MDS cells impair osteolineage differentiation of MSCs via extracellular vesicles to suppress normal hematopoiesis

Myelodysplastic syndrome (MDS) is a clonal disorder of hematopoietic stem cells (HSCs), characterized by ineffective hematopoiesis and frequent progression to leukemia. It has long remained unresolved how MDS cells, which are less proliferative, inhibit normal hematopoiesis and eventually dominate the bone marrow space. Despite several studies implicating mesenchymal stromal or stem cells (MSCs), a principal component of the HSC niche, in the inhibition of normal hematopoiesis, the molecular mechanisms underlying this process remain unclear. Here, we demonstrate that both human and mouse MDS cells perturb bone metabolism by suppressing the osteolineage differentiation of MSCs, which impairs the ability of MSCs to support normal HSCs. Enforced MSC differentiation rescues the suppressed normal hematopoiesis in both in vivo and in vitro MDS models. Intriguingly, the suppression effect is reversible and mediated by extracellular vesicles (EVs) derived from MDS cells. These findings shed light on the novel MDS EV-MSC axis in ineffective hematopoiesis.

Combined epigenetic and metabolic treatments overcome differentiation blockade in acute myeloid leukemia

A hallmark of acute myeloid leukemia (AML) is the inability of self-renewing malignant cells to mature into a non-dividing terminally differentiated state. This differentiation block has been linked to dysregulation of multiple cellular processes, including transcriptional, chromatin, and metabolic regulation. The transcription factor HOXA9 and the histone demethylase LSD1 are examples of such regulators that promote differentiation blockade in AML. To identify metabolic targets that interact with LSD1 inhibition to promote myeloid maturation, we screened a small molecule library to identify druggable substrates. We found that differentiation caused by LSD1 inhibition is enhanced by combined perturbation of purine nucleotide salvage and de novo lipogenesis pathways, and identified multiple lines of evidence to support the specificity of these pathways and suggest a potential basis of how perturbation of these pathways may interact synergistically to promote myeloid differentiation. In sum, these findings suggest potential drug combination strategies in the treatment of AML.

Chemotherapy Induces Senescence-Like Resilient Cells Capable of Initiating AML Recurrence

Patients with acute myeloid leukemia (AML) frequently relapse after chemotherapy, yet the mechanism by which AML reemerges is not fully understood. Herein, we show that primary AML cells enter a senescence-like phenotype following chemotherapy in vitro and in vivo. This is accompanied by induction of senescence/inflammatory and embryonic diapause transcriptional programs, with downregulation of MYC and leukemia stem cell genes. Single-cell RNA sequencing suggested depletion of leukemia stem cells in vitro and in vivo, and enrichment for subpopulations with distinct senescence-like cells. This senescence effect was transient and conferred superior colony-forming and engraftment potential. Entry into this senescence-like phenotype was dependent on ATR, and persistence of AML cells was severely impaired by ATR inhibitors. Altogether, we propose that AML relapse is facilitated by a senescence-like resilience phenotype that occurs regardless of their stem cell status. Upon recovery, these post-senescence AML cells give rise to relapsed AMLs with increased stem cell potential. SIGNIFICANCE: Despite entering complete remission after chemotherapy, relapse occurs in many patients with AML. Thus, there is an urgent need to understand the relapse mechanism in AML and the development of targeted treatments to improve outcome. Here, we identified a senescence-like resilience phenotype through which AML cells can survive and repopulate leukemia.This article is highlighted in the In This Issue feature, p. 1307.

High expression of ABCG2 induced by EZH2 disruption has pivotal roles in MDS pathogenesis

Both proto-oncogenic and tumor-suppressive functions have been reported for enhancer of zeste homolog 2 (EZH2). To investigate the effects of its inactivation, a mutant EZH2 lacking its catalytic domain was prepared (EZH2-dSET). In a mouse bone marrow transplant model, EZH2-dSET expression in bone marrow cells induced a myelodysplastic syndrome (MDS)-like disease in transplanted mice. Analysis of these mice identified Abcg2 as a direct target of EZH2. Intriguingly, Abcg2 expression alone induced the same disease in the transplanted mice, where stemness genes were enriched. Interestingly, ABCG2 expression is specifically high in MDS patients. The present results indicate that ABCG2 de-repression induced by EZH2 mutations have crucial roles in MDS pathogenesis.

