Abstract LB-112: Unraveling biology and identifying targets with functional genomics approaches using lentiarray crispr libraries

The ability of CRISPR/Cas9, to efficiently and precisely edit a cell’s DNA and introduce a complete genetic knockout, while minimizing off-target effects, offers an improved approach to target identification. Identifying and validating targets that underlie disease mechanisms still remains a significant challenge in the drug discovery and development process. The transcription factor NF-κB is a crucial player in many aspects of cancer initiation and progression. Here we demonstrate a knock-out screening approach that utilizes an arrayed CRISPR library to interrogate the impact of individual gene knock-outs on the NF-κB pathway in a cervical carcinoma cell line as measured by a functional cell-based assay. We describe the library design concepts, the assay development, screening results and validation of specific identified hits. We tested the approach using a library that targets the human kinome and developed a loss-of-function assay using CellSensor™ NF-κB-bla ME-180 cell line, which stably expresses Cas9. The CellSensor™ reporter assay enables easy identification of genomic targets associated with the NF-κB pathway. We elucidate the key factors in developing a robust assay including both transduction and assay optimization to achieve the highest levels of transduction efficiency and assay window. Using the optimized parameters, we screened the LentiArray CRISPR human kinome library that targets >800 kinases and demonstrate how we followed-up on and validated a subset of the identified hits. The hits were further validated by confirming nucleotide(s) deletion/ insertion in target DNA and additional screening using target specific siRNA and chemical compounds. The ability to scale this approach through the generation of genome wide CRISPR libraries, coupled with the use of lentiviral delivery methods enables high-throughput (HTP) loss-of-function screens to be performed rapidly and identify genes whose activity is important for the specific endpoint being measured. These screenings can provide a wealth of data on the normal functioning of a target and in turn, should yield better validated targets for progression into full drug discovery. Ultimately, the hits from the HTP screenings can be followed up by using CRISPR technology to generate animal knockout models that would support translating the screen to the pre-clinical trials. This in turn could provide better correlation to the clinical setting and thereby reduce candidate drug attrition.

Citation Format: Chetana M. Revankar, Julia Braun, Jian-Ping Yang, Justin Wetter, Tufan Gokirmak, Namritha Ravinder, Jon Chesnut, David Piper. Unraveling biology and identifying targets with functional genomics approaches using lentiarray crispr libraries [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-112.

Abstract 4499: Genomic editing of EWS-FLI1 and its targets, and its therapeutic potential in treatment of Ewing sarcoma

Ewing sarcoma (ES) is an aggressive bone and soft tissue tumor. These tumors have a low mutational burden and are mainly driven by chimeric transcription factor, EWS-FLI1 or equivalent fusion gene. Tumor dependency on EWS-FLI1 makes it an ideal therapeutic target but targeting this disordered protein has proven to be a challenge. Here we show that CRISPR-Cas9-mediated genomic editing can a) be employed to effectively target EWS-FLI1 and its target genes, both coding and non-coding, and b) be safely delivered to tumors in mice, thus highlighting its potential as a therapeutic agent. All CRISPR-Cas9 reagents (ThermoFisher Scientific) including Cas9 protein, single guide RNA (sgRNA) directed against various gene targets and controls, and LentiArrayTM lentiviruses were obtained and optimized for in vitro and in vivo use. Cas9-sgRNA ribonucleoprotein (RNP) complexes were transfected using CRISPRMAXTM or delivered to ES cells by CD99-targeted nanoparticles. We were able to achieve about 70% genomic cleavage when we targeted the fusion gene, EWS-FLI1, and its coding target, NR0B1, using two different sgRNA delivered as RNP complexes. Loss of EWS-FLI1 slowed down cell growth, but increased cell adhesion, and cell invasion. For EWS-FLI1 target, long non-coding RNA, FEZF1-AS1, we used paired sgRNA targeting two different regions of the RNA to achieve effective genomic editing (70%) and loss of transcript expression. We further used a lentiviral-generated stable Cas9 expressing A673 cell line to effectively knockout FEZF1-AS1 using lentiviruses expressing paired sgRNA. Both the RNP and lentiviral mediated FEZF1-AS1 knockdown did not affect cell growth but significantly decreased cell invasion. Dual knockdown of EWS-FLI1 and FEZF1-AS1 decreased cell growth and cell invasion. Mice with ES tumors in their flanks were treated with intravenous injections of CD99-targeted nanoparticles carrying Cas9-sgRNA RNP. Mice injected with Cas9-scrambled gRNA RNP had regular tumor growth. Cas9-EWSR1 sgRNA RNP-treated mice had nearly completely suppressed tumor growth for the duration of treatment but failed to totally eliminate the tumors. In conclusion, we show that CRISPR-Cas9 mediated genomic editing can effectively target EWS-FLI1 and disrupt its function in ES pathogenesis. We further demonstrate that modified CRISPR strategy can effectively knockout non-coding RNA and that the combined targeting of EWS-FLI1 and its lncRNA target had an added therapeutic response in controlling tumor growth and invasion. The pilot animal studies (more detailed studies underway) emphasize the importance of developing CRISPR-Cas9 as a therapeutic tool provided we can safely deliver it in vivo using vehicles like targeted nanoparticles to help reduce toxicity caused by untoward targeting effects.

