MFN2 mutations in Charcot-Marie-Tooth disease alter mitochondria-associated ER membrane function but do not impair bioenergetics.

Delfina Larrea,Diego De Stefani,Marta Pera,H Orhan Akman,Kevin R Velasco,Rita Horvath,Federica Dal Bello,Rubén Quintana–Cabrera,Marta Giacomello,Estela Area-Gomez,Michael E Shy,Cristina Guardia-Laguarta,Adriano Gonnelli,Eric A Schon

doi:10.1093/hmg/ddz008

Abstract

Charcot–Marie–Tooth disease (CMT) type 2A is a form of peripheral neuropathy, due almost exclusively to dominant mutations in the nuclear gene encoding the mitochondrial protein mitofusin-2 (MFN2). However, there is no understanding of the relationship of clinical phenotype to genotype. MFN2 has two functions: it promotes inter-mitochondrial fusion and mediates endoplasmic reticulum (ER)–mitochondrial tethering at mitochondria-associated ER membranes (MAM). MAM regulates a number of key cellular functions, including lipid and calcium homeostasis, and mitochondrial behavior. To date, no studies have been performed to address whether mutations in MFN2 in CMT2A patient cells affect MAM function, which might provide insight into pathogenesis. Using fibroblasts from three CMT2AMFN2 patients with different mutations in MFN2, we found that some, but not all, examined aspects of ER–mitochondrial connectivity and of MAM function were indeed altered, and correlated with disease severity. Notably, however, respiratory chain function in those cells was unimpaired. Our results suggest that CMT2AMFN2 is a MAM-related disorder but is not a respiratory chain-deficiency disease. The alterations in MAM function described here could also provide insight into the pathogenesis of other forms of CMT.

Highlights

Charcot–Marie–Tooth disease Type 2A (CMT2A; OMIM #609260) is a neurological disorder that presents as a peripheral neuropathy [1] but involves the central nervous system [2]
CMT2A is a dominant neuropathy in which patients are heterozygous for MFN2 mutations, but it is not known whether the CMT2A phenotype results from MFN2 haploinsufficiency is a dominant negative effect on the WT allele or is semidominant
We quantified the steady-state levels of MFN2 protein in total homogenates of controls and the three CMT2AMFN2 patients by western blot

Summary

Introduction

Charcot–Marie–Tooth disease Type 2A (CMT2A; OMIM #609260) is a neurological disorder that presents as a peripheral neuropathy [1] but involves the central nervous system [2] This autosomal-dominant disorder is due to heterozygous mutations in the gene encoding mitofusin-2 (MFN2) [3] but occasionally MFN2 mutations are recessive [4] or semi-dominant [5]. MFN2 and its paralog MFN1 localize to the outer mitochondrial membrane to promote inter-mitochondrial fusion [11]. Both mitofusins protrude into the cytoplasm and establish homotypic and heterotypic interactions [12]. A re-evaluation of MFN2 topology showed that it has a single membrane-spanning domain where redox-mediated disulfide modifications could drive MFN2 oligomerization and mitochondrial fusion [16,17]

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Conclusion