Nicolas Goardon

Coexistence of LMPP-like and GMP-like Leukemia Stem Cells in Acute Myeloid Leukemia

The relationships between normal and leukemic stem/progenitor cells are unclear. We show that in ∼80% of primary human CD34+ acute myeloid leukemia (AML), two expanded populations with hemopoietic progenitor immunophenotype coexist in most patients. Both populations have leukemic stem cell (LSC) activity and are hierarchically ordered; one LSC population gives rise to the other. Global gene expression profiling shows the LSC populations are molecularly distinct and resemble normal progenitors but not stem cells. The more mature LSC population most closely mirrors normal granulocyte-macrophage progenitors (GMP) and the immature LSC population a previously uncharacterized progenitor functionally similar to lymphoid-primed multipotential progenitors (LMPPs). This suggests that in most cases primary CD34+ AML is a progenitor disease where LSCs acquire abnormal self-renewal potential.

ETO2 coordinates cellular proliferation and differentiation during erythropoiesis

The passage from proliferation to terminal differentiation is critical for normal development and is often perturbed in malignancies. To define the molecular mechanisms that govern this process during erythropoiesis, we have used tagging/proteomics approaches and characterized protein complexes nucleated by TAL‐1/SCL, a basic helix–loop–helix transcription factor that specifies the erythrocytic lineage. In addition to known TAL‐1 partners, GATA‐1, E2A, HEB, LMO2 and Ldb1, we identify the ETO2 repressor as a novel component recruited to TAL‐1 complexes through interaction with E2A/HEB. Ectopic expression and siRNA knockdown experiments in hematopoietic progenitor cells show that ETO2 actively represses erythroid TAL‐1 target genes and governs the expansion of erythroid progenitors. At the onset of erythroid differentiation, a change in the stoichiometry of ETO2 within the TAL‐1 complex activates the expression of known erythroid‐specific TAL‐1 target genes and of Gfi‐1b and p21Cip, encoding two essential regulators of erythroid cell proliferation. These results suggest that the dynamics of ETO2 recruitment within nuclear complexes couple cell proliferation to cell differentiation and determine the onset of terminal erythroid maturation.

NKX3.1 is a direct TAL1 target gene that mediates proliferation of TAL1-expressing human T cell acute lymphoblastic leukemia

TAL1 (also known as SCL) is expressed in >40% of human T cell acute lymphoblastic leukemias (T-ALLs). TAL1 encodes a basic helix-loop-helix transcription factor that can interfere with the transcriptional activity of E2A and HEB during T cell leukemogenesis; however, the oncogenic pathways directly activated by TAL1 are not characterized. In this study, we show that, in human TAL1–expressing T-ALL cell lines, TAL1 directly activates NKX3.1, a tumor suppressor gene required for prostate stem cell maintenance. In human T-ALL cell lines, NKX3.1 gene activation is mediated by a TAL1–LMO–Ldb1 complex that is recruited by GATA-3 bound to an NKX3.1 gene promoter regulatory sequence. TAL1-induced NKX3.1 activation is associated with suppression of HP1-α (heterochromatin protein 1 α) binding and opening of chromatin on the NKX3.1 gene promoter. NKX3.1 is necessary for T-ALL proliferation, can partially restore proliferation in TAL1 knockdown cells, and directly regulates miR-17-92. In primary human TAL1-expressing leukemic cells, the NKX3.1 gene is expressed independently of the Notch pathway, and its inactivation impairs proliferation. Finally, TAL1 or NKX3.1 knockdown abrogates the ability of human T-ALL cells to efficiently induce leukemia development in mice. These results suggest that tumor suppressor or oncogenic activity of NKX3.1 depends on tissue expression.

FLT3-ITDs Instruct a Myeloid Differentiation and Transformation Bias in Lymphomyeloid Multipotent Progenitors.

Summary Whether signals mediated via growth factor receptors (GFRs) might influence lineage fate in multipotent progenitors (MPPs) is unclear. We explored this issue in a mouse knockin model of gain-of-function Flt3-ITD mutation because FLT3-ITDs are paradoxically restricted to acute myeloid leukemia even though Flt3 primarily promotes lymphoid development during normal hematopoiesis. When expressed in MPPs, Flt3-ITD collaborated with Runx1 mutation to induce high-penetrance aggressive leukemias that were exclusively of the myeloid phenotype. Flt3-ITDs preferentially expanded MPPs with reduced lymphoid and increased myeloid transcriptional priming while compromising early B and T lymphopoiesis. Flt3-ITD-induced myeloid lineage bias involved upregulation of the transcription factor Pu.1, which is a direct target gene of Stat3, an aberrantly activated target of Flt3-ITDs, further establishing how lineage bias can be inflicted on MPPs through aberrant GFR signaling. Collectively, these findings provide new insights into how oncogenic mutations might subvert the normal process of lineage commitment and dictate the phenotype of resulting malignancies.

