Alternative titles; symbols
HGNC Approved Gene Symbol: TTN
SNOMEDCT: 698846009, 702343002, 702373006, 725042001;
Cytogenetic location: 2q31.2 Genomic coordinates (GRCh38): 2:178,525,989-178,807,423 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 2q31.2 | Cardiomyopathy, dilated, 1G | 604145 | Autosomal dominant | 3 |
| Cardiomyopathy, familial hypertrophic, 9 | 613765 | Autosomal dominant | 3 | |
| Congenital myopathy 5 with cardiomyopathy | 611705 | Autosomal recessive | 3 | |
| Muscular dystrophy, limb-girdle, autosomal recessive 10 | 608807 | Autosomal recessive | 3 | |
| Myopathy, myofibrillar, 9, with early respiratory failure | 603689 | Autosomal dominant | 3 | |
| Tibial muscular dystrophy, tardive | 600334 | Autosomal dominant | 3 |
Titin, or connectin, is a giant muscle protein expressed in the cardiac and skeletal muscles that spans half of the sarcomere from Z line to M line. Titin plays a key role in muscle assembly, force transmission at the Z line, and maintenance of resting tension in the I band region (Itoh-Satoh et al., 2002).
Labeit et al. (1990) showed that partial titin cDNAs encode a regular pattern of 2 types of 100-residue motif, each of which probably folds into a separate domain type. Such motifs are present in several evolutionarily divergent proteins, all of which are likely to interact with myosin.
Labeit and Kolmerer (1995) determined the cDNA sequence of human cardiac titin. The 82-kb cDNA predicted a 26,926-amino acid protein with a molecular mass of 2,993 kD. Ninety percent of the mass is contained in a repetitive structure composed of 244 copies of 100-residue repeats that encode 112 immunoglobulin-like and 132 fibronectin type III domains. Alternative splicing accounts for tissue-specific titin isoforms. In the central part of I band titin, cardiac and skeletal titins branch into distinct isoforms; in heart, differential splicing includes about 3.5 kb of cDNA within the I band region of titin, whereas in skeletal muscle, 22.5 kb of cDNA is included. In addition, a sequence element rich in proline (P), glutamic acid (E), lysine (K), and valine (V) residues, referred to as the PEVK domain, comprises 163 residues in cardiac titin and 2,174 residues in skeletal titin.
Bang et al. (2001) determined that the complete sequence of human titin encodes a 38,138-amino acid protein with a molecular mass of 4,200 kD.
Bang et al. (2001) determined that titin has 363 exons.
Titin contains 6 M band-encoding exons at the C terminus, exons 358 to 363, referred to as Mex1 to Mex6. These exons are constitutively expressed in both skeletal and cardiac muscle (Carmignac et al., 2007).
Labeit et al. (1990) suggested that the I band of titin makes elastic connections between the thick filament and the Z line within the sarcomere. The A band of titin appears to bind to the thick filament, where it may regulate filament length and assembly. The architecture of sequences in the A band region of titin suggested to Labeit and Kolmerer (1995) why thick filament structure is conserved among vertebrates. In the I band region, comparison of titin sequences from muscles of different passive tension identified 2 elements that correlate with tissue stiffness, suggesting that titin may act as 2 springs in series. The differential expression of the springs provides a molecular explanation for the diversity of sarcomere length and resting tension in vertebrate striated muscles.
Ma and Wang (2002) presented evidence that the PEVK segment of titin, which contains numerous SH3-binding motifs, and the Z line protein myopalladin (MYPN; 608517) may play signaling roles in targeting and orienting nebulin (NEB; 161650) to the Z line during sarcomere assembly.
The I band region of titin contains tandem arrays of immunoglobulin domains. Immunoglobulin domain-27 (I27) unfolds through an intermediate under force in which the A-strand is detached. The lengthening of I27 without unfolding forms a stable intermediate that is believed to be an important component of titin elasticity (Marszalek et al., 1999). Williams et al. (2003) used mutant titins to study the role of the partly unfolded intermediate of titin. Under physiologic forces, the partly unfolded intermediate of immunoglobulin domain-27 does not contribute to mechanical strength. Williams et al. (2003) also proposed a unified forced unfolding model of all I27 analogs studied, and concluded that I27 can withstand higher forces in muscle than had previously been predicted.
