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Recent advances in single-cell molecular analytical methods and clonal growth assays are enabling more refined models of human hematopoietic lineage restriction processes to be conceptualized. Here, we report the results of integrating single-cell proteome measurements with clonally determined lymphoid, neutrophilic/monocytic, and/or erythroid progeny outputs from >1000 index-sorted CD34+human cord blood cells in short-term cultures with and without stromal cells. Surface phenotypes of functionally examined cells were individually mapped onto a molecular landscape of the entire CD34+compartment constructed from single-cell mass cytometric measurements of 14 cell surface markers, 20 signaling/cell cycle proteins, and 6 transcription factors in ∼300 000 cells. This analysis showed that conventionally defined subsets of CD34+cord blood cells are heterogeneous in their functional properties, transcription factor content, and signaling activities. Importantly, this molecular heterogeneity was reduced but not eliminated in phenotypes that were found to display highly restricted lineage outputs. Integration of the complete proteomic and functional data sets obtained revealed a continuous probabilistic topology of change that includes a multiplicity of lineage restriction trajectories. Each of these reflects progressive but variable changes in the levels of specific signaling intermediates and transcription factors but shared features of decreasing quiescence. Taken together, our results suggest a model in which increasingly narrowed hematopoietic output capabilities in neonatal CD34+cord blood cells are determined by a history of external stimulation in combination with innately programmed cell state changes.

Myeloproliferative neoplasms (MPNs) are associated with recurrent activating mutations of signaling proteins such as Janus kinase 2 (JAK2). However, the actual downstream signaling events and how these alter myeloid homeostasis are poorly understood. We developed an assay to measure basal levels of phosphorylated signaling intermediates by flow cytometry during myeloid differentiation in MPN patients. Our study provides the first systematic demonstration of specific signaling events and their comparison with disease phenotype and JAK2 mutation status. We demonstrate increased basal signaling in MPN patients, which occurs in both early and later stages of myeloid differentiation. In addition, the pattern of signaling is not correlated with JAK2 mutation status and signaling intensity is poorly correlated with mutant JAK2 allele burden. In contrast, signaling differences are detected between different MPN disease phenotypes. Finally, we demonstrate that signaling can be inhibited by a JAK2-selective small molecule, but that this inhibition is not JAK2 V617F specific, because MPN patients with mutant JAK2, wild-type JAK2, and control patients were inhibited to a similar degree. Our data suggest that, in addition to JAK2 mutations, other factors contribute significantly to the MPN phenotype, results that are relevant to both the pathogenesis and therapy of MPN.

The 46/1 JAK2haplotype predisposes to V617F-positive myeloproliferative neoplasms, but the underlying mechanism is obscure. We analyzed essential thrombocythemia patients entered into the PT-1 studies and, as expected, found that 46/1 was overrepresented in V617F-positive cases (n = 404) versus controls (n = 1492, P= 3.9 × 10−11). The 46/1 haplotype was also overrepresented in cases without V617F (n = 347, P= .009), with an excess seen for both MPLexon 10 mutated and V617F, MPLexon 10 nonmutated cases. Analysis of further MPL-positive, V617F-negative cases confirmed an excess of 46/1 (n = 176, P= .002), but no association between MPLmutations and MPLhaplotype was seen. An excess of 46/1 was also seen in JAK2exon 12 mutated cases (n = 69, P= .002), and these mutations preferentially arose on the 46/1 chromosome (P= .029). No association between 46/1 and clinical or laboratory features was seen in the PT-1 cohort either with or without V617F. The excess of 46/1 in JAK2exon 12 cases is compatible with both the “hypermutability” and “fertile ground” hypotheses, but the excess in MPL-mutated cases argues against the former. No difference in sequence, splicing, or expression of JAK2was found on 46/1 compared with other haplotypes, suggesting that any functional difference of JAK2on 46/1, if it exists, must be relatively subtle.

