Mapping variant effects on anti-tumor hallmarks of primary human T cells with base-editing screens.

Single-nucleotide variants (SNVs) in key T cell genes can drive clinical pathologies and could be repurposed to improve cellular cancer immunotherapies. Here, we perform massively parallel base-editing screens to generate thousands of variants at gene loci annotated with known or potential clinical relevance. We discover a broad landscape of putative gain-of-function (GOF) and loss-of-function (LOF) mutations, including in PIK3CD and the gene encoding its regulatory subunit, PIK3R1, LCK, SOS1, AKT1 and RHOA. Base editing of PIK3CD and PIK3R1 variants in T cells with an engineered T cell receptor specific to a melanoma epitope or in different generations of CD19 chimeric antigen receptor (CAR) T cells demonstrates that discovered GOF variants, but not LOF or silent mutation controls, enhanced signaling, cytokine production and lysis of cognate melanoma and leukemia cell models, respectively. Additionally, we show that generations of CD19 CAR T cells engineered with PIK3CD GOF mutations demonstrate enhanced antigen-specific signaling, cytokine production and leukemia cell killing, including when benchmarked against other recent strategies.

SMARCAL1 is a dual regulator of innate immune signaling and PD-L1 expression that promotes tumor immune evasion.

Genomic instability can trigger cancer-intrinsic innate immune responses that promote tumor rejection. However, cancer cells often evade these responses by overexpressing immune checkpoint regulators, such as PD-L1. Here, we identify the SNF2-family DNA translocase SMARCAL1 as a factor that favors tumor immune evasion by a dual mechanism involving both the suppression of innate immune signaling and the induction of PD-L1-mediated immune checkpoint responses. Mechanistically, SMARCAL1 limits endogenous DNA damage, thereby suppressing cGAS-STING-dependent signaling during cancer cell growth. Simultaneously, it cooperates with the AP-1 family member JUN to maintain chromatin accessibility at a PD-L1 transcriptional regulatory element, thereby promoting PD-L1 expression in cancer cells. SMARCAL1 loss hinders the ability of tumor cells to induce PD-L1 in response to genomic instability, enhances anti-tumor immune responses and sensitizes tumors to immune checkpoint blockade in a mouse melanoma model. Collectively, these studies uncover SMARCAL1 as a promising target for cancer immunotherapy.

The DNA Damage Response and Inflammation in Cancer.

Genomic stability in normal cells is crucial to avoid oncogenesis. Accordingly, multiple components of the DNA damage response (DDR) operate as bona fide tumor suppressor proteins by preserving genomic stability, eliciting the demise of cells with unrepairable DNA lesions, and engaging cell-extrinsic oncosuppression via immunosurveillance. That said, DDR sig-naling can also favor tumor progression and resistance to therapy. Indeed, DDR signaling in cancer cells has been consistently linked to the inhibition of tumor-targeting immune responses. Here, we discuss the complex interactions between the DDR and inflammation in the context of oncogenesis, tumor progression, and response to therapy. SIGNIFICANCE: Accumulating preclinical and clinical evidence indicates that DDR is intimately connected to the emission of immunomodulatory signals by normal and malignant cells, as part of a cell-extrinsic program to preserve organismal homeostasis. DDR-driven inflammation, however, can have diametrically opposed effects on tumor-targeting immunity. Understanding the links between the DDR and inflammation in normal and malignant cells may unlock novel immunotherapeutic paradigms to treat cancer.

Massively parallel base editing screens to map variant effects on anti-tumor hallmarks of primary human T cells.

Base editing enables generation of single nucleotide variants, but large-scale screening in primary human T cells is limited due to low editing efficiency, among other challenges 1 . Here, we developed a high-throughput approach for high-efficiency and massively parallel adenine and cytosine base-editor screening in primary human T cells. We performed multiple large-scale screens editing 102 genes with central functions in T cells and full-length tiling mutagenesis of selected genes, and read out variant effects on hallmarks of T cell anti-tumor immunity, including activation, proliferation, and cytokine production. We discovered a broad landscape of gain- and loss-of-function mutations, including in PIK3CD and its regulatory subunit encoded by PIK3R1, LCK , AKT1, CTLA-4 and JAK1 . We identified variants that affected several (e.g., PIK3CD C416R) or only selected (e.g. LCK Y505C) hallmarks of T cell activity, and functionally validated several hits by probing downstream signaling nodes and testing their impact on T cell polyfunctionality and proliferation. Using primary human T cells in which we engineered a T cell receptor (TCR) specific to a commonly presented tumor testis antigen as a model for cellular immunotherapy, we demonstrate that base edits identified in our screens can tune specific or broad T cell functions and ultimately improve tumor elimination while exerting minimal off-target activity. In summary, we present the first large-scale base editing screen in primary human T cells and provide a framework for scalable and targeted base editing at high efficiency. Coupled with multi-modal phenotypic mapping, we accurately nominate variants that produce a desirable T cell state and leverage these synthetic proteins to improve models of cellular cancer immunotherapies.