Citation Format: Sheetal A. Mitra, Namritha Ravinder, Veronica Magnon, Jon Nagy, Timothy J. Triche. Genomic editing of EWS-FLI1 and its targets, and its therapeutic potential in treatment of Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4499.

Site-specific phosphorylation and caspase cleavage of GFAP are new markers of Alexander disease severity

Alexander disease (AxD) is a fatal neurodegenerative disorder caused by mutations in glial fibrillary acidic protein (GFAP), which supports the structural integrity of astrocytes. Over 70 GFAP missense mutations cause AxD, but the mechanism linking different mutations to disease-relevant phenotypes remains unknown. We used AxD patient brain tissue and induced pluripotent stem cell (iPSC)-derived astrocytes to investigate the hypothesis that AxD-causing mutations perturb key post-translational modifications (PTMs) on GFAP. Our findings reveal selective phosphorylation of GFAP-Ser13 in patients who died young, independently of the mutation they carried. AxD iPSC-astrocytes accumulated pSer13-GFAP in cytoplasmic aggregates within deep nuclear invaginations, resembling the hallmark Rosenthal fibers observed in vivo. Ser13 phosphorylation facilitated GFAP aggregation and was associated with increased GFAP proteolysis by caspase-6. Furthermore, caspase-6 was selectively expressed in young AxD patients, and correlated with the presence of cleaved GFAP. We reveal a novel PTM signature linking different GFAP mutations in infantile AxD.

Abstract LB-116: Functional genomics screening using LentiArray™ CRISPR libraries and CellSensor™ assays

Identifying and validating targets that underlie disease mechanisms and can be addressed to provide efficacious therapies remains a significant challenge in the drug discovery and development process. Use of siRNA and shRNA to knock-down RNA and suppress gene function, have provided insights into mechanism of action, but depending on the nature of the targets, cells, biology and end-point assays these approaches may suffer variously from their transient nature, design complexity, incomplete knock-down or off-target effects. The use of CRISPR (clustered regularly interspaced short palindromic repeat)-associated Cas9 nuclease and guide RNA (gRNA) provides a strong alternative that can produce long-lasting impact, straightforward design, knock-out of genes and increased specificity. A number of laboratories have already published reports demonstrating how pools of gRNA can be delivered to cells and “hits” can be established through enrichment or depletion of cells following a “survival” assay and identified by sequencing the introduced gRNAs in the remaining cell population. Here we demonstrate a knock-out screening approach that utilizes the Invitrogen™ LentiArray™ CRISPR library to interrogate the impact of individual gene knock-outs on the NFκB pathway as measured by a functional cell-based assay. We describe the library design concepts, the assay development, initial screening results and validation of specific identified hits. The gRNAs are designed to primarily 5’ coding exons of a target gene using our CRISPR design tool to maximize knock-out efficiency and minimize off-target effects. Each gRNA is delivered as a separate lentiviral particle including an antibiotic-resistant marker and each gene is targeted by 4 gRNAs per well, delivered in a 96-well plate. We tested the approach using a library that targets the human kinome and developed a loss-of-function assay using our CellSensor® NF-κB-bla ME180 cell line, which is based on the ratiometric blue/green reporter assay and easily enables identification of genomic targets associated with the NF-κB pathway. We elucidate the key factors in developing a robust assay including both transduction and assay optimization to achieve the highest levels of transduction efficiency and assay window. Using these optimized parameters, we screened the Invitrogen™ LentiArray™ CRISPR kinome library that targets >800 kinases and demonstrate how we followed-up on and validated a subset of the identified hits. We expect these approaches to be scalable to the entire human genome and portable to multiple cell types and end-point assays including both high-throughput plate-based assays and high-content imaging based assays.

