Germline Neurofibromin 1 mutation enhances the anti-tumour immune response and decreases juvenile myelomonocytic leukaemia tumourigenicity.

Juvenile myelomonocytic leukaemia (JMML) is an aggressive paediatric leukaemia characterized by mutations in five canonical RAS pathway genes, including the NF1 gene. JMML is driven by germline NF1 gene mutations, with additional somatic aberrations resulting in the NF1 biallelic inactivation, leading to disease progression. Germline mutations in the NF1 gene alone primarily cause benign neurofibromatosis type 1 (NF1) tumours rather than malignant JMML, yet the underlying mechanism remains unclear. Here, we demonstrate that with reduced NF1 gene dose, immune cells are promoted in anti-tumour immune response. Comparing the biological properties of JMML and NF1 patients, we found that not only JMML but also NF1 patients driven by NF1 mutations could increase monocytes generation. But monocytes cannot further malignant development in NF1 patients. Utilizing haematopoietic and macrophage differentiation from iPSCs, we revealed that NF1 mutations or knockout (KO) recapitulated the classical haematopoietic pathological features of JMML with reduced NF1 gene dose. NF1 mutations or KO promoted the proliferation and immune function of NK cells and iMacs derived from iPSCs. Moreover, NF1-mutated iNKs had a high capacity to kill NF1-KO iMacs. NF1-mutated or KO iNKs administration delayed leukaemia progression in a xenograft animal model. Our findings demonstrate that germline NF1 mutations alone cannot directly drive JMML development and suggest a potential cell immunotherapy for JMML patients.

Potential clinical use of azacitidine and MEK inhibitor combination therapy in PTPN11-mutated juvenile myelomonocytic leukemia.

Juvenile myelomonocytic leukemia (JMML) is a rare myeloproliferative neoplasm of childhood. The molecular hallmark of JMML is hyperactivation of the Ras/MAPK pathway with the most common cause being mutations in the gene PTPN11, encoding the protein tyrosine phosphatase SHP2. Current strategies for treating JMML include using the hypomethylating agent, 5-azacitidine (5-Aza) or MEK inhibitors trametinib and PD0325901 (PD-901), but none of these are curative as monotherapy. Utilizing an Shp2E76K/+ murine model of JMML, we show that the combination of 5-Aza and PD-901 modulates several hematologic abnormalities often seen in JMML patients, in part by reducing the burden of leukemic hematopoietic stem and progenitor cells (HSC/Ps). The reduced JMML features in drug-treated mice were associated with a decrease in p-MEK and p-ERK levels in Shp2E76K/+ mice treated with the combination of 5-Aza and PD-901. RNA-sequencing analysis revealed a reduction in several RAS and MAPK signaling-related genes. Additionally, a decrease in the expression of genes associated with inflammation and myeloid leukemia was also observed in Shp2E76K/+ mice treated with the combination of the two drugs. Finally, we report two patients with JMML and PTPN11 mutations treated with 5-Aza, trametinib, and chemotherapy who experienced a clinical response because of the combination treatment.

Targeted therapy in juvenile myelomonocytic leukemia: Where are we now?

Juvenile myelomonocytic leukemia (JMML) is a rare and aggressive clonal neoplasm of early childhood, classified as an overlap myeloproliferative/myelodysplastic neoplasm by the World Health Organization. In 90% of the patients with JMML, typical initiating mutations in the canonical Ras pathway genes NF1, PTPN11, NRAS, KRAS, and CBL can be identified. Hematopoietic stem cell transplantation (HSCT) currently is the established standard of care in most patients, although long-term survival is still only 50-60%. Given the limited therapeutic options and the important morbidity and mortality associated with HSCT, new therapeutic approaches are urgently needed. Hyperactivation of the Ras pathway as disease mechanism in JMML lends itself to the use of targeted therapy. Targeted therapy could play an important role in the future treatment of patients with JMML. This review presents a comprehensive overview of targeted therapies already developed and evaluated in vitro and in vivo in patients with JMML.

Progenitor Hierarchy of Chronic Myelomonocytic Leukemia Identifies Inflammatory Monocytic-Biased Trajectory Linked to Worse Outcomes.

