STEEP mediates STING ER exit and activation of signaling.

STING is essential for control of infections and for tumor immunosurveillance, but it can also drive pathological inflammation. STING resides on the endoplasmic reticulum (ER) and traffics following stimulation to the ERGIC/Golgi, where signaling occurs. Although STING ER exit is the rate-limiting step in STING signaling, the mechanism that drives this process is not understood. Here we identify STEEP as a positive regulator of STING signaling. STEEP was associated with STING and promoted trafficking from the ER. This was mediated through stimulation of phosphatidylinositol-3-phosphate (PtdIns(3)P) production and ER membrane curvature formation, thus inducing COPII-mediated ER-to-Golgi trafficking of STING. Depletion of STEEP impaired STING-driven gene expression in response to virus infection in brain tissue and in cells from patients with STING-associated diseases. Interestingly, STING gain-of-function mutants from patients interacted strongly with STEEP, leading to increased ER PtdIns(3)P levels and membrane curvature. Thus, STEEP enables STING signaling by promoting ER exit.

Author Correction: STEEP mediates STING ER exit and activation of signaling.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

ER stress induces caspase-2-tBID-GSDME-dependent cell death in neurons lytically infected with herpes simplex virus type 2.

Neurotropic viruses, including herpes simplex virus (HSV) types 1 and 2, have the capacity to infect neurons and can cause severe diseases. This is associated with neuronal cell death, which may contribute to morbidity or even mortality if the infection is not controlled. However, the mechanistic details of HSV-induced neuronal cell death remain enigmatic. Here, we report that lytic HSV-2 infection of human neuron-like SH-SY5Y cells and primary human and murine brain cells leads to cell death mediated by gasdermin E (GSDME). HSV-2-induced GSDME-mediated cell death occurs downstream of replication-induced endoplasmic reticulum stress driven by inositol-requiring kinase 1α (IRE1α), leading to activation of caspase-2, cleavage of the pro-apoptotic protein BH3-interacting domain death agonist (BID), and mitochondria-dependent activation of caspase-3. Finally, necrotic neurons released alarmins, which activated inflammatory responses in human iPSC-derived microglia. In conclusion, lytic HSV infection in neurons activates an ER stress-driven pathway to execute GSDME-mediated cell death and promote inflammation.

CRISPR activation to characterize splice-altering variants in easily accessible cells.

Genetic variants that affect mRNA splicing are a major cause of hereditary disorders, but the spliceogenicity of variants is challenging to predict. RNA diagnostics of clinically accessible tissues enable rapid functional characterization of splice-altering variants within their natural genetic context. However, this analysis cannot be offered to all individuals as one in five human disease genes are not expressed in easily accessible cell types. To overcome this problem, we have used CRISPR activation (CRISPRa) based on a dCas9-VPR mRNA-based delivery platform to induce expression of the gene of interest in skin fibroblasts from individuals with suspected monogenic disorders. Using this ex vivo splicing assay, we characterized the splicing patterns associated with germline variants in the myelin protein zero gene (MPZ), which is exclusively expressed in Schwann cells of the peripheral nerves, and the spastin gene (SPAST), which is predominantly expressed in the central nervous system. After overnight incubation, CRISPRa strongly upregulated MPZ and SPAST transcription in skin fibroblasts, which enabled splice variant profiling using reverse transcription polymerase chain reaction, next-generation sequencing, and long-read sequencing. Our investigations show proof of principle of a promising genetic diagnostic tool that involves CRISPRa to activate gene expression in easily accessible cells to study the functional impact of genetic variants. The procedure is easy to perform in a diagnostic laboratory with equipment and reagents all readily available.

TLR2 and TLR7 mediate distinct immunopathological and antiviral plasmacytoid dendritic cell responses to SARS-CoV-2 infection.

Understanding the molecular pathways driving the acute antiviral and inflammatory response to SARS-CoV-2 infection is critical for developing treatments for severe COVID-19. Here, we find decreasing number of circulating plasmacytoid dendritic cells (pDCs) in COVID-19 patients early after symptom onset, correlating with disease severity. pDC depletion is transient and coincides with decreased expression of antiviral type I IFNα and of systemic inflammatory cytokines CXCL10 and IL-6. Using an in vitro stem cell-based human pDC model, we further demonstrate that pDCs, while not supporting SARS-CoV-2 replication, directly sense the virus and in response produce multiple antiviral (interferons: IFNα and IFNλ1) and inflammatory (IL-6, IL-8, CXCL10) cytokines that protect epithelial cells from de novo SARS-CoV-2 infection. Via targeted deletion of virus-recognition innate immune pathways, we identify TLR7-MyD88 signaling as crucial for production of antiviral interferons (IFNs), whereas Toll-like receptor (TLR)2 is responsible for the inflammatory IL-6 response. We further show that SARS-CoV-2 engages the receptor neuropilin-1 on pDCs to selectively mitigate the antiviral interferon response, but not the IL-6 response, suggesting neuropilin-1 as potential therapeutic target for stimulation of TLR7-mediated antiviral protection.

