Genome-wide CRISPR Knockout Screens Research Articles

Targeting the GPI transamidase subunit GPAA1 abrogates the CD24 immune checkpoint in ovarian cancer

CD24 is frequently overexpressed in ovarian cancer and promotes immune evasion by interacting with its receptor Siglec10, present on tumor-associated macrophages, providing a "don't eat me" signal that prevents targeting and phagocytosis by macrophages. Factors promoting CD24 expression could represent novel immunotherapeutic targets for ovarian cancer. Here, using a genome-wide CRISPR knockout screen, we identify GPAA1 (glycosylphosphatidylinositol anchor attachment 1), a factor that catalyzes the attachment of a glycosylphosphatidylinositol (GPI) lipid anchor to substrate proteins, as a positive regulator of CD24 cell surface expression. Genetic ablation of GPAA1 abolishes CD24 cell surface expression, enhances macrophage-mediated phagocytosis, and inhibits ovarian tumor growth in mice. GPAA1 shares structural similarities with aminopeptidases. Consequently, we show that bestatin, a clinically advanced aminopeptidase inhibitor, binds to GPAA1 and blocks GPI attachment, resulting in reduced CD24 cell surface expression, increased macrophage-mediated phagocytosis, and suppressed growth of ovarian tumors. Our study highlights the potential of targeting GPAA1 as an immunotherapeutic approach for CD24+ ovarian cancers.

Abstract 37: IL-4 drives CART cell exhaustion in a CD4 independent manner

Abstract While chimeric antigen receptor T (CART) cell therapy has shown remarkable success, the development of exhaustion limits durable response. We identified a role for interleukin (IL)-4 in the development of CART cell exhaustion through three independent approaches including: 1) a genome-wide CRISPR knockout screen using healthy donor CART cells in an in vitro model for exhaustion, 2) RNA and ATAC sequencing on freshly produced and chronically stimulated healthy donor CART cells, and 3) RNA and ATAC sequencing on pre-infusion CART cell products from responders and non-responders in the Zuma-1 clinical trial that led to the FDA approval of axi-cel. Further, in vitro validation studies revealed that CD19 directed CART (CART19) cells chronically stimulated in the presence of human recombinant IL-4 (hrIL-4) displayed signs of exhaustion such as 1) decreased proliferation (p = 0.01), 2) increased coexpression of inhibitory receptors (p = 0.01), and 3) decreased production of IL-2 and interferon (IFN)-γ (p= 0.02, p = 0.002). Encouragingly, CART19 cells combined with an IL-4 monoclonal antibody improved antitumor activity (p = 0.045) and expansion (p = 0.01) while also decreasing the co-expression of inhibitory receptors (p = 0.02) in a mantle cell lymphoma xenograft mouse model. Building on these results, we asked if IL-4 driven exhaustion results from a direct impact of IL-4 on CART cells. To test this, we used a tumor-free assay where CART19 cells were chronically stimulated with CD19 beads in the presence of hrIL-4 or diluent. CART cells treated with hrIL-4 displayed an exhausted phenotype characterized by increased co-expression of inhibitory receptors (p = 0.04) and decreased production of IL-2 (p = 0.01). Next, we asked if IL-4 driven CART cell exhaustion is dependent on the costimulatory domain. We tested the impact of IL-4 on both CD28ζ and 41BBζ costimulated CART19 cells. Similar to our previous studies with CART19-28ζ cells, chronic stimulation of CART19-BBζ cells in the presence of hrIL-4 enhanced the exhausted phenotype as seen by increased co-expression of inhibitory receptors (p = 0.04) and decreased production of IL-2 and IFN-γ (p = 0.08 and p = 0.007). Finally, we asked if IL-4 induces exhaustion independently of its classic role in Th2 polarization of CD4 CART cells. Following CART production, we isolated CD8 cells and chronically stimulated them in the presence of hrIL-4 or diluent. CD8 CART cells treated with hrIL-4 displayed an enhanced exhausted profile as seen by 1) decreased proliferative ability (p < 0.0001), 2) increased co-expression of inhibitory receptors (p = 0.01), and 3) decreased production of IL-2 and IFN-γ (p < 0.0001, p = 0.004). Together, our data indicates a novel role for IL-4 in the development of CART cell exhaustion that is independent of tumor cells, costimulatory domain, and CD4 cells. As such, we believe IL-4 neutralization may be a widely applicable and actionable approach to improve the durable response to CART cell therapy. Citation Format: Carli M. Stewart, Elizabeth L. Siegler, Truc N. Huynh, R. Leo Sakemura, Brooke Kimball, Long Mai, Kun Yun, James H. Girsch, Jennifer Feigin, Omar Gutierrez Ruiz, Makena Rodriguez, Ekene Ogbodo, Ismail Can, Claudia Manriquez Roman, Olivia Sirpilla, Hong Xia, Jenny Kim, Justin Budka, Mike Mattie, Nathalie Scholler, Simone Filosto, Saad S. Kenderian. IL-4 drives CART cell exhaustion in a CD4 independent manner [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 37.