Acute myeloid leukemia (AML) may follow a JAK2-positive myeloproliferative neoplasm (MPN), although the mechanisms of disease evolution, often involving loss of mutant JAK2, remain obscure. We studied 16 patients with JAK2-mutant (7 of 16) or JAK2wild-type (9 of 16) AML after a JAK2-mutant MPN. Primary myelofibrosis or myelofibrotic transformation preceded all 7 JAK2-mutant but only 1 of 9 JAK2wild-type AMLs (P= .001), implying that JAK2-mutant AML is preceded by mutation(s) that give rise to a “myelofibrosis” phenotype. Loss of the JAK2mutation by mitotic recombination, gene conversion, or deletion was excluded in all wild-type AMLs. A search for additional mutations identified alterations of RUNX1, WT1, TP53, CBL, NRAS, and TET2, without significant differences between JAK2-mutant and wild-type leukemias. In 4 patients, mutations in TP53, CBL, or TET2were present in JAK2wild-type leukemic blasts but absent from the JAK2-mutant MPN. By contrast in a chronic-phase patient, clones harboring mutations in JAK2or MPLrepresented the progeny of a shared TET2-mutant ancestral clone. These results indicate that different pathogenetic mechanisms underlie transformation to JAK2wild-type and JAK2-mutant AML, show that TET2mutations may be present in a clone distinct from that harboring a JAK2mutation, and emphasize the clonal heterogeneity of the MPNs.

Activating mutations of MPLexon 10 have been described in a minority of patients with idiopathic myelofibrosis (IMF) or essential thrombocythemia (ET), but their prevalence and clinical significance are unclear. Here we demonstrate that MPLmutations outside exon 10 are uncommon in platelet cDNA and identify 4 different exon 10 mutations in granulocyte DNA from a retrospective cohort of 200 patients with ET or IMF. Allele-specific polymerase chain reaction was then used to genotype 776 samples from patients with ET entered into the PT-1 studies. MPLmutations were identified in 8.5% of JAK2 V617F−patients and a single V617F+patient. Patients carrying the W515Kallele had a significantly higher allele burden than did those with the W515Lallele, suggesting a functional difference between the 2 variants. Compared with V617F+ET patients, those with MPLmutations displayed lower hemoglobin and higher platelet levels at diagnosis, higher serum erythropoietin levels, endogenous megakaryocytic but not erythroid colony growth, and reduced bone marrow erythroid and overall cellularity. Compared with V617F−patients, those with MPLmutations were older with reduced bone marrow cellularity but could not be identified as a discrete clinicopathologic subgroup. MPLmutations lacked prognostic significance with respect to thrombosis, major hemorrhage, myelofibrotic transformation or survival.

The identification of an acquired mutation of JAK2in patients with myeloproliferative disorders has raised questions about the relationship between mutation-positive and mutation-negative subtypes, timing of the JAK2mutation, and molecular mechanisms of disease progression. Here we demonstrate that patients with V617F-essential thrombocythemia do not commonly progress to become V617F+. Consistent with the concept of distinct pathogenetic mechanisms, we show that patients with and without the JAK2mutation have different patterns of cytogenetic abnormality, with virtually all patients carrying the 20q deletion or trisomy 9 being V617F+. We also investigated the existence of a “pre-JAK2” phase by comparing the proportion of clonally derived granulocytes, estimated from X-chromosome inactivation patterns (XCIPs), with the proportion of V617F+granulocytes. Our results demonstrate that inherent XCIP variability between granulocytes and T cells produces a systematically biased pattern of results that may be misinterpreted as evidence for an excess of clonally derived granulocytes, an observation that limits the utility of XCIP analysis in this context. Lastly, we studied 4 patients with V617F+myeloproliferative disorders who subsequently developed acute myeloid leukemia. In 3 patients the leukemic cells were V617F-, suggesting that in these patients the leukemia arose in a V617F-cell.

