Selected Publications
Vorselen D*, Kamber RA*, Labitigan RLD, van Loon AP, Peterman E, Delgado MK, Lin S, Rasmussen JP, Bassik MC**, Theriot JA**.
in review, bioRxiv
Macrophages phagocytose and thereby eliminate a wide array of extracellular threats, ranging from antibody-coated bacteria to apoptotic cells. Precision modulation of phagocytosis has emerged as a therapeutic strategy across a range of diseases, but is limited by our incomplete understanding of how macrophages recognize, engulf, and respond to different phagocytic targets. Here, we undertook a systematic investigation of the morphological, biophysical and regulatory differences between two major types of phagocytosis: an immunostimulatory form of phagocytosis triggered by antibody-coated targets and an immunosuppressive form triggered by phosphatidylserine (PS)-coated targets. We confirmed classic observations that antibody-mediated phagocytosis involves the extension of thin actin-rich protrusions around the target, but find that PS-mediated phagocytosis involves an unexpected combination of filopodial probing, piecemeal phagocytosis and a distinct ‘sinking’ mechanism of uptake. Using a genome-wide screening approach, we identified genes specifically required for each form of phagocytosis, including actin regulators, cell surface receptors and intracellular signaling molecules. Three cell surface receptors - TREM2, CD14 and integrin αMβ2 - were revealed as essential for PS-mediated uptake. Strikingly, each receptor exhibited a distinct pattern of localization at the plasma membrane and contributed uniquely to the organization of the PS-dependent phagocytic cup. Overall, this work reveals divergent genetic requirements for the morphologically and mechanically distinct forms of PS-mediated and antibody-mediated phagocytosis, thereby informing therapeutic strategies for substrate-specific phagocytosis modulation.
Tycko J*, Van MV*, Aradhana, DelRosso N, Yao D, Xu X, Ludwig C, Spees K, Liu K, Hess GT, Gu M, Mukund AX, Suzuki PH, Kamber RA, Qi LS, Bintu L**, Bassik MC**
In review, bioRxiv
Human nuclear proteins contain >1000 transcriptional effector domains that can activate or repress transcription of target genes. We lack a systematic understanding of which effector domains regulate transcription robustly across genomic, cell-type, and DNA-binding domain (DBD) contexts. Here, we developed dCas9-mediated high-throughput recruitment (HT-recruit), a pooled screening method for quantifying effector function at endogenous targets, and tested effector function for a library containing 5092 nuclear protein Pfam domains across varied contexts. We find many effectors depend on target and DBD contexts, such as HLH domains that can act as either activators or repressors. We then confirm these findings and further map context dependencies of effectors drawn from unannotated protein regions using a larger library containing 114,288 sequences tiling chromatin regulators and transcription factors. To enable efficient perturbations, we select effectors that are potent in diverse contexts, and engineer (1) improved ZNF705 KRAB CRISPRi tools to silence promoters and enhancers, and (2) a compact human activator combination NFZ for better CRISPRa and inducible circuit delivery. Together, this effector-by-context functional map reveals context-dependence across human effectors and guides effector selection for robustly manipulating transcription.
SLC12A9 is a lysosome-detoxifying ammonium – chloride co-transporter
Levin-Konigsberg R, Mitra K, Nigam A, Spees K, Hivare P, Liu K, Kundaje A, Krishnan Y, Bassik MC
In review, bioRxiv
Ammonia is a ubiquitous, toxic by-product of cell metabolism. Its high membrane permeability and proton affinity causes ammonia to accumulate inside acidic lysosomes in its poorly membrane-permeant form: ammonium (NH4+). Ammonium buildup compromises lysosomal function, suggesting the existence of mechanisms that protect cells from ammonium toxicity. Here, we identified SLC12A9 as a lysosomal ammonium exporter that preserves lysosomal homeostasis. SLC12A9 knockout cells showed grossly enlarged lysosomes and elevated ammonium content. These phenotypes were reversed upon removal of the metabolic source of ammonium or dissipation of the lysosomal pH gradient. Lysosomal chloride increased in SLC12A9 knockout cells and chloride binding by SLC12A9 was required for ammonium transport. Our data indicate that SLC12A9 is a chloride-driven ammonium co-transporter that is central in an unappreciated, fundamental mechanism of lysosomal physiology that may have special relevance in tissues with elevated ammonia, such as tumors.
