MFN2 mutations cause severe phenotypes in most patients with CMT2A.

BACKGROUND: Charcot-Marie-Tooth disease type 2A (CMT2A), the most common form of CMT2, is caused by mutations in the mitofusin 2 gene (MFN2), a nuclear encoded gene essential for mitochondrial fusion and tethering the endoplasmic reticulum to mitochondria. Published CMT2A phenotypes have differed widely in severity. METHODS: To determine the prevalence and phenotypes of CMT2A within our clinics we performed genetic testing on 99 patients with CMT2 evaluated at Wayne State University in Detroit and on 27 patients with CMT2 evaluated in the National Hospital for Neurology and Neurosurgery in London. We then preformed a cross-sectional analysis on our patients with CMT2A. RESULTS: Twenty-one percent of patients had MFN2 mutations. Most of 27 patients evaluated with CMT2A had an earlier onset and more severe impairment than patients without CMT2A. CMT2A accounted for 91% of all our severely impaired patients with CMT2 but only 11% of mildly or moderately impaired patients. Twenty-three of 27 patients with CMT2A were nonambulatory prior to age 20 whereas just one of 78 non-CMT2A patients was nonambulatory after this age. Eleven patients with CMT2A had a pure motor neuropathy while another 5 also had profound proprioception loss. MFN2 mutations were in the GTPase domain, the coiled-coil domains, or the highly conserved R3 domain of the protein. CONCLUSIONS: We find MFN2 mutations particularly likely to cause severe neuropathy that may be primarily motor or motor accompanied by prominent proprioception loss. Disruption of functional domains of the protein was particularly likely to cause neuropathy.

Recessive axonal Charcot-Marie-Tooth disease due to compound heterozygous mitofusin 2 mutations.

OBJECTIVE: Mutations in mitofusin 2 (MFN2) are the most common cause of axonal Charcot-Marie-Tooth disease (CMT2). Over 50 mutations have been reported, mainly causing autosomal dominant disease, though families with homozygous or compound heterozygous mutations have been described. We present 3 families with early-onset CMT2 associated with compound heterozygous MFN2 mutations. Transcriptional analysis was performed to investigate the effects of the mutations. METHODS: Patients were examined clinically and electrophysiologically; parents were also examined where available. Genetic investigations included MFN2 DNA sequencing and dosage analysis by multiplex ligation-dependent probe amplification. MFN2 mRNA transcripts from blood lymphocytes were analyzed in 2 families. RESULTS: Compound heterozygosity for MFN2 mutations was associated with early-onset CMT2 of varying severity between pedigrees. Parents, where examined, were unaffected and were heterozygous for the expected mutations. Four novel mutations were detected (one missense, one nonsense, an intragenic deletion of exons 7 + 8, and a 3-base pair deletion), as well as 2 previously reported missense mutations. Transcriptional analysis demonstrated aberrant splicing of the exonic deletion and indicated nonsense-mediated decay of mutant alleles with premature truncating mutations. CONCLUSIONS: Our findings confirm that MFN2 mutations can cause early-onset CMT2 with apparent recessive inheritance. Novel genetic findings include an intragenic MFN2 deletion and nonsense-mediated decay. Carrier parents were asymptomatic, suggesting that MFN2 null alleles can be nonpathogenic unless coinherited with another mutation.