AID expression is correlated with Bcr-Abl expression in CML-LBC and can be down-regulated by As₂O₃ and/or imatinib

Abstract

Activation-induced deaminase (AID), a cytidine deaminase, can accelerate the acquisition of BCR-ABL1 kinase domain mutations in human CML. In the present study, we investigated the expression of AID and Bcr-Abl in CML cells derived from 35 clinical patients. We found that both AID and Bcr-Abl were correlatively over-expressed in CML-LBC (lymphoid blast crisis) cells as compared with those in CML-CP (chronic phase) cells. AID expression was significantly decreased in CML-LBC cells after treated with arsenic trioxide, especially together with imatinib. We also observed satisfied therapy effects of As₂O₃ and imatinib on patients with CML blast crisis. These data suggest that decreasing AID expression in CML-LBC by As₂O₃ may be a promising approach to CML treatment.

Introduction

Chronic myelocytic leukemia (CML) develops from a hematopoietic stem cell and consequently displays multilineage differentiation potential. If not efficiently treated, CML follows a triphasic clinical course with an initial indolent chronic phase (CML-CP; 5–15 years) followed by an intermediate accelerated phase and eventually a blast crisis of myeloid (CML-MBC; ∼60% incidence), B lymphoid (CML-LBC; ∼30% incidence), or biphenotypic myeloid/lymphoid (∼10% incidence) lineage [1]. How to prolong the life span of CML patients and prevent CML progressing into blast crisis always puzzled us.

Recent works showed that some kinds of oncogene and tumor suppressor gene were correlated with the evolution of CML to blast crisis. For instance, the fuse gene Bcr-Abl expression level increased consistently in CML blast crisis [2], bcl-2, PDGF and c-myc were up-regulated dramatically in CML blast crisis [3]. In addition, Wnt signal pathway was aberrantly activated in CML blast crisis [4]. However, the fundamental mechanism of how CML progresses to blast crisis is still unclear.

More than 90% of patients with CML in chronic phase have the Philadelphia (Ph) chromosome, and about 75–80% of patients with CML in acceleration or blast crisis phase have morphological changes of chromosomes, such as additional chromosome (+8, +19), chromosome translocation (+22q−) and inversion (i17q). Actually, chromosome instability plays an important role in CML progressing into blast crisis [5]. Activation-induced cytidine deaminase (AID) promotes somatic hypermutation and class switch recombination of immunoglobulin (Ig) genes in germinal center (GC) B cells. AID initiates both processes by deaminating Ig gene cytidines into uracils, thereby creating DNA mismatches that ultimately progressed into mutations or DNA breaks [6]. On the one hand, the U:G mismatch produced by uracil improves the efficiency and affinity of the antibody response; on the other hand, uracil can be removed by uracil-N-glycosylase (UNG) to produce an abasic site that enables both transition mutations at G:C pairs and endonuclease cleavage, leading to the generation of a DNA double strand break (DSB)[7]. In eukaryotes, DSB are repaired through two pathways: homologous recombination (HR) and non-homologous end-joining (NHEJ). NHEJ is an error-prone process that operates primarily during the G1-phase of the cell cycle to rejoin DNA ends bearing little or no homology, which may result in chromosome translocation [8].

Previous studies have reported that AID was strictly expressed in activated B cells. AID played an important role in the progression of lymphoma and lymphoblastic leukemia. For instance, Bcl-1, Bcl-2 and c-myc to Ig translocations found in mantle-zone lymphoma, follicular lymphoma and Burkitt lymphoma, respectively, all needed the action of AID [9]. In the absence of AID, IL-6 transgenic mice developed hyperplastic lymph nodes and plasmacytosis without c-myc/IgH translocation, indicating that AID is indeed required for translocation in this model. AID increased the frequency of amplification and deletion in BCR-ABL1 leukemia and induced aberrant somatic hypermutation [6]. However, results from previous investigations suggested that AID was not exclusively expressed in activated B cells. AID was detected in the gastric tissues infected by Helicobacter pylori, in liver tissues of HCV patients, as well as in some kinds of tumors, such as breast cancer, hepatoma and prostatic carcinoma [10]. Klemm also reported significant expression of AID mRNA in CML-LBC blasts, but little or no expression in CML-CP cells. Moreover, AID can accelerate the acquisition of Bcr-Abl kinase domain mutations in human CML and lead to imatinib resistance [1].

Here, we investigate the interrelationship between AID and Bcr-Abl in CML progressing into blast crisis, and the effects of As₂O₃ on AID expression in CML-LBC cells and on imatinib resistance in CML-LBC patients.