Molecular dissection of the Schwann cell specific promoter of the PMP22 gene.

PMP22, one of the major components of myelin, is overexpressed in Charcot-Marie-Tooth type 1A (CMT1A) patients. In an attempt to determine the mechanisms by which the expression of this gene is regulated (with a view to lowering its expression in CMT1A patients), we subcloned genomic fragments covering 6kb of the promoter region in an expression vector containing the beta-galactosidase gene as reporter, and used these in transfection assays. We show that the 300bp upstream of the transcription start contain the elements required for Schwann cell specific expression of the reporter gene. This minimal promoter activity appears to be under the control of a silencer element sensitive to cAMP, located between -0.3kb and -3. 5kb from the start of transcription. Computer analysis of 2kb of the promoter predicted the presence of transcription factor binding sites, including CREB (which may be involved in the response of PMP22 expression to cAMP stimulation) and steroid receptors. Using constructs with or without the CREB sites, we were able to demonstrate that these sites are involved in silencing the PMP22 promoter activity. Lastly, we identified a region containing blocks of polymorphic CA repeats, located close to the CREB binding site, which may further influence the transcriptional activity of PMP22.