Data from Exon Array Analyses across the NCI-60 Reveal Potential Regulation of TOP1 by Transcription Pausing at Guanosine Quartets in the First Intron

Abstract

<div>Abstract<p>Because topoisomerase 1 (TOP1) is critical for the relaxation of DNA supercoils and because it is the target for the anticancer activity of camptothecins, we assessed <i>TOP1</i> transcript levels in the 60 cell line panel (the NCI-60) of the National Cancer Institute's anticancer drug screen. <i>TOP1</i> 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 <i>TOP1</i> transcripts could be conveniently used to monitor Top1 protein levels and activity in tissues. Assessment of the <i>TOP1</i> 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 <i>TOP1</i> transcript levels (<i>P</i> < 0.01). Further analyses of <i>TOP1</i> expression on an exon-specific basis revealed that exon 1 expression was generally higher and less variable than expression of the other exons, suggesting some form of transcriptional pausing regulation between exons 1 and 2. Accordingly, we found the presence of multiple evolutionarily conserved potential G-quadruplex–forming sequences in the first <i>TOP1</i> intron. Physicochemical 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 <i>TOP1</i> gene associated with a quadruplex-prone region. Cancer Res; 70(6); 2191–203</p></div>