RUNX1/CBFβ Dosage Is Critical for MLL Leukemias Development

Transcription factors RUNX1/CBFβ play critical roles in hematopoiesis. Both of them are frequently involved in chromosomal translocations, point mutations, or deletions in acute leukemia. The mixed lineage leukemia (MLL) gene is also frequently involved in chromosomal translocations or partial tandem duplication in acute leukemia. We have previously shown that MLL, RUNX1, and CBFβ interact and form a regulatory complex to regulate downstream target genes. However, the functional consequence of MLL fusions on RUNX1/CBFβ activity remains unknown.

To determine the impact of MLL fusion protein on RUNX1/CBFβ, we introduced either MLL, MLL-BP (longer N-terminal Flag-tagged MLL construct which contains CXXC domain; 1-1406), or MLL-fusions together with RUNX1, CBFβ, or both RUNX1 and CBFβ into 293T cells. MLL-BP and MLL fusions significantly decreased RUNX1 levels compared with controls (empty vector and MLL). CBFβ protein was mildly decreased by MLL-BP and MLL-fusions when expressed alone. However, when CBFβ was co-expressed with RUNX1, it was significantly decreased compared with controls. The expression levels of RUNX1 and CBFβ proteins in LSK cells from Mll-Af9 knock-in mice were significantly lower than those from wild-type (WT) mice. To confirm these findings in human acute myeloid leukemia (AML), we measured the expression of RUNX1 and CBFβ at both mRNA and protein levels in various leukemia cell lines. The expression levels of RUNX1 and CBFβ proteins were significantly decreased in AML cells with MLL fusion and MLL partial tandem duplication (MLL-PTD) compared with those in AML cells without MLL aberrations. MLL fusions still have CXXC domain. In MLL-PTD, the CXXC domain is duplicated. Our data showed that RUNX1 protein is not only down-regulated by MLL fusion proteins, but also by MLL-BP. Thus, to determine which region is involved in the down-regulation of RUNX1, we introduced a series of MLL deletion mutants into 293T cells and measured RUNX1 protein expression. MLL deletion mutants without CXXC domain had no effect on RUNX1 stability. The construct which contains point mutations in CXXC domain also lacked the ability to reduce RUNX1 expression. Furthermore, overexpression of only CXXC domain and flanking regions could down-regulate RUNX1 protein expression. These results suggest that MLL fusion proteins and the N-terminal MLL portion of MLL fusions down-regulate RUNX1 and CBFβ protein expression via the MLL CXXC domain and flanking regions.

To understand the impact of RUNX1/CBFβ down-regulation on hematopoietic stem and progenitor cells (HSPCs), we generated RUNX1^+/–/CBFβ^+/– mice as a hypomorph model. The percentage of bone marrow (BM) LSK cells from RUNX1^+/–/CBFβ^+/– mice was significantly increased compared with that from WT mice. Using BM cells from these mice, we performed in vitro CFU assay and in vivo bone marrow transplantation (BMT) assay. BM cells from RUNX1^+/–/CBFβ^+/– mice provided more colonies in CFU assay compared with those from WT mice. To determine whether restoration of RUNX1 could repress the MLL mediated leukemogenesis, we retrovirally overexpressed WT RUNX1 in BM cells from Mll-Af9 knock-in mice. Using transduced BM cells, we performed in vitro CFU assay and in vivo BMT assay. RUNX1 overexpressed Mll-Af9 (Mll-Af9/RUNX1) cells underwent terminal differentiation after 2 times replating, while control vector transduced Mll-Af9 (Mll-Af9/Control) cells could still be replated more than 4 times. All the recipient mice transplanted with Mll-Af9/Control cells developed AML. In contrast, all the recipient mice transplanted with Mll-Af9/RUNX1 never develop AML. Furthermore, when we treated MLL leukemia cell lines with DOT1L inhibitor (EPZ-5676), RUNX1 protein levels in these MLL leukemia cell lines were significantly increased 48 hours after the treatment in comparing with controls treated with DMSO. However, there was no significant mRNA expression level change of RUNX1within 48 hours. Future studies are needed to fully understand the mechanism of whether this increasing RUNX1 protein level by DOT1L inhibitor is through blocking CXXC domain and flanking regions mediated degradation.

In conclusion, MLL aberrations down-regulate RUNX1/CBFβ via their CXXC domain and flanking regions. Down-regulation of RUNX1/CBFβ plays critical role for MLL mediated leukemia development. Targeting RUNX1/CBFβ levels allows us to test novel therapies for MLL leukemias.