Fetal and neonatal hematopoietic progenitors are functionally and transcriptionally resistant to Flt3-ITD mutations

Kmt2c restricts G-CSF-driven HSC mobilization and granulocyte production in a methyltransferase-independent manner

Granulocyte colony-stimulating factor (G-CSF) is widely used to enhance myeloid recovery after chemotherapy and to mobilize hematopoietic stem cells (HSCs) for transplantation. Unfortunately, through the course of chemotherapy, cancer patients can acquire leukemogenic mutations that cause therapy-related myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). This raises the question of whether therapeutic G-CSF might potentiate therapy-related MDS/AML by disproportionately stimulating mutant HSCs and other myeloid progenitors. A common mutation in therapy-related MDS/AML involves chromosome 7 deletions that inactivate many tumor suppressor genes, including KMT2C. Here, we show that Kmt2c deletions hypersensitize murine HSCs and myeloid progenitors to G-CSF, as evidenced by increased HSC mobilization and enhanced granulocyte production from granulocyte-monocyte progenitors (GMPs). Furthermore, Kmt2c attenuates the G-CSF response independently from its SET methyltransferase function. Altogether, the data raise concerns that monosomy 7 can hypersensitize progenitors to G-CSF, such that clinical use of G-CSF may amplify the risk of therapy-related MDS/AML.

HES1 is required for mouse fetal hematopoiesis

BACKGROUND

Hematopoiesis in mammal is a complex and highly regulated process in which hematopoietic stem cells (HSCs) give rise to all types of differentiated blood cells. Previous studies have shown that hairy and enhancer of split (HES) repressors are essential regulators of adult HSC development downstream of Notch signaling.

METHODS

In this study, we investigated the role of HES1, a member of HES family, in fetal hematopoiesis using an embryonic hematopoietic specific Hes1 conditional knockout mouse model by using phenotypic flow cytometry, histopathology analysis, and functional in vitro colony forming unit (CFU) assay and in vivo bone marrow transplant (BMT) assay.

RESULTS

We found that loss of Hes1 in early embryonic stage leads to smaller embryos and fetal livers, decreases hematopoietic stem progenitor cell (HSPC) pool, results in defective multi-lineage differentiation. Functionally, fetal hematopoietic cells deficient for Hes1 exhibit reduced in vitro progenitor activity and compromised in vivo repopulation capacity in the transplanted recipients. Further analysis shows that fetal hematopoiesis defects in Hes1fl/flFlt3Cre embryos are resulted from decreased proliferation and elevated apoptosis, associated with de-repressed HES1 targets, p27 and PTEN in Hes1-KO fetal HSPCs. Finally, pharmacological inhibition of p27 or PTEN improves fetal HSPCs function both in vitro and in vivo.

CONCLUSION

Together, our findings reveal a previously unappreciated role for HES1 in regulating fetal hematopoiesis, and provide new insight into the differences between fetal and adult HSC maintenance.

Nodal Metastases Associated With Fusion Oncogenes Are Age Dependent in Young Adult Patients With Thyroid Cancer

CONTEXT

Fusion oncogenes, especially those involving RET or NTRK, are known drivers of papillary thyroid cancer (PTC). They are prevalent in pediatric patients and correlate with aggressive tumor behavior.

OBJECTIVE

We explored the age dependence of fusion oncogenes and aggressive tumor behavior in young adult PTC patients.

EXPERIMENTAL DESIGN

We examined 150 tumors from 142 PTC patients aged between 17∼35 years old with established tumor-node-metastasis stages. Oncogenic drivers and the thyroid differentiation score (TDS) were determined by DNA and RNA sequencing of a target panel. Transcriptome analysis was performed in PTCs with RET fusions.

RESULTS

Among 150 PTCs, we detected BRAF V600E (n = 105), RET fusions (n = 15), NTRK3 fusions (n = 8), and BRAF fusions (n = 4). We found that fusion oncogenes were associated with nodal metastasis when age was tiered into 3 groups: <25 years, 25∼29 years, and 30∼35 years. Patients under 25 years old showed a marginal increase in tumor stage compared to those over 25 years (75.00% vs 21.74%, P = .0646). Risk of lateral lymph node metastasis increased with younger age (75.00% vs 27.27% vs 8.33%, P = .0369). As with advanced tumor and node stage, patients harboring fusion oncogenes and aged under 25 years showed the lowest TDS; genes associated with immunoglobulin production and production of molecular mediators of the immune response were significantly upregulated.

