EVI1 drives leukemogenesis through aberrant ERG activation.

Chromosomal rearrangements involving the MDS1 and EVI1 complex locus (MECOM) on chromosome 3q26 define an aggressive subtype of acute myeloid leukemia (AML) that is associated with chemotherapy resistance and dismal prognosis. Established treatment regimens commonly fail in these patients, therefore, there is an urgent need for new therapeutic concepts that will require a better understanding of the molecular and cellular functions of the ecotropic viral integration site 1 (EVI1) oncogene. To characterize gene regulatory functions of EVI1 and associated dependencies in AML, we developed experimentally tractable human and murine disease models, investigated the transcriptional consequences of EVI1 withdrawal in vitro and in vivo, and performed the first genome-wide CRISPR screens in EVI1-dependent AML. By integrating conserved transcriptional targets with genetic dependency data, we identified and characterized the ETS transcription factor ERG as a direct transcriptional target of EVI1 that is aberrantly expressed and selectively required in both human and murine EVI1-driven AML. EVI1 controls the expression of ERG and occupies a conserved intragenic enhancer region in AML cell lines and samples from patients with primary AML. Suppression of ERG induces terminal differentiation of EVI1-driven AML cells, whereas ectopic expression of ERG abrogates their dependence on EVI1, indicating that the major oncogenic functions of EVI1 are mediated through aberrant transcriptional activation of ERG. Interfering with this regulatory axis may provide entry points for the development of rational targeted therapies.

Multilayered VBC score predicts sgRNAs that efficiently generate loss-of-function alleles.

CRISPR-Cas9 screens have emerged as a transformative approach to systematically probe gene functions. The quality and success of these screens depends on the frequencies of loss-of-function alleles, particularly in negative-selection screens widely applied for probing essential genes. Using optimized screening workflows, we performed essentialome screens in cancer cell lines and embryonic stem cells and achieved dropout efficiencies that could not be explained by common frameshift frequencies. We find that these superior effect sizes are mainly determined by the impact of in-frame mutations on protein function, which can be predicted based on amino acid composition and conservation. We integrate protein features into a 'Bioscore' and fuse it with improved predictors of single-guide RNA activity and indel formation to establish a score that captures all relevant processes in CRISPR-Cas9 mutagenesis. This Vienna Bioactivity CRISPR score (www.vbc-score.org) outperforms previous prediction tools and enables the selection of sgRNAs that effectively produce loss-of-function alleles.

Rational discovery of molecular glue degraders via scalable chemical profiling.

Targeted protein degradation is a new therapeutic modality based on drugs that destabilize proteins by inducing their proximity to E3 ubiquitin ligases. Of particular interest are molecular glues that can degrade otherwise unligandable proteins by orchestrating direct interactions between target and ligase. However, their discovery has so far been serendipitous, thus hampering broad translational efforts. Here, we describe a scalable strategy toward glue degrader discovery that is based on chemical screening in hyponeddylated cells coupled to a multi-omics target deconvolution campaign. This approach led us to identify compounds that induce ubiquitination and degradation of cyclin K by prompting an interaction of CDK12-cyclin K with a CRL4B ligase complex. Notably, this interaction is independent of a dedicated substrate receptor, thus functionally segregating this mechanism from all described degraders. Collectively, our data outline a versatile and broadly applicable strategy to identify degraders with nonobvious mechanisms and thus empower future drug discovery efforts.

The histone chaperone CAF-1 safeguards somatic cell identity.

Cellular differentiation involves profound remodelling of chromatic landscapes, yet the mechanisms by which somatic cell identity is subsequently maintained remain incompletely understood. To further elucidate regulatory pathways that safeguard the somatic state, we performed two comprehensive RNA interference (RNAi) screens targeting chromatin factors during transcription-factor-mediated reprogramming of mouse fibroblasts to induced pluripotent stem cells (iPS cells). Subunits of the chromatin assembly factor-1 (CAF-1) complex, including Chaf1a and Chaf1b, emerged as the most prominent hits from both screens, followed by modulators of lysine sumoylation and heterochromatin maintenance. Optimal modulation of both CAF-1 and transcription factor levels increased reprogramming efficiency by several orders of magnitude and facilitated iPS cell formation in as little as 4 days. Mechanistically, CAF-1 suppression led to a more accessible chromatin structure at enhancer elements early during reprogramming. These changes were accompanied by a decrease in somatic heterochromatin domains, increased binding of Sox2 to pluripotency-specific targets and activation of associated genes. Notably, suppression of CAF-1 also enhanced the direct conversion of B cells into macrophages and fibroblasts into neurons. Together, our findings reveal the histone chaperone CAF-1 to be a novel regulator of somatic cell identity during transcription-factor-induced cell-fate transitions and provide a potential strategy to modulate cellular plasticity in a regenerative setting.

