Targeting PRC1 in leukemia

Abstract Acute leukemia is an aggressive disease resulting from various genetic abnormalities and is characterized by the presence of undifferentiated blood cells. Emerging evidence shows that recurrence of acute leukemia results from the activity of leukemic stem cells (LSCs), which are resistant to chemotherapy, supporting the need for new pharmacological agents inducing differentiation of LSCs. The self-renewal and differentiation capacity of stem cells, including LSCs, are dependent on the activity of the Polycomb repressive complex 1 (PRC1). The core of all PRC1 complexes comprises heterodimeric complexes involving RING1B or RING1A and one of the PCGF1-6 paralogs, which catalyze monubiquitination of histone H2A (H2Aub) constituting a strong repressive mark. Simultaneous depletion of core PRC1 components RING1A/B results in a loss of H2Aub deposition, cell growth arrest and differentiation of acute leukemia cells driven by the oncogenes associated with expression of HOXA9. Therefore, blocking PRC1 activity with small molecules could lead to eradication of LSCs and differentiation of leukemic blasts. The major goal of this project is to develop potent small molecule inhibitors blocking the activity of PRC1 by a direct inhibition of the RING1B-BMI1 E3 ubiquitin ligase to induce differentiation of leukemic cells. To this end, we employed fragment-based screening followed by extensive medicinal chemistry to develop the first-in-class small molecules that directly bind to the RING1A/B proteins and inhibit PRC1 activity through blocking the interaction of the PRC1 complex with nucleosomes. Our first-generation inhibitor RB-3 decreases global level of H2Aub and induces differentiation in leukemia cells and primary AML samples. The goal of this proposal is to optimize this class of compounds and develop potent PRC1 inhibitors with optimized drug-like properties and pronounced in vivo efficacy in animal models of acute leukemia. We will employ extensive medicinal chemistry and structure-based design approach to optimize PRC1 inhibitors. All inhibitors will be characterized to determine binding affinity, inhibitory activity, and cellular activity to inhibit H2Aub. Best compounds will be extensively evaluated in a panel of leukemia cell lines. We will assess the mechanism of action of the PRC1 inhibitors and test their activity in the models of LSCs and in the primary AML patient samples. The optimized compounds will be assessed in vivo for their potential to block development of leukemia in mouse models of AML. Our studies will explore a new approach to induce differentiation of leukemia cells and LSCs and may lead to the development of highly valuable chemical probe compounds or novel pharmacologic agents for acute leukemia patients.