Biochemical characterization and regulation of transcription of Polycomb Group RING Finger 5

The Polycomb Group (PcG) is a highly conserved group of genes which serve to repress transcription via specific modifications of histones in chromatin. The PcG has well-established roles in development and is involved, by mutation or dysregulation, in many human diseases including cancer. This study identifies the gene PCGF5, which is a paralogue of the oncogene Bmi1, as a transcriptional target of Notch signalling in T cell acute lymphoblastic leukemia (T-ALL). Evidence suggests that this regulation is direct and that the Notch transactivation complex binds DNA at several regions near the PCGF5 gene. PCGF5 is found to be expressed at a higher level in T-ALL than other hematopoietic malignancies. PCGF5 is found to associate with the PcG proteins RING1A and RING1B and its overexpression results in increased ubiquitylation of histone H2A, suggesting it shares functional similarity to Bmi1. Despite their similarities, Bmi1 and PCGF5 have a different spectrum of binding partners and are targeted to different locations in the genome. Overexpression of PCGF5 does not significantly alter hematopoietic development in vivo; however, enforced expression of PCGF5 in bone marrow progenitors results in the generation of fewer colonies in a myeloid colony forming assay. This study suggests that PCGF5 may have as yet unappreciated roles in PcG biology and merits further study into its effects on development and hematopoietic neoplasia.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat