Increasing Role of Titin Mutations in Neuromuscular Disorders

Genetic Markers of Cardiovascular Disease

Cardiovascular Disorders (CVDs) are the leading cause mortality in developed as well as developing nations, and has now emerged as one of the leading causes of disability and mortality around the globe. According to the World Health Organization, four out of every five patients with cardiovascular disease die from a myocardial infarction each year. Numerous genes have been linked to coronary artery disease, influencing mechanisms such as blood pressure regulation, lipid metabolism, inflammation, and cardiac activity. Genetic variations or mutations in these genes can affect lipid metabolism, blood pressure management, and heart function, increasing the risk of obesity, metabolic disorders, and resulting in the development of cardiovascular disease. Understanding the role of genes and related complications are essential for the identification, management, and prevention of cardiovascular conditions. Performing a genetic test for variations in the gene may help identify people as well as their families who are at a greater risk of heart disease, which enables risk identification and timely intervention. . This article investigates the applications of genetic biomarkers in cardiac disorders such as coronary artery disease, hypertension, arrhythmias, cardiomyopathy, and heart failure, with an emphasis on individual genes and their effects on mutation.

Identification of four TTN variants in three families with fetal akinesia deformation sequence

BACKGROUND

TTN is a complex gene with large genomic size and highly repetitive structure. Pathogenic variants in TTN have been reported to cause a range of skeletal muscle and cardiac disorders. Homozygous or compound heterozygous mutations tend to cause a wide spectrum of phenotypes with congenital or childhood onset. The onset and severity of the features were considered to be correlated with the types and location of the TTN variants.

METHODS

Whole-exome sequencing was performed on three unrelated families presenting with fetal akinesia deformation sequence (FADS), mainly characterized by reduced fetal movements and limb contractures. Sanger sequencing was performed to confirm the variants. RT-PCR analysis was performed.

RESULTS

TTN c.38,876-2 A > C, a meta transcript-only variant, with a second pathogenic or likely pathogenic variant in trans, was observed in five affected fetuses from the three families. Sanger sequencing showed that all the fetal variants were inherited from the parents. RT-PCR analysis showed two kinds of abnormal splicing, including intron 199 extension and skipping of 8 bases.

CONCLUSIONS

Here we report on three unrelated families presenting with FADS caused by four TTN variants. In addition, our study demonstrates that pathogenic meta transcript-only TTN variant can lead to defects which is recognizable prenatally in a recessive manner.

DNMT3B splicing dysregulation mediated by SMCHD1 loss contributes to DUX4 overexpression and FSHD pathogenesis

Structural maintenance of chromosomes flexible hinge domain-containing 1 (SMCHD1) is a noncanonical SMC protein and an epigenetic regulator. Mutations in SMCHD1 cause facioscapulohumeral muscular dystrophy (FSHD), by overexpressing DUX4 in muscle cells. Here, we demonstrate that SMCHD1 is a key regulator of alternative splicing in various cell types. We show how SMCHD1 loss causes splicing alterations of DNMT3B, which can lead to hypomethylation and DUX4 overexpression. Analyzing RNA sequencing data from muscle biopsies of patients with FSHD and Smchd1 knocked out cells, we found mis-splicing of hundreds of genes upon SMCHD1 loss. We conducted a high-throughput screen of splicing factors, revealing the involvement of the splicing factor RBM5 in the mis-splicing of DNMT3B. Subsequent RNA immunoprecipitation experiments confirmed that SMCHD1 is required for RBM5 recruitment. Last, we show that mis-splicing of DNMT3B leads to hypomethylation of the D4Z4 region and to DUX4 overexpression. These results suggest that DNMT3B mis-splicing due to SMCHD1 loss plays a major role in FSHD pathogenesis.

Titin copy number variations associated with dominant inherited phenotypes

BACKGROUND

Titinopathies are caused by mutations in the titin gene (TTN). Titin is the largest known human protein; its gene has the longest coding phase with 364 exons. Titinopathies are very complex neuromuscular pathologies due to the variable age of onset of symptoms, the great diversity of pathological and muscular impairment patterns (cardiac, skeletal muscle or mixed) and both autosomal dominant and recessive modes of transmission. Until now, only few CNVs in TTN have been reported without clear genotype-phenotype associations.

