Genomic structure and chromosomal mapping of the gene coding for ICBP90, aprotein involved in the regulation of the topoisomerase II alpha gene expression

We have recently identified a novel CCAAT box binding protein (ICBP90) invo lved in the regulation of topoisomerase II alpha gene expression. We have o bserved that it is expressed in non-tumoral proliferating human lung fibrob last cells whereas in HeLa cells, a tumoral cell line, ICBP90 was still pre sent even when cells were at confluence. In the present study, we have dete rmined the ICBP90 gene structure by screening of a human placenta genomic l ibrary and PCR analysis. We report that the ICBP90 gene spans about 35.8 kb and contains six coding exons named A to F. In the 5 ' upstream sequence o f the region containing the coding exons. two additional exons (I and II) w ere found. Additionally, an internal splicing site was found in exon A. A p romoter region, including three putative Spl binding sites between exons I and A, was identified by transient transfection. Northern blot analysis of several cancer cell lines revealed the existence of two ICBP90 mRNA species of 5.1 and 4.3 kb that are transcribed from the gene. The relative amounts of these mRNAs depended on the cell type. in MOLT-4 cells and Burkitts lym phoma Raji cells, the 4.3 kb or the 5.1 kb transcripts were mainly observed , respectively. In other cell lines, such as HL-60 cells, chronic myelogeno us leukaemia K-562, lung carcinoma A549. HeLa or colorectal SW480, both 3.3 and 5.1 kb forms of ICBP90 mRNA could be detected, interestingly, western blot analysis showed several ICBP90 protein bands in HeLa but only a single band in MOLT-4 cell extracts. Taken together our results are consistent wi th the ICBP90 gene exhibiting alternative splicing and promoter usage in a cell-specific manner. (C) 2001 Elsevier Science B.V. All rights reserved.