C.P.9 A novel dynamin-2 gene mutation associated with a late-onset centronuclear myopathy with unusual clinical presentation and necklace fibres

Abstract

Nuclear centralisation and internalisation, sarcoplasmic radiating strands and type 1 muscle fibre predominance and hypotrophy are morphologic features of centronuclear myopathy (CNM) related to dynamin-2 (DNM2) gene defects, whereas necklace fibres characterise late-onset myopathy associated with myotubularin-1 (MTM1) gene defects. We report a 40-year-old woman with 1-year history of pain and paresthesia in the left shoulder and arm that was clinically interpreted as brachial plexus neuritis. Electromyography revealed both myopathic and neuropathic abnormalities, and because of the myopathic changes a muscle biopsy was performed. The typical morphologic features of dynamin-2 CNM with additional numerous necklace fibres were found in the muscle biopsy. Sequencing of the DNM2 and MTM1 genes revealed a not previously described heterozygous missense mutation in exon 18 of DNM2 leading to replacement of highly conserved Proline in position 647 by Arginine. The muscle symptoms have not progressed during the two-year follow-up, but the patient has developed bilateral subtle lens opacities. Necklace fibres were originally described as fibres that had usually a small diameter and internalized nuclei aligned in a basophilic ring at a few micrometers beneath the sarcolemma. They were described in association with myopathies caused by MTM1 mutations, and similar but not identical fibres have also been reported in a case of DNM2 associated CNM. Our findings support the concept that necklace fibres are not specific but indicate common pathogenic mechanisms in DNM2 and MTM1 associated CNM. This case report expands the clinical, morphological and molecular genetic variability of DNM2 associated CNM.