Atypical phenotypes in titinopathies explained by second titin mutations

MRI for the diagnosis of limb girdle muscular dystrophies

PURPOSE OF REVIEW

In the last 30 years, there have many publications describing the pattern of muscle involvement of different neuromuscular diseases leading to an increase in the information available for diagnosis. A high degree of expertise is needed to remember all the patterns described. Some attempts to use artificial intelligence or analysing muscle MRIs have been developed. We review the main patterns of involvement in limb girdle muscular dystrophies (LGMDs) and summarize the strategies for using artificial intelligence tools in this field.

RECENT FINDINGS

The most frequent LGMDs have a widely described pattern of muscle involvement; however, for those rarer diseases, there is still not too much information available. patients. Most of the articles still include only pelvic and lower limbs muscles, which provide an incomplete picture of the diseases. AI tools have efficiently demonstrated to predict diagnosis of a limited number of disease with high accuracy.

SUMMARY

Muscle MRI continues being a useful tool supporting the diagnosis of patients with LGMD and other neuromuscular diseases. However, the huge variety of patterns described makes their use in clinics a complicated task. Artificial intelligence tools are helping in that regard and there are already some accessible machine learning algorithms that can be used by the global medical community.

Exploring TTN variants as genetic insights into cardiomyopathy pathogenesis and potential emerging clues to molecular mechanisms in cardiomyopathies

The giant protein titin (TTN) is a sarcomeric protein that forms the myofibrillar backbone for the components of the contractile machinery which plays a crucial role in muscle disorders and cardiomyopathies. Diagnosing TTN pathogenic variants has important implications for patient management and genetic counseling. Genetic testing for TTN variants can help identify individuals at risk for developing cardiomyopathies, allowing for early intervention and personalized treatment strategies. Furthermore, identifying TTN variants can inform prognosis and guide therapeutic decisions. Deciphering the intricate genotype-phenotype correlations between TTN variants and their pathologic traits in cardiomyopathies is imperative for gene-based diagnosis, risk assessment, and personalized clinical management. With the increasing use of next-generation sequencing (NGS), a high number of variants in the TTN gene have been detected in patients with cardiomyopathies. However, not all TTN variants detected in cardiomyopathy cohorts can be assumed to be disease-causing. The interpretation of TTN variants remains challenging due to high background population variation. This narrative review aimed to comprehensively summarize current evidence on TTN variants identified in published cardiomyopathy studies and determine which specific variants are likely pathogenic contributors to cardiomyopathy development.

Digenic inheritance involving a muscle-specific protein kinase and the giant titin protein causes a skeletal muscle myopathy

In digenic inheritance, pathogenic variants in two genes must be inherited together to cause disease. Only very few examples of digenic inheritance have been described in the neuromuscular disease field. Here we show that predicted deleterious variants in SRPK3, encoding the X-linked serine/argenine protein kinase 3, lead to a progressive early onset skeletal muscle myopathy only when in combination with heterozygous variants in the TTN gene. The co-occurrence of predicted deleterious SRPK3/TTN variants was not seen among 76,702 healthy male individuals, and statistical modeling strongly supported digenic inheritance as the best-fitting model. Furthermore, double-mutant zebrafish (srpk3-/-; ttn.1+/-) replicated the myopathic phenotype and showed myofibrillar disorganization. Transcriptome data suggest that the interaction of srpk3 and ttn.1 in zebrafish occurs at a post-transcriptional level. We propose that digenic inheritance of deleterious changes impacting both the protein kinase SRPK3 and the giant muscle protein titin causes a skeletal myopathy and might serve as a model for other genetic diseases.

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.

Biallelic truncating TTN variants in M-band encoding exons cause a fetal lethal titinopathy

To report two novel TTN variants associated with fetal recessive titinopathy, thereby broadening the range of TTN variants that can lead to titinopathy. Clinical information on the fetus and parents was gathered, and genomic DNAs were extracted from the fetal tissue and family members' peripheral blood samples. Exome sequencing on fetal DNA was performed and following bioinformatics analysis, the suspected pathogenic variants were confirmed through Sanger sequencing. Prenatal ultrasound performed at 29 weeks of gestation revealed hydrops fetalis, decreased fetal movements, multiple joint contractures and polyhydramnios. Intrauterine fetal death was noted in the third trimester. Exome sequencing revealed compound heterozygous variants in the TTN gene: a paternally inherited allele c.101227C>T (p.Arg33743Ter) and a maternally inherited c.104254C>T (p.Gln34752Ter) allele. These variants have not been previously reported and are evaluated to be likely pathogenic according to the American College of Medical Genetics and Genomics guidelines. We report a fetus with hydrops fetalis and arthrogryposis multiplex congenita associated with a compound heterozygote in the TTN gene. Our report broadens the clinical and genetic spectrum associated with the TTN-related conditions.