Titin interacts with many sarcomeric proteins: telethonin (TCAP; 604488) and alpha-actinin (e.g., 102575) at the Z line region; calpain-3 (CAPN3; 114240) and obscurin (OBSCN; 608616) at the I band region; and myosin-binding protein C (MYBPC3; 600958), calmodulin (CALM1; 114180), and CAPN3 at the M line region (Bang et al., 2001). In a review, Hackman et al. (2003) noted that titin has at least 2 different CAPN3-binding sites: one is in region N2A in I band titin and the other is in the Mex5 exon of M line titin. Obscurin (608616) interacts with both the NH2-terminal of Z disc titin and the M line titin during different phases of myofibrillogenesis, and MURF1 (606131) binds titin close to the kinase domain at the periphery of the M line titin.
Lange et al. (2005) identified a signaling complex where the titin protein kinase domain (TK) interacts with the zinc finger protein NBR1 (166945) through a mechanically inducible conformation. NBR1 targets the ubiquitin-associated p62/SQSTM1 (601530) to sarcomeres, and p62 in turn interacts with MURF2 (606469), a muscle-specific RING-B-box E3 ligase and ligand of the transactivation domain of the serum response transcription factor (SRF; 600589). Nuclear translocation of MURF2 was induced by mechanical inactivity and caused reduction of nuclear SRF and repression of transcription.
Sarparanta et al. (2010) observed that M-band-localized myospryn (CMYA5; 612193) was in close proximity (less than 40 nm) to the M-band-associated titin C terminus in mouse muscle sections. Yeast 2-hybrid analysis of human fetal and adult skeletal muscle cDNA libraries showed that C-terminal domains of titin interacted with a C-terminal fragment of myospryn. Reciprocal coimmunoprecipitation analysis confirmed the interaction between the titin and myospryn fragments.
Li et al. (2002) used protein engineering and single-molecule atomic force microscopy to examine the mechanical components that form the elastic region of human cardiac titin. They showed that when these mechanical elements are combined, they explain the macroscopic behavior of titin in intact muscle.
Using x-ray crystallography, Zou et al. (2006) showed how the amino terminus of the longest filament component in the Z disc of muscle, the giant muscle protein titin, is assembled into an antiparallel (2:1) sandwich complex by the Z disc ligand telethonin. The pseudosymmetric structure of telethonin mediates a unique palindromic arrangement of 2 titin filaments, a type of molecular assembly previously found only in protein-DNA complexes. Zou et al. (2006) confirmed its unique architecture in vivo by protein complementation assays, and in vitro by experiments using fluorescence resonance energy transfer. Zou et al. (2006) proposed a model that provides a molecular paradigm of how major sarcomeric filaments are crosslinked, anchored, and aligned within complex cytoskeletal networks.
By studies of DNA from a panel of Chinese hamster/human hybrid cell lines, Labeit et al. (1990) assigned the TTN locus to 2q13-q33. Another myofibrillar protein, nebulin, maps to 2q31-q32. The fact that the 2 genes are close together suggests that their regulation may be coordinated, possibly to control the ratio of the proteins. In the mouse, the titin gene was also mapped to chromosome 2. Muller-Seitz et al. (1993) showed that the murine equivalents of the human TTN, NEB, and CHRNA1 (100690) genes are all on mouse chromosome 2.
Using radiation hybrid mapping, Pelin et al. (1997) reassigned the titin gene to the vicinity of the markers D2S384 and D2S364 on 2q24.3. They concluded that the TTN gene lies outside the candidate region for NEM2 (256030), the autosomal recessive form of nemaline myopathy.
Carmignac et al. (2007) noted that the TTN gene maps to chromosome 2q31.2.
