Mutations of the same sequence of the myelin P0 gene causing two different phenotypes.

Abstract

The hereditary motor and sensory neuropathy type 1 (HMSNI) or Charcot-Marie-Tooth type 1 (CMT1) is the most common inherited demyelinating peripheral neuropathy in humans (Skre, 1974). The disease is characterized by distal muscle atrophy and weakness, pes cavus, and lowered nerve conduction velocities (NVC) below 38m/s, and is mostly inherited as an autosomal dominant trait. The major locus, CMTlA, has been mapped on the basis of cosegregation of CMTl disease with 1.5-Mb duplication (17pl1.2-pl2) (Lupsky et al., 1991) including the peripheral myelin protein-22 (PMP-22) gene (Pate1 et al., 1992) or, in a few nonduplicated cases, with point mutations of the gene itself (Valenijn et al., 1992; Roa et al., 1993a). The minor locus (CMTlB) has been assigned by linkage analysis to chromosome lq21923 (Bird et al., 1982) and mutations have been found in the candidate gene encoding the peripheral myelin protein zero (PO) (Hoogendijk et al., 1992; Nelis et al., 1994). Mutations in PMP-22 and PO genes have also been detected in patients with HMSN type I11 or DejerineSottas syndrome (DSS) (Roa et al., 199313; Hayasaka et al., 1993a). This relatively rare disorder shows clinical, histopathologic, and electrophysiologic findings similar to those of CMTI, although the severity is higher and the age of onset is earlier. NCV is greatly reduced. In this study, mutation screening of the PO gene was performed in two patients diagnosed respectively as CMT 1 and DSS, both of whom had been excluded from having either CMTlA duplication or point mutations in the PMP-22 gene. Patient A1 was a 49-year-old man with CMT type 1. The onset of the disease was at 42 years of age, when he began to suffer from generalized asthenia. Electromyographic (EMG) examination showed signs of polyneuropathy with slightly decreased motor nerve conduction velocity: median 3 7 m/s, SPE 3 1 m/s. Family history was positive: patient Al's son (A3), who had neuromuscular problems, was confirmed to be affected by CMTl after electromyographic evaluation (Fig. 1A). Patient B3 was a 13-year-old girl with DSS; she had pes cavus, mild wasting and weakness of the distal part of the legs, and also involvement of the hands. Mild weakness was also found in the shoulders, pelvic girdle and proximal limb muscles. EMG examination revealed severe polyneuropathy with markedly slowed NCV: median and anterior tibia1 6 m/s. Neurologic examination and nerve conduction were normal in the unrelated parents (Fig. 1B). The coding regions of the PO gene of both patients were screened for point mutations by PCR-SSCP under various conditions of electrophoresis temperature and gel glycerol percentage. The 370-bp fragment corresponding to exon 3 of the PO gene showed a mobility shift of single strands in both patients (Fig. 1). Sequencing of the 370-bp fragments, which had been previously purified and cloned, revealed, in patient Al , a 371C+T transition, leading to Thr-to-Met substitution at codon 124 (T124M) in the PO protein. Patient B3 had a 6-bp deletion from nucleotide positions 369 to 374 (369de16), predicting the loss of Thr and Phe amino acids at codons 124 and 125 in the PO protein. Both normal and mutant alleles were present, indicating