Impaired sCFU-E expansion prevents JAK2(V617F)-driven erythrocytosis in MPN. (A) Blood cell counts in transplanted mice expressing EpoR or EpoR(core) together with JAK2 or JAK2(V617F) 3 months after transplantation. (B-C) JAK2(V617F)-driven sCFU-E expansion is defective in transplant-recipient mice expressing EpoR(core). Representative flow plots are shown in (B) and quantifications in (C). (D) Expression of EpoR(core) prevents JAK2(V617F)-induced splenomegaly in JAK2(V617F)KI mice. (E) The numbers of sCFU-E, CFU-E, and Ter119⁺ cells are reduced significantly in EpoR(core)/JAK2(V617F)KI mice compared with EpoR/JAK2(V617F)KI mice. (F) Sorted BFU-E and sCFU-E cells from EpoR(core)/JAK2(V617F)KI mice fail to generate Ter119⁺ progenies in vitro. Cells were cultured in media with SCF, but devoid of Epo. (G) In mice expressing wild-type (WT) EpoR, JAK2(V617F) increases IRS2 messenger RNA (mRNA) expression in sCFU-E and CFU-E cells. (H) IRS2 mRNA expression is reduced significantly in sCFU-E cells from EpoR(core)/JAK2(V617F) mice. (I) IRS2 knockdown inhibits sCFU-E and erythroid progeny growth in vitro. GFP⁺ cells are gated for analyses. sCFU-E fold changes are normalized to shControl, and the relative growth of Ter119⁺ cells are normalized to cell numbers at 24 hours. BM, bone marrow; HCT, hematocrit; SP, spleen; VF, JAK2(V617F); WBC, white blood cell. ∗P < .05, ∗∗P < .01, 2-way ANOVA.