The myosin-binding protein C, cardiac-type is a protein that in humans is encoded by the MYBPC3 gene. [5] This isoform is expressed exclusively in heart muscle during human and mouse development, [6] and is distinct from those expressed in slow skeletal muscle (MYBPC1) and fast skeletal muscle (MYBPC2).
cMyBP-C is a 140.5 kDa protein composed of 1273 amino acids. [7] [8] [9] cMyBP-C is a myosin-associated protein that binds at 43 nm intervals along the myosin thick filament backbone, stretching for 200 nm on either side of the M-line within the crossbridge-bearing zone (C-region) of the A band in striated muscle. [10] The approximate stoichiometry of cMyBP-C along the thick filament is 1 per 9-10 myosin molecules, or 37 cMyBP-C molecules per thick filament. [11] In addition to myosin, cMyBP-C also binds titin and actin. [12] [13] The cMyBP-C isoform expressed in cardiac muscle differs from those expressed in slow and fast skeletal muscle (MYBPC1 and MYBPC2, respectively) by three features: (1) an additional immunoglobulin (Ig)-like domain on the N-terminus, (2) a linker region between the second and third Ig domains, and (3) an additional loop in the sixth Ig domain. [14] cMyBP-C appears necessary for normal order, filament length and lattice spacing within the structure of the sarcomere. [15] [16]
cMyBP-C is not essential for sarcomere formation during embryogenesis, but is crucial for sarcomere organization and maintenance of normal cardiac function. Absence of cMyBP-C (Mybpc3-targeted knock-out mice) results in severe cardiac hypertrophy, increased heart-weight-to-body-weight-ratios, enlargement of ventricles, increased myofilament Ca2+ sensitivity and depressed diastolic and systolic function. [17] [18] [19] Histologically, Mybpc3-targeted knock-out hearts display structural rearrangements with cardiac myocyte disarray and increased interstitial fibrosis similar to patients with hypertrophic cardiomyopathy, without obvious alterations in shape or size of single cardiac myocytes. Ultrastructural examination revealed a loss of lateral alignment of adjacent myofibrils with their Z-lines misaligned. [17] [18] [20] [21]
cMyBP-C appears to act as a brake on cardiac contraction, as loaded shortening, power and cycling kinetics all increase in cMyBP-C knockout mice. [22] Consistent with this notion, cMyBP-C knockout mice exhibit an abnormal systolic timecourse, with a shortened elastance timecourse and lower peak elastance in vivo, [23] and an accelerated force development in isolated, skinned cardiac fibers [24] suggesting that cMyBP-C is required to constrain the crossbridges in order to sustain a normal ejection.
cMyBP-C regulates the positioning of myosin and actin for interaction and acts as a tether to the myosin S1 heads, limiting their mobility. This results in a decreased number of crossbridges formed, which hinders force generation, due to its N-terminal C1-M-C2 region interacting with the myosin-S2 domain. [25] [26] [27] [28] Furthermore, cMyBP-C contributes to the regulation of cardiac contraction at short sarcomere length and is required for complete relaxation in diastole. [19] [29]
Interactions of cMyBP-C with its binding partners vary with its posttranslational modification status. At least three extensively characterized phosphorylation sites (Ser273, 282 and 302; numbering refers to the mouse sequence) are localized in the M motif of cMyBP-C and are targeted by protein kinases in a hierarchical order of events. In its dephosphorylated state, cMyBP-C binds predominantly to myosin S2 and brakes crossbridge formation, however, when phosphorylated in response to β-adrenergic stimulation through activating cAMP-dependent protein kinase (PKA), it favours binding to actin, then accelerating crossbridge formation, enhancing force development and promoting relaxation. [30] Protein kinases identified thus far to phosphorylate cMyBP-C in the M motif are PKA, [31] [32] [33] [34] [35] Ca2+/calmodulin-dependent kinase II (CaMKII), [36] ribosomal s6 kinase (RSK), [37] protein kinase D (PKD), [38] [39] and protein kinase C (PKC). [34] Furthermore, GSK3β was described as another protein kinase to phosphorylate cMyBP-C outside the M-domain in the proline-alanine-rich actin-binding site at Ser133 in human myocardium (mouse Ser131). [40] Phosphorylation is required for normal cardiac function and cMyBP-C stability, [41] [42] and overall phosphorylation levels of cMyBP-C are reduced in human and experimental heart failure. [43] [44] Other posttranslational modifications of cMyBP-C exist, which occur throughout the protein and are not thoroughly characterised yet, such as acetylation, [45] citrullination, [46] S-glutathiolation, [47] [48] [49] [50] S-nitrosylation [51] and carbonylation. [52]