Novel working hypothesis for pathogenesis of hematological malignancies: combination of mutations-induced cellular phenotypes determines the disease (cMIP-DD)

Recent progress in high-speed sequencing technology has revealed that tumors harbor novel mutations in a variety of genes including those for molecules involved in epigenetics and splicing, some of which were not categorized to previously thought malignancy-related genes. However, despite thorough identification of mutations in solid tumors and hematological malignancies, how these mutations induce cell transformation still remains elusive. In addition, each tumor usually contains multiple mutations or sometimes consists of multiple clones, which makes functional analysis difficult. Fifteen years ago, it was proposed that combination of two types of mutations induce acute leukemia; Class I mutations induce cell growth or inhibit apoptosis while class II mutations block differentiation, co-operating in inducing acute leukemia. This notion has been proven using a variety of mouse models, however most of recently found mutations are not typical class I/II mutations. Although some novel mutations have been found to functionally work as class I or II mutation in leukemogenesis, the classical class I/II theory seems to be too simple to explain the whole story. We here overview the molecular basis of hematological malignancies based on clinical and experimental results, and propose a new working hypothesis for leukemogenesis.

Role played by Prx1-dependent extracellular matrix properties in vascular smooth muscle development in embryonic lungs

Although there are many studies focusing on the molecular pathways underlying lung vascular morphogenesis, the extracellular matrix (ECM)-dependent regulation of mesenchymal cell differentiation in vascular smooth muscle development needs better understanding. In this study, we demonstrate that the paired related homeobox gene transcription factor Prx1 maintains the elastic ECM properties, which are essential for vascular smooth muscle precursor cell differentiation. We have found that Prx1(null) mouse lungs exhibit defective vascular smooth muscle development, downregulated elastic ECM expression, and compromised transforming growth factor (TGF)-β localization and signaling. Further characterization of ECM properties using decellularized lung ECM scaffolds derived from Prx1 mice demonstrated that Prx1 is required to maintain lung ECM stiffness. The results of cell culture using stiffness-controlled 2-D and 3-D synthetic substrates confirmed that Prx1-dependent ECM stiffness is essential for promotion of smooth muscle precursor differentiation for effective TGF-β stimulation. Supporting these results, both decellularized Prx1(null) lung ECM and Prx1(WT) (wild type) ECM scaffolds with blocked TGF-β failed to support mesenchymal cell to 3-D smooth muscle cell differentiation. These results suggest a novel ECM-dependent regulatory pathway of lung vascular development wherein Prx1 regulates lung vascular smooth muscle precursor development by coordinating the ECM biophysical and biochemical properties.

A C-terminal mutant of CCAAT-enhancer-binding protein α (C/EBPα-Cm) downregulates Csf1r, a potent accelerator in the progression of acute myeloid leukemia with C/EBPα-Cm

Two types of CCAAT-enhancer-binding protein α (C/EBPα) mutants are found in acute myeloid leukemia (AML) patients: N-terminal frame-shift mutants (C/EBPα-N(m)) generating p30 as a dominant form and C-terminal basic leucine zipper domain mutants (C/EBPα-C(m)). We have previously shown that C/EBPα-K304_R323dup belonging to C/EBPα-C(m), but not C/EBPα-T60fsX159 belonging to C/EBPα-N(m), alone induced AML in mouse bone marrow transplantation (BMT) models. Here we show that various C/EBPα-C(m) mutations have a similar, but not identical, potential in myeloid leukemogenesis. Notably, like C/EBPα-K304_R323dup, any type of C/EBPα-C(m) tested (C/EBPα-S299_K304dup, K313dup, or N321D) by itself induced AML, albeit with different latencies after BMT; C/EBPα-N321D induced AML with the shortest latency. By analyzing the gene expression profiles of C/EBPα-N321D- and mock-transduced c-kit(+)Sca-1(+)Lin(-) cells, we identified Csf1r as a gene downregulated by C/EBPα-N321D. In addition, leukemic cells expressing C/EBPα-C(m) exhibited low levels of colony stimulating factor 1 receptor in mice. On the other hand, transduction with C/EBPα-N(m) did not influence Csf1r expression in c-kit(+)Sca-1(+)Lin(-) cells, implying a unique role for C/EBPα-C(m) in downregulating Csf1r. Importantly, Csf1r overexpression collaborated with C/EBPα-N321D to induce fulminant AML with leukocytosis in mouse BMT models to a greater extent than did C/EBPα-N321D alone. Collectively, these results suggest that C/EBPα-C(m)-mediated downregulation of Csf1r has a negative, rather than a positive, impact on the progression of AML involving C/EBPα-C(m), which might possibly be accelerated by additional genetic and/or epigenetic alterations inducing Csf1r upregulation.

