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Cap myopathy is a congenital myopathy characterized by peripheral, well-demarcated ‘caps’ of disorganized thin filaments in muscle fibers. The beta-tropomyosin gene TPM2 was first implicated as the genetic cause in an autosomal dominant cap myopathy patient with a heterozygous in-frame deletion p.Glu139del (PMID:19345583). A subsequent multi-patient study confirmed TPM2 variants in three unrelated individuals with similar histopathology and early-onset muscle weakness (PMID:19047562).
Genetic evidence supports an autosomal dominant inheritance mode with five unrelated probands (PMID:19345583; PMID:19047562). All variants occurred de novo or segregated with disease, and no additional affected relatives were reported. Case series have described in-frame deletions (p.Lys49del, p.Gly52dup, p.Glu139del) and a missense substitution (p.Asn202Lys), indicating a spectrum of dominant-negative and structure-altering alleles.
Functional assays demonstrate that aberrant β-tropomyosins incorporate into sarcomeres and disrupt normal thin filament assembly. In vitro studies revealed altered actin-binding affinity and impaired calcium-sensitive regulation (PMID:22084935; PMID:15562513). Animal and cellular models, including Drosophila and zebrafish myogenesis assays, recapitulate muscle developmental defects and contractile dysfunction.
The predominant pathogenic mechanism is a dominant-negative effect of mutant tropomyosin on sarcomeric thin filament dynamics, leading to cap formation and muscle weakness. Concordant histopathological and functional data reinforce the causal link between TPM2 variants and cap myopathy.
Together, the genetic and experimental findings establish a strong gene-disease relationship for TPM2 in Cap Myopathy. Molecular testing of TPM2 is clinically indicated for patients with suggestive histology and congenital muscle weakness. Key Take-home: Autosomal dominant TPM2 variants are a well-validated cause of cap myopathy and should be included in congenital myopathy diagnostic panels.
Gene–Disease AssociationStrong5 unrelated probands with de novo heterozygous TPM2 variants, functional evidence concordant Genetic EvidenceStrong5 probands with heterozygous in-frame deletions and missense variants occurring de novo and segregating in families, autosomal dominant inheritance Functional EvidenceModerateMultiple in vitro and in vivo studies demonstrate altered actin binding and Ca2+-sensitivity consistent with dominant-negative mechanism |