Genotype-phenotype correlations in Charcot-Marie-Tooth disease type 2 caused by mitofusin 2 mutations

Abstract

Background: Mutations in the gene encoding mitofusin 2 (MFN2) cause Charcot-Marie-Tooth disease type 2 (CMT2), with heterogeneity concerning severity and associated clinical features.

Objective: To describe MFN2 mutations and associated phenotypes in patients with hereditary motor and sensory neuropathy (HMSN).

Design: Direct sequencing of the MFN2 gene and clinical investigations of patients with MFN2 mutations.

Setting: Molecular genetics laboratory of a university hospital and the Limoges National Referral Center for Rare Peripheral Neuropathies.

Patients: One hundred fifty index patients with HMSN and a median motor nerve conduction velocity of 25 m/s or greater and without mutations in the genes encoding connexin 32 and myelin protein zero.

Main outcome measures: Results of genetic analyses and phenotypic observations.

Results: Twenty different missense mutations were identified in 20 index patients. Mutation frequency was 19 of 107 (17.8%) in patients with CMT2 and 1 of 43 (2.3%) in patients with a median motor nerve conduction velocity less than 38 m/s. Four patients had proven de novo mutations, 8 families had autosomal dominant inheritance, and 3 had autosomal recessive inheritance. The remaining 5 patients were sporadic cases with heterozygous mutations. Phenotypes varied from mild forms to early-onset severe forms. Additional features were encountered in 8 patients (32%). Six patients underwent sural nerve biopsy: electronic microscopy showed prominent mitochondrial abnormalities on longitudinal sections.

Conclusions: MFN2 mutations are a frequent cause of CMT2, with variable severity and either dominant or recessive inheritance. MFN2 gene testing must be a first-line analysis in axonal HMSN irrespective of the mode of inheritance or the severity of the peripheral neuropathy.