Homozygous JAK2V617F expression compromises HSC self-renewal. (A) Representative flow cytometry plot for analysis of the E-SLAM HSC population (EPCR⁺CD45⁺CD150⁺CD48⁻). The histogram shows significantly reduced frequency of E-SLAM HSCs in BM from JAK2^R/R mice at 6 to 8 weeks of age compared with WT and JAK2^R/+. (B) JAK2^R/R chimerism (homozygous JAK2V617F allele burden) was reduced in noncompetitive transplant recipients at 4 months compared with 1 month; (C) the reduction of the allele burden was associated with a decline in hematocrit levels in 9 individual recipients of JAK2^R/R BM cells. (D) Histograms show the percentage of total donor chimerism (% of CD45.2⁺ cells) in noncompetitive transplant recipients over time. (E) Histograms show the percentage of donor chimerism in myeloid cells (% of CD45.2⁺ cells in total Mac-1⁺ and/or Ly6G⁺ cells) in noncompetitive transplant recipients over time. (F) Histograms show reduced total repopulating capacity of BM cells from 6- to 8-week-old donor mice (backcrossed 10 generations onto C57/Bl6 background) in competitive transplant recipients. Histograms are presented as a ratio of donor/(donor+comp). (G,H) Histograms show the repopulating capacity of donor cells in the myeloid and lymphoid lineages in competitive transplant recipients (ratio of donor/(donor+comp) in total Mac-1⁺ and/or Ly6G⁺ cells). Asterisks indicate significant differences by Student t test (*P < .05; **P < .01). Data are shown as mean ± SEM. (I) HSCs from JAK2^R/R mice display a repopulating defect. Ten E-SLAM cells from 8- to 10-week-old JAK2^+/+, JAK2^R/+, and JAK2^R/R mice on a C57Bl/6 background were injected per recipient (C57Bl/6 CD45.1), with total of 5 recipients for each genotype. The table shows data from 6 months posttransplantation in which donor cells were assessed for overall chimerism as well as contributions to lymphoid and myeloid elements of the blood system. Animals were considered to have received an HSC if they had >1% contribution to each lineage at 24 weeks posttransplantation. JAK2^R/R HSC recipients showed lower overall repopulation levels and an absence (<0.5%) of granulocytes and macrophages.