Analysis of somatic NF1 promoter methylation in plexiform neurofibromas and Schwann cells.

Neurofibromatosis 1 (NF1) is an autosomal dominant disorder with the characteristic feature being the neurofibroma. It is believed that both NF1 alleles must be inactivated as the first step in tumorigenesis. However, often the somatic mutations are not identified, suggesting that epigenetic changes such as methylation could account for the "second hit" in some tumors. The literature reports that the region of the NF1 promoter surrounding the transcription start site is completely unmethylated in several normal tissues and some NF1-related dermal and plexiform neurofibromas. We analyzed the methylation state of the NF1 promoter in normal Schwann cells (the cell type clonally expanded in neurofibromas) and in NF1-related plexiform tumor samples with unidentified somatic mutations. In a region of 451 bp surrounding the transcription start site, a low level of methylation was found at several specific cytosines in 12 of 18 tumor samples. Overall, epigenetic silencing through methylation does not appear to be a major mechanism for the second hit. However, this study, which analyzed the largest number of NF1-related plexiform tumors and is the first to include Schwann cell-enriched tumor cultures, detected greater methylation than in any previous reports. This suggests that methylation, especially at potential transcription factor binding sites, is moderately perturbed in some plexiform neurofibromas and should be investigated further.