Long-range transcriptional regulation by the p110 CUX1 homeodomain protein on theENCODE array

Abstract

Background: Overexpression of the Cut homeobox 1 gene, CUX1, inverselycorrelates with patient survival in breast cancers. Cell-based assays andmolecular studies have revealed that transcriptional regulation byCUX1 involves mostly the proteolytically processed p110isoform. As there is no antibody specific to p110 CUX1 only, an alternatestrategy must be employed to identify its targets. Results: We expressed physiological levels of a tagged-p110 CUX1 protein and performedchromatin affinity purification followed by hybridization on ENCODE andpromoter arrays. Targets were validated by chromatin immunoprecipitation andtranscriptional regulation by CUX1 was analyzed in expression profiling andRT-qPCR assays following CUX1 knockdown or p110 CUX1 overexpression.Approximately 47% and 14% of CUX1 binding sites were respectively mappedless than 4 Kbp, or more than 40 Kbp, away from a transcription start site.More genes exhibited changes in expression following CUX1 knockdown thanp110 CUX1 overexpression. CUX1 directly activated or repressed 7.4% and 8.4%of putative targets identified on the ENCODE and promoter arraysrespectively. This proportion increased to 11.2% for targets with 2 bindingsites or more. Transcriptional repression was observed in a slightly higherproportion of target genes. The CUX1 consensus binding motif, ATCRAT, wasfound at 47.2% of the CUX1 binding sites, yet only 8.3% of the CUX1consensus motifs present on the array were bound in vivo. Thepresence of a consensus binding motif did not have an impact on whether atarget gene was repressed or activated. Interestingly, the distance betweena binding site and a transcription start site did not significantly reducedthe ability of CUX1 to regulate a target gene. Moreover, CUX1 not only wasable to regulate the next adjacent gene, but also regulated the gene locatedbeyond this one as well as the gene located further away in the oppositedirection. Conclusion: Our results demonstrate that p110 CUX1 can activate or repress transcriptionwhen bound at a distance and can regulate more than one gene on certaingenomic loci.