Tropomyosin Congenital Myopathy (TCM) is an early-onset muscular disorder due to mutations on TPM2 and TPM3 genes encoding β-tropomyosin and γ-tropomyosin proteins that are highly expressed in skeletal muscles and play a critical role in regulating the functions of actin filaments. Mutations in TPM2 and TPM3 result in at least six diseases (or conditions) defined by muscle biopsy or clinical evaluation including nemaline myopathy, core-rod myopathy, cap disease, congenital fiber-type disproportion, distal arthrogryposis and Escobar syndrome. Around seventy different mutations, mainly dominant missense, have been identified in patients who present an undefined heterogeneous spectrum of clinical manifestations from mild to severe cases. To date, there is no cure for this extremely rare muscle disease.
We studied a Tpm3 K6X zebrafish carrying a nonsense mutation in the exon 1a of tpm3 that leads to ablation of tpm3.6 and tpm3.12 isoform expressions. Tpm3 K6X fish manifest clear hypotonia at an early development stage since stimulated swimming performance was significantly decreased at 5 days post-fertilization. Further experiments showed persistent muscle weakness at adult stages. Tpm3 K6X adult fish presented a symptomatic body shape including elongated face and constant opened mouth. Electron microscopy displayed numerous severe myopathic features, including atrophied myofibers, breakdown of the sarcomeres, alteration of the mitochondrial network, lobulated and intratubular structures in the slow muscle fibers exclusively. Tpm3 K6X fish died prematurely at up to one year of life.
This tpm3 zebrafish model recapitulates some of the muscle features seen in a patient carrying TPM3 Q31X from a nonsense mutation in the exon 1a (Tan et al., 1999, Neuromuscul Disord). Motor development of the patient was extremely delayed and impaired. Muscle biopsy showed an exclusive alteration of slow-twitch muscle tissue. The patient died prematurely at the age of 21 months.
We aim to generate additional TCM zebrafish models to recapitulate features of the human condition and consider therapeutics by developing an in vivo phenotype-based drug screening assay.