Alteration of the c-mos locus in "normal" tissues from humans exposed to radium

The structure of a number of human protooncogenes of persons with internal systemic exposure to radium was analyzed by restriction enzyme digestion and Southern blotting of their DNA. Two extra c-mos EcoRI restriction fragment length bands of 5.0 and 5.5 kilobases were found in tissue DNA from six of seven such individuals. The extra c-mos bands were detected in DNA from many, but not all, of the tissues of the individuals exposed to radium. Kidney DNA, however, from three of four individuals exposed to radium contained these alterations; kidney DNA from six age-matched controls did not. The 5.0- and 5.5-kilobase bands, which were of a similar intensity, varied in their intensity with respect to that of the normal 2.5-kilobase band of the c-mos gene. The DNAs that have the polymorphic bands also appear to have a more complex c-mos methylation pattern. Our results suggest that the c-mos restriction fragment length alterations found in individuals exposed to radium were induced rather than inherited, are epigenetic in origin, and most likely result from changes in the methylation of bases surrounding the single exon of the c-mos protooncogene.