Abstract 4636: Genome-wide drug anchor screens identify CAAP1 and AKAP17A as regulators of PRMT5 inhibitor sensitivity

Abstract MTA-cooperative PRMT5 inhibitors are an emerging treatment option for patients with one of the 10-15% of all human cancers harboring MTAP homozygous deletion. To identify potential regulators of sensitivity to PRMT5 inhibitors, we performed genome-wide CRISPR knockout screens in the presence and absence of an MTA-cooperative PRMT5 inhibitor. Knockout of CAAP1 and AKAP17A were among the strongest sensitizing hits across multiple MTAP-deleted cancer cell lines representing different histologies. Strikingly, the CAAP1 gene co-localizes with MTAP and CDKN2A on chromosome 9p21. Co-deletion of CAAP1 is reported in 20 percent of MTAP-deleted cancers in the TCGA PanCancer Atlas. CAAP1 or AKAP17A knockout in MTAP-deleted cancer cell lines sensitized the cells to PRMT5 inhibitors including the clinical stage MTA-cooperative inhibitors, TNG908 and TNG462, and the non-MTA-cooperative inhibitor, GSK3326595. Moreover, we discovered that CAAP1 and AKAP17A protein levels are interdependent, as knockout of either gene caused decreased protein levels for the other. Consistent with this finding, CAAP1 reconstitution in CAAP1-deleted cell lines led to increased AKAP17A levels. Endogenous CAAP1 and AKAP17A protein levels are positively correlated across a panel of cancer cell lines and MTAP-deleted patient-derived xenograft models. Consistent with a previous report (Ni et al., 2023), exogenous CAAP1 and AKAP17A co-immunoprecipitation studies suggest that the proteins form a protein complex. AKAP17A and CAAP1 are not well-characterized proteins, but PRMT5 inhibitors induce global alternative splicing events (ASEs) in cancer cells, and based on preliminary studies a possible function for the CAAP1/AKAP17A complex could be to mitigate ASEs induced by PRMT5 inhibition. Collectively, these data indicate that CAAP1 and AKAP17A exist interdependently and mediate sensitivity to PRMT5 inhibitors. The colocalization and 20 percent incidence of CAAP1 deletion in the setting of MTAP deletion may suggest that such patients will have improved responses to PRMT5-targeted therapy. S. Yoda and M. R. Tonini contributed equally. Citation Format: Satoshi Yoda, Matthew R. Tonini, Hilary E. Nicolson, Samuel Poleretzky, Silvia Fenoglio, Steven Lombardo, Lauren Grove, Samuel R. Meier, Ashley Choi, Yi Yu, Kevin M. Cottrell, John P. Maxwell, Teng, Jannik N. Andersen, Kimberly J. Briggs. Genome-wide drug anchor screens identify CAAP1 and AKAP17A as regulators of PRMT5 inhibitor sensitivity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4636.

Validation of Candidate Host Cell Entry Factors for Bovine Herpes Virus Type-1 Based on a Genome-Wide CRISPR Knockout Screen.

To identify host factors that affect Bovine Herpes Virus Type 1 (BoHV-1) infection we previously applied a genome wide CRISPR knockout screen targeting all bovine protein coding genes. By doing so we compiled a list of both pro-viral and anti-viral proteins involved in BoHV-1 replication. Here we provide further analysis of those that are potentially involved in viral entry into the host cell. We first generated single cell knockout clones deficient in some of the candidate genes for validation. We provide evidence that Polio Virus Receptor-related protein (PVRL2) serves as a receptor for BoHV-1, mediating more efficient entry than the previously identified Polio Virus Receptor (PVR). By knocking out two enzymes that catalyze HSPG chain elongation, HST2ST1 and GLCE, we further demonstrate the significance of HSPG in BoHV-1 entry. Another intriguing cluster of candidate genes, COG1, COG2 and COG4-7 encode six subunits of the Conserved Oligomeric Golgi (COG) complex. MDBK cells lacking COG6 produced fewer but bigger plaques compared to control cells, suggesting more efficient release of newly produced virions from these COG6 knockout cells, due to impaired HSPG biosynthesis. We further observed that viruses produced by the COG6 knockout cells consist of protein(s) with reduced N-glycosylation, potentially explaining their lower infectivity. To facilitate candidate validation, we also detailed a one-step multiplex CRISPR interference (CRISPRi) system, an orthogonal method to KO that enables quick and simultaneous deployment of three CRISPRs for efficient gene inactivation. Using CRISPR3i, we verified eight candidates that have been implicated in the synthesis of surface heparan sulfate proteoglycans (HSPGs). In summary, our experiments confirmed the two receptors PVR and PVRL2 for BoHV-1 entry into the host cell and other factors that affect this process, likely through the direct or indirect roles they play during HSPG synthesis and glycosylation of viral proteins.

CRISPR screen for protein inclusion formation uncovers a role for SRRD in the regulation of intermediate filament dynamics and aggresome assembly.

