CPG ISLANDS AND DOUBLE-MINUTE CHROMOSOMES

Double-minute chromosomes (DMs) simplify oncogenes in human tumors. Th e organization of genomic DNA in four independently isolated DMs ampli fying the DHFR (dihydrofolate reductase) gene has been compared by map ping locations of CpG islands. When cleaved with methylation-sensitive rare-cutting restriction endonucleases, three hypomethylated GC-rich DNA sequences were frequently found in specific regions in these DMs. One such zone was in the CpG island containing the divergently transcr ibed promoter separating the DHFR and the Rep-3 genes. The other two s ites were approximately 500 kb upstream and 300 kb downstream of the D HFR gene. An approximately 800-kb amplified core genomic region contai ning the DHFR gene using DM-specific probes has been identified in thi s study. All the DMs consisted of the core amplified region combined w ith additional DNA fragments. These additional fragments are different for each DM. Therefore, while the DNAs in each of the DMs are differe nt, they have common hypomethylated regions in similar locations. Thes e results suggest a role for the location of hypomethylated GC-rich si tes such as the CpG islands in genesis of DMs. (C) 1998 Academic Press .