Impaired GATA-1 expression and myelofibrosis in an animal model

Mesenchymal stem cells in fibrotic diseases-the two sides of the same coin

Fibrosis is caused by extensive deposition of extracellular matrix (ECM) components, which play a crucial role in injury repair. Fibrosis attributes to ~45% of all deaths worldwide. The molecular pathology of different fibrotic diseases varies, and a number of bioactive factors are involved in the pathogenic process. Mesenchymal stem cells (MSCs) are a type of multipotent stem cells that have promising therapeutic effects in the treatment of different diseases. Current updates of fibrotic pathogenesis reveal that residential MSCs may differentiate into myofibroblasts which lead to the fibrosis development. However, preclinical and clinical trials with autologous or allogeneic MSCs infusion demonstrate that MSCs can relieve the fibrotic diseases by modulating inflammation, regenerating damaged tissues, remodeling the ECMs, and modulating the death of stressed cells after implantation. A variety of animal models were developed to study the mechanisms behind different fibrotic tissues and test the preclinical efficacy of MSC therapy in these diseases. Furthermore, MSCs have been used for treating liver cirrhosis and pulmonary fibrosis patients in several clinical trials, leading to satisfactory clinical efficacy without severe adverse events. This review discusses the two opposite roles of residential MSCs and external MSCs in fibrotic diseases, and summarizes the current perspective of therapeutic mechanism of MSCs in fibrosis, through both laboratory study and clinical trials.

G6b-B regulates an essential step in megakaryocyte maturation

Loss of G6b-B leads to an unexpected megakaryocyte development defect resulting in severe macrothrombocytopenia.

G6b-B–deficient mice display reduced levels of MK-specific transcripts, surface receptors, GATA-1, and thrombopoietin signaling.

G6b-B is a megakaryocyte lineage-specific immunoreceptor tyrosine-based inhibition motif–containing receptor, essential for platelet homeostasis. Mice with a genomic deletion of the entire Mpig6b locus develop severe macrothrombocytopenia and myelofibrosis, which is reflected in humans with null mutations in MPIG6B. The current model proposes that megakaryocytes lacking G6b-B develop normally, whereas proplatelet release is hampered, but the underlying molecular mechanism remains unclear. We report on a spontaneous recessive single nucleotide mutation in C57BL/6 mice, localized within the intronic region of the Mpig6b locus that abolishes G6b-B expression and reproduces macrothrombocytopenia, myelofibrosis, and osteosclerosis. As the mutation is based on a single-nucleotide exchange, Mpig6b^mut mice represent an ideal model to study the role of G6b-B. Megakaryocytes from these mice were smaller, displayed a less-developed demarcation membrane system, and had a reduced expression of receptors. RNA sequencing revealed a striking global reduction in the level of megakaryocyte-specific transcripts, in conjunction with decreased protein levels of the transcription factor GATA-1 and impaired thrombopoietin signaling. The reduced number of mature MKs in the bone marrow was corroborated on a newly developed Mpig6b-null mouse strain. Our findings highlight an unexpected essential role of G6b-B in the early differentiation within the megakaryocytic lineage.

GATA1 Expression in BCR/ABL1-negative Myeloproliferative Neoplasms

Background

This study aimed to determine GATA1 expression levels to better characterize subgroups in BCR/ABL1-negative myeloproliferative neoplasms (MPNs).

Methods

This study enrolled 49 patients diagnosed as having BCR/ABL1-negative MPN on the basis of the 2016 World Health Organization classification : nine polycythemia vera (PV), 17 essential thrombocythemia (ET), 12 prefibrotic primary myelofibrosis (prePMF), and 11 overt primary myelofibrosis (PMF). Relevant clinical and laboratory data were retrieved from the medical records. The molecular analysis of CALR and MPL mutations and quantification of JAK2 V617F allele burden were performed. GATA1 expression was assessed by an immunohistochemical assay on bone marrow biopsy. GATA1 expression was analyzed serially in 18 patients.

Results

GATA1 expression decreased significantly in PMF compared with that in other subtypes, while no statistical difference was identified between ET and prePMF. GATA1 expression did not differ according to the mutation profiles or the allele burden of JAK2 V617F, but it decreased significantly in patients with overt fibrosis or leukemic transformation.

Conclusions

Our results suggest that GATA1 expression is significantly low in PMF and decreases with progressive fibrosis and possibly with leukemic transformation, although our attempt to accurately distinguish between subgroups using GATA1 immunohistochemical approach did not achieve statistical significance. A large patient cohort with long term follow-up is required to evaluate the prognostic value of GATA1 expression.

