C.O.2 DNM2 mutations cause multiple mtDNA deletions in muscle: A novel disorder of mtDNA maintenance

• Published in: Neuromuscular disorders : NMD 2012-10, Vol.22 (9), p.839-839
• Main Authors: Krishnan, K.J, Nelson, G, Romero, N.B, Ratnaike, T, Blakely, E.L, Ziyadeh-Isleem, A, Miller, J, Murphy, J.L, Horvath, R, Lochmuller, H, Flanigan, K, Turnbull, D.M, Guicheney, P, Bitoun, M, Taylor, R.W
• Format: Article
• Language: English
• Subjects: Animal models; Biopsy; Confocal microscopy; Cytochrome-c oxidase; Cytoskeleton; Data processing; Dynamin; Fibroblasts; Guanosinetriphosphatase; Membrane fusion; membrane trafficking; Mitochondria; Mitochondrial DNA; Muscles; Mutation; Myopathy; Neurology; neuromuscular system; Ophthalmoplegia; siRNA; Transfection
• ISSN: 0960-8966; 1873-2364
• DOI: 10.1016/j.nmd.2012.06.124

Abstract Centronuclear myopathy (CNM) is a congenital myopathy characterised by delayed motor milestones, progressive facial weakness, ptosis, ophthalmoplegia and centrally-located myonuclei caused by autosomal dominant mutations in the dynamin-2 gene, DNM2 . Dnm2 is a large GTPase protein, one of three classical dynamins that is ubiquitously expressed, and key to regulating cytoskeleton and membrane trafficking within cells. Recently, a DNM2 (KI- Dnm2R465W ) animal model was shown to develop phenotypic and morphological abnormalities similar to those observed in the human disease, with abnormal central accumulation of mitochondria in the muscle fibres of the heterozygous mice. Furthermore, mutations in OPA1 and MFN2 – also dynamin-like GTPase proteins which are involved in mitochondrial membrane fusion – have recently been shown to be associated with impaired mitochondrial DNA (mtDNA) stability, inferring a crucial role for these proteins in regulating mitochondrial networks and mtDNA maintenance. We investigated five patients with dominant DNM2 mutations. In addition to the typical features of CNM, the muscle biopsies demonstrated variable amount of cytochrome c oxidase (COX)-deficient muscle fibres indicative of mitochondrial dysfunction. We show here that these focal mitochondrial biochemical defects are due to clonally-expanded mtDNA deletions, characteristic of mtDNA maintenance abnormality. Confocal microscopy studies of patient fibroblasts reveal a quantitative disruption of the dynamic mitochondrial network associated with the p.R369W DNM2 mutation, which interestingly corresponded with the most severe mitochondrial defect in muscle. Similar results were observed in HeLa cells following Dnm2 siRNA knockdown, whilst transfection of NIH3T3 cells with mutant p.R369W Dnm2 led to a significant decrease in mitochondrial objects compared to controls. Together, our data suggest an important and emerging role for Dnm2 in mtDNA maintenance and stability.