EditR: A Method to Quantify Base Editing from Sanger Sequencing

CRISPR-Cas9-Cytidine deaminase fusion enzymes—termed “base editors”—allow targeted editing of genomic deoxycytidine to deoxythymidine (C:G→T:A) without the need for double-stranded break induction. Base editors represent a paradigm shift in gene editing technology due to their unprecedented efficiency to mediate targeted, single-base conversion. However, current analysis of base editing outcomes rely on methods that are either imprecise or expensive and time-consuming. To overcome these limitations, we developed a simple, cost-effective, and accurate program to measure base editing efficiency from fluorescence-based Sanger sequencing, termed “EditR.” We provide EditR as a free online tool or downloadable desktop application requiring a single Sanger sequencing file and guide RNA sequence. EditR is more accurate than enzymatic assays, and provides added insight to the position, type, and efficiency of base editing. Furthermore, EditR is likely amenable to quantify base editing from the recently developed adenosine deaminase base editors that act on either DNA (adenosine deaminase base editors [ABEs]) or RNA (REPAIRs) (catalyzes A:T→G:C). Collectively, we demonstrate that EditR is a robust, inexpensive tool that will facilitate the broad application of base editing technology, thereby fostering further innovation in this burgeoning field.

A Genetically Engineered Primary Human Natural Killer Cell Platform for Cancer Immunotherapy

Enhancing natural killer (NK) cell cytotoxicity by blocking inhibitory signaling could lead to improved NK-based cancer immunotherapy. Thus, we have developed a highly efficient method for editing the genome of human NK cells using CRISPR/Cas9 to knock out inhibitory signaling molecules. Our method efficiently edits up to 90% of primary peripheral blood NK cells. As a proof-of-principle we demonstrate highly efficient knockout of ADAM17 and PDCD1, genes that have a functional impact on NK cells, and demonstrate that these gene-edited NK cells have significantly improved activity, cytokine production, and cancer cell cytotoxicity. Furthermore, we were able to expand cells to clinically relevant numbers, without loss of activity. We also demonstrate that our CRISPR/Cas9 method can be used for efficient knockin of genes by delivering homologous recombination template DNA using recombinant adeno-associated virus serotype 6 (rAAV6). Our platform represents a feasible method for generating engineered primary NK cells as a universal therapeutic for cancer immunotherapy.

CRISPR-Cas9 cytidine and adenosine base editing of splice-sites mediates highly-efficient disruption of proteins in primary and immortalized cells

CRISPR-Cas9 cytidine and adenosine base editors (CBEs and ABEs) can disrupt genes without introducing double-stranded breaks by inactivating splice sites (BE-splice) or by introducing premature stop (pmSTOP) codons. However, no in-depth comparison of these methods or a modular tool for designing BE-splice sgRNAs exists. To address these needs, we develop SpliceR ( http://z.umn.edu/spliceR ) to design and rank BE-splice sgRNAs for any Ensembl annotated genome, and compared disruption approaches in T cells using a screen against the TCR-CD3 MHC Class I immune synapse. Among the targeted genes, we find that targeting splice-donors is the most reliable disruption method, followed by targeting splice-acceptors, and introducing pmSTOPs. Further, the CBE BE4 is more effective for disruption than the ABE ABE7.10, however this disparity is eliminated by employing ABE8e. Collectively, we demonstrate a robust method for gene disruption, accompanied by a modular design tool that is of use to basic and translational researchers alike.

