Myelodysplastic syndromes and related disorders (myelodysplasia) are a heterogeneous group of myeloid neoplasms showing deregulated blood cell production with evidence of myeloid dysplasia and a predisposition to acute myeloid leukaemia, whose pathogenesis is only incompletely understood. Here we report whole-exome sequencing of 29 myelodysplasia specimens, which unexpectedly revealed novel pathway mutations involving multiple components of the RNA splicing machinery, including U2AF35, ZRSR2, SRSF2 and SF3B1. In a large series analysis, these splicing pathway mutations were frequent (∼45 to ∼85%) in, and highly specific to, myeloid neoplasms showing features of myelodysplasia. Conspicuously, most of the mutations, which occurred in a mutually exclusive manner, affected genes involved in the 3'-splice site recognition during pre-mRNA processing, inducing abnormal RNA splicing and compromised haematopoiesis. Our results provide the first evidence indicating that genetic alterations of the major splicing components could be involved in human pathogenesis, also implicating a novel therapeutic possibility for myelodysplasia.

Infection with the human immunodeficiency virus type 1 (HIV-1) requires the presence of a CD4 receptor and a chemokine receptor, principally chemokine receptor 5 (CCR5). Homozygosity for a 32-bp deletion in the CCR5 allele provides resistance against HIV-1 acquisition. We transplanted stem cells from a donor who was homozygous for CCR5 delta32 in a patient with acute myeloid leukemia and HIV-1 infection. The patient remained without viral rebound 20 months after transplantation and discontinuation of antiretroviral therapy. This outcome demonstrates the critical role CCR5 plays in maintaining HIV-1 infection.

Blood formation is believed to occur through step-wise progression of haematopoietic stem cells (HSCs) following a tree-like hierarchy of oligo-, bi- and unipotent progenitors. However, this model is based on the analysis of predefined flow-sorted cell populations. Here we integrated flow cytometric, transcriptomic and functional data at single-cell resolution to quantitatively map early differentiation of human HSCs towards lineage commitment. During homeostasis, individual HSCs gradually acquire lineage biases along multiple directions without passing through discrete hierarchically organized progenitor populations. Instead, unilineage-restricted cells emerge directly from a “Continuum of LOw primed UnDifferentiated hematopoietic stem- and progenitor cells” (CLOUD-HSPCs). Distinct gene expression modules operate in a combinatorial manner to control stemness, early lineage priming and the subsequent progression into all major branches of haematopoiesis. These data reveal a continuous landscape of human steady state haematopoiesis downstream of HSCs and provide a basis for the understanding of hematopoietic malignancies.

Myelodysplastic syndromes (MDSs) are a heterogeneous group of myeloid neoplasms with defects in hematopoietic stem and progenitor cells (HSPCs) and possibly the HSPC niche. Here, we show that patient-derived mesenchymal stromal cells (MDS MSCs) display a disturbed differentiation program and are essential for the propagation of MDS-initiating Lin(-)CD34(+)CD38(-) stem cells in orthotopic xenografts. Overproduction of niche factors such as CDH2 (N-Cadherin), IGFBP2, VEGFA, and LIF is associated with the ability of MDS MSCs to enhance MDS expansion. These factors represent putative therapeutic targets in order to disrupt critical hematopoietic-stromal interactions in MDS. Finally, healthy MSCs adopt MDS MSC-like molecular features when exposed to hematopoietic MDS cells, indicative of an instructive remodeling of the microenvironment. Therefore, this patient-derived xenograft model provides functional and molecular evidence that MDS is a complex disease that involves both the hematopoietic and stromal compartments. The resulting deregulated expression of niche factors may well also be a feature of other hematopoietic malignancies.

IntroductionA characteristic abnormality of leukemia cells is that they are blocked at an early stage of their development and fail to differentiate into functional mature cells. During the 1970s and 1980s, several scientific achievements popularized the strategy of inducing malignant cells to overcome their block of differentiation and enter the apoptotic pathways as an elegant alternative to killing cancer cells by cytotoxic therapies. This intervention could theoretically limit exposure to unwanted side effects of cytotoxic chemotherapy, and more importantly, improve complete remission and cure rates. Pioneering reports included studies demonstrating the differentiating capability of dimethylsulfoxide (DMSO) on erythropoiesis, 1 efforts to elucidate substances to control the differentiation of myeloid leukemia, 2 and the first evidence of the differentiating properties of retinoic acid. 3,4 The initial promising preclinical results of this approach prompted a review article by us 25 years ago concerning the possibilities and therapeutic implications of differentiation induction in leukemia. 5 At that stage, cell line models for in vitro differentiation experiments were described. Substances such as phorbol diesters, teleocidins, polar planar drugs, cytokines, retinoids, and vitamin D metabolites showed dramatic potential to differentiate cell lines such as HL-60, KG-1, ML-3, or K562 in vitro and fueled the hope of developing a new approach to treat cancers by overcoming their blocked differentiation. Today, 25 years later, we discuss the progress and the clinical achievements of this therapeutic approach. Ligands of nuclear hormone receptorsPoster child of success: complete remissions of APL by differentiation therapyThe potential for differentiating therapy to improve cure rates in leukemia is exemplified by the development of all-trans retinoic acid (ATRA) for the targeted treatment of acute promyelocytic leukemia (APL). One of the most remarkable results of initial in vitro experiments was achieved in differentiating HL-60 cells with ATRA, which produced terminal differentiation in 90% of cells with 10 Ϫ6 M retinoic acid. 3 Investigators soon realized that ATRA was specifically effective in APL cells carrying a typical chromosomal translocation between chromosomes 15 and 17 [t(15;17)(q22; q21)] 6 but not in other leukemias. 4 The first APL patients treated with ATRA in the early 1980s achieved encouraging remissions by the new therapy. [7][8][9][10] The first clinical trial of ATRA reported 16 newly diagnosed, and 8 anthracycline refractory patients who were induced with single-agent ATRA. Complete hematologic remission was induced in all patients; more than 90% of samples from these individuals demonstrated in vitro evidence of blast differentiation. 10 This seminal trial changed the management and prognosis of APL, and introduced a paradigm for success of cell differentiation therapy. Subsequently, APL therapy was improved stepwise through elucidation of the most effective combination regimen of ATRA wit...

Cohesin is a multimeric protein complex that is involved in the cohesion of sister chromatids, post-replicative DNA repair and transcriptional regulation. Here we report recurrent mutations and deletions involving multiple components of the cohesin complex, including STAG2, RAD21, SMC1A and SMC3, in different myeloid neoplasms. These mutations and deletions were mostly mutually exclusive and occurred in 12.1% (19/157) of acute myeloid leukemia, 8.0% (18/224) of myelodysplastic syndromes, 10.2% (9/88) of chronic myelomonocytic leukemia, 6.3% (4/64) of chronic myelogenous leukemia and 1.3% (1/77) of classical myeloproliferative neoplasms. Cohesin-mutated leukemic cells showed reduced amounts of chromatin-bound cohesin components, suggesting a substantial loss of cohesin binding sites on chromatin. The growth of leukemic cell lines harboring a mutation in RAD21 (Kasumi-1 cells) or having severely reduced expression of RAD21 and STAG2 (MOLM-13 cells) was suppressed by forced expression of wild-type RAD21 and wild-type RAD21 and STAG2, respectively. These findings suggest a role for compromised cohesin functions in myeloid leukemogenesis.

Key Points Mutational trajectories are defined by complex patterns of molecular heterogeneity in MDS, including lower-risk cases. Therapeutic intervention dynamically reshapes mutational patterns often resulting in branched or independent evolution of MDS clones.

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