The presence of large protein inclusions is a hallmark of neurodegeneration, and yet the precise molecular factors that contribute to their formation remain poorly understood. Screens using aggregation-prone proteins have commonly relied on downstream toxicity as a readout rather than the direct formation of aggregates. Here, we combined a genome-wide CRISPR knockout screen with Pulse Shape Analysis, a FACS-based method for inclusion detection, to identify direct modifiers of TDP-43 aggregation in human cells. Our screen revealed both canonical and novel proteostasis genes, and unearthed SRRD, a poorly characterized protein, as a top regulator of protein inclusion formation. APEX biotin labeling reveals that SRRD resides in proximity to proteins that are involved in the formation and breakage of disulfide bonds and to intermediate filaments, suggesting a role in regulation of the spatial dynamics of the intermediate filament network. Indeed, loss of SRRD results in aberrant intermediate filament fibrils and the impaired formation of aggresomes, including blunted vimentin cage structure, during proteotoxic stress. Interestingly, SRRD also localizes to aggresomes and unfolded proteins, and rescues proteotoxicity in yeast whereby its N-terminal low complexity domain is sufficient to induce this affect. Altogether this suggests an unanticipated and broad role for SRRD in cytoskeletal organization and cellular proteostasis.

GARP and EARP are required for efficient BoHV-1 replication as identified by a genome wide CRISPR knockout screen.

The advances in gene editing bring unprecedented opportunities in high throughput functional genomics to animal research. Here we describe a genome wide CRISPR knockout library, btCRISPRko.v1, targeting all protein coding genes in the cattle genome. Using it, we conducted genome wide screens during Bovine Herpes Virus type 1 (BoHV-1) replication and compiled a list of pro-viral and anti-viral candidates. These candidates might influence multiple aspects of BoHV-1 biology such as viral entry, genome replication and transcription, viral protein trafficking and virion maturation in the cytoplasm. Some of the most intriguing examples are VPS51, VPS52 and VPS53 that code for subunits of two membrane tethering complexes, the endosome-associated recycling protein (EARP) complex and the Golgi-associated retrograde protein (GARP) complex. These complexes mediate endosomal recycling and retrograde trafficking to the trans Golgi Network (TGN). Simultaneous loss of both complexes in MDBKs resulted in greatly reduced production of infectious BoHV-1 virions. We also found that viruses released by these deficient cells severely lack VP8, the most abundant tegument protein of BoHV-1 that are crucial for its virulence. In combination with previous reports, our data suggest vital roles GARP and EARP play during viral protein packaging and capsid re-envelopment in the cytoplasm. It also contributes to evidence that both the TGN and the recycling endosomes are recruited in this process, mediated by these complexes. The btCRISPRko.v1 library generated here has been controlled for quality and shown to be effective in host gene discovery. We hope it will facilitate efforts in the study of other pathogens and various aspects of cell biology in cattle.

A Multi-Modal Analysis of Acquired Resistance to Acalabrutinib and Pirtobrutinib Provides Potential Strategies to Augment Treatment Outcome with BTKi Drugs

Introduction: Acalabrutinib (acala), a covalent BTK inhibitor (BTKi), exhibits excellent efficacy and safety in CLL and other B cell malignancies. However, long term efficacy is often compromised by the acquisition of mutations at the acala-binding residue C481, the most prevalent being C481S. Pirtobrutinib (pirto), a reversible BTKi, is active clinically in patients who have progressed on a covalent BTKi with or without C481S mutations. Besides BTK mutations, several additional pathways are hypothesized to contribute to BTKi resistance and may be highly relevant to the sequencing and combining of CLL-directed therapies. To understand acquired resistance to both acala and pirto, we employed multiple approaches, including a genome-wide CRISPR knockout (KO) screen and the generation of BTK mutant drug resistant cell lines. Methods: Genome-wide CRISPR KO screen was performed in the BTKi-sensitive CD79B-MYD88 double mutant ABC-DLBCL cell line TMD8 with 3 guides/gene (best Vienna BioScore). Pooled library-transduced cells at 750x target coverage were cultured for 14 days with sublethal acala or pirto concentrations to achieve ~60% growth inhibition over DMSO. NGS/BAGAL/MAGeCK analyses identified enriched/depleted guides and corresponding resistance/sensitizing genes (FDR<0.1). Hits were validated by single gene KO (Cas9 RNP + 3-plex gRNA nucleofection) and 10-day growth assays with BTKi treatment. BTK mutant cell lines resistant to acala or pirto were derived in TMD8 and the BTKi-sensitive Karpas1718 MZL cell line by culturing through dose escalation. Exome/RNAseq was used to identify BTK mutations. TMD8 BTK C481R cell line was generated by CRISPR-HDR. BTK kinase and pathway activities were assayed by Western/Flow. Results: CRISPR screening results correlated highly between acala and pirto (Pearson's r=0.66), indicating shared pathways that impact response to both BTKi's. Collectively we identified 133 resistance and 57 sensitizing genes that span both known pathways (BCR/NFκB signaling) and novel modulators (autophagy, g-secretase, PBAF complex) of BTKi response. Systematic hit validation by single gene KO confirmed resistance phenotype for 6 genes: TNFAIP3 and TRAF3 negative regulators of canonical/non-canonical NFκB pathways; GRB2 adaptor protein for BCR signaling; PSEN1 in γ-secretase, PHF10 in PBAF complex; and SPOP substrate recognition protein for ubiquitin ligase. Interrogating the Project GENIE patient database identified loss-of-function mutations in these genes (8.6% CLL, 19.3% MCL, 25% MZL), supporting the hypothesis that they may be biomarkers for resistance to both BTKis. Sensitizing genes included CBL adaptor protein, PTPRCAP a CD45 tyrosine phosphatase, and RRAS2 small GTPase; KO of these genes did not alter TMD8 cell growth but enhanced the potency of both BTKis. Genomic analysis of BTKi-adaptive resistant cell lines identified causal BTK mutations, including nonclassical binding site mutation C481F for acala resistance and 2 novel mutations F442L and L460S for pirto resistance. C481F and C481R are kinase-inactive but cells still retain BCR signaling, pointing to BTK scaffolding activity. HCK is a proposed mediator of BTK scaffolding activity (Dhami et al., 2022; Yuan et al., 2022). However, while HCK activation was detected in C481R cells, HCK KO did not impair cell growth. In addition, no HCK activation was detected in C481F cells. Thus, scaffolding activity by C481F/R mutants does not depend on HCK. For the 2 pirto-resistant BTK mutations, F442L is kinase active but has reduced response to pirto, whereas L460S is kinase impaired. All our BTK mutant cell lines were still sensitive to the other BTKi, supporting a rationale for treatment sequencing between BTKi drugs to overcome resistance due to BTK mutations. Conclusions: We uncovered known and novel BTK resistance mutations and demonstrated BTK scaffolding activity independent of HCK, highlighting the need for other strategies to disrupt scaffolding-mediated BCR signaling. Beyond BTK mutations, our CRISPR KO screens illustrate a map of genetic modifiers of BTKi response and point to several potential resistance biomarkers for both acala and pirto. A better understanding of resistance mechanisms in the presence and absence of BTK mutations will help augment the use of BTKi treatments in patients.