Durrant MG*, Fanton A*, Tycko J*, Hinks M, Chandrasekaran S, Perry NT, Schaepe J, Du PP, Bassik MC**, Bintu L**, Bhatt AS**, Hsu PD**.
Nat Biotechnol. 2022 Oct 10. doi: 10.1038/s41587-022-01494-w.
Large serine recombinases (LSRs) are DNA integrases that facilitate the site-specific integration of mobile genetic elements into bacterial genomes. Only a few LSRs, such as Bxb1 and PhiC31, have been characterized to date, with limited efficiency as tools for DNA integration in human cells. In this study, we developed a computational approach to identify thousands of LSRs and their DNA attachment sites, expanding known LSR diversity by >100-fold and enabling the prediction of their insertion site specificities. We tested their recombination activity in human cells, classifying them as landing pad, genome-targeting or multi-targeting LSRs. Overall, we achieved up to seven-fold higher recombination than Bxb1 and genome integration efficiencies of 40-75% with cargo sizes over 7 kb. We also demonstrate virus-free, direct integration of plasmid or amplicon libraries for improved functional genomics applications. This systematic discovery of recombinases directly from microbial sequencing data provides a resource of over 60 LSRs experimentally characterized in human cells for large-payload genome insertion without exposed DNA double-stranded breaks.
Inter-cellular CRISPR screens reveal regulators of cancer cell phagocytosis.
Kamber RA, Nishiga Y, Morton B, Banuelos AM, Barkal AA, Vences-Catalán F, Gu M, Fernandez D, Seoane JA, Yao D, Liu K, Lin S, Spees K, Curtis C, Jerby-Arnon L, Weissman IL, Sage J, Bassik MC.
Nature. 2021 Sep;597(7877):549-554. doi: 10.1038/s41586-021-03879-4.
Monoclonal antibody therapies targeting tumour antigens drive cancer cell elimination in large part by triggering macrophage phagocytosis of cancer cells. However, cancer cells evade phagocytosis using mechanisms that are incompletely understood. Here we develop a platform for unbiased identification of factors that impede antibody-dependent cellular phagocytosis (ADCP) using complementary genome-wide CRISPR knockout and overexpression screens in both cancer cells and macrophages. In cancer cells, beyond known factors such as CD47, we identify many regulators of susceptibility to ADCP, including the poorly characterized enzyme adipocyte plasma membrane-associated protein (APMAP). We find that loss of APMAP synergizes with tumour antigen-targeting monoclonal antibodies and/or CD47-blocking monoclonal antibodies to drive markedly increased phagocytosis across a wide range of cancer cell types, including those that are otherwise resistant to ADCP. Additionally, we show that APMAP loss synergizes with several different tumour-targeting monoclonal antibodies to inhibit tumour growth in mice. Using genome-wide counterscreens in macrophages, we find that the G-protein-coupled receptor GPR84 mediates enhanced phagocytosis of APMAP-deficient cancer cells. This work reveals a cancer-intrinsic regulator of susceptibility to antibody-driven phagocytosis and, more broadly, expands our knowledge of the mechanisms governing cancer resistance to macrophage phagocytosis.
Coukos R*, Yao D*, Sanchez MI, Strand ET, Olive ME, Udeshi ND, Weissman JS, Carr SA, Bassik MC**, Ting AY**.
Elife. 2021 Aug 20;10:e69142. doi: 10.7554/eLife.69142.
The trafficking of specific protein cohorts to correct subcellular locations at correct times is essential for every signaling and regulatory process in biology. Gene perturbation screens could provide a powerful approach to probe the molecular mechanisms of protein trafficking, but only if protein localization or mislocalization can be tied to a simple and robust phenotype for cell selection, such as cell proliferation or fluorescence-activated cell sorting (FACS). To empower the study of protein trafficking processes with gene perturbation, we developed a genetically encoded molecular tool named HiLITR (High-throughput Localization Indicator with Transcriptional Readout). HiLITR converts protein colocalization into proteolytic release of a membrane-anchored transcription factor, which drives the expression of a chosen reporter gene. Using HiLITR in combination with FACS-based CRISPRi screening in human cell lines, we identified genes that influence the trafficking of mitochondrial and ER tail-anchored proteins. We show that loss of the SUMO E1 component SAE1 results in mislocalization and destabilization of many mitochondrial tail-anchored proteins. We also demonstrate a distinct regulatory role for EMC10 in the ER membrane complex, opposing the transmembrane-domain insertion activity of the complex. Through transcriptional integration of complex cellular functions, HiLITR expands the scope of biological processes that can be studied by genetic perturbation screening technologies.