Aberrant mRNA processing caused by splicing mutations in TTN-related neuromuscular disorders

Mutations in the TTN gene have been reported to be responsible for a range of neuromuscular disorders, including recessive distal myopathy and congenital myopathy (CM). Only five splicing mutations have been identified to induce aberrant mRNA splicing in TTN-related neuromuscular disorders. In our study, we described detailed clinical characteristics, muscle pathology and genetic analysis of two probands with TTN-related autosomal recessive neuromuscular disorders. Besides, we identified two novel intronic mutations, c.107377+1 G > C in intron 362 and c.19994-2 A > G in intron 68, in the two probands. Through cDNA analysis, we revealed the c.107377+1 G > C mutation induced retention of the entire intron 362, and the c.19994-2 A > G mutation triggered skipping of the first 11 bp of exon 69. Our study broadens the aberrant splicing spectrum of neuromuscular disorders caused by splicing mutations in the TTN gene.

The crucial role of titin in fetal development: recurrent miscarriages and bone, heart and muscle anomalies characterise the severe end of titinopathies spectrum

BACKGROUND

Titin truncating variants (TTNtvs) have been associated with several forms of myopathies and/or cardiomyopathies. In homozygosity or in compound heterozygosity, they cause a wide spectrum of recessive phenotypes with a congenital or childhood onset. Most recessive phenotypes showing a congenital or childhood onset have been described in subjects carrying biallelic TTNtv in specific exons. Often karyotype or chromosomal microarray analyses are the only tests performed when prenatal anomalies are identified. Thereby, many cases caused by TTN defects might be missed in the diagnostic evaluations. In this study, we aimed to dissect the most severe end of the titinopathies spectrum.

METHODS

We performed a retrospective study analysing an international cohort of 93 published and 10 unpublished cases carrying biallelic TTNtv.

RESULTS

We identified recurrent clinical features showing a significant correlation with the genotype, including fetal akinesia (up to 62%), arthrogryposis (up to 85%), facial dysmorphisms (up to 73%), joint (up to 17%), bone (up to 22%) and heart anomalies (up to 27%) resembling complex, syndromic phenotypes.

CONCLUSION

We suggest TTN to be carefully evaluated in any diagnostic process involving patients with these prenatal signs. This step will be essential to improve diagnostic performance, expand our knowledge and optimise prenatal genetic counselling.

The Spectrum of the Heterozygous Effect in Biallelic Mendelian Diseases-The Symptomatic Heterozygote Issue

Heterozygous carriers of pathogenic/likely pathogenic variants in autosomal recessive disorders seem to be asymptomatic. However, in recent years, an increasing number of case reports have suggested that mild and unspecific symptoms can occur in some heterozygotes, as symptomatic heterozygotes have been identified across different disease types, including neurological, neuromuscular, hematological, and pulmonary diseases. The symptoms are usually milder in heterozygotes than in biallelic variants and occur "later in life". The status of symptomatic heterozygotes as separate entities is often disputed, and alternative diagnoses are considered. Indeed, often only a thin line exists between dual, dominant, and recessive modes of inheritance and symptomatic heterozygosity. Interestingly, recent population studies have found global disease effects in heterozygous carriers of some genetic variants. What makes the few heterozygotes symptomatic, while the majority show no symptoms? The molecular basis of this phenomenon is still unknown. Possible explanations include undiscovered deep-splicing variants, genetic and environmental modifiers, digenic/oligogenic inheritance, skewed methylation patterns, and mutational burden. Symptomatic heterozygotes are rarely reported in the literature, mainly because most did not undergo the complete diagnostic procedure, so alternative diagnoses could not be conclusively excluded. However, despite the increasing accessibility to high-throughput technologies, there still seems to be a small group of patients with mild symptoms and just one variant of autosomes in biallelic diseases. Here, we present some examples, the current state of knowledge, and possible explanations for this phenomenon, and thus argue against the existing dominant/recessive classification.

Cardiac MRI as an Imaging Tool in Titin Variant-Related Dilated Cardiomyopathy

Dilated Cardiomyopathy is a common myocardial disease characterized by dilation and loss of function of one or both ventricles. A variety of etiologies have been implicated including genetic variation. Advancement in genetic sequencing, and diagnostic imaging allows for detection of genetic mutations in sarcomere protein titin (TTN) and high resolution assessment of cardiac function. This review article discusses the role of cardiac MRI in diagnosing dilated cardiomyopathy in patients with TTN variant related cardiomyopathy.

