Enforced differentiation of Dnmt3a-KO HSCs leads to bone marrow failure resembling MDS. Noncompetitive transplantation of control (n = 25), Dnmt3a-HET (n = 10), and Dnmt3a-KO (n = 25) WBM. Compiled data are derived from 2 independent transplant cohorts. Control WBM in the first cohort was Mx1-CRE^-:Dnmt3a^fl/fl (n = 10) and in the second cohort Mx1-CRE⁺:Dnmt3a^+/+ (n = 15). Dnmt3a-HET WBM was Mx1-CRE⁺:Dnmt3a^fl/+ (n = 10). (A) Kaplan-Meier survival curve of all mice. † indicates death of a mouse from nonhematopoietic complications. The remainder of the figure shows data only for the Dnmt3a-KO (3a-KO) mice that were diagnosed with MDS at time of sacrifice. Data for control (Ctl) and Dnmt3a-HET (3a-HET) are from day 500 posttransplant. (B) Bone marrow pathology of the MDS phenotype. Erythroid dysplasia indicates orthochromic pronormoblasts with irregular nuclear contours and nuclear budding indicative of dyserythropoiesis. Myeloid dysplasia indicates lobated, hypersegmented neutrophils with pale blue cytoplasm indicative of dysmyelopoiesis. Red arrows indicate cells with described phenotypes. All photomicrographs taken at original magnification ×100. Scale bar represents 10 μm. (C) Peripheral blood counts show leukopenia, neutropenia, and anemia in recipients of Dnmt3a-KO WBM. Dnmt3a-KO MDS mice also showed bone marrow hypercellularity (D) and increased HSC (Lineage⁻ CD45.2⁺ Sca-1⁺ c-Kit⁺ CD34⁻ Flk2⁻) frequency (E). (F) FACS plots showing increase of phenotypically defined HSCs in Dnmt3a-KO recipient mice. Numbers represent percentage of that cell fraction in total bone marrow. Dnmt3a-KO MDS mice showed splenomegaly (G-H) with increased myeloid cell (Gr-1⁺ Mac-1⁺) cell infiltration (I-J). * P < .05, ** P < .01, *** P < .001, **** P < .0001.