Myeloblast expansion is a hallmark of disease progression and comprises CD34+ hematopoietic stem and progenitor cells (HSPC). How this compartment evolves during disease progression in chronic myeloid neoplasms is unknown. Using single-cell RNA sequencing and high-parameter flow cytometry, we show that chronic myelomonocytic leukemia (CMML) CD34+ HSPC can be classified into three differentiation trajectories: monocytic, megakaryocyte-erythroid progenitor (MEP), and normal-like. Hallmarks of monocytic-biased trajectory were enrichment of CD120b+ inflammatory granulocyte-macrophage progenitor (GMP)-like cells, activated cytokine receptor signaling, phenotypic hematopoietic stem cell (HSC) depletion, and adverse outcomes. Cytokine receptor diversity was generally an adverse feature and elevated in CD120b+ GMPs. Hypomethylating agents decreased monocytic-biased cells in CMML patients. Given the enrichment of RAS pathway mutations in monocytic-biased cells, NRAS-competitive transplants and LPS-treated xenograft models recapitulated monocytic-biased CMML, suggesting that hematopoietic stress precipitates the monocytic-biased state. Deconvolution of HSPC compartments in other myeloid neoplasms and identifying therapeutic strategies to mitigate the monocytic-biased differentiation trajectory should be explored. SIGNIFICANCE: Our findings establish that multiple differentiation states underlie CMML disease progression. These states are negatively augmented by inflammation and positively affected by hypomethylating agents. Furthermore, we identify HSC depletion and expansion of GMP-like cells with increased cytokine receptor diversity as a feature of myeloblast expansion in inflammatory chronic myeloid neoplasms. This article is highlighted in the In This Issue feature, p. 476.

Cooperation between KDM6B overexpression and TET2 deficiency in the pathogenesis of chronic myelomonocytic leukemia.

Loss-of-function TET2 mutations are recurrent somatic lesions in chronic myelomonocytic leukemia (CMML). KDM6B encodes a histone demethylase involved in innate immune regulation that is overexpressed in CMML. We conducted genomic and transcriptomic analyses in treatment naïve CMML patients and observed that the patients carrying both TET2 mutations and KDM6B overexpression constituted 18% of the cohort and 42% of patients with TET2 mutations. We therefore hypothesized that KDM6B overexpression cooperated with TET2 deficiency in CMML pathogenesis. We developed a double-lesion mouse model with both aberrations, and discovered that the mice exhibited a more prominent CMML-like phenotype than mice with either Tet2 deficiency or KDM6B overexpression alone. The phenotype includes monocytosis, anemia, splenomegaly, and increased frequencies and repopulating activity of bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs). Significant transcriptional alterations were identified in double-lesion mice, which were associated with activation of proinflammatory signals and repression of signals maintaining genome stability. Finally, KDM6B inhibitor reduced BM repopulating activity of double-lesion mice and tumor burden in mice transplanted with BM-HSPCs from CMML patients with TET2 mutations. These data indicate that TET2 deficiency and KDM6B overexpression cooperate in CMML pathogenesis of and that KDM6B could serve as a potential therapeutic target in this disease.

Inflammatory response in hematopoietic stem and progenitor cells triggered by activating SHP2 mutations evokes blood defects.

Gain-of-function mutations in the protein-tyrosine phosphatase SHP2 are the most frequently occurring mutations in sporadic juvenile myelomonocytic leukemia (JMML) and JMML-like myeloproliferative neoplasm (MPN) associated with Noonan syndrome (NS). Hematopoietic stem and progenitor cells (HSPCs) are the disease propagating cells of JMML. Here, we explored transcriptomes of HSPCs with SHP2 mutations derived from JMML patients and a novel NS zebrafish model. In addition to major NS traits, CRISPR/Cas9 knock-in Shp2D61G mutant zebrafish recapitulated a JMML-like MPN phenotype, including myeloid lineage hyperproliferation, ex vivo growth of myeloid colonies, and in vivo transplantability of HSPCs. Single-cell mRNA sequencing of HSPCs from Shp2D61G zebrafish embryos and bulk sequencing of HSPCs from JMML patients revealed an overlapping inflammatory gene expression pattern. Strikingly, an anti-inflammatory agent rescued JMML-like MPN in Shp2D61G zebrafish embryos. Our results indicate that a common inflammatory response was triggered in the HSPCs from sporadic JMML patients and syndromic NS zebrafish, which potentiated MPN and may represent a future target for JMML therapies.