Long-term favorable prognosis in late onset dominant distal titinopathy: Tibial muscular dystrophy

BACKGROUND AND PURPOSE

Tibial muscular dystrophy (TMD) is a dominant late onset distal titinopathy. It was first described in Finnish patients 3 decades ago. TMD patients with several other TTN mutations occur in many European populations. In this retrospective study, we were able to obtain longitudinal follow-up data of the disease progression over 15 years in 137 TMD patients.

METHODS

We retrieved clinical data retrospectively from three examinations spanning a period of 15 years. The data were analyzed in R. Frequencies, percentages, and median values were used to describe data. Probability values were determined with the chi-squared test.

RESULTS

In the cohort, the first symptoms were walking difficulties (97.8%) and weakness in distal lower limbs (98.5%). The progression of the weakness in distal lower limbs was moderate, and in the proximal lower limbs and proximal upper limbs it was mild. The distal upper limbs were not affected. Magnetic resonance imaging results indicated fatty degeneration preferentially in lower leg anterior muscles, gluteus minimus, and hamstring muscles. Serum creatine kinase values in the cohort were mostly normal (40.7%) or mildly elevated (53.7%). The data suggest that 50% of patients need walking aids by the age of 88 years.

CONCLUSIONS

Despite individual variability of severity, the overall disability due to walking difficulties and upper limb weakness remained moderate even at very advanced ages, and cardiomyopathy did not develop due to the titin defect alone. The acquired results promote the correct identification of TMD, and the obtained trajectories of disease evolution can be used as natural history data for any therapeutic intervention.

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.

Distal myopathy

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

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.

TTN as a candidate gene for distal arthrogryposis type 10 pathogenesis

Background

Arthrogryposis is a medical term used to describe congenital contractures which often affect multiple limbs. Distal arthrogryposis (DA) is one of the major categories of arthrogryposis that primarily affects the distal parts of the body, i.e., the hands and the legs. Although ten different types and several subtypes of DAs have been described, the genes associated with each of these DAs are yet to be characterized. Distal arthrogryposis type 10 (DA10) is a rare genetic disease, which is distinguished from the other arthrogryposis types by plantar flexion contractures resulting in toe-walking during infancy as well as variability in contractures of the hip, hamstring, elbow, wrist and finger joints with no ocular or neurological abnormalities. Symptoms of DA10 indicate impairment specifically in the musculoskeletal system. DA10 is still poorly studied.

Aim

The objective of this study was to identify the candidate gene for DA10 by scrutinizing the protein-protein interaction (PPI) networks using in silico tools.

Results

Among the genes that reside within the previously reported genomic coordinates (human chromosome assembly 38 or GRCh38 coordinates 2:179,700,000–188,500,000) of the causative agent of DA10, only TTN (the gene that codes for the protein Titin or TTN) follows the expression pattern similar to the other known DA associated genes and its expression is predominant in the skeletal and heart muscles. Titin also participates in biological pathways and processes relevant to arthrogryposes. TTN-related known skeletal muscle disorders follow the autosomal-dominant pattern of inheritance, which is a common characteristic of distal arthrogryposes as well.

Conclusion

Based on the findings of the analyses and their correlation with previous reports, TTN appears to be the candidate gene for DA10. Our attempt to discover a potential candidate gene may eventually lead to an understanding of disease mechanism and possible treatment strategies, as well as demonstrate the suitability of PPI in the search for candidate genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00405-5.

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.

Novel dominant distal titinopathy phenotype associated with copy number variation

The aim of this study was to analyze patients from two distinct families with a novel distal titinopathy phenotype associated with exactly the same CNV in the TTN gene. We used an integrated strategy combining deep phenotyping and complete molecular analyses in patients. The CNV is the most proximal out-of-frame TTN variant reported and leads to aberrant splicing transcripts leading to a frameshift. In this case, the dominant effect would be due to dominant-negative and/or haploinsufficiency. Few CNV in TTN have been reported to date. Our data represent a novel phenotype-genotype association and provides hypotheses for its dominant effects.

The constantly evolving spectrum of phenotypes in titinopathies - will it ever stop?

PURPOSE OF REVIEW

The last few years have confirmed previous assumptions of an enormous impact of the titin gene (TTN) on the occurrence of muscle disease, cardiomyopathy, or both together. The reason for this rather late understanding of its importance is because of the huge size which prevented sequencing of the whole gene by the previous Sanger technique in the individual cases. An update of the advances in diagnosing titinopathies is the main focus of this review.

RECENT FINDINGS

High throughput methods are now widely available for TTN sequencing and a corresponding explosion of different types of identified titinopathies is observed and published in the literature, although final confirmation is lacking in many cases with recessive missense variants.

