SIMILAR PATTERNS OF HYPOMETHYLATION IN THE BETA-HYDROXY-BETA-METHYLGLUTARYL COENZYME-A REDUCTASE GENE IN HEPATIC NODULES INDUCED BY DIFFERENT CARCINOGENS

Our earlier studies demonstrated that the beta-hydroxy-beta-methylglut aryl coenzyme A (H M G CoA) reductase gene is hypomethylated and overe xpressed in hepatic nodules initiated by 1,2-dimethylhydrazine (1,2-DM H). The study presented here examined whether the pattern of DNA methy lation of the HMG CoA reductase gene in hepatic nodules reflected carc inogen-DNA interaction during initiation. Accordingly, hepatic nodules were generated in male Fischer 344 rats with either 1,2-DMH or aristo lochic acid (AA), which interact predominantly with the guanine and ad enine bases in DNA, respectively. DNA from individual nodules was rest ricted with Hpall, Mspl, and Hhal, and the fragments obtained were hyb ridized to a cDNA probe for HMG CoA reductase. The results indicated t hat the hypomethylation pattern in the reductase gene in the nodules i nitiated with these two carcinogens was similar, although they interac ted with different bases in the DNA. The question still remained wheth er the DNA fragments obtained by digesting the two sets of nodules wit h the restriction endonucleases were from the same domains in the geno me of HMG CoA reductase. To examine this, probes for the different dom ains of the HMG CoA reductase gene were generated from the cDNA using the restriction enzyme Accl. Three probes were obtained: (i) a 5'-end fragment corresponding to the membrane-spanning region, (ii) a second fragment corresponding to the 3'-end of the protein, and (iii) a third fragment spanning the region between (i) and (ii). Each of these prob es was radiolabeled and hybridized to the Hpall- and Hhal-generated fr agments from the DNA of hepatic nodules initiated with 1,2-DMH and AA. Irrespective of the carcinogen used for initiation, hypomethylation o ccurred in ail three domains of the gene. More important, the pattern of hypomethylation was similar in the nodules initiated by the two car cinogens. Furthermore, an overall similarity was seen in the hypomethy lation patterns in the c-myc and c-Ha-ras genes in the DNA of nodules initiated with either 1,2-DMH or AA. These results raise the possibili ty that the pattern of hypomethylation established in the hepatic nodu les may not directly reflect the interaction between the initiating ca rcinogen and DNA but may represent a unique phenotype of hepatic nodul es. (C) 1994 Wiley-Liss, Inc.