Active growth signaling promotes senescence and cancer cell sensitivity to CDK7 inhibition

Tumor growth is driven by continued cellular growth and proliferation. Cyclin-dependent kinase 7's (CDK7) role in activating mitotic CDKs and global gene expression makes it therefore an attractive target for cancer therapies. However, what makes cancer cells particularly sensitive to CDK7 inhibition (CDK7i) remains unclear. Here, we address this question. We show that CDK7i, by samuraciclib, induces a permanent cell-cycle exit, known as senescence, without promoting DNA damage signaling or cell death. A chemogenetic genome-wide CRISPR knockout screen identified that active mTOR (mammalian target of rapamycin) signaling promotes samuraciclib-induced senescence. mTOR inhibition decreases samuraciclib sensitivity, and increased mTOR-dependent growth signaling correlates with sensitivity in cancer cell lines. Reverting a growth-promoting mutation in PIK3CA to wild type decreases sensitivity to CDK7i. Our work establishes that enhanced growth alone promotes CDK7i sensitivity, providing an explanation for why some cancers are more sensitive to CDK inhibition than normally growing cells.

N-terminal acetylation shields proteins from degradation and promotes age-dependent motility and longevity

Most eukaryotic proteins are N-terminally acetylated, but the functional impact on a global scale has remained obscure. Using genome-wide CRISPR knockout screens in human cells, we reveal a strong genetic dependency between a major N-terminal acetyltransferase and specific ubiquitin ligases. Biochemical analyses uncover that both the ubiquitin ligase complex UBR4-KCMF1 and the acetyltransferase NatC recognize proteins bearing an unacetylated N-terminal methionine followed by a hydrophobic residue. NatC KO-induced protein degradation and phenotypes are reversed by UBR knockdown, demonstrating the central cellular role of this interplay. We reveal that loss of Drosophila NatC is associated with male sterility, reduced longevity, and age-dependent loss of motility due to developmental muscle defects. Remarkably, muscle-specific overexpression of UbcE2M, one of the proteins targeted for NatC KO-mediated degradation, suppresses defects of NatC deletion. In conclusion, NatC-mediated N-terminal acetylation acts as a protective mechanism against protein degradation, which is relevant for increased longevity and motility.

Genome-scale functional genomics screening highlights genes impacting protein fucosylation in Chinese hamster ovary cells

N-linked glycosylation is a common post-translational modification that has various effects on multiple types of proteins. The extent to which an N-linked glycoprotein is modified and the identity of glycans species involved is of great interest to the biopharmaceutical industry, since glycosylation can impact the efficacy and safety of therapeutic monoclonal antibodies (mAbs). mAbs lacking core fucose, for example, display enhanced clinical efficacy through increased antibody-dependent cellular cytotoxicity. We performed a genome-wide CRISPR knockout screen in Chinese hamster ovary (CHO) cells, the workhorse cell culture system for industrial production of mAbs, aimed at identifying novel regulators of protein fucosylation. Using a lectin binding assay, we identified 224 gene perturbations that significantly alter protein fucosylation, including well-known glycosylation genes. This functional genomics framework could readily be extended and applied to study the genetic pathways involved in regulation of other glycoforms. We hope this resource will provide useful guidance toward the development of next generation CHO cell lines and mAb therapeutics.

