MLL fusion proteins preferentially regulate a subset of wild-type MLL target genes in the leukemic genome

Abstract

IntroductionChromosomal translocations often lead to creation of chimeric fusion genes that define cancer subtypes and act as initiating events in oncogenesis. Fusion genes can acquire novel biologic features via their fusion partners, yet retain important functions of the wild-type protein. When chromosomal rearrangements involve genes encoding DNA-binding proteins, chimeric fusion proteins can serve as aberrant transcription factors leading to genome-wide dysregulation in gene expression. Studies have shown that fusion transcription factors may gain novel specificity in target gene selection. The EWS/FLI1 oncogenic transcription factor modulates a specific set of genes that are not bound by wild-type FLI1 in vivo. 1 The AML1-ETO fusion protein preferentially binds to duplicated AML-1 DNA-binding sequences. 2 In acute promyelocytic leukemia (APL), the PML/RAR␣ fusion protein is selectively targeted to regions containing PU.1 consensus and RARE (retinoic acid response elements) half sites. The majority of those sites do not correspond to canonical RAREs. 3 Chromosomal translocations involving the MLL (mixed-lineage leukemia) gene result in the formation of a chimeric transcription factor. It remains unknown whether the MLL fusion protein shares the same set of target genes compared with the wild-type MLL protein.The histone methyltransferase MLL gene is frequently targeted by chromosomal translocations in acute myeloid, lymphoid, and biphenotypic leukemias, and rearrangement of MLL is associated with a poor prognosis. Wild-type MLL is critical for the maintenance of expression of its target genes. This activity is mediated by its carboxyl-terminal SET domain, which acts as a histone 3 lysine-4 (H3K4) methyltransferase. 4,5 The critical consequence of 11q23 chromosomal translocations is the formation of a chimeric oncogenic transcription factor that retains the N-terminus of MLL but replaces carboxyl-terminal domains with sequences from its partner proteins. As a result of 11q23 gene rearrangements, MLL fuses in frame with Ͼ 60 different partner proteins. 6 The most common partners are AF4, AF9, ENL, AF10, and ELL, which together account for Ͼ 85% of all MLL-rearranged leukemias. AF4, ENL, AF9, and AF10 form a complex that promotes H3K79 methylation through recruitment of histone methyltransferase DOT1L. 7,8 Aberrant H3K79 methylation has been shown to be a key molecular mechanism in MLL fusion-induced dysregulation of gene expression. 9,10 MLL is targeted to a specific set of gene loci, presumably via its own DNA binding domain, recognition of local histone modifications, and/or recruitment by sequence-specific transcription factors. 11 However, the target genes of MLL fusion proteins compared with wild-type MLL remain poorly understood. In MLL-AF4-expressing mouse acute lymphoid leukemia cells, thousands of gene promoters exhibited increased levels of H3K79 methylation in comparison to normal control lymphocytes, suggesting MLL-AF4 has widespread targets in the leukemic genome. 12 In contrast, by looking for co...

Eya2, a Target Activated by Plzf, Is Critical for PLZF-RARA-Induced Leukemogenesis

“…In fly eye development (45), Drosophila Eya constitutes a regulatory network with the orthologue molecules Pax and Dach, which are also implicated in leukemogenesis (46,47), as well as a critical partner, Six. Interestingly, one of the Eya family genes, Eya1, is highly expressed in mouse hematopoietic immature progenitor cells (48) and was recently reported to be a direct target gene of MLL-fusion protein as well as wild-type MLL (49). This is analogous to our findings of activation of Eya2 by PLZF-RARA and PLZF.…”

“…This is analogous to our findings of activation of Eya2 by PLZF-RARA and PLZF. However, the roles of Eya1 in MLL-fusion-mediated leukemogenesis and normal hematopoiesis remain unclear, although Eya1 was found to immortalize mouse HSPCs in colony replating assays (49). Our study of Eya2 has unveiled several important properties, in addition to the immortalization potential of HSPCs.…”

MicroRNA-30a regulates zebrafish myogenesis via targeting the transcription factor Six1

“…Interestingly, the expression of miR30a decreases with the progression of lymphoma, leukemia, lung, ovarian, breast and colon cancer (Cheng et al, 2012;González-Gugel et al, 2013;Guan et al, 2012;Lee et al, 2012;Liu et al, 2013;Ma et al, 2012;Võsa et al, 2013), all cancers in which SIX1 overexpression has been detected (Behbakht et al, 2007;Ford et al, 1998;Mimae et al, 2012;Ono et al, 2012;Wang et al, 2011). Importantly, increased SIX1 expression enhances progression of RMS and several additional tumor types by re-activating many of the same signaling networks that SIX1 turns on developmentally to promote cell survival, proliferation and migration (Christensen et al, 2008;Khan et al, 1999;Yu et al, 2006;Yu et al, 2004).…”

The trithorax protein partner menin acts in tandem with EZH2 to suppress C/EBP and differentiation in MLL-AF9 leukemia

“…25,26 In addition, global analysis has identified over 200 direct MLL fusion protein target genes, some of which could also have a role in blocking differentiation. 12 C/EBPα is a leucine zipper transcription factor that promotes myeloid differentiation in part through the activation of differ- Trithorax and polycomb group proteins antagonistically regulate the transcription of many genes, and cancer can result from the disruption of this regulation. Deregulation of trithorax function occurs through chromosomal translocations involving the trithorax gene MLL, leading to the expression of MLL fusion proteins and acute leukemia.…”

“…6,7 MLL fusion proteins promote the expression of a subset of wild-type (WT) MLL target genes, including HOX genes, through recruitment of the histone H3K79 methyltransferase Dot1L and the pTEFb complex, enhancing transcriptional elongation. [8][9][10][11][12] MLL fusion proteins lack a large C-terminal portion of the WT MLL protein, including the histone H3 lysine 4 (H3K4)-methylating SET domain. This functional deficiency is remedied by expression of WT MLL from the non-translocated MLL allele.…”