METHODS

Our study includes eight families with dominant titinopathies. We performed next-generation sequencing or comparative genomic hybridisation array analyses and found CNVs in the TTN gene. We characterised these CNVs by RNA sequencing (RNAseq) analyses in six patients' muscles and performed genotype-phenotype inheritance association study by combining the clinical and biological data of these eight families.

RESULTS

Seven deletion-type CNVs in the TTN gene were identified among these families. Genotype and RNAseq results showed that five deletions do not alter the reading frame and one is out-of-reading frame. The main phenotype identified was distal myopathy associated with contractures. The analysis of morphological, clinical and genetic data and imaging let us draw new genotype-phenotype associations of titinopathies.

CONCLUSION

Identifying TTN CNVs will further increase diagnostic sensitivity in these complex neuromuscular pathologies. Our cohort of patients enabled us to identify new deletion-type CNVs in the TTN gene, with unexpected autosomal dominant transmission. This is valuable in establishing new genotype-phenotype associations of titinopathies, mainly distal myopathy in most of the patients.

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.

Gustavson syndrome is caused by an in-frame deletion in RBMX associated with potentially disturbed SH3 domain interactions

RNA binding motif protein X-linked (RBMX) encodes the heterogeneous nuclear ribonucleoprotein G (hnRNP G) that regulates splicing, sister chromatid cohesion and genome stability. RBMX knock down experiments in various model organisms highlight the gene's importance for brain development. Deletion of the RGG/RG motif in hnRNP G has previously been associated with Shashi syndrome, however involvement of other hnRNP G domains in intellectual disability remain unknown. In the current study, we present the underlying genetic and molecular cause of Gustavson syndrome. Gustavson syndrome was first reported in 1993 in a large Swedish five-generation family presented with profound X-linked intellectual disability and an early death. Extensive genomic analyses of the family revealed hemizygosity for a novel in-frame deletion in RBMX in affected individuals (NM_002139.4; c.484_486del, p.(Pro162del)). Carrier females were asymptomatic and presented with skewed X-chromosome inactivation, indicating silencing of the pathogenic allele. Affected individuals presented minor phenotypic overlap with Shashi syndrome, indicating a different disease-causing mechanism. Investigation of the variant effect in a neuronal cell line (SH-SY5Y) revealed differentially expressed genes enriched for transcription factors involved in RNA polymerase II transcription. Prediction tools and a fluorescence polarization assay imply a novel SH3-binding motif of hnRNP G, and potentially a reduced affinity to SH3 domains caused by the deletion. In conclusion, we present a novel in-frame deletion in RBMX segregating with Gustavson syndrome, leading to disturbed RNA polymerase II transcription, and potentially reduced SH3 binding. The results indicate that disruption of different protein domains affects the severity of RBMX-associated intellectual disabilities.

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.

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.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

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.

Clinical and functional characterization of a long survivor congenital titinopathy patient with a novel metatranscript-only titin variant

Congenital titinopathies are an emerging group of a potentially severe form of congenital myopathies caused by biallelic mutations in titin, encoding the largest existing human protein involved in the formation and stability of sarcomeres. In this study we describe a patient with a congenital myopathy characterized by multiple contractures, a rigid spine, non progressive muscular weakness, and a novel homozygous TTN pathogenic variant in a metatranscript-only exon: the c.36400A > T, p.Lys12134*. Muscle biopsies showed increased internalized nuclei, variability in fiber size, mild fibrosis, type 1 fiber predominance, and a slight increase in the number of satellite cells. RNA studies revealed the retention of intron 170 and 171 in the open reading frame, and immunoflourescence and western blot studies, a normal titin content. Single fiber functional studies showed a slight decrease in absolute maximal force and a cross-sectional area with no decreases in tension, suggesting that weakness is not sarcomere-based but due to hypotrophy. Passive properties of single fibers were not affected, but the observed increased calcium sensitivity of force generation might contribute to the contractural phenotype and rigid spine of the patient. Our findings provide evidence for a pathogenic, causative role of a metatranscript-only titin variant in a long survivor congenital titinopathy patient with distal arthrogryposis and rigid spine.