Cardiomyopathy
In 1 of 82 patients with hypertrophic cardiomyopathy (CMH) who had no mutation in known disease genes, Satoh et al. (1999) identified a mutation in the TTN gene (188840.0001) that was not found in more than 500 normal chromosomes and increased the binding affinity of titin to alpha-actinin (see 102575) in the yeast 2-hybrid assay. The form of hypertrophic cardiomyopathy due to mutation in the TTN gene has been designated CMH9 (613765).
In 2 unrelated families with autosomal dominant dilated cardiomyopathy (CMD) linked to 2q31 (CMD1G; 604145), Gerull et al. (2002) identified 2 different heterozygous mutations in the TTN gene (188840.0002; 188840.0003).
In 4 patients with dilated cardiomyopathy, Itoh-Satoh et al. (2002) identified 4 different mutations in the TTN gene (188840.0007-188840.0010). Two of the cases were familial.
Herman et al. (2012) used next-generation sequencing to analyze the TTN gene in 203 individuals with dilated cardiomyopathy, 231 with hypertrophic cardiomyopathy, and 249 controls. The frequency of TTN mutations was significantly higher among individuals with CMD (27%) than among those with CMH (1%) or controls (3%). In the 3 patients with CMH in whom TTN truncating or splicing mutations were identified, concurrent analyses revealed a pathogenic mutation in the known CMH genes MYH7 (160760) or MYBPC3 (600958). In CMD families, TTN mutations cosegregated with dilated cardiomyopathy, with highly observed penetrance after the age of 40 years. Mutations associated with CMD were overrepresented in the titin A-band but were absent from the Z-disc and M-band regions of titin. Herman et al. (2012) concluded that TTN truncating mutations are a common cause of dilated cardiomyopathy, occurring in approximately 25% of familial CMD cases and in 18% of sporadic cases, and suggested that TTN truncations rarely, if ever, cause hypertrophic cardiomyopathy.
Lopes et al. (2013) analyzed the coding, intronic, and regulatory regions of 41 cardiovascular genes in 223 unrelated patients with CMH using high-throughput sequencing technology. They found 219 rare variants in 142 (63.6%) of the patients: 30 patients (13%) had titin candidate variants in isolation, 22 (10%) had titin variants only in association with desmosomal gene candidate variants or ion channel disease-associated variants, and 171 (77%) carried a TTN candidate variant in association with sarcomere, Z-disc, or calcium-handling gene variants. Lopes et al. (2013) noted that titin has been difficult to sequence and study due to its size, large number of isoforms, and unsolved tertiary structure. All of the individual variants present in this cohort occurred with a frequency of less than 0.5% in the 1000 Genomes Project, suggesting that a proportion of them might be, at the very least, modulators of the phenotype. However, the overall frequency of variants in the CMH cohort was actually lower than that seen in the control exome population. Lopes et al. (2013) concluded that further work on understanding the role of titin in CMH was necessary.
Vikhorev et al. (2017) compared contractility and passive stiffness of cardiac myofibril samples from 3 unrelated patients with dilated cardiomyopathy (DCM) and 2 different truncation mutations in titin, 3 unrelated DCM patients with mutations in different contractile proteins (lys36 to gln in TNNI3 (191044.0012), gly159 to asp in TNNC1 (191040.0001), and glu1426 to lys in MYH7), and controls. All 3 contractile protein mutations, but not the titin mutations, had faster relaxation kinetics than controls. Myofibril passive stiffness was reduced by about 38% in all DCM samples compared with controls, but there was no change in maximum force or titin N2BA/N2B isoform ratio, and there was no titin haploinsufficiency. The authors concluded that decreased myofibril passive stiffness, a common feature in all DCM samples, may be a causative of DCM.