A genome-wide screen reveals new regulators of the 2-cell-like cell state.

In mammals, only the zygote and blastomeres of the early embryo are totipotent. This totipotency is mirrored in vitro by mouse '2-cell-like cells' (2CLCs), which appear at low frequency in cultures of embryonic stem cells (ESCs). Because totipotency is not completely understood, we carried out a genome-wide CRISPR knockout screen in mouse ESCs, searching for mutants that reactivate the expression of Dazl, a gene expressed in 2CLCs. Here we report the identification of four mutants that reactivate Dazl and a broader 2-cell-like signature: the E3 ubiquitin ligase adaptor SPOP, the Zinc-Finger transcription factor ZBTB14, MCM3AP, a component of the RNA processing complex TREX-2, and the lysine demethylase KDM5C. All four factors function upstream of DPPA2 and DUX, but not via p53. In addition, SPOP binds DPPA2, and KDM5C interacts with ncPRC1.6 and inhibits 2CLC gene expression in a catalytic-independent manner. These results extend our knowledge of totipotency, a key phase of organismal life.

Mislocalization of pathogenic RBM20 variants in dilated cardiomyopathy is caused by loss-of-interaction with Transportin-3

Severe forms of dilated cardiomyopathy (DCM) are associated with point mutations in the alternative splicing regulator RBM20 that are frequently located in the arginine/serine-rich domain (RS-domain). Such mutations can cause defective splicing and cytoplasmic mislocalization, which leads to the formation of detrimental cytoplasmic granules. Successful development of personalized therapies requires identifying the direct mechanisms of pathogenic RBM20 variants. Here, we decipher the molecular mechanism of RBM20 mislocalization and its specific role in DCM pathogenesis. We demonstrate that mislocalized RBM20 RS-domain variants retain their splice regulatory activity, which reveals that aberrant cellular localization is the main driver of their pathological phenotype. A genome-wide CRISPR knockout screen combined with image-enabled cell sorting identified Transportin-3 (TNPO3) as the main nuclear importer of RBM20. We show that the direct RBM20-TNPO3 interaction involves the RS-domain, and is disrupted by pathogenic variants. Relocalization of pathogenic RBM20 variants to the nucleus restores alternative splicing and dissolves cytoplasmic granules in cell culture and animal models. These findings provide proof-of-principle for developing therapeutic strategies to restore RBM20’s nuclear localization in RBM20-DCM patients.

Abstract B015: HOXA9 promotes enzalutamide resistance in RB-p53 deficient prostate cancer

Abstract Introduction: Castration resistant prostate cancer (CRPC) cells can acquire resistance to the androgen receptor (AR) inhibitor enzalutamide (EZ). These cells can switch lineages from an adenocarcinoma to a neuroendocrine (NE) cell type that proliferate independently of the AR signaling pathway. Cancer genomic and molecular studies identified that co-deletion of the retinoblastoma (RB1) and TP53 genes can promote the acquisition of EZ resistance and neuroendocrine features. However, RB and p53 are both tumour suppressor and are therefore difficult to target pharmacologically. The purpose of this study is to identify an actionable molecular factor downstream of RB and p53 that drives EZ resistance in CRPC. Methods and Results: To characterize functional and molecular features of RB and p53 loss, CRISPR-Cas9 was used to generate a double knockout (DKO) line in LNCaP prostate cancer cells. Compared with LNCaP wild type (WT) cells, only DKOs formed distinct colonies over a 4-week colony forming assay under EZ treatment. RNA sequencing of DKO and WT cells, followed by gene ontology (GO) analysis, revealed NE and stemness genes, including HOXA9, were significantly upregulated in DKO cells. To categorize gene loss events in EZ-treated LNCaP cells, a genome-wide CRISPR knockout screen was performed. Pools of KO cells were treated with either EZ or DMSO and then analyzed by next generation sequencing to identify gene mutations that confer increased resistance or sensitivity to EZ. GO analysis of de-enriched genes following EZ treatment identified stemness genes, including HOXA9, highlighting the potential importance of a stem-like phenotype for acquiring EZ resistance EZ. To further investigate the functional significance of HOXA9 we analyzed EZ-resistant prostate tumour genomic data. HOXA9 is mutated or mis-expressed in 10% of cases. Importantly, HOXA9 is either amplified or overexpressed in virtually all these cases, and is associated with poorer prognosis, suggesting an oncogenic role for HOXA9 in CRPC. HOXA9 transcript levels were positively correlated with neuroendocrine features and negatively correlated with RB1expression in these tumour samples. LNCaP WT and DKO cells we then engineered to overexpress HOXA9 displayed increased IC50 values following a 6-day EZ treatment, compared with either parental line. DKO cells that overexpress HOXA9 also formed significantly more colonies following a 4-week EZ treatment, compared with parentals. In contrast, shRNA knockdown of HOXA9 caused a reduction in IC50 values and formed fewer drug resistant colonies compared with control cells. Finally, DKO and WT cells were co-treated with varying concentrations of the HOXA9 inhibitor DB818 and EZ. DKO cells were more sensitive to DB818 alone at high concentrations and displayed higher synergy as measured by a ZIP synergy score when co-treated with EZ. Conclusions: Overall, these results suggest that HOXA9 regulates EZ resistance in prostate cancer. Furthermore, HOXA9 inhibition may be of therapeutic benefit for treating EZ-resistant CRPC. Citation Format: Michael V. Roes, Fred A. Dick. HOXA9 promotes enzalutamide resistance in RB-p53 deficient prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B015.