Citation Format: Chetana M. Revankar, Justin Wetter, Julia Braun, Natasha Roark, Veronica Magnon, LaiYee Wong, Yanfei Zou, Namritha Ravinder, Jian-Ping Yang, Jonathan Chesnut, David Piper. Functional genomics screening using LentiArray™ CRISPR libraries and CellSensor™ assays [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-116. doi:10.1158/1538-7445.AM2017-LB-116

Enhanced CRISPR/Cas9-mediated precise genome editing by improved design and delivery of gRNA, Cas9 nuclease, and donor DNA

While CRISPR-based gene knock out in mammalian cells has proven to be very efficient, precise insertion of genetic elements via the cellular homology directed repair (HDR) pathway remains a rate-limiting step to seamless genome editing. Under the conditions described here, we achieved up to 56% targeted integration efficiency with up to a six-nucleotide insertion in HEK293 cells. In induced pluripotent stem cells (iPSCs), we achieved precise genome editing rates of up to 45% by co-delivering the Cas9 RNP and donor DNA. In addition, the use of a short double stranded DNA oligonucleotide with 3' overhangs allowed integration of a longer FLAG epitope tag along with a restriction site at rates of up to 50%. We propose a model that favors the design of donor DNAs with the change as close to the cleavage site as possible. For small changes such as SNPs or short insertions, asymmetric single stranded donor molecules with 30 base homology arms 3' to the insertion/repair cassette and greater than 40 bases of homology on the 5' end seems to be favored. For larger insertions such as an epitope tag, a dsDNA donor with protruding 3' homology arms of 30 bases is favored. In both cases, protecting the ends of the donor DNA with phosphorothioate modifications improves the editing efficiency.

Abstract LB-105: High-throughput target identification using CRISPR/Cas9

One of the major challenges of the drug discovery process is the identification of novel, validated targets, whose pharmacological modulation may yield the desired therapeutic outcomes. The use of CRISPR (clustered regularly interspaced short palindromic repeat)-associated Cas9 nuclease and guide RNA can potentially impact the entire drug discovery process from generation of models based on clinical findings through to the target identification. The ability of CRISPR/Cas9, to efficiently and precisely edit a cell's DNA and introduce a complete genetic knockout, while minimizing off-target effects, offers an improved approach to target identification. Moreover, the ability to scale this approach through the generation of genome wide CRISPR libraries, coupled with the use of lentiviral delivery methods enables high-throughput (HTP) loss-of-function screens to be performed rapidly and identify genes whose activity is important for the specific endpoint being measured. For example, in this study, we have used CellSensor® cell lines targeting various signaling pathways (AP-1, c-Fos and NF-kB) to identify key targets critical to these pathways. Transcription factors like AP-1, c-Fos and NF-kB have shown to play an important role in cancer initiation and progression. Using the key signaling molecules along these pathways as controls we have data that supports HTP target identification. Furthermore, we will present our results from screening of a subset of a CRISPR library targeting 160 different kinases. Eventually, we plan to screen the entire CRISPR library targeting 750 kinases with 4 gRNA per gene against each of these signaling pathways. Similar screenings can be performed using other functional assay formats, like the cell health assays or high content imaging. These screenings can provide a wealth of data on the normal functioning of a target and in turn, should yield better validated targets for progression into full drug discovery. Ultimately, the hits from the HTP screenings can be followed up by using CRISPR technology to generate animal knockout models that would support translating the screen to the pre-clinical trials. This in turn could provide better correlation to the clinical setting and thereby reduce candidate drug attrition.

Citation Format: Chetana M. Revankar, Julia Braun, LaiYee Wong, Justin Wetter, Jian-Ping Yang, Namritha Ravinder, Jon Chesnut, David Piper. High-throughput target identification using CRISPR/Cas9. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-105.

Improved delivery of Cas9 protein/gRNA complexes using lipofectamine CRISPRMAX

Objectives

To identify the best lipid nanoparticles for delivery of purified Cas9 protein and gRNA complexes (Cas9 RNPs) into mammalian cells and to establish the optimal conditions for transfection.

Results

Using a systematic approach, we screened 60 transfection reagents using six commonly-used mammalian cell lines and identified a novel transfection reagent (named Lipofectamine CRISPRMAX). Based on statistical analysis, the genome modification efficiencies in Lipofectamine CRISPRMAX-transfected cell lines were 40 or 15 % higher than those in Lipofectamine 3000 or RNAiMAX-transfected cell lines, respectively. Upon optimization of transfection conditions, we observed 85, 75 or 55 % genome editing efficiencies in HEK293FT cells, mouse ES cells, or human iPSCs, respectively. Furthermore, we were able to co-deliver donor DNA with Cas9 RNPs into a disrupted EmGFP stable cell line, resulting in the generation of up to 17 % EmGFP-positive cells.