Highly multiplexed genome engineering using CRISPR/Cas9 gRNA arrays

The CRISPR/Cas9 system is an RNA guided nuclease system that evolved as a mechanism of adaptive immunity in bacteria. This system has been adopted for numerous genome engineering applications in research and recently, therapeutics. The CRISPR/Cas9 system has been largely implemented by delivery of Cas9 as protein, RNA, or plasmid along with a chimeric crRNA-tracrRNA guide RNA (gRNA) under the expression of a pol III promoter, such as U6. Using this approach, multiplex genome engineering has been achieved by delivering several U6-gRNA plasmids targeting multiple loci. However, this approach is limited due to the efficiently of delivering multiple plasmids to a single cell at one time. To augment the capability and accessibility of multiplexed genome engineering, we developed an efficient golden gate based method to assemble gRNAs linked by optimal Csy4 ribonuclease sequences to deliver up to 10 gRNAs as a single gRNA array transcript. Here we report the optimal expression of our guide RNA array under a strong pol II promoter. This system can be implemented alongside the myriad of CRISPR applications, allowing users to model complex biological processes requiring numerous gRNAs.

MultiEditR: The first tool for the detection and quantification of RNA editing from Sanger sequencing demonstrates comparable fidelity to RNA-seq

We present MultiEditR (Multiple Edit Deconvolution by Inference of Traces in R), the first algorithm specifically designed to detect and quantify RNA editing from Sanger sequencing (z.umn.edu/multieditr). Although RNA editing is routinely evaluated by measuring the heights of peaks from Sanger sequencing traces, the accuracy and precision of this approach has yet to be evaluated against gold standard next-generation sequencing methods. Through a comprehensive comparison to RNA sequencing (RNA-seq) and amplicon-based deep sequencing, we show that MultiEditR is accurate, precise, and reliable for detecting endogenous and programmable RNA editing.

C-to-U RNA Editing: From Computational Detection to Experimental Validation

The AID/APOBEC family of enzymes are cytidine deaminases that act upon DNA and RNA. Among APOBECs, the best characterized family member to act on RNA is the enzyme APOBEC1. APOBEC1-mediated RNA editing plays a key role in lipid metabolism and in maintenance of brain homeostasis. Editing can be easily detected in RNA-seq data as a cytosine to thymine (C-to-T) change with regard to the reference. However, there are many other sources of base conversions relative to reference, such as PCR errors, SNPs, and even DNA editing by mutator APOBECs. Furthermore, APOBEC1 exhibits disparate activity in different cell types, with respect to which transcripts are edited and the level to which they are edited. When considering these potential sources of error and variability, an RNA-seq comparison between wild-type APOBEC1 sample and a matched control with an APOBEC1 knockout is a reliable method for the discrimination of true sites edited by APOBEC1. Here we present a detailed description of a method for studying APOBEC1 RNA editing, specifically in the murine macrophage cell line RAW 264.7. Our method covers the production of an APOBEC1 knockout cell line using the CRISPR/Cas9 system, through to experimental validation and quantification of editing sites (where we discuss a recently published algorithm (termed MultiEditR) which allows for the detection and quantification of RNA editing from Sanger sequencing). Importantly, this same protocol can be adapted to any RNA modification detectable by RNA-seq analysis for which the responsible protein is known.

Signaling via a CD27-TRAF2-SHP-1 axis during naive T cell activation promotes memory-associated gene regulatory networks

The interaction of the tumor necrosis factor receptor (TNFR) family member CD27 on naive CD8+ T (Tn) cells with homotrimeric CD70 on antigen-presenting cells (APCs) is necessary for T cell memory fate determination. Here, we examined CD27 signaling during Tn cell activation and differentiation. In conjunction with T cell receptor (TCR) stimulation, ligation of CD27 by a synthetic trimeric CD70 ligand triggered CD27 internalization and degradation, suggesting active regulation of this signaling axis. Internalized CD27 recruited the signaling adaptor TRAF2 and the phosphatase SHP-1, thereby modulating TCR and CD28 signals. CD27-mediated modulation of TCR signals promoted transcription factor circuits that induced memory rather than effector associated gene programs, which are induced by CD28 costimulation. CD27-costimulated chimeric antigen receptor (CAR)-engineered T cells exhibited improved tumor control compared with CD28-costimulated CAR-T cells. Thus, CD27 signaling during Tn cell activation promotes memory properties with relevance to T cell immunotherapy.