A genome-wide almanac of co-essential modules assigns function to uncharacterized genes.
Wainberg M*, Kamber RA*, Balsubramani A*, Meyers RM, Sinnott-Armstrong N, Hornburg D, Jiang L, Chan J, Jian R, Gu M, Shcherbina A, Dubreuil MM, Spees K, Snyder MP, Bassik MC**, Kundaje A**.
Nat Genet. 2021 May;53(5):638-649. doi: 10.1038/s41588-021-00840-z.
A central question in the post-genomic era is how genes interact to form biological pathways. Measurements of gene dependency across hundreds of cell lines have been used to cluster genes into 'co-essential' pathways, but this approach has been limited by ubiquitous false positives. In the present study, we develop a statistical method that enables robust identification of gene co-essentiality and yields a genome-wide set of functional modules. This atlas recapitulates diverse pathways and protein complexes, and predicts the functions of 108 uncharacterized genes. Validating top predictions, we show that TMEM189 encodes plasmanylethanolamine desaturase, a key enzyme for plasmalogen synthesis. We also show that C15orf57 encodes a protein that binds the AP2 complex, localizes to clathrin-coated pits and enables efficient transferrin uptake. Finally, we provide an interactive webtool for the community to explore our results, which establish co-essentiality profiling as a powerful resource for biological pathway identification and discovery of new gene functions.
High-throughput discovery and characterization of human transcriptional effectors.
Tycko J, DelRosso N, Hess GT, Aradhana, Banerjee A, Mukund A, Van MV, Ego BK, David Yao, Spees K, Suzuki P, Marinov GK, Kundaje A, Bassik MC**, Bintu L**.
Cell. 2020 Dec 9;S0092-8674(20)31541-5.
Thousands of proteins localize to the nucleus; however, it remains unclear which contain transcriptional effectors. Here, we develop HT-recruit, a pooled assay where protein libraries are recruited to a reporter, and their transcriptional effects are measured by sequencing. Using this approach, we measure gene silencing and activation for thousands of domains. We find a relationship between repressor function and evolutionary age for the KRAB domains, discover that Homeodomain repressor strength is collinear with Hox genetic organization, and identify activities for several domains of unknown function. Deep mutational scanning of the CRISPRi KRAB maps the co-repressor binding surface and identifies substitutions that improve stability/silencing. By tiling 238 proteins, we find repressors as short as ten amino acids. Finally, we report new activator domains, including a divergent KRAB. These results provide a resource of 600 human proteins containing effectors and demonstrate a scalable strategy for assigning functions to protein domains.
CRISPR screens in cancer spheroids identify 3D growth-specific vulnerabilities.
Han K, Pierce SE, Li A, Spees K, Anderson GR, Seoane JA, Lo YH, Dubreuil M, Olivas M, Kamber RA, Wainberg M, Kostyrko K, Kelly MR, Yousefi M, Simpkins SW, Yao D, Lee K, Kuo CJ, Jackson PK, Sweet-Cordero A, Kundaje A, Gentles AJ, Curtis C, Winslow MM, Bassik MC.
Nature. 2020 Apr;580(7801):136-141. doi: 10.1038/s41586-020-2099-x
Cancer genomics studies have identified thousands of putative cancer driver genes. Development of high-throughput and accurate models to define the functions of these genes is a major challenge. Here we devised a scalable cancer-spheroid model and performed genome-wide CRISPR screens in 2D monolayers and 3D lung-cancer spheroids. CRISPR phenotypes in 3D more accurately recapitulated those of in vivo tumours, and genes with differential sensitivities between 2D and 3D conditions were highly enriched for genes that are mutated in lung cancers. These analyses also revealed drivers that are essential for cancer growth in 3D and in vivo, but not in 2D. Notably, we found that carboxypeptidase D is responsible for removal of a C-terminal RKRR motif2 from the α-chain of the insulin-like growth factor 1 receptor that is critical for receptor activity. Carboxypeptidase D expression correlates with patient outcomes in patients with lung cancer, and loss of carboxypeptidase D reduced tumour growth. Our results reveal key differences between 2D and 3D cancer models, and establish a generalizable strategy for performing CRISPR screens in spheroids to reveal cancer vulnerabilities.