SETBP1 mutations drive leukemic transformation in ASXL1-mutated MDS

Mutations in ASXL1 are frequent in patients with myelodysplastic syndrome (MDS) and associated with adverse survival yet the molecular pathogenesis of ASXL1 mutations are not fully understood. Recently it has been found that deletion of Asxl1 or expression of C-terminal-truncating ASXL1 mutations (ASXL1-MT) inhibit myeloid differentiation and induce MDS-like disease in mice. Here, we find that SETBP1 mutations (SETBP1-MT) are enriched among patients with ASXL1-mutated MDS patients and associated with increased incidence of leukemic transformation as well as shorter survival, suggesting SETBP1-MT play a critical role in leukemic transformation of MDS. We identify that SETBP1-MT inhibit ubiquitination and subsequent degradation of SETBP1, resulting in increased expression. Expression of SETBP1-MT, in turn, inhibited Pp2a activity, leading to Akt activation and enhanced expression of posterior Hoxa genes in ASXL1 mutant cells. Biologically, SETBP1-MT augmented ASXL1-MT-induced differentiation block, inhibited apoptosis, and enhanced myeloid colony output. SETBP1-MT collaborated with ASXL1-MT in inducing AML in vivo. The combination of ASXL1-MT and SETBP1-MT activated a stem cell signature and repressed the TGF-β signaling pathway, in contrast to the ASXL1-MT-induced MDS model. These data reveal that SETBP1-MT are critical drivers of ASXL1-mutated MDS and identify several deregulated pathways as potential therapeutic targets in high-risk MDS.

Hes1 upregulation contributes to the development of FIP1L1-PDGRA-positive leukemia in blast crisis

We have previously shown that elevated expression of Hairy enhancer of split 1 (Hes1) contributes to blast crisis transition in Bcr-Abl-positive chronic myelogenous leukemia. Here we investigate whether Hes1 is involved in the development of other myeloid neoplasms. Notably, Hes1 expression was elevated in only a few cases of 65 samples with different types of myeloid neoplasms. Interestingly, elevated expression of Hes1 was found in two of five samples of Fip1-like1 platelet-derived growth factor receptor-α (FIP1L1-PDGFA)-positive myeloid neoplasms associated with eosinophilia. Whereas FIP1L1-PDGFRα alone induced acute T-cell leukemia or myeloproliferative neoplasms in mouse bone marrow transplantation models, mice transplanted with bone marrow cells expressing both Hes1 and FIP1L1-PDGFRα developed acute leukemia characterized by an expansion of myeloid blasts and leukemic cells without eosinophilic granules. FIP1L1-PDGFRα conferred cytokine-independent growth to Hes1-transduced common myeloid progenitors, interleukin-3-dependent cells. Imatinib inhibited the growth of common myeloid progenitors expressing Hes1 with FIP1L1-PDGFRα, but not with imatinib-resistant FIP1L1-PDGFRα mutants harboring T674I or D842V. In contrast, ponatinib efficiently eradicated leukemic cells expressing Hes1 and the imatinib-resistant FLP1L1-PDGFRΑ mutant in vitro and in vivo. Thus, we have established mouse models of FIP1L1-PDGFRA-positive leukemia in myeloid blast crisis, which will help elucidate the pathogenesis of the disease and develop a new treatment for it.

Autologous stem cell transplantation using MEAM regimen for relapsed AIDS-related lymphoma patients who received highly active anti-retroviral therapy: a report of three cases

AIDS-related lymphoma (ARL) is a serious complication of HIV infection. We performed MEAM (MCNU + etoposide + cytarabine + L-PAM) regimen with autologous stem cell transplantation (ASCT) for three patients with refractory or relapsed ARL. All three patients had been treated with highly active anti-retroviral therapy (HAART) during the course of the treatment regimen and ASCT. The regimen was well tolerable, and no uncontrollable infection was noted. All patients are still alive and maintain complete remission at 24, 20 and 9 months after transplantation. ASCT using MEAM regimen as a conditioning regimen was feasible for our patients with refractory or relapsed ARL.