title: Regulatory dissection of the CBX5 and hnRNPA1 bi-directional promoter in human breast cancer cells reveals novel transcript variants differentially associated with HP1α down-regulation in metastatic cells.
abstract: BACKGROUND
  The three members of the human heterochromatin protein 1 (HP1) family of proteins, HP1α, HP1β, and HPγ, are involved in chromatin packing and epigenetic gene regulation. HP1α is encoded from the CBX5 gene and is a suppressor of metastasis. CBX5 is down-regulated at the transcriptional and protein level in metastatic compared to non-metastatic breast cancer. CBX5 shares a bi-directional promoter structure with the hnRNPA1 gene. But whereas CBX5 expression is down-regulated in metastatic cells, hnRNAP1 expression is constant. Here, we address the regulation of CBX5 in human breast cancer.
  
  
  METHODS
  Transient transfection and transposon mediated integration of dual-reporter mini-genes containing the bi-directional hnRNPA1 and CBX5 promoter was performed to investigate transcriptional regulation in breast cancer cell lines. Bioinformatics and functional analysis were performed to characterize transcriptional events specifically regulating CBX5 expression. TSA treatment and Chromatin Immunoprecipitation (ChIP) were performed to investigate the chromatin structure along CBX5 in breast cancer cells. Finally, expression of hnRNPA1 and CBX5 mRNA isoforms were measured by quantitative reverse transcriptase PCR (qRT-PCR) in breast cancer tissue samples.
  
  
  RESULTS
  We demonstrate that an hnRNPA1 and CBX5 bi-directional core promoter fragment does not comprise intrinsic capacity for specific CBX5 down-regulation in metastatic cells. Characterization of transcriptional events in the 20 kb CBX5 intron 1 revealed existence of several novel CBX5 transcripts. Two of these encode consensus HP1α protein but used autonomous promoters in intron 1 by which HP1α expression could be de-coupled from the bi-directional promoter. In addition, another CBX5 transcriptional isoform, STET, was discovered. This transcript includes CBX5 exon 1 and part of intron 1 sequences but lacks inclusion of HP1α encoding exons. Inverse correlation between STET and HP1α coding CBX5 mRNA expression was observed in breast cancer cell lines and tissue samples from breast cancer patients.
  
  
  CONCLUSIONS
  We find that HP1α is down-regulated in a mechanism involving CBX5 promoter downstream sequences and that regulation through alternative polyadenylation and splicing generates a transcript, STET, with potential importance in carcinogenesis.
authors.affiliation: Department of Biomedicine, The Bartholin building, Aarhus University, DK-8000, Aarhus C, Denmark.
authors.forename: Johan
authors.initials: J
authors.lastname: Vad-Nielsen
authors.affiliation: Department of Biomedicine, The Bartholin building, Aarhus University, DK-8000, Aarhus C, Denmark.|Department of Clinical-Biochemistry, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Kristine Raaby
authors.initials: KR
authors.lastname: Jakobsen
authors.affiliation: Department of Biomedicine, The Bartholin building, Aarhus University, DK-8000, Aarhus C, Denmark.
authors.forename: Tina Fuglsang
authors.initials: TF
authors.lastname: Daugaard
authors.affiliation: Department of Biomedicine, The Bartholin building, Aarhus University, DK-8000, Aarhus C, Denmark.
authors.forename: Rune
authors.initials: R
authors.lastname: Thomsen
authors.affiliation: Department of Pathology, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Anja
authors.initials: A
authors.lastname: Brügmann
authors.affiliation: Department of Clinical-Biochemistry, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Boe Sandahl
authors.initials: BS
authors.lastname: Sørensen
authors.affiliation: Department of Biomedicine, The Bartholin building, Aarhus University, DK-8000, Aarhus C, Denmark. aln@biomed.au.dk.
authors.forename: Anders Lade
authors.initials: AL
authors.lastname: Nielsen
mesh_terms: D001943:Breast Neoplasms / Q000235:genetics* / Q000473:pathology; D045744:Cell Line, Tumor; D002843:Chromatin / Q000235:genetics; D000090266:Chromobox Protein Homolog 5; D002868:Chromosomal Proteins, Non-Histone / Q000096:biosynthesis* / Q000235:genetics; D005091:Exons / Q000235:genetics; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B / Q000096:biosynthesis* / Q000235:genetics; D006801:Humans; D007438:Introns / Q000235:genetics; D009362:Neoplasm Metastasis; D011401:Promoter Regions, Genetic; D012326:RNA Splicing / Q000235:genetics
chemical_list: C000715593:CBX1 protein, human; C000715594:CBX5 protein, human; D002843:Chromatin; D002868:Chromosomal Proteins, Non-Histone; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; C000619656:hnRNPA1 protein, human; D000090266:Chromobox Protein Homolog 5