A Novel Variant in TPM3 Causing Muscle Weakness and Concomitant Hypercontractile Phenotype

Area of Science:

• Genetics
• Molecular Biology
• Neuromuscular Disorders

Background:

• Nemaline rod myopathy is a muscle disorder characterized by muscle weakness and the presence of rod-like inclusions in muscle fibers.
• The TPM3 gene encodes Tpm3.12, a tropomyosin isoform crucial for slow skeletal muscle function.
• A novel variant, c.8A > G (p.Glu3Gly) in TPM3, was identified in patients with early-onset muscle weakness and contractures.

Purpose of the Study:

• To investigate the pathogenicity of the novel TPM3 c.8A > G variant.
• To elucidate the molecular mechanisms underlying the observed muscle weakness and contractures.

Main Methods:

• Genetic analysis to identify the TPM3 variant in affected families.
• Histopathological examination of muscle biopsies.
• Biochemical in vitro assays using reconstituted thin filaments to assess molecular interactions.

Main Results:

• The TPM3 c.8A > G variant leads to increased polymerization of Tpm3.12 and altered interactions with actin and troponin.
• The mutation impairs Ca2+-dependent regulation of actin-myosin interactions, causing increased Ca2+ sensitivity and reduced relaxation.
• These molecular defects likely explain the hypercontractile phenotype, contributing to muscle weakness and joint contractures.

Conclusions:

• The study provides strong evidence for the pathogenicity of the TPM3 c.8A > G variant.
• The findings support the classification of this variant as likely pathogenic, aiding in the diagnosis of nemaline rod myopathy.
• Further research is needed to fully understand the genotype-phenotype correlations and the mild severity of contractures.