CRISPRmap: Sequencing-free optical pooled screens mapping multi-omic phenotypes in cells and tissue.

Pooled genetic screens are powerful tools to study gene function in a high-throughput manner. Typically, sequencing-based screens require cell lysis, which limits the examination of critical phenotypes such as cell morphology, protein subcellular localization, and cell-cell/tissue interactions. In contrast, emerging optical pooled screening methods enable the investigation of these spatial phenotypes in response to targeted CRISPR perturbations. In this study, we report a multi-omic optical pooled CRISPR screening method, which we have named CRISPRmap. Our method combines a novel in situ CRISPR guide identifying barcode readout approach with concurrent multiplexed immunofluorescence and in situ RNA detection. CRISPRmap barcodes are detected and read out through combinatorial hybridization of DNA oligos, enhancing barcode detection efficiency, while reducing both dependency on third party proprietary sequencing reagents and assay cost. Notably, we conducted a multi-omic base-editing screen in a breast cancer cell line on core DNA damage repair genes involved in the homologous recombination and Fanconi anemia pathways investigating how nucleotide variants in those genes influence DNA damage signaling and cell cycle regulation following treatment with ionizing radiation or DNA damaging agents commonly used for cancer therapy. Approximately a million cells were profiled with our multi-omic approach, providing a comprehensive phenotypic assessment of the functional consequences of the studied variants. CRISPRmap enabled us to pinpoint likely-pathogenic patient-derived mutations that were previously classified as variants of unknown clinical significance. Furthermore, our approach effectively distinguished barcodes of a pooled library in tumor tissue, and we coupled it with cell-type and molecular phenotyping by cyclic immunofluorescence. Multi-omic spatial analysis of how CRISPR-perturbed cells respond to various environmental cues in the tissue context offers the potential to significantly expand our understanding of tissue biology in both health and disease.

Partial loss-of-function mutations in GINS4 lead to NK cell deficiency with neutropenia.

Human NK cell deficiency (NKD) is a primary immunodeficiency in which the main clinically relevant immunological defect involves missing or dysfunctional NK cells. Here, we describe a familial NKD case in which 2 siblings had a substantive NKD and neutropenia in the absence of other immune system abnormalities. Exome sequencing identified compound heterozygous variants in Go-Ichi-Ni-San (GINS) complex subunit 4 (GINS4, also known as SLD5), an essential component of the human replicative helicase, which we demonstrate to have a damaging impact upon the expression and assembly of the GINS complex. Cells derived from affected individuals and a GINS4-knockdown cell line demonstrate delayed cell cycle progression, without signs of improper DNA synthesis or increased replication stress. By modeling partial GINS4 depletion in differentiating NK cells in vitro, we demonstrate the causal relationship between the genotype and the NK cell phenotype, as well as a cell-intrinsic defect in NK cell development. Thus, biallelic partial loss-of-function mutations in GINS4 define a potentially novel disease-causing gene underlying NKD with neutropenia. Together with the previously described mutations in other helicase genes causing NKD, and with the mild defects observed in other human cells, these variants underscore the importance of this pathway in NK cell biology.

Histone methylation antagonism drives tumor immune evasion in squamous cell carcinomas.

How cancer-associated chromatin abnormalities shape tumor-immune interaction remains incompletely understood. Recent studies have linked DNA hypomethylation and de-repression of retrotransposons to anti-tumor immunity through the induction of interferon response. Here, we report that inactivation of the histone H3K36 methyltransferase NSD1, which is frequently found in squamous cell carcinomas (SCCs) and induces DNA hypomethylation, unexpectedly results in diminished tumor immune infiltration. In syngeneic and genetically engineered mouse models of head and neck SCCs, NSD1-deficient tumors exhibit immune exclusion and reduced interferon response despite high retrotransposon expression. Mechanistically, NSD1 loss results in silencing of innate immunity genes, including the type III interferon receptor IFNLR1, through depletion of H3K36 di-methylation (H3K36me2) and gain of H3K27 tri-methylation (H3K27me3). Inhibition of EZH2 restores immune infiltration and impairs the growth of Nsd1-mutant tumors. Thus, our work uncovers a druggable chromatin cross talk that regulates the viral mimicry response and enables immune evasion of DNA hypomethylated tumors.

