Dysregulated EVI1 expression in myeloid malignancies

The ecotropic virus integration site-1 (EVI1) protooncogene was first identified as a common site for retroviral integration in myeloid leukemias in susceptible mice [1,2]. The integrations occurred near or in 50 noncoding exons of the EVI1gene and led to activation of transcription from this locus. Subsequently, EVI1 was found to be activated in human myeloid malignancies, particularly acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) with chromosome 3q26 rearrangements [3 – 6]. Structural abnormalities of the long arm of chromosome 3, frequently presenting as inv(3)(q21q26) and t(3;3)(q21;q26), are detected in approximately 2% of AML/MDS cases, and are associated with the 3q21q26 syndrome that is commonly but not invariably characterized by a normal or elevated platelet count at diagnosis, hyperplasia with dysplasia of megakaryocytes, and a poor prognosis, even with aggressive therapies [7 – 11]. The poor response to therapy correlates with frequent association with recurring poor-risk chromosomal abnormalities such as monosomy 7, 7q, 5q or 11q23 abnormalities in such cases [12,13]. Other studies have shown (over)expression of EVI1 in AML/MDS in the absence of 3q26 rearrangements [3,14]. However, these previous studies were conducted using conventional reversetranscriptase PCR (RT-PCR), and failed to distinguish between EVI1 and its longer mRNA isoform, MDS1-EVI1, generated from in-frame splicing of the small gene myelodysplasia syndrome 1 (MDS1) to EVI1 [15]. This distinction is thought to be biologically important given the known ‘yin-yang’ properties of PR-domain containing proteins in carcinogenesis based on the existence of alternate isoforms (e.g. MDS1-EVI1/EVI1) [16]. The mammalian PR-domain containing family comprises at least 16 members, each characterized by zinc finger domains and an N-terminal PR-domain, which is a highly conserved sequence of approximately 100 amino acids, and which has homology with the methyltransferase activity-possessing SET domain present in proteins such as Su(var)3-9, Enhancerof-zeste and Trithorax. PR-domain containing proteins have a unique characteristic in that they exist in pairs (e.g. EVI1/MDS1-EVI1, RIZ1/RIZ2, BLIMP1/ BLIMPb), where the smaller protein in each pair is generated from an internal gene promoter and consequently lacks the PR-domain, but is otherwise near identical to the full length product. Although not definitively proven in every case, the frequent disruption of the PR-domain by either viral integration or chromosomal translocation events is thought to destroy the tumor suppressive properties of the PR-domain-containing longer protein isoform (e.g. MDS1-EVI1), leading to dysregulated overexpression of the shorter isoform (e.g. EVI1) in tumor tissues, which contributes to tumorigenesis by incompletely understood mechanisms. In the present study, Weisser et al. [17] use a quantitative RT-PCR assay to measure combined EVI1/MDS1-EVI1 transcript levels in 424 AML bone marrow samples. They find highest median EVI1/MDS1-EVI1 expression levels in cases with rearrangements involving 3q26 (n1⁄4 58) or 11q23 (n1⁄4 62), as well as those with chromosome 7 abnormalities (n1⁄4 29) and trisomy 8 (n1⁄4 10). These data essentially confirm the results of an earlier study of 319 de novo AML patients that used an independent quantitative RT-PCR assay that was additionally capable of discriminating between EVI1 and MDS1-EVI1 transcript isoforms [13]. In the latter study, high EVI1 expression (with or without concurrent MDS1-EVI1 expression) was seen in 32