Project description:In the recent years, massively parallel sequencing approaches identified hundreds of mutated genes in cancer providing an unprecedented amount of information about mechanisms of cancer cell maintenance and progression. However, while (it is widely accepted that) transformation processes result from oncogenic cooperation between deregulated genes and pathways, the functional characterization of candidate key players is mostly performed at the single gene level which is generally inadequate to identify these oncogene circuitries. In addition, studies aimed at depicting oncogenic cooperation involve the generation of challenging mouse models or the deployment of tedious screening pipelines. Genome wide mapping of epigenomic modifications on histone tails or binding of factors such as MED1 and BRD4 allowed identification of clusters of regulatory elements, also termed Super-Enhancers (SE). Functional annotation of these regions revealed their high relevance during normal tissue development and cancer ontogeny. An interesting paradigm of the tumorigenic function of these SE regions comes from ETO2-GLIS2-driven acute megakaryoblastic leukemia (AMKL) in which the fusion protein ETO2-GLIS2 is sufficient to promote an aberrant transcriptional network by the rewiring of SE regions4. We thus hypothesized that important regulatory regions could control simultaneously expression of genes cooperating in functional modules to promote cancer development. In an effort to identify such modules, we deployed a genome-wide CRISPRi-based screening approach and nominated SE regions that are functionally linked to leukemia maintenance. In particular, we pinpointed a novel SE region regulating the expression of both tyrosine kinases KIT and PDGFRA. Whereas the inhibition of each kinase alone affected modestly cancer cell growth, combined inhibition of both receptors synergizes to impair leukemia cell growth and survival. Our results demonstrate that genome-wide screening of regulatory DNA elements can identify co-regulated genes collaborating to promote cancer and could open new avenues to the concept of combined gene inhibition upon single hit targeting.

Project description:The ETO-family transcriptional corepressors, including ETO, ETO2 and MTGR1, are all involved in leukemia-causing chromosomal translocations. In every case, an ETO-family corepressor acquires a DNA-binding domain (DBD) to form a typical transcription factor – the DBD binds to target genes, while the ETO moiety contributes essentially to its transcriptional property. A direct comparative study of these “homologous” fusion transcription factors may clarify their similarities and differences in regulating transcription and leukemogenesis. Here, we performed a side-by-side comparison between AML1-ETO and ETO2-GLIS2, the most common fusion proteins in the M2 and M7 subtypes of acute myeloid leukemia, respectively, by inducible expression of them in U937 leukemia cells. We found that, although AML1-ETO and ETO2-GLIS2 can use their own DBDs (i.e., the Runt domain of AML1 and the zinc finger domain of GLIS2) to bind DNA, they actually share a large proportion of genome-wide binding regions dependent on other cooperative transcription factors such as ETS- and CEBP-family proteins. Functionally, AML1-ETO acts as either transcriptional repressor or activator, whereas ETO2-GLIS2 mainly acts as an activator. The repressor-versus-activator functions of AML1-ETO is determined by the abundance of cooperative transcription factors/cofactors on the target genes. Through these mechanisms, AML1-ETO and ETO2-GLIS2 differentially regulate several key transcription factors that are essential for myeloid differentiation. Indeed, AML1-ETO inhibits myeloid differentiation of U937 cells, whereas ETO2-GLIS2 facilitates it. Taken together, this study is the first direct comparative study between AML1-ETO and ETO2-GLIS2 in the same cellular context, and the results provide new insights into the context-dependent transcriptional regulatory mechanisms that may underlie how these seemingly “homologous” fusion transcription factors exert distinct functions to drive different subtypes of leukemia.

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Transcriptome analysis of M07e cells and patient derived AMKL cells upon SEKIT inhibition by CRISPRi

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