Targeted regulation of transcription in primary cells using CRISPRa and CRISPRi.

Targeted transcriptional activation or interference can be induced with the CRISPR-Cas9 system (CRISPRa/CRISPRi) using nuclease-deactivated Cas9 fused to transcriptional effector molecules. These technologies have been used in cancer cell lines, particularly for genome-wide functional genetic screens using lentiviral vectors. However, CRISPRa and CRISPRi have not yet been widely applied to ex vivo cultured primary cells with therapeutic relevance owing to a lack of effective and nontoxic delivery modalities. Here we develop CRISPRa and CRISPRi platforms based on RNA or ribonucleoprotein (RNP) delivery by electroporation and show transient, programmable gene regulation in primary cells, including human CD34+ hematopoietic stem and progenitor cells (HSPCs) and human CD3+ T cells. We show multiplex and orthogonal gene modulation using multiple sgRNAs and CRISPR systems from different bacterial species, and we show that CRISPRa can be applied to manipulate differentiation trajectories of HSPCs. These platforms constitute simple and effective means to transiently control transcription and are easily adopted and reprogrammed to new target genes by synthetic sgRNAs. We believe these technologies will find wide use in engineering the transcriptome for studies of stem cell biology and gene function, and we foresee that they will be implemented to develop and enhance cellular therapeutics.

CRISPR-Cas-mediated transcriptional modulation: The therapeutic promises of CRISPRa and CRISPRi.

The CRISPR-Cas system is commonly known for its ability to cleave DNA in a programmable manner, which has democratized gene editing and facilitated recent breakthroughs in gene therapy. However, newer iterations of the technology using nuclease-disabled Cas enzymes have spurred a variety of different types of genetic engineering platforms such as transcriptional modulation using the CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) systems. This review introduces the creation of these programmable transcriptional modulators, various methods of delivery utilized for these systems, and recent technological developments. CRISPRa and CRISPRi have also been implemented in genetic screens for interrogating gene function and discovering genes involved in various biological pathways. We describe recent compelling examples of how these tools have become powerful means to unravel genetic networks and uncovering important information about devastating diseases. Finally, we provide an overview of preclinical studies in which transcriptional modulation has been used therapeutically, and we discuss potential future directions of these novel modalities.

Enrichment strategies to enhance genome editing.

Genome editing technologies hold great promise for numerous applications including the understanding of cellular and disease mechanisms and the development of gene and cellular therapies. Achieving high editing frequencies is critical to these research areas and to achieve the overall goal of being able to manipulate any target with any desired genetic outcome. However, gene editing technologies sometimes suffer from low editing efficiencies due to several challenges. This is often the case for emerging gene editing technologies, which require assistance for translation into broader applications. Enrichment strategies can support this goal by selecting gene edited cells from non-edited cells. In this review, we elucidate the different enrichment strategies, their many applications in non-clinical and clinical settings, and the remaining need for novel strategies to further improve genome research and gene and cellular therapy studies.

A truncated reverse transcriptase enhances prime editing by split AAV vectors.

Prime editing is a new CRISPR-based, genome-editing technology that relies on the prime editor (PE), a fusion protein of Cas9-nickase and M-MLV reverse transcriptase (RT), and a prime editing guide RNA (pegRNA) that serves both to target PE to the desired genomic locus and to carry the edit to be introduced. Here, we make advancements to the RT moiety to improve prime editing efficiencies and truncations to mitigate issues with adeno-associated virus (AAV) viral vector size limitations, which currently do not support efficient delivery of the large prime editing components. These efforts include RT variant screening, codon optimization, and PE truncation by removal of the RNase H domain and further trimming. This led to a codon-optimized and size-minimized PE that has an expression advantage (1.4-fold) and size advantage (621 bp shorter). In addition, we optimize the split intein PE system and identify Rma-based Cas9 split sites (573-574 and 673-674) that combined with the truncated PE delivered by dual AAVs result in superior AAV titer and prime editing efficiency. We also show that this minimized PE gives rise to superior lentiviral vector titers (46-fold) over the regular PE in an all-in-one PE lentiviral vector. We finally deliver the minimized PE to mouse liver by dual AAV8 vectors and show up to 6% precise editing of the PCSK9 gene, thereby demonstrating the value of this truncated split PE system for in vivo applications.