The identification of an acquired mutation of JAK2 in patients with myeloproliferative disorders has raised questions about the relationship between mutation-positive and mutation-negative subtypes, timing of the JAK2 mutation, and molecular mechanisms of disease progression. Here we demonstrate that patients with V617F- essential thrombocythemia do not commonly progress to become V617F+. Consistent with the concept of distinct pathogenetic mechanisms, we show that patients with and without the JAK2 mutation have different patterns of cytogenetic abnormality, with virtually all patients carrying the 20q deletion or trisomy 9 being V617F+. We also investigated the existence of a “pre-JAK2” phase by comparing the proportion of clonally derived granulocytes, estimated from X-chromosome inactivation patterns (XCIPs), with the proportion of V617F+ granulocytes. Our results demonstrate that inherent XCIP variability between granulocytes and T cells produces a systematically biased pattern of results that may be misinterpreted as evidence for an excess of clonally derived granulocytes, an observation that limits the utility of XCIP analysis in this context. Lastly, we studied 4 patients with V617F+ myeloproliferative disorders who subsequently developed acute myeloid leukemia. In 3 patients the leukemic cells were V617F-, suggesting that in these patients the leukemia arose in a V617F- cell.

Erythrocytosis can arise from deregulation of the erythropoietin (Epo) axis resulting from defects in the oxygen-sensing pathway. Epo synthesis is controlled by the hypoxia inducible factor (HIF) complex, composed of an α and a β subunit. There are 2 main α subunits, HIF-1α and HIF-2α. Recently, a HIF-2α Gly537Trp mutation was identified in a family with erythrocytosis. This raises the possibility of HIF2Amutations being associated with other cases of erythrocytosis. We now report a subsequent analysis of HIF2Ain a cohort of 75 erythrocytosis patients and identify 4 additional patients with novel heterozygous Met535Val and Gly537Arg mutations. All patients presented at a young age with elevated serum Epo. Mutations at Gly-537 account for 4 of 5 HIF2Amutations associated with erythrocytosis. These findings support the importance of HIF-2α in human Epo regulation and warrant investigation of HIF2Ain patients with unexplained erythrocytosis.

Background:Assessment of the growth and differentiation of human hematopoietic cells transplanted into immunodeficient mice has become an important method for evaluating the numbers and properties of cells with engraftment potential and the progeny they can generate in an in vivosetting. Such xenograft models have undergone numerous modifications and have also been applied with increasing success to primary sources of many types of malignant as well as normal human hematopoietic cells. These advances include the incorporation of cDNAs encoding various human-specific growth factor genes into the genomes of the widely used and long-lived NOD-scid-IL2Rγcnull(NSG) mouse, the creation of regenerated human bone ossicles within the mice to more closely mimic the human marrow microenvironment, recognition of the superior supportive activity of female mice, and more recently of NSG mice additionally compromised by their genetic acquisition of a c-kitdeficiency. Exploitation of these modifications to recipients of human hematopoietic cells has greatly increased our understanding of the complex composition of both normal and leukemic populations and their responses to various treatments. Nevertheless, many examples remain of both acute and chronic human myeloid leukemias that do not engraft or engraft very poorly, even in the most permissive mice thus far tested. These include samples of many cases of chronic phase chronic myeloid leukemia (CP-CML), myelodysplastic syndromes (MDS), and approximately one third of all cases of acute myeloid leukemia (AML). We have now created 2 new strains of immunodeficient mice that have the B, T, NK immunodeficiency determined by a SirpαNOD-Rag1-/--IL2Rγc-/-genotype (which is equivalent to that of NSG mice) and a c-kitdeficiency obtained by replacement of the wild-type c-kitgene with a homozygous W41/W41genotype (SRG-W41 mice). The second strain has the same SRG-W41 genotype but is also transgenic for human IL3, GM-CSF, and SCF production (SRG-W41-3GS). NRG/SRG mice were chosen as the parental strain because they support similar levels of chimerism as NSG mice when given a radiobiologically equivalent dose of radiation, but can benefit from split or low dose rate irradiation protocols.

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