Dubreuil MM, Morgens DW, Okumoto K, Honsho M, Contrepois K, Lee-McMullen B, Traber G, Sood RS, Dixon SJ, Snyder MP, Fujiki Y, and Bassik MC.
Cell Rep. 2020 Feb 4;30(5):1417-1433.e7.
Reactive oxygen species (ROS) play critical roles in metabolism and disease, yet a comprehensive analysis of the cellular response to oxidative stress is lacking. To systematically identify regulators of oxidative stress, we conducted genome-wide Cas9/CRISPR and shRNA screens. This revealed a detailed picture of diverse pathways that control oxidative stress response, ranging from the TCA cycle and DNA repair machineries to iron transport, trafficking, and metabolism. Paradoxically, disrupting the pentose phosphate pathway (PPP) at the level of phosphogluconate dehydrogenase (PGD) protects cells against ROS. This dramatically alters metabolites in the PPP, consistent with rewiring of upper glycolysis to promote antioxidant production. In addition, disruption of peroxisomal import unexpectedly increases resistance to oxidative stress by altering the localization of catalase. Together, these studies provide insights into the roles of peroxisomal matrix import and the PPP in redox biology and represent a rich resource for understanding the cellular response to oxidative stress.
Morgens DW*, Chan C*, Kane AJ, Weir NR, Li A, Tsui CK, Hess GT, Lavertu A, Han K, Polyakov N, Handy EL, Alabi P, Dombroski A, Yao D, Altman RB, Sello JK**, Denic V**, Bassik MC**.
Elife. 2019 Nov 1;8:e48434. doi: 10.7554/eLife.48434.
The small molecule Retro-2 prevents ricin toxicity through a poorly-defined mechanism of action (MOA), which involves halting retrograde vesicle transport to the endoplasmic reticulum (ER). CRISPRi genetic interaction analysis revealed Retro-2 activity resembles disruption of the transmembrane domain recognition complex (TRC) pathway, which mediates post-translational ER-targeting and insertion of tail-anchored (TA) proteins, including SNAREs required for retrograde transport. Cell-based and in vitro assays show that Retro-2 blocks delivery of newly-synthesized TA-proteins to the ER-targeting factor ASNA1 (TRC40). An ASNA1 point mutant identified using CRISPR-mediated mutagenesis abolishes both the cytoprotective effect of Retro-2 against ricin and its inhibitory effect on ASNA1-mediated ER-targeting. Together, our work explains how Retro-2 prevents retrograde trafficking of toxins by inhibiting TA-protein targeting, describes a general CRISPR strategy for predicting the MOA of small molecules, and paves the way for drugging the TRC pathway to treat broad classes of viruses known to be inhibited by Retro-2.
Jeng EE, Bhadkamkar V, Ibe NU, Gause H, Jiang L, Chan J, Jian R, Jimenez-Morales D, Stevenson E, Krogan NJ, Swaney DL, Snyder MP, Mukherjee S*, Bassik MC*.
Cell Host Microbe. 2019 Oct 9;26(4):551-563.e6. doi: 10.1016/j.chom.2019.08.017.
During infection, Legionella pneumophila translocates over 300 effector proteins into the host cytosol, allowing the pathogen to establish an endoplasmic reticulum (ER)-like Legionella-containing vacuole (LCV) that supports bacterial replication. Here, we perform a genome-wide CRISPR-Cas9 screen and secondary targeted screens in U937 human monocyte/macrophage-like cells to systematically identify host factors that regulate killing by L. pneumophila. The screens reveal known host factors hijacked by L. pneumophila, as well as genes spanning diverse trafficking and signaling pathways previously not linked to L. pneumophila pathogenesis. We further characterize C1orf43 and KIAA1109 as regulators of phagocytosis and show that RAB10 and its chaperone RABIF are required for optimal L. pneumophila replication and ER recruitment to the LCV. Finally, we show that Rab10 protein is recruited to the LCV and ubiquitinated by the effectors SidC/SdcA. Collectively, our results provide a wealth of previously undescribed insights into L. pneumophila pathogenesis and mammalian cell function.
Mitigation of Off-Target Toxicity in CRISPR-Cas9 Screens for Essential Non-Coding Elements.