SUMMARY

The implications of these findings for clinical practice are easy to understand: patients with previously undiagnosed muscle disease can now have a correct diagnosis and subsequently receive a likely prognosis, can have accurate genetic counseling for the whole family and early treatment for predictable complications from the heart and respiratory muscles. In addition not to forget, they can avoid wrong diagnoses leading to wrong treatments.

Genotype phenotype analysis in a family carrying truncating mutations in the titin gene

We report a family carrying a previously described truncating mutation, NM_001267550.2(TTN):c.107889del p.(Lys35963Asnfs*9) in exon 364, and a novel truncating mutation, NM_001267550.1:c.100704C > A p.(Tyr33568*) in exon 358 in the titin gene. The c.107889del mutation, which was maternally transmitted, has been previously described in patients from the Iberian Peninsula. The mother was of Peruvian descent suggesting a potential European ancestral origin of this mutation. In this family, a daughter, who is a compound heterozygote carrying both these mutations, developed a peripartum cardiomyopathy during her second pregnancy. Subsequently, she was diagnosed with a myopathy following electromyography testing and a muscle biopsy which showed fiber type disproportion. Her brother, who carries only the paternally inherited c.100704C > A mutation, developed a cardiomyopathy following a suspected viral illness. Their father, who transmitted this mutation, has no evidence of a cardiomyopathy. We hypothesize that the c.100704C > A mutation confers susceptibility to the development of cardiomyopathy which may be brought on by cardiovascular stress. Our study of this family expands the genotype and phenotype spectrum of disorders that can be associated with mutations in the titin gene.

Is Gene-Size an Issue for the Diagnosis of Skeletal Muscle Disorders?

Human genes have a variable length. Those having a coding sequence of extraordinary length and a high number of exons were almost impossible to sequence using the traditional Sanger-based gene-by-gene approach. High-throughput sequencing has partly overcome the size-related technical issues, enabling a straightforward, rapid and relatively inexpensive analysis of large genes.

Several large genes (e.g. TTN, NEB, RYR1, DMD) are recognized as disease-causing in patients with skeletal muscle diseases. However, because of their sheer size, the clinical interpretation of variants in these genes is probably the most challenging aspect of the high-throughput genetic investigation in the field of skeletal muscle diseases.

The main aim of this review is to discuss the technical and interpretative issues related to the diagnostic investigation of large genes and to reflect upon the current state of the art and the future advancements in the field.

Digenic Variants in the TTN and TRAPPC11 Genes Co-segregating With a Limb-Girdle Muscular Dystrophy in a Han Chinese Family

Limb-girdle muscular dystrophies (LGMD) are hereditary genetic disorders characterized by progressive muscle impairment which predominantly include proximal muscle weaknesses in the pelvic and shoulder girdles. This article describes an attempt to identify genetic cause(s) for a LGMD pedigree via a combination of whole exome sequencing and Sanger sequencing. Digenic variants, the titin gene (TTN) c.19481T>G (p.Leu6494Arg) and the trafficking protein particle complex 11 gene (TRAPPC11) c.3092C>G (p.Pro1031Arg), co-segregated with the disease phenotype in the family, suggesting their possible pathogenicity.

Panorama of the distal myopathies

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

Improved Criteria for the Classification of Titin Variants in Inherited Skeletal Myopathies

BACKGROUND

Extensive genetic screening results in the identification of thousands of rare variants that are difficult to interpret. Because of its sheer size, rare variants in the titin gene (TTN) are detected frequently in any individual. Unambiguous interpretation of molecular findings is almost impossible in many patients with myopathies or cardiomyopathies.

OBJECTIVE

To refine the current classification framework for TTN-associated skeletal muscle disorders and standardize the interpretation of TTN variants.

METHODS

We used the guidelines issued by the American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) to re-analyze TTN genetic findings from our patient cohort.

RESULTS

We identified in the classification guidelines three rules that are not applicable to titin-related skeletal muscle disorders; six rules that require disease-/gene-specific adjustments and four rules requiring quantitative thresholds for a proper use. In three cases, the rule strength need to be modified.

CONCLUSIONS

We suggest adjustments are made to the guidelines. We provide frequency thresholds to facilitate filtering of candidate causative variants and guidance for the use and interpretation of functional data and co-segregation evidence. We expect that the variant classification framework for TTN-related skeletal muscle disorders will be further improved along with a better understanding of these diseases.

