A new missense mutation in exon 6 of the proteolipid protein gene in a patient with Pelizaeus-Merzbacher disease.

Abstract

Pelizaeus-Merzbacher disease (PMD) is an X-linked recessive disorder with dysmyelination restricted to the central nervous system. Proteolipid protein (PLP), the major myelin protein in the brain, is markedly decreased in the brains of PMD patients; molecular analysis has revealed various PLP gene defects in independent PMD families (Hodes et al., 1993). We report a new point mutation in exon 6 of the PLP gene in a Japanese patient with PMD. The proband is a 2-year old Japanese boy. Nystagmus was first detected during the neonatal period. He showed truncal hypotonia and could not hold his head upright until recently. T2-weighted magnetic resonance imaging (MRI) revealed a severe dysmyelinating pattern in the white matter of the entire brain, and only the first waves were detected in the auditory brainstem response. Polymerase chain reaction (PCR) and hetero-duplex analysis to screen single-base transitions in PCR products were performed as described previously (Osaka et al., 1995). Heteroduplex analysis revealed heteroduplex patterns with the mixed PCR product containing exon 6 of the proband. A G-to-C substitution was observed at nucleotide position 721 by sequencing DNA, resulting in an A241P substitution. The mutation was confirmed by digesting PCR products with the restriction enzyme, BsoFI. The normal product has three BsoFI sites that cut the 277-bp fragment into 140-, 119-, 15-, and 3-bp fragments. The mutation in the proband results in the loss of one site, so that 155- and 119-bp fragments were separated on a 3% NuSieve electrophoresis gel (FMC Bio Products) after digestion with BsoFI. The mother showed the herozygous pattern. Then, 120 times; chromosomes from unrelated Japanese subjects were examined to check the absence of the transition; it was not present in any of the alleles analyzed. A241 is conceivably located in a putative transmembrane domain of PLP (Pham-Dinh et al., 1991). Mutant PLP in our proband may be dysfunctional because of aberrant peptide structure induced by the replacement of Ala with Pro, a residue known to disrupt α-helices. Gow et al. (1994) examined the subcellular localization of wild-type and five mutant forms of PLP, including the A242V mutant, which was observed in the jimpymsd mutant mouse (Gencic and Hudson, 1990), using Cos-7 cells transiently transfected with wild-type and mutant PLP cDNA. Unlike the wild-type, all mutant PLP molecules were retained in the endoplasmic reticulum and were not expressed on the cell surface. A241P substitution may also affect the transport and processing of the protein.