Gene mutation analysis and quantitation of DNA topoisomerase I in previously untreated non-small cell lung carcinomas

To elucidate whether gene alterations of topoisomerase I (topo I) exist in untreated non-small cell lung carcinomas (NSCLC), polymerase chain reaction-single strand conformation polymorphism analysis was performed in forty-four NSCLC tissue samples. Gene alterations of topo I were sought in three regions, near codons 361 and 363, 533, and 722 and 729, where point mutations have been found in resistant tumor cell lines selected by chronic camptothecin exposure. In addition, nuclear topo I contents were determined by immunoblotting. No mobility shifts were observed compared to the pattern observed in a normal control at any of the three regions in any sample, whereas topo I levels showed an approximately 12-fold variation. The variation is remarkably large compared to those seen in previous in vitro and in vivo studies. The results suggest that mutations of topo I may not contribute to intrinsic resistance of NSCLC to camptothecins, but low topo I levels may account, at least in part, for the resistance.