CRISPR/Cas9 ß-globin gene targeting in human haematopoietic stem cells.

The ß-haemoglobinopathies, such as sickle cell disease and ß-thalassaemia, are caused by mutations in the ß-globin (HBB) gene and affect millions of people worldwide. Ex vivo gene correction in patient-derived haematopoietic stem cells followed by autologous transplantation could be used to cure ß-haemoglobinopathies. Here we present a CRISPR/Cas9 gene-editing system that combines Cas9 ribonucleoproteins and adeno-associated viral vector delivery of a homologous donor to achieve homologous recombination at the HBB gene in haematopoietic stem cells. Notably, we devise an enrichment model to purify a population of haematopoietic stem and progenitor cells with more than 90% targeted integration. We also show efficient correction of the Glu6Val mutation responsible for sickle cell disease by using patient-derived stem and progenitor cells that, after differentiation into erythrocytes, express adult ß-globin (HbA) messenger RNA, which confirms intact transcriptional regulation of edited HBB alleles. Collectively, these preclinical studies outline a CRISPR-based methodology for targeting haematopoietic stem cells by homologous recombination at the HBB locus to advance the development of next-generation therapies for ß-haemoglobinopathies.

Targeted Knockout of the Vegfa Gene in the Retina by Subretinal Injection of RNP Complexes Containing Cas9 Protein and Modified sgRNAs.

The therapeutic effect of retinal gene therapy using CRISPR/Cas9-mediated genome editing and knockout applications is dependent on efficient and safe delivery of gene-modifying tool kits. Recently, transient administration of single guide RNAs (sgRNAs) and SpCas9 proteins delivered as ribonucleoproteins (RNPs) has provided potent gene knockout in vitro. To improve efficacy of CRISPR-based gene therapy, we delivered RNPs containing SpCas9 protein complexed to chemically modified sgRNAs (msgRNAs). In K562 cells, msgRNAs significantly increased the insertion/deletion (indel) frequency (25%) compared with unmodified counterparts leading to robust knockout of the VEGFA gene encoding vascular endothelial growth factor A (96% indels). Likewise, in HEK293 cells, lipoplexes containing varying amounts of RNP and EGFP mRNA showed efficient VEGFA knockout (43% indels) and strong EGFP expression, indicative of efficacious functional knockout using small amounts of RNP. In mice, subretinal injections of equivalent lipoplexes yielded 6% indels in Vegfa of isolated EGFP-positive RPE cells. However, signs of toxicity following delivery of lipoplexes containing high amounts of RNP were observed. Although the mechanism resulting in the varying efficacy remains to be elucidated, our data suggest that a single subretinal injection of RNPs carrying msgRNAs and SpCas9 induces targeted retinal indel formation, thus providing a clinically relevant strategy relying on nonviral delivery of short-lived nuclease activity.

Plasmacytoid Dendritic Cells as a Novel Cell-Based Cancer Immunotherapy.

Plasmacytoid dendritic cells (pDCs) are multifaceted immune cells with a wide range of innate and adaptive immunological functions. They constitute the first line of defence against multiple viral infections and have also been reported to actively participate in antitumor immune responses. The clinical implication of the presence of pDCs in the tumor microenvironment (TME) is still ambiguous, but it is clear that pDCs possess the ability to modulate tumor-specific T cell responses and direct cytotoxic functions. Therapeutic strategies designed to exploit these qualities of pDCs to boost tumor-specific immune responses could represent an attractive alternative compared to conventional therapeutic approaches in the future, and promising antitumor effects have already been reported in phase I/II clinical trials. Here, we review the many roles of pDCs in cancer and present current advances in developing pDC-based immunotherapeutic approaches for treating cancer.

Gene Editing on Center Stage.

