Potential cytotoxicity of truncated slow skeletal muscle troponin T (ssTnT) in a loss of function TNNT1 myopathy mouse model.

A nonsense mutation in codon Glu₁₈₀ of the TNNT1 gene, which encodes the slow skeletal muscle isoform of troponin T (ssTnT), causes a recessively inherited myopathy (the Amish Nemaline Myopathy, ANM). A ssTnT knockout (ssTnT-KO) mouse model produced the loss of ssTnT function phenotypes of ANM with slow fiber atrophy and decreased fatigue resistance of soleus muscle. We further developed a Tnnt1 p.Glu180* knock-in (ANM-KI) mouse model to precisely mimic the human mutation. In addition to reproducing the loss of function phenotypes, ANM-KI mice exhibit more severe myopathy than that of ssTnT-KO mice. Compared with wild-type controls, ANM-KI and ssTnT-KO soleus muscles show different changes in gene expression profiles, of which gene ontology analysis indicated inflammatory activation in ANM-KI soleus muscle. The mutant Tnnt1 mRNA was readily detectable in ANM-KI soleus muscle. However, the truncated ssTnT_1-179 fragment cannot be detected in western blot, indicating its very low level due to the active proteolytic clearance of non-myofilament-incorporated TnT in muscle cells. Nonetheless, the more severe myopathic impacts of the ANM-KI allele with more fiber number loss and muscle activity/injury-caused hypertrophy support a potent cytotoxicity of the ssTnT fragment, as shown in previous cell culture studies, which is further supported by activity-dependent and age-progressing myopathy with more active regeneration. The notion that non-myofilament-incorporated ssTnT fragments may potentially contribute to the pathogenesis and progression of myopathy merits further investigation.