Abstract C149: Exon array analyses across the NCI-60 reveals potential regulation of TOP1 by transcription pausing at guanosine quartets in the first intron

Abstract

Abstract Because Topoisomerase I (TOP1) is critical for the relaxation of DNA supercoils and because it is the target for anticancer activity of camptothecins, we assessed TOP1 transcript levels in the 60 cell line panel (the NCI-60) of the National Cancer Institute's anticancer drug screen. TOP1 expression levels varied over a 5.7-fold range across the NCI-60. HCT116 colon and MCF-7 breast cancer cells were the highest expressers; SK-MEL-28 melanoma and HS578T breast carcinoma cells were the lowest. TOP1 mRNA expression was highly correlated with Top1 protein levels, indicating that TOP1 transcripts could be conveniently used to monitor Top1 protein levels and activity in tissues. Assessment of the TOP1 locus by array comparative genomic hybridization across the NCI-60 showed copy numbers ranging from 1.71 to 4.13 and a statistically significant correlation with TOP1 transcript levels (p<0.01). Further analyses of TOP1 expression on an exon-specific basis revealed that exon 1 expression appeared to be generally higher, and less variable than the other exons, suggesting some form of transcriptional pausing regulation between exons one and two. Accordingly, we found the presence of multiple evolutionarilly-conserved potential G-quadruplex-forming sequences in the first TOP1 intron. Physico-chemical tests for actual quadruplex formation by several of those sequences yielded quadruplex formation for two of them and duplex formation for one. The observations reported here suggest the hypothesis that there is a conserved negative transcription regulator within intron 1 of the TOP1 gene, and the regulator is a quadruplex-prone region. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C149.