VIEWS IN THE SPOTLIGHT MEF2D Fusions Drive Oncogenic Pre-BCR Signaling in B-ALL Teresa Sadras and Markus Müschen Summary: Chimeric fusion proteins involving transcriptional regulators are a common feature in pre-B acute lymphoblastic leukemia (B-ALL). However, systematic dissection of the core regulatory circuits by which these fusions exert their oncogenic effects is still required. Using chromatin immunoprecipitation sequencing and robust functional assays, Tsuzuki and colleagues identify the core transcription factor network directed by MEF2D fusions in B-ALL. The new findings demonstrate how activation of MEF2D fusions ultimately converge on pre-BCR signaling and lipid metabolism to drive malignant B-cell transformation. See related article by Tsuzuki et al., p. 82 (5). B acute lymphoblastic leukemia (B-ALL) is a heterogenous fusions may be further enhanced by translocation-mediated disease characterized by recurrent chromosomal alterations loss of miRNA sites that negatively regulate MEF2D expres- that result in chimeric fusions commonly involving molecules sion (3). Although large-scale gene expression studies indicate implicated in tumor suppression, kinase signaling, chroma- that MEF2D-rearranged B-ALL represents a distinct subtype tin remodeling, and B-cell differentiation. Fusions involving of B-ALL, several groups have observed a strong overlap with the transcription factor MEF2D are found in approximately patients harboring the TCF3–PBX1 fusion (1, 4); however, the 5% of B-ALL cases and are associated with poor outcome (1). significance of this was not clear. MEF2D belongs to the myocyte enhancer factor (MEF2) To unravel the genome-wide occupancy of MEF2D family of transcription factors (TF) which includes 4 para- fusions in B-ALL, Tsuzuki and colleagues (5) utilized logs in vertebrates (MEF2A–D). MEF2 proteins, originally CRISPR editing to insert a HA-tag in-frame to the endog- identified as critical players in muscle and neuronal differ- enously expressed MEF2D–HNRNPUL1 fusion in Kasumi-7 entiation, are pleiotropic gene regulators characterized by B-ALL cells. This allowed them to perform fusion-specific highly conserved N-terminal dimerization and DNA-binding chromatin immunoprecipitation sequencing (ChIP-seq) domains. In a tissue- and context-dependent fashion, MEF2 that uncovered 7,077 unique MEF2D–HNRNPUL1 bind- TFs act in multimeric complexes with transcriptional coacti- ing sites in proximity (<3 kb) of recognized transcription vators/repressors and epigenetic modifiers such as EP300 and start sites (TSS) and defined super-enhancer (SE) elements. HDACs to alter target gene expression. Highlighting a role for Strikingly, MEF2D–HNRNPUL1 binding peaks were highly MEF2 in normal B-lymphopoiesis, conditional deletion of enriched for genes associated with pre–B-cell receptor (pre- Mef2c/d in B-cells results in a block at the pre–B-cell stage by BCR) signaling, including PI3KCD, IGLL1, VPREB1, CD79A, impeding activation of normal transcriptional differentiation and BCL6 (6, 7). Deletion of these pre-BCR components was programs (2). toxic to MEF2D-rearranged B-ALL cells consistent with a To date, the MEF2D fusions identified in B-ALL invariably functional dependence on this pathway. fuse the N-terminus of MEF2D with a range of 3′ partners, Assembly of the pre-BCR marks the first important check- the most common of which are BCL9 and HNRNPUL1 (1). point of B-cell development. Following productive rearrange- Although a gene-specific function for the ′3 partners can- ment of immunoglobulin (Ig) V, D, and J gene segments, not be excluded, the extreme similarity in gene expression expression of a functional immunoglobulin μ-heavy chain observed in patients with different MEF2D fusions suggests paired with the surrogate light chain (SLC) initiates signal- that the central malignant phenotype is driven by the MEF2D ing via SYK, PI3K, and MAPK to trigger clonal expansion of TF itself. Studies that examined protein expression of the pre-B cells. Importantly, the authors demonstrate unmistak- MEF2D fusions found these were more active and expressed able expression of surface pre-BCR components in MEF2D- at significantly higher levels than nonrearrangedMEF2D , and rearranged B-ALL samples, which was dependent on intact it is thought that the 3′ partners act largely to stabilize the MEF2D–HNRNPUL1-binding sites around the TSS of these MEF2D-containing fusion (1, 3). The stability of the MEF2D elements. Further in line with positive regulation of the pre- BCR itself, enforced expression of MEF2D–BCL9 in murine pro-B cell progenitors gave rise to pre–BCR expressing clones Department of Systems Biology, City of Hope Comprehensive Cancer in vivo, which ultimately developed into aggressive pre–B-cell