Muscular Dystrophy
Tibial muscular dystrophy (TMD; 600334) is an autosomal dominant late-onset distal myopathy characterized by weakness and atrophy usually confined to the anterior compartment of the lower leg. Cardiomyopathy has not been diagnosed in patients with TMD. In 81 Finnish patients with TMD from 12 unrelated families, Hackman et al. (2002) identified an 11-bp deletion/insertion (188840.0004) in Mex6, the last exon (exon 363) of the TTN gene. Mex6 encodes an Ig domain that, in situ, is localized at the periphery of the M line lattice. Mex6 and Mex5 are in the region determining the calpain-3 binding site of M line titin. Three patients with a more severe phenotype, autosomal recessive limb-girdle muscular dystrophy-10 (LGMDR10; 608807), previously symbolized LGMD2J, were homozygous for the 11-bp deletion. In a French family with TMD, a leu34315-to-pro mutation in Mex6 (188840.0005) was discovered.
In a 29-year-old man of Romanian and Hungarian origin with a relatively mild form of autosomal recessive limb-girdle muscular dystrophy-10 (LGMDR10; 608807), Dabby et al. (2015) identified compound heterozygous missense mutations at conserved residues in the TTN gene: K26320T (188840.0017) and A6135G (188840.0018). The mutations, which were found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variants were not performed, but patient skeletal muscle biopsy showed a reduction in C-terminal titin compared to controls.
In 3 sibs from a consanguineous Han Chinese family with LGMDR10, Zheng et al. (2016) identified a homozygous missense mutation in the TTN gene (W35930R; 188840.0019). The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed.
Myofibrillar Myopathy with Early Respiratory Failure
In affected members of Swedish families with myofibrillar myopathy-9 with early respiratory failure (MFM9; 603689), Lange et al. (2005) identified a heterozygous missense mutation in the titin protein kinase domain (R279W; 188840.0011).
In affected members of 3 unrelated English families with MFM9, Pfeffer et al. (2012) identified a heterozygous missense mutation in the TTN gene (C30071R; 188840.0016). The mutation, which was found by a combination of linkage analysis and whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the families. Haplotype analysis indicated a founder effect. Functional studies of the variant were not performed, but the size and abundance of the titin protein was normal in affected muscle. However, affected muscle showed decreased levels of the titin binding partner calpain-3 (CAPN3; 114240). Family A had previously been reported by Chinnery et al. (2001) and Birchall et al. (2005), and family B had previously been reported by Birchall et al. (2005).
Pfeffer et al. (2014) identified the C30071R mutation in affected members of 5 additional families with HMERF. The patients were ascertained from 127 probands with a clinical presentation compatible with myofibrillar myopathy, thus accounting for 3.9% of the cohort. The families with the mutation were of English descent, and haplotype analysis indicated a founder effect. Pfeffer et al. (2014) suggested that the disorder is likely underrecognized.
Congenital Myopathy 5 with Cardiomyopathy
Carmignac et al. (2007) identified 2 different homozygous deletions in the TTN gene (188840.0012, 188840.0013, respectively) in affected members of 2 unrelated families with autosomal recessive congenital myopathy-5 with cardiomyopathy (CMYP5; 611705). The deletions occurred in Mex1 and Mex3, truncating the C-terminal region of the protein; the kinase portion was preserved.
Chauveau et al. (2014) identified 7 novel homozygous or compound heterozygous TTN mutations (5 in the M-line; 5 truncating; see, e.g., 188840.0014 and 188840.0015) in 5 patients in 4 of 23 families with congenital myopathy and cardiomyopathy.
The zebrafish embryo is transparent and can tolerate absence of blood flow because its oxygen is delivered by diffusion rather than by the cardiovascular system. It is, therefore, possible to attribute cardiac failure directly to particular genes by ruling out the possibility that it is due to a secondary effect of hypoxia. Xu et al. (2002) studied a recessive lethal mutation, called 'pickwick' (pik), discovered in a large-scale genetic screen. The heart of the pik mutant develops normally but is poorly contractile from the first beat. Aside from the edema that inevitably accompanies cardiac dysfunction, development is normal during the first 3 days. Xu et al. (2002) showed by positional cloning that the 'causative' mutation is in an alternatively spliced exon of the titin gene. Titin is the biggest known protein and spans the half-sarcomere from the Z disc to M line in heart and skeletal muscle. It appears to provide a scaffold for the assembly of thick and thin filaments and to provide elastic recoil engendered by stretch during diastole. Xu et al. (2002) found that nascent myofibrils form in pik mutants, but normal sarcomeres are absent. Mutant cells transplanted to wildtype hearts remained thin and bulged outwards as individual cell aneurysms without affecting nearby wildtype cardiomyocytes, indicating that the contractile deficiency is cell-autonomous. Absence of titin function thus results in blockage of sarcomere assembly and causes a functional disorder resembling human dilated cardiomyopathies, one form of which was shown to be caused in the human by mutations in the TTN gene (Gerull et al., 2002).
