Oligomerization of Evi-1 Contributes to Recruitment of Transcriptional Corepressor CtBP and Repression of TGF-β Signaling.

Aberrant oligomerization of transcription factors has recently emerged as a prevalent mechanism for activating their oncogenic potential in hematopoietic malignancies. Furthermore, for some chimeric transcription factors generated in leukemia-associated chromosomal translocations including PML-RARα and AML1-ETO, it has also been proposed that aberrant recruitment of corepressor complexes caused by oligomerization is critical for the development of leukemia. Among leukemia-associated transcription factors is Evi-1, which is frequently activated in cases with myeloid malignancies. Evi-1a and Evi-1c, with the latter also called MDS1-Evi-1, are two alternative forms derived from the Evi-1 gene. Evi-1a is a Zn finger transcription factor which recruits CtBP as a transcriptional corepressor and possesses the ability to repress TGF-β signaling. Amino-terminal to the Evi-1a, Evi-1c harbors an additional sequence similar to the PR domain, which was originally identified in PRDI-BF1 and RIZ1. In contrast to frequent upregulation of Evi-1a in myeloid malignancies, expression of Evi-1c widely varies depending on cases. Thus, although several lines of evidence suggest that Evi-1a is involved in leukemic transformation of hematopoietic cells, a role for Evi-1c in leukemogenesis has remained elusive. To elucidate the mechanism that underlies leukemogenesis by Evi-1, we investigated the ability of Evi-1 proteins to form oligomeric complexes. We found that Evi-1a forms a homo-oligomer in mammalian cells, whereas Evi-1c exclusively exists as a monomer. Remarkably, Evi-1c has lost the ability to interact with CtBP and failed to efficiently repress TGF-β signaling. We previously reported that AML1-Evi-1, a chimeric transcription factor generated in t(3;21) leukemia, interacts with CtBP as Evi-1a does. In the current study, we also found that AML1-Evi-1 forms a homo-oligomer, suggesting a close relationship between oligomer formation and CtBP binding. Taken together, these data indicate that homo-oligomerization may contribute to the oncogenic potential of the Evi-1 proteins by regulating the interaction with CtBP. These results also identify a novel function of PR domain to dictate oligomerization of transcription factors.