Compact and highly active next-generation libraries for CRISPR-mediated gene repression and activation

Genetically encoded fluorescent reporter for polyamines

Polyamines are abundant and evolutionarily conserved metabolites that are essential for life. Dietary polyamine supplementation extends life-span and health-span. Dysregulation of polyamine homeostasis is linked to Parkinson's disease and cancer, driving interest in therapeutically targeting this pathway. However, measuring cellular polyamine levels, which vary across cell types and states, remains challenging. We introduce a genetically encoded polyamine reporter for real-time measurement of polyamine concentrations in single living cells. This reporter utilizes the polyamine-responsive ribosomal frameshift motif from the OAZ1 gene. We demonstrate broad applicability of this approach and reveal dynamic changes in polyamine levels in response to genetic and pharmacological perturbations. Using this reporter, we conduct a genome-wide CRISPR screen and uncover an unexpected link between mitochondrial respiration and polyamine import, which are both risk factors for Parkinson's disease. By offering a lens to examine polyamine biology, this reporter may advance our understanding of these ubiquitous metabolites and accelerate therapy development.

RNA-binding proteins DND1 and NANOS3 cooperatively suppress the entry of germ cell lineage

Specification of primordial germ cells (PGCs) establishes germline development during early embryogenesis, yet the underlying mechanisms in humans remain largely unknown. Here, we reveal the functional roles of germline-specific RNA-binding protein (RBP) DND1 in human PGC (hPGC) specification. We discovered that DND1 forms a complex with another RBP, NANOS3, to restrict hPGC specification. Furthermore, by analyzing the mRNAs bound by DND1 and NANOS3, we found that DND1 facilitates the binding of NANOS3 to hPGC-like cells-related mRNAs. We identified SOX4 mRNAs as the key downstream factor for the DND1 and NANOS3 complex. Mechanistically, DND1 and NANOS3 function in processing bodies (P-bodies) to repress the translation of SOX4 mRNAs, with NANOS3 mediating the interaction between DND1 and the translational repressor 4E-T. Altogether, these findings identify the RBP complex formed by DND1 and NANOS3 functioning as a "braking system" to restrict the entry of germ cell fate in humans.

In vivo CRISPR activation screen identifies acyl-CoA-binding protein as a driver of bone metastasis

One of the most common sites of cancer metastasis is to the bone. Bone metastasis is associated with substantial morbidity and mortality, and current therapeutic interventions remain largely palliative. Metastasizing tumor cells need to reprogram their metabolic states to adapt to the nutrient environment of distant organs; however, the role and translational relevance of lipid metabolism in bone metastasis remain unclear. Here, we used an in vivo CRISPR activation screening system coupled with positive selection to identify acyl-coenzyme A (CoA) binding protein (ACBP) as a bone metastasis driver. In nonmetastatic and weakly metastatic cancer cells, overexpression of wild-type ACBP, but not the acyl-CoA-binding deficient mutant, stimulated fatty acid oxidation (FAO) and bone metastasis. Conversely, knockout of ACBP in highly bone metastatic cancer cells abrogated metastatic bone colonization. Mechanistically, ACBP-mediated FAO increased ATP and NADPH production, reduced reactive oxygen species, and inhibited lipid peroxidation and ferroptosis. We found that ACBP expression correlated with metabolic signaling, bone metastatic ability, and poor clinical outcomes. In mouse models, pharmacological blockade of FAO or treatment with a ferroptosis inducer inhibited bone metastasis. Together, our findings reveal the role of lipid metabolism in tumor cells adapting and thriving in the bone and identify ACBP as a key regulator of this process. Agents that target FAO or induce ferroptosis represent a promising therapeutic approach for treating bone metastases.

MicroRNA mechanisms instructing Purkinje cell specification

MicroRNAs (miRNAs) are critical for brain development; however, if, when, and how miRNAs drive neuronal subtype specification remains poorly understood. To address this, we engineered technologies with vastly improved spatiotemporal resolution that allow the dissection of cell-type-specific miRNA-target networks. Fast and reversible miRNA loss of function showed that miRNAs are necessary for Purkinje cell (PC) differentiation, which previously appeared to be miRNA independent, and identified distinct critical miRNA windows for dendritogenesis and climbing fiber synaptogenesis, structural features defining PC identity. Using new mouse models that enable miRNA-target network mapping in rare cell types, we uncovered PC-specific post-transcriptional programs. Manipulation of these programs revealed that the PC-enriched miR-206 and targets Shank3, Prag1, En2, and Vash1, which are uniquely repressed in PCs, are critical regulators of PC-specific dendritogenesis and synaptogenesis, with miR-206 knockdown and target overexpression partially phenocopying miRNA loss of function. Our results suggest that gene expression regulation by miRNAs, beyond transcription, is critical for neuronal subtype specification.

Neddylation as a target in PIK3CA-mutated head and neck cancer

PIK3CA encodes the catalytic subunit of phosphoinositide 3-kinase (PI3K) enzyme and is the most commonly mutated oncogene in head and neck squamous cell carcinoma (HNSCC). This study aimed to identify potential therapeutic targets in HNSCC harboring mutant PIK3CA. We used CRISPR interference (CRISPRi)-based genome-wide screening methodology to reveal targetable genetic dependencies in PIK3CA-mutated HNSCC. Screening was conducted in an HPV-positive HNSCC cell line, UM-SCC-47, engineered to express the canonical E545K PIK3CA mutant. We identified 34 genes co-dependent on PIK3CA E545K mutation, including 5 genes in the neddylation pathway (NEDD8, NEDD8-MDP-1 and NAE1, USP8, UBA3). Validation experiments confirmed the essential role of NEDD8, NEDD8-MDP-1, and NAE1, indicating a novel regulatory mechanism in PIK3CA E545K-mutated HNSCC. Our findings suggest that PIK3CA mutation may serve as a predictive biomarker for neddylation inhibitor therapy in a subpopulation of HNSCC.

Elucidating the genetic mechanisms governing cytosine base editing outcomes through CRISPRi screens

Cytosine base editors enable programmable and efficient genome editing using an intermediate featuring a U•G mismatch across from a DNA nick. This intermediate facilitates two major outcomes, C•G to T•A and C•G to G•C point mutations, and it is not currently well-understood which DNA repair factors are involved. Here, we couple reporters for cytosine base editing activity with knockdown of 2015 DNA processing genes to identify genes involved in these two outcomes. Our data suggest that mismatch repair factors facilitate C•G to T•A outcomes, while C•G to G•C outcomes are mediated by RFWD3, an E3 ubiquitin ligase. We also propose that XPF, a 3'-flap endonuclease, and LIG3, a DNA ligase, are involved in repairing the intermediate back to the original C•G base pair. Our results demonstrate that competition and collaboration among different DNA repair pathways shape cytosine base editing outcomes.

ZNF574 is a quality control factor for defective ribosome biogenesis intermediates

Eukaryotic ribosome assembly is an intricate process that involves four ribosomal RNAs, 80 ribosomal proteins, and over 200 biogenesis factors that participate in numerous interdependent steps. The complexity and essentiality of this process create opportunities for deleterious mutations to occur, accumulate, and impact downstream cellular processes. "Dead-end" ribosome intermediates that result from biogenesis errors are rapidly degraded, affirming the existence of quality control (QC) pathway(s) that monitor ribosome assembly. However, the factors that differentiate between on-path and dead-end intermediates are unknown. We engineered a system to perturb ribosome assembly in human cells and discovered that faulty ribosomes are degraded via the ubiquitin-proteasome system. We identified ZNF574 as a key component of a QC pathway, which we term the ribosome assembly surveillance pathway (RASP). In an animal model, loss of ZNF574 leads to developmental defects, emphasizing the importance of RASP in organismal health.