CONCLUSIONS

Adult PTC patients under 25 years with fusion oncogenes showed a tendency toward advanced tumor stage and lower thyroid differentiation. Integrating onset age together with oncogenic alterations is worthwhile when managing adult PTC patients.

Cancers make their own luck: theories of cancer origins

Cancer has been a leading cause of death for decades. This dismal statistic has increased efforts to prevent the disease or to detect it early, when treatment is less invasive, relatively inexpensive and more likely to cure. But precisely how tissues are transformed continues to provoke controversy and debate, hindering cancer prevention and early intervention strategies. Various theories of cancer origins have emerged, including the suggestion that it is 'bad luck': the inevitable consequence of random mutations in proliferating stem cells. In this Review, we discuss the principal theories of cancer origins and the relative importance of the factors that underpin them. The body of available evidence suggests that developing and ageing tissues 'walk a tightrope', retaining adequate levels of cell plasticity to generate and maintain tissues while avoiding overstepping into transformation. Rather than viewing cancer as 'bad luck', understanding the complex choreography of cell intrinsic and extrinsic factors that characterize transformation holds promise to discover effective new ways to prevent, detect and stop cancer before it becomes incurable.

Pathogenic human variant that dislocates GATA2 zinc fingers disrupts hematopoietic gene expression and signaling networks

Although certain human genetic variants are conspicuously loss of function, decoding the impact of many variants is challenging. Previously, we described a patient with leukemia predisposition syndrome (GATA2 deficiency) with a germline GATA2 variant that inserts 9 amino acids between the 2 zinc fingers (9aa-Ins). Here, we conducted mechanistic analyses using genomic technologies and a genetic rescue system with Gata2 enhancer-mutant hematopoietic progenitor cells to compare how GATA2 and 9aa-Ins function genome-wide. Despite nuclear localization, 9aa-Ins was severely defective in occupying and remodeling chromatin and regulating transcription. Variation of the inter-zinc finger spacer length revealed that insertions were more deleterious to activation than repression. GATA2 deficiency generated a lineage-diverting gene expression program and a hematopoiesis-disrupting signaling network in progenitors with reduced granulocyte-macrophage colony-stimulating factor (GM-CSF) and elevated IL-6 signaling. As insufficient GM-CSF signaling caused pulmonary alveolar proteinosis and excessive IL-6 signaling promoted bone marrow failure and GATA2 deficiency patient phenotypes, these results provide insight into mechanisms underlying GATA2-linked pathologies.

FLT3ITD drives context-specific changes in cell identity and variable interferon dependence during AML initiation

Acute myeloid leukemia (AML) initiation requires multiple rate-limiting mutations to cooperatively reprogram progenitor cell identity. For example, FLT3 internal tandem duplication (FLT3ITD) mutations cooperate with a variety of different initiating mutations to reprogram myeloid progenitor fate. These initiating mutations often skew toward either pediatric or adult AML patient populations, though FLT3ITD itself occurs at similar frequencies in both age groups. This raises the question of whether FLT3ITD might induce distinct transcriptional programs and unmask distinct therapeutic vulnerabilities when paired with pediatric, as opposed to adult AML-initiating mutations. To explore this possibility, we compared AML evolution in mice that carried Flt3ITD/NUP98-HOXD13 (NHD13) or Flt3ITD/Runx1DEL mutation pairs, which are respectively most common in pediatric and adult AML. Single-cell analyses and epigenome profiling revealed distinct interactions between Flt3ITD and its cooperating mutations. Whereas Flt3ITD and Flt3ITD/Runx1DEL caused aberrant expansion of myeloid progenitors, Flt3ITD/NHD13 drove the emergence of a pre-AML population that did not resemble normal hematopoietic progenitors. Differences between Flt3ITD/Runx1DEL and Flt3ITD/NHD13 cooperative target gene expression extended to fully transformed AML as well. Flt3ITD/NHD13 cooperative target genes were enriched in human NUP98-translocated AML. Flt3ITD/NHD13 selectively hijacked type I interferon signaling to drive expansion of the pre-AML population. Blocking interferon signaling delayed AML initiation and extended survival. Thus, common AML driver mutations, such as FLT3ITD, can coopt different mechanisms of transformation in different genetic contexts. Furthermore, pediatric-biased NUP98 fusions convey actionable interferon dependence.