The cloning of the human MYBPC3 cDNA and localization of the gene on human chromosome 11p11.2 has assisted the structure and function of cMyBP-C. [53] MYBPC3 became therefore the “best” candidate gene for the CMH4 locus for hypertrophic cardiomyopathy that was initially mapped by the group of Schwartz. [54] MYBPC3 mutations segregating in families with hypertrophic cardiomyopathy have been identified. [55] [56] MYBPC3 was thus the fourth gene for hypertrophic cardiomyopathy, following MYH7, encoding β-myosin heavy chain, TNNT2 and TPM1, encoding cardiac troponin T and α-tropomyosin, respectively, earmarking hypertrophic cardiomyopathy (HCM) as a disease of the sarcomere. Truncation mutations in MYBPC3 stand as the primary cause of HCM. [57]
To date, roughly 350 mutations in MYBPC3 have been identified, and in large part, the mutations result in protein truncation, shifts in reading frames, and premature termination codons. [58] [59] Genetic studies have revealed significant overlap between genotypes and phenotypes as MYBPC3 mutations can lead to various forms of cardiomyopathies, such as dilated cardiomyopathy [60] and left ventricular noncompaction cardiomyopathy. [61] In patients with isolated or familial cases of dilated cardiomyoathy, MYBPC3 mutations represented the second highest number of known mutations. [60] Furthermore, a 25-bp intronic MYBPC3 deletion leading to protein truncation is present in 4% of the population in South India and is associated with a higher risk to develop heart failure. [62] Founder MYBPC3 mutations have been reported in Iceland, Italy, The Netherlands, Japan, France and Finland, where they represent a large percentage of cases with hypertrophic cardiomyopathy. All of them are truncating mutations, resulting in a shorter protein, lacking the regulatory phosphorylatable M motif and/or major binding domains to other sarcomeric proteins. [63] [64] [65] [66] [67] [68] [69] A body of evidence indicates that patients with more than 1 mutation often develop a more severe phenotype, [70] and a significant fraction of childhood-onset hypertrophic cardiomyopathy (14%) is caused by compound genetic variants. [71] This suggests that a gene-dosage effect might be responsible for manifestations at a younger age. A total of 51 cases of homozygotes or compound heterozygotes have been reported, most of them with double truncating MYBPC3 mutations and associated with severe cardiomyopathy, leading to heart failure and death within the first year of life. [72]
A great understanding of how MYBPC3 mutations lead to the development of inherited cardiomyopathy came from the analyses of human myocardial samples, gene transfer in different cell lines, naturally-occurring or transgenic animal models and more recently disease modeling using induced pluripotent stem cells (iPSC)-derived cardiac myocytes. [73] [74] Although access to human myocardial samples is difficult, at least some studies provided evidence that truncated cMyBP-Cs, resulting from truncating MYBPC3 mutations are not detectable in human patient samples by Western-immunoblot analysis. [75] [76] [77] [78] This was supported in heterozygous Mybpc3-targeted knock-in mice, [79] carrying the human c.772G>A transition (i.e. founder mutation in Tuscany [67] These data suggest haploinsufficiency as the main disease mechanism for heterozygous truncating mutations. [80] [81] A body of evidence exists that the mechanisms regulating the expression of mutant allele involve the nonsense-mediated mRNA decay, the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway after gene transfer of mutant MYBPC3 in cardiac myocytes or in mice in vivo. [82] [83] [79] [84] [85] [86] In contrast to truncating mutations, missense mutations lead, in most of the cases (although difficult to specifically detect), to stable mutant cMyBP-Cs that are, at least in part, incorporated into the sarcomere and could act as poison polypeptides on the structure and/or function of the sarcomere. Homozygous or compound heterozygous mutations are therefore likely subject to differential regulation depending on whether they are double missense, double truncating or mixed missense/truncating mutations. The homozygous Mybpc3-targeted knock-in mice, which genetically mimic the situation of severe neonatal cardiomyopathy are born without phenotype and soon after birth develop systolic dysfunction followed by (compensatory) cardiac hypertrophy. [87] [88] The human c.772G>A transition results in low levels of three different mutant Mybpc3 mRNAs and cMyBP-Cs in homozygous mice, suggesting a combination of haploinsufficiency and polypeptide poisoning as disease mechanism in the homozygous state. [79] In addition, the combination of external stress (such as neurohumoral stress or aging) and Mybpc3 mutations have been shown to impair the UPS in mice, [89] [90] and proteasomal activities were also depressed in patients with hypertrophic cardiomyopathy or dilated cardiomyopathy. [91]
