[A family of X-linked motor and sensory neuropathy with a new type of connexin32 mutation]

Abstract

A 28-year-old man had complaints of muscle weakness in both his legs and fingers. Moderate degrees of symmetrical atrophy adn weakness of the bilateral lower limbs, moderate degree of muscle atrophy was also noticed distal to the lower one third of the upper thigh. A moderate degree of weakness of the anterior tibial, extensor digitorum and peroneus muscles was also noted. Pes cavus deformity was evident bilaterally. Knee jerk was normal, and Achilles tendon reflex was absent without pathologic reflexes. He could not walk on his heels. Vibratory sensation was severely decreased in the toes, and both touch and pain sensations were slightly decreased on the dorsum of the feet. The median motor nerve conduction velocity was 28.9 m/sec with a prolonged distal latency. An amplitude of M-wave evoked by electrical stimulation of the median nerve 1 mV. No M-wave was obtained from stimulation of the tibial nerve, and no sensory nerve action potentials were elicited from electrical stimulation of the median and sural nerves. Histologic studies of the biopsied sural nerve revealed the occasional presence of internodes with a thin myelin sheath and a decrease in the density of large myelinated fibers. Small and atypical onion-bulbs were occasionally observed by electron microscopy. Based on the neurological examination and nerve conduction study of the family members, a sister, mother and grandmother of the proband were found to be mildly affected without any disability in their daily activities. However, the father and an uncle on the mother's side of the proband were normal. Therefore, we concluded clinically that this family had HMSN type I with autosomal dominant inheritance or X-linked HMSN. In the studies of fluorescence in situ hybridization and restriction fragment length polymorphism of the genomic DNA of the proband, a DNA duplication in chromosome 17p11.2-12 was not observed. A single-strand conformational polymorphism analysis of the genomic DNA encoding connexin32 (Cx32) revealed the abnormal band different from that of the control. A sequence analysis of the genomic DNA obtained by use of the polymerase chain reaction was also performed. It revealed that there was a mutant allele, a cytosine to thymine substitution of the nucleotide position 140, which caused a substitution of leucine for serine at amino acid position 26. The proband's mother was heterozygous for the mutant allele and the normal allele. This type of Cx32 mutation was different from any type of Cx32 mutation reported in the literature. The mutation in this family is located in the first transmembrane portion of Cx32, and may alter the function of Cx32 protein, as well as lead to the functional and structural abnormalities of the myelin sheath at the nodes of Ranvier and Schmidt-Lanterman's incisures, where Cx32 is present. This is the first Japanese X-linked HMSN family showing a new type of mutation of Cx32 gene with clinical findings and a histologic evaluation of the sural nerve.