Developmental cues license megakaryocyte priming in murine hematopoietic stem cells

The fetal-to-adult switch in hematopoietic stem cell (HSC) behavior is characterized by alterations in lineage output and entry into deep quiescence. Here we identify the emergence of megakaryocyte (Mk)-biased HSCs as an event coinciding with this developmental switch. Single-cell chromatin accessibility analysis reveals a ubiquitous acquisition of Mk lineage priming signatures in HSCs during the fetal-to-adult transition. These molecular changes functionally coincide with increased amplitude of early Mk differentiation events after acute inflammatory insult. Importantly, we identify LIN28B, known for its role in promoting fetal-like self-renewal, as an insulator against the establishment of an Mk-biased HSC pool. LIN28B protein is developmentally silenced in the third week of life, and its prolonged expression delays emergency platelet output in young adult mice. We propose that developmental regulation of Mk priming may represent a switch for HSCs to toggle between prioritizing self-renewal in the fetus and increased host protection in postnatal life.

One Size Does Not Fit All: Heterogeneity in Developmental Hematopoiesis

Our knowledge of the complexity of the developing hematopoietic system has dramatically expanded over the course of the last few decades. We now know that, while hematopoietic stem cells (HSCs) firmly reside at the top of the adult hematopoietic hierarchy, multiple HSC-independent progenitor populations play variegated and fundamental roles during fetal life, which reflect on adult physiology and can lead to disease if subject to perturbations. The importance of obtaining a high-resolution picture of the mechanisms by which the developing embryo establishes a functional hematopoietic system is demonstrated by many recent indications showing that ontogeny is a primary determinant of function of multiple critical cell types. This review will specifically focus on exploring the diversity of hematopoietic stem and progenitor cells unique to embryonic and fetal life. We will initially examine the evidence demonstrating heterogeneity within the hemogenic endothelium, precursor to all definitive hematopoietic cells. Next, we will summarize the dynamics and characteristics of the so-called "hematopoietic waves" taking place during vertebrate development. For each of these waves, we will define the cellular identities of their components, the extent and relevance of their respective contributions as well as potential drivers of heterogeneity.

GATA2 deficiency elevates interferon regulatory factor-8 to subvert a progenitor cell differentiation program

Cell type-specific transcription factors control stem and progenitor cell transitions by establishing networks containing hundreds of genes and proteins. Network complexity renders it challenging to discover essential versus modulatory or redundant components. This scenario is exemplified by GATA2 regulation of hematopoiesis during embryogenesis. Loss of a far upstream Gata2 enhancer (-77) disrupts the GATA2-dependent transcriptome governing hematopoietic progenitor cell differentiation. The aberrant transcriptome includes the transcription factor interferon regulatory factor 8 (IRF8) and a host of innate immune regulators. Mutant progenitors lose the capacity to balance production of diverse hematopoietic progeny. To elucidate mechanisms, we asked if IRF8 is essential, contributory, or not required. Reducing Irf8, in the context of the -77 mutant allele, reversed granulocytic deficiencies and the excessive accumulation of dendritic cell committed progenitors. Despite many dysregulated components that control vital transcriptional, signaling, and immune processes, the aberrant elevation of a single transcription factor deconstructed the differentiation program.

Transcriptional reprogramming in neonatal hematopoietic stem and progenitor cells

Hematopoietic stem cells (HSCs) and lineage-committed hematopoietic progenitor cells (HPCs) undergo profound shifts in gene expression during the neonatal and juvenile stages of life. Temporal changes in HSC/HPC gene expression underlie concomitant changes in self-renewal capacity, lineage biases, and hematopoietic output. Moreover, they can modify disease phenotypes. For example, childhood leukemias have distinct driver mutation profiles relative to adult leukemias, and they may arise from distinct cells of origin. The putative relationship between neonatal HSC/HPC ontogeny and childhood blood disorders highlights the importance of understanding how, at a mechanistic level, HSCs transition from fetal to adult transcriptional states. In this perspective piece, we summarize recent work indicating that the transition is uncoordinated and imprecisely timed. We discuss implications of these findings, including mechanisms that might enable neonatal HSCs and HPCs to acquire adultlike properties over a drawn-out period, in lieu of precise gene regulatory networks. The transition from fetal to adult transcriptional programs coincides with a pulse of type I interferon signaling that activates many genes associated with the adultlike state. This pulse may sensitize HSCs/HPCs to mutations that drive leukemogenesis shortly after birth. If we can understand how developmental switches modulate HSC and HPC fate after birth-both under normal circumstances and in the setting of disease-causing mutations-we can potentially reprogram these switches to treat or prevent childhood leukemias.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

