NEW GENES AND DISEASES: EP.301 The alpha2-subunit of the AP2 clathrin adaptor as the causal gene in an atypical myopathy with granulofilamentous inclusions

Abstract

Clathrin-mediated endocytosis is the best characterized process for the entry of proteins and lipids at the plasma membrane of eukaryotic cells. We have previously shown that at costameres, muscle adhesion and force transmission sites, clathrin recruited by the AP2 adaptor forms flat plaques required for actin and desmin intermediate filament organization. Here, we describe a patient presenting a progressive muscular atrophy with loss of ambulation, complete tetraplegia, ophthalmoplegia, severe scoliosis and lower limb contractures and no cardiac involvement. Electron microscopy of muscle biopsy revealed inclusions composed of filamentous material, vacuoles, honeycomb structures and thin granular material surcharge. This case was classified as a myofibrillar myopathy with granulofilamentous inclusion bodies despite no mutation in DES, FHL1, CRYAB and other usual suspect genes. Whole exome sequencing identified two compound heterozygote single-nucleotide variants in the AP2A2 gene encoding the AP2 a2-subunit. Immunofluorescent labeling on muscle sections displayed intracellular aggregates positive for AP2, clathrin and desmin. Depletion of total AP2 in mouse muscle induced a dystrophic phenotype with severe force loss, subsarcolemmal disorganization, vacuoles and neuromuscular junction defects while specifically depleting AP2 a2-subunit induced small subsarcolemmal disorganization and light neuromuscular junction defects. Lastly, transferrin uptake assays of endocytosis function in the patient fibroblasts showed a decrease in endocytosis rates compared to fibroblasts from his healthy parents. Altogether, our results suggest that AP2A2 could be the causal gene in this family and AP2A2 mutations leading to a severe but atypical myopathy. Mutations in AP2A2 could represent the first case in a wider family of atypical myopathies with desmin inclusions caused by mutations in genes encoding proteins of the endocytosis machinery.