Identification of a Distal Enhancer That Regulates TGF-β-Induced SNAI1 Expression

Snail is a zinc finger transcription factor encoded by the SNAI1 gene and triggers a cellular process termed epithelial-mesenchymal transition (EMT) upon its increased expression and/or functional activation. Snail expression and activity are regulated by various extracellular stimuli, including cytokines and environmental factors. Transforming growth factor-β (TGF-β) is a Snail inducer that functions via Smad3-mediated transcriptional activation. In the present study, we identified a distal enhancer that modulates TGF-β-induced SNAI1 expression. ChIP-seq and Hi-C analyses showed that the enhancer is located 46 kb downstream of the SNAI1 gene; in TGF-β-stimulated cells, it associates with Smad3 and interacts with the SNAI1 proximal promoter. Inhibiting the activity of the enhancer using CRISPRi attenuated TGF-β-induced SNAI1 expression, stress fiber formation, and cell motility enhancement, suggesting that the enhancer mediates TGF-β-induced EMT. The enhancer contains a Smad-binding CAGA motif and an activator protein-1 (AP-1) binding motif that function in transcriptional activation. Ras-responsive element binding protein 1 (RREB1), a transcription factor required for TGF-β-induced Snail expression, regulated the basal activity of the enhancer but not its inducibility by TGF-β. In contrast to the enhancer, the association of Smad3 with the proximal promoter was not evident. These findings suggest that the proximal promoter and the distal enhancer respond to distinct signaling cues, integrate them, and cooperatively function to drive SNAI1 expression.

Genome-wide CRISPR activation screen identifies ARL11 as a sensitivity determinant of PARP inhibitor therapy

Resistance to poly-(ADP)-ribose polymerase inhibitors (PARPi) remains a significant challenge in clinical practice, leading to treatment failure in many patients. It is crucial to better understand the molecular mechanisms that underlie PARPi resistance. In this study, utilizing a genome-wide CRISPR activation screen with olaparib, we identified ARL11 as a potential modulator of PARPi treatment response in BRCA-wild-type MDA-MB-231 cells. Mechanistically, ARL11 interacts with STING to enhance innate immunity and forms positive feedback with type I interferon (IFN) induction, which induces ARL11 up-regulation and contributes to resistance to PARPi therapy. Additionally, we observed that ARL11 interacts with the RUVBL1 and RUVBL2 (RUVBL1/2) complex, the key DNA double-strand repair proteins, facilitating DNA homologous recombination (HR) repair and significantly reducing PARPi-induced DNA double-strand damages. Clinical sample analysis reveals that the expression levels of ARL11 and RUVBL1/2 are significantly elevated in breast cancer patients compared to healthy controls. Collectively, our findings suggested that ARL11 and RUVBL1/2 may be promising therapeutic targets to sensitize breast cancer cells to PARPi therapy.

Serotonin and neurotensin inputs in the vCA1 dictate opposing social valence

The ability to evaluate valence of a social agent based on social experience is essential for an animal's survival in its social group1. Although hippocampal circuits have been implicated in distinguishing novel and familiar conspecifics2-7, it remains unclear how social valence is constructed on the basis of social history and what mechanisms underlie the heightened valence versatility in dynamic relationships. Here we demonstrate that the ventral (v)CA1 integrates serotonin (5-HT) inputs from the dorsal raphe and neurotensin inputs from the paraventricular nucleus of the thalamus (PVT) to determine positive or negative valence of conspecific representations. Specifically, during an appetitive social interaction 5-HT is released into the vCA1 and disinhibits pyramidal neurons through 5-HT1B receptors, whereas neurotensin is released during an aversive social interaction and potentiates vCA1 neurons directly through NTR1s. Optogenetic silencing of dorsal raphe 5-HT and PVT neurotensin inputs into the vCA1 impairs positive and negative social valence, respectively, and excitation flexibly switches valence assignment. These results show how aversive and rewarding social experiences are linked to conspecific identity through converging dorsal raphe 5-HT and PVT neurotensin signals in the vCA1 that instruct opposing valence, and represent a synaptic switch for flexible social valence computation.

An ultraconserved snoRNA-like element in long noncoding RNA CRNDE promotes ribosome biogenesis and cell proliferation

Cancer cells frequently upregulate ribosome production to support tumorigenesis. While small nucleolar RNAs (snoRNAs) are critical for ribosome biogenesis, the roles of other classes of noncoding RNAs in this process remain largely unknown. Here, we performed CRISPR interference (CRISPRi) screens to identify essential long noncoding RNAs (lncRNAs) in renal cell carcinoma (RCC) cells. This revealed that an alternatively spliced isoform of lncRNA colorectal neoplasia differentially expressed (CRNDE) containing an ultraconserved element (UCE), referred to as CRNDEUCE, is required for RCC cell proliferation. CRNDEUCE localizes to the nucleolus and promotes 60S ribosomal subunit biogenesis. The UCE of CRNDE functions as an unprocessed C/D box snoRNA that directly interacts with ribosomal RNA precursors. This facilitates delivery of eukaryotic initiation factor 6 (eIF6), a key 60S biogenesis factor, which binds to CRNDEUCE through a sequence element adjacent to the UCE. These findings highlight the functional versatility of snoRNA sequences and expand the known mechanisms through which noncoding RNAs orchestrate ribosome biogenesis.

Mitochondria-organelle crosstalk in establishing compartmentalized metabolic homeostasis

Mitochondria serve as central hubs in cellular metabolism by sensing, integrating, and responding to metabolic demands. This integrative function is achieved through inter-organellar communication, involving the exchange of metabolites, lipids, and signaling molecules. The functional diversity of metabolite exchange and pathway interactions is enabled by compartmentalization within organelle membranes. Membrane contact sites (MCSs) are critical for facilitating mitochondria-organelle communication, creating specialized microdomains that enhance the efficiency of metabolite and lipid exchange. MCS dynamics, regulated by tethering proteins, adapt to changing cellular conditions. Dysregulation of mitochondrial-organelle interactions at MCSs is increasingly recognized as a contributing factor in the pathogenesis of multiple diseases. Emerging technologies, such as advanced microscopy, biosensors, chemical-biology tools, and functional genomics, are revolutionizing our understanding of inter-organellar communication. These approaches provide novel insights into the role of these interactions in both normal cellular physiology and disease states. This review will highlight the roles of metabolite transporters, lipid-transfer proteins, and mitochondria-organelle interfaces in the coordination of metabolism and transport.

