Altered transcription of the stem cell leukemia gene in myelofibrosis with myeloid metaplasia

Immunoproteasome Genes Are Modulated in CD34+ JAK2V617F Mutated Cells from Primary Myelofibrosis Patients

Primary myelofibrosis (PMF) is a rare myeloproliferative neoplasm characterized by stem-cell-derived clonal over-proliferation of mature myeloid lineages, bone marrow fibrosis, osteosclerosis, defective erythropoiesis, and pro-inflammatory cytokine over-expression. The aim of the present study was to highlight possible differences in the transcriptome among CD34⁺ cells from peripheral blood (PB) of PMF patients. Therefore, we merged two microarray datasets of healthy control subjects and PMF (34 JAK2^V617F MUTATED and 28 JAK2 wild-type). The GO analysis of upregulated genes revealed enrichment for JAK2/STAT1 pathway gene set in PB CD34⁺ cells of PMF patients with and without the JAK2^V617F mutation comparing to the healthy control subjects, and in particular a significant upregulation of immunoproteasome (IP)-belonging genes as PSMB8, PSMB9, and PSMB10. A more detailed investigation of the IFN-gamma (IFNG) pathway also revealed that IFNG, IRF1, and IFNGR2 were significantly upregulated in PB CD34⁺ cells of PMF patients carrying the mutation for JAK2^V617F compared to JAK2 wild-type PMF patients. Finally, we showed an upregulation of HLA-class I genes in PB CD34⁺ cells from PMF JAK2^V617F mutated patients compared to JAK2 wild-type and healthy controls. In conclusion, our results demonstrate that IPs and IFNG pathways could be involved in PMF disease and in particular in patients carrying the JAK2^V617F mutation.

New insights into the pathogenesis and drug treatment of myelofibrosis

PURPOSE OF REVIEW

Myelofibrosis with myeloid metaplasia was first described in 1879, classified as a myeloproliferative disorder in 1951, and characterized as a clonal stem cell disorder in 1978. Despite the passing of time, the molecular basis of the disease has remained elusive although substantial progress has been made regarding the pathogenesis of the associated bone marrow stromal reaction. Advances have also been meager in terms of treatment for disease complications, including anemia, splenomegaly, and leukemic transformation.

RECENT FINDINGS

At the molecular level, a JAK2 tyrosine kinase mutation (JAK2) has recently been described in a spectrum of myeloproliferative disorders including myelofibrosis with myeloid metaplasia with the reported mutational frequency ranging from 35% to 57% with 9-29% homozygosity. To date, the presence of JAK2 in myelofibrosis with myeloid metaplasia has not been shown to have prognostic relevance. Other recent observations of potential pathogenetic relevance in this disease include the description of a highly specific chromosomal translocation {der(6)t(1;6)(q23;p21)}, the demonstration of an epigenetic downregulation of the retinoic acid receptor-beta2 expression in CD34 cells, and the direct implication of transforming growth factor-beta1 in thrombopoietin-driven experimental myelofibrosis in mice. From a therapeutic standpoint, benefit to a subset of patients has been demonstrated for both allogeneic stem cell transplantation and novel drugs, including thalidomide and lenalidomide.

SUMMARY

Recent advances in the pathogenesis of myelofibrosis with myeloid metaplasia are expected to facilitate the development of molecularly targeted therapy. In the mean time, current management strategies include observation, participation in experimental drug therapy, and allogeneic stem cell transplantation for low-risk, intermediate-risk, and high-risk disease, respectively.

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.

IL-8 and its CXCR1 and CXCR2 receptors participate in the control of megakaryocytic proliferation, differentiation, and ploidy in myeloid metaplasia with myelofibrosis

Myeloproliferation, myelofibrosis, and neoangiogenesis are the 3 major intrinsic pathophysiologic features of myeloid metaplasia with myelofibrosis (MMM). The myeloproliferation is characterized by an increased number of circulating CD34+ progenitors with the prominent amplification of dystrophic megakaryocytic (MK) cells and myeloid metaplasia in the spleen and liver. The various biologic activities of interleukin 8 (IL-8) in hematopoietic progenitor proliferation and mobilization as well as in neoangiogenesis prompted us to analyze its potential role in MMM. We showed that the level of IL-8 chemokine is significantly increased in the serum of patients and that various hematopoietic cells, including platelets, participate in its production. In vitro inhibition of autocrine IL-8 expressed by CD34+ cells with either a neutralizing or an antisense anti-IL-8 treatment increases the proliferation of MMM CD34(+)-derived cells and stimulates their MK differentiation. Moreover, addition of neutralizing anti-IL-8 receptor (CXC chemokine receptor 1 [CXCR1] or 2 [CXCR2]) antibodies to MMM CD34+ cells cultured under MK liquid culture conditions increases the proliferation and differentiation of MMM CD41+ MK cells and restores their polyploidization. Our results suggest that IL-8 and its receptors participate in the altered MK growth that features MMM and open new therapeutic prospects for this still incurable disease.