UC Santa Cruz

Background: MLL-rearranged leukemias are a clinically challenging and biologically unique subtype of leukemias associated with poor prognosis. While novel therapeutic strategies have been primarily directed at epigenetic dysregulation, post-transcriptional gene regulatory mechanisms have emerged as important mediators in leukemogenesis and have the unexplored potential to be potent combinatorial therapeutic targets. Previously, we found that the RNA-binding protein IGF2BP3 is a critical regulator of MLL-AF4 leukemogenesis and represents a promising therapeutic target. Methods: We studied the combined effects of targeting IGF2BP3 and the Menin-MLL interaction in MLL-AF4 driven leukemia in vitro and in vivo, using genetic inhibition through CRISPR-Cas9 mediated deletion of Igf2bp3 and pharmacologic inhibition of the Menin-MLL interaction with commercially available inhibitors MI-503, MI-463, and MI-538. In vitro, we tested the human B-cell acute lymphoblastic leukemia cell lines, SEM, RS4;11 and NALM6, and MLL-Af4-transformed murine hematopoietic stem and progenitor cells (herein referred to as MLL-Af4 Lin-), derived from bone marrow of Cas9 mice. Results: Depletion of Igf2bp3 sensitized MLL-AF4 leukemia to the negative effects of Menin-MLL inhibition on leukemic cell growth, colony formation and leukemic initiating cells in vitro. Mechanistically, we found that both Igf2bp3 depletion and Menin-MLL inhibition led to increased differentiation in vitro and in vivo in functional readouts and by gene expression analyses. Both MI-503 treatment and IGF2BP3 knockdown in MLL-Af4 Lin- cells showed a shift towards more differentiated colony morphologies in colony formation assays, decreased expression of c-Kit and increased expression of maturation markers by flow cytometry, and morphologic changes consistent with increased differentiation. To gain insight into these phenotypic findings, we examined gene expression from MLL-Af4 Lin- cells with IGF2BP3 knockdown and treated with MI-503. Metascape analysis revealed significant enrichment in pathways involved in cell differentiation and activation, particularly in leukocytes. This was observed in both I3KO and MI-503 treated cells, with significant overlap in shared differentially expressed genes in both conditions. Next, we looked at the overlap between differentially expressed genes with MI-503 treatment and IGF2BP3 knockdown with targets identified from MLL-AF4 ChIP and IGF2BP3 CLIP in CD11b+ and MLL-Af4 Lin- cells. We found significant overlap between differentially expressed genes with MI-503 treatment and IGF2BP3 CLIP targets, suggesting that IGF2BP3 directly regulates genes that are affected by Menin-MLL inhibition. Furthermore, we saw overlap between differentially expressed genes with IGF2BP3 knockdown and MLL-AF4 ChIP targets identified in SEM and RS4;11 cells. These patterns highlight the interaction between the MLL-AF4 and IGF2BP3 transcriptomes and suggest a mechanism for the synergistic inhibition of leukemia by targeting both MLL-AF4 and IGF2BP3. To examine the combined effects of IGF2BP3 knockdown and Menin-MLL inhibition on leukemic engraftment and survival in vivo, we transplanted MLL-Af4 Lin- cells, depleted (or non-depleted) for IGF2BP3 and treated with MI-503 at 0.5 μM for 5 days, to initiate leukemia. We observed a significant decrease in peripheral blood leukemic engraftment with MI-503 treatment and IGF2BP3 knockdown. IGF2BP3 knockdown had a greater effect on survival and attenuating disease than MI-503 alone and showed enhanced anti-leukemic effects in combination. Conclusions: Our work shows that IGF2BP3 is an oncogenic amplifier of MLL-AF4 mediated leukemogenesis and is a potent therapeutic target and suggests a novel combinatorial approach to targeting leukemia at both the transcriptional and post-transcriptional level.