Muscular dystrophy with myositis (mdm) is a recessive mouse mutation that causes severe and progressive muscular degeneration. Garvey et al. (2002) identified the mdm mutation as a complex rearrangement that includes a deletion and LINE insertion in the titin gene. Mutant allele-specific splicing results in the deletion of 83 amino acids from the N2A region of TTN, a domain thought to bind CAPN3. Western blot analysis detected a 50 to 60% reduction in the amount of CAPN3 in affected muscles. Garvey et al. (2002) concluded that the mdm mouse is a model for tibial muscular dystrophy.
The giant protein titin serves a primary role as a scaffold for sarcomere assembly; one potential mediator of this process is calpain-3 (CAPN3; 114240). To test the hypothesis that calpain-3 mediates remodeling during myofibrillogenesis, Kramerova et al. (2004) generated CAPN3 knockout (C3KO) mice. The mice were atrophic, with small foci of muscular necrosis. Myogenic cells fused normally in vitro, but lacked well-organized sarcomeres, as visualized by electron microscopy. Titin distribution was normal in longitudinal sections from the C3KO mice; however, electron microscopy of muscle fibers showed misaligned A bands. In vitro studies revealed that calpain-3 can bind and cleave titin and that some mutations that are pathogenic in human muscular dystrophy result in reduced affinity of calpain-3 for titin. The authors suggested a role for calpain-3 in myofibrillogenesis and sarcomere remodeling.
Huebsch et al. (2005) generated CAPN3 overexpressing transgenic (C3Tg) and C3KO mice and showed that overexpression of CAPN3 exacerbated mdm disease, leading to a shorter life span and more severe muscular dystrophy. However, C3KO/mdm double-mutant mice showed no change in the progression or severity of disease, indicating that aberrant CAPN3 activity is not a primary mechanism in this disease. The authors examined the treadmill locomotion of heterozygous +/mdm mice and detected a significant increase in stride time with a concomitant increase in stance time. These altered gait parameters were completely corrected by CAPN3 overexpression in C3Tg/+/mdm mice, suggesting a CAPN3-dependent role for the N2A domain of TTN in the dynamics of muscle contraction.
The N2B region of cardiac titin is thought to modulate elasticity of the titin filament and may be important for hypertrophy signaling and ischemic stress response through its binding to FHL2 (602633) and alpha-B crystallin (CRYAB; 123590), respectively. Radke et al. (2007) deleted the N2B-encoding exon 49 of the titin gene in mice, leaving the remainder of the gene intact. Mutant mice survived to adulthood and were fertile. Although mutant hearts were small, they produced normal ejection volumes because of an increased ejection fraction. Mutant mice has significantly reduced Fhl2 protein levels, consistent with the reduced size of mutant hearts. Ultrastructural analysis revealed increased extension of the remaining spring elements of titin (tandem Ig segments and the PEVK region), resulting in reduced sarcomere length and increased passive tension in skinned cardiomyocytes and diastolic dysfunction. Radke et al. (2007) concluded that the titin N2B region is dispensable for cardiac development and systolic properties, but it is important to integrate trophic and elastic functions of the heart.
In a patient with hypertrophic cardiomyopathy (CMH9; 613765) and no mutations in any of the 8 genes associated with this disorder, Satoh et al. (1999) identified a heterozygous 740G-T transversion in the TTN gene, resulting in an arg740-to-leu (R740L) substitution. The parents were deceased, and the few relatives available were unaffected. Functional expression studies showed that the mutation resulted in increased titin binding affinity for alpha-actinin (102575). See 188840.0007 for a nearby mutation (A743V) that causes decreased titin binding affinity to alpha-actinin, resulting in dilated cardiomyopathy (CMD1G; 604145).