Abstract A036: Multiplex human T-cell engineering by Cas9 base editor technology

Chimeric antigen receptor (CAR)-engineered T-cells have mediated impressive outcomes in a subset of hematologic malignancies, yet this therapy remains highly personalized and largely ineffective against solid tumors. Genome editing strategies using targeted nucleases could overcome these limitations and have begun to enter clinical application. Multiplex editing strategies to develop off-the- shelf therapies are of high interest but remain limited by concerns of off-target effects and chromosomal translocations formed by simultaneous double-strand break (DSB) induction at multiple loci. The risk of genotoxic side effects is amplified when combining multiplex DSB induction with randomly integrating platforms for antigen-specific receptor delivery. An ideal strategy would allow for multigene disruption and targeted integration of antigen-specific receptors without introduction of multiple genomic DSBs. To this end, we evaluated the application of third- and fourth-generation Cas9 base editor technologies for gene disruption and integration in primary human T-cells. Chemically modified gRNAs and Cas9 base editor mRNA were delivered to stimulated T-cells by electroporation, followed by viral transduction for delivery of a DNA repair template as recombinant adeno-associated virus (rAAV). Base editing efficiencies were determined on the genomic level by PCR amplification, Sanger sequencing, and analysis of resultant traces using the EditR web app. Gene knock-out and knock-in efficiencies were analyzed on the protein level by flow cytometry. Through systematic reagent and dose optimization efforts, we achieved highly efficient C>T base conversion and consequent protein knockout at multiple therapeutically relevant loci including TRAC (KO = 83.6 ± 3.3%), PD-1 (KO = 78.6 ± 2.3%), and B2M (KO = 80 ± 1.8%). We observed that fourth-generation base editor (BE4) achieved consistently higher C>T conversion rates with reduced non-canonical editing (i.e., C>A/G) compared to third-generation base editor (BE3). Targeted disruption of splice acceptor (SA) and splice donor (SD) sites resulted in higher frequency of protein knockout vs. induction of premature stop codons at all loci examined. Importantly, while multiplex editing using Cas9 nuclease resulted in detectable translocations between the targeted sites, we were unable to detect these translocations using BE3 and BE4 as measured by PCR. Finally, we exploited the single-strand nickase function of the base editors in conjunction with rAAV delivery to achieve simultaneous targeted integration of a gene expression cassette at the AAVS1 safe harbor locus. Staphylococcus aureus Cas9 (SaCas9) was also employed as an orthogonal approach to AAVS1 targeting, and yielded higher rates of donor integration. Collectively, we demonstrate that Cas9 base editor technology can be utilized to mediate efficient, multiplex gene disruption and targeted gene integration in primary human T-cells without associated translocations. This streamlined approach to genome engineering may be broadly applied for the development of safe and effective cell therapies.

Citation Format: Cara Lonetree, Beau R. Webber, Miechaleen D. Diers, Walker S. Lahr, Mitchell Kluesner, Mark J. Osborn, Matthew J. Johnson, Branden S. Moriarity. Multiplex human T-cell engineering by Cas9 base editor technology [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A036.

Developing Bottom-Up Induced Pluripotent Stem Cell Derived Solid Tumor Models Using Precision Genome Editing Technologies

Advances in genome and tissue engineering have spurred significant progress and opportunity for innovation in cancer modeling. Human induced pluripotent stem cells (iPSCs) are an established and powerful tool to study cellular processes in the context of disease-specific genetic backgrounds; however, their application to cancer has been limited by the resistance of many transformed cells to undergo successful reprogramming. Here, we review the status of human iPSC modeling of solid tumors in the context of genetic engineering, including how base and prime editing can be incorporated into "bottom-up" cancer modeling, a term we coined for iPSC-based cancer models using genetic engineering to induce transformation. This approach circumvents the need to reprogram cancer cells while allowing for dissection of the genetic mechanisms underlying transformation, progression, and metastasis with a high degree of precision and control. We also discuss the strengths and limitations of respective engineering approaches and outline experimental considerations for establishing future models.