Juvenile myelomonocytic leukaemia is a childhood blood cancer. It is more common in children with a genetic condition called Noonan Syndrome, which causes problems with development in many parts of the body. The most frequent cause is a mutation in a protein called Src homology region 2 domain-containing phosphatase-2, or SHP2 for short. Juvenile myelomonocytic leukaemia starts in the stem cells that normally become blood cells. In children with Noonan Syndrome, these cells show signs of problems before leukaemia begins. Recreating Noonan Syndrome in an animal could shed light on how this childhood cancer develops, but doing this is not straightforward. One option is to use zebrafish, a species of fish in which the embryos are transparent, allowing scientists to watch their blood cells developing under a microscope. They also share many genes with humans, including SHP2. Solman et al. genetically modified zebrafish so they would carry one of the most common mutations seen in children with Noonan Syndrome in the SHP2 protein. The fish had many of the typical features of the condition, including problems producing blood cells. Single cell analysis of the stem cells that become these blood cells showed that, in the mutated fish, these cells had abnormally high levels of activity in genes involved in inflammation. Treating the fish with an anti-inflammatory drug, dexamethasone, reversed the problem. When Solman et al. investigated stem cells from human patients with juvenile myelomonocytic leukaemia, they found the same high levels of activity in inflammatory genes. The current treatment for juvenile myelomonocytic leukaemia is a stem cell transplant, which is only successful in around half of cases. Finding a way to prevent the cancer from developing altogether could save lives. This new line of zebrafish allows researchers to study Noonan Syndrome in more detail, and to test new treatments. A next step could be to find out whether anti-inflammatory drugs have the same effects in mammals as they do in fish.

Inhibition of BTK and PI3Kδ impairs the development of human JMML stem and progenitor cells.

Juvenile myelomonocytic leukemia (JMML) is an aggressive myeloproliferative neoplasia that lacks effective targeted chemotherapies. Clinically, JMML manifests as monocytic leukocytosis, splenomegaly with consequential thrombocytopenia. Most commonly, patients have gain-of-function (GOF) oncogenic mutations in PTPN11 (SHP2), leading to Erk and Akt hyperactivation. Mechanism(s) involved in co-regulation of Erk and Akt in the context of GOF SHP2 are poorly understood. Here, we show that Bruton's tyrosine kinase (BTK) is hyperphosphorylated in GOF Shp2-bearing cells and utilizes B cell adaptor for PI3K to cooperate with p110δ, the catalytic subunit of PI3K. Dual inhibition of BTK and p110δ reduces the activation of both Erk and Akt. In vivo, individual targeting of BTK or p110δ in a mouse model of human JMML equally reduces monocytosis and splenomegaly; however, the combined treatment results in a more robust inhibition and uniquely rescues anemia and thrombocytopenia. RNA-seq analysis of drug-treated mice showed a profound reduction in the expression of genes associated with leukemic cell migration and inflammation, leading to correction in the infiltration of leukemic cells in the lung, liver, and spleen. Remarkably, in a patient derived xenograft model of JMML, leukemia-initiating stem and progenitor cells were potently inhibited in response to the dual drug treatment.

JMML tumor cells disrupt normal hematopoietic stem cells by imposing inflammatory stress through overproduction of IL-1ß.

Development of normal blood cells is often suppressed in juvenile myelomonocytic leukemia (JMML), a myeloproliferative neoplasm (MPN) of childhood, causing complications and impacting therapeutic outcomes. However, the mechanism underlying this phenomenon remains uncharacterized. To address this question, we induced the most common mutation identified in JMML (Ptpn11E76K) specifically in the myeloid lineage with hematopoietic stem cells (HSCs) spared. These mice uniformly developed a JMML-like MPN. Importantly, HSCs in the same bone marrow (BM) microenvironment were aberrantly activated and differentiated at the expense of self-renewal. As a result, HSCs lost quiescence and became exhausted. A similar result was observed in wild-type (WT) donor HSCs when co-transplanted with Ptpn11E76K/+ BM cells into WT mice. Co-culture testing demonstrated that JMML/MPN cells robustly accelerated differentiation in mouse and human normal hematopoietic stem/progenitor cells. Cytokine profiling revealed that Ptpn11E76K/+ MPN cells produced excessive IL-1ß, but not IL-6, T NF-α, IFN-γ, IL-1α, or other inflammatory cytokines. Depletion of the IL-1ß receptor effectively restored HSC quiescence, normalized their pool size, and rescued them from exhaustion in Ptpn11E76K/+/IL-1R-/- double mutant mice. These findings suggest IL-1ß signaling as a potential therapeutic target for preserving normal hematopoietic development in JMML.

