Cardiac myosin-binding protein C (MYBPC3) in cardiac pathophysiology

Abstract

More than 350 individual MYPBC3 mutations have been identified in patients with inherited hypertrophic cardiomyopathy (HCM), thus representing 40–50% of all HCM mutations, making it the most frequently mutated gene in HCM. HCM is considered a disease of the sarcomere and is characterized by left ventricular hypertrophy, myocyte disarray and diastolic dysfunction. MYBPC3 encodes for the thick filament associated protein cardiac myosin-binding protein C (cMyBP-C), a signaling node in cardiac myocytes that contributes to the maintenance of sarcomeric structure and regulation of contraction and relaxation. This review aims to provide a succinct overview of how mutations in MYBPC3 are considered to affect the physiological function of cMyBP-C, thus causing the deleterious consequences observed inHCM patients. Importantly, recent advances to causally treat HCM by repairing MYBPC3 mutations by gene therapy are discussed here, providing a promising alternative to heart transplantation for patients with a fatal form of neonatal cardiomyopathy due to bi-allelic truncating MYBPC3 mutations.

Keywords: Cardiac myosin-binding protein C; Cardiomyopathies; Gene therapy; Hypertrophy postranslational modifications; MYBPC3.

Figure 1.. Schematic representation of MYBPC3 gene, mRNA and protein structure, its interaction partners (interactome) and sites of posttranslational modifications (PTMs).

MYBPC3 encompasses 21 kbp and is composed of 35 exons, which is transcribed into a 3824-bp transcript. cMyBP-C is a multi-modular protein composed of 8 immunoglobulin-like (C0, C1, C2, C3, C4, C5, C8, C10) and 3 fibronectin-type III (C6, C7, C9) domains. The cardiac isoform differs from the slow-skeletal and the fast-skeletal isoforms by cardiac-specific regions (C0, M, 28-amino acid insertion in C5) that are highlighted in yellow. Between C0 and C1 exists a proline-alanine rich domain (Pro-Ala; PA). The linker between C4 and C5 domains is indicated by a black oval. Anchoring of cMyBP-C to the C-zones of sarcomeric A-bands is mediated by domains C6 to C10. The NH₂-terminus of cMyBP-C interacts with F-actin (C0-M) and the myosin-S2 domain (C1-M-C2). The COOH-terminus of cMyBP-C interacts with members of the four-and-a-half-LIM-domain family (C6-C10), titin (C8-C10) and light meromyosin (LMM; C10). cMyBP-C is subject to a variety of PTMs that are depicted with different coloured diamonds for acetylation (purple), phosphorylation (blue), calpain cleavage (red), S-glutathiolation (yellow), citrullination (orange), and S-nitrosylation (green). Involved protein kinases are shown below the symbols. Abbreviations used: CaMK, Ca²⁺/calmodulin dependent protein kinase; CK2, casein kinase 2; GSK3β, glycogen-synthase kinase isoform 3β; M, MyBP-C motif; PKA, cAMP-dependent protein kinase; M, MyBP-C motif; PKD, protein kinase D; PKC, protein kinase C; RNS, reactive nitrogen species; ROS, reactive oxygen species; RSK, p90 ribosomal S6 kinase. Numbering of amino acids refers to the mouse sequence.

Figure 2.. Schematic diagram depicting sarcomeric localization of cMyBP-C.

cMyBP-C (dark blue) localization is restricted to the C-zones of sarcomeric A-bands in 7–9 transverse stripes at both sites of the M-line. Thin filament (light blue), thick filament (middle blue), titin (orange).

Figure 3.. Schematic representation of MYBPC3 gene, mutations and options for causal therapy.

MYBPC3 encompasses 21 kbp and is composed of 35 exons. Exonic missense mutations (blue, total 129) and exonic and intronic truncating mutations (red, total 202) are shown on top of each exon. Causal therapies are shown as a reverse pyramide: i) CRISPR/Cas9 targeting single or a few mutations (purple dotted line), ii) exon skipping targeting all mutations present in one or two exons and associated introns (dark grey), iii) trans-splicing targeting either all 5’ mutations (exons 1 to 21; left yellow box) or 3’ mutations (exons 22 to 35; right yellow box), and finally iv) gene replacement targeting all mutations at once by gene transfer of the full-length MYBPC3 cDNA (green box). Abbreviations used: CRISPR/Cas9, clustered regularly interspaced short palindromic repeat-associated system. Number of mutations are taken from (Behrens-Gawlik et al., 2014).