A Novel Asp121Asn Mutation of Myelin Protein Zero Is Associated with Late-Onset Axonal Charcot-Marie-Tooth Disease, Hearing Loss and Pupil Abnormalities

Abstract

Myelin protein zero (MPZ) is a major component of compact myelin in peripheral nerves. Mutations in MPZ have been associated with different Charcot-Marie-Tooth disease (CMT) phenotypes (CMT1B, CMT2I/J, CMTDI), Dejerine-Sottas syndrome, and congenital hypomyelination neuropathy. Here, we report phenotypic variability in a four-generation Chinese family with the MPZ mutation Asp121Asn. Genetic testing was performed on nine family members and 200 controls. Clinical, electrophysiological and skeletal muscle MRI assessments were available for review in six family members. A novel heterozygous missense mutation, Asp121Asn, was observed in five affected members of the family. Unaffected relatives and 200 normal controls were without the mutation. Four of the affected members of the family displayed late-onset, predominantly axonal sensory and motor neuropathy, pupil abnormalities, and progressive sensorineural hearing loss. One young affected member presented with Argyll-Robertson pupils and diminished deep tendon reflexes in the lower limbs. The MPZ mutation Asp121Asn may be associated with late-onset axonal neuropathy, early onset hearing loss and pupil abnormalities. Our report expands the number and phenotypic spectrum of MPZ mutations.

Keywords: Charcot–Marie–Tooth disease; hearing loss; myelin protein zero; novel mutation; pupil abnormalities.

FIGURE 1

(A) Family tree of a Chinese family of Asian descent affected by peripheral neuropathy, pupil abnormalities, and progressive sensorineural hearing loss. The family included 7 affected and 10 unaffected members over four generations. The proband is member III-5. The other family members, II-1, II-3 III-1-4, III-6, and IV-7, were examined clinically or electrophysiologically and received genetic testing. (B) A novel heterozygous missense mutation c.361G>A was identified in MPZ exon 3, which caused the Asp121Asn substitution in the extracellular domain. (C) The position of the mutation is highly conserved in vertebrates, juxtaposed with known mutations, and located within the critical disulfide bonding domain responsible for the adhesive properties of MPZ.