Epigenetic function of BMI-1 in the regulation of hematopoietic stem cell self-renewal

Abstract

Hematopoietic stem cell (HSC) is a multipotent cell type, which can self-renew and differentiate into all blood cell lineages. These unique features of HSCs are vital for replenishing the stem cell pool within an individual. However, the underlying molecular mechanisms of HSC function remain to be elucidated. Recent studies have revealed the polycomb group protein, Bmi-1, has an essential role in regulating HSC function. Bmi-1 is a core component of the polycomb repressive complex 1, which silences gene expression by introducing histone H2A ubiquitination (H2AK119ub) mark at target gene loci. We have established a primary cell system to study the function of Bmi-1 in hematopoiesis. Overexpression of Bmi-1 led to the suppression of known Bmi-1 target genes p16Ink4a and p19Arf in primary isolated murine HSCs. The colony forming potential was significantly enhanced through the promotion of HSC proliferation. In contrast, Bmi-1 knockdown in HSCs was accompanied by de-repression of p16Ink4a and p19Arf genes and by a significant reduction of colony formation due to inhibited cell proliferation. Bmi-1 enhances HSC self-renewal is associated with its epigenetic function on H2A ubiquitination. Treatment of HSC with a BMI-1-RING1A/B E3 ubiquitin ligase inhibitor PRT4165, which efficiently and specifically depletes global H2AK119ub level, led to over 50% reduction of colony number. However, overexpression of Bmi-1 in HSCs can maintain the colony forming potential after the inhibitor treatment, indicating that H2A ubiquitination is crucial for HSC self-renewal. To investigate how Bmi-1 regulates gene transcriptional machinery in HSCs, we performed RNA-Seq analysis of HSCs with Bmi-1 overexpression. Bmi-1 overexpression mediated the downregulation of 722 genes and upregulation of 541 genes. Gene Ontology analysis indicated that Bmi-1 was involved in the regulation of proliferation, differentiation and various cell signaling pathways, including Wnt, cytokine-associated and Mapk pathways, which are crucial for the HSC biology. Further investigation of the Bmi-1 targeted regions and the genome-wide H2K119ub pattern would allow us to dissect the underlying epigenetic mechanism that controls the global transcription machinery of HSCs and provides novel insights on the role of Bmi-1 in HSC self-renewal.