In a large family with autosomal dominant dilated cardiomyopathy and linkage to 2q31 (CMD1G; 604145), Gerull et al. (2002) found a 2-bp insertion mutation (43628AT) in exon 326 of the TTN gene, causing a frameshift that truncated A band titin. The premature stop codon occurred after the addition of 4 novel amino acid residues. Puzzling was the absence of any clinically detectable phenotype in skeletal muscle. The 2 exons found to be affected in dilated cardiomyopathy by Gerull et al. (2002), namely exons 18 and 326, are both expressed in cardiac and noncardiac muscle isoforms. The truncated protein of approximately 2 mD was expressed in skeletal muscle, but Western blot studies with epitope-specific anti-titin antibodies suggested that the mutant protein was truncated to a 1.14-mD subfragment by site-specific cleavage. Clinical characteristics were described by Siu et al. (1999).
In a large family with autosomal dominant dilated cardiomyopathy mapping to 2q31 (Siu et al., 1999; CMD1G, 604145), Gerull et al. (2002) found a TTN missense mutation, trp930-to-arg (W930R), predicted to disrupt a highly conserved hydrophobic core sequence of an immunoglobulin fold located in the Z disc/I band transition zone. In this kindred, reduced penetrance of the mutation was observed, as was the case also in the family with the 2-bp insertion mutation (188840.0002).
Variant Function
Using cardiac microtissues (CMTs) engineered from human induced pluripotent stem (iPS) cells to evaluate the pathogenicity of titin gene variants, Hinson et al. (2015) observed that CMTs with TTN A-band truncating variants and the Z/I junction missense mutation W930R (which the authors referred to as TRP976ARG) showed comparable force deficits in contractile function assays.
Tardive Tibial Muscular Dystrophy
In 81 Finnish patients with tardive tibial muscular dystrophy (TMD; 600334) from 12 unrelated families, Hackman et al. (2002) found a heterozygous 11-bp deletion/insertion mutation located at position 293269-293279 in the TTN sequence. The mutation changed 4 amino acids close to the C-terminal end of the titin protein but did not cause a frameshift or a stop codon. Each of the 4 amino acids was changed to an amino acid of another charge, and the overall charge was changed from acidic to basic. The mutation was not found in 216 Finnish control samples.
Limb-Girdle Muscular Dystrophy, Autosomal Recessive 10
In 3 patients with limb-girdle muscular dystrophy type 2J (LGMDR10; 608807) from a large consanguineous Finnish family, Hackman et al. (2002) identified homozygosity for the TTN 11-bp deletion/insertion. Other members in the same family who were heterozygous for the 11-bp deletion/insertion manifested the less severe TMD phenotype.
Variant Function
Using yeast 2-hybrid analysis, Sarparanta et al. (2010) found that titin containing this mutation failed to interact with myospryn (CMYA5; 612193).
In a French family in which tardive tibial muscular dystrophy (TMD; 600334) was shown to be linked to 2q31, Hackman et al. (2002) identified a 293357T-C transition in the TTN sequence, resulting in a leu34315-to-pro (L3431P) substitution in the last exon.
In affected members of a Belgian family with tibial muscular dystrophy (TMD; 600334), Van den Bergh et al. (2003) identified a heterozygous 293329T-A change in the Mex6 exon of the TTN gene, resulting in an ile-to-asn substitution. The family showed incomplete disease penetrance.
