The giant sarcomeric protein titin contains a protein kinase domain (TK) ideally positioned to sense mechanical load. We identified a signaling complex where TK interacts with the zinc-finger protein nbr1 through a mechanically inducible conformation. Nbr1 targets the ubiquitin-associated p62/SQSTM1 to sarcomeres, and p62 in turn interacts with MuRF2, a muscle-specific RING-B-box E3 ligase and ligand of the transactivation domain of the serum response transcription factor (SRF). Nuclear translocation of MuRF2 was induced by mechanical inactivity and caused reduction of nuclear SRF and repression of transcription. A human mutation in the titin protein kinase domain causes hereditary muscle disease by disrupting this pathway.During muscle differentiation, a specific program of gene expression leads to the translation of myofibrillar proteins and their assembly into contractile units, the sarcomeres, which are constantly remodeled to adapt to changes in mechanical load. The giant protein titin (also known as connectin) acts as a molecular blueprint for sarcomere assembly by providing specific attachment sites for numerous sarcomeric proteins, as well as acting as a molecular spring (1, 2). Titin also contains a catalytic serine-threonine kinase domain (TK), which is inhibited by a specific dual mechanism (3). However, the upstream elements controlling TK activation, its range of cellular substrates, and particularly the role of TK in mature muscle are largely unknown. Spanning half sarcomeres from Z disk to M band, titin is in a unique position to sense mechanical strain along the sarcomere (1). The elastic properties of the titin molecule and the mechanical deformation of the M band during stretch and contraction (4) suggest that the signaling properties of TK might be modulated by mechanically induced conformational changes. Molecular dynamics simulations suggest that mechanical strain can induce a catalytically active conformation of TK (5).The catalytic kinase domain of titin interacts with nbr1. We searched for further elements of a putative signaling pathway that might recognize mechanically induced conformational intermediates of titin's catalytic domain. In a systematic two-hybrid screening approach with various structure-based open states of the catalytic site [kin1, kin2, and kin3 (6)], we identified the zinc-finger protein nbr1 (7) as a TK ligand, which interacted via its Nterminal PB1 domain with the semiopened construct kin3 (Fig. 1, A and B). This interaction was also seen in precipitation experiments with nbr1 and TK-kin3 ( fig. S1A). Kin1, where the complete regulatory domain closes the active site, and kin2, where the a helix R1 (3) is deleted, did not interact. Thus, aR1 was necessary but not sufficient for nbr1 binding, which also required a semiopened catalyt-

Limb-girdle muscular dystrophy type 1D (LGMD1D) was linked to 7q36 over a decade ago1, but its genetic cause has remained elusive. We have studied nine LGMD families from Finland, the U.S., and Italy, and identified four dominant missense mutations leading to p.Phe93Leu or p.Phe89Ile changes in the ubiquitously expressed co-chaperone DNAJB6. Functional testing in vivo showed that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. Further, we show that DNAJB6 interacts with members of the CASA complex, including the myofibrillar-myopathy-causing protein BAG3. Our data provide the genetic cause of LGMD1D, suggest that the pathogenesis is mediated by defective chaperone function, and highlight how mutations expressed ubiquitously can exert their effect in a tissue-, cellular compartment-, and isoform-specific manner.

Tibial muscular dystrophy (TMD) is an autosomal dominant late-onset distal myopathy linked to chromosome 2q31. The linked region includes the giant TTN gene, which encodes the central sarcomeric protein, titin. We have previously shown a secondary calpain-3 defect to be associated with TMD, which further underscored that titin is the candidate. We now report the first mutations in TTN to cause a human skeletal-muscle disease, TMD. In Mex6, the last exon of TTN, a unique 11-bp deletion/insertion mutation, changing four amino acid residues, completely cosegregated with all tested 81 Finnish patients with TMD in 12 unrelated families. The mutation was not found in 216 Finnish control samples. In a French family with TMD, a Leu-->Pro mutation at position 293,357 in Mex6 was discovered. Mex6 is adjacent to the known calpain-3 binding site Mex5 of M-line titin. Immunohistochemical analysis using two exon-specific antibodies directed to the M-line region of titin demonstrated the specific loss of carboxy-terminal titin epitopes in the TMD muscle samples that we studied, thus implicating a functional defect of the M-line titin in the genesis of the TMD disease phenotype.

Obscurin, a giant modular muscle protein implicated in G-protein and protein-kinase signalling, can localize to both sarcomeric Z-disks and M-bands. Interaction of obscurin with the Z-disk is mediated by Z-disk titin. Here, we unravel the molecular basis for the unusual localization of obscurin, a Z-disk-associated protein, to the M-band, where its invertebrate analogue UNC-89 is also localized. The first three domains of the N-terminus of obscurin bind to the most C-terminal domain of M-band titin, as well as to the M-band protein myomesin. Both proteins also interact with the N-terminal domains of obscurin-like 1 (Obsl1), a small homologue of obscurin. Downregulation of myomesin by siRNA interference disrupts obscurin–M-band integration in neonatal cardiomyocytes, as does overexpression of the binding sites on either myomesin, obscurin or Obsl1. Furthermore, all titin mutations that have been linked to limb-girdle muscular dystrophy 2J (LGMD2J) or Salih myopathy weaken or abrogate titin-obscurin and titin-Obsl1 binding, and lead to obscurin mislocalization, suggesting that interference with the interaction of these proteins might be of pathogenic relevance for human disease.

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

In 2001, we described an autosomal dominant myopathy characterized by neuromuscular ventilatory failure in ambulant patients. Here we describe the underlying genetic basis for the disorder, and we define the neuromuscular, respiratory and radiological phenotype in a study of 31 mutation carriers followed for up to 31 years. A combination of genome-wide linkage and whole exome sequencing revealed the likely causal genetic variant in the titin (TTN) gene (g.274375T>C; p.Cys30071Arg) within a shared haplotype of 2.93 Mbp on chromosome 2. This segregated with the phenotype in 21 individuals from the original family, nine subjects in a second family with the same highly selective pattern of muscle involvement on magnetic resonance imaging and a third familial case with a similar phenotype. Comparing the mutation carriers revealed novel features not apparent in our original report. The clinical presentation included predominant distal, proximal or respiratory muscle weakness. The age of onset was highly variable, from early adulthood, and including a mild phenotype in advanced age. Muscle weakness was earlier onset and more severe in the lower extremities in nearly all patients. Seven patients also had axial muscle weakness. Respiratory function studies demonstrated a gradual deterioration over time, reflecting the progressive nature of this condition. Cardiomyopathy was not present in any of our patients despite up to 31 years of follow-up. Magnetic resonance muscle imaging was performed in 21 affected patients and revealed characteristic abnormalities with semitendinosus involvement in 20 of 21 patients studied, including 3 patients who were presymptomatic. Diagnostic muscle histopathology most frequently revealed eosinophilic inclusions (inclusion bodies) and rimmed vacuoles, but was non-specific in a minority of patients. These findings have important clinical implications. This disease should be considered in patients with adult-onset proximal or distal myopathy and early respiratory failure, even in the presence of non-specific muscle pathology. Muscle magnetic resonance imaging findings are characteristic and should be considered as an initial investigation, and if positive should prompt screening for mutations in TTN. With 363 exons, screening TTN presented a major challenge until recently. However, whole exome sequencing provides a reliable cost-effective approach, providing the gene of interest is adequately captured.

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.

WDM is caused by mutated TIA1 through a dominant pathomechanism probably involving altered stress granule dynamics.