New functions for ecotropic viral integration site 1 (EVI1), an oncogene causing aggressive malignant disease.

The oncogene Evi1, which codes for a sequence-specific transcription factor, was cloned as a target of activating retroviral insertions in murine myeloid tumors 24 y ago. Soon thereafter, its human homolog was found to be transcriptionally activated through rearrangements of chromosome band 3q26,, which are associated with a poor prognosis in acute myeloid leukemia (AML). Since then, EVI1 has emerged repeatedly as retroviral integration site in different organisms, and a role of its overexpression as an indicator of poor prognosis in a variety of hematopoietic malignancies, as well as some solid tumors, has been firmly established. Because of its well-documented clinical importance, understanding the mechanism(s) of action of EVI1 is of high significance. However, EVI1 reveals its secrets only slowly, with merely 475 articles containing the search terms “EVI1,” “EVI-1” or “MECOM” (the officially assigned, but still rarely used, gene name) deposited in PubMed at the time of writing.

One of the most recent occasions on which EVI1 gained negative prominence was in the context of a human gene therapy trial for X-linked chronic granulomatous disease (X-CGD). Transplantation of gene-modified, autologous hematopoietic stem cells initially provided substantial clinical benefit to two young adults with X-CGD. However, over time, hematopoiesis became dominated by clones with vector integrations into, and transcriptional activation of, the EVI1 locus. These clones finally evolved into myeloid malignancies, acquiring monosomy 7, a chromosome aberration frequently associated with EVI1 overexpression in AML, during this process. As a preliminary mechanistic explanation for these findings, experimental expression of EVI1 in human diploid fibroblasts increased the numbers of cells with aberrant centrosome numbers.

In the September 15th issue of Cell Cycle, Karakaya and colleagues confirmed and extended these observations. Using U2OS osteosarcoma cells inducibly expressing EVI1 as a model, they show that only 72 h after induction of EVI1, 17% of the cells contained centrosome amplifications vs. 5% of the control cells. Analysis of nuclear morphology and time-lapse video microscopy suggested that supernumerary centrosomes resulted from a cytokinesis defect that is known to activate a p53-dependent tetraploidy checkpoint. Indeed, induction of EVI1 upregulated p53 and siRNA-mediated p53 depletion increased the percentage of polyploid cells after EVI1 induction. Furthermore, the vast majority of EVI1-overexpressing cells with centrosome amplification had low or undetectable levels of the proliferation marker Ki67, indicating that EVI1-induced centrosome aberrations were largely confined to cells in G0 or early G1, and confirming that EVI1-induced tetraploidization caused a cell cycle arrest.

These data, together with those from the X-CGD study, establish genomic instability as a not previously reported consequence of EVI1 overexpression, which also might explain its frequent association with monosomy 7. It should be pointed out, though, that chromosome aberrations can also emerge through selection on the background of normal missegregation rates rather than through increased genetic instability. In fact, in the Mitelman database, in ~39% of AML cases with a 3q26 aberration this was the sole cytogentic anomaly, and for ~82% of 3q26 rearranged cases, only a single clone was reported [compared with ~43% and ~83%, respectively, for the prognostically favorable t(8;21)(q22;q22)]. These data do not argue for greatly increased rates of chromosome instability as a consequence of EVI1 overexpression. However, excessive instability would likely pose the danger of lethal genetic aberrations and thus not be beneficial to a tumor cell, while low-level instability may become apparent from cytogenetic data only through more detailed analyses. In addition to eventual subtle effects on karyotype at the time of diagnosis, modest EVI1-induced chromosome instability raises the intriguing possibility that EVI1 overexpression may cause poor prognosis at least in part by facilitating acquisition of aberrations that allow tumor cells to escape chemotherapy-induced apoptosis. This hypothesis can be tested through quantitative evaluation of chromosome aberrations and clones at diagnosis and relapse (which is caused by cells that survived the initial therapy) in patients with 3q26 rearranged/EVI1 overexpressing AML and suitable controls. The work of Karakaya et al., in addition to the novel information it provides as it stands, therefore opens new perspectives that may lead to substantial advances in our understanding of how overexpression of EVI1 contributes to poor prognosis in AML and other malignancies.