Alteration of mosaic methylation of the repeat unit of the human ribosomal RNA genes in lung cancer.

We have investigated the methylation status of the repeat unit of the human ribosomal RNA genes in lung cancer. Using a Southern blot analysis approach we have determined that the non-transcribed region of these genes was generally heavily methylated, while the transcribed region was not methylated in either tumor or normal DNA. Our study also revealed that, in one tumor, the boundary of mosaic methylation of the repeat unit was not distinct. In the same tumor, both the non-transcribed ribosomal spacer region and the L1 interspersed repeat sequences became partially demethylated. In tumor cells, the methylation status of DNA can be altered, but the methylation of subtelomeric repeats was found to be maintained. These results suggest that the mosaic methylation of the repeat unit is not necessarily maintained in tumor DNA, while subtelomeric repeats escape tumor-specific wave of demethylation.