FLT3-ITD mutations in acute myeloid leukaemia - molecular characteristics, distribution and numerical variation

Recurrent somatic internal tandem duplications (ITD) in the FMS-like tyrosine kinase 3 (FLT3) gene characterise approximately one third of patients with acute myeloid leukaemia (AML), and FLT3-ITD mutation status guides risk-adapted treatment strategies. The aim of this work was to characterise FLT3-ITD variant distribution in relation to molecular and clinical features, and overall survival in adult AML patients. We performed two parallel retrospective cohort studies investigating FLT3-ITD length and expression by cDNA fragment analysis, followed by Sanger sequencing in a subset of samples. In the two cohorts, a total of 139 and 172 mutant alleles were identified in 111 and 123 patients, respectively, with 22% and 28% of patients presenting with more than one mutated allele. Further, 15% and 32% of samples had a FLT3-ITD total variant allele frequency (VAF) < 0.3, while 24% and 16% had a total VAF ≥ 0.7. Most of the assessed clinical features did not significantly correlate to FLT3-ITD numerical variation nor VAF. Low VAF was, however, associated with lower white blood cell count, while increasing VAF correlated with inferior overall survival in one of the cohorts. In the other cohort, ITD length above 50 bp was identified to correlate with inferior overall survival. Our report corroborates the poor prognostic association with high FLT3-ITD disease burden, as well as extensive inter- and intrapatient heterogeneity in the molecular features of FLT3-ITD. We suggest that future use of FLT3-targeted therapy could be accompanied with thorough molecular diagnostics and follow-up to better predict optimal therapy responders.

Single-Cell Analysis of Neonatal HSC Ontogeny Reveals Gradual and Uncoordinated Transcriptional Reprogramming that Begins before Birth

Fetal and adult hematopoietic stem cells (HSCs) have distinct proliferation rates, lineage biases, gene expression profiles, and gene dependencies. Although these differences are widely recognized, it is not clear how the transition from fetal to adult identity is coordinated. Here we show that murine HSCs and committed hematopoietic progenitor cells (HPCs) undergo a gradual, rather than precipitous, transition from fetal to adult transcriptional states. The transition begins prior to birth and is punctuated by a late prenatal spike in type I interferon signaling that promotes perinatal HPC expansion and sensitizes progenitors to the leukemogenic FLT3ITD mutation. Most other changes in gene expression and enhancer activation are imprecisely timed and poorly coordinated. Thus, heterochronic enhancer elements, and their associated transcripts, are activated independently of one another rather than as part of a robust network. This simplifies the regulatory programs that guide neonatal HSC/HPC ontogeny, but it creates heterogeneity within these populations.

The efficiency of murine MLL-ENL-driven leukemia initiation changes with age and peaks during neonatal development

MLL rearrangements are translocation mutations that cause both acute lymphoblastic leukemia and acute myeloid leukemia (AML). These translocations can occur as sole clonal driver mutations in infant leukemias, suggesting that fetal or neonatal hematopoietic progenitors may be exquisitely sensitive to transformation by MLL fusion proteins. To test this possibility, we used transgenic mice to induce one translocation product, MLL-ENL, during fetal, neonatal, juvenile and adult stages of life. When MLL-ENL was induced in fetal or neonatal mice, almost all died of AML. In contrast, when MLL-ENL was induced in adult mice, most survived for >1 year despite sustained transgene expression. AML initiation was most efficient when MLL-ENL was induced in neonates, and even transient suppression of MLL-ENL in neonates could prevent AML in most mice. MLL-ENL target genes were induced more efficiently in neonatal progenitors than in adult progenitors, consistent with the distinct AML initiation efficiencies. Interestingly, transplantation stress mitigated the developmental barrier to leukemogenesis. Since fetal/neonatal progenitors were highly competent to initiate MLL-ENL-driven AML, we tested whether Lin28b, a fetal master regulator, could accelerate leukemogenesis. Surprisingly, Lin28b suppressed AML initiation rather than accelerating it. This may explain why MLL rearrangements often occur before birth in human infant leukemia patients, but transformation usually does not occur until after birth, when Lin28b levels decline. Our findings show that the efficiency of MLL-ENL-driven AML initiation changes through the course of pre- and postnatal development, and developmental programs can be manipulated to impede transformation.