A spectrum of clinical severity of recessive titinopathies in prenatal

Variants in TTN are associated with a broad range of clinical phenotypes, from dominant adult-onset dilated cardiomyopathy to recessive infantile-onset myopathy. However, few foetal cases have been reported for multiple reasons. Next-generation sequencing has facilitated the prenatal identification of a growing number of suspected titinopathy variants. We investigated six affected foetuses from three families, completed the intrauterine course of the serial phenotypic spectrum of TTN, and discussed the genotype-phenotype correlations from a broader perspective. The recognizable prenatal feature onset at the second trimester was started with reduced movement, then contracture 3-6 weeks later, followed with/without hydrops, finally at late pregnancy was accompanied with polyhydramnio (major) or oligohydramnios. Two cases with typical arthrogryposis-hydrops sequences identified a meta-only transcript variant c.36203-1G>T. Deleterious transcriptional consequences of the substitution were verified by minigene splicing analysis. Case 3 identified a homozygous splicing variant in the constitutively expressed Z-disc. It presented a milder phenotype than expected, which was presumably saved by the isoform of corons. A summary of the foetal-onset titinopathy cases implied that variants in TTN present with a series of signs and a spectrum of clinical severity, which followed the dosage/positional effect; the meta-only transcript allele involvement may be a prerequisite for the development of fatal hydrops.

Comprehensive analysis of RNA-binding protein SRSF2-dependent alternative splicing signature in malignant proliferation of colorectal carcinoma

Aberrant expression of serine/arginine-rich splicing factor 2 (SRSF2) can lead to tumorigenesis, but its molecular mechanism in colorectal cancer is currently unknown. Herein, we found SRSF2 to be highly expressed in human colorectal cancer (CRC) samples compared with normal tissues. Both in vitro and in vivo, SRSF2 significantly accelerated the proliferation of colon cancer cells. Using RNA-seq, we screened and identified 33 alternative splicing events regulated by SRSF2. Knockdown of SLMAP-L or CETN3-S splice isoform could suppress the growth of colon cancer cells, predicting their role in malignant proliferation of colon cancer cells. Mechanistically, the in vivo crosslinking immunoprecipitation assay demonstrated the direct binding of the RNA recognition motif of SRSF2 protein to SLMAP and CETN3 pre-mRNAs. SRSF2 activated the inclusion of SLMAP alternative exon 24 by binding to constitutive exon 25, while SRSF2 facilitated the exclusion of CETN3 alternative exon 5 by binding to neighboring exon 6. Knockdown of SRSF2, its splicing targets SLMAP-L, or CETN3-S caused colon cancer cells to arrest in G1 phase of the cell cycle. Rescue of SLMAP-L or CETN3-S splice isoform in SRSF2 knockdown colon cancer cells could effectively reverse the inhibition of cell proliferation by SRSF2 knockdown through mediating cell cycle progression. Importantly, the percentage of SLMAP exon 24 inclusion increased and CETN3 exon 5 inclusion decreased in CRC samples compared to paired normal samples. Collectively, our findings identify that SRSF2 dysregulates colorectal carcinoma proliferation at the molecular level of splicing regulation and reveal potential splicing targets in CRC patients.

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.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

Whole-Exome Sequencing Identifies Genetic Variants for Severe Adolescent Idiopathic Scoliosis in a Taiwanese Population

Adolescent idiopathic scoliosis (AIS) is a three-dimensional spinal curvature deformity that appears in the adolescent period. In this study, we performed whole-exome sequencing on 11 unrelated Taiwanese patients with a Cobb's angle greater than 40 degrees. Our results identified more than 200 potential pathogenic rare variants, however, most of which were carried only by one individual. By in silico pathogenicity annotation studies, we found that TTN, CLCN1, and SOX8 were the most important genes, as multiple pathogenic variants were within these genes. Furthermore, biological functional annotation indicated critical roles of these AIS candidate genes in the skeletal muscle. Importantly, a pathogenic variant on SOX8 was shared by over 35% of the patients. These results highlighted TTN, CLCN1, and SOX8 as the most likely susceptibility genes for severe AIS.

