DNA discontinuities in the domain of amplified human MYC oncogenes

COLO320DM and COLO320HSR are cell lines derived from a human colon carcinoma. Both lines have an amplification of the MYC oncogene: COLO320DM in the form of double minute chromosomes, COLO320HSR as a large marker chromosome with homogeneously staining regions. We have used pulsed field gel electrophoresis (PFGE) to analyze undigested DNA from both COLO320 cell lines. Upon blotting and hybridization with a MYC probe, the autoradiograms showed the existence of discontinuities in the amplified DNA domains. Additional evidence indicating a preferential concentration of DNA breaks in the region containing the MYC amplicons was obtained with the alkaline unwinding technique. We propose that amplicons are connected by hairpin formation or Hoogsteen pairing between free DNA ends.