Genome-wide CRISPR-Cas9 screening to identify CDS2 as an essential gene in uveal melanoma.

e21585 Background: Despite low rates of local recurrence in primary uveal melanomas following radical treatment, approximately half of all patients will develop metastatic disease. There are limited effective systemic therapeutic options and survival outcomes remain poor. Genome-wide CRISPR-Cas9 knockout screening enables appropriate molecular targets and pathways that underlie cancer development to be identified. Methods: Genome-wide single gRNA CRISPR knockout screens were performed using Human Improved Genome-wide Knockout CRISPR Library v1.1. Eight human uveal melanoma cell lines were screened for 14 days to identify gene dependencies. Prior to screening, all cell lines underwent RNA-Seq to quantify gene expression. CRISPR screen hits were validated using single gRNA competitive growth assays. In vitro functional assessment of CDS2 knockout was performed using inducible CDS2 knockout isogenic cell lines in colony forming assays and Annexin V apoptosis assays. To determine the effects of CDS2 loss on phosphoinositide synthesis, quantification of phosphatidic acid and phosphoinositides was performed using HPLC-MS. To test the association between CDS2 essentiality and CDS1 expression across all cancer types, we analysed gene essentiality data from Project Score and Project Achilles with gene expression data obtained from Cell Model Passports and the DepMap download portal (22Q4). Cell lines with CDS1 expression log2(TPM+1) < 1 were annotated as low-expressors, and the rest were annotated as high-expressors. Single group data were assessed using Student's t test. Results: We identified CDS2, which encodes enzymes that maintain phosphoinositide levels during PLC signalling, to be an essential gene in 7 out of 8 screened cell lines. All cell lines were found to have low expression of its paralogous gene, CDS1. Doxycycline-inducible CDS2 knockout clones were generated from cell lines MP41 and OMM2.5. Reduced colony formation, with a significant decrease in surviving fraction was observed after doxycycline compared with treatment with a DMSO control in both cell lines (P < 0.0001). Apoptotic and dead cells increased significantly when doxycycline was added in MP41(13.6 ± 0.9% to 47.4 ± 3.3%; P= 0.0006) and OMM2.5 (3.7 ± 0.4% to 60.4± 0.7%, P< 0.0001). CDS2 knockout resulted in an increase in the precursor phosphatidic acid, and a significant reduction of the phosphoinositides PI (MP41, P= 0.018; OMM2.5, P= 0.0005) and PIP (MP41, P= 0.033; OMM2.5, P= 0.006). CDS2 was more essential in CDS1 low-expressors in both Project Score ( P< 0.0001) and Project Achilles ( P< 0.0001) datasets, compared with CDS1 high-expressors. Conclusions: Disruption of CDS2 results in reduced cellular fitness in uveal melanoma. Given the synthetic lethal relationship between CDS1 and CDS2, in the context of low CDS1 gene expression as observed in uveal melanomas and in other tumour types, CDS2 is a potential novel therapeutic target.

A genome-wide optical pooled screen reveals regulators of cellular antiviral responses

The infection of mammalian cells by viruses and innate immune responses to infection are spatiotemporally organized processes. Cytosolic RNA sensors trigger nuclear translocation of the transcription factor interferon regulatory factor 3 (IRF3) and consequent induction of host immune responses to RNA viruses. Previous genetic screens for factors involved in viral sensing did not resolve changes in the subcellular localization of host or viral proteins. Here, we increased the throughput of our optical pooled screening technology by over fourfold. This allowed us to carry out a genome-wide CRISPR knockout screen using high-resolution multiparameter imaging of cellular responses to Sendai virus infection coupled with insitu cDNA sequencing by synthesis (SBS) to identify 80,408 single guide RNAs (sgRNAs) in 10,366,390 cells-over an order of magnitude more genomic perturbations than demonstrated previously using an insitu SBS readout. By ranking perturbations using human-designed and deep learning image feature scores, we identified regulators of IRF3 translocation, Sendai virus localization, and peroxisomal biogenesis. Among the hits, we found that ATP13A1, an ER-localized P5A-type ATPase, is essential for viral sensing and is required for targeting of mitochondrial antiviral signaling protein (MAVS) to mitochondrial membranes where MAVS must be localized for effective signaling through retinoic acid-inducible gene I (RIG-I). The ability to carry out genome-wide pooled screens with complex high-resolution image-based phenotyping dramatically expands the scope of functional genomics approaches.

