The ornithine decarboxylase (ODC; EC gene in parental, dexamethasone-resistant and 2-difluoromethylornithine (DFMO)-resistant human IgG-myeloma-cell lines was studied with the aid of methylation-sensitive restriction endonucleases and probes recognizing different parts of the gene. In all cell lines the promoter region of the ODC gene appeared to be heavily methylated, whereas the first long intron was unmethylated. Methylation analyses of several clones from the parental cell line revealed that these cells are heterogeneous with respect to the methylation status of the ODC gene, whereas all clones from DFMO-resistant cell lines displayed the same methylation pattern. Two of the parental clones represented a hypomethylated type very close to that exclusively found among the DFMO-resistant clones with ODC gene amplification. This typical methylation pattern was due to decreased methylation of a few CCGG sequences in the 3′-flanking region of the gene. It is possible that this kind of hypomethylation favours the initiation of the gene-amplification process in certain individual cells. This hypothesis was supported by the finding that no hypomethylation was present in the ODC gene of another human myeloma cell line that had acquired resistance to DFMO without gene amplification. In a dexamethasone-resistant cell line that overproduced ODC mRNA at normal gene dosage there were some minor differences between the methylation pattern of the ODC gene of different clones, but no such hypomethylation could be found in clones from the parental cell line. In dexamethasone-resistant cells the ODC gene was hypomethylated around the two HpaII sites and three CfoI sites in the coding region and also, as well as in cells with amplified ODC sequences, in the 3′-flanking region of the gene. Some hypomethylation in the distant 5′-flanking region was also observed.

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