KCTD20 suppression mitigates excitotoxicity in tauopathy patient organoids

Excitotoxicity is a major pathologic mechanism in patients with tauopathy and other neurodegenerative diseases. However, the key neurotoxic drivers and the most effective strategies for mitigating these degenerative processes are unclear. Here, we show that glutamate treatment of induced pluripotent stem cell (iPSC)-derived cerebral organoids induces tau oligomerization and neurodegeneration and that these phenotypes are enhanced in organoids derived from tauopathy patients. Using a genome-wide CRISPR interference (CRISPRi) screen, we find that the suppression of KCTD20 potently ameliorates tau pathology and neurodegeneration in glutamate-treated organoids and mice, as well as in transgenic mice overexpressing mutant human tau. KCTD20 suppression reduces oligomeric tau and improves neuron survival by activating lysosomal exocytosis, which clears pathological tau. Our results show that glutamate signaling can induce neuronal tau pathology and identify KCTD20 suppression and lysosomal exocytosis as effective strategies for clearing neurotoxic tau species.

PTPσ-mediated PI3P regulation modulates neurodegeneration in C9ORF72-ALS/FTD

The most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the repeat expansion in C9ORF72. Dipeptide repeat (DPR) proteins translated from both sense and antisense repeats, especially arginine-rich DPRs (R-DPRs), contribute to neurodegeneration. Through CRISPR interference (CRISPRi) screening in human-derived neurons, we identified receptor-type tyrosine-protein phosphatase S (PTPσ) as a strong modifier of poly-GR-mediated toxicity. We showed that reducing PTPσ promotes the survival of both poly-GR- and poly-PR-expressing neurons by elevating phosphatidylinositol 3-phosphate (PI3P), accompanied by restored early endosomes and lysosomes. Remarkably, PTPσ knockdown or inhibition substantially rescues the PI3P-endolysosomal defects and improves the survival of C9ORF72-ALS/FTD patient-derived neurons. Furthermore, the PTPσ inhibitor diminishes GR toxicity and rescues pathological and behavioral phenotypes in mice. Overall, these findings emphasize the critical role of PI3P-mediated endolysosomal deficits induced by R-DPRs in disease pathogenesis and reveal the therapeutic potential of targeting PTPσ in C9ORF72-ALS/FTD.

EIF3D safeguards the homeostasis of key signaling pathways in human primed pluripotency

Although pluripotent stem cell (PSC) properties, such as differentiation and infinite proliferation, have been well documented within the frameworks of transcription factor networks, epigenomes, and signal transduction, they remain unclear and fragmented. Directing attention toward translational regulation as a bridge between these events can yield additional insights into previously unexplained mechanisms. Our functional CRISPR interference screen-based approach revealed that EIF3D, a translation initiation factor, is crucial for maintaining primed pluripotency. Loss of EIF3D disrupted the balance of pluripotency-associated signaling pathways, thereby compromising primed pluripotency. Moreover, EIF3D ensured robust proliferation by controlling the translation of various p53 regulators, which maintain low p53 activity in the undifferentiated state. In this way, EIF3D-mediated translation contributes to tuning the homeostasis of the primed pluripotency networks, ensuring the maintenance of an undifferentiated state with high proliferative potential. This study provides further insights into the translation network in maintaining pluripotency.

Commander complex regulates lysosomal function and is implicated in Parkinson's disease risk

Variants in GBA1 resulting in decreased lysosomal glucocerebrosidase (GCase) activity are a common risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Incomplete penetrance of GBA1 variants suggests that additional genes contribute to PD and DLB manifestation. By using a pooled genome-wide CRISPR interference screen, we identified copper metabolism MURR1 domain-containing 3 (COMMD3) protein, a component of the COMMD/coiled-coil domain-containing protein 22 (CCDC22)/CCDC93 (CCC) and Commander complexes, as a modifier of GCase and lysosomal activity. Loss of COMMD3 increased the release of lysosomal proteins through extracellular vesicles, leading to their impaired delivery to endolysosomes and consequent lysosomal dysfunction. Rare variants in the Commander gene family were associated with increased PD risk. Thus, COMMD genes and related complexes regulate lysosomal homeostasis and may represent modifiers in PD and other neurodegenerative diseases associated with lysosomal dysfunction.

Cell type-specific enhancers regulate IL-22 expression in innate and adaptive lymphoid cells

IL-22, a signature cytokine of type 3 lymphoid cells, mediates epithelial homeostasis and protective pathogen responses in barrier tissues, while its deregulated expression drives chronic inflammation associated with colitis and psoriasis. Despite its therapeutic value, little is known about regulatory elements for IL-22 expression. We identify two conserved enhancers, E22-1 and E22-2, which differentially regulate Il22 in type 3 lymphoid subsets. These enhancers are required for steady-state expression of gut antimicrobial peptides, protection from C. rodentium infection, and development of IL-22-mediated psoriasis. E22-1 resembles many known enhancers, functioning in both Th-ILC counterparts. However, E22-2 is only required for IL-22 expression in ILC3s. Its ILC3 restriction relies on multiple Runx3 sites, combined with the lack of a functional RORγt motif, which is present in E22-1. Thus, although responding to similar stimuli, type 3 lymphoid cells use distinct cis-elements for IL-22 expression, with E22-2 likely serving as a homeostatic enhancer in barrier tissues.

CRISPRi-based screen of autism spectrum disorder risk genes in microglia uncovers roles of ADNP in microglia endocytosis and synaptic pruning

Autism Spectrum Disorders (ASD) are a set of neurodevelopmental disorders with complex biology. The identification of ASD risk genes from exome-wide association studies and de novo variation analyses has enabled mechanistic investigations into how ASD-risk genes alter development. Most functional genomics studies have focused on the role of these genes in neurons and neural progenitor cells. However, roles for ASD risk genes in other cell types are largely uncharacterized. There is evidence from postmortem tissue that microglia, the resident immune cells of the brain, appear activated in ASD. Here, we used CRISPRi-based functional genomics to systematically assess the impact of ASD risk gene knockdown on microglia activation and phagocytosis. We developed an iPSC-derived microglia-neuron coculture system and high-throughput flow cytometry readout for synaptic pruning to enable parallel CRISPRi-based screening of phagocytosis of beads, synaptosomes, and synaptic pruning. Our screen identified ADNP, a high-confidence ASD risk genes, as a modifier of microglial synaptic pruning. We found that microglia with ADNP loss have altered endocytic trafficking, remodeled proteomes, and increased motility in coculture.

Genome-coverage single-cell histone modifications for embryo lineage tracing

Substantial epigenetic resetting during early embryo development from fertilization to blastocyst formation ensures zygotic genome activation and leads to progressive cellular heterogeneities1-3. Mapping single-cell epigenomic profiles of core histone modifications that cover each individual cell is a fundamental goal in developmental biology. Here we develop target chromatin indexing and tagmentation (TACIT), a method that enabled genome-coverage single-cell profiling of seven histone modifications across mouse early embryos. We integrated these single-cell histone modifications with single-cell RNA sequencing data to chart a single-cell resolution epigenetic landscape. Multimodal chromatin-state annotations showed that the onset of zygotic genome activation at the early two-cell stage already primes heterogeneities in totipotency. We used machine learning to identify totipotency gene regulatory networks, including stage-specific transposable elements and putative transcription factors. CRISPR activation of a combination of these identified transcription factors induced totipotency activation in mouse embryonic stem cells. Together with single-cell co-profiles of multiple histone modifications, we developed a model that predicts the earliest cell branching towards the inner cell mass and the trophectoderm in latent multimodal space and identifies regulatory elements and previously unknown lineage-specifying transcription factors. Our work provides insights into single-cell epigenetic reprogramming, multimodal regulation of cellular lineages and cell-fate priming during mouse pre-implantation development.