Mutational landscape of Juvenile Myelomonocytic Leukemia (JMML)-A real-world context.

INTRODUCTION: Juvenile myelomonocytic leukemia (JMML) is a rare childhood neoplasm (<5% cases), which has been categorized under myelodysplastic/myeloproliferative neoplasms (MDS/MPN) in the recent classification by the World Health Organization. METHODS: We developed a 51-gene (151.5kB) low-cost targeted myeloid panel based on single-molecule molecular inversion probes to comprehensively evaluate the genomic profile of Juvenile myelomonocytic leukemia (JMML). RESULTS: A total of 50 children with clinical and pathological features of JMML were sequenced at high coverage. Among the 50 patients, 44(88%) harbored mutations in one of the RAS/MAPK-pathway genes, most frequently in NRAS (32%), followed by PTPN11 (28%) and NF1 (22%). One-fifth of children had more than one mutation, with 5 cases harboring two RAS pathway mutations. Monosomy 7 was detected in 32% (16) patients, and five of these did not harbor any RAS pathway mutations. Children with monosomy 7 showed shorter overall survival compared with their wild-type counterparts (P = .02). CONCLUSION: Our study highlights that comprehensive genomic profiling identifies at least one mutation in almost 90% of JMML patients. Performing genomic analysis at baseline might help in triaging children with JMML for allogenic stem cell transplant in resource-constrained settings.

Mutant SETBP1 enhances NRAS-driven MAPK pathway activation to promote aggressive leukemia.

Mutations in SET-binding protein 1 (SETBP1) are associated with poor outcomes in myeloid leukemias. In the Ras-driven leukemia, juvenile myelomonocytic leukemia, SETBP1 mutations are enriched in relapsed disease. While some mechanisms for SETBP1-driven oncogenesis have been established, it remains unclear how SETBP1 specifically modulates the biology of Ras-driven leukemias. In this study, we found that when co-expressed with Ras pathway mutations, SETBP1 promoted oncogenic transformation of murine bone marrow in vitro and aggressive myeloid leukemia in vivo. We demonstrate that SETBP1 enhances the NRAS gene expression signature, driving upregulation of mitogen-activated protein kinase (MAPK) signaling and downregulation of differentiation pathways. SETBP1 also enhances NRAS-driven phosphorylation of MAPK proteins. Cells expressing NRAS and SETBP1 are sensitive to inhibitors of the MAPK pathway, and treatment with the MEK inhibitor trametinib conferred a survival benefit in a mouse model of NRAS/SETBP1-mutant disease. Our data demonstrate that despite driving enhanced MAPK signaling, SETBP1-mutant cells remain susceptible to trametinib in vitro and in vivo, providing encouraging preclinical data for the use of trametinib in SETBP1-mutant disease.

Pediatric Neoplasms Presenting with Monocytosis.

PURPOSE OF REVIEW: Juvenile myelomonocytic leukemia (JMML) is a rare but severe pediatric neoplasm with hematopoietic stem cell transplant as its only established curative option. The development of targeted therapeutics for JMML is being guided by an understanding of the pathobiology of this condition. Here, we review JMML with an emphasis on genetics in order to (i) demonstrate the relationship between JMML genotype and clinical phenotype and (ii) explore potential genetic targets of novel JMML therapies. RECENT FINDINGS: DNA hypermethylation studies have demonstrated consistently that methylation is related to disease severity. Increasing understanding of methylation in JMML may open the door to novel therapies, such as DNA methyltransferase inhibitors. The PI3K/AKT/MTOR, JAK/STAT, and RAF/MEK/ERK pathways are being investigated as therapeutic targets for JMML. Future therapy for JMML will be driven by an increased understanding of pathobiology. Targeted therapeutic approaches hold potential for improving outcomes in patients with JMML.

Myelodysplastic syndrome/myeloproliferative neoplasm overlap syndromes: a focused review.