The importance of an integrated genotype-phenotype strategy to unravel the molecular bases of titinopathies

Next generation sequencing (NGS) has allowed the titin gene (TTN) to be identified as a major contributor to neuromuscular disorders, with high clinical heterogeneity. The mechanisms underlying the phenotypic variability and the dominant or recessive pattern of inheritance are unclear. Titin is involved in the formation and stability of the sarcomeres. The effects of the different TTN variants can be harmless or pathogenic (recessive or dominant) but the interpretation is tricky because the current bioinformatics tools can not predict their functional impact effectively. Moreover, TTN variants are very frequent in the general population. The combination of deep phenotyping associated with RNA molecular analyses, western blot (WB) and functional studies is often essential for the interpretation of genetic variants in patients suspected of titinopathy. In line with the current guidelines and suggestions, we implemented for patients with skeletal myopathy and with potentially disease causing TTN variant(s) an integrated genotype-transcripts-protein-phenotype approach, associated with phenotype and variants segregation studies in relatives and confrontation with published data on titinopathies to evaluate pathogenic effects of TTN variants (even truncating ones) on titin transcripts, amount, size and functionality. We illustrate this integrated approach in four patients with recessive congenital myopathy.

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.

Genetics of Muscle Stiffness, Muscle Elasticity and Explosive Strength

Muscle stiffness, muscle elasticity and explosive strength are the main components of athletes' performance and they show a sex-based as well as ethnicity variation. Muscle stiffness is thought to be one of the risk factors associated with sports injuries and is less common in females than in males. These observations may be explained by circulating levels of sex hormones and their specific receptors. It has been shown that higher levels of estrogen are associated with lower muscle stiffness responsible for suppression of collagen synthesis. It is thought that these properties, at least in part, depend on genetic factors. Particularly, the gene encoding estrogen receptor 1 (ESR1) is one of the candidates that may be associated with muscle stiffness. Muscle elasticity increases with aging and there is evidence suggesting that titin (encoded by the TTN gene), a protein that is expressed in cardiac and skeletal muscles, is one of the factors responsible for elastic properties of the muscles. Mutations in the TTN gene result in some types of muscular dystrophy or cardiomyopathy. In this context, TTN may be regarded as a promising candidate for studying the elastic properties of muscles in athletes. The physiological background of explosive strength depends not only on the muscle architecture and muscle fiber composition, but also on the central nervous system and functionality of neuromuscular units. These properties are, at least partly, genetically determined. In this context, the ACTN3 gene code for α-actinin 3 has been widely researched.

Novel heterozygous truncating titin variants affecting the A-band are associated with cardiomyopathy and myopathy/muscular dystrophy

BACKGROUND

Variants in TTN are frequently identified in the genetic evaluation of skeletal myopathy or cardiomyopathy. However, due to the high frequency of TTN variants in the general population, incomplete penetrance, and limited understanding of the spectrum of disease, interpretation of TTN variants is often difficult for laboratories and clinicians. Currently, cardiomyopathy is associated with heterozygous A-band TTN variants, whereas skeletal myopathy is largely associated with homozygous or compound heterozygous TTN variants. Recent reports show pathogenic variants in TTN may result in a broader phenotypic spectrum than previously recognized.

METHODS

Here we report the results of a multisite study that characterized the phenotypes of probands with variants in TTN. We investigated TTN genotype-phenotype correlations in probands with skeletal myopathy and/or cardiomyopathy. Probands with TTN truncating variants (TTNtv) or pathogenic missense variants were ascertained from two academic medical centers. Variants were identified via clinical genetic testing and reviewed according to the American College of Medical Genetics criteria. Clinical and family history data were documented via retrospective chart review. Family studies were performed for probands with atypical phenotypes.

RESULTS

Forty-nine probands were identified with TTNtv or pathogenic missense variants. Probands were classified by clinical presentation: cardiac (n = 30), skeletal muscle (n = 12), or both (cardioskeletal, n = 7). Within the cardioskeletal group, 5/7 probands had heterozygous TTNtv predicted to affect the distal (3') end of the A-band. All cardioskeletal probands had onset of proximal-predominant muscle weakness before diagnosis of cardiovascular disease, five pedigrees support dominant transmission.

CONCLUSION

Although heterozygous TTNtv in the A-band is known to cause dilated cardiomyopathy, we present evidence that these variants may in some cases cause a novel, dominant skeletal myopathy with a limb-girdle pattern of weakness. These findings emphasize the importance of multidisciplinary care for patients with A-band TTNtv who may be at risk for multisystem disease.

Genotype-phenotype correlations in recessive titinopathies

PURPOSE

High throughput sequencing analysis has facilitated the rapid analysis of the entire titin (TTN) coding sequence. This has resulted in the identification of a growing number of recessive titinopathy patients. The aim of this study was to (1) characterize the causative genetic variants and clinical features of the largest cohort of recessive titinopathy patients reported to date and (2) to evaluate genotype-phenotype correlations in this cohort.

