Installation of a Cancer Promoting WNT/SIX1 Signaling Axis by the Oncofusion Protein MLL-AF9

Abstract

Background: Chromosomal translocation-induced expression of the chromatin modifying oncofusion protein MLL-AF9 promotes acute myelocytic leukemia (AML). Whereas WNT/β-catenin signaling has previously been shown to support MLL-AF9-driven leukemogenesis, the mechanism underlying this relationship remains unclear. Methods: We used two novel small molecules targeting WNT signaling as well as a genetically modified mouse model that allow targeted deletion of the WNT protein chaperone Wntless (WLS) to evaluate the role of WNT signaling in AML progression. ATAC-seq and transcriptome profiling were deployed to understand the cellular consequences of disrupting a WNT signaling in leukemic initiating cells (LICs). Findings: We identified Six1 to be a WNT-controlled target gene in MLL-AF9-transformed leukemic initiating cells (LICs). MLL-AF9 alters the accessibility of Six1 DNA to the transcriptional effector TCF7L2, a transducer of WNT/βcatenin gene expression changes. Disruption of WNT/SIX1 signaling using inhibitors of the Wnt signaling delays the development of AML. Interpretation: By rendering TCF/LEF-binding elements controlling Six1 accessible to TCF7L2, MLL-AF9 promotes WNT/β-catenin-dependent growth of LICs. Small molecules disrupting WNT/β-catenin signaling block Six1 expression thereby disrupting leukemia driven by MLL fusion proteins.

BCOR Binding to MLL-AF9 Is Essential for Leukemia via Altered EYA1, SIX, and MYC Activity

“…Indeed, Eya1 overexpression immortalizes hematopoietic progenitor cells, and heterodimerization with Six1 augments this effect (26). Furthermore, two recent reports demonstrate a reduced LIC population in Six1 knockout or knockdown mouse cells expressing MLL-AF9 versus in a Six1 WT background (48,49). The unexpected observation that we see increased proliferation in the colony formation assay with MLL-AF9(E531R) could in part be due to increased expression of Hoxa7 ( Fig.…”

Retinal determination gene networks: from biological functions to therapeutic strategies

“…MLL-AF9 is known to be a driver of acute myeloid leukemia (AML). Zhang et al confirmed that SIX1 was the target gene of leukemic initiating cells (LICs) confusion protein (MLL-AF9), and blocking WNT/SIX1 signaling by Wnt signaling inhibitors suppressed the progression of AML [61]. EYA1 and SIX1 are induced by MLL-ENL, EYA1 could immortalize hematopoietic progenitor cells, and SIX1 enhanced the transforming capacity of EYA1 [175].…”

“…Furthermore, Zhang et al illustrated that SIX1 directly affected the expression activity of SIX1 and EYA1 in MLL-AF9. With the administration of WNT signaling inhibitors, WNT/ SIX1 signaling was disrupted, which delayed the progression of AML [61]. Monteiro et al proposed that the methylation of the SIX1 promoter could be an independent prognostic factor for overall survival in melanoma patients [201].…”

The multiple ways Wnt signaling contributes to acute leukemia pathogenesis

“…Therefore, this study showed that the activation of the Wnt/β‐catenin signaling pathway is necessary for AML induction by HoxA9 and Meis1, as well as by KMT2A ‐ AF9 rearrangement. Moreover, the fusion protein KMT2A‐AF9 facilitates the accessibility of the Wnt transcriptional effector TCF‐4 to the transcriptional regulatory elements of SIX1 , a growth‐promoting homeobox gene, contributing to the growth of leukemic initiating cells (LICs) 60 …”

“…Moreover, the fusion protein KMT2A-AF9 facilitates the accessibility of the Wnt transcriptional effector TCF-4 to the transcriptional regulatory elements of SIX1, a growth-promoting homeobox gene, contributing to the growth of leukemic initiating cells (LICs). 60 The oncogenic potential of other fusion proteins has also been linked to Wnt signaling activation in AML. 61 in RUNX1-RUNX1T1 AML.…”