Captopril mitigates splenomegaly and myelofibrosis in the Gata1low murine model of myelofibrosis

Allogeneic stem cell transplantation is currently the only curative therapy for primary myelofibrosis (MF), while the JAK2 inhibitor, ruxolitinib. Has been approved only for palliation. Other therapies are desperately needed to reverse life-threatening MF. However, the cell(s) and cytokine(s) that promote MF remain unclear. Several reports have demonstrated that captopril, an inhibitor of angiotensin-converting enzyme that blocks the production of angiotensin II (Ang II), mitigates fibrosis in heart, lung, skin and kidney. Here, we show that captopril can mitigate the development of MF in the Gata1low mouse model of primary MF. Gata1low mice were treated with 79 mg/kg/d captopril in the drinking water from 10 to 12 months of age. At 13 months of age, bone marrows were examined for fibrosis, megakaryocytosis and collagen expression; spleens were examined for megakaryocytosis, splenomegaly and collagen expression. Treatment of Gata1low mice with captopril in the drinking water was associated with normalization of the bone marrow cellularity; reduced reticulin fibres, splenomegaly and megakaryocytosis; and decreased collagen expression. Our findings suggest that treating with the ACE inhibitors captopril has a significant benefit in overcoming pathological changes associated with MF.

JAK2 inhibitors in the treatment of myeloproliferative neoplasms

INTRODUCTION

Dysregulation of JAK-STAT signaling is a pathogenetic hallmark of myeloproliferative neoplasms (MPNs) arising from several distinct molecular aberrations, including mutations in JAK2, the thrombopoietin receptor (MPL), mutations in negative regulators of JAK-STAT signaling, such as lymphocyte-specific adapter protein (SH2B3), and epigenetic dysregulation as seen with Suppressor of Cytokine Signaling (SOCS) proteins. In addition, growth factor/cytokine stimulatory events activate JAK-STAT signaling independent of mutations.

AREAS COVERED

The various mutations and molecular events activating JAK-STAT signaling in MPNs are reviewed. Detailed inhibitory kinase profiles of the currently developed JAK inhibitors are presented. Clinical trial results for currently developed JAK targeting agents are comprehensively summarized. The limitations of JAK-STAT targeting in MPNs, as well as potential rational combination therapies with JAK2 inhibitors, are discussed.

EXPERT OPINION

Aberrant JAK-STAT signaling is an underlying theme in the pathogenesis of MPNs. While JAK2 inhibitors are active in JAK2V617F and wild-type JAK2 MPNs, JAK2V617F mutation-specific or JAK2-selective inhibitors may possess unique clinical attributes. Complimentary targeting of parallel pathways operating in MPNs may offer novel therapeutic approaches in combination with JAK inhibition. Understanding the intricacies of JAK-STAT pathway activation, including growth factor/cytokine-driven signaling, will open new avenues for therapeutic intervention at known and novel molecular vulnerabilities of MPNs.

Reflections on the European Union Eurythron Network Meeting "Molecular Control of Erythropoiesis," September 22-23, 2005, Istituto Superiore di Sanità, Rome, Italy

Red blood cells (RBCs) mediate oxygen transport throughout the body, a function that is essential for life. RBCs are continuously produced via a process called erythropoiesis. Anemias (insufficient numbers of functional RBCs), caused by failure of erythropoiesis, are a major cause of disease worldwide. Hereditary anemias constitute the most common human genetic disorders; they have no effective cure yet. The European research training network Eurythron follows a multidisciplinary approach to clarify the important molecular mechanisms in normal and pathological erythropoiesis, with a view to develop novel therapies to cure the anemias. The aim is to generate a comprehensive molecular description of mechanisms governing specification of hematopoietic stem cells in embryogenesis, lineage commitment, differentiation, and postmitotic maturation of RBCs. We report on the Eurythron meeting in Rome, in which novel approaches in stem cell and erythroid cell biology, including in vitro expansion of primary cells, biochemistry of receptor/signal transduction complexes and transcription factors, and (epi)genetics, were discussed.

Pathogenesis of myelofibrosis with myeloid metaplasia

The primary disease process in myelofibrosis with myeloid metaplasia (MMM) is clonal myeloproliferation with varying degrees of phenotypic differentiation. This is characteristically accompanied by secondary intramedullary collagen fibrosis, osteosclerosis, angiogenesis, and extramedullary hematopoiesis. Modern clonality studies have confirmed the multipotent stem-cell origin of the neoplastic process in MMM. The nature of the specific oncogenic mutation(s) is currently being unraveled with the recent discovery of an association between a somatic point mutation of JAK2 tyrosine kinase (V617F) and bcr/abl-negative myeloproliferative disorders, including MMM. The pathogenetic mechanisms that underlie the secondary bone marrow stromal changes in MMM are also incompletely understood. Mouse models of this latter disease aspect have been constructed by either in vivo overexpression of thrombopoietin (TPOhigh mice) or megakaryocyte lineage restricted underexpression of the transcription factor GATA-1 (GATA-1low mice). Gene knockout experiments using such animal models have suggested the essential role of hematopoietic cell-derived transforming growth factor beta1 in inducing bone marrow fibrosis and stromal cell-derived osteoprotegerin in promoting osteosclerosis. However, experimental myelofibrosis in mice does not recapitulate clonal myeloproliferation that is fundamental to human MMM. Other cytokines that are implicated in mediating myelofibrosis and angiogenesis in MMM include basic fibroblast, platelet-derived, and vascular endothelial growth factors. It is currently assumed that such cytokines are abnormally released from clonal megakaryocytes as a result of a pathologic interaction with neutrophils (eg, emperipolesis). This latter phenomenon, through neutrophil-derived elastase, could also underlie the abnormal peripheral-blood egress of myeloid progenitors in MMM.