METHODS

We analyzed clinical and genetic data in a cohort of patients with biallelic pathogenic or likely pathogenic TTN variants. The cohort included both previously reported cases (100 patients from 81 unrelated families) and unreported cases (23 patients from 20 unrelated families).

RESULTS

Overall, 132 causative variants were identified in cohort members. More than half of the cases had hypotonia at birth or muscle weakness and a delayed motor development within the first 12 months of life (congenital myopathy) with causative variants located along the entire gene. The remaining patients had a distal or proximal phenotype and a childhood or later (noncongenital) onset. All noncongenital cases had at least one pathogenic variant in one of the final three TTN exons (362-364).

CONCLUSION

Our findings suggest a novel association between the location of nonsense variants and the clinical severity of the disease.

Targeted Next-Generation Sequencing in a Large Cohort of Genetically Undiagnosed Patients with Neuromuscular Disorders in Spain

The term neuromuscular disorder (NMD) includes many genetic and acquired diseases and differential diagnosis can be challenging. Next-generation sequencing (NGS) is especially useful in this setting given the large number of possible candidate genes, the clinical, pathological, and genetic heterogeneity, the absence of an established genotype-phenotype correlation, and the exceptionally large size of some causative genes such as TTN, NEB and RYR1. We evaluated the diagnostic value of a custom targeted next-generation sequencing gene panel to study the mutational spectrum of a subset of NMD patients in Spain. In an NMD cohort of 207 patients with congenital myopathies, distal myopathies, congenital and adult-onset muscular dystrophies, and congenital myasthenic syndromes, we detected causative mutations in 102 patients (49.3%), involving 42 NMD-related genes. The most common causative genes, TTN and RYR1, accounted for almost 30% of cases. Thirty-two of the 207 patients (15.4%) carried variants of uncertain significance or had an unidentified second mutation to explain the genetic cause of the disease. In the remaining 73 patients (35.3%), no candidate variant was identified. In combination with patients' clinical and myopathological data, the custom gene panel designed in our lab proved to be a powerful tool to diagnose patients with myopathies, muscular dystrophies and congenital myasthenic syndromes. Targeted NGS approaches enable a rapid and cost-effective analysis of NMD- related genes, offering reliable results in a short time and relegating invasive techniques to a second tier.

A new congenital multicore titinopathy associated with fast myosin heavy chain deficiency

Congenital titinopathies are myopathies with variable phenotypes and inheritance modes. Here, we fully characterized, using an integrated approach (deep phenotyping, muscle morphology, mRNA and protein evaluation in muscle biopsies), two siblings with congenital multicore myopathy harboring three TTN variants predicted to affect titin stability and titin-myosin interactions. Muscle biopsies showed multicores, type 1 fiber uniformity and sarcomeric structure disruption with some thick filament loss. Immunohistochemistry and Western blotting revealed a marked reduction of fast myosin heavy chain isoforms. This is the first observation of a titinopathy suggesting that titin defect leads to secondary loss of fast myosin heavy chain isoforms.

Analysis of spinal muscular atrophy-like patients by targeted resequencing

BACKGROUND

Several effective therapies have been developed for spinal muscular atrophy (SMA), but there are multiple diseases that show SMA-like symptoms, necessitating efficient differential genetic diagnostic methods. Advancements in next-generation sequencing (NGS) technology have facilitated the successful diagnosis of many undiagnosed genetic diseases. Here, we applied NGS along with conventional methods for the molecular diagnosis of undiagnosed patients with lower motor neuron (LMN) symptoms who were initially suspected to have SMA.

METHODS

We enrolled 157 patients with LMN symptoms who visited the Institute of Medical Genetics, Tokyo Women's Medical University, between 2005 and 2016. We excluded 86 patients diagnosed with SMA after confirming the causative SMN1 gene deletion or variants. Finally, we examined 12 undiagnosed patients from eight families by targeted resequencing using NGS. Variants were selected on the basis of literature search and databases, and mutations in a gene where loss of function is a known mechanism of disease were considered as pathogenic. Candidate variants were validated by Sanger sequencing.

RESULTS

We detected novel variants for three patients from two families. Patients 1 and 2 (siblings) showed compound heterozygous TTN variants (c.6621delG, p.W2207Cfs*28 and c.23718T>A, p.F7906L), while patient 3 displayed compound heterozygous KIF1A variants of (c.3871C>T, p.R1291C and c.3898G>A, p.V1300M).

CONCLUSIONS

We detected appropriate variants using our approach of obtaining candidate pathogenic variants by targeted resequencing through NGS and narrowed down the variants in light of patient clinical symptoms. We successfully identified novel causative variants in three undiagnosed patients, which indicated the effectiveness of our approach.

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.

