Mutations in MME Cause an Autosomal-recessive Charcot-Marie-Tooth Disease Type 2

Objective: The objective of this study was to identify new causes of Charcot-Marie-Tooth (CMT) disease in patients with autosomal-recessive (AR) CMT.

Methods: To efficiently identify novel causative genes for AR-CMT, we analyzed 303 unrelated Japanese patients with CMT using whole-exome sequencing and extracted recessive variants/genes shared among multiple patients. We performed mutation screening of the newly identified membrane metalloendopeptidase (MME) gene in 354 additional patients with CMT. We clinically, genetically, pathologically, and radiologically examined 10 patients with the MME mutation.

Results: We identified recessive mutations in MME in 10 patients. The MME gene encodes neprilysin (NEP), which is well known to be one of the most prominent beta-amyloid (Aβ)-degrading enzymes. All patients had a similar phenotype consistent with late-onset axonal neuropathy. They showed muscle weakness, atrophy, and sensory disturbance in the lower extremities. All the MME mutations could be loss-of-function mutations, and we confirmed a lack/decrease of NEP protein expression in a peripheral nerve. No patients showed symptoms of dementia, and 1 patient showed no excess Aβ in Pittsburgh compound-B positron emission tomography imaging.

Interpretation: Our results indicate that loss-of-function MME mutations are the most frequent cause of adult-onset AR-CMT2 in Japan, and we propose that this new disease should be termed AR-CMT2T. A loss-of-function MME mutation did not cause early-onset Alzheimer's disease. Identifying the MME mutation responsible for AR-CMT could improve the rate of molecular diagnosis and the understanding of the molecular mechanisms of CMT.