In a father and daughter with dilated cardiomyopathy (CMD1G; 604145), Itoh-Satoh et al. (2002) identified a heterozygous C-to-T transition in the TTN gene, resulting in an ala743-to-val (A743V) substitution. Both patients had a history of cardiac arrhythmias (premature atrial or ventricular contraction and atrioventricular conduction block) before they developed dilated cardiomyopathy or congestive heart failure. The A743V mutation is located in the alpha-actinin (102575)-binding domain of titin, and functional studies showed that the mutation decreased the affinity of titin Z-repeats to alpha-actinin by about 40% compared to normal. The authors noted that the A743V mutation is located near the R740L (188840.0001) mutation, which was found in a patient with hypertrophic cardiomyopathy and results in increased titin-binding affinity to alpha-actinin.
In a 19-year-old woman with dilated cardiomyopathy (CMD1G; 604145) whose father had died from the same disorder, Itoh-Satoh et al. (2002) identified a heterozygous G-to-A transition in the TTN gene, resulting in a val54-to-met (V54M) substitution at a well-conserved residue in the Z1 domain. The V54M mutation is located in the telethonin (604488)-binding domain of titin, and functional studies showed that the V54M mutation decreased the affinity of titin for telethonin to about 60% of normal.
In a 45-year-old man with severe heart failure and cardiac dilatation (CMD1G; 604145) without signs of muscle disease, Itoh-Satoh et al. (2002) identified a heterozygous C-to-T transition in the TTN gene, resulting in a gln4053-to-ter (Q4053X) nonsense mutation. The mutation occurred in the N2-B domain of the titin protein, which is known to be expressed only in cardiac muscle.
In a 51-year-old man with dilated cardiomyopathy (CMD1G; 604145), Itoh-Satoh et al. (2002) identified a heterozygous G-to-A transition in the TTN gene, resulting in a ser4465-to-asn (S4465N) substitution. The mutation occurred in the N2-B domain of the titin protein, which is known to be expressed only in cardiac muscle.
In 2 large unrelated Swedish families described by Nicolao et al. (1999) segregating autosomal dominant myofibrillar myopathy-9 with early respiratory failure (MFM9; 603689), Lange et al. (2005) identified a C-to-T transition in the TTN gene resulting in an arg-to-trp substitution at codon 279 (R279W) in the alpha-R1 region of the protein kinase regulatory tail of titin. This mutation showed complete segregation with the disease in the 2 families. The mutation was not reported in single-nucleotide polymorphism (SNP) databases and was not found in 200 normal Swedish controls. An additional Swedish patient with an identical phenotype but without known genealogic relation to anyone in the 2 original families was found to have the same mutation on the same haplotype, indicating a common ancestry. The R279W mutant protein kinase domain (TK) showed no difference in calmodulin (114180)-stimulated catalytic activity when compared with wildtype TK. However, the interaction of TK with NBR1 (166945) was dramatically reduced. In patient biopsies, NBR1 was localized abnormally diffusely in diseased muscle instead of being M band- and Z disc-associated, although in HMERF 50% of TK was expected to be wildtype. This suggested a dominant-negative mechanism of action for this mutation.
In 3 sibs with autosomal recessive congenital myopathy-5 with cardiomyopathy (CMYP5; 611705), born of consanguineous Moroccan parents, Carmignac et al. (2007) analyzed genomic DNA and identified a homozygous 1-bp deletion (g.291394delA) in exon 380 (Mex3) of the TTN gene, resulting in the loss of 447 C-terminal residues and disruption of the sarcomeric M line protein complex. Absence of this part of titin had been expected to be lethal. The heterozygous parents were clinically unaffected. (The original article erroneously labeled the mutation g.291297delA.)
In 2 sibs, born of consanguineous Sudanese parents, with congenital myopathy-5 with cardiomyopathy (CMYP5; 611705), Carmignac et al. (2007) analyzed genomic DNA and identified a homozygous 8-bp deletion (g.289385delACCAAGTG) in exon 358 (Mex1) of the TTN gene, resulting in the loss of 808 C-terminal residues and disruption of the sarcomeric M line protein complex. Absence of this part of titin had been expected to be lethal. The heterozygous parents were clinically unaffected.