Abstract 1153: IL-4 depletion leads to the improvement of CART cell therapy

Abstract While chimeric antigen receptor T-cell therapy targeting CD19 (CART19) has shown remarkable success in the treatment of hematological malignancies, the durable response rates remain approximately 40% and there are limited solutions for CART cell therapy in the treatment of solid tumors. To further understand mechanisms of resistance, including CART cell exhaustion, we employed three independent approaches: 1) RNA and ATAC sequencing on unstimulated vs. exhausted healthy donor CART19 cells by utilizing an in vitro model for exhaustion, 2) RNA and ATAC sequencing on pre-infusion CART19 cell products from responders and non-responders in the Zuma-1 clinical trial that led to the FDA approval of axi-cel CART19 therapy, and 3) a genome-wide CRISPR knockout screen in healthy donor CART19 cells using our in vitro model for exhaustion. In each of these approaches, IL-4 was identified as a regulator of CART cell dysfunction. In approach 1, ingenuity pathway analysis of genes that were both differentially accessible and expressed in exhausted compared with unstimulated CART19 cells revealed IL-4 as a top upstream regulator (p = 5E-6). In approach 2, IL-4 was one of two genes that were both upregulated and more accessible in CART19 cell products from non-responders (p < 5E-2). Finally, in approach 3, gene ontology enrichment analysis of genes that were positively selected during the genome-wide CRISPR knockout screen, revealed regulation of the IL-4 pathway as one of the top affected pathways (p = 1E-4). Together, our data indicates a role for IL-4 in CART cell dysfunction caused by exhaustion. Investigating this mechanism further, we saw an increase in the production of IL-4 as CART cells became exhausted (p = 4E-3). Treatment of CART19 cells with human recombinant IL-4 (hrIL-4) resulted in dysfunction as evident by a decrease in antigen specific cytotoxicity (p = 4E-3) and proliferative ability (p= 6.5E-2), as well as exhaustion-specific signs of dysfunction such as an increase in the expression of the inhibitory receptor, TIM-3 (p = 3E-3) and an increase in the transcription of the exhaustion-related transcription factor EOMES (p = 1E-2). Finally, we tested whether IL-4 neutralization enhances CART19 cell functions. Using a CD19+ JeKo-1 xenograft mouse model, we compared the combination treatment of CART19 cells and an IL-4 neutralizing monoclonal antibody (10 mg/Kg, clone # MP4-25D2) to CART19 cells and an IgG control. IL-4 neutralization in combination with CART19 cells resulted in reduced tumor burden (p = 4.6E-2), increased CART cell proliferation (p = 8E-3), and prolonged overall survival (p= 5E-2). In summary, our data indicates that 1) IL-4 induces CART cell dysfunction through a state of exhaustion and 2) IL-4 neutralization with a monoclonal antibody enhances CART cell therapy. As such, this novel combination therapy holds the potential to be translated to the clinic to improve durable responses from CART cell therapy. Citation Format: Carli Stewart, Michelle J. Cox, Reona Sakemura, Ekene J. Ogbodo, Ismail Can, Claudia Manriquez Roman, Kun Yun, Olivia Sirpilla, James H. Girsch, Truc Huynh, Elizabeth L. Siegler, Jenny J. Kim, Mike Mattie, Nathalie Scholler, Simone Filosto, Saad S. Kenderian. IL-4 depletion leads to the improvement of CART cell therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1153.

Abstract 251: Identification of therapeutic vulnerabilities by genome-wide CRISPR knockout library screening of colon cancer organoids

Abstract The promise of precision medicine is based on the idea that genetic alterations present only in tumor cells create vulnerabilities that can be targeted for therapeutic intent. In some instances, somatically mutated driver genes such as KRAS, BRAF or PIK3CA can be targeted with small molecule inhibitors specific for mutant oncoproteins. In other cases therapeutic vulnerability is an indirect consequence, such as response to immune checkpoint inhibitors caused by somatic inactivation of mismatch repair genes and subsequent creation of neo-antigens in tumors. Synthetic lethal vulnerabilities dependent on common somatic alterations would be highly specific therapeutic targets applicable to a large number of cancer patients. We sought to identify potential therapeutic vulnerabilities for colon cancer by performing a genome wide CRISPR knockout screen in a colon cancer organoid in both TP53-Wild-Type and TP53-Knockout backgrounds. We identified 1784 gene knockouts with TP53-dependent effects on Darwinian fitness. Examples include MDM2 and PPM1D knockouts, both of which selectively harmed the TP53 WT organoids. 250 gene knockouts selectively harmed the TP53 KO organoids and represent novel avenues for development of TP53 synthetic lethal targeted therapeutics. We also identified ~1000 gene knockouts under significant negative Darwinian selection in both TP53 WT and TP53 KO organoids which are not known to be common essential genes. Several of these organoid model specific vulnerabilities are in pathways that are downstream of the somatic mutations present in the tumor and are therefore candidates for highly-specific targeted therapeutic intervention. Examples include Werner’s pathway dependencies (WRN, EME1, MUS81) which are known to result from mismatch repair deficiency, WNT pathway dependencies (CTNNB1, PORCN) which result from RNF43 inactivating somatic mutation, and BRAF dependency resulting from its own activating oncogenic somatic mutation. In summary, genome-wide CRISPR knockout library screening of human colon cancer organoids can provide a comprehensive overview of dependencies interacting with either natural or engineered mutations and are a promising novel platform for discovering personalized or pathway based therapeutic targets. Future work will focus on target validation and additional primary screens using our biobank of tumor-normal organoid pairs. Citation Format: Phillip J. Buckhaults, Sana Khalili, Carolyn Banister, Prashanth R. Gokare, Dave Pocalyko, Kurtis Bachman. Identification of therapeutic vulnerabilities by genome-wide CRISPR knockout library screening of colon cancer organoids [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 251.