Skinned trabeculae or cardiac myocytes obtained from human patients carrying a MYBPC3 mutation or from heterozygous and homozygous Mybpc3-targeted knock-in mice exhibited higher myofilament Ca2+ sensitivity than controls. [92] [78] [93] [94] [95] Disease-modeling by engineered heart tissue (EHT) technology with cardiac cells from heterozygous or homozygous Mybpc3-targeted knock-in mice reproduced observations made in human and mouse studies displaying abbreviated contractions, greater sensitivity to external Ca2+ and smaller inotropic responses to various drugs (isoprenaline, EMD 57033 and verapamil) compared to wild-type control EHTs. [96] Therefore, EHTs are suitable to model the disease phenotype and recapitulate functional alterations found in mice with hypertrophic cardiomyopathy. Another good system for modeling cardiomyopathies in the cell culture dish is the derivation of cardiac myocytes from iPSC. Reports of human iPSC models of sarcomeric cardiomyopathies showed cellular hypertrophy in most of the cases, [97] [98] [99] [100] including one with the c.2995_3010del MYBPC3 mutation that exhibited in addition to hypertrophy contractile variability in the presence of endothelin-1. [100]
Because of their tissue selectivity and persistent expression recombinant adeno-associated viruses (AAV) have therapeutic potential in the treatment of inherited cardiomyopathy resulting from MYBPC3 mutations- [101] Several targeting approaches have been developed. [102] [103] The most recent is genome editing to correct a mutation by CRISPR/Cas9 technology. [104] Naturally existing as part of the prokaryotic immune system, the CRISPR/Cas9 system has been used for correction of mutations in the mammalian genome. [105] By inducing nicks in the double-stranded DNA and providing a template DNA sequence, it is possible to repair mutations by homologous recombination. This approach has not yet been evaluated for MYBPC3 mutations, but it could be used for each single or clustered mutation, and therefore applied preferentially for frequent founder MYBPC3 mutations.
Other strategies targeting the mutant pre-mRNA by exon skipping and/or spliceosome-mediated RNA trans-splicing (SMaRT) have been evaluated for MYBPC3. Exon skipping can be achieved using antisense oligonucleotide (AON) masking exonic splicing enhancer sequences and therefore preventing binding of the splicing machinery and therefore resulting in exclusion of the exon from the mRNA. [106] [107] This approach can be applied when the resulting shorter, but in-frame translated protein maintains its function. Proof-of-concept of exon skipping was recently shown in homozygous Mybpc3-targeted knock-in mice. [87] Systemic administration of AAV-based AONs to Mybpc3-targeted knock-in newborn mice prevented both systolic dysfunction and left ventricular hypertrophy, at least for the duration of the investigated period. [87] For the human MYBPC3 gene, skipping of 6 single exons or 5 double exons with specific AONs would result in shortened in-frame cMyBP-Cs, allowing the preservation of the functionally important phosphorylation and protein interaction sites. With this approach, about half of missense or exonic/intronic truncating mutations could be removed, including 35 mutations in exon 25. The other strategy targeting the mutant pre-mRNA is SMaRT. Hereby, two independently transcribed molecules, the mutant pre-mRNA and the therapeutic pre-trans-splicing molecule carrying the wild-type sequence are spliced together to give rise to a repaired full-length mRNA. [108] Recently, the feasibility of this method was shown both in isolated cardiac myocytes and in vivo in the heart of homozygous Mybpc3-targeted knock-in mice, although the efficiency of the process was low and the amount of repaired protein was not sufficient to prevent the development of the cardiac disease phenotype. [88] In principle, however, this SmART strategy is superior to exon skipping or CRISPR/Cas9 genome editing and still attractive, because only two pre-trans-splicing molecules, targeting the 5’ and the 3’ of MYBPC3 pre-mRNA would be sufficient to bypass all MYBPC3 mutations associated with cardiomyopathies and therefore repair the mRNA.