Systematic genetic analysis of muscle morphogenesis and function in Drosophila.

Systematic genetic approaches have provided deep insight into the molecular and cellular mechanisms that operate in simple unicellular organisms. For multicellular organisms, however, the pleiotropy of gene function has largely restricted such approaches to the study of early embryogenesis. With the availability of genome-wide transgenic RNA interference (RNAi) libraries in Drosophila, it is now possible to perform a systematic genetic dissection of any cell or tissue type at any stage of the lifespan. Here we apply these methods to define the genetic basis for formation and function of the Drosophila muscle. We identify a role in muscle for 2,785 genes, many of which we assign to specific functions in the organization of muscles, myofibrils or sarcomeres. Many of these genes are phylogenetically conserved, including genes implicated in mammalian sarcomere organization and human muscle diseases.

Alkylamine-tethered molecules recruit FBXO22 for targeted protein degradation.

Targeted protein degradation (TPD) relies on small molecules to recruit proteins to E3 ligases to induce their ubiquitylation and degradation by the proteasome. Only a few of the approximately 600 human E3 ligases are currently amenable to this strategy. This limits the actionable target space and clinical opportunities and thus establishes the necessity to expand to additional ligases. Here we identify and characterize SP3N, a specific degrader of the prolyl isomerase FKBP12. SP3N features a minimal design, where a known FKBP12 ligand is appended with a flexible alkylamine tail that conveys degradation properties. We found that SP3N is a precursor and that the alkylamine is metabolized to an active aldehyde species that recruits the SCFFBXO22 ligase for FKBP12 degradation. Target engagement occurs via covalent adduction of Cys326 in the FBXO22 C-terminal domain, which is critical for ternary complex formation, ubiquitylation and degradation. This mechanism is conserved for two recently reported alkylamine-based degraders of NSD2 and XIAP, thus establishing alkylamine tethering and covalent hijacking of FBXO22 as a generalizable TPD strategy.

A genome-wide transgenic RNAi library for conditional gene inactivation in Drosophila.

Forward genetic screens in model organisms have provided important insights into numerous aspects of development, physiology and pathology. With the availability of complete genome sequences and the introduction of RNA-mediated gene interference (RNAi), systematic reverse genetic screens are now also possible. Until now, such genome-wide RNAi screens have mostly been restricted to cultured cells and ubiquitous gene inactivation in Caenorhabditis elegans. This powerful approach has not yet been applied in a tissue-specific manner. Here we report the generation and validation of a genome-wide library of Drosophila melanogaster RNAi transgenes, enabling the conditional inactivation of gene function in specific tissues of the intact organism. Our RNAi transgenes consist of short gene fragments cloned as inverted repeats and expressed using the binary GAL4/UAS system. We generated 22,270 transgenic lines, covering 88% of the predicted protein-coding genes in the Drosophila genome. Molecular and phenotypic assays indicate that the majority of these transgenes are functional. Our transgenic RNAi library thus opens up the prospect of systematically analysing gene functions in any tissue and at any stage of the Drosophila lifespan.

Precision RNAi using synthetic shRNAmir target sites.

Loss-of-function genetic tools are widely applied for validating therapeutic targets, but their utility remains limited by incomplete on- and uncontrolled off-target effects. We describe artificial RNA interference (ARTi) based on synthetic, ultra-potent, off-target-free shRNAs that enable efficient and inducible suppression of any gene upon introduction of a synthetic target sequence into non-coding transcript regions. ARTi establishes a scalable loss-of-function tool with full control over on- and off-target effects.

Comprehensive CRISPR-Cas9 screen identifies factors which are important for plasmablast development.

Plasma cells (PCs) and their progenitors plasmablasts (PBs) are essential for the acute and long-term protection of the host against infections by providing vast levels of highly specific antibodies. Several transcription factors, like Blimp1 and Irf4, are already known to be essential for PC and PB differentiation and survival. We set out to identify additional genes, that are essential for PB development by CRISPR-Cas9 screening of 3,000 genes for the loss of PBs by employing the in vitro-inducible germinal center B cell (iGB) culture system and Rosa26Cas9/+ mice. Identified hits in the screen were Mau2 and Nipbl, which are known to contribute to the loop extrusion function of the cohesin complex. Other examples of promising hits were Taf6, Stat3, Ppp6c and Pgs1. We thus provide a new set of genes, which are important for PB development.

High-resolution, high-throughput SNP mapping in Drosophila melanogaster.

