Transcriptional startpoints and methylation patterns in the PMP22 promoters of peripheral nerve, leukocytes and tumor cell lines

PMP22 is a dosage sensitive gene responsible for Charcot-Marie-Tooth type 1A (CMT1A) neuropathy and hereditary neuropathy with liability to pressure palsies (HNPP). PMP22 is expressed in myelinating Schwann cells in the peripheral nerve, but also in a variety of other tissues. PMP22 expression is regulated by alternatively used promoters, the relative expression of the different PMP22 transcripts is tissue-specific. At first we analysed the transcriptional startpoints of the different PMP22 transcripts. Transcript 1A starts from a distinct nucleotide, whereas transcript 1B and the here described transcript 1C revealed multiple transcriptional startpoints in sciatic nerve as well as in the osteosarcoma and glioblastoma cell lines, RH30 and SF763. Using promoter specific primers we identified transcripts from each of the three promoters in sciatic nerve and RH30, whereas transcript 1B is absent in SF763. Leukocytes do not express PMP22 at all. Additionally, we determined the methylation pattern of CpG islands present in the PMP22 promoters 1B and 1C for leukocytes, sciatic nerve, SF763 and RH30, the latter carrying multiple copies of the PMP22 gene. We observed that there was no methylation in promoter 1B and 1C in sciatic nerve and leukocytes. However, hypermethylation of promoter 1B was discovered in SF763 and indicates a silencing effect. In RH30 most copies of promoters 1B and 1C were methylated but the few remaining hypomethylated copies were sufficient for strong expression of PMP22. These results indicate that the transcriptional control in tumor cell lines is probably different from leukocytes and sciatic nerve.