Functional features of EVI1 and EVI1Δ324 isoforms of MECOM gene in genome-wide transcription regulation and oncogenicity

The MDS1 and ecotropic viral integration site 1 (EVI1) complex locus (MECOM) gene encodes several transcription factor variants including MDS1-EVI1, EVI1 and EVI1Δ324. Although MDS1-EVI1 has been associated with tumor-suppressing activity, EVI1 is a known oncogene in various cancers, whose expression is associated with poor patient survival. Although EVI1Δ324 is co-transcribed with EVI1, its activity in cancer cells is not fully understood. Previous reports described that unlike EVI1, EVI1Δ324 protein cannot transform fibroblasts because of its disrupted N-terminal zinc finger (ZNF) domain. To better understand EVI1Δ324 biology and function, we obtained genome-wide binding occupancies and expression data in ovarian cancer cells. We characterized its DNA-binding sites, binding motif and target genes. Comparative analyses with previous study show that EVI1 and EVI1Δ324 share similar transcriptional activities linked to their common C-terminus ZNF domain. They bind to an E-twenty-six family (ETS)-like motif, target to a large extent the same genes and cooperate with AP1 transcription factor. EVI1Δ324-occupied genes were 70.7% similar to EVI1-bound genes. More strikingly, EVI1 and EVI1Δ324 differentially expressed genes were 99.87% identical, indicating comparable transcriptional regulatory functions. Consistently with gene ontologies linked to these target genes, EVI1Δ324 expression in HeLa cells could enhance anchorage-independent growth, such as EVI1, showing that EVI1Δ324 expression also lead to pro-oncogenic effects. The main specific feature of EVI1 variant is its N-terminus ZNF domain that binds DNA through GATA-like motif. We found that most GATA-like EVI1 chromatin immunoprecipitation sequencing peaks are far from genes and are not involved in transcriptional regulation. These genomic regions were enriched in simple sequence repeats and displayed high meiotic recombination rates. Overall, our genomics analyses uncovered common and specific features of two major MECOM isoforms. Their influence on transcription and downstream cell proliferation was comparable. However, EVI1-specific GATA-like binding sites, from its N-terminus ZNF domain, associated with high recombination rates, suggesting possible additional oncogenic potential for EVI1 in modulating genomic stability.