Transcriptomic profiles of muscular dystrophy with myositis (mdm) in extensor digitorum longus, psoas, and soleus muscles from mice

Background

Titinopathies are inherited muscular diseases triggered by genetic mutations in the titin gene. Muscular dystrophy with myositis (mdm) is one such disease caused by a LINE repeat insertion, leading to exon skipping and an 83-amino acid residue deletion in the N2A-PEVK region of mouse titin. This region has been implicated in a number of titin—titin ligand interactions, hence are important for myocyte signaling and health. Mice with this mdm mutation develop a severe and progressive muscle degeneration. The range of phenotypic differences observed in mdm mice shows that the deletion of this region induces a cascade of transcriptional changes extending to numerous signaling pathways affected by the titin filament. Previous research has focused on correlating phenotypic differences with muscle function in mdm mice. These studies have provided understanding of the downstream physiological effects resulting from the mdm mutation but only provide insights on processes that can be physiologically observed and measured. We used differential gene expression (DGE) to compare the transcriptomes of extensor digitorum longus (EDL), psoas and soleus muscles from wild-type and mdm mice to develop a deeper understand of these tissue-specific responses.

Results

The overall expression pattern observed shows a well-differentiated transcriptional signature in mdm muscles compared to wild type. Muscle-specific clusters observed within the mdm transcriptome highlight the level of variability of each muscle to the deletion. Differential gene expression and weighted gene co-expression network analysis showed a strong directional response in oxidative respiration-associated mitochondrial genes, which aligns with the poor shivering and non-shivering thermogenesis previously observed. Sln, which is a marker associated with shivering and non-shivering thermogenesis, showed the strongest expression change in fast-fibered muscles. No drastic changes in MYH expression levels were reported, which indicated an absence of major fiber-type switching events. Overall expression shifts in MYH isoforms, MARPs, and extracellular matrix associated genes demonstrated the transcriptional complexity associated with mdm mutation. The expression alterations in mitochondrial respiration and metabolism related genes in the mdm muscle dominated over other transcriptomic changes, and likely account for the late stage cellular responses in the mdm muscles.

Conclusions

We were able to demonstrate that the complex nature of mdm mutation extends beyond a simple rearrangement in titin gene. EDL, psoas and soleus exemplify unique response modes observed in skeletal muscles with mdm mutation. Our data also raises the possibility that failure to maintain proper energy homeostasis in mdm muscles may contribute to the pathogenesis of the degenerative phenotype in mdm mice. Understanding the full disease-causing molecular cascade is difficult using bulk RNA sequencing techniques due to intricate nature of the disease. The development of the mdm phenotype is temporally and spatially regulated, hence future studies should focus on single fiber level investigations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08873-2.

Use of animal models to understand titin physiology and pathology

In recent years, increasing attention has been paid to titin (TTN) and its mutations. Heterozygous TTN truncating variants (TTNtv) increase the risk of a cardiomyopathy. At the same time, TTNtv and few missense variants have been identified in patients with mainly recessive skeletal muscle diseases. The pathogenic mechanisms underlying titin‐related diseases are still partly unknown. Similarly, the titin mechanical and functional role in the muscle contraction are far from being exhaustively clarified. In the last few years, several animal models carrying variants in the titin gene have been developed and characterized to study the structural and mechanical properties of specific titin domains or to mimic patients' mutations. This review describes the main animal models so far characterized, including eight mice models and three fish models (Medaka and Zebrafish) and discusses the useful insights provided by a thorough characterization of the cell‐, tissue‐ and organism‐phenotypes in these models.