Smithies et al. (1985) and Jasin and colleagues (1994) provided proof of concept that homologous recombination (HR) could be applied to the treatment of human disease and that its efficiency could be improved by the induction of double-strand breaks (DSBs). A key advance was the discovery of engineered nucleases, such as zinc-finger nucleases (ZFNs) and transcription activator-like (TAL) effector nucleases (TALENs), that can generate site-specific DSBs. The democratization and widespread use of genome editing was enabled by the discovery of the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 nuclease system. While genome editing using ZFNs and TALENs has already reached clinical trials, the pace at which genome editing enters human trials is bound to accelerate in the next several years with multiple promising preclinical studies heralding cures for monogenic diseases that are currently difficult to manage or even incurable. Here we review recent advances and current limitations and discuss the path forward using genome editing to understand, treat, and cure genetic diseases.

Therapeutic gene editing in haematological disorders with CRISPR/Cas9.

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated (Cas)9 platform offers an efficient way of making precise genetic changes to the human genome. This can be employed for disruption, addition and correction of genes, thereby enabling a new class of genetic therapies that can be applied to haematological disorders. Here we review recent technological advances in the CRISPR/Cas9 methodology and applications in haematology for curing monogenic genetic disorders and for engineering novel chimeric antigen receptor (CAR) T cells to treat haematological malignancies. Furthermore, we discuss current challenges for full clinical implementation of CRISPR/Cas9, and reflect on future trajectories of the technology.

Improved microRNA suppression by WPRE-linked tough decoy microRNA sponges.

Our genes are post-transcriptionally regulated by microRNAs (miRNAs) inducing translational suppression and degradation of targeted mRNAs. Strategies to inhibit miRNAs in a spatiotemporal manner in a desired cell type or tissue, or at a desired developmental stage, can be crucial for understanding miRNA function and for pushing forward miRNA suppression as a feasible rationale for genetic treatment of disease. For such purposes, RNA polymerase II (RNA Pol II)-transcribed tough decoy (TuD) miRNA inhibitors are particularly attractive. Here, we demonstrate augmented miRNA suppression capacity of TuD RNA hairpins linked to the Woodchuck hepatitis virus post-transcriptional regulatory element (WPRE). This effect is position-dependent and evident only when the WPRE is positioned upstream of the TuD. In accordance, inclusion of the WPRE does not change nuclear export, translation, total levels of TuD-containing RNA transcripts, or cytoplasmic P-body localization, suggesting that previously reported WPRE functions are negligible for improved TuD function. Notably, deletion analysis of TuD-fused WPRE unveils truncated WPRE variants resulting in optimized miRNA suppression. Together, our findings add to the guidelines for production of WPRE-supported anti-miRNA TuDs.

Global Transcriptional Response to CRISPR/Cas9-AAV6-Based Genome Editing in CD34+ Hematopoietic Stem and Progenitor Cells.

Genome-editing technologies are currently being translated to the clinic. However, cellular effects of the editing machinery have yet to be fully elucidated. Here, we performed global microarray-based gene expression measurements on human CD34+ hematopoietic stem and progenitor cells that underwent editing. We probed effects of the entire editing process as well as each component individually, including electroporation, Cas9 (mRNA or protein) with chemically modified sgRNA, and AAV6 transduction. We identified differentially expressed genes relative to control treatments, which displayed enrichment for particular biological processes. All editing machinery components elicited immune, stress, and apoptotic responses. Cas9 mRNA invoked the greatest amount of transcriptional change, eliciting a distinct viral response and global transcriptional downregulation, particularly of metabolic and cell cycle processes. Electroporation also induced significant transcriptional change, with notable downregulation of metabolic processes. Surprisingly, AAV6 evoked no detectable viral response. We also found Cas9/sgRNA ribonucleoprotein treatment to be well tolerated, in spite of eliciting a DNA damage signature. Overall, this data establishes a benchmark for cellular tolerance of CRISPR/Cas9-AAV6-based genome editing, ensuring that the clinical protocol is as safe and efficient as possible.

DNA transposition by protein transduction of the piggyBac transposase from lentiviral Gag precursors.

DNA transposon-based vectors have emerged as gene vehicles with a wide biomedical and therapeutic potential. So far, genomic insertion of such vectors has relied on the co-delivery of genetic material encoding the gene-inserting transposase protein, raising concerns related to persistent expression, insertional mutagenesis and cytotoxicity. This report describes potent DNA transposition achieved by direct delivery of transposase protein. By adapting integrase-deficient lentiviral particles (LPs) as carriers of the hyperactive piggyBac transposase protein (hyPBase), we demonstrate rates of DNA transposition that are comparable with the efficiency of a conventional plasmid-based strategy. Embedded in the Gag polypeptide, hyPBase is robustly incorporated into LPs and liberated from the viral proteins by the viral protease during particle maturation. We demonstrate lentiviral co-delivery of the transposase protein and vector RNA carrying the transposon sequence, allowing robust DNA transposition in a variety of cell types. Importantly, this novel delivery method facilitates a balanced cellular uptake of hyPBase, as shown by confocal microscopy, and allows high-efficiency production of clones harboring a single transposon insertion. Our findings establish engineered LPs as a new tool for transposase delivery. We believe that protein transduction methods will increase applicability and safety of DNA transposon-based vector technologies.

