The MN1 oncoprotein synergizes with coactivators RAC3 and p300 in RAR-RXR-mediated transcription

Abstract

The t(12;22) creates an MN1-TEL fusion gene leading to acute myeloid leukemia. The fusion partner TEL (ETV6) is a member of the ETS family of transcription factors. The nature of the other fusion partner, MN1, has not been investigated in detail until now. We recently described that MN1 activates the transcription activity of the moloney sarcoma virus long terminal repeat, indicating that this protein itself may act as a transcription factor. We show here that MN1 comprises multiple transcription activating domains. A search for a bound DNA sequence revealed that MN1 has affinity for retinoic acid responsive elements. A DR5 retinoic acid responsive element was observed in the LTR. The combination of MN1 and ligand-activated retinoic acid receptor leads to a synergistic induction of expression directed by the LTR. Cotransfection of MN1 with RAC3 or p300, known coactivators of retinoic acid receptors, leads to a further synergistic induction of transcription. In addition, the effect of MN1 can be inhibited by the wild-type adenovirus ElA protein that inhibits p300 function, but not by an E1A mutant lacking the p300-binding site. GAL4-MN1-mediated transcription can be enhanced directly by RAC3 and p300. Taken together, our results indicate that MN1 is a transcription coactivator rather than a sequence-specific transcription factor, and that it may stimulate RAR/RXR-mediated transcription through interaction with p160 and p300.

The MN1-TEL myeloid leukemia-associated fusion protein has a dominant-negative effect on RAR-RXR-mediated transcription

“…Both MN1 and MN1-TEL stimulate transcription from the murine sarcoma virus long terminal repeat (MSV-LTR), and MN1 synergizes with RAR-RXR, p160 and p300 in the activation of this promoter (Buijs et al, 2000;van Wely et al, 2003). The MSV-LTR comprises multiple transcription factor binding sites including a direct repeat 5 (DR5) ATRA-responsive element (van Wely et al, 2003) and several binding sites for ETS transcription factors (Gunther et al, 1990).…”

“…The MSV-LTR comprises multiple transcription factor binding sites including a direct repeat 5 (DR5) ATRA-responsive element (van Wely et al, 2003) and several binding sites for ETS transcription factors (Gunther et al, 1990). To determine whether MN1-TEL synergizes with RAR-RXR when stimulating the MSV promoter, we cotransfected human Hep3B cells with the MSV1 reporter and an MN1-TEL expression construct.…”

“…Stimulation of the MSV-LTR by MN1 is enhanced in a synergistic manner by p160 and p300, indicating that these coactivators mediate the synergy of MN1 and RAR-RXR (van Wely et al, 2003). To determine whether MN1-TEL synergizes with p160 and p300, increasing amounts of RAC3 or p300 were co-transfected with either MN1 or MN1-TEL.…”

“…The encoded fusion protein comprises nearly all of MN1 fused to the central part and DNA-binding domain of TEL. MN1, a protein of 1319 amino acids, is a transcription coactivator of the nuclear hormone receptors for all-trans retinoic acid (ATRA) and vitamin D (van Wely et al, 2003;Sutton et al, 2005). Targeted disruption of the Mn1 gene causes a defect in the development of cranial membranous bones and death just after birth (Meester-Smoor et al, 2005).…”

“…Although these studies have established the capacity of MN1-TEL to promote leukemia, they did not provide a molecular basis for its malignant effects. We therefore studied the fusion protein in a model system with established MN1 activity (van Wely et al, 2003). MN1-TEL acts as an altered transcription factor compared to TEL, as it stimulates transcription from a reporter carrying responsive elements normally repressed by TEL (Buijs et al, 2000).…”