Nuclear Vav3 is required for canonical PRC1 activity in B-cell acute lymphoblastic leukemia

Acute B-cell lymphoblastic leukemia (B-ALL) represents a multiclonal evolution of B-cell progenitors with tumor-initiating and tumor-propagating activity. We have previously shown that the activation and overexpression of both the guanine nucleotide exchange factor (GEF) Vav3 and Rac GTPases are required for BCR-ABL mediated leukemogenesis. Here, we identified that upon oncogene expression, Vav3 expression becomes predominantly nuclear and interacts with Rac and canonical polycomb repression complex proteins such as Ezh2, Bmi1 and the E3-ubiquitin ligase Ring1B. The Vav3 GEF domain is required for B-cell proliferation and Bmi1-dependent B-cell progenitor self-renewal, nuclear Rac activation and Bmi1-dependent H2A(K119) mono-ubiquitination. Canonical Bmi1/PRC1.4 dependent downstream targets Cdkn2a and Cdkn2b are upregulated in Vav3 deficient leukemic B-progenitor cells and the histone-dependent transcriptional repression of their promoters is significantly reduced in Vav3 deficient leukemic progenitors. Further, the CUT&RUN sequencing shows the de-repression of Cdkn2a and Cdkn2b loci in Vav3 deficient cells. Whole exome sequencing was carried out to evaluate the mutations and/or deletions of loci known to be modified in the advance stages of the disease. Vav3 deficiency results in nuclear Akt activation and Bmi1 phosphorylation leading to reduced PRC1.4 activity. Nuclear Akt activation in Vav3-/- B-cell progenitors associates with decreased Phlpp2 expression, and forced expression of Phlpp2 significantly restores leukemogenesis of Vav3-deficient B-cell progenitors. These data indicate that nuclear Vav3 GEF activity is required for oncogenic, canonical PRC activity and regulates the epigenetic repression of key factors involved in B-cell differentiation, proliferation and survival. This is the first nuclear GEF activity associated with an oncogenic polycomb repression program, opening a novel area of research for the understanding of the role of Rho GTPases in leukemogenesis.