Efficient multiplex non-viral engineering and expansion of polyclonal γδ CAR-T cells for immunotherapy

Gamma delta (γδ) T cells are defined by their unique ability to recognize a limited repertoire of non-peptide, non-MHC-associated antigens on transformed and pathogen-infected cells. In addition to their lack of alloreactivity, γδ T cells exhibit properties distinct from other lymphocyte subsets, prompting significant interest in their development as an off-the-shelf cellular immunotherapeutic. However, their low abundance in circulation, heterogeneity, limited methods for ex vivo expansion, and under-developed methodologies for genetic modification have hindered basic study and clinical application of γδ T cells. Here, we implement a feeder-free, scalable approach for ex vivo manufacture of polyclonal, non-virally modified, gene edited chimeric antigen receptor (CAR)-γδ T cells in support of therapeutic application. Engineered CAR-γδ T cells demonstrate high function in vitro and and in vivo. Longitudinal in vivo pharmacokinetic profiling of adoptively transferred polyclonal CAR-γδ T cells uncover subset-specific responses to IL-15 cytokine armoring and multiplex base editing. Our results present a robust platform for genetic modification of polyclonal CAR-γδ T cells and present unique opportunities to further define synergy and the contribution of discrete, engineered CAR-γδ T cell subsets to therapeutic efficacy in vivo.

Design of sensitive monospecific and bispecific synthetic chimeric T cell receptors for cancer therapy

The adoptive transfer of T cells expressing chimeric antigen receptors (CARs) is effective in B cell malignancies. However, the persistence of cancer cells with low levels or complete absence of the target antigen, thereby evading detection by CAR T cells, leads to relapse. These evasion mechanisms highlight the need for receptors with enhanced sensitivity and multispecificity. We introduce a synthetic chimeric T cell receptor (ChTCR) that confers superior antigen sensitivity compared with CARS and previous hybrid TCR designs and is readily adapted for bispecific targeting. ChTCRs replicate the structure of natural TCRs, form classical immune synapses and demonstrate TCR-like signaling. T cells expressing bispecific ChTCRs (Bi-ChTCRs) are more effective than bispecific CAR T cells in eradicating tumors with heterogeneous antigen expression in vivo in female mice. The Bi-ChTCR architecture is resilient and can be designed to target pairs of B cell and multiple myeloma antigens. These findings provide a widely applicable strategy to combat tumor heterogeneity and prevent relapse.

Correction of Fanconi Anemia Mutations Using Digital Genome Engineering

Fanconi anemia (FA) is a rare genetic disease in which genes essential for DNA repair are mutated. Both the interstrand crosslink (ICL) and double-strand break (DSB) repair pathways are disrupted in FA, leading to patient bone marrow failure (BMF) and cancer predisposition. The only curative therapy for the hematological manifestations of FA is an allogeneic hematopoietic cell transplant (HCT); however, many (>70%) patients lack a suitable human leukocyte antigen (HLA)-matched donor, often resulting in increased rates of graft-versus-host disease (GvHD) and, potentially, the exacerbation of cancer risk. Successful engraftment of gene-corrected autologous hematopoietic stem cells (HSC) circumvents the need for an allogeneic HCT and has been achieved in other genetic diseases using targeted nucleases to induce site specific DSBs and the correction of mutated genes through homology-directed repair (HDR). However, this process is extremely inefficient in FA cells, as they are inherently deficient in DNA repair. Here, we demonstrate the correction of FANCA mutations in primary patient cells using ‘digital’ genome editing with the cytosine and adenine base editors (BEs). These Cas9-based tools allow for C:G > T:A or A:T > C:G base transitions without the induction of a toxic DSB or the need for a DNA donor molecule. These genetic corrections or conservative codon substitution strategies lead to phenotypic rescue as illustrated by a resistance to the alkylating crosslinking agent Mitomycin C (MMC). Further, FANCA protein expression was restored, and an intact FA pathway was demonstrated by downstream FANCD2 monoubiquitination induction. This BE digital correction strategy will enable the use of gene-corrected FA patient hematopoietic stem and progenitor cells (HSPCs) for autologous HCT, obviating the risks associated with allogeneic HCT and DSB induction during autologous HSC gene therapy.