In a patient (P5) with congenital myopathy-5 with cardiomyopathy (CMYP5; 611705), Chauveau et al. (2014) identified compound heterozygous mutations in the TTN gene: a single nucleotide deletion (c.G9388+1C, NM_001267550.1) at the splice donor site of exon 38, resulting in a frameshift and premature termination (Glu2989GlufsTer4), inherited from the mother, and a c.102439T-C transition, resulting in a trp34072-to-arg (W34072R; 188840.0015) substitution at a highly conserved amino acid acid in the core of the titin kinase (TK) domain (TK-W260R), inherited from the father. For the maternally inherited mutation, deep sequencing of the patient's myocardium confirmed the coexistence of a wildtype and a shorter transcript corresponding to use of an alternative splice donor site and leading to titin truncation after 2,950 N-terminal amino acids, therefore devoid of the TK domain. Semiquantitative protein gels confirmed a moderate reduction of the global titin amount in the patient's samples. For the paternally inherited mutation, genetic interaction assays showed that TK-R260W abrogated interactions with 2 known TK ligands. Circular dichroism spectroscopy demonstrated reduced stability of the TK-W260R mutant.
For discussion of the c.102439T-C transition (c.102439T-C, NM_001267550.1) in the TTN gene, resulting in a trp34072-to-arg (W34072R) substitution, that was found in compound heterozygous state in a patient with congenital myopathy-5 with cardiomyopathy (CMYP5; 611705) by Chauveau et al. (2014), see 188840.0014.
In 18 affected members of 3 families from England with myofibrillar myopathy-9 with early respiratory failure (MFM9; 603689), Pfeffer et al. (2012) identified a heterozygous A-to-G transition in exon 343 of the TTN gene, resulting in a cys30071-to-arg (C30071R) substitution at a conserved residue in the fibronectin III/Ig domain just proximal to the kinase domain in the A-band. The mutation, which was found by a combination of linkage analysis and whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the families. The mutation was not found in 182 controls (364 chromosomes). Haplotype analysis indicated a founder effect. Functional studies of the variant were not performed, but the size and abundance of the titin protein was normal in affected muscle. However, affected muscle showed decreased levels of the titin binding partner calpain-3 (CAPN3; 114240). Family A had previously been reported by Chinnery et al. (2001) and Birchall et al. (2005), and family B had previously been reported by Birchall et al. (2005).
Pfeffer et al. (2014) identified the C30071R mutation (g.179410829A-G, GRCh37) in affected members of 5 additional families with HMERF. The patients were ascertained from 127 probands with a clinical presentation compatible with myofibrillar myopathy, thus accounting for 3.9% of the cohort. The families with the mutation were of English descent, and haplotype analysis indicated a founder effect.
In a 29-year-old man of Romanian and Hungarian origin, with a relatively mild form of autosomal recessive limb-girdle muscular dystrophy-10 (LGMDR10; 608807), Dabby et al. (2015) identified compound heterozygous missense mutations at conserved residues in TTN gene: lys26320-to-thr (K26320T) and ala6135-to-gly (A6135G; 188840.0018). The K26320T substitution arose from a change at g.179395188. The mutations, which were found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. The K26320T change was not found in the 1000 Genomes Project or Exome Variant Server, but A6135G was found in the heterozygous state at a low frequency (0.06%). Functional studies of the variants were not performed, but patient skeletal muscle biopsy showed a reduction in C-terminal titin compared to controls.
For discussion of the ala6135-to-gly (A6135G) substitution arising from a change at g.179485846 in the TTN gene that was found in compound heterozygous state in a patient with a relatively mild form of autosomal recessive limb-girdle muscular dystrophy-10 (LGMDR10; 608807) by Dabby et al. (2015), see 188840.0017.
In 3 sibs from a consanguineous Han Chinese family with autosomal recessive limb-girdle muscular dystrophy-10 (LGMDR10; 608807), Zheng et al. (2016) identified a homozygous c.107788T-C transition in the TTN gene, resulting in a trp35930-to-arg (W35930R) substitution. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not found in the dbSNP (build 137), 1000 Genomes Project, or Exome Sequencing Project databases, or in 700 Chinese controls. Functional studies of the variant and studies of patient cells were not performed.
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