Mapping of amplified c-myb oncogene, sister chromatid exchanges, and karyotypic analysis of the COLO 205 colon carcinoma cell line.

We have studied molecular and chromosomal details of cytogenetic status in a human tumor cell line COLO 205 that shows a stable, approximately tenfold amplification of the c-myb oncogene. The amplified copies of c-myb reside in two marker chromosomes that may have evolved from chromosome #6 by complex chromosomal rearrangements. No homogeneously staining regions can be discerned at the site of c-myb amplification. We suggest that c-myb was amplified in situ in a chromosomal segment (6q22-24) that became a part of the marker chromosome, possibly through isochromosome formation followed by duplication, and without the extrachromosomal intermediate form of double minute chromosomes. There is an enhanced frequency of sister chromatid exchanges at the site of amplified c-myb. These results are discussed in the context of models for gene amplification and oncogene activation.