Tycko J*, Wainberg M*, Marinov GK*, Ursu O, Hess GT, Ego BK, Aradhana, Li A, Truong A, Trevino AE, Spees K, Yao D, Kaplow IM, Greenside PG, Morgens DW, Phanstiel DH, Snyder MP, Bintu L, Greenleaf WJ**, Kundaje A**, Bassik MC**.
Nature Communications. 2019 Sep 6;10(1):4063. doi: 10.1038/s41467-019-11955-7.
Pooled CRISPR-Cas9 screens are a powerful method for functionally characterizing regulatory elements in the non-coding genome, but off-target effects in these experiments have not been systematically evaluated. Here, we investigate Cas9, dCas9, and CRISPRi/a off-target activity in screens for essential regulatory elements. The sgRNAs with the largest effects in genome-scale screens for essential CTCF loop anchors in K562 cells were not single guide RNAs (sgRNAs) that disrupted gene expression near the on-target CTCF anchor. Rather, these sgRNAs had high off-target activity that, while only weakly correlated with absolute off-target site number, could be predicted by the recently developed GuideScan specificity score. Screens conducted in parallel with CRISPRi/a, which do not induce double-stranded DNA breaks, revealed that a distinct set of off-targets also cause strong confounding fitness effects with these epigenome-editing tools. Promisingly, filtering of CRISPRi libraries using GuideScan specificity scores removed these confounded sgRNAs and enabled identification of essential regulatory elements.
CRISPR-Cas9 Screens Identify Regulators of Antibody-Drug Conjugate Toxicity.
Tsui CK, Barfield RM, Fischer CR, Morgens DW, Li A, Smith BAH, Gray MA, Bertozzi CR, Rabuka D, Bassik MC.
Nature Chemical Biology. 2019 Oct;15(10):949-958. doi: 10.1038/s41589-019-0342-2.
Antibody-drug conjugates (ADCs) selectively deliver chemotherapeutic agents to target cells and are important cancer therapeutics. However, the mechanisms by which ADCs are internalized and activated remain unclear. Using CRISPR-Cas9 screens, we uncover many known and novel endolysosomal regulators as modulators of ADC toxicity. We identify and characterize C18ORF8/RMC1 as a regulator of ADC toxicity through its role in endosomal maturation. Through comparative analysis of screens with ADCs bearing different linkers, we show that a subset of late endolysosomal regulators selectively influence toxicity of noncleavable linker ADCs. Surprisingly, we find cleavable valine-citrulline linkers can be processed rapidly after internalization without lysosomal delivery. Lastly, we show that sialic acid depletion enhances ADC lysosomal delivery and killing in diverse cancer cell types, including with FDA (US Food and Drug Administration)-approved trastuzumab emtansine (T-DM1) in Her2-positive breast cancer cells. Together, these results reveal new regulators of endolysosomal trafficking, provide important insights for ADC design and identify candidate combination therapy targets.
Identification of phagocytosis regulators using magnetic genome-wide CRISPR screens
Haney MS, Bohlen CJ, Morgens DW, Ousey JA, Barkal AA, Tsui CK, Ego B, Levin R, Kamber R, Collins H, Tucker A, Li A, Vorselen D, Labitigan L, Crane E, Boyle E, Jiang L, Chan J, Rincón E, Greenleaf WJ, Li B, Snyder MP, Weissman IL, Theriot JA, Collins SR, Barres BA, Bassik MC
Nat Genet. 2018, Dec;50(12):1716-1727.
Phagocytosis is required for a broad range of physiological functions, from pathogen defense to tissue homeostasis, but the mechanisms required for phagocytosis of diverse substrates remain incompletely understood. Here, we developed a rapid magnet-based phenotypic screening strategy, and performed eight genome-wide CRISPR screens in human cells to identify genes regulating phagocytosis of distinct substrates. After validating select hits in focused miniscreens, orthogonal assays and primary human macrophages, we show that (1) the previously uncharacterized gene NHLRC2 is a central player in phagocytosis, regulating RhoA-Rac1 signaling cascades that control actin polymerization and filopodia formation, (2) very-long-chain fatty acids are essential for efficient phagocytosis of certain substrates and (3) the previously uncharacterized Alzheimer's disease-associated gene TM2D3 can preferentially influence uptake of amyloid-β aggregates. These findings illuminate new regulators and core principles of phagocytosis, and more generally establish an efficient method for unbiased identification of cellular uptake mechanisms across diverse physiological and pathological contexts.
