Sarcomeric deficits underlie MYBPC1-associated myopathy with myogenic tremor
Janelle Geist Hauserman, … , Christopher Ward, Aikaterini Kontrogianni-Konstantopoulos
Janelle Geist Hauserman, … , Christopher Ward, Aikaterini Kontrogianni-Konstantopoulos
Published August 26, 2021
Citation Information: JCI Insight. 2021;6(19):e147612. https://doi.org/10.1172/jci.insight.147612.
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Research Article Cell biology Muscle biology

Sarcomeric deficits underlie MYBPC1-associated myopathy with myogenic tremor

Abstract

Myosin binding protein-C slow (sMyBP-C) comprises a subfamily of cytoskeletal proteins encoded by MYBPC1 that is expressed in skeletal muscles where it contributes to myosin thick filament stabilization and actomyosin cross-bridge regulation. Recently, our group described the causal association of dominant missense pathogenic variants in MYBPC1 with an early-onset myopathy characterized by generalized muscle weakness, hypotonia, dysmorphia, skeletal deformities, and myogenic tremor, occurring in the absence of neuropathy. To mechanistically interrogate the etiologies of this MYBPC1-associated myopathy in vivo, we generated a knock-in mouse model carrying the E248K pathogenic variant. Using a battery of phenotypic, behavioral, and physiological measurements spanning neonatal to young adult life, we found that heterozygous E248K mice faithfully recapitulated the onset and progression of generalized myopathy, tremor occurrence, and skeletal deformities seen in human carriers. Moreover, using a combination of biochemical, ultrastructural, and contractile assessments at the level of the tissue, cell, and myofilaments, we show that the loss-of-function phenotype observed in mutant muscles is primarily driven by disordered and misaligned sarcomeres containing fragmented and out-of-register internal membranes that result in reduced force production and tremor initiation. Collectively, our findings provide mechanistic insights underscoring the E248K-disease pathogenesis and offer a relevant preclinical model for therapeutic discovery.

Authors

Janelle Geist Hauserman, Janis Stavusis, Humberto C. Joca, Joel C. Robinett, Laurin Hanft, Jack Vandermeulen, Runchen Zhao, Joseph P. Stains, Konstantinos Konstantopoulos, Kerry S. McDonald, Christopher Ward, Aikaterini Kontrogianni-Konstantopoulos

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Figure 7

Structural evaluation of heterozygous KI soleus and diaphragm muscles at 4 weeks of age.

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Structural evaluation of heterozygous KI soleus and diaphragm muscles at...
(A) sMyBP-C exhibited its expected localization at the C-zone of the A-band in WT and heterozygous KI soleus muscles; however, similar to 4-week heterozygous KI EDL muscle, bundles of split myofibrils and misaligned sarcomeres were observed in the latter; n = 4 WT (2 male and 2 female) and n = 4 heterozygous KI (2 male and 2 female). Scale bar: 5 µm. (B) Electron micrographs of longitudinal sections of WT soleus muscles revealed the presence of structurally intact and registered sarcomeres with orderly internal membranes forming typical triads (open arrowheads) at the level of A/I junctions. In contrast, heterozygous KI soleus and diaphragm muscles displayed less compact Z-discs and frequent Z-disc streaming; hazy A-, I-, and M-bands; enlarged and more abundant mitochondria (asterisks); and fragmented and/or misaligned internal membranes (closed arrowheads); n = 4 WT (2 male and 2 female) and n = 4 heterozygous KI (2 male and 2 female). Scale bar: 500 nm. (C and D) Cross-sectional electron micrographs of WT and heterozygous KI soleus muscles contained typical hexagonal arrays of myosin filaments (C); however, both the thick filament CSA and interfilament distance were significantly reduced as quantified by FFT analysis (D); n = 2 WT (2 male) and n = 2 heterozygous KI (2 male). Scale bar: 100 nm. Two electron micrographs were analyzed per muscle by quantifying 3 randomly selected regions per micrograph including > 100 myofilaments. (E) Electron micrographs of longitudinal sections of WT and heterozygous KI diaphragm muscles showed similar structural alterations as those seen in heterozygous KI EDL and soleus muscles; n = 4 WT (2 male and 2 female) and n = 4 heterozygous KI (2 male and 2 female). Scale bar: 500 nm. Statistical significance was calculated with unpaired t test (D; area) or Mann Whitney U test (D; distance). #P < 0.05, *P < 0.01, ****P < 0.0001.