Myelodysplastic syndrome (MDS)/myeloproliferative neoplasm (MPN) overlap syndromes are unique myeloid neoplasms, with overlapping features of MDS and MPN. They consist of four adult onset entities including chronic myelomonocytic leukemia (CMML), MDS/MPN-ring sideroblasts-thrombocytosis (MDS/MPN-RS-T), BCR-ABL1 negative atypical chronic myeloid leukemia (aCML) and MDS/MPN-unclassifiable (MDS/MPN-U); with juvenile myelomonocytic leukemia (JMML) being the only pediatric onset entity. Among these overlap neoplasms, CMML is the most frequent and is hallmarked by the presence of sustained peripheral blood monocytosis with recurrent mutations involving TET2 (60%), SRSF2 (50%) and ASXL1 (40%); with RAS pathway mutations and JAK2V617F being relatively enriched in proliferative CMML subtypes (WBC ≥13 × 109/L). CMML usually presents in the 7th decade of life, with a male preponderance and is associated with a median overall survival of <36 months. Adverse prognosticators in CMML include increasing age, high WBC, presence of circulating immature myeloid cells, anemia, thrombocytopenia and truncating ASXL1 mutations. While allogeneic stem cell transplantation remains the only curative option, given the late onset of this neoplasm and high frequency of comorbidities, most patients remain ineligible. Hypomethylating agents such as azacitidine, decitabine and oral decitabine/cedazuridine have been US FDA approved for the management of CMML, with overall response rates of 40-50% and complete remission rates of <20%. While these agents epigenetically restore hematopoiesis in a subset of responding patients, they do not impact mutational allele burdens and eventual disease progression to AML remains inevitable. Newer treatment modalities exploiting epigenetic, signaling and splicing abnormalities commonly seen in CMML are much needed.

Molecular genetics of MDS/MPN overlap syndromes.

The myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are a heterogenous group of myeloid malignancies hallmarked by clinicopathologic features that overlap with myelodysplastic syndromes and myeloproliferative neoplasms. Formally recognized by the World Health Organization, this group includes the entities chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, atypical chronic myeloid leukemia, MDS/MPN with ring sideroblasts and thrombocytosis and MDS/MPN, unclassifiable. Advancements in next generation sequencing have begun to unravel the molecular underpinnings of these diseases, identifying an array of recurrently mutated genes involved in epigenetic regulation, RNA splicing, transcription, and cell signaling. Despite molecular overlap with other myeloid malignancies, each entity displays a unique spectrum of somatic mutations supporting their unique pathobiology and clinical features. Importantly, molecular profiling is becoming an integral tool utilized in routine clinical practice. This review summarizes our current understanding of the molecular pathogenesis of overlap syndromes and details the impact of somatic mutations in diagnostic, prognostic, and therapeutic decision-making.

CD14/16 monocyte profiling in juvenile myelomonocytic leukemia.

Monocyte subset analysis by flow cytometry has been shown to be a useful diagnostic tool in chronic myelomonocytic leukemia in adults. An increase in the classical monocyte fraction (CD14++/CD16-) greater than 94.0% of total monocytes is considered highly sensitive and specific in distinguishing chronic myelomonocytic leukemia from other myeloproliferative disorders. In a pilot study of juvenile myelomonocytic leukemia cases, we noted that CD14++/CD16- monocyte fraction was >95% in de novo juvenile myelomonocytic leukemia (JMML) with somatic PTPN11 mutations but normal in those with monosomy 7 or Noonan syndrome. Monocyte subgroup profiling by itself is not diagnostic of JMML but may distinguish molecular subgroups within JMML.

Juvenile myelomonocytic leukemia - A bona fide RASopathy syndrome.

Juvenile myelomonocytic leukemia (JMML) is a pediatric myelodysplastic/myeloproliferative neoplasm overlap syndrome with sustained peripheral blood monocytosis, aggressive features, and poor outcomes. In >90% of cases JMML is driven by germline or somatic mutations involving the canonical RAS pathway (PTPN11, NRAS, CBL, KRAS and NF1), with somatic mutations/alterations in RAS pathway genes (second hit), SETBP1, ASXL1 and JAK3 resulting in disease progression. While spontaneous regression has been seen in germline PTPN11 and CBL mutant JMML, in most patients, allogeneic stem cell transplant is the only curative modality. JMML shares several phenotypic features with its adult counterpart proliferative, chronic myelomonocytic leukemia (pCMML). pCMML largely occurs due to RAS pathway mutations that occur in the context of age related clonal hematopoiesis (TET2, SRSF2, ASXL1), while JMML is a bona fide RASopathy, with additional somatic mutations, including in epigenetic regulators genes resulting in disease progression.