Comparative characterization of human accelerated regions in neurons

Human accelerated regions (HARs) are conserved genomic loci that have experienced rapid nucleotide substitutions following the divergence from chimpanzees1,2. HARs are enriched in candidate regulatory regions near neurodevelopmental genes, suggesting their roles in gene regulation3. However, their target genes and functional contributions to human brain development remain largely uncharacterized. Here we elucidate the cis-regulatory functions of HARs in human and chimpanzee induced pluripotent stem (iPS) cell-induced excitatory neurons. Using genomic4 and chromatin looping information, we prioritized 20 HARs and their chimpanzee orthologues for functional characterization via single-cell CRISPR interference, and demonstrated their species-specific gene regulatory functions. Our findings reveal diverse functional outcomes of HAR-mediated cis-regulation in human neurons, including attenuated NPAS3 expression by altering the binding affinities of multiple transcription factors in HAR202 and maintaining iPS cell pluripotency and neuronal differentiation capacities through the upregulation of PUM2 by 2xHAR.319. Finally, we used prime editing to demonstrate differential enhancer activity caused by several HAR26;2xHAR.178 variants. In particular, we link one variant in HAR26;2xHAR.178 to elevated SOCS2 expression and increased neurite outgrowth in human neurons. Thus, our study sheds new light on the endogenous gene regulatory functions of HARs and their potential contribution to human brain evolution.

Comprehensive interrogation of synthetic lethality in the DNA damage response

The DNA damage response (DDR) is a multifaceted network of pathways that preserves genome stability1,2. Unravelling the complementary interplay between these pathways remains a challenge3,4. Here we used CRISPR interference (CRISPRi) screening to comprehensively map the genetic interactions required for survival during normal human cell homeostasis across all core DDR genes. We captured known interactions and discovered myriad new connections that are available online. We defined the molecular mechanism of two of the strongest interactions. First, we found that WDR48 works with USP1 to restrain PCNA degradation in FEN1/LIG1-deficient cells. Second, we found that SMARCAL1 and FANCM directly unwind TA-rich DNA cruciforms, preventing catastrophic chromosome breakage by the ERCC1-ERCC4 complex. Our data yield fundamental insights into genome maintenance, provide a springboard for mechanistic investigations into new connections between DDR factors and pinpoint synthetic vulnerabilities that could be exploited in cancer therapy.

FOS binding sites are a hub for the evolution of activity-dependent gene regulatory programs in human neurons

After birth, sensory inputs to neurons trigger the induction of activity-dependent genes (ADGs) that mediate many aspects of neuronal maturation and plasticity. To identify human-specific ADGs, we characterized these genes in human-chimpanzee tetraploid neurons. We identified 235 ADGs that are differentially expressed between human and chimpanzee neurons and found that their nearby regulatory sites are species-biased in their binding of the transcription factor FOS. An assessment of these sites revealed that many are enriched for single nucleotide variants that promote or eliminate FOS binding in human neurons. Disrupting the function of individual species-biased FOS-bound enhancers diminishes expression of nearby genes and affects the firing dynamics of human neurons. Our findings indicate that FOS-bound enhancers are frequent sites of evolution and that they regulate human-specific ADGs that may contribute to the unusually protracted and complex process of postnatal human brain development.

A quiescence-like/TGF-β1-specific CRISPRi screen reveals drug uptake transporters as secondary targets of kinase inhibitors in AML

Relapse in acute myeloid leukemia (AML) is driven by resistant subclones that survive chemotherapy. It is assumed that these resilient leukemic cells can modify their proliferative behavior by entering a quiescent-like state, similar to healthy hematopoietic stem cells (HSCs). These dormant cells can evade the effects of cytostatic drugs that primarily target actively dividing cells. Although quiescence has been extensively studied in healthy hematopoiesis and various solid cancers, its role in AML has remained unexplored. In this study, we applied an HSC-derived quiescence-associated gene signature to an AML patient cohort and found it to be strongly correlated with poor prognosis and active TGF-β signaling. In vitro treatment with TGF-β1 induces a quiescence-like phenotype, resulting in a G0 shift and reduced sensitivity to cytarabine. To find potential therapeutic targets that prevent AML-associated quiescence and improve response to cytarabine, we conducted a comprehensive CRISPR interference (CRISPRi) screen combined with TGF-β1 stimulation. This approach identified TGFBR1 inhibitors, like vactosertib, as effective agents for preventing the G0 shift in AML cell models. However, pretreatment with vactosertib unexpectedly induced complete resistance to cytarabine. To elucidate the underlying mechanism, we performed a multi-faceted approach combining a second CRISPRi screen, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and in silico analysis. Our findings revealed that TGFBR1 inhibitors unintentionally target the nucleoside transporter SLC29A1 (ENT1), leading to reduced intracellular cytarabine levels. Importantly, we found that this drug interaction is not unique to TGFBR1 inhibitors, but extends to other clinically significant kinase inhibitors, such as the FLT3 inhibitor midostaurin. These findings may have important implications for optimizing combination therapies in AML treatment.

The histone modifier KAT2A presents a selective target in a subset of well-differentiated microsatellite-stable colorectal cancers

Lysine acetyltransferase 2 A (KAT2A) plays a pivotal role in epigenetic gene regulation across various types of cancer. In colorectal cancer (CRC), increased KAT2A expression is associated with a more aggressive phenotype. Our study aims to elucidate the molecular underpinnings of KAT2A dependency in CRC and assess the consequences of KAT2A depletion. We conducted a comprehensive analysis by integrating CRISPR-Cas9 screening data with genomics, transcriptomics, and global acetylation patterns in CRC cell lines to pinpoint molecular markers indicative of KAT2A dependency. Additionally, we characterized the phenotypic effect of a CRISPR-interference-mediated KAT2A knockdown in CRC cell lines and patient-derived 3D spheroid cultures. Moreover, we assessed the effect of KAT2A depletion within a patient-derived xenograft mouse model in vivo. Our findings reveal that KAT2A dependency is closely associated with microsatellite stability, lower mutational burden, and increased molecular differentiation signatures in CRC, independent of the KAT2A expression levels. KAT2A-dependent CRC cells display higher gene expression levels and enriched H3K27ac marks at gene loci linked to enterocytic differentiation. Furthermore, loss of KAT2A leads to decreased cell growth and viability in vitro and in vivo, downregulation of proliferation- and stem cell-associated genes, and induction of differentiation markers. Altogether, our data show that a specific subset of CRCs with a more differentiated phenotype relies on KAT2A. For these CRC cases, KAT2A might represent a promising novel therapeutic target.

