Characterization of the topoisomerase I locus in human colorectal cancer

DNA topoisomerase I (topo I) is the principle target for Camptothecin and i ts analogues. The topo I gene is located on chromosome 20q11.2-q13.1 and va riation in topo I gene copy number has been shown to have impact on the in vitro sensitivity to topoisomerase I inhibitor chemotherapy. Fluorescence i n situ hybridization (FISH) rr-as used to detect and compare the TOPO I gen e copy number between metaphase and interphase nuclei in a panel of 7 color ectal cancer cell lines. TOPO I gene copy number varied from 2 to 8 between cell lines, and signal in interphase nuclei demonstrated a linear relation ship with that detected in metaphase nuclei. The structure of gene amplific ation included isochromosome formation, amplicon extension, and marker chro mosome generation. Comparative genomic hybridization (CGH) was then used to further define the region of gain on chromosome 20. The region of gain con tained the topo I gene and involved nearly all of 20q in most cases. This d emonstrates a high degree of intrinsic variation in topo I gene copy number and the involvement of a 20q amplicon in colorectal cancer, which may have important implications for colorectal tumorigenesis and the use of chemoth erapy. (C) 2000 Elsevier Science Inc. All rights reserved.