Differential Expression of Genes Related to JAK-STAT Pathway Activation or Megakaryocyte Differentiation/Maturation in CD34+ Cells from Patients with Refractory Anemia with Ringed Sideroblasts and Thrombocytosis (RARS-T).

Abstract

A peculiar feature of the myelodysplastic syndrome (MDS) defined as refractory anemia with ringed sideroblasts (RARS) is that CD34+ cells have a distinct expression profile characterized by up-regulation of mitochondrial-related genes and, in particular, of those of heme synthesis, e.g., ALAS2 [Blood 2006; 108:337–45]. RARS-T is a poorly defined myelodysplastic/myeloproliferative variant of RARS. RARS-T patients may carry the JAK2 (V617F) mutation, and this might considerably affect gene expression. We did studies of JAK2 and MPL mutation analysis, and of gene expression profiling in 16 RARS-T patients. We employed a real time PCR-based allelic discrimination assay for the detection of JAK2 (V617F), and direct sequencing for the analysis of JAK2 exon 12 and MPL exon 10 mutations. The Affymetrix microarray technology (U133 Plus2.0 chips) was used for gene expression profiling of purified bone marrow CD34+ cells. Mutations of JAK2 and/or MPL were detected in granulocytes but not T-lymphocytes from 7/16 patients (44%). Five patients carried JAK2 (V617F), one patient had the JAK2 exon 12 E543-D544del, and the remaining patient had both JAK2 (V617F) and MPL (W515L) mutant alleles. Granulocytes carrying mutant alleles represented only a minor fraction of clonal granulocytes, as determined by X-chromosome inactivation patterns. By comparing gene expression profiles of CD34+ cells from 12 RARS and 6 RARS-T patients (4 of these latter carrying mutant JAK2 alleles), 255 genes were found to be differentially expressed. Within the 165 up-regulated genes, the most represented ones included genes related to transcription regulation, cell proliferation, and cytoskeleton organization. Within the 90 down-regulated genes, those related to cell cycle arrest and cell adhesion were the most recurrent ones. Forty-six genes were found to be differentially expressed in all 6 RARS-T cases, and 8 of these showed a relative expression ratio >2. In particular, CXCR4, a gene encoding a CXC chemokine receptor specific for SDF-1 and reported to be down-regulated in primary myelofibrosis, was markedly down-regulated in RARS-T patients. JCTSG (encoding cathepsin) and JLTF (encoding lactoferrin and inhibited by STAT5 activation) were also markedly down-regulated. A significant up-regulation of TNRC15, a gene involved in the signal transduction by insulin-like growth factor (IGF-I), was observed. Finally, a few genes involved in cytoskeleton organization and megakaryocyte differentiation/maturation (like CDC2L5, ARHGAP12, and PLDN) were found to be differentially expressed. Thus, in refractory anemia with ringed sideroblasts, clonal hematopoietic cells may acquire not only JAK2 (V617F), but also of JAK2 exon 12 and MPL exon 10 mutations. The occurrence of JAK2 and MPL mutations, and possibly of additional ones, results in CD34+ cell expression profiles consistent with activation of the JAK-STAT pathway and enhancement of megakaryocytopoiesis.