Redirecting cytotoxic lymphocytes to breast cancer tumors via metabolite-sensing receptors

Insufficient infiltration of cytotoxic lymphocytes to solid tumors limits the efficacy of immunotherapies and cell therapies. Here, we report a programmable mechanism to mobilize Natural Killer (NK) and T cells to breast cancer tumors by engineering these cells to express orphan and metabolite-sensing G protein-coupled receptors (GPCRs). First, in vivo and in vitro CRISPR activation screens in NK-92 cells identified GPR183, GPR84, GPR34, GPR18, FPR3, and LPAR2 as top enhancers of both tumor infiltration and chemotaxis to breast cancer. These genes equip NK and T cells with the ability to sense and migrate to chemoattracting metabolites such as 7α,25-dihydroxycholesterol and other factors released from breast cancer. Based on Perturb-seq and functional investigations, GPR183 also enhances effector functions, such that engineering NK and CAR NK cells to express GPR183 enhances their ability to migrate to, infiltrate, and control breast cancer tumors. Our study uncovered metabolite-based tumor immune recruitment mechanisms, opening avenues for spatially targeted cell therapies.

Optimizing in vitro Transcribed CRISPR-Cas9 Single-Guide RNA Libraries for Improved Uniformity and Affordability

We describe a scalable and cost-effective sgRNA synthesis workflow that reduces costs by over 70% through the use of large pools of microarray-derived oligos encoding unique sgRNA spacers. These subpool oligos are assembled into full-length dsDNA templates via Golden Gate Assembly before in vitro transcription with T7 RNA polymerase. RNA-seq analysis reveals severe biases in spacer representation, with some spacers being highly overrepresented while others are completely absent. Consistent with previous studies, we identify guanine-rich sequences within the first four nucleotides of the spacer, immediately downstream of the T7 promoter, as the primary driver of this bias. To address this issue, we introduced a guanine tetramer upstream of all spacers, which reduced bias by an average of 19% in sgRNA libraries containing 389 spacers. However, this modification also increased the presence of high-molecular-weight RNA species after transcription. We also tested two alternative bias-reduction strategies: compartmentalizing spacers within emulsions and optimizing DNA input and reaction volumes. Both methods independently reduced bias in 2,626-plex sgRNA libraries, though to a lesser extent than the guanine tetramer approach. These advancements enhance both the affordability and uniformity of sgRNA libraries, with broad implications for improving CRISPR-Cas9 screens and optimizing guide RNA design for other CRISPR and nuclease systems.

Engineering a New Generation of Gene Editors: Integrating Synthetic Biology and AI Innovations

CRISPR-Cas technology has revolutionized biology by enabling precise DNA and RNA edits with ease. However, significant challenges remain for translating this technology into clinical applications. Traditional protein engineering methods, such as rational design, mutagenesis screens, and directed evolution, have been used to address issues like low efficacy, specificity, and high immunogenicity. These methods are labor-intensive, time-consuming, and resource-intensive and often require detailed structural knowledge. Recently, computational strategies have emerged as powerful solutions to these limitations. Using artificial intelligence (AI) and machine learning (ML), the discovery and design of novel gene-editing enzymes can be streamlined. AI/ML models predict activity, specificity, and immunogenicity while also enhancing mutagenesis screens and directed evolution. These approaches not only accelerate rational design but also create new opportunities for developing safer and more efficient genome-editing tools, which could eventually be translated into the clinic.

EGFR-induced lncRNA TRIDENT promotes drug resistance in non-small cell lung cancer via phospho-TRIM28-mediated DNA damage repair

Long noncoding RNAs (lncRNAs) play numerous roles in cellular biology and alterations in lncRNA expression profiles have been implicated in a variety of cancers. Here, we identify and characterize a lncRNA, TRIM28 Interacting DNA damage repair Enhancing Noncoding Transcript (TRIDENT), whose expression is induced upon epithelial growth factor receptor (EGFR) activation, and which exerts pro-oncogenic functions in EGFR-driven non-small cell lung cancer. Knocking down TRIDENT leads to decreased tumor-cell proliferation in both in vitro and in vivo model systems and induces sensitization to chemotherapeutic drugs. Using ChIRP-MS analysis we identified TRIM28 as a protein interactor of TRIDENT. TRIDENT promotes phosphorylation of TRIM28 and knocking down TRIDENT leads to accumulation of DNA damage in cancer cells via decreased TRIM28 phosphorylation. Altogether, our results reveal a molecular pathway in which TRIDENT regulates TRIM28 phosphorylation to promote tumor cell growth and drug resistance. Our findings suggest that TRIDENT can be developed as a biomarker or therapeutic target for EGFR mutant non-small cell lung cancer.

Large-scale CRISPRi screens link metabolic stress to glioblastoma chemoresistance

BACKGROUND

Glioblastoma (GBM) patients frequently develop resistance to temozolomide (TMZ), the standard chemotherapy. While targeting cancer metabolism shows promise, the relationship between metabolic perturbation and drug resistance remains poorly understood.

METHODS

We performed high-throughput CRISPR interference screens in GBM cells to identify genes modulating TMZ sensitivity. Findings were validated using multiple GBM cell lines, patient-derived glioma stem cells, and clinical data. Molecular mechanisms were investigated through transcriptome analysis, metabolic profiling, and functional assays.

RESULTS

We identified phosphoglycerate kinase 1 (PGK1) as a key determinant of TMZ sensitivity. Paradoxically, while PGK1 inhibition suppressed tumor growth, it enhanced TMZ resistance by inducing metabolic stress. This activated AMPK and HIF-1α pathways, leading to enhanced DNA damage repair through 53BP1. PGK1 expression levels correlated with TMZ sensitivity across multiple GBM models and patient samples.

CONCLUSIONS

Our study reveals an unexpected link between metabolic stress and chemoresistance, demonstrating how metabolic adaptation can promote therapeutic resistance. These findings caution against single-agent metabolic targeting and suggest PGK1 as a potential biomarker for TMZ response in GBM.

CRISPRoff epigenome editing for programmable gene silencing in human cell lines and primary T cells

The advent of CRISPR-based technologies has enabled the rapid advancement of programmable gene manipulation in cells, tissues, and whole organisms. An emerging platform for targeted gene perturbation is epigenetic editing, the direct editing of chemical modifications on DNA and histones that ultimately results in repression or activation of the targeted gene. In contrast to CRISPR nucleases, epigenetic editors modulate gene expression without inducing DNA breaks or altering the genomic sequence of host cells. Recently, we developed the CRISPRoff epigenetic editing technology that simultaneously establishes DNA methylation and repressive histone modifications at targeted gene promoters. Transient expression of CRISPRoff and the accompanying single guide RNAs in mammalian cells results in transcriptional repression of targeted genes that is memorized heritably by cells through cell division and differentiation. Here, we describe our protocol for the delivery of CRISPRoff through plasmid DNA transfection, as well as the delivery of CRISPRoff mRNA, into transformed human cell lines and primary immune cells. We also provide guidance on evaluating target gene silencing and highlight key considerations when utilizing CRISPRoff for gene perturbations. Our protocols are broadly applicable to other CRISPR-based epigenetic editing technologies, as programmable genome manipulation tools continue to evolve rapidly.

CRISPRi-based screens in iAssembloids to elucidate neuron-glia interactions

The complexity of the human brain makes it challenging to understand the molecular mechanisms underlying brain function. Genome-wide association studies have uncovered variants associated with neurological phenotypes. Single-cell transcriptomics have provided descriptions of changes brain cells undergo during disease. However, these approaches do not establish molecular mechanism. To facilitate the scalable interrogation of causal molecular mechanisms in brain cell types, we developed a 3D co-culture system of induced pluripotent stem cell (iPSC)-derived neurons and glia, termed iAssembloids. Using iAssembloids, we ask how glial and neuronal cells interact to control neuronal death and survival. Our CRISPRi-based screens identified that GSK3β inhibits the protective NRF2-mediated oxidative stress response elicited by high neuronal activity. We then investigate the role of APOE-ε4, a risk variant for Alzheimer's disease, on neuronal survival. We find that APOE-ε4-expressing astrocytes may promote neuronal hyperactivity as compared with APOE-ε3-expressing astrocytes. This platform allows for the unbiased identification of mechanisms of neuron-glia cell interactions.