Center, Monrovia, California. leukemias. Corresponding Author: Markus Müschen, City of Hope Comprehensive As MEF2 TFs largely function by forming multicomponent Cancer Center, 1218 South Fifth Avenue, Monrovia, CA 91016. Phone: regulatory complexes, Tsuzuki and colleagues hypothesized 626-218-5171; Fax: 626-218-5172; E-mail: [email protected] that MEF2D fusions may integrate into a core autoregula- Blood Cancer Discov 2020;1:18–20 tory circuit with B-cell transcription factors. Through detailed doi: 10.1158/2643-3249.BCD-20-0078 functional and mapping experiments, they were able to nar- ©2020 American Association for Cancer Research. row down a “core regulatory circuit” (CRC) of four key TFs 18 | BLOOD CANCER DISCOVERY JULY 2020 AACRJournals.org Downloaded from https://bloodcancerdiscov.aacrjournals.org by guest on October 2, 2021. Copyright 2020 American Association for Cancer Research. VIEWS DLBCL, the mutations in MEF2B were found to enhance its transcriptional activity and were directly linked to increased Pre-BCR expression of BCL6 (10). In this way, it is possible that genetic CD79A aberrations affecting MEF2 TFs may result in common tran- SRC SYK MAPK scriptional programs in early (pre-B) and mature (GC-derived) B-cells to invoke expression of factors required for (pre-) BCR signaling and promote malignant B-cell survival. MEF2D fusion CREB The development of targeted inhibitors in B-ALL has shown remarkable scope for fusions involving tyrosine kinases (e.g., dasatinib for ABL1 and PDGFR fusions; ruxolitinib for JAK2 EGR1 lesions), but has proven more complex for oncogenic fusions FOS SREBF1 Pre-BCR genes MEF2D fusion BCL6 involving TFs. However, consistent with activation of pre- BCR signaling in TCF3–PBX1 B-ALL, a screen of 51 kinase Autoregulation inhibitors currently in clinical use showed these samples were selectively sensitive to inhibitors of the pre-BCR pathway, including inhibitors targeting the pre-BCR proximal kinases Figure 1. Feed-forward CRC directed by MEF2D fusions in B-ALL. SYK, SRC, and BTK (7). In line with the new classification of MEF2D-rearranged B-ALL as pre-BCR+, this study found that, compared with pre-BCR− cells, MEF2D-rearranged B-ALL cell acting in concert with the MEF2D fusion, namely SREBF1, lines and patient-derived B-ALL cells were also significantly FOS, EGR1, and BCL6. In addition to occupying genomic more sensitive to inhibition of SYK and SRC kinases. To fur- regions near TSSs of genes involved in pre-BCR signaling, co- ther determine whether destabilization of the MEF2D fusion occupancy of the MEF2D fusion with the four CRC TFs was at its core could serve as an additional targetable node, the confirmed near the TSSs of their respective gene loci, indicat- authors focused on one CRC TF, SREBF1. SREBF1 activ- ing positive self-feedback regulation. Previously, tonic pre-BCR ity depends on cellular lipid levels and hence is susceptible signaling was shown to enhance the transcriptional activity of to drug inhibition. SREBF1 is normally anchored to the MEF2C in normal pre-B cells, through ERK-mediated phos- endoplasmic reticulum membrane in the shape of a precur- phorylation (2). Similarly, in this study, activation of the cAMP sor protein, and, in response to low levels of lipids such as response element-binding protein (CREB) downstream of cholesterol and fatty acids, translocates to the Golgi body ERK kinases was shown to further promote expression of the where it is cleaved to release a mature functional TF. Remark- MEF2D–HNRNPUL1 fusion in Kasumi-7 cells. Together, these ably, treatment of MEF2D ALL cells with SREBF1 inhibitors results suggest that MEF2D fusions form part of a coherent such as fatostatin and FGH10019 reduced the amount of feed-forward loop involving a CRC of B-cell TFs and the pre- cleaved SREBF1, MEF2D fusion expression, and also expres- BCR signaling axis to drive the growth of B-ALL cells (Fig. 1). sion of the remaining CRC TFs (BCL6, FOS, and EGR1) Although B-cell precursors depend on survival signals with subsequent induction of cell death, reflecting the tight from a functional pre-BCR, in the majority of B-ALL cases intradependence of the CRC TFs required for maintaining these signals are delivered by oncogenic mimics of this path- survival of MEF2D-rearranged B-ALL cells. SREBF1 has not way (e.g., BCR–ABL1; ref. 8). Active pre-BCR activity is found been implicated in the phenotype of TCF3–PBX1 ALL, and in only approximately 13.5% B-ALLs (defined as pre-BCR+ accordingly SREBF1 inhibitors were not effective on these B-ALL), including cases with the TCF3–PBX1 fusion (7). In cells, suggesting that unlike targeting of the pre-BCR itself, a similar way to MEF2D-rearranged B-ALL, TCF3–PBX1 inhibition of SREBF1 is a unique vulnerability of MEF2D- B-ALLs have increased