IFI16 senses DNA forms of the lentiviral replication cycle and controls HIV-1 replication.

Replication of lentiviruses generates different DNA forms, including RNA:DNA hybrids, ssDNA, and dsDNA. Nucleic acids stimulate innate immune responses, and pattern recognition receptors detecting dsDNA have been identified. However, sensors for ssDNA have not been reported, and the ability of RNA:DNA hybrids to stimulate innate immune responses is controversial. Using ssDNAs derived from HIV-1 proviral DNA, we report that this DNA form potently induces the expression of IFNs in primary human macrophages. This response was stimulated by stem regions in the DNA structure and was dependent on IFN-inducible protein 16 (IFI16), which bound immunostimulatory DNA directly and activated the stimulator of IFN genes -TANK-binding kinase 1 - IFN regulatory factors 3/7 (STING-TBK1-IRF3/7) pathway. Importantly, IFI16 colocalized and associated with lentiviral DNA in the cytoplasm in macrophages, and IFI16 knockdown in this cell type augmented lentiviral transduction and also HIV-1 replication. Thus, IFI16 is a sensor for DNA forms produced during the lentiviral replication cycle and regulates HIV-1 replication in macrophages.

Potent microRNA suppression by RNA Pol II-transcribed 'Tough Decoy' inhibitors.

MicroRNAs (miRNAs) are key regulators of gene expression and modulators of diverse biological pathways. Analyses of miRNA function as well as therapeutic managing of miRNAs rely on cellular administration of miRNA inhibitors which may be achieved by the use of viral vehicles. This study explores the miRNA-suppressive capacity of inhibitors expressed intracellularly from lentivirus-derived gene vectors. Superior activity of two decoy-type inhibitors, a "Bulged Sponge" with eight miRNA recognition sites and a hairpin-shaped "Tough Decoy" containing two miRNA recognition sites, is demonstrated in a side-by-side comparison of seven types of miRNA inhibitors transcribed as short RNAs from an RNA Pol III promoter. We find that lentiviral vectors expressing Tough Decoy inhibitors are less vulnerable than Bulged Sponge-encoding vectors to targeting by the cognate miRNA and less prone, therefore, to reductions in transfer efficiency. Importantly, it is demonstrated that Tough Decoy inhibitors retain their miRNA suppression capacity in the context of longer RNA transcripts expressed from an RNA Pol II promoter. Such RNA Pol II-transcribed Tough Decoy inhibitors are new tools in managing of miRNAs and may have potential for temporal and spatial regulation of miRNA activity as well as for therapeutic targeting of miRNAs that are aberrantly expressed in human disease.

Managing microRNAs with vector-encoded decoy-type inhibitors.

A rapidly growing understanding of the complex circuitry of microRNA (miRNA)-mediated gene regulation is attracting attention to miRNAs as new drug targets. Targeted miRNA suppression is achieved in a sequence-specific manner by antisense RNA "decoy" molecules. Such synthetic miRNA inhibitors have reached the clinic with remarkable pace and may soon appear as new therapeutic modalities in several diseases. Shortcomings, however, include high production costs, the requirement for repeated administration, and difficulty achieving tissue-specific delivery. With the many recent landmark achievements in clinical gene therapy, new and refined vector-encoded miRNA suppression technologies are attractive for many applications, not least as tools in innumerable daily studies of miRNA biology in laboratories worldwide. Here, we provide an overview of the strategies that have been used to adapt vector-encoded inhibitors for miRNA suppression and discuss advantages related to spatiotemporal and long-term miRNA attenuation. With the remarkable new discovery of miRNA management by naturally occurring circular RNAs, RNA circles generated by trans-splicing mechanisms may prove to be well-suited carriers of decoy-type miRNA inhibitors. The community will aspire to combine circles with high-affinity miRNA decoy methodologies, and such "vectorized" RNA circles may represent new solid ways to deliver miRNA inhibitors, perhaps even with therapeutic applications.