Functionally conserved inner mitochondrial membrane proteins CCDC51 and Mdm33 demarcate a subset of fission events

While extensive work has examined the mechanisms of mitochondrial fission, it remains unclear whether internal mitochondrial proteins in metazoans play a direct role in the process. Previously, the yeast inner membrane protein Mdm33 was shown to be required for normal mitochondrial morphology and has been hypothesized to be involved in mitochondrial fission. However, it is unknown whether Mdm33 plays a direct role, and it is not thought to have a mammalian homolog. Here, we use a bioinformatic approach to identify a structural ortholog of Mdm33 in humans, CCDC51 (also called MITOK), whose depletion phenocopies loss of Mdm33. We find that knockdown of CCDC51 also leads to reduced rates of mitochondrial fission. Further, we spatially and temporally resolve Mdm33 and CCDC51 to a subset of mitochondrial fission events. Finally, we show that CCDC51 overexpression promotes its spatial association with Drp1 and induces mitochondrial fragmentation, suggesting it is a positive effector of mitochondrial fission. Together, our data reveal that Mdm33 and CCDC51 are functionally conserved and suggest that internal mitochondrial proteins are directly involved in at least a subset of mitochondrial fission events in human cells.

Androgen Receptor Inhibition Increases MHC Class I Expression and Improves Immune Response in Prostate Cancer

SIGNIFICANCE

Immunotherapy options for immune cold tumors, like prostate cancer, are limited. We show that AR downregulates MHCI expression/antigen presentation and that AR inhibition improves T-cell responses and tumor control. This suggests that treatments combining AR inhibitors and checkpoint blockade may improve tumor immune surveillance and antitumor immunity in patients.

Functional screen identifies RBM42 as a mediator of oncogenic mRNA translation specificity

Oncogenic protein dosage is tightly regulated to enable cancer formation but how this is regulated by translational control remains unknown. The Myc oncogene is a paradigm of an exquisitely regulated oncogene and a driver of pancreatic ductal adenocarcinoma (PDAC). Here we use a CRISPR interference screen in PDAC cells to identify activators of selective MYC translation. The top hit, the RNA-binding protein RBM42, is highly expressed in PDAC and predicts poor survival. We show that RBM42 binds and selectively regulates the translation of MYC and a precise suite of pro-oncogenic transcripts, including JUN and EGFR. Mechanistically, we find that RBM42 binds and remodels the MYC 5' untranslated region structure, facilitating the formation of the translation pre-initiation complex. Importantly, RBM42 is necessary for PDAC tumorigenesis in a Myc-dependent manner in vivo. This work transforms the understanding of the translational code in cancer and illuminates therapeutic openings to target the expression of oncogenes.

Bidirectional epigenetic editing reveals hierarchies in gene regulation

CRISPR perturbation methods are limited in their ability to study non-coding elements and genetic interactions. In this study, we developed a system for bidirectional epigenetic editing, called CRISPRai, in which we apply activating (CRISPRa) and repressive (CRISPRi) perturbations to two loci simultaneously in the same cell. We developed CRISPRai Perturb-seq by coupling dual perturbation gRNA detection with single-cell RNA sequencing, enabling study of pooled perturbations in a mixed single-cell population. We applied this platform to study the genetic interaction between two hematopoietic lineage transcription factors, SPI1 and GATA1, and discovered novel characteristics of their co-regulation on downstream target genes, including differences in SPI1 and GATA1 occupancy at genes that are regulated through different modes. We also studied the regulatory landscape of IL2 (interleukin-2) in Jurkat T cells, primary T cells and chimeric antigen receptor (CAR) T cells and elucidated mechanisms of enhancer-mediated IL2 gene regulation. CRISPRai facilitates investigation of context-specific genetic interactions, provides new insights into gene regulation and will enable exploration of non-coding disease-associated variants.

An engineered trafficking biosensor reveals a role for DNAJC13 in DOR downregulation

Trafficking of G protein-coupled receptors (GPCRs) through the endosomal-lysosomal pathway is critical to homeostatic regulation of GPCRs following activation with agonist. Identifying the genes involved in GPCR trafficking is challenging due to the complexity of sorting operations and the large number of cellular proteins involved in the process. Here, we developed a high-sensitivity biosensor for GPCR expression and agonist-induced trafficking to the lysosome by leveraging the ability of the engineered peroxidase APEX2 to activate the fluorogenic substrate Amplex UltraRed (AUR). We used the GPCR-APEX2/AUR assay to perform a genome-wide CRISPR interference screen focused on identifying genes regulating expression and trafficking of the δ-opioid receptor (DOR). We identified 492 genes consisting of both known and new regulators of DOR function. We demonstrate that one new regulator, DNAJC13, controls trafficking of multiple GPCRs, including DOR, through the endosomal-lysosomal pathway by regulating the composition of the endosomal proteome and endosomal homeostasis.

Engineered CRISPR-Cas12a for higher-order combinatorial chromatin perturbations

Multiplexed genetic perturbations are critical for testing functional interactions among coding or non-coding genetic elements. Compared to double-stranded DNA cutting, repressive chromatin formation using CRISPR interference (CRISPRi) avoids genotoxicity and is more effective for perturbing non-coding regulatory elements in pooled assays. However, current CRISPRi pooled screening approaches are limited to targeting one to three genomic sites per cell. We engineer an Acidaminococcus Cas12a (AsCas12a) variant, multiplexed transcriptional interference AsCas12a (multiAsCas12a), that incorporates R1226A, a mutation that stabilizes the ribonucleoprotein-DNA complex via DNA nicking. The multiAsCas12a-KRAB fusion improves CRISPRi activity over DNase-dead AsCas12a-KRAB fusions, often rescuing the activities of lentivirally delivered CRISPR RNAs (crRNA) that are inactive when used with the latter. multiAsCas12a-KRAB supports CRISPRi using 6-plex crRNA arrays in high-throughput pooled screens. Using multiAsCas12a-KRAB, we discover enhancer elements and dissect the combinatorial function of cis-regulatory elements in human cells. These results instantiate a group testing framework for efficiently surveying numerous combinations of chromatin perturbations for biological discovery and engineering.

Genome-wide CRISPR guide RNA design and specificity analysis with GuideScan2

We present GuideScan2 for memory-efficient, parallelizable construction of high-specificity CRISPR guide RNA (gRNA) databases and user-friendly design and analysis of individual gRNAs and gRNA libraries for targeting coding and non-coding regions in custom genomes. GuideScan2 analysis identifies widespread confounding effects of low-specificity gRNAs in published CRISPR screens and enables construction of a gRNA library that reduces off-target effects in a gene essentiality screen. GuideScan2 also enables the design and experimental validation of allele-specific gRNAs in a hybrid mouse genome. GuideScan2 will facilitate CRISPR experiments across a wide range of applications.

