[A family of hereditary motor and sensory neuropathy type I with a mutation (Arg98-->His) in myelin Po--report on a second Japanese family].
A 46-year-old housewife had complaints of insidiously progressive muscle weakness and paresthesia in the distal lower limbs. On neurological examination, a slight to moderate degree of muscle weakness with slight atrophy was observed in the bilateral intrinsic hand muscles. A severe degree of muscle weakness with moderate atrophy was observed in tibialis anterior, gastrocnemius and soleus muscles. Muscle stretch reflexes were decreased in the upper limbs and absent in the lower limbs, without pathologic reflexes. She had a steppage gait. Vibratory sensation was slightly decreased in the hands and moderately decreased in the feet. Touch, pain and temperature sensations were also moderately decreased only in the feet. On laboratory examination, glycosuria (5.6g/dl) was noted. Fasting blood sugar was 226mg/dl with an elevated hemoglobin A1C level (12.7%). The right median motor and sensory nerve conduction velocities were 14.8 and 20.3 m/sec, respectively, with a markedly prolonged distal latency. No muscle action potential was obtained from stimulation of the right tibial nerve. Also, no nerve action potential was elicited from stimulation of the right sural nerve. A fascicular biopsy of the right sural nerve revealed the presence of both demyelinated and remyelinated axons, and an onion-bulb formation with a marked decrease in the density of the myelinated fibers. Based on the neurological examination and nerve conduction studies of the family members, a younger sister, younger brother and an elder daughter of the proband were found to be affected by demyelinating polyneuropathy. Diabetes mellitus was not found among the family members with laboratory evidences of demyelinating polyneuropathy. Based on the family history, an uncle on the mother's side of the proband, the proband's grandmother and a younger daughter of a proband's brother were considered to be affected. The uncle and grandmother had diabetes mellitus. Therefore, we concluded that this family had HMSN type I with autosomal dominant inheritance. In the studies on fluorescence in situ hybridization, and restriction fragment length polymorphism of the genomic DNA of the proband, a DNA duplication in the 17p11.2-12 region was not observed. However, the direct sequencing analysis of DNA fragments from genomic DNA encoding the Po gene of the proband revealed a substitution of histidine for arginine at the codon 98 in the extramembranous domain of Po. She was heterozygous for the mutant allele and normal allele. Alterations in the tertiary structure of the extramembranous domain of Po may result in an impairment of the peripheral myelin compaction. This is the second Japanese family with the same mutation (Arg98-->His) of myelin Po as reported previously by us, and this type of case is rare in the literature. Therefore, the mutation at the codon 98 may play a critical role in the development of the myelin abnormality in HMSN type IB.