p15^Ink4b is a critical target mediating the function of KDM2b/JHDM1b in Hoxa9-Meis1-induced AML. (A) Loss of p15^Ink4b abrogates the effect of Kdm2b/Jhdm1b KD on Hoxa9-Meis1-induced leukemic transformation. Serial methylcellulose replating assay shows that similar numbers of colonies were obtained from p15^Ink4b-null HPCs transduced with Hoxa9-Meis1 regardless of whether Kdm2b/Jhdm1b was knocked down. (B) Splenomegaly is observed in mice that received a transplant with both control KD (CKD/HMG) and Kdm2b/Jhdm1b KD (J1bKD-HMG) Hoxa9-Mesi1-transduced p15^Ink4b-null hematopoietic progenitors. (C) Flow cytometric analysis shows that both J1bKD-HMG and CKD/HMG transduced p15^Ink4b-null hematopoietic progenitor donor cells can repopulate and dominate peripheral blood of recipient mice 4 weeks after transplantation. (D) Survival curve shows that mice that received a transplant with p15^Ink4b-null HPCs transduced with either CKD-HMG or J1bKD-HMG die within 90 days after transplantation. (E) Proposed model for epigenetic regulation of the Ink4b locus in LSCs. In LSCs, p15^Ink4b is suppressed by multiple epigenetic modifiers, including KDM2b/JHDM1b and Polycomb group proteins. In this model, Kdm2b/Jhdm1b is up-regulated by oncogenic stimuli and maintains at a high level in LSCs. Demethylation of H3K36 by KDM2b/JHDM1b and concomitant H2A ubiquitylation by the associated Polycomb group of proteins results in silencing of p15^Ink4b. Conversely, depletion of Kdm2b/Jhdm1b causes an increase in the H3K36me2 level concomitant with the loss of Polycomb group proteins, leading to de-repression of p15^Ink4b, resulting in defects in leukemic cell proliferation and LSC self-renewal.