REV1-Polζ maintains the viability of homologous recombination-deficient cancer cells through mutagenic repair of PRIMPOL-dependent ssDNA gaps.

BRCA1/2 mutant tumor cells display an elevated mutation burden, the etiology of which remains unclear. Here, we report that these cells accumulate ssDNA gaps and spontaneous mutations during unperturbed DNA replication due to repriming by the DNA primase-polymerase PRIMPOL. Gap accumulation requires the DNA glycosylase SMUG1 and is exacerbated by depletion of the translesion synthesis (TLS) factor RAD18 or inhibition of the error-prone TLS polymerase complex REV1-Polζ by the small molecule JH-RE-06. JH-RE-06 treatment of BRCA1/2-deficient cells results in reduced mutation rates and PRIMPOL- and SMUG1-dependent loss of viability. Through cellular and animal studies, we demonstrate that JH-RE-06 is preferentially toxic toward HR-deficient cancer cells. Furthermore, JH-RE-06 remains effective toward PARP inhibitor (PARPi)-resistant BRCA1 mutant cells and displays additive toxicity with crosslinking agents or PARPi. Collectively, these studies identify a protective and mutagenic role for REV1-Polζ in BRCA1/2 mutant cells and provide the rationale for using REV1-Polζ inhibitors to treat BRCA1/2 mutant tumors.

Functional interrogation of DNA damage response variants with base editing screens.

Mutations in DNA damage response (DDR) genes endanger genome integrity and predispose to cancer and genetic disorders. Here, using CRISPR-dependent cytosine base editing screens, we identify > 2,000 sgRNAs that generate nucleotide variants in 86 DDR genes, resulting in altered cellular fitness upon DNA damage. Among those variants, we discover loss- and gain-of-function mutants in the Tudor domain of the DDR regulator 53BP1 that define a non-canonical surface required for binding the deubiquitinase USP28. Moreover, we characterize variants of the TRAIP ubiquitin ligase that define a domain, whose loss renders cells resistant to topoisomerase I inhibition. Finally, we identify mutations in the ATM kinase with opposing genome stability phenotypes and loss-of-function mutations in the CHK2 kinase previously categorized as variants of uncertain significance for breast cancer. We anticipate that this resource will enable the discovery of additional DDR gene functions and expedite studies of DDR variants in human disease.

HATtracting Nucleases to Stalled Forks.

In this issue of Molecular Cell, Kim et al., 2020 report that PCAF is a fork-associated histone acetyltransferase (HAT) that regulates the stability of stalled forks and the response to PARP inhibition in BRCA1/2-deficient cells.

Time for remodeling: SNF2-family DNA translocases in replication fork metabolism and human disease.

Over the course of DNA replication, DNA lesions, transcriptional intermediates and protein-DNA complexes can impair the progression of replication forks, thus resulting in replication stress. Failure to maintain replication fork integrity in response to replication stress leads to genomic instability and predisposes to the development of cancer and other genetic disorders. Multiple DNA damage and repair pathways have evolved to allow completion of DNA replication following replication stress, thus preserving genomic integrity. One of the processes commonly induced in response to replication stress is fork reversal, which consists in the remodeling of stalled replication forks into four-way DNA junctions. In normal conditions, fork reversal slows down replication fork progression to ensure accurate repair of DNA lesions and facilitates replication fork restart once the DNA lesions have been removed. However, in certain pathological situations, such as the deficiency of DNA repair factors that protect regressed forks from nuclease-mediated degradation, fork reversal can cause genomic instability. In this review, we describe the complex molecular mechanisms regulating fork reversal, with a focus on the role of the SNF2-family fork remodelers SMARCAL1, ZRANB3 and HLTF, and highlight the implications of fork reversal for tumorigenesis and cancer therapy.

MCM8IP activates the MCM8-9 helicase to promote DNA synthesis and homologous recombination upon DNA damage.

Homologous recombination (HR) mediates the error-free repair of DNA double-strand breaks to maintain genomic stability. Here we characterize C17orf53/MCM8IP, an OB-fold containing protein that binds ssDNA, as a DNA repair factor involved in HR. MCM8IP-deficient cells exhibit HR defects, especially in long-tract gene conversion, occurring downstream of RAD51 loading, consistent with a role for MCM8IP in HR-dependent DNA synthesis. Moreover, loss of MCM8IP confers cellular sensitivity to crosslinking agents and PARP inhibition. Importantly, we report that MCM8IP directly associates with MCM8-9, a helicase complex mutated in primary ovarian insufficiency, and RPA1. We additionally show that the interactions of MCM8IP with MCM8-9 and RPA facilitate HR and promote replication fork progression and cellular viability in response to treatment with crosslinking agents. Mechanistically, MCM8IP stimulates the helicase activity of MCM8-9. Collectively, our work identifies MCM8IP as a key regulator of MCM8-9-dependent DNA synthesis during DNA recombination and replication.