Contributions of Titin and Collagen to Passive Stress in Muscles from mdm Mice with a Small Deletion in Titin’s Molecular Spring

Muscular dystrophy with myositis (mdm) is a naturally occurring mutation in the mouse Ttn gene that results in higher passive stress in muscle fibers and intact muscles compared to wild-type (WT). The goal of this study was to test whether alternative splicing of titin exons occurs in mdm muscles, which contain a small deletion in the N2A-PEVK regions of titin, and to test whether splicing changes are associated with an increase in titin-based passive tension. Although higher levels of collagen have been reported previously in mdm muscles, here we demonstrate alternative splicing of titin in mdm skeletal muscle fibers. We identified Z-band, PEVK, and C-terminus Mex5 exons as splicing hotspots in mdm titin using RNA sequencing data and further reported upregulation in ECM-associated genes. We also treated skinned mdm soleus fiber bundles with trypsin, trypsin + KCl, and trypsin + KCL + KI to degrade titin. The results showed that passive stress dropped significantly more after trypsin treatment in mdm fibers (11 ± 1.6 mN/mm²) than in WT fibers (4.8 ± 1 mN/mm²; p = 0.0004). The finding that treatment with trypsin reduces titin-based passive tension more in mdm than in WT fibers supports the hypothesis that exon splicing leads to the expression of a stiffer and shorter titin isoform in mdm fibers. After titin extraction by trypsin + KCl + KI, mdm fibers (6.7 ± 1.27 mN/mm²) had significantly higher collagen-based passive stress remaining than WT fibers (2.6 ± 1.3 mN/mm²; p = 0.0014). We conclude that both titin and collagen contribute to higher passive tension of mdm muscles.

Dysregulation of Tweak and Fn14 in skeletal muscle of spinal muscular atrophy mice

BACKGROUND

Spinal muscular atrophy (SMA) is a childhood neuromuscular disorder caused by depletion of the survival motor neuron (SMN) protein. SMA is characterized by the selective death of spinal cord motor neurons, leading to progressive muscle wasting. Loss of skeletal muscle in SMA is a combination of denervation-induced muscle atrophy and intrinsic muscle pathologies. Elucidation of the pathways involved is essential to identify the key molecules that contribute to and sustain muscle pathology. The tumor necrosis factor-like weak inducer of apoptosis (TWEAK)/TNF receptor superfamily member fibroblast growth factor-inducible 14 (Fn14) pathway has been shown to play a critical role in the regulation of denervation-induced muscle atrophy as well as muscle proliferation, differentiation, and metabolism in adults. However, it is not clear whether this pathway would be important in highly dynamic and developing muscle.

METHODS

We thus investigated the potential role of the TWEAK/Fn14 pathway in SMA muscle pathology, using the severe Taiwanese Smn^-/-; SMN2 and the less severe Smn^2B/- SMA mice, which undergo a progressive neuromuscular decline in the first three post-natal weeks. We also used experimental models of denervation and muscle injury in pre-weaned wild-type (WT) animals and siRNA-mediated knockdown in C2C12 muscle cells to conduct additional mechanistic investigations.

RESULTS

Here, we report significantly dysregulated expression of Tweak, Fn14, and previously proposed downstream effectors during disease progression in skeletal muscle of the two SMA mouse models. In addition, siRNA-mediated Smn knockdown in C2C12 myoblasts suggests a genetic interaction between Smn and the TWEAK/Fn14 pathway. Further analyses of SMA, Tweak^-/-, and Fn14^-/- mice revealed dysregulated myopathy, myogenesis, and glucose metabolism pathways as a common skeletal muscle feature, providing further evidence in support of a relationship between the TWEAK/Fn14 pathway and Smn. Finally, administration of the TWEAK/Fn14 agonist Fc-TWEAK improved disease phenotypes in the two SMA mouse models.

CONCLUSIONS

Our study provides mechanistic insights into potential molecular players that contribute to muscle pathology in SMA and into likely differential responses of the TWEAK/Fn14 pathway in developing muscle.

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.

Exploring the Potential of Symmetric Exon Deletion to Treat Non-Ischemic Dilated Cardiomyopathy by Removing Frameshift Mutations in TTN