AAV-mediated gene transfer of the full-length Mybpc3 (defined as “gene replacement”) dose-dependently prevents the development of cardiac hypertrophy and dysfunction in homozygous Mybpc3-targeted knock-in mice. [109] The dose-dependent expression of exogenous Mybpc3 was associated with the down-regulation of endogenous mutant Mybpc3. Additional expression of a sarcomeric protein is expected to replace partially or completely the endogenous protein level in the sarcomere, as it has been shown in transgenic mice expressing sarcomeric proteins. [73]
 | The 2015 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Lucie Carrier, Giulia Mearini, Konstantina Stathopoulou, Friederike Cuello (7 September 2015). "Cardiac myosin-binding protein C (MYBPC3) in cardiac pathophysiology". Gene . Gene Wiki Review Series. 573 (2): 188–197. doi:10.1016/J.GENE.2015.09.008. ISSN 0378-1119. PMC 6660134 . PMID 26358504. Wikidata Q38584470. |
Hypertrophic cardiomyopathy is a condition in which muscle tissues of the heart become thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ventricles. This results in the heart being less able to pump blood effectively and also may cause electrical conduction problems. Specifically, within the bundle branches that conduct impulses through the interventricular septum and into the Purkinje fibers, as these are responsible for the depolarization of contractile cells of both ventricles.
Titin is a protein that in humans is encoded by the TTN gene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring that is responsible for the passive elasticity of muscle. It comprises 244 individually folded protein domains connected by unstructured peptide sequences. These domains unfold when the protein is stretched and refold when the tension is removed.
MYH7 is a gene encoding a myosin heavy chain beta (MHC-β) isoform expressed primarily in the heart, but also in skeletal muscles. This isoform is distinct from the fast isoform of cardiac myosin heavy chain, MYH6, referred to as MHC-α. MHC-β is the major protein comprising the thick filament that forms the sarcomeres in cardiac muscle and plays a major role in cardiac muscle contraction.
Troponin I, cardiac muscle is a protein that in humans is encoded by the TNNI3 gene. It is a tissue-specific subtype of troponin I, which in turn is a part of the troponin complex.
Cardiac muscle troponin T (cTnT) is a protein that in humans is encoded by the TNNT2 gene. Cardiac TnT is the tropomyosin-binding subunit of the troponin complex, which is located on the thin filament of striated muscles and regulates muscle contraction in response to alterations in intracellular calcium ion concentration.
Ryanodine receptor 2 (RYR2) is one of a class of ryanodine receptors and a protein found primarily in cardiac muscle. In humans, it is encoded by the RYR2 gene. In the process of cardiac calcium-induced calcium release, RYR2 is the major mediator for sarcoplasmic release of stored calcium ions.
Tropomyosin alpha-1 chain is a protein that in humans is encoded by the TPM1 gene. This gene is a member of the tropomyosin (Tm) family of highly conserved, widely distributed actin-binding proteins involved in the contractile system of striated and smooth muscles and the cytoskeleton of non-muscle cells.
ACTC1 encodes cardiac muscle alpha actin. This isoform differs from the alpha actin that is expressed in skeletal muscle, ACTA1. Alpha cardiac actin is the major protein of the thin filament in cardiac sarcomeres, which are responsible for muscle contraction and generation of force to support the pump function of the heart.
