Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium
Martijn van de Locht, … , Carsten G. Bönnemann, Coen A.C. Ottenheijm
Martijn van de Locht, … , Carsten G. Bönnemann, Coen A.C. Ottenheijm
Published March 23, 2021
Citation Information: J Clin Invest. 2021;131(9):e145700. https://doi.org/10.1172/JCI145700.
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Research Article Genetics Muscle biology

Pathogenic variants in TNNC2 cause congenital myopathy due to an impaired force response to calcium

Abstract

Troponin C (TnC) is a critical regulator of skeletal muscle contraction; it binds Ca2+ to activate muscle contraction. Surprisingly, the gene encoding fast skeletal TnC (TNNC2) has not yet been implicated in muscle disease. Here, we report 2 families with pathogenic variants in TNNC2. Patients present with a distinct, dominantly inherited congenital muscle disease. Molecular dynamics simulations suggested that the pathomechanisms by which the variants cause muscle disease include disruption of the binding sites for Ca2+ and for troponin I. In line with these findings, physiological studies in myofibers isolated from patients’ biopsies revealed a markedly reduced force response of the sarcomeres to [Ca2+]. This pathomechanism was further confirmed in experiments in which contractile dysfunction was evoked by replacing TnC in myofibers from healthy control subjects with recombinant, mutant TnC. Conversely, the contractile dysfunction of myofibers from patients was repaired by replacing endogenous, mutant TnC with recombinant, wild-type TnC. Finally, we tested the therapeutic potential of the fast skeletal muscle troponin activator tirasemtiv in patients’ myofibers and showed that the contractile dysfunction was repaired. Thus, our data reveal that pathogenic variants in TNNC2 cause congenital muscle disease, and they provide therapeutic angles to repair muscle contractility.

Authors

Martijn van de Locht, Sandra Donkervoort, Josine M. de Winter, Stefan Conijn, Leon Begthel, Benno Kusters, Payam Mohassel, Ying Hu, Livija Medne, Colin Quinn, Steven A. Moore, A. Reghan Foley, Gwimoon Seo, Darren T. Hwee, Fady I. Malik, Thomas Irving, Weikang Ma, Henk L. Granzier, Erik-Jan Kamsteeg, Kalyan Immadisetty, Peter Kekenes-Huskey, José R. Pinto, Nicol Voermans, Carsten G. Bönnemann, Coen A.C. Ottenheijm

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Figure 4

Results of the molecular dynamics simulations.

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Results of the molecular dynamics simulations. (A and B) Superposition o...
(A and B) Superposition of N-domains of MD-predicted apo (A) and holo (B) variants. WT, D34Y, and M79I variants are colored purple, green, and orange, respectively. Helices N (residues 2–11), A (residues 14–27), B (residues 40–48), C (residues 54–63), and D (residues 74–85) of the N-domain are labeled. Protein oxygens within 2.5 Å of Ca2+ are shown as spheres and their respective residues as sticks. (C and D) Root mean squared fluctuations (RMSFs) of N-terminal domain residues. Holo and apo systems are represented as lines and broken lines, respectively. WT is compared against D34Y and M79I in C and D, respectively. Shaded regions reflect standard deviations. (E–H) Radial distribution of protein (E and G) and solvent (F and H) oxygens around bound Ca2+ ions in the fsTnC N-terminal domains from the final 50 ns of each trajectory. gPO(r) and gWO(r) are the radial distribution functions of the protein and water oxygen atoms around the bound calcium, respectively. N:CA1, N-terminal calcium-binding pocket 1; N:CA2, N-terminal calcium-binding pocket 2. (I and J) Principal component (PC) analysis of the fsTnC2 N-domain MD trajectory data (J). Square displacements signify the relative contribution of each amino acid to PC1 (solid) or PC2 (dashed) (K). PC1 reflects the displacement of helices B–D, while PC2 corresponds to the loop connecting the C and D helices. These PCs demonstrate that the apo structures exhibit different displacements, or conformations, than the holo structures, and the D34Y variant samples a very different conformation than the WT and the M79I variants. (K) Cartoon diagram of the MD-predicted structures for holo WT-fsTnC (purple), holo M79I-fsTnC (orange), and holo cTnC with switch peptide (cyan). The structures of WT- and M79I-fsTnC are from the MD simulations; for cTnC the PDB 1MXL structure was used. Residues that are within 4 Å of the TnI switch peptide in cTnC are shown as cyan sticks, and residues within 4 Å of M79/I79 in holo M79I-fsTnC are shown as orange sticks. Residues that overlap between the 2 are shown as thick sticks. M79/I79 is shown in ball and stick representation.