After 95 years, it's time to eRASe JMML.

Juvenile myelomonocytic leukaemia (JMML) is a rare clonal disorder of early childhood. Constitutive activation of the RAS pathway is the initial event in JMML. Around 90% of patients diagnosed with JMML carry a mutation in the PTPN11, NRAS, KRAS, NF1 or CBL genes. It has been demonstrated that after this first genetic event, an additional somatic mutation or epigenetic modification is involved in disease progression. The available genetic and clinical data have enabled researchers to establish relationships between JMML and several clinical conditions, including Noonan syndrome, Ras-associated lymphoproliferative disease, and Moyamoya disease. Despite scientific progress and the development of more effective treatments, JMML is still a deadly disease: the 5-year survival rate is ~50%. Here, we report on recent research having led to a better understanding of the genetic and molecular mechanisms involved in JMML.

Essential role of PTPN11 mutation in enhanced haematopoietic differentiation potential of induced pluripotent stem cells of juvenile myelomonocytic leukaemia.

We established mutated and non-mutated induced pluripotent stem cell (iPSC) clones from a patient with PTPN11 (c.226G>A)-mutated juvenile myelomonocytic leukaemia (JMML). Both types of iPSCs fulfilled the quality criteria. Mutated iPSC colonies generated significantly more CD34+ and CD34+ CD45+ cells compared to non-mutated iPSC colonies in a culture coated with irradiated AGM-S3 cells to which four growth factors were added sequentially or simultaneously. The haematopoietic differentiation potential of non-mutated JMML iPSC colonies was similar to or lower than that of iPSC colonies from a healthy individual. The PTPN11 mutation coexisted with the OSBP2 c.389C>T mutation. Zinc-finger nuclease-mediated homologous recombination revealed that correction of PTPN11 mutation in iPSCs with PTPN11 and OSBP2 mutations resulted in reduced CD34+ cell generation to a level similar to that obtained with JMML iPSC colonies with the wild-type of both genes, and interestingly, to that obtained with normal iPSC colonies. Transduction of the PTPN11 mutation into JMML iPSCs with the wild-type of both genes increased CD34+ cell production to a level comparable to that obtained with JMML iPSC colonies harbouring the two genetic mutations. Thus, PTPN11 mutation may be the most essential abnormality to confer an aberrant haematopoietic differentiation potential in this disorder.

JMML genomics and decisions.

Juvenile myelomonocytic leukemia (JMML) is a unique clonal hematopoietic disorder of early childhood characterized by hyperactivation of the RAS signal transduction pathway. Approximately 90% of patients harbor molecular alteration in 1 of 5 genes (PTPN11, NRAS, KRAS, NF1, CBL), which define genetically and clinically distinct JMML subtypes. Three subtypes, PTPN11- , NRAS-, and KRAS-mutated JMML, are characterized by heterozygous somatic gain-of-function mutations in non syndromic children, while two subtypes, JMML in neurofibromatosis type 1 and in JMML in children with CBL syndrome, are characterized by germ line RAS disease and acquired biallelic inactivation of the respective tumor suppressor genes in hematopoietic cells. In addition to the initiating RAS pathway lesion, secondary genetic alterations within and outside of the RAS pathway are detected in about half the patients. Most recently, genome-wide DNA methylation profiles identified distinct methylation signatures correlating with clinical and genetic features and highly predictive of outcome. JMML is a stem cell disorder, and most JMML patients require allogeneic stem cell transplantation for long-term survival. However, spontaneous disease regression is noted in the majority of children with CBL-mutated JMML and in some NRAS-mutated cases. In the absence of 1 of the 5 canonical RAS pathway alteration, rare mutations in other RAS genes and non-JMML myeloproliferative disorders need to be excluded. Understanding the genetic basis of myeloproliferative disorders in early childhood will greatly improve clinical decision making.