Kramer NJ*, Haney MS*, Morgens DW, Jovičić A, Couthouis J, Li A, Ousey J, Ma R, Bieri G, Bassik MC**, Gitler AD**
Nat. Genet.. 2018 Mar 5. doi: 10.1038/s41588-018-0070-7.
Hexanucleotide-repeat expansions in the C9ORF72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). The nucleotide-repeat expansions are translated into dipeptide-repeat (DPR) proteins, which are aggregation prone and may contribute to neurodegeneration. We used the CRISPR-Cas9 system to perform genome-wide gene-knockout screens for suppressors and enhancers of C9ORF72 DPR toxicity in human cells. We validated hits by performing secondary CRISPR-Cas9 screens in primary mouse neurons. We uncovered potent modifiers of DPR toxicity whose gene products function in nucleocytoplasmic transport, the endoplasmic reticulum (ER), proteasome, RNA-processing pathways, and chromatin modification. One modifier, TMX2, modulated the ER-stress signature elicited by C9ORF72 DPRs in neurons and improved survival of human induced motor neurons from patients with C9ORF72 ALS. Together, our results demonstrate the promise of CRISPR-Cas9 screens in defining mechanisms of neurodegenerative diseases.
Diverse Controls of LINE-1 Retrotransposition in Human Cells Revealed by A Genome-wide CRISPR Screen
Liu N,* Lee CH,* Swigut T, Grow E, Gu B, Bassik MC**, Wysocka J**
Nature. 2017 doi:10.1038/nature25179, published online 06 December.
Transposable elements, also known as transposons, are now recognized not only as parasitic DNA, the spread of which in the genome must be controlled by the host, but also as major players in genome evolution and regulation. Long interspersed element-1 (LINE-1, also known as L1), the only currently autonomous mobile transposon in humans, occupies 17% of the genome and generates inter- and intra-individual genetic variation, in some cases resulting in disease. However, how L1 activity is controlled and the function of L1s in host gene regulation are not completely understood. Here we use CRISPR-Cas9 screening strategies in two distinct human cell lines to provide a genome-wide survey of genes involved in the control of L1 retrotransposition. We identify functionally diverse genes that either promote or restrict L1 retrotransposition. These genes, which are often associated with human diseases, control the L1 life cycle at the transcriptional or the post-transcriptional level in a manner that can depend on the endogenous L1 nucleotide sequence, underscoring the complexity of L1 regulation. We further investigate the restriction of L1 by the protein MORC2 and by the human silencing hub (HUSH) complex subunits MPP8 and TASOR. HUSH and MORC2 can selectively bind evolutionarily young, full-length L1s located within transcriptionally permissive euchromatic environments, and promote deposition of histone H3 Lys9 trimethylation (H3K9me3) for transcriptional silencing. Notably, these silencing events often occur within introns of transcriptionally active genes, and lead to the downregulation of host gene expression in a HUSH-, MORC2-, and L1-dependent manner. Together, these results provide a rich resource for studies of L1 retrotransposition, elucidate a novel L1 restriction pathway and illustrate how epigenetic silencing of transposable elements rewires host gene expression programs.
Genome-scale measurement of off-target activity using Cas9 toxicity in high-throughput screens
Morgens DW, Wainberg M, Boyle EA, Ursu O, Araya CL, Tsui CK, Haney MS, Hess GT, Han K, Jeng EE, Li A, Snyder MP, Greenleaf WJ, Kundaje A, Bassik MC
Nat. Commun.. 2017 May 5;8:15178.
CRISPR-Cas9 screens are powerful tools for high-throughput interrogation of genome function, but can be confounded by nuclease-induced toxicity at both on- and off-target sites, likely due to DNA damage. Here, to test potential solutions to this issue, we design and analyse a CRISPR-Cas9 library with 10 variable-length guides per gene and thousands of negative controls targeting non-functional, non-genic regions (termed safe-targeting guides), in addition to non-targeting controls. We find this library has excellent performance in identifying genes affecting growth and sensitivity to the ricin toxin. The safe-targeting guides allow for proper control of toxicity from on-target DNA damage. Using this toxicity as a proxy to measure off-target cutting, we demonstrate with tens of thousands of guides both the nucleotide position-dependent sensitivity to single mismatches and the reduction of off-target cutting using truncated guides. Our results demonstrate a simple strategy for high-throughput evaluation of target specificity and nuclease toxicity in Cas9 screens.
