[A family of hereditary motor and sensory neuropathy type I with a mutation (Arg98-->His) in myelin Po--report on a second Japanese family]

Steroid responsive polyneuropathy in a family with a novel myelin protein zero mutation

OBJECTIVE

To report a novel hereditary motor and sensory neuropathy (HMSN) phenotype, with partial steroid responsiveness, caused by a novel dominant mutation in the myelin protein zero (MPZ) gene. Most MPZ mutations lead to the HMSN type I phenotype, with recent reports of Déjérine-Sottas, congenital hypomyelination, and HMSN II also ascribed to MPZ mutations. Differing phenotypes may reflect the effect of particular mutations on MPZ structure and adhesivity.

METHODS

Clinical, neurophysiological, neuropathological, and molecular genetic analysis of a family presenting with an unusual hereditary neuropathy.

RESULTS

Progressive disabling weakness, with positive sensory phenomena and areflexia, occurred in the proband with raised CSF protein and initial steroid responsiveness. Nerve biopsy in a less severely affected sibling disclosed a demyelinating process with disruption of compacted myelin. The younger generation were so far less severely affected, becoming symptomatic only after 30 years. All affected family members were heterozygous for a novel MPZ mutation (Ile99Thr), in a conserved residue.

CONCLUSIONS

This broadens the range of familial neuropathy associated with MPZ mutations to include steroid responsive neuropathy, initially diagnosed as chronic inflammatory demyelinating polyneuropathy.

De novo mutation (Arg98-->Cys) of the myelin P0 gene and uncompaction of the major dense line of the myelin sheath in a severe variant of Charcot-Marie-Tooth disease type 1B

A point mutation (Arg98-->Cys) of exon 3 coding for the extracellular domain of the myelin protein zero (P0) gene was found in a sporadic case of an eighteen year old Japanese man with a severe variant of Charcot-Marie-Tooth disease type 1B (CMT1B). A de novo mutation was established by parentage testing and analyses of the P0 gene in the family. This patient showed delayed motor development, nonprogressive limb weakness and kyphoscoliosis. In addition to the nerve biopsy findings typical of CMT1B, such as segmental demyelination, marked decrease in the density of myelinated fibers, and frequent onion-bulb formation, ultrastructural examination disclosed uncompaction of the major dense lines with slight widening of the intraperiod distance in the inner layers of the myelin sheath. Although mutations in the extracellular domain of P0 should affect homophilic adhesion between external surfaces of Schwann cell processes, resulting in the separation at the intraperiod lines, our study shows uncompacted major dense lines as a main myelin abnormality where the cytoplasmic domain of P0 resides.