SCL-mediated regulation of the cell cycle regulator p21 is critical for murine megakaryopoiesis

Megakaryopoiesis is a complex process that involves major cellular and nuclear changes and relies on controlled coordination of cellular proliferation and differentiation. These mechanisms are orchestrated in part by transcriptional regulators. The key hematopoietic transcription factor stem cell leukemia (SCL)/TAL1 is required in early hematopoietic progenitors for specification of the megakaryocytic lineage. These early functions have, so far, prevented full investigation of its role in megakaryocyte development in loss-of-function studies. Here, we report that SCL critically controls terminal megakaryocyte maturation. In vivo deletion of Scl specifically in the megakaryocytic lineage affects all key attributes of megakaryocyte progenitors (MkPs), namely, proliferation, ploidization, cytoplasmic maturation, and platelet release. Genome-wide expression analysis reveals increased expression of the cell-cycle regulator p21 in Scl-deleted MkPs. Importantly, p21 knockdown-mediated rescue of Scl-mutant MkPs shows full restoration of cell-cycle progression and partial rescue of the nuclear and cytoplasmic maturation defects. Therefore, SCL-mediated transcriptional control of p21 is essential for terminal maturation of MkPs. Our study provides a mechanistic link between a major hematopoietic transcriptional regulator, cell-cycle progression, and megakaryocytic differentiation.

Genetically distinct leukemic stem cells in human CD34- acute myeloid leukemia are arrested at a hemopoietic precursor-like stage.

Quek and colleagues identify human leukemic stem cells (LSCs) present in CD34− AML. In-depth characterization of the functional and clonal aspects of CD34− LSCs indicates that most are similar to myeloid precursors.

Reduced CD38 expression on CD34+ cells as a diagnostic test in myelodysplastic syndromes

This report indicates that a reduced mean fluorescence intensity of CD38 expression on CD34+ cells can be used as a surrogate marker for abnormalities in the CD34+ compartment of patients with myelodysplastic syndrome. See related perspective article on page 1041. Diagnosis of myelodysplastic syndrome can be difficult especially in cases with a low blast count and a normal karyotype. Flow cytometry has been used to distinguish myelodysplastic syndrome from non-clonal cytopenias. No one single simple flow cytometric parameter has been proposed to be diagnostic of myelodysplastic syndrome. We have studied samples from 100 myelodysplastic syndrome patients and as control samples; 70 non-clonal cytopenias, 5 subjects with normal hematology, 31 patients with acute myeloid leukemia and 11 with chronic myelomonocytic leukemia or myeloproliferative disorder. We show that reduced relative mean fluorescence of CD38 below a threshold value on CD34+ cells diagnosed low-grade myelodysplastic syndrome with 95% sensitivity (95% confidence interval, 87–99%) and 92% specificity (95% confidence interval, 82–97%). This simple flow cytometric test may be of value in the routine clinical diagnosis of myelodysplastic syndrome, especially in cases with a low blast count and normal karyotype.

Single-cell analysis reveals the continuum of human lympho-myeloid progenitor cells.

The human hemopoietic progenitor hierarchy producing lymphoid and granulocytic-monocytic (myeloid) lineages is unclear. Multiple progenitor populations produce lymphoid and myeloid cells, but remain incompletely characterized. Here, we demonstrated cord blood lympho-myeloid containing progenitor populations - the lymphoid-primed multi-potential progenitor (LMPP), granulocyte-macrophage progenitor (GMP) and multi-lymphoid progenitor (MLP) - were functionally and transcriptionally distinct and heterogeneous at the clonal level, with progenitors of many different functional potentials present. Though most progenitors had uni-lineage myeloid or lymphoid potential, bi- and rarer multi-lineage progenitors occurred in LMPP, GMP and MLP. This, coupled with single cell expression analyses, suggested a continuum of progenitors execute lymphoid and myeloid differentiation rather than only uni-lineage progenitors being present downstream of stem cells.

Heterogeneous leukemia stem cells in myeloid blast phase chronic myeloid leukemia

Chronic myeloid leukemia (CML) is an excellent model of the multistep processes in cancer. Initiating BCR-ABL mutations are required for the initial phase of the disease (chronic phase, CP-CML). Some CP-CML patients acquire additional mutation(s) that transforms CP-CML to poor prognosis, hard to treat, acute myeloid or lymphoid leukemia or blast phase CML (BP-CML). It is unclear where in the hemopoietic hierarchy additional mutations are acquired in BP-CML, how the hemopoietic hierarchy is altered as a consequence, and the cellular identity of the resulting leukemia-propagating stem cell (LSC) populations. Here, we show that myeloid BP-CML is associated with expanded populations that have the immunophenotype of normal progenitor populations that vary between patients. Serial transplantation in immunodeficient mice demonstrated functional LSCs reside in multiple populations with the immunophenotype of normal progenitor as well as stem cells. Multicolor fluorescence in situ hybridization detected serial acquisition of cytogenetic abnormalities of chromosome 17, associated with transformation to BP-CML, that is detected with equal frequency in all functional LSC compartments. New effective myeloid BP-CML therapies will likely have to target all these LSC populations.