1. Department of Medicine, Division of Genomic Medicine, George Washington University Medical Center, Washington, DC, USA
2. Center for Vaccine Development, University of Maryland, Baltimore, MD, USA
3. Department of Anesthesiology, George Washington University Medical Center, Washington, DC, USA
4. Armed Forces Institute of Pathology, Washington, DC, USA
5. Department of Medicine, Division of Hematology and Oncology, George Washington University Medical Center, Washington, DC, USA
6. Department of Biochemistry & Molecular Biology, George Washington University Medical Center, Washington, DC, USA
7. Mammalian Genetics Section, NIH, Bethesda, MD, USA
Introduction: Several lines of evidence point to an important role for BP1, an isoform of DLX4 homeobox gene, in breast carcinogenesis and progression. BRCA1 is a well-known player in the etiology of breast cancer. While familial breast cancer is often marked by BRCA1 mutation and subsequent loss of heterozygosity, sporadic breast cancers exhibit reduced expression of wild type BRCA1, and loss of BRCA1 expression may result in tumor development and progression.
Methods: The Cister algorithm and Genomatix program were used to identify potential BP1 binding sites in BRCA1 gene. Real-time PCR, Western blot and immunohistochemistry analysis were performed to verify the expression of BRCA1 and BP1 in cell lines and breast cancer tissues. Double-stranded siRNA transfection was carried out for silencing BP1 expression. ChIP and EMSA were used to confirm that BP1 specifically binds to BRCA1.
Results: A putative BP1 binding site was identified in the first intron of BRCA1, which was confirmed by chromatin immunoprecipiation and electrophoresis mobility shift assay. BP1 and BRCA1 expression were inversely correlated in breast cancer cell lines and tissues, suggesting that BP1 may suppress BRCA1 transcription through consensus sequence binding.
Conclusions: BP1 homeoprotein represses BRCA1 expression through direct binding to its first intron, which is consistent with a previous study which identified a novel transcriptional repressor element located more than 500 base pairs into the first intron of BRCA1, suggesting that the first intron plays an important role in the negative regulation of BRCA1. Although further functional studies are necessary to confirm its repressor activity towards BRCA1, the elucidation of the role of BP1 in breast tumorigenesis holds great promise in establishing BP1 as a novel target for drug therapy.
Keywords: BP1, DLX4, homeoprotein, BRCA1, breast cancer.