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

Katarzyna Robaszkiewicz, Małgorzata Siatkowska, Renske I Wadman, Erik-Jan Kamsteeg, Zhiyong Chen, Ashirwad Merve, Matthew Parton, Enrico Bugiardini, Charlotte de Bie, Joanna Moraczewska*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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Abstract

A novel variant of unknown significance c.8A > G (p.Glu3Gly) in TPM3 was detected in two unrelated families. TPM3 encodes the transcript variant Tpm3.12 (NM_152263.4), the tropomyosin isoform specifically expressed in slow skeletal muscle fibers. The patients presented with slowly progressive muscle weakness associated with Achilles tendon contractures of early childhood onset. Histopathology revealed features consistent with a nemaline rod myopathy. Biochemical in vitro assays performed with reconstituted thin filaments revealed defects in the assembly of the thin filament and regulation of actin-myosin interactions. The substitution p.Glu3Gly increased polymerization of Tpm3.12, but did not significantly change its affinity to actin alone. Affinity of Tpm3.12 to actin in the presence of troponin ± Ca2+ was decreased by the mutation, which was due to reduced interactions with troponin. Altered molecular interactions affected Ca2+-dependent regulation of the thin filament interactions with myosin, resulting in increased Ca2+ sensitivity and decreased relaxation of the actin-activated myosin ATPase activity. The hypercontractile molecular phenotype probably explains the distal joint contractions observed in the patients, but additional research is needed to explain the relatively mild severity of the contractures. The slowly progressive muscle weakness is most likely caused by the lack of relaxation and prolonged contractions which cause muscle wasting. This work provides evidence for the pathogenicity of the TPM3 c.8A > G variant, which allows for its classification as (likely) pathogenic.

Original languageEnglish
Article number16147
JournalInternational Journal of Molecular Sciences
Volume24
Issue number22
DOIs
Publication statusPublished - 9 Nov 2023

Keywords

  • Actins/genetics
  • Child, Preschool
  • Contracture/pathology
  • Humans
  • Muscle Weakness/genetics
  • Muscle, Skeletal/pathology
  • Mutation
  • Myopathies, Nemaline/genetics
  • Myosins/genetics
  • Phenotype
  • Tropomyosin/genetics
  • Troponin/genetics

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