Myopathic EDS (mEDS) is characterized by muscle weakness that manifests in childhood with proximal contractures of the large joints and distal joint hypermobility. Typically, muscle weakness improves in young adulthood, although it may deteriorate again in the fourth decade of life. The spectrum of disorders characterized by muscle weakness, hypotonia, myopathy, and connective tissue symptomatology was originally associated with collagen type VI disorders.
Pathogenic variants in COL12A1, which encodes the α1-chain of type XII collagen, are the molecular cause of mEDS. Collagen XII is present as a homotrimer on the surface of type I collagen fibers. It forms a link between type I collagen fibers and extracellular matrix components such as decorin, fibromodulin, and tenascin. To date, mEDS has been described in both an autosomal dominantly inherited form with heterozygous missense variants and an autosomal recessively inherited form with homozygous frameshift variants.
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