Single nucleotide polymorphisms (SNPs) are useful markers for genetic mapping experiments in model organisms. Here we report the establishment of a high-density SNP map and high-throughput genotyping assays for Drosophila melanogaster. Our map comprises 27,367 SNPs in common laboratory Drosophila stocks. These SNPs were clustered within 2,238 amplifiable markers at an average density of 1 marker every 50.3 kb, or 6.3 genes. We have also constructed a set of 62 Drosophila stocks, each of which facilitates the generation of recombinants within a defined genetic interval of 1-2 Mb. For flexible, high-throughput SNP genotyping, we used fluorescent tag-array mini-sequencing (TAMS) assays. We designed and validated TAMS assays for 293 SNPs at an average resolution of 391.3 kb, and demonstrated the utility of these tools by rapidly mapping 14 mutations that disrupt embryonic muscle patterning. These resources enable high-resolution high-throughput genetic mapping in Drosophila.

FLYSNPdb: a high-density SNP database of Drosophila melanogaster.

FLYSNPdb provides high-resolution single nucleotide polymorphism (SNP) data of Drosophila melanogaster. The database currently contains 27,367 polymorphisms, including >3700 indels (insertions/deletions), covering all major chromsomes. These SNPs are clustered into 2238 markers, which are evenly distributed with an average density of one marker every 50.3 kb or 6.6 genes. SNPs were identified automatically, filtered for high quality and partly manually curated. The database provides detailed information on the SNP data including molecular and cytological locations (genome Releases 3-5), alleles of up to five commonly used laboratory stocks, flanking sequences, SNP marker amplification primers, quality scores and genotyping assays. Data specific for a certain region, particular stocks or a certain genome assembly version are easily retrievable through the interface of a publicly accessible website (http://flysnp.imp.ac.at/flysnpdb.php).

Isolating live cell clones from barcoded populations using CRISPRa-inducible reporters.

We developed a functional lineage tracing tool termed CaTCH (CRISPRa tracing of clones in heterogeneous cell populations). CaTCH combines precise clonal tracing of millions of cells with the ability to retrospectively isolate founding clones alive before and during selection, allowing functional experiments. Using CaTCH, we captured rare clones representing as little as 0.001% of a population and investigated the emergence of resistance to targeted melanoma therapy in vivo.

Inhibition of CBP synergizes with the RNA-dependent mechanisms of Azacitidine by limiting protein synthesis.

The nucleotide analogue azacitidine (AZA) is currently the best treatment option for patients with high-risk myelodysplastic syndromes (MDS). However, only half of treated patients respond and of these almost all eventually relapse. New treatment options are urgently needed to improve the clinical management of these patients. Here, we perform a loss-of-function shRNA screen and identify the histone acetyl transferase and transcriptional co-activator, CREB binding protein (CBP), as a major regulator of AZA sensitivity. Compounds inhibiting the activity of CBP and the closely related p300 synergistically reduce viability of MDS-derived AML cell lines when combined with AZA. Importantly, this effect is specific for the RNA-dependent functions of AZA and not observed with the related compound decitabine that is only incorporated into DNA. The identification of immediate target genes leads us to the unexpected finding that the effect of CBP/p300 inhibition is mediated by globally down regulating protein synthesis.

Selective Requirement of MYB for Oncogenic Hyperactivation of a Translocated Enhancer in Leukemia.

In acute myeloid leukemia (AML) with inv(3)(q21;q26) or t(3;3)(q21;q26), a translocated GATA2 enhancer drives oncogenic expression of EVI1. We generated an EVI1-GFP AML model and applied an unbiased CRISPR/Cas9 enhancer scan to uncover sequence motifs essential for EVI1 transcription. Using this approach, we pinpointed a single regulatory element in the translocated GATA2 enhancer that is critically required for aberrant EVI1 expression. This element contained a DNA-binding motif for the transcription factor MYB, which specifically occupied this site at the translocated allele and was dispensable for GATA2 expression. MYB knockout as well as peptidomimetic blockade of CBP/p300-dependent MYB functions resulted in downregulation of EVI1 but not of GATA2. Targeting MYB or mutating its DNA-binding motif within the GATA2 enhancer resulted in myeloid differentiation and cell death, suggesting that interference with MYB-driven EVI1 transcription provides a potential entry point for therapy of inv(3)/t(3;3) AMLs. SIGNIFICANCE: We show a novel paradigm in which chromosomal aberrations reveal critical regulatory elements that are nonfunctional at their endogenous locus. This knowledge provides a rationale to develop new compounds to selectively interfere with oncogenic enhancer activity.This article is highlighted in the In This Issue feature, p. 2659.