Interpreting Genetic Variants in Titin in Patients With Muscle Disorders

Importance

Mutations in the titin gene (TTN) cause a wide spectrum of genetic diseases. The interpretation of the numerous rare variants identified in TTN is a difficult challenge given its large size.

Objective

To identify genetic variants in titin in a cohort of patients with muscle disorders.

Design, Setting, and Participants

In this case series, 9 patients with titinopathy and 4 other patients with possibly disease-causing variants in TTN were identified. Titin mutations were detected through targeted resequencing performed on DNA from 504 patients with muscular dystrophy, congenital myopathy, or other skeletal muscle disorders. Patients were enrolled from 10 clinical centers in April 2012 to December 2013. All of them had not received a diagnosis after undergoing an extensive investigation, including Sanger sequencing of candidate genes. The data analysis was performed between September 2013 and January 2017. Sequencing data were analyzed using an internal custom bioinformatics pipeline.

Main Outcomes and Measures

The identification of novel mutations in the TTN gene and novel patients with titinopathy. We performed an evaluation of putative causative variants in the TTN gene, combining genetic, clinical, and imaging data with messenger RNA and/or protein studies.

Results

Of the 9 novel patients with titinopathy, 5 (55.5%) were men and the mean (SD) age at onset was 25 (15.8) years (range, 0-46 years). Of the 4 other patients (3 men and 1 woman) with possibly disease-causing TTN variants, 2 (50%) had a congenital myopathy and 2 (50%) had a slowly progressive distal myopathy with onset in the second decade. Most of the identified mutations were previously unreported. However, all the variants, even the already described mutations, require careful clinical and molecular evaluation of probands and relatives. Heterozygous truncating variants or unique missense changes are not sufficient to make a diagnosis of titinopathy.

Conclusions and Relevance

The interpretation of TTN variants often requires further analyses, including a comprehensive evaluation of the clinical phenotype (deep phenotyping) as well as messenger RNA and protein studies. We propose a specific workflow for the clinical interpretation of genetic findings in titin.

A novel TTN deletion in a family with skeletal myopathy, facial weakness, and dilated cardiomyopathy

BACKGROUND

Pathogenic variants in TTN (OMIM 188840), encoding the largest human protein, are known to cause dilated cardiomyopathy and several forms of skeletal myopathy. The clinical interpretation of TTN variants is challenging, however, due to the frequency of missense changes, variable testing and reporting practices in commercial laboratories, and incomplete understanding of the spectrum of TTN-related disease.

METHODS

We report a heterozygous TTN deletion segregating in a family with an unusual skeletal myopathy phenotype associated with facial weakness, gait abnormality, and dilated cardiomyopathy.

RESULTS

A novel 16.430 kb heterozygous deletion spanning part of the A- and M-bands of TTN was identified in the proband and his symptomatic son, as well as in an additional son whose symptoms were identified on clinical evaluation. The deletion was found to be de novo in the proband.

CONCLUSION

Pathogenic variants in TTN may be an unrecognized cause of skeletal myopathy phenotypes, particularly when accompanied by dilated cardiomyopathy.

Novel TTN mutations and muscle imaging characteristics in congenital titinopathy

Objective

We present clinical features, muscle imaging findings, and genetic characteristics of five unrelated Chinese patients with congenital titinopathy, emphasizing the diagnostic role of muscle MRI.

Methods

Five patients who recessive titinopathies were recruited. All patients received muscle biopsies. Mutations were detected by panel massively parallel sequencing and confirmed by Sanger sequencing. Western blotting of muscle proteins was performed. Leg muscle MRIs were performed in four patients.

Results

Four patients aged 1–4 years old showed delayed motor development from early infancy, while a 17‐year‐old boy showed only a 1‐year history of exercise intolerance. Physical examination showed proximal weakness in three patients. Muscle biopsies demonstrated multiple myopathological changes, including increased internalized nuclei, multicores, central cores, and dystrophic changes. Genetic sequencing revealed compound heterozygous or homozygous novel TTN mutations, including six frameshift mutations, one nonsense mutation, two missense mutations, one splicing mutation, and one small nonframeshift deletion. Protein analyses revealed significant decrease of full‐length titin in all patients. Thigh muscle MRIs in four patients showed prominent fatty infiltration in the upper portion of semitendinosus and the peripheral portion of gluteus medius, while the sartorius and gracilis were relatively preserved.

Interpretation

These cases provided further evidence that TTN mutations are likely responsible for an increasing proportion of congenital myopathies than currently recognized. The novel mutations reported expand the mutation spectrum of the TTN gene. There is a characteristic pattern of muscle involvement in congenital titinopathy regardless of clinical or pathological phenotype, providing valuable clues for guiding a genetic diagnosis workup.

