Telomeres, telomerase activity and gene amplification

Abstract

COLO320DM and COLO320HSR are cell lines derived from a human malignant neuroendocrine colon carcinoma showing amplification of MYC oncogene. Previous reports showed that in COLO320DM the amplification is cytogenetically visible as DMs while in COLO320HSR the amplified DNA domains are contained in an HSR of a large marker chromosome. In this report, we demonstrate the presence of amplicon clusters in 3-4 additional chromosomes in both cell lines. Amplicons from COLO320HSR cells contain normal MYC genes while in COLO320DM cells half of the amplicons comprise normal MYCs and half of them contain rearranged PVT/MYC chimeras. We propose that normal MYC genes occur in HSR amplicons while PVT/MYC rearrangements are specific of amplicons in DM chromosomes. Most chromosome ends in COLO320HSR cells contain hexamer repeats producing telomeres with an average length of 6.5kb whereas COLO320DM cells have no more than 25-30% of chromosome ends with telomeres of 4.0kb average size. Both cell lines exhibited similar levels of telomerase activity indicating that telomere differences between COLO320HSR and DM chromosomes are due to a mechanism other than telomerase concentration. HSR and DM amplicons did not show telomere structures when tested with FISH and a telomere-specific probe. The lack of telomeres in DM chromosomes supports the idea that these elements are formed by circular DNA molecules. Moreover, differences in size and number of amplicons in ring DM chromosomes can be easily explained by the occurrence of odd numbers of SCEs.