Non-ischemic dilated cardiomyopathy (DCM) is one of the most frequent pathologies requiring cardiac transplants. Even though the etiology of this disease is complex, frameshift mutations in the giant sarcomeric protein Titin could explain up to 25% of the familial and 18% of the sporadic cases of DCM. Many studies have shown the potential of genome editing using CRISPR/Cas9 to correct truncating mutations in sarcomeric proteins and have established the grounds for myoediting. However, these therapies are still in an immature state, with only few studies showing an efficient treatment of cardiac diseases. This publication hypothesizes that the Titin (TTN)-specific gene structure allows the application of myoediting approaches in a broad range of locations to reframe TTNtvvariants and to treat DCM patients. Additionally, to pave the way for the generation of efficient myoediting approaches for DCM, we screened and selected promising target locations in TTN. We conceptually explored the deletion of symmetric exons as a therapeutic approach to restore TTN’s reading frame in cases of frameshift mutations. We identified a set of 94 potential candidate exons of TTN that we consider particularly suitable for this therapeutic deletion. With this study, we aim to contribute to the development of new therapies to efficiently treat titinopathies and other diseases caused by mutations in genes encoding proteins with modular structures, e.g., Obscurin.

Genetic Profile of Patients with Limb-Girdle Muscle Weakness in the Chilean Population

Hereditary myopathies are a group of genetically determined muscle disorders comprising more than 300 entities. In Chile, there are no specific registries of the distinct forms of these myopathies. We now report the genetic findings of a series of Chilean patients presenting with limb-girdle muscle weakness of unknown etiology. Eighty-two patients were explored using high-throughput sequencing approaches with neuromuscular gene panels, establishing a definite genetic diagnosis in 49 patients (59.8%) and a highly probable genetic diagnosis in eight additional cases (9.8%). The most frequent causative genes identified were DYSF and CAPN3, accounting for 22% and 8.5% of the cases, respectively, followed by DMD (4.9%) and RYR1 (4.9%). The remaining 17 causative genes were present in one or two cases only. Twelve novel variants were identified. Five patients (6.1%) carried a variant of uncertain significance in genes partially matching the clinical phenotype. Twenty patients (24.4%) did not carry a pathogenic or likely pathogenic variant in the phenotypically related genes, including five patients (6.1%) presenting an autoimmune neuromuscular disorder. The relative frequency of the different forms of myopathy in Chile is like that of other series reported from different regions of the world with perhaps a relatively higher incidence of dysferlinopathy.

Long-Reads Sequencing Strategy to Localize Variants in TTN Repeated Domains

Titin protein is responsible for muscle elasticity. The TTN gene, composed of 364 exons, is subjected to extensive alternative splicing and leads to different isoforms expressed in skeletal and cardiac muscle. Variants in TTN are responsible for myopathies with a wide phenotypic spectrum and autosomal dominant or recessive transmission. The I-band coding domain, highly subject to alternative splicing, contains a three-zone block of repeated sequences with 99% homology. Sequencing and localization of variants in these areas are complex when using short-reads sequencing, a second-generation sequencing technique. We have implemented a protocol based on the third-generation sequencing technology (long-reads sequencing). This new method allows us to localize variants in these repeated areas to improve the diagnosis of TTN-related myopathies and offer the analysis of relatives in postnatal or in prenatal screening.

Heterozygous missense variant in the TTN gene causing Tibial muscular dystrophy

Background

Tibial muscular dystrophy (TMD), tardive, is a dominantly inherited mild degenerative disorder of anterior tibial muscles. Mutations of Titin (TTN) have been reported in patients with different phenotypes such as skeletal muscular abnormalities or complex overlapping disorders of muscles. Titin (TTN) is a large 363 exon gene that encodes an abundant protein (the longest polypeptide known in nature) expressed in the heart and skeletal muscles.

Methods

DNA from peripheral blood sample was extracted, whole exome sequencing (WES) was performed, and a neuromuscular disorders related gene-filtering strategy was used to analyse the disease-causing mutations. Further, sanger sequencing was applied to confirm the variant.

Results

A novel missense variant (c.41529G > C;p.Arg13843Ser) of TTN gene was identified in a patient with lower limb weakness, occasional tongue fasciculation and mild scoliosis. This variant leads to a substitution of arginine with serine, causing structural changes in titin protein that is responsible for the TMD disease.

Conclusion

The novel variant detected has widened the genetic spectrum of TTN-associated diseases, further functional studies will aid in establishing the clinical diagnosis.