Troponin C, also known as TN-C or TnC, is a protein that resides in the troponin complex on actin thin filaments of striated muscle and is responsible for binding calcium to activate muscle contraction. Troponin C is encoded by the TNNC1 gene in humans for both cardiac and slow skeletal muscle. In slow skeletal muscle. structural analysis,anlaizie;10.164.138.220 Hotspot in for phone lunch everyday. Troponin C, also known as TN-C or TnC, is a protein that resides in the troponin complex on actin thin filaments of striated muscle and is responsible for binding
Telethonin, also known as Tcap, is a protein that in humans is encoded by the TCAP gene. Telethonin is expressed in cardiac and skeletal muscle at Z-discs and functions to regulate sarcomere assembly, T-tubule function and apoptosis. Telethonin has been implicated in several diseases, including limb-girdle muscular dystrophy, hypertrophic cardiomyopathy, dilated cardiomyopathy and idiopathic cardiomyopathy.
Myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC-2) also known as the regulatory light chain of myosin (RLC) is a protein that in humans is encoded by the MYL2 gene. This cardiac ventricular RLC isoform is distinct from that expressed in skeletal muscle (MYLPF), smooth muscle (MYL12B) and cardiac atrial muscle (MYL7).
Myosin heavy chain, α isoform (MHC-α) is a protein that in humans is encoded by the MYH6 gene. This isoform is distinct from the ventricular/slow myosin heavy chain isoform, MYH7, referred to as MHC-β. MHC-α isoform is expressed predominantly in human cardiac atria, exhibiting only minor expression in human cardiac ventricles. It is the major protein comprising the cardiac muscle thick filament, and functions in cardiac muscle contraction. Mutations in MYH6 have been associated with late-onset hypertrophic cardiomyopathy, atrial septal defects and sick sinus syndrome.
Myosin essential light chain (ELC), ventricular/cardiac isoform is a protein that in humans is encoded by the MYL3 gene. This cardiac ventricular/slow skeletal ELC isoform is distinct from that expressed in fast skeletal muscle (MYL1) and cardiac atrial muscle (MYL4). Ventricular ELC is part of the myosin molecule and is important in modulating cardiac muscle contraction.
Atrial Light Chain-1 (ALC-1), also known as Essential Light Chain, Atrial is a protein that in humans is encoded by the MYL4 gene. ALC-1 is expressed in fetal cardiac ventricular and fetal skeletal muscle, as well as fetal and adult cardiac atrial tissue. ALC-1 expression is reactivated in human ventricular myocardium in various cardiac muscle diseases, including hypertrophic cardiomyopathy, dilated cardiomyopathy, ischemic cardiomyopathy and congenital heart diseases.
Obscurin is a protein that in humans is encoded by the OBSCN gene. Obscurin belongs to the family of giant sarcomeric signaling proteins that includes titin and nebulin. Obscurin is expressed in cardiac and skeletal muscle, and plays a role in the organization of myofibrils during sarcomere assembly. A mutation in the OBSCN gene has been associated with hypertrophic cardiomyopathy and altered obscurin protein properties have been associated with other muscle diseases.
Ankyrin repeat domain-containing protein 1, or Cardiac ankyrin repeat protein is a protein that in humans is encoded by the ANKRD1 gene also known as CARP. CARP is highly expressed in cardiac and skeletal muscle, and is a transcription factor involved in development and under conditions of stress. CARP has been implicated in several diseases, including dilated cardiomyopathy, hypertrophic cardiomyopathy, and several skeletal muscle myopathies.
Cysteine and glycine-rich protein 3 also known as cardiac LIM protein (CLP) or muscle LIM protein (MLP) is a protein that in humans is encoded by the CSRP3 gene.
Myozenin-2, also referred to as Calsarcin-1, is a protein that in humans is encoded by the MYOZ2 gene. The Calsarcin-1 isoform is a muscle protein expressed in cardiac muscle and slow-twitch skeletal muscle, which functions to tether calcineurin to alpha-actinin at Z-discs, and inhibit the pathological cardiac hypertrophic response. This differs from the fast-skeletal muscle isoform, calsarcin-2.
Atrial Light Chain-2 (ALC-2) also known as Myosin regulatory light chain 2, atrial isoform (MLC2a) is a protein that in humans is encoded by the MYL7 gene. ALC-2 expression is restricted to cardiac muscle atria in healthy individuals, where it functions to modulate cardiac development and contractility. In human diseases, including hypertrophic cardiomyopathy, dilated cardiomyopathy, ischemic cardiomyopathy and others, ALC-2 expression is altered.
Myosin binding protein C, fast type is a protein that in humans is encoded by the MYBPC2 gene.
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