Stimulation of CRISPR-mediated homology-directed repair by an engineered RAD18 variant.

Precise editing of genomic DNA can be achieved upon repair of CRISPR-induced DNA double-stranded breaks (DSBs) by homology-directed repair (HDR). However, the efficiency of this process is limited by DSB repair pathways competing with HDR, such as non-homologous end joining (NHEJ). Here we individually express in human cells 204 open reading frames involved in the DNA damage response (DDR) and determine their impact on CRISPR-mediated HDR. From these studies, we identify RAD18 as a stimulator of CRISPR-mediated HDR. By defining the RAD18 domains required to promote HDR, we derive an enhanced RAD18 variant (e18) that stimulates CRISPR-mediated HDR in multiple human cell types, including embryonic stem cells. Mechanistically, e18 induces HDR by suppressing the localization of the NHEJ-promoting factor 53BP1 to DSBs. Altogether, this study identifies e18 as an enhancer of CRISPR-mediated HDR and highlights the promise of engineering DDR factors to augment the efficiency of precision genome editing.

The BRCT Domains of the BRCA1 and BARD1 Tumor Suppressors Differentially Regulate Homology-Directed Repair and Stalled Fork Protection.

The BRCA1 tumor suppressor preserves genome integrity through both homology-directed repair (HDR) and stalled fork protection (SFP). In vivo, BRCA1 exists as a heterodimer with the BARD1 tumor suppressor, and both proteins harbor a phosphate-binding BRCT domain. Here, we compare mice with mutations that ablate BRCT phospho-recognition by Bard1 (Bard1S563F and Bard1K607A) or Brca1 (Brca1S1598F). Brca1S1598F abrogates both HDR and SFP, suggesting that both pathways are likely impaired in most BRCA1 mutant tumors. Although not affecting HDR, the Bard1 mutations ablate poly(ADP-ribose)-dependent recruitment of BRCA1/BARD1 to stalled replication forks, resulting in fork degradation and chromosome instability. Nonetheless, Bard1S563F/S563F and Bard1K607A/K607A mice, unlike Brca1S1598F/S1598F mice, are not tumor prone, indicating that HDR alone is sufficient to suppress tumor formation in the absence of SFP. Nevertheless, because SFP, unlike HDR, is also impaired in heterozygous Brca1/Bard1 mutant cells, SFP and HDR may contribute to distinct stages of tumorigenesis in BRCA1/BARD1 mutation carriers.

CSA and CSB play a role in the response to DNA breaks.

CS proteins have been involved in the repair of a wide variety of DNA lesions. Here, we analyse the role of CS proteins in DNA break repair by studying histone H2AX phosphorylation in different cell cycle phases and DNA break repair by comet assay in CS-A and CS-B primary and transformed cells. Following methyl methane sulphate treatment a significant accumulation of unrepaired single strand breaks was detected in CS cells as compared to normal cells, leading to accumulation of double strand breaks in S and G2 phases. A delay in DSBs repair and accumulation in S and G2 phases were also observed following IR exposure. These data confirm the role of CSB in the suppression of NHEJ in S and G2 phase cells and extend this function to CSA. However, the repair kinetics of double strand breaks showed unique features for CS-A and CS-B cells suggesting that these proteins may act at different times along DNA break repair. The involvement of CS proteins in the repair of DNA breaks may play an important role in the clinical features of CS patients.

Crosstalk between mismatch repair and base excision repair in human gastric cancer.

DNA repair gene expression in a set of gastric cancers suggested an inverse association between the expression of the mismatch repair (MMR) gene MLH1 and that of the base excision repair (BER) gene DNA polymerase ß (Polß). To gain insight into possible crosstalk of these two repair pathways in cancer, we analysed human gastric adenocarcinoma AGS cells over-expressing Polß or Polß active site mutants, alone or in combination with MLH1 silencing. Next, we investigated the cellular response to the alkylating agent methyl methanesulfonate (MMS) and the purine analogue 6-thioguanine (6-TG), agents that induce lesions that are substrates for BER and/or MMR. AGS cells over-expressing Polß were resistant to 6-TG to a similar extent as when MLH1 was inactivated while inhibition of O6-methylguanine-DNA methyltransferase (MGMT) was required to detect resistance to MMS. Upon either treatment, the association with MLH1 down-regulation further amplified the resistant phenotype. Moreover, AGS cells mutated in Polß were hypersensitive to both 6-TG and MMS killing and their sensitivity was partially rescued by MLH1 silencing. We provide evidence that the critical lethal lesions in this new pathway are double strand breaks that are exacerbated when Polß is defective and relieved when MLH1 is silenced. In conclusion, we provide evidence of crosstalk between MLH1 and Polß that modulates the response to alkylation damage. These studies suggest that the Polß/MLH1 status should be taken into consideration when designing chemotherapeutic approaches for gastric cancer.

