family members were reanalyzed. The titin protein was analyzed by Western blotting and TTN gene by reverse transcription polymerase chain reaction (RT-PCR) and Sanger sequencing.

RESULTS

Western blotting showed more pronounced C-terminal titin abnormality than expected for heterozygous probands, suggesting the existence of additional TTN mutations. RT-PCR indicated unequal mRNA expression of the TTN alleles in biopsies of 6 patients, 3 with an limb-girdle muscular dystrophy type 2J (LGMD2J) phenotype. Novel frameshift mutations were identified in 5 patients. A novel A-band titin mutation, c.92167C>T (p.P30723S), was found in 1 patient, and 1 Portuguese patient with a severe TMD phenotype proved to be homozygous for the previously reported Iberian TMD mutation.

INTERPRETATION

The unequal expression levels of TTN transcripts in 5 probands suggested severely reduced expression of the frameshift mutated allele, probably through nonsense-mediated decay, explaining the more severe phenotypes. The Iberian TMD mutation may cause a more severe TMD rather than LGMD2J when homozygous. The Finnish patient compound heterozygous for the FINmaj TMD mutation and the novel A-band titin missense mutation showed a phenotype completely different from previously described titinopathies. Our results further expand the complexity of muscular dystrophies caused by TTN mutations and suggest that the coexistence of second mutations may constitute a more common general mechanism explaining phenotype variability.

A new disease allele for the p.C30071R mutation in titin causing hereditary myopathy with early respiratory failure

Hereditary myopathy with early respiratory failure is an autosomal dominant myopathy caused by mutations in the 119th fibronectin-3 domain of titin. To date all reported patients with the most common mutation in this domain (p.C30071R) appear to share ancestral disease alleles. We undertook this study of two families with the p.C30071R mutation to determine whether they share the same haplotype as previously reported British families or whether the mutation arose as a de novo event. We sequenced the 119th fibronectin-3 domain in these two probands and flanking polymorphisms associated with the British haplotype in hereditary myopathy with early respiratory failure. A family of Indian descent had a haplotype that was not compatible with the British shared haplotype. Cloning of the 119th fibronectin-3 domain in this patient demonstrated polymorphisms rs191484894 and novel noncoding variant c.90225C>T on the same allele as the mutation, which is distinct from previously reported British families. This proves that the p.C30071R mutation itself (rather than the haplotype containing this mutation) causes hereditary myopathy with early respiratory failure and suggests its independent origin in different ethnic groups.

Recessive TTN truncating mutations define novel forms of core myopathy with heart disease

Core myopathies (CM), the main non-dystrophic myopathies in childhood, remain genetically unexplained in many cases. Heart disease is not considered part of the typical CM spectrum. No congenital heart defect has been reported, and childhood-onset cardiomyopathy has been documented in only two CM families with homozygous mutations of the TTN gene. TTN encodes titin, a giant protein of striated muscles. Recently, heterozygous TTN truncating mutations have also been reported as a major cause of dominant dilated cardiomyopathy. However, relatively few TTN mutations and phenotypes are known, and titin pathophysiological role in cardiac and skeletal muscle conditions is incompletely understood. We analyzed a series of 23 families with congenital CM and primary heart disease using TTN M-line-targeted sequencing followed in selected patients by whole-exome sequencing and functional studies. We identified seven novel homozygous or compound heterozygous TTN mutations (five in the M-line, five truncating) in 17% patients. Heterozygous parents were healthy. Phenotype analysis identified four novel titinopathies, including cardiac septal defects, left ventricular non-compaction, Emery-Dreifuss muscular dystrophy or arthrogryposis. Additionally, in vitro studies documented the first-reported absence of a functional titin kinase domain in humans, leading to a severe antenatal phenotype. We establish that CM are associated with a large range of heart conditions of which TTN mutations are a major cause, thereby expanding the TTN mutational and phenotypic spectrum. Additionally, our results suggest titin kinase implication in cardiac morphogenesis and demonstrate that heterozygous TTN truncating mutations may not manifest unless associated with a second mutation, reassessing the paradigm of their dominant expression.