Genetic predisposition study of heart failure and its association with cardiomyopathy

Heart failure (HF) is a clinical condition distinguished by structural and functional defects in the myocardium, which genetic and environmental factors can induce. HF is caused by various genetic factors that are both heterogeneous and complex. The incidence of HF varies depending on the definition and area, but it is calculated to be between 1 and 2% in developed countries. There are several factors associated with the progression of HF, ranging from coronary artery disease to hypertension, of which observed the most common genetic cause to be cardiomyopathy. The main objective of this study is to investigate heart failure and its association with cardiomyopathy with their genetic variants. The selected novel genes that have been linked to human inherited cardiomyopathy play a critical role in the pathogenesis and progression of HF. Research sources collected from the human gene mutation and several databases revealed that numerous genes are linked to cardiomyopathy and thus explained the hereditary influence of such a condition. Our findings support the understanding of the genetics aspect of HF and will provide more accurate evidence of the role of changing disease accuracy. Furthermore, a better knowledge of the molecular pathophysiology of genetically caused HF could contribute to the emergence of personalized therapeutics in future.

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

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

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.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Ratio of urinary N-terminal titin fragment to urinary creatinine is a novel biomarker for amyotrophic lateral sclerosis

OBJECTIVE

We aimed to investigate the validity of urinary N-terminal titin fragment as a biomarker for amyotrophic lateral sclerosis (ALS).

METHODS

We consecutively enrolled patients with ALS (n=70) and healthy controls (HC) (n=43). We assessed the urinary titin N-terminal fragment, urinary neurotrophin receptor p75 extracellular domain, serum neurofilament light chain (NfL), motor functional measurements and prognosis. We used urinary creatinine (Cr) levels to normalise the urinary levels of titin fragment.

RESULTS

Compared with HC, patients with ALS had significantly increased urinary levels of titin N-terminal fragment normalised with Cr (titin/Cr) (ALS, 27.2 pmol/mg/dL; HC, 5.8 pmol/mg/dL; p<0.001), which were correlated with the scores of the Revised Amyotrophic Lateral Sclerosis Functional Rating Scale (r=-0.422, p<0.001). A Cox proportional hazards model demonstrated that the high urinary level of titin/Cr was a survival predictor in patients with ALS. Multivariate analysis of prognostic factors showed that the urinary titin/Cr and serum NfL were independent factors for poor prognosis.

CONCLUSIONS

Our findings indicate that urinary N-terminal titin fragment is a non-invasive measure of muscle damage in ALS, which could be applied in disease monitoring and prediction of disease progression, in combination with serum NfL.

Clinicopathological features of titinopathy from a Chinese neuromuscular center

Titin, one of the largest proteins in humans, is a major component of muscle sarcomeres. Pathogenic variants in the titin gene (TTN) have been reported to cause a range of skeletal muscle diseases, collectively known as titinopathy. Titinopathy is a heterogeneous group of disabling diseases characterized by muscle weakness. In our study, we aimed to establish the clinicopathological-genetic spectrum of titinopathy from a single neuromuscular center. Three patients were diagnosed as having definite titinopathy, and additional three patients were diagnosed as having possible titinopathy according to the diagnostic criteria. All the patients showed initial symptoms from age one to 40 years. Physical examination revealed that five patients had muscle weakness, and that one patient experienced behavioral changes. Muscle biopsy specimens obtained from all six patients demonstrated multiple myopathological changes, including increased fiber size variation, muscle fiber hypertrophy or atrophy, formation of centralized cell nuclei, necklace cytoplasmic bodies, and formation of rimmed vacuoles and cores. Genetic testing revealed 11 different TTN alterations, including missense (6/11), nonsense (2/11), frameshift (2/11), and splicing (1/11) mutations. Our study provides further evidence that TTN mutations are more likely to be responsible for an increasing proportion of various myopathies, such as hereditary myopathy with early respiratory failure (HMERF), core myopathy, and distal myopathy with rimmed vacuoles, than currently recognized mutations. Our findings expand the clinical, pathohistological and genetic spectrum of titinopathy.

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.