Restoration of Replication Fork Stability in BRCA1- and BRCA2-Deficient Cells by Inactivation of SNF2-Family Fork Remodelers.

To ensure the completion of DNA replication and maintenance of genome integrity, DNA repair factors protect stalled replication forks upon replication stress. Previous studies have identified a critical role for the tumor suppressors BRCA1 and BRCA2 in preventing the degradation of nascent DNA by the MRE11 nuclease after replication stress. Here we show that depletion of SMARCAL1, a SNF2-family DNA translocase that remodels stalled forks, restores replication fork stability and reduces the formation of replication stress-induced DNA breaks and chromosomal aberrations in BRCA1/2-deficient cells. In addition to SMARCAL1, other SNF2-family fork remodelers, including ZRANB3 and HLTF, cause nascent DNA degradation and genomic instability in BRCA1/2-deficient cells upon replication stress. Our observations indicate that nascent DNA degradation in BRCA1/2-deficient cells occurs as a consequence of MRE11-dependent nucleolytic processing of reversed forks generated by fork remodelers. These studies provide mechanistic insights into the processes that cause genome instability in BRCA1/2-deficient cells.

WRNIP1: A new guardian of genome integrity at stalled replication forks.

Failure to protect and/or restart stalled replication forks contributes to genomic instability. Radiation-sensitive 51 (RAD51) recombinase defends stalled forks from nucleolytic attack, which otherwise can threaten their integrity. Recently, we have uncovered a novel and key function of Werner helicase interacting protein 1 (WRNIP1) as a fork-protective factor working in conjunction with RAD51 in response to replication stress.

WRNIP1 protects stalled forks from degradation and promotes fork restart after replication stress.

Accurate handling of stalled replication forks is crucial for the maintenance of genome stability. RAD51 defends stalled replication forks from nucleolytic attack, which otherwise can threaten genome stability. However, the identity of other factors that can collaborate with RAD51 in this task is poorly elucidated. Here, we establish that human Werner helicase interacting protein 1 (WRNIP1) is localized to stalled replication forks and cooperates with RAD51 to safeguard fork integrity. We show that WRNIP1 is directly involved in preventing uncontrolled MRE11-mediated degradation of stalled replication forks by promoting RAD51 stabilization on ssDNA We further demonstrate that replication fork protection does not require the ATPase activity of WRNIP1 that is however essential to achieve the recovery of perturbed replication forks. Loss of WRNIP1 or its catalytic activity causes extensive DNA damage and chromosomal aberrations. Intriguingly, downregulation of the anti-recombinase FBH1 can compensate for loss of WRNIP1 activity, since it attenuates replication fork degradation and chromosomal aberrations in WRNIP1-deficient cells. Therefore, these findings unveil a unique role for WRNIP1 as a replication fork-protective factor in maintaining genome stability.

Checkpoint-dependent and independent roles of the Werner syndrome protein in preserving genome integrity in response to mild replication stress.

Werner syndrome (WS) is a human chromosomal instability disorder associated with cancer predisposition and caused by mutations in the WRN gene. WRN helicase activity is crucial in limiting breakage at common fragile sites (CFS), which are the preferential targets of genome instability in precancerous lesions. However, the precise function of WRN in response to mild replication stress, like that commonly used to induce breaks at CFS, is still missing. Here, we establish that WRN plays a role in mediating CHK1 activation under moderate replication stress. We provide evidence that phosphorylation of CHK1 relies on the ATR-mediated phosphorylation of WRN, but not on WRN helicase activity. Analysis of replication fork dynamics shows that loss of WRN checkpoint mediator function as well as of WRN helicase activity hamper replication fork progression, and lead to new origin activation to allow recovery from replication slowing upon replication stress. Furthermore, bypass of WRN checkpoint mediator function through overexpression of a phospho-mimic form of CHK1 restores fork progression and chromosome stability to the wild-type levels. Together, these findings are the first demonstration that WRN regulates the ATR-checkpoint activation upon mild replication stress, preventing chromosome fragility.