Abstract 6089: Identification of novel YAP/TAZ pathway regulators in the triple-negative breast cancer cell line MDA-MB231 using single-cell CRISPR screening

Abstract The Hippo signaling cascade is a major pathway that integrates a broad spectrum of mechanosensory signals at the plasma membrane and regulates response via control of cell proliferation, self-renewal, differentiation, and apoptosis. Dysregulation of this pathway has been observed across a range of cancer types and results in an altered activity of its primary downstream effectors, the oncogenic transcription factors YAP/TAZ. For example, both germline and somatic loss-of-function mutations in the tumor suppressor gene NF2, a component of Hippo, induce hyperactivation of YAP/TAZ, transcriptional changes and ultimately result in tumor growth. The Hippo signaling pathway is an attractive target for drug discovery efforts, however, it is highly complex and still incompletely understood. Hence it is indispensable to get a deeper insight into the Hippo - YAP/TAZ signaling axis. To this end, we performed a genome-wide CRISPR knockout screen in the triple-negative breast cancer (TNBC) cell line MDA-MB231 (NF2LOF) expressing a YAP/TAZ reporter construct. We identified both negative and positive regulators of YAP/TAZ in breast cancer cells. In a second step, screening hits were further characterized in a focused single-cell CRISPR screen (Perturb-Seq), aiming at better understanding of the effects on YAP/TAZ activity regulation and downstream effects on gene expression. Here we present the technical details of our screening approaches and the results of perturbing known and novel regulators of YAP/TAZ on single cell level. We discuss the use of Perturb-Seq in the initial validation of hits from genome-wide screens and provide data that may serve as a basis for future drug discovery efforts, seeking for novel and effective treatments for triple-negative breast cancers and other malignancies with Hippo pathway alterations. Citation Format: Mareike Berlak, Zuzanna Makowska, Filippos Klironomos, Julia Kuehnlenz, Atanas Kamburov, Andreas Steffen, Martin Lange, Barbara Nicke, Ralf Lesche, Peter Staller, Charlotte Kopitz, Jan Naujoks. Identification of novel YAP/TAZ pathway regulators in the triple-negative breast cancer cell line MDA-MB231 using single-cell CRISPR screening [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6089.

Deletion of the transcriptional regulator TFAP4 accelerates c-MYC-driven lymphomagenesis

Many lymphoid malignancies arise from deregulated c-MYC expression in cooperation with additional genetic lesions. While many of these cooperative genetic lesions have been discovered and their functions characterised, DNA sequence data of primary patient samples suggest that many more do exist. However, the nature of their contributions to c-MYC driven lymphomagenesis have not yet been investigated. We identified TFAP4 as a potent suppressor of c-MYC driven lymphoma development in a previous genome-wide CRISPR knockout screen in primary cells in vivo [1]. CRISPR deletion of TFAP4 in Eµ-MYC transgenic haematopoietic stem and progenitor cells (HSPCs) and transplantation of these manipulated HSPCs into lethally irradiated animals significantly accelerated c-MYC-driven lymphoma development. Interestingly, TFAP4 deficient Eµ-MYC lymphomas all arose at the pre-B cell stage of B cell development. This observation prompted us to characterise the transcriptional profile of pre-B cells from pre-leukaemic mice transplanted with Eµ-MYC/Cas9 HSPCs that had been transduced with sgRNAs targeting TFAP4. This analysis revealed that TFAP4 deletion reduced expression of several master regulators of B cell differentiation, such as Spi1, SpiB and Pax5, which are direct target genes of both TFAP4 and MYC. We therefore conclude that loss of TFAP4 leads to a block in differentiation during early B cell development, thereby accelerating c-MYC-driven lymphoma development.

Survival-based CRISPR genetic screens across a panel of permissive cell lines identify common and cell-specific SARS-CoV-2 host factors

SARS-CoV-2 depends on host cell components for infection and replication. Identification of virus-host dependencies offers an effective way to elucidate mechanisms involved in viral infection and replication. If druggable, host factor dependencies may present an attractive strategy for anti-viral therapy. In this study, we performed genome wide CRISPR knockout screens in Vero E6 cells and four human cell lines including Calu-3, UM-UC-4, HEK-293 and HuH-7 to identify genetic regulators of SARS-CoV-2 infection. Our findings identified only ACE2, the cognate SARS-CoV-2 entry receptor, as a common host dependency factor across all cell lines, while other host genes identified were largely cell line specific, including known factors TMPRSS2 and CTSL. Several of the discovered host-dependency factors converged on pathways involved in cell signalling, immune-related pathways, and chromatin modification. Notably, the chromatin modifier gene KMT2C in Calu-3 cells had the strongest impact in preventing SARS-CoV-2 infection when perturbed.