The Myc/Max/Mxd Network Is a Target of Mutated Flt3 Signaling in Hematopoietic Stem Cells in Flt3-ITD-Induced Myeloproliferative Disease

Acute myeloid leukemia (AML) has poor prognosis due to various mutations, e.g., in the FLT3 gene. Therefore, it is important to identify pathways regulated by the activated Flt3 receptor for the discovery of new therapeutic targets. The Myc network of oncogenes and tumor suppressor genes is involved in mechanisms regulating proliferation and survival of cells, including that of the hematopoietic system. In this study, we evaluated the expression of the Myc oncogenes and Mxd antagonists in hematopoietic stem cell and myeloid progenitor populations in the Flt3-ITD-knockin myeloproliferative mouse model. Our data shows that the expression of Myc network genes is changed in Flt3-ITD mice compared with the wild type. Mycn is increased in multipotent progenitors and in the pre-GM compartment of myeloid progenitors in the ITD mice while the expression of several genes in the tumor suppressor Mxd family, including Mxd1, Mxd2, and Mxd4, is concomitantly downregulated, as well as the expression of the Mxd-related gene Mnt and the transcriptional activator Miz-1. LSKCD150⁺CD48⁻ hematopoietic long-term stem cells are decreased in the Flt3-ITD cells while multipotent progenitors are increased. Of note, PKC412-mediated inhibition of Flt3-ITD signaling results in downregulation of cMyc and upregulation of the Myc antagonists Mxd1, Mxd2, and Mxd4. Our data provides new mechanistic insights into downstream alterations upon aberrant Flt3 signaling and rationale for combination therapies for tyrosine kinase inhibitors with Myc antagonists in treating AML.

MEF2C Phosphorylation Is Required for Chemotherapy Resistance in Acute Myeloid Leukemia

In acute myeloid leukemia (AML), chemotherapy resistance remains prevalent and poorly understood. Using functional proteomics of patient AML specimens, we identified MEF2C S222 phosphorylation as a specific marker of primary chemoresistance. We found that Mef2cS222A/S222A knock-in mutant mice engineered to block MEF2C phosphorylation exhibited normal hematopoiesis, but were resistant to leukemogenesis induced by MLL-AF9 MEF2C phosphorylation was required for leukemia stem cell maintenance and induced by MARK kinases in cells. Treatment with the selective MARK/SIK inhibitor MRT199665 caused apoptosis and conferred chemosensitivity in MEF2C-activated human AML cell lines and primary patient specimens, but not those lacking MEF2C phosphorylation. These findings identify kinase-dependent dysregulation of transcription factor control as a determinant of therapy response in AML, with immediate potential for improved diagnosis and therapy for this disease.Significance: Functional proteomics identifies phosphorylation of MEF2C in the majority of primary chemotherapy-resistant AML. Kinase-dependent dysregulation of this transcription factor confers susceptibility to MARK/SIK kinase inhibition in preclinical models, substantiating its clinical investigation for improved diagnosis and therapy of AML. Cancer Discov; 8(4); 478-97. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 371.

Stat5 deficiency decreases transcriptional heterogeneity and supports emergence of hematopoietic sub-populations

Aging is associated with significant changes in hematopoiesis, including clonal dominance, anemia, myeloid malignancies, and reduced activation of signal transducer and activator of transcription 5 (Stat5). In previous studies, Stat5 deletion surprisingly amplified FLT3/ITD+ myeloid expansion or Myc-driven lymphoid expansion. Here we show that Stat5 deficiency has a strong impact upon transcriptional heterogeneity in single sorted c-Kit+Lin-Sca-1+ (KLS) cells or CD150+CD48- KLS long-term repopulating hematopoietic stem cells (LT-HSC). Single cell polymerase chain reaction (PCR) was performed on selected regulators of multi-lineage hematopoiesis. At least two dominant sub-populations were identified by increased expression of cell cycle regulatory and leukemia-associated genes. Furthermore, in the top expressing quartile of cells, the majority of genes were proportionally overrepresented. In wild-type KLS cells, Stat5 mRNA levels were also strongly correlated with several genes. Since heterogeneity decreases with age or inflammatory or oncogenic stress, these results provide a potential mechanistic linkage to Stat5 expression.