Multiplexed CRISPRi Reveals a Transcriptional Switch Between KLF Activators and Repressors in the Maturing Neocortex

A critical phase of mammalian brain development takes place after birth. Neurons of the mouse neocortex undergo dramatic changes in their morphology, physiology, and synaptic connections during the first postnatal month, while properties of immature neurons, such as the capacity for robust axon outgrowth, are lost. The genetic and epigenetic programs controlling prenatal development are well studied, but our understanding of the transcriptional mechanisms that regulate postnatal neuronal maturation is comparatively lacking. By integrating chromatin accessibility and gene expression data from two subtypes of neocortical pyramidal neurons in the neonatal and maturing brain, we predicted a role for the Krüppel-Like Factor (KLF) family of Transcription Factors in the developmental regulation of neonatally expressed genes. Using a multiplexed CRISPR Interference (CRISPRi) knockdown strategy, we found that a shift in expression from KLF activators (Klf6, Klf7) to repressors (Klf9, Klf13) during early postnatal development functions as a transcriptional 'switch' to first activate, then repress a set of shared targets with cytoskeletal functions including Tubb2b and Dpysl3. We demonstrate that this switch is buffered by redundancy between KLF paralogs, which our multiplexed CRISPRi strategy is equipped to overcome and study. Our results indicate that competition between activators and repressors within the KLF family regulates a conserved component of the postnatal maturation program that may underlie the loss of intrinsic axon growth in maturing neurons. This could facilitate the transition from axon growth to synaptic refinement required to stabilize mature circuits.

Massively parallel in vivo Perturb-seq screening

Advances in genomics have identified thousands of risk genes impacting human health and diseases, but the functions of these genes and their mechanistic contribution to disease are often unclear. Moving beyond identification to actionable biological pathways requires dissecting risk gene function and cell type-specific action in intact tissues. This gap can in part be addressed by in vivo Perturb-seq, a method that combines state-of-the-art gene editing tools for programmable perturbation of genes with high-content, high-resolution single-cell genomic assays as phenotypic readouts. Here we describe a detailed protocol to perform massively parallel in vivo Perturb-seq using several versatile adeno-associated virus (AAV) vectors and provide guidance for conducting successful downstream analyses. Expertise in mouse work, AAV production and single-cell genomics is required. We discuss key parameters for designing in vivo Perturb-seq experiments across diverse biological questions and contexts. We further detail the step-by-step procedure, from designing a perturbation library to producing and administering AAV, highlighting where quality control checks can offer critical go-no-go points for this time- and cost-expensive method. Finally, we discuss data analysis options and available software. In vivo Perturb-seq has the potential to greatly accelerate functional genomics studies in mammalian systems, and this protocol will help others adopt it to answer a broad array of biological questions. From guide RNA design to tissue collection and data collection, this protocol is expected to take 9-15 weeks to complete, followed by data analysis.

Activation of the imprinted Prader-Willi syndrome locus by CRISPR-based epigenome editing

Epigenome editing with DNA-targeting technologies such as CRISPR-dCas9 can be used to dissect gene regulatory mechanisms and potentially treat associated disorders. For example, Prader-Willi syndrome (PWS) results from loss of paternally expressed imprinted genes on chromosome 15q11.2-q13.3, although the maternal allele is intact but epigenetically silenced. Using CRISPR repression and activation screens in human induced pluripotent stem cells (iPSCs), we identified genomic elements that control the expression of the PWS gene SNRPN from the paternal and maternal chromosomes. We showed that either targeted transcriptional activation or DNA demethylation can activate the silenced maternal SNRPN and downstream PWS transcripts. However, these two approaches function at unique regions, preferentially activating different transcript variants and involving distinct epigenetic reprogramming mechanisms. Remarkably, transient expression of the targeted demethylase leads to stable, long-term maternal SNRPN expression in PWS iPSCs. This work uncovers targeted epigenetic manipulations to reprogram a disease-associated imprinted locus and suggests possible therapeutic interventions.

A Massively Parallel CRISPR-Based Screening Platform for Modifiers of Neuronal Activity

Understanding the complex interplay between gene expression and neuronal activity is crucial for unraveling the molecular mechanisms underlying cognitive function and neurological disorders. Here, we developed pooled screens for neuronal activity, using CRISPR interference (CRISPRi) and the fluorescent calcium integrator CaMPARI2. Using this screening method, we evaluated 1343 genes for their effect on excitability in human iPSC-derived neurons, revealing potential links to neurodegenerative and neurodevelopmental disorders. These genes include known regulators of neuronal excitability, such as TARPs and ion channels, as well as genes associated with autism spectrum disorder and Alzheimer's disease not previously described to affect neuronal excitability. This CRISPRi-based screening platform offers a versatile tool to uncover molecular mechanisms controlling neuronal activity in health and disease.

Transitioning from wet lab to artificial intelligence: a systematic review of AI predictors in CRISPR

The revolutionary CRISPR-Cas9 system leverages a programmable guide RNA (gRNA) and Cas9 proteins to precisely cleave problematic regions within DNA sequences. This groundbreaking technology holds immense potential for the development of targeted therapies for a wide range of diseases, including cancers, genetic disorders, and hereditary diseases. CRISPR-Cas9 based genome editing is a multi-step process such as designing a precise gRNA, selecting the appropriate Cas protein, and thoroughly evaluating both on-target and off-target activity of the Cas9-gRNA complex. To ensure the accuracy and effectiveness of CRISPR-Cas9 system, after the targeted DNA cleavage, the process requires careful analysis of the resultant outcomes such as indels and deletions. Following the success of artificial intelligence (AI) in various fields, researchers are now leveraging AI algorithms to catalyze and optimize the multi-step process of CRISPR-Cas9 system. To achieve this goal AI-driven applications are being integrated into each step, but existing AI predictors have limited performance and many steps still rely on expensive and time-consuming wet-lab experiments. The primary reason behind low performance of AI predictors is the gap between CRISPR and AI fields. Effective integration of AI into multi-step CRISPR-Cas9 system demands comprehensive knowledge of both domains. This paper bridges the knowledge gap between AI and CRISPR-Cas9 research. It offers a unique platform for AI researchers to grasp deep understanding of the biological foundations behind each step in the CRISPR-Cas9 multi-step process. Furthermore, it provides details of 80 available CRISPR-Cas9 system-related datasets that can be utilized to develop AI-driven applications. Within the landscape of AI predictors in CRISPR-Cas9 multi-step process, it provides insights of representation learning methods, machine and deep learning methods trends, and performance values of existing 50 predictive pipelines. In the context of representation learning methods and classifiers/regressors, a thorough analysis of existing predictive pipelines is utilized for recommendations to develop more robust and precise predictive pipelines.

