Ophir Shalem

Genome-scale CRISPR-Cas9 knockout screening in human cells.

Improving Whole-Genome Screens Improved methods are needed for the knockout of individual genes in genome-scale functional screens. Wang et al. (p. 80, published online 12 December) and Shalem et al. (p. 84, published online 12 December) used the bacterial CRISPR/Cas9 system to power-screen protocols that avoid several of the pitfalls associated with small interfering RNA (siRNA) screens. Genome editing by these methods completely disrupts target genes, thus avoiding weak signals that can occur when transcript abundance is partially decreased by siRNA. Furthermore, gene targeting by the CRISPR system is more precise and appears to produce substantially fewer off-target effects than existing methods. Genome-editing technology allows improved positive or negative selection screens. The simplicity of programming the CRISPR (clustered regularly interspaced short palindromic repeats)–associated nuclease Cas9 to modify specific genomic loci suggests a new way to interrogate gene function on a genome-wide scale. We show that lentiviral delivery of a genome-scale CRISPR-Cas9 knockout (GeCKO) library targeting 18,080 genes with 64,751 unique guide sequences enables both negative and positive selection screening in human cells. First, we used the GeCKO library to identify genes essential for cell viability in cancer and pluripotent stem cells. Next, in a melanoma model, we screened for genes whose loss is involved in resistance to vemurafenib, a therapeutic RAF inhibitor. Our highest-ranking candidates include previously validated genes NF1 and MED12, as well as novel hits NF2, CUL3, TADA2B, and TADA1. We observe a high level of consistency between independent guide RNAs targeting the same gene and a high rate of hit confirmation, demonstrating the promise of genome-scale screening with Cas9.

Improved vectors and genome-wide libraries for CRISPR screening

Genome-wide, targeted loss-of-function pooled screens using the CRISPR (clustered regularly interspaced short palindrome repeats)–associated nuclease Cas9 in human and mouse cells provide an alternative screening system to RNA interference (RNAi) and have been used to reveal new mechanisms in diverse biological models1-4. Previously, we used a Genome-scale CRISPR Knock-Out (GeCKO) library to identify loss-of-function mutations conferring vemurafenib resistance in a melanoma model1. However, initial lentiviral delivery systems for CRISPR screening had low viral titer or required a cell line already expressing Cas9, limiting the range of biological systems amenable to screening.

Genome-wide CRISPR screen in a mouse model of tumor growth and metastasis

Genetic screens are powerful tools for identifying genes responsible for diverse phenotypes. Here we describe a genome-wide CRISPR/Cas9-mediated loss-of-function screen in tumor growth and metastasis. We mutagenized a non-metastatic mouse cancer cell line using a genome-scale library with 67,405 single-guide RNAs (sgRNAs). The mutant cell pool rapidly generates metastases when transplanted into immunocompromised mice. Enriched sgRNAs in lung metastases and late-stage primary tumors were found to target a small set of genes, suggesting that specific loss-of-function mutations drive tumor growth and metastasis. Individual sgRNAs and a small pool of 624 sgRNAs targeting the top-scoring genes from the primary screen dramatically accelerate metastasis. In all of these experiments, the effect of mutations on primary tumor growth positively correlates with the development of metastases. Our study demonstrates Cas9-based screening as a robust method to systematically assay gene phenotypes in cancer evolution in vivo.

A Genome-wide CRISPR Screen in Primary Immune Cells to Dissect Regulatory Networks

Finding the components of cellular circuits and determining their functions systematically remains a major challenge in mammalian cells. Here, we introduced genome-wide pooled CRISPR-Cas9 libraries into dendritic cells (DCs) to identify genes that control the induction of tumor necrosis factor (Tnf) by bacterial lipopolysaccharide (LPS), a key process in the host response to pathogens, mediated by the Tlr4 pathway. We found many of the known regulators of Tlr4 signaling, as well as dozens of previously unknown candidates that we validated. By measuring protein markers and mRNA profiles in DCs that are deficient in known or candidate genes, we classified the genes into three functional modules with distinct effects on the canonical responses to LPS and highlighted functions for the PAF complex and oligosaccharyltransferase (OST) complex. Our findings uncover new facets of innate immune circuits in primary cells and provide a genetic approach for dissection of mammalian cell circuits.

Axonal transcription factors signal retrogradely in lesioned peripheral nerve.

Retrograde axonal injury signalling stimulates cell body responses in lesioned peripheral neurons. The involvement of importins in retrograde transport suggests that transcription factors (TFs) might be directly involved in axonal injury signalling. Here, we show that multiple TFs are found in axons and associate with dynein in axoplasm from injured nerve. Biochemical and functional validation for one TF family establishes that axonal STAT3 is locally translated and activated upon injury, and is transported retrogradely with dynein and importin α5 to modulate survival of peripheral sensory neurons after injury. Hence, retrograde transport of TFs from axonal lesion sites provides a direct link between axon and nucleus.

Transient transcriptional responses to stress are generated by opposing effects of mRNA production and degradation

The state of the transcriptome reflects a balance between mRNA production and degradation. Yet how these two regulatory arms interact in shaping the kinetics of the transcriptome in response to environmental changes is not known. We subjected yeast to two stresses, one that induces a fast and transient response, and another that triggers a slow enduring response. We then used microarrays following transcriptional arrest to measure genome‐wide decay profiles under each condition. We found condition‐specific changes in mRNA decay rates and coordination between mRNA production and degradation. In the transient response, most induced genes were surprisingly destabilized, whereas repressed genes were somewhat stabilized, exhibiting counteraction between production and degradation. This strategy can reconcile high steady‐state level with short response time among induced genes. In contrast, the stress that induces the slow response displays the more expected behavior, whereby most induced genes are stabilized, and repressed genes are destabilized. Our results show genome‐wide interplay between mRNA production and degradation, and that alternative modes of such interplay determine the kinetics of the transcriptome in response to stress.