Protein nanomechanics in biological context

How proteins respond to pulling forces, or protein nanomechanics, is a key contributor to the form and function of biological systems. Indeed, the conventional view that proteins are able to diffuse in solution does not apply to the many polypeptides that are anchored to rigid supramolecular structures. These tethered proteins typically have important mechanical roles that enable cells to generate, sense, and transduce mechanical forces. To fully comprehend the interplay between mechanical forces and biology, we must understand how protein nanomechanics emerge in living matter. This endeavor is definitely challenging and only recently has it started to appear tractable. Here, I introduce the main in vitro single-molecule biophysics methods that have been instrumental to investigate protein nanomechanics over the last 2 decades. Then, I present the contemporary view on how mechanical force shapes the free energy of tethered proteins, as well as the effect of biological factors such as post-translational modifications and mutations. To illustrate the contribution of protein nanomechanics to biological function, I review current knowledge on the mechanobiology of selected muscle and cell adhesion proteins including titin, talin, and bacterial pilins. Finally, I discuss emerging methods to modulate protein nanomechanics in living matter, for instance by inducing specific mechanical loss-of-function (mLOF). By interrogating biological systems in a causative manner, these new tools can contribute to further place protein nanomechanics in a biological context.

The Sarcomeric Spring Protein Titin: Biophysical Properties, Molecular Mechanisms, and Genetic Mutations Associated with Heart Failure and Cardiomyopathy

PURPOSE OF REVIEW

The giant protein titin forms the "elastic" filament of the sarcomere, essential for the mechanical compliance of the heart muscle. Titin serves a biological spring, and therefore structural modifications of titin affect function of the myocardium and are associated with heart failure and cardiomyopathy.

RECENT FINDINGS

In this review, we discuss the current understanding of titin's biophysical properties and how modifications contribute to cardiac function and heart failure. In addition, we review the most recent data on the clinical impact and phenotype heterogeneity of TTN truncating variants, including diseases involving striated muscles, and prospects for future therapies. Because of the giant structure of the titin protein and the complexity of its function, titin's role in health and disease is not yet completely understood. Future research efforts need to focus on novel therapeutic approaches able to modulate titin transcriptional and post-translational modification.

Myopathic Cardiac Genotypes Increase Risk for Myocarditis

Impairments in certain cardiac genes confer risk for myocarditis in children. To determine the extent of this association, we performed genomic sequencing in predominantly adult patients with acute myocarditis and matched control subjects. Putatively deleterious variants in a broad set of cardiac genes were found in 19 of 117 acute myocarditis cases vs 34 of 468 control subjects (P = 0.003). Thirteen genes classically associated with cardiomyopathy or neuromuscular disorders with cardiac involvement were implicated, including >1 associated damaging variant in DYSF, DSP, and TTN. Phenotypes of subjects who have acute myocarditis with or without deleterious variants were similar, indicating that genetic testing is necessary to differentiate them.

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.

Making sense of missense variants in TTN-related congenital myopathies

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

Gene Size Matters: An Analysis of Gene Length in the Human Genome

While it is expected for gene length to be associated with factors such as intron number and evolutionary conservation, we are yet to understand the connections between gene length and function in the human genome. In this study, we show that, as expected, there is a strong positive correlation between gene length, transcript length, and protein size as well as a correlation with the number of genetic variants and introns. Among tissue-specific genes, we find that the longest transcripts tend to be expressed in the blood vessels, nerves, thyroid, cervix uteri, and the brain, while the smallest transcripts tend to be expressed in the pancreas, skin, stomach, vagina, and testis. We report, as shown previously, that natural selection suppresses changes for genes with longer transcripts and promotes changes for genes with smaller transcripts. We also observe that genes with longer transcripts tend to have a higher number of co-expressed genes and protein-protein interactions, as well as more associated publications. In the functional analysis, we show that bigger transcripts are often associated with neuronal development, while smaller transcripts tend to play roles in skin development and in the immune system. Furthermore, pathways related to cancer, neurons, and heart diseases tend to have genes with longer transcripts, with smaller transcripts being present in pathways related to immune responses and neurodegenerative diseases. Based on our results, we hypothesize that longer genes tend to be associated with functions that are important in the early development stages, while smaller genes tend to play a role in functions that are important throughout the whole life, like the immune system, which requires fast responses.

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.