Han K, Jeng EE, Hess GT, Morgens DW, Li A, Bassik MC
Nat Biotechnol. 2017 Mar 20. doi: 10.1038/nbt.3834.
Identification of effective combination therapies is critical to address the emergence of drug-resistant cancers, but direct screening of all possible drug combinations is infeasible. Here we introduce a CRISPR-based double knockout (CDKO) system that improves the efficiency of combinatorial genetic screening using an effective strategy for cloning and sequencing paired single guide RNA (sgRNA) libraries and a robust statistical scoring method for calculating genetic interactions (GIs) from CRISPR-deleted gene pairs. We applied CDKO to generate a large-scale human GI map, comprising 490,000 double-sgRNAs directed against 21,321 pairs of drug targets in K562 leukemia cells and identified synthetic lethal drug target pairs for which corresponding drugs exhibit synergistic killing. These included the BCL2L1 and MCL1 combination, which was also effective in imatinib-resistant cells. We further validated this system by identifying known and previously unidentified GIs between modifiers of ricin toxicity. This work provides an effective strategy to screen synergistic drug combinations in high-throughput and a CRISPR-based tool to dissect functional GI networks.
Directed evolution using dCas9-targeted somatic hypermutation in mammalian cells
Hess GT, Frésard L, Han K, Lee CH, Li A, Cimprich KA, Montgomery SB, Bassik MC
Nat Methods. 2016 Dec;13(12):1036-1042. doi: 10.1038/nmeth.4038.
Engineering and study of protein function by directed evolution has been limited by the technical requirement to use global mutagenesis or introduce DNA libraries. Here, we develop CRISPR-X, a strategy to repurpose the somatic hypermutation machinery for protein engineering in situ. Using catalytically inactive dCas9 to recruit variants of cytidine deaminase (AID) with MS2-modified sgRNAs, we can specifically mutagenize endogenous targets with limited off-target damage. This generates diverse libraries of localized point mutations and can target multiple genomic locations simultaneously. We mutagenize GFP and select for spectrum-shifted variants, including EGFP. Additionally, we mutate the target of the cancer therapeutic bortezomib, PSMB5, and identify known and novel mutations that confer bortezomib resistance. Finally, using a hyperactive AID variant, we mutagenize loci both upstream and downstream of transcriptional start sites. These experiments illustrate a powerful approach to create complex libraries of genetic variants in native context, which is broadly applicable to investigate and improve protein function.
Systematic comparison of CRISPR/Cas9 and RNAi screens for essential genes
Morgens DW, Deans RM, Li A, Bassik MC
Nat Biotechnol2016 Jun;34(6):634-6. doi: 10.1038/nbt.3567.
We compared the ability of short hairpin RNA (shRNA) and CRISPR/Cas9 screens to identify essential genes in the human chronic myelogenous leukemia cell line K562. We found that the precision of the two libraries in detecting essential genes was similar and that combining data from both screens improved performance. Notably, results from the two screens showed little correlation, which can be partially explained by the identification of distinct essential biological processes with each technology.
Parallel shRNA and CRISPR-Cas9 screens enable antiviral drug target identification
Deans RM, Morgens DW, Ökesli A, Pillay S, Horlbeck MA, Kampmann M, Gilbert LA, Li A, Mateo R, Smith M, Glenn JS, Carette JE, Khosla C, Bassik MC
Nat Chem Biol. 2016 May;12(5):361-6. doi: 10.1038/nchembio.2050.
Broad-spectrum antiviral drugs targeting host processes could potentially treat a wide range of viruses while reducing the likelihood of emergent resistance. Despite great promise as therapeutics, such drugs remain largely elusive. Here we used parallel genome-wide high-coverage short hairpin RNA (shRNA) and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 screens to identify the cellular target and mechanism of action of GSK983, a potent broad-spectrum antiviral with unexplained cytotoxicity. We found that GSK983 blocked cell proliferation and dengue virus replication by inhibiting the pyrimidine biosynthesis enzyme dihydroorotate dehydrogenase (DHODH). Guided by mechanistic insights from both genomic screens, we found that exogenous deoxycytidine markedly reduced GSK983 cytotoxicity but not antiviral activity, providing an attractive new approach to improve the therapeutic window of DHODH inhibitors against RNA viruses. Our results highlight the distinct advantages and limitations of each screening method for identifying drug targets, and demonstrate the utility of parallel knockdown and knockout screens for comprehensive probing of drug activity.