C-terminal amides mark proteins for degradation via SCF-FBXO31

During normal cellular homeostasis, unfolded and mislocalized proteins are recognized and removed, preventing the build-up of toxic byproducts1. When protein homeostasis is perturbed during ageing, neurodegeneration or cellular stress, proteins can accumulate several forms of chemical damage through reactive metabolites2,3. Such modifications have been proposed to trigger the selective removal of chemically marked proteins3-6; however, identifying modifications that are sufficient to induce protein degradation has remained challenging. Here, using a semi-synthetic chemical biology approach coupled to cellular assays, we found that C-terminal amide-bearing proteins (CTAPs) are rapidly cleared from human cells. A CRISPR screen identified FBXO31 as a reader of C-terminal amides. FBXO31 is a substrate receptor for the SKP1-CUL1-F-box protein (SCF) ubiquitin ligase SCF-FBXO31, which ubiquitylates CTAPs for subsequent proteasomal degradation. A conserved binding pocket enables FBXO31 to bind to almost any C-terminal peptide bearing an amide while retaining exquisite selectivity over non-modified clients. This mechanism facilitates binding and turnover of endogenous CTAPs that are formed after oxidative stress. A dominant human mutation found in neurodevelopmental disorders reverses CTAP recognition, such that non-amidated neosubstrates are now degraded and FBXO31 becomes markedly toxic. We propose that CTAPs may represent the vanguard of a largely unexplored class of modified amino acid degrons that could provide a general strategy for selective yet broad surveillance of chemically damaged proteins.

SLC25A38 is required for mitochondrial pyridoxal 5'-phosphate (PLP) accumulation

Many essential proteins require pyridoxal 5'-phosphate, the active form of vitamin B6, as a cofactor for their activity. These include enzymes important for amino acid metabolism, one-carbon metabolism, polyamine synthesis, erythropoiesis, and neurotransmitter metabolism. A third of all mammalian pyridoxal 5'-phosphate-dependent enzymes are localized in the mitochondria; however, the molecular machinery involved in the regulation of mitochondrial pyridoxal 5'-phosphate levels in mammals remains unknown. In this study, we used a genome-wide CRISPR interference screen in erythroleukemia cells and organellar metabolomics to identify the mitochondrial inner membrane protein SLC25A38 as a regulator of mitochondrial pyridoxal 5'-phosphate. Loss of SLC25A38 causes depletion of mitochondrial, but not cellular, pyridoxal 5'-phosphate, and impairs cellular proliferation under both physiological and low vitamin B6 conditions. Metabolic changes associated with SLC25A38 loss suggest impaired mitochondrial pyridoxal 5'-phosphate-dependent enzymatic reactions, including serine to glycine conversion catalyzed by serine hydroxymethyltransferase-2 as well as ornithine aminotransferase. The proliferation defect of SLC25A38-null K562 cells in physiological and low vitamin B6 media can be explained by the loss of serine hydroxymethyltransferase-2-dependent production of one-carbon units and downstream de novo nucleotide synthesis. Our work points to a role for SLC25A38 in mitochondrial pyridoxal 5'-phosphate accumulation and provides insights into the pathology of congenital sideroblastic anemia.

Loss of Fanconi anemia proteins causes a reliance on lysosomal exocytosis

Mutations in the FA pathway lead to a rare genetic disease that increases risk of bone marrow failure, acute myeloid leukemia, and solid tumors. FA patients have a 500 to 800-fold increase in head and neck squamous cell carcinoma compared to the general population and the treatment for these malignancies are ineffective and limited due to the deficiency in DNA damage repair. Using unbiased CRISPR-interference screening, we found the loss of FA function renders cells dependent on key exocytosis genes such as SNAP23. Further investigation revealed that loss of FA pathway function induced deficiencies in lysosomal health, dysregulation of autophagy and increased lysosomal exocytosis. The compromised cellular state caused by the loss of FA genes is accompanied with decreased lysosome abundance and increased lysosomal membrane permeabilization in cells. We found these signatures in vitro across multiple cell types and cell lines and in clinically relevant FA patient cancers. Our findings are the first to connect the FA pathway to lysosomal exocytosis and thus expands our understanding of FA as a disease and of induced dependencies in FA mutant cancers.

Benchmarking and optimizing Perturb-seq in differentiating human pluripotent stem cells

Perturb-seq is a powerful approach to systematically assess how genes and enhancers impact the molecular and cellular pathways of development and disease. However, technical challenges have limited its application in stem cell-based systems. Here, we benchmarked Perturb-seq across multiple CRISPRi modalities, on diverse genomic targets, in multiple human pluripotent stem cells, during directed differentiation to multiple lineages, and across multiple sgRNA delivery systems. To ensure cost-effective production of large-scale Perturb-seq datasets as part of the Impact of Genomic Variants on Function (IGVF) consortium, our optimized protocol dynamically assesses experiment quality across the weeks-long procedure. Our analysis of 1,996,260 sequenced cells across benchmarking datasets reveals shared regulatory networks linking disease-associated enhancers and genes with downstream targets during cardiomyocyte differentiation. This study establishes open tools and resources for interrogating genome function during stem cell differentiation.

A systematic screening assay identifies efficient small guide RNAs for CRISPR activation

CRISPR-mediated gene activation (CRISPRa) encompasses a growing field of biotechnological approaches with exciting implications for gene therapy. However, there is a lack of experimental validation tools for selecting efficient sgRNAs for downstream applications. Here, we present a screening assay capable of identifying efficient single- and double sgRNAs through fluorescence quantification in vitro. In addition, we provide a tailored Golden Gate cloning workflow for streamlined incorporation of selected sgRNA candidates into lentiviral (LVs) or adeno-associated viral vectors (AAVs). The overall workflow was validated using therapeutically relevant genes for neurodegenerative diseases, including Tfeb, Adam17, and Sirt1. The most efficient sgRNAs also demonstrated activation of endogenous gene expression at mRNA level. Correlation analysis of gene activation relative to sgRNA binding site distance to transcription start-site or nearby transcription factor binding sites failed to detect common characteristics influencing gene activation in the selected promoter regions. This data demonstrates the potential of the screening assay to identify functionally efficient sgRNA candidates across multiple genes along with streamlined cloning of viral vectors and may assist in accelerating future developments of CRISPRa-focused applications.

Post-transcriptional cross- and auto-regulation buffer expression of the human RNA helicases DDX3X and DDX3Y

The Y-linked gene DDX3Y and its X-linked homolog DDX3X survived the evolution of the human sex chromosomes from ordinary autosomes. DDX3X encodes a multifunctional RNA helicase, with mutations causing developmental disorders and cancers. We find that, among X-linked genes with surviving Y homologs, DDX3X is extraordinarily dosage sensitive. Studying cells of individuals with sex chromosome aneuploidy, we observe that when the number of Y Chromosomes increases, DDX3X transcript levels fall; conversely, when the number of X Chromosomes increases, DDX3Y transcript levels fall. In 46,XY cells, CRISPRi knockdown of either DDX3X or DDX3Y causes transcript levels of the homologous gene to rise. In 46,XX cells, chemical inhibition of DDX3X protein activity elicits an increase in DDX3X transcript levels. Thus, perturbation of either DDX3X or DDX3Y expression is buffered: by negative cross-regulation of DDX3X and DDX3Y in 46,XY cells and by negative auto-regulation of DDX3X in 46,XX cells. DDX3X-DDX3Y cross-regulation is mediated through mRNA destabilization-as shown by metabolic labeling of newly transcribed RNA-and buffers total levels of DDX3X and DDX3Y protein in human cells. We infer that post-transcriptional auto-regulation of the ancestral (autosomal) DDX3X gene transmuted into auto- and cross-regulation of DDX3X and DDX3Y as these sex-linked genes evolved from ordinary alleles of their autosomal precursor.