Bmi1 – A Path to Targeting Cancer Stem Cells

Abstract

T he Polycomb group (PcG) genes encode for proteins comprising two multiprotein complexes, Polycomb repressive complex 1 (PRC1) and Polycomb repressive complex 2 (PRC2). Although the initial discovery of PcG genes was made in Drosophila, as transcriptional repressors of homeotic (HOX) genes. Polycomb repressive complexes have been since implicated in regulating a wide range of cellular processes, including differentiation and self-renewal in normal and cancer stem cells. Bmi1, a subunit of PRC1, has been long implicated in driving self-renewal, the key property of stem cells. Subsequent studies showing upregulation of Bmi1 in several cancers correlated with increased aggressiveness, radioresistance and metastatic potential, provided rationale for development of targeted therapies against Bmi1. Although Bmi1 activity can be reduced through transcriptional, post-transcriptional and post-translational regulation, to date, the most promising approach has been through small molecule inhibitors targeting Bmi1 activity. The post-translational targeting of Bmi1 in colorectal carcinoma, lung adenocarcinoma, multiple myeloma and medulloblastoma have led to significant reduction of self-renewal capacity of cancer stem cells, leading to slower tumour progression and reduced extent of metastatic spread. Further value of Bmi1 targeting in cancer can be established through trials evaluating the combinatorial effect of Bmi1 inhibition with current 'gold standard' therapies. KeywordsPolycomb group (PcG) genes, Bmi1, Mel-18, cancer stem cells (CSCs), self-renewal Disclosure: David Bakhshinyan, Ashley A Adile, Chitra Venugopal and Sheila K Singh have nothing to disclose in relation to this article.Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Compliance with Ethics:This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors. 1 PRC2 initiates gene silencing through the activity of histone deacetylase (HDAC), and histone methyltransferases that can methylate lysine 9 and 27 residues on histone H3 and lysine 26 on histone H1.2-6 The stable gene repression is then maintained by PRC1 through recognition of tri-methylated H3K27 (H3K27me3).3 The canonical repression pathway is initiated through trimethylation of H3K27 on the promoter of target gene by PRC2 subunit EZH1 or its paralog EZH2.2-4 The H3K27me3 mark is readily recognised by PRC1 through chromatin binding ability of its subunit CBX. The repression is further maintained through ubiquitination of lysine 119 residue on histone H2A (H2AK119ub) by RING1, a subunit of PRC1, or chromatin condensation. [7][8][9] Promoters of the polycomb target genes have been generally characterised as CpG-rich DNA sequences that are lacking other epigenetic markers.10-14 However, several other models of context depen...