Identification and Characterization of a Deletion Mutant of DNA Topoisomerase I mRNA in a Camptothecin‐resistant Subline of Human Colon Carcinoma

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Abstract
In previous studies, we established two camptothecin (CPT)‐resistant sublines, HT‐29/CPT and St‐4/CPT, from the human colon cancer cell line HT‐29 and the human stomach cancer cell line St‐4, respectively. Cellular contents of DNA topoisomerase I (topo I) in the resistant cells were eight‐fold less than those in the corresponding parental lines. In this study, we have shown expression of two species of the TOP1 mRNA in HT‐29/CPT. The longer mRNA (4.0 kb) is the wild‐type TOP1 mRNA, and the shorter mRNA (3.3 kb) proved to have a deletion of 672 bp (nucleotides 58–729 or 59–730) that caused the in‐frame deletion of amino acids 20–243 of human topo I. The deleted region is identical to exons 3–9 of the TOP1 gene. The expression level of the 3.3‐kb mRNA was similar to that of the wild‐type mRNA in HT‐29/CPT. St‐4/CPT expressed only the wild‐type TOP1 mRNA in lesser amounts than did St‐4. Mouse NIH3T3 cells transfected with the wild‐type TOP1 cDNA showed higher sensitivity to CPT than the parental cells, whereas those transfected with the deleted TOP1 cDNA showed levels similar to those of the parental cells. Expression of the exogenous TOP1 mRNA was confirmed; however, expression of the truncated topo I was not detected in cells transfected with the deleted TOP1 cDNA. These results suggest that the expression of the deleted TOP1 mRNA led to the low expression of CPT‐sensitive topo I in the resistant cells.