216 Multiplex base editing of NK cell to enhance cancer immunotherapy

Background

Natural killer (NK) cells have many unique features that have gained attention in cancer immunotherapy. NK cells can kill in antigen independent and dependent fashion, can be used as an allogeneic product, and perform antibody-dependent cell-mediated cytotoxicity (ADCC). However, NK cell function is regulated by many activating and inhibitory receptors, which cancer cells take advantage of to avoid being killed by NK cells. NK cells are also known for their technical and biological challenges which result in low editing efficiencies, compared to T cells and other immune cells.

Methods

Base editing (BE) is a CRISPR-Cas9 based genome editing technology that allows precise single base transitions. Previously, we reported a high efficiency method for multiplex engineering of T cells using BE and thus reasoned that applying similar concepts in NK cells may offer an opportunity to alter many genes simultaneously at higher efficiency through multiplex base editing. We thus selected a panel of genes bearing critical roles in NK cell function for immunotherapy, including inhibitory intracellular regulator AHR and CISH, inhibitory checkpoint receptor KLRG1, TIGIT, KLRC1, and PDCD1, and Fc receptor CD16A. CD16A is responsible for NK cell ADCC and is regulated via cleavage upon NK activation. Non-cleavable CD16A improves ADCC killing and can be achieved through single-base substitution with BE.

Results

Using the adenosine BE (ABE8e), we achieved multiplex editing (6 genes) rates up to 99% and 95% editing/knockout at DNA and protein levels, respectively. Notably, we assessed for reduction in editing efficiency when additional genes were targeted and found no significant reduction in editing efficiencies when targeting up to 6 genes simultaneously. Moreover, functional evaluation of non-cleavable CD16A NK cells revealed up to 35% increase of cytotoxicity against Raji cells.

Conclusions

We were able to achieve high multiplex editing efficiency in primary human NK cells using ABE8eand there was no significant decrease of editing efficiency as the number of gene of interest increases, up to 6 genes in total. Functional assay confirmed increased NK cell cytotoxicity against tumor cells. Our end goal is to achieve high efficiency multiplex editing in CAR-expressing NK cells to further improve NK cell activity and toxicity for cancer immunotherapy.

Reference

Webber B, Lonetree C, Kluesner M, et al. Highly efficient multiplex human T cell engineering without double-strand breaks usingCas9 base editors. Nat Commun 2019;10:5222.

Precision Enhancement of CAR-NK Cells through Non-Viral Engineering and Highly Multiplexed Base Editing

Natural killer (NK) cells' unique ability to kill transformed cells expressing stress ligands or lacking major histocompatibility complexes (MHC) has prompted their development for immunotherapy. However, NK cells have demonstrated only moderate responses against cancer in clinical trials and likely require advanced genome engineering to reach their full potential as a cancer therapeutic. Multiplex genome editing with CRISPR/Cas9 base editors (BE) has been used to enhance T cell function and has already entered clinical trials but has not been reported in human NK cells. Here, we report the first application of BE in primary NK cells to achieve both loss-of-function and gain-of-function mutations. We observed highly efficient single and multiplex base editing, resulting in significantly enhanced NK cell function. Next, we combined multiplex BE with non-viral TcBuster transposon-based integration to generate IL-15 armored CD19 CAR-NK cells with significantly improved functionality in a highly suppressive model of Burkitt's lymphoma both in vitro and in vivo. The use of concomitant non-viral transposon engineering with multiplex base editing thus represents a highly versatile and efficient platform to generate CAR-NK products for cell-based immunotherapy and affords the flexibility to tailor multiple gene edits to maximize the effectiveness of the therapy for the cancer type being treated.