HYPOMETHYLATION OF CCGG SITES IN THE 3' REGION OF H-RAS PROTOONCOGENEIS FREQUENT AND IS ASSOCIATED WITH H-RAS ALLELE LOSS IN NONSMALL CELLLUNG-CANCER

The methylation of MspI/HpaII sites flanking a variable tandem repeat in the 3' region of the c-Ha-ras protooncogene was analyzed in 74 DNA samples of non-small cell lung carcinomas and control lung tissues. Of 39 informative samples, 7 allelic deletions (18%) were found at the c -aa-ras locus and of these, five (71%) showed hypomethylation of the n ondelated allele. Heterozygous DNA samples without allele loss reveale d hypomethylation in 37% (12 of 32). Among 35 homozygotes, 9 showed hy pomethylation (26%). We also analyzed c-Ha-ras mutations at codons 12, 13, and 61 by polymerase chain reaction and designed restriction frag ment Length polymorphism and found no mutation. Thus, c-Ha-ras mutatio ns are not associated with the development of the detected abnormaliti es. We conclude that hypomethylation at specific sites in the 3' regio n is associated with loss of heterozygosity for the c-Ha-ras gene in n on-small cell lung cancer. The finding that, in informative samples, h ypomethylation occurs 2-3 times more frequently than allelic loss sugg ests that it might be a change contributing or predisposing to a genet ic instability that can ultimately lead to c-Ha-ras allelic deletions found in tumor DNA.