A Human IPS Model Implicates Embryonic B-Myeloid Fate Restriction as Developmental Susceptibility to B Acute Lymphoblastic Leukemia-Associated ETV6-RUNX1

ETV6-RUNX1 is associated with childhood acute B-lymphoblastic leukemia (cALL) functioning as a first-hit mutation that initiates a clinically silent pre-leukemia in utero. Because lineage commitment hierarchies differ between embryo and adult, and the impact of oncogenes is cell-context dependent, we hypothesized that the childhood affiliation of ETV6-RUNX1 cALL reflects its origins in a progenitor unique to embryonic life. We characterize the first emerging B cells in first-trimester human embryos, identifying a developmentally restricted CD19^-IL-7R⁺ progenitor compartment, which transitions from a myeloid to lymphoid program during ontogeny. This developmental series is recapitulated in differentiating human pluripotent stem cells (hPSCs), thereby providing a model for the initiation of cALL. Genome-engineered hPSCs expressing ETV6-RUNX1 from the endogenous ETV6 locus show expansion of the CD19^-IL-7R⁺ compartment, show a partial block in B lineage commitment, and produce proB cells with aberrant myeloid gene expression signatures and potential: features (collectively) consistent with a pre-leukemic state.

Mice expressing KrasG12D in hematopoietic multipotent progenitor cells develop neonatal myeloid leukemia

Juvenile myelomonocytic leukemia (JMML) is a pediatric myeloproliferative neoplasm that bears distinct characteristics associated with abnormal fetal development. JMML has been extensively modeled in mice expressing the oncogenic KrasG12D mutation. However, these models have struggled to recapitulate the defining features of JMML due to in utero lethality, nonhematopoietic expression, and the pervasive emergence of T cell acute lymphoblastic leukemia. Here, we have developed a model of JMML using mice that express KrasG12D in multipotent progenitor cells (Flt3Cre+ KrasG12D mice). These mice express KrasG12D in utero, are born at normal Mendelian ratios, develop hepatosplenomegaly, anemia, and thrombocytopenia, and succumb to a rapidly progressing and fully penetrant neonatal myeloid disease. Mutant mice have altered hematopoietic stem and progenitor cell populations in the BM and spleen that are hypersensitive to granulocyte macrophage-CSF due to hyperactive RAS/ERK signaling. Biased differentiation in these progenitors results in an expansion of neutrophils and DCs and a concomitant decrease in T lymphocytes. Flt3Cre+ KrasG12D fetal liver hematopoietic progenitors give rise to a myeloid disease upon transplantation. In summary, we describe a KrasG12D mouse model that reproducibly develops JMML-like disease. This model will prove useful for preclinical drug studies and for elucidating the developmental origins of pediatric neoplasms.

PRKCH regulates hematopoietic stem cell function and predicts poor prognosis in acute myeloid leukemia

Acute myeloid leukemia (AML) cells often co-opt normal hematopoietic stem cell (HSC) programs to drive neoplastic proliferation, and HSC-related gene expression signatures have been identified as biomarkers for poor prognosis in AML patients. We sought to identify new regulators of HSCs and AML cells from previously published HSC and leukemia stem cell (LSC) gene expression signatures. We identified PRKCH (protein kinase C eta) as a gene that is highly expressed in both mouse and human HSCs, as well as in LSCs from independent cohorts of AML patients. Prkch deletion in mice resulted in impaired HSC function. PRKCH was most highly expressed in undifferentiated (FAB M0) subtype AML, and high expression correlated with TP53 and RUNX1 mutations, high-risk cytogenetic features, and poor overall survival. Prkch deletion in an Flt3-ITD/Runx1 mutant mouse AML model did not extend survival. Thus, PRKCH is necessary for normal HSC function; its expression predicts poor survival in AML patients, but it is not required for AML to develop.