Signaling to transcription networks in the neuronal retrograde injury response.

Robustness in nerve injury responses results from control of axon-to-soma signaling networks by multiple regulatory components. Calling In the Repair Crew The ability of a damaged neuron to regenerate depends on the initiation of a repair program in the cell body, so that the injured neuron switches from a “growth-as-normal” mode to an “injury-response” mode. Initiation of such a repair program depends in turn on the receipt by the cell body of injury signals from the lesion. Michaelevski et al. combined phosphoproteomic analyses of injured and uninjured rat sciatic nerve with microarray analyses of transcripts in the dorsal root ganglia to identify retrograde signaling networks implicated in activating the transcriptional response to axonal injury. Pharmacological manipulation of various protein kinases that appeared in many of these networks and were predicted to play a key role in affecting signaling network size and connectivity affected neurite outgrowth of cultured sensory neurons. Paradoxically, the combined manipulation of pairs of these kinases was sometimes less effective at affecting neurite outgrowth than manipulation of either alone—an observation that has substantial implications for development of appropriate therapies for treating nerve injury. Retrograde signaling from axon to soma activates intrinsic regeneration mechanisms in lesioned peripheral sensory neurons; however, the links between axonal injury signaling and the cell body response are not well understood. Here, we used phosphoproteomics and microarrays to implicate ~900 phosphoproteins in retrograde injury signaling in rat sciatic nerve axons in vivo and ~4500 transcripts in the in vivo response to injury in the dorsal root ganglia. Computational analyses of these data sets identified ~400 redundant axonal signaling networks connected to 39 transcription factors implicated in the sensory neuron response to axonal injury. Experimental perturbation of individual overrepresented signaling hub proteins, including Abl, AKT, p38, and protein kinase C, affected neurite outgrowth in sensory neurons. Paradoxically, however, combined perturbation of Abl together with other hub proteins had a reduced effect relative to perturbation of individual proteins. Our data indicate that nerve injury responses are controlled by multiple regulatory components, and suggest that network redundancies provide robustness to the injury response.

Hypoxia as a therapy for mitochondrial disease

Thriving on a breath of low oxygen Mitochondrial diseases are debilitating and largely untreatable. Most are caused by genetic mutations that impair the mitochondrial respiratory chain, which generates cellular energy. Because these diseases do not affect all tissues equally, it is thought that endogenous mechanisms exist that can help cells cope with mitochondrial defects. Jain et al. identified the hypoxia response, a mechanism that helps cells adapt when oxygen is limited, as a potent suppressor of mitochondrial dysfunction (see the Perspective by Shoubridge). Mouse models of the mitochondrial disease Leigh syndrome showed fewer symptoms and a dramatically extended life span when raised in a hypoxic environment. Science, this issue p. 54; see also p. 31 Chronic exposure to low oxygen levels improves the health and extends the survival of mice that model a human mitochondrial disease. [Also see Perspective by Shoubridge] Defects in the mitochondrial respiratory chain (RC) underlie a spectrum of human conditions, ranging from devastating inborn errors of metabolism to aging. We performed a genome-wide Cas9-mediated screen to identify factors that are protective during RC inhibition. Our results highlight the hypoxia response, an endogenous program evolved to adapt to limited oxygen availability. Genetic or small-molecule activation of the hypoxia response is protective against mitochondrial toxicity in cultured cells and zebrafish models. Chronic hypoxia leads to a marked improvement in survival, body weight, body temperature, behavior, neuropathology, and disease biomarkers in a genetic mouse model of Leigh syndrome, the most common pediatric manifestation of mitochondrial disease. Further preclinical studies are required to assess whether hypoxic exposure can be developed into a safe and effective treatment for human diseases associated with mitochondrial dysfunction.

Identification of essential genes for cancer immunotherapy.

Somatic gene mutations can alter the vulnerability of cancer cells to T-cell-based immunotherapies. Here we perturbed genes in human melanoma cells to mimic loss-of-function mutations involved in resistance to these therapies, by using a genome-scale CRISPR–Cas9 library that consisted of around 123,000 single-guide RNAs, and profiled genes whose loss in tumour cells impaired the effector function of CD8+ T cells. The genes that were most enriched in the screen have key roles in antigen presentation and interferon-γ signalling, and correlate with cytolytic activity in patient tumours from The Cancer Genome Atlas. Among the genes validated using different cancer cell lines and antigens, we identified multiple loss-of-function mutations in APLNR, encoding the apelin receptor, in patient tumours that were refractory to immunotherapy. We show that APLNR interacts with JAK1, modulating interferon-γ responses in tumours, and that its functional loss reduces the efficacy of adoptive cell transfer and checkpoint blockade immunotherapies in mouse models. Our results link the loss of essential genes for the effector function of CD8+ T cells with the resistance or non-responsiveness of cancer to immunotherapies.

High-resolution interrogation of functional elements in the noncoding genome

The noncoding genome affects gene regulation and disease, yet we lack tools for rapid identification and manipulation of noncoding elements. We developed a CRISPR screen using ~18,000 single guide RNAs targeting >700 kilobases surrounding the genes NF1, NF2, and CUL3, which are involved in BRAF inhibitor resistance in melanoma. We find that noncoding locations that modulate drug resistance also harbor predictive hallmarks of noncoding function. With a subset of regions at the CUL3 locus, we demonstrate that engineered mutations alter transcription factor occupancy and long-range and local epigenetic environments, implicating these sites in gene regulation and chemotherapeutic resistance. Through our expansion of the potential of pooled CRISPR screens, we provide tools for genomic discovery and for elucidating biologically relevant mechanisms of gene regulation.