Hypertrophic cardiomyopathy-linked Tnnc1 variants are associated with distinct myopathic changes in slow skeletal muscle of mice

Inherited hypertrophic cardiomyopathy (HCM) is considered a disease of the cardiac sarcomere and caused by pathogenic variants present in genes that encode sarcomeric proteins. The human TNNC1 gene is a designated HCM-susceptibility gene encoding the troponin C (TnC) protein, which is expressed in both cardiac and type I slow skeletal muscles and abbreviated as cTnC and ssTnC respectively. HCM patients have been reported to exhibit skeletal muscle weakness and reduced exercise tolerance. Patients bearing TNNC1 cardiac pathogenic variants also express it in type I fibers of their slow skeletal muscles. We hypothesized that the presence of TNNC1 HCM variants in type I fibers may decrease force generating capabilities and alter fatigue resistance of slow skeletal muscles. To address this, we examined the impact of HCM Tnnc1 variants in the soleus muscles of two HCM knock-in mouse models A8V+/-, A8V-/-, and C84Y+/- and their respective wild type (WT) controls. At high stimulation frequencies, we found that A8V-/- soleus muscles had lower tetanic force production than WT, however no differences in fatigue when comparing fatigue indices for either of the variants. The muscles were evaluated by comparing % specific force (N/cm 2 )-variant and both A8V-/- and C84+/- had increased responsiveness at low stimulation frequencies. Fiber type analysis uncovered an increase in abundance of type I fibers in A8V-/- soleus, however no other differences were observed in fiber-type percentage of either mouse model. Comparison of the cross-sectional areas (CSA) of type I fibers in soleus muscles revealed lower average values for A8V+/+ and A8V-/-, however an increase in C84Y+/- relative to controls. Examination of the CSA of type IIa fibers uncovered decreased values for both A8V+/- and A8V-/- with no changes in C84Y+/- soleus. Histological evaluation of A8V+/- and A8V-/- soleus muscles revealed central nucleation and the detection of embryonic myosin heavy chain by immunofluorescence suggested the presence of mild regeneration. In contrast, no histopathological changes were detected in the C84Y+/- soleus muscle. Serum myokines were also measured to assess systemic impacts of the pathogenic variants and alterations in the physical activity of the mice but no significant changes were found. Taken together our results suggest that the HCM A8V variant alters force production in soleus muscle that may be attributed to fiber atrophy and heightened pathophysiology.