text: INTRODUCTION
  Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder with a typical three phased course (chronic, accelerated and blastic phase) reflecting the loss of differentiation and malignant progress which inevitably leads to death after the blastic phase (1,2). The hallmark genetic aberration of CML is a reciprocal chromosomal translocation t(9;22) leading to expression of a bcr-abl fusion gene, an aberrant activated tyrosine kinase (2). Treatment with interferon alpha (IFN-alpha) prolongs survival of CML patients and is associated with a complete cytogenetic response in 5-33% of CML patients (1,2). Recently, we described an impaired expression of the interferon regulatory factor 4 (IRF-4) in CML, correlating with poor response to IFN-alpha treatment (3). The cause of the silencing of IRF-4 level remained unclear. 
  Interferon regulatory factors (IRFs) are a family of transcriptional regulators defined by a characteristic homology in their DNA-binding domain. They play an important role in the regulation of various genes (such as IFNs, interleukins, MHC class I/II), apoptosis and differentiation/maturation (4-6). IRF-4 (ICSAT/Pip/MUM1/LSIRF) is one member with very restricted expression pattern: Predominately B- and activated T-lymphocytes are IRF-4 positive (7-11). In contrast to other IRFs, expression of IRF-4 cannot be induced by IFNs, but by antigen stimulation, crosslinking of T- or B-cell receptors or phorbol-myristate-acetate (10,11). Consistent with the restriction of expression to immunocompetent cells, mice with deletion of IRF-4 failed to develop mature and functionally active B- and T-lymphocytes (12), and the impaired expression of IRF-4 in CML was predominately found in T-cells (3). These data suggest a crucial role for IRF-4 in the function of immune cells. 
  Methylation of dinucleotide cytosine-guanosine motifs (CpG), especially in CpG islands located in promoter regions, is one of the mechanisms of gene regulation in mammals and a common event of gene silencing in human neoplasias (13,14). As opposed to normal cells, hypermethylation of CpG islands is a frequently observed phenomenon in every cancer type. De novo DNA methylation of genes such as cell cycle, DNA repair, apoptosis and tumor suppressor genes is therefore thought to be involved in tumorigenesis (15-17). Examples for such aberrated genes are MGMT, DAPK, p14ARF, p15INK4b, p16INK4a, BRCA1, CDH13 and APAF-1 (17-19). In CML, methylation is known to regulate expression of the c-abl, the bcr gene and others (20-23), and the extent of methylation in the c-abl promoter has been shown to be associated with advanced disease (24). Hypermethylation due to overexpression of DNA methyltransferases (DNMTs) remains one possible explanation for de novo methylation in tumorigenesis. Recently, DNMTs have been shown to be up-regulated in hematopoietic malignancies (25). Methyl-CpG-binding proteins (MBPs) are thought to inhibit the binding of transcriptional factors to the promoter and are therefore discussed as one mechanism of transcription inhibition by hypermethylation (26). 
  In this work, we studied mechanisms of IRF-4 gene expression silencing in leukemic cells. We analyzed the IRF-4 promoter region for genetic aberrations and methylational status in IRF-4-positive and -negative hematopoietic cells. 
  