Titin in muscular dystrophy and cardiomyopathy: Urinary titin as a novel marker

Titin, encoded by the gene TTN, is the largest human protein, and plays central roles in sarcomeric structures and functions in skeletal and cardiac muscles. Mutations of TTN are causally related to specific types of muscular dystrophies and cardiomyopathies. A developed methodology of next generation sequencing has recently led to the identification of novel TTN mutations in such diseases. The clinical significance of titin is now emerging as a target for genetic strategies. Titin-related muscular dystrophies include tibial muscular dystrophy, limb-girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy, hereditary myopathy with early respiratory failure, central core myopathy, centronuclear myopathies, and Salih myopathy. Truncation mutations of TTN have been identified as the most frequent genetic cause of dilated cardiomyopathy. In this review article, we highlight the role of titin and impact of TTN mutations in the pathogenesis of muscular dystrophies and cardiomyopathies. Recently, a novel sensitive sandwich enzyme-linked immunosorbent assay (ELISA) for the detection of the urinary titin N-terminal fragments (U-TN) has been established. We discuss the clinical significance of U-TN in the diagnosis of muscular dystrophies and differential diagnosis of cardiomyopathies, as well as risk stratification in dilated cardiomyopathy.

The unfolding spectrum of inherited distal myopathies

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

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.

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.

TITINdb-a computational tool to assess titin's role as a disease gene

SUMMARY

Large numbers of rare and unique titin missense variants have been discovered in both healthy and disease cohorts, thus the correct classification of variants as pathogenic or non-pathogenic has become imperative. Due to titin's large size (363 coding exons), current web applications are unable to map titin variants to domain structures. Here, we present a web application, TITINdb, which integrates titin structure, variant, sequence and isoform information, along with pre-computed predictions of the impact of non-synonymous single nucleotide variants, to facilitate the correct classification of titin variants.

AVAILABILITY AND IMPLEMENTATION

TITINdb can be freely accessed at http://fraternalilab.kcl.ac.uk/TITINdb.

CONTACT

franca.fraternali@kcl.ac.uk.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

A novel FLNC frameshift and an OBSCN variant in a family with distal muscular dystrophy

A novel FLNC c.5161delG (p.Gly1722ValfsTer61) mutation was identified in two members of a French family affected by distal myopathy and in one healthy relative. This FLNC c.5161delG mutation is one nucleotide away from a previously reported FLNC mutation (c.5160delC) that was identified in patients and in asymptomatic carriers of three Bulgarian families with distal muscular dystrophy, indicating a low penetrance of the FLNC frameshift mutations. Given these similarities, we believe that the two FLNC mutations alone can be causative of distal myopathy without full penetrance. Moreover, comparative analysis of the clinical manifestations indicates that patients of the French family show an earlier onset and a complete segregation of the disease. As a possible explanation of this, the two French patients also carry a OBSCN c.13330C>T (p.Arg4444Trp) mutation. The p.Arg4444Trp variant is localized within the OBSCN Ig59 domain that, together with Ig58, binds to the ZIg9/ZIg10 domains of titin at Z-disks. Structural and functional studies indicate that this OBSCN p.Arg4444Trp mutation decreases titin binding by ~15-fold. On this line, we suggest that the combination of the OBSCN p.Arg4444Trp variant and of the FLNC c.5161delG mutation, can cooperatively affect myofibril stability and increase the penetrance of muscular dystrophy in the French family.

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.

A novel recessive TTN founder variant is a common cause of distal myopathy in the Serbian population

Variants in the TTN gene have been associated with distal myopathies and other distinctive phenotypes involving skeletal and cardiac muscle. Through whole-exome sequencing we identified a novel stop-gain variant (c.107635C>T, p.(Gln35879Ter)) in the TTN gene, coding a part of the M-line of titin, in 14 patients with autosomal recessive distal myopathy and Serbian ancestry. All patients share a common 1 Mb core haplotype associated with c.107635C>T, suggesting a founder variant. In compound heterozygotes, nine other TTN variants were identified: four stop-gain, three frameshift, one missense and one splice donor variant. Patients homozygous for the common variant did not show significant clinical differences to the compound heterozygous patients. The clinical presentation of all patients was an adult onset distal myopathy with predominant lower limb involvement. In addition, most patients had normal to mildly elevated serum creatine kinase levels, myopathic electromyograms, normal cardiologic and respiratory tests and muscle pathology consistent with a dystrophic process. In this study, we describe a distinct phenotype for patients with distal myopathy associated with novel recessive TTN variants including a Serbian founder variant. Our results expand the phenotypic and genetic spectrum of titinopathies and will facilitate the diagnosis of this condition in patients of Serbian origin.