Mitofusin 2 (MFN2) Mutations Cause Mitochondrial DNA Instability In Charcot-Marie-Tooth Disease

Abstract

Purpose: : Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of inherited peripheral neuropathies and it affects at least 1 in 2,500 individuals. A specific autosomal-dominant axonal CMT subtype, hereditary motor and sensory neuropathy type VI (HMSN-VI, OMIM 601152), is caused by mutations in mitofusin 2 (MFN2, 1p36.2). In addition to early-onset severe peripheral neuropathy, affected individuals also develop progressive optic nerve dysfunction in later childhood. MFN2 is a critical mitochondrial outer membrane protein and it shares a remarkable degree of structural and functional complementarity with OPA1, a mitochondrial inner membrane protein implicated in the majority of cases of autosomal-dominant optic atrophy (ADOA, OMIM 165500). The aim of this study was to provide new mechanistic insights by investigating the deleterious influence of MFN2 mutations on mitochondrial biogenesis.

Methods: : Tissue samples were available from 48 patients with confirmed MFN2 mutations. Using well-established molecular protocols, various markers of mitochondrial dysfunction were assayed including: (i) mitochondrial DNA (mtDNA) proliferation in blood leukocytes, (ii) the presence of biochemically-defective cytochrome c oxidase (COX) fibers in skeletal muscle biopsies, and (iii) the kinetics of mtDNA repopulation in fibroblast cultures after ethidium bromide-induced depletion.

Results: : Blood leukocytes from MFN2-positive patients had significantly increased mtDNA copy numbers (Mean = 205.9, n = 48) compared with age-matched controls (Mean = 60.9, n = 131; p < 0.0001). Significant mtDNA proliferation was also evident in skeletal muscle fibers from MFN2-positive patients. COX-negative skeletal muscle fibers were present and these harboured high levels of somatic mtDNA deletions, accounting for the observed respiratory chain defect. MFN2-positive fibroblasts had significantly impaired rates of mtDNA repopulation following a period of increased cellular stress.

Conclusions: : The MFN2 protein is intricately involved in mtDNA maintenance and the faithful replication of the mitochondrial genome. Pathogenic MFN2 mutations ultimately lead to neuronal cell loss by triggering a mitochondrial biochemical defect.

Keywords: mitochondria • neuro-ophthalmology: optic nerve • genetics