Functional restoration of genetically inherited neuronal disorder - Molecular biological approach using heriditary mutant mice -

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The mouse proteolipid protein (PLP) gene was cloned into the phage Charon 4A. The organization and the nucleotide sequence of the exons of the mouse PLP gene were quite similar to those of their human counterparts, consisting of seven exons. The transcription of the plp gene started from multiple sites. There was a unique sequence tandemly repeated four times, upstream from the transcribed region.The jimpy mouse, an X-linked recessive dysmyelinating mutant, has been shown to contain abnormal PLP-mRNA. In order to understand the molecular basis of the mutation, we analyzed the structure of PLP-nRNA by an RNase-mapping procedure, using the probes specific to each exon of the mouse plp gene. We found that the fifth exon of the PLP gene is not utilized in the jimpy. We analyzed the jimpy PLP-mRNA and showed that the transcription initiated from the same sites as those in normal mice. Cloning and sequencing of the 5'-flanking region of the jimpy PLP gene revealed that there were no mutations in the promoter region of the jimpy PLP gene. The upstream region of the mouse PLP gene from wild-type and jimpy mice was subcloned into a pIP111 vector, with which the promoter activity of the inserted fragments within the cells can be measured. The plasmids were introduced into rat C6 glioma cells, since the PLP-mRNA was detected in C6 cells and its expression increased greatly following serum deprivation. The PLP-promoter activity was three times higher in C6 cells than in NIH3T3 cells and it increased six times upon serum deprivation. Thus this fragment contained cis-acting element(s) conferring tissue-restricted expression of the PLP gene. However, there was no difference in the promoter activity between the wild type and the jimpy type PLP gene.Therefore, it is likely that an "A" to "G" conversion existing at the splicing acceptor site of the fifth exon of the jimpy PLP gene caused the skipping of the fifth exon and directly affected the mRNA level.