Conclusion

Lipofectamine CRISPRMAX was characterized as the best lipid nanoparticles for the delivery of Cas9 RNPs into a variety of mammalian cell lines, including mouse ES cells and iPSCs.

Electronic supplementary material

The online version of this article (doi:10.1007/s10529-016-2064-9) contains supplementary material, which is available to authorized users.

Rapid and highly efficient mammalian cell engineering via Cas9 protein transfection

CRISPR-Cas9 systems provide a platform for high efficiency genome editing that are enabling innovative applications of mammalian cell engineering. However, the delivery of Cas9 and synthesis of guide RNA (gRNA) remain as steps that can limit overall efficiency and ease of use. Here we describe methods for rapid synthesis of gRNA and for delivery of Cas9 protein/gRNA ribonucleoprotein complexes (Cas9 RNPs) into a variety of mammalian cells through liposome-mediated transfection or electroporation. Using these methods, we report nuclease-mediated indel rates of up to 94% in Jurkat T cells and 87% in induced pluripotent stem cells (iPSC) for a single target. When we used this approach for multigene targeting in Jurkat cells we found that two-locus and three-locus indels were achieved in approximately 93% and 65% of the resulting isolated cell lines, respectively. Further, we found that the off-target cleavage rate is reduced using Cas9 protein when compared to plasmid DNA transfection. Taken together, we present a streamlined cell engineering workflow that enables gRNA design to analysis of edited cells in as little as four days and results in highly efficient genome modulation in hard-to-transfect cells. The reagent preparation and delivery to cells is amenable to high throughput, multiplexed genome-wide cell engineering.

Identification of PTM5 protein interaction partners, a MADS-box gene involved in aspen tree vegetative development

In a past article, our lab described the identification and characterization of a novel vegetative MADS-box gene from quaking aspen trees, Populus tremuloides MADS-box 5 (PTM5). PTM5 was shown to be a member of the SOC1/TM3 class of MADS-box genes with a seasonal expression pattern specific to developing vascular tissues including the vascular cambium, the precursor to all woody branches, stems, and roots. Since the proper function of MADS-box proteins is dependent on specific interactions with other regulatory proteins, we further examined PTM5 protein-protein interactions as a means to better understand its function. Through yeast two-hybrid analyses, it was demonstrated that, like other SOC1/TM3 class proteins, PTM5 is capable of interacting with itself as well as other MADS-box proteins from aspen. In addition, yeast two-hybrid library screening revealed that PTM5 interacts with two non-MADS proteins, an actin depolymerizing factor (PtADF) and a novel leucine-rich repeat protein (PtLRR). In situ RNA localization was used to verify the overlapping expression patterns of these genes, and transgenic studies showed that over-expression of PTM5 in aspen causes alterations in root vasculature and root biomass development consistent with the cell growth and expansion functions of related ADF and LRR genes. These results suggest that the interaction of vegetative MADS-box genes with specific protein cofactors is a key step in the mechanisms that control woody tissue development in trees.

SEP-class genes in Populus tremuloides and their likely role in reproductive survival of poplar trees

One of the most important processes to the survival of a species is its ability to reproduce. In plants, SEPALLATA-class MADS-box genes have been found to control the development of the inner whorls of flowers. However, while much is known about floral development in herbaceous plants, similar systems in woody trees remain poorly understood. Populus tremuloides (trembling aspen) is a widespread North American tree having important economic value, and its floral development differs from that of well-studied species in that the flowers have only two whorls and are truly unisexual. Sequence based analyses indicate that PTM3 (Populus tremuloides MADS-box 3), and a duplicate gene PTM4, are related to the SEPALLATA1-and 2-class of MADS-box genes. Another gene, PTM6, is related to SEP3, and each of these genes has a counterpart in the poplar genomic database along with additional members of the A, B, C, D, and E-classes of MADS-box genes. PTM3/4 and 6 are expressed in all stages of male and female aspen floral development. However, PTM3/4 is also expressed in the terminal buds, young leaves, and young stems. In situ RNA localization identified PTM3/4 and 6 transcripts predominantly in the inner, sexual whorl, within developing ovules of female flowers and anther primordia of male flowers. Tree researchers often use heterologous systems to help study tree floral development due to the long juvenile periods found in most trees. We found that the participation of PTM3/4 in floral development is supported by transgenic experiments in both P. tremuloides and heterologous systems such as tobacco and Arabidopsis. However, phenotypic artifacts were observed in the heterologous systems. Together the results suggest a role for poplar SEP-class genes in reproductive viability.