Stephen Philip Jackson

Many cancer cells exhibit elevated replication stress and are highly dependent on ataxia telangiectasia and Rad3-related (ATR) kinase activity to maintain genomic stability and survival. While ATR inhibitors (ATRi) have great promise as therapeutic agents, intrinsic or acquired resistance will likely be a significant challenge. We previously showed that loss of the RNA polymerase II Mediator subunits CDK8 and Cyclin C (CCNC) confers resistance to ATRi by suppressing transcription-dependent replication stress. To identify vulnerabilities that could be exploited to restore ATRi sensitivity in these resistant settings, we performed genome-wide CRISPR screens in wild-type and CDK8-deficient cells. These screens revealed impairment of epigenetic components HDAC3 or the PRC2 complex as synthetic vulnerabilities that enhance ATRi sensitivity, particularly in contexts of loss of CDK8 or CCNC. This ATRi sensitivity is driven by the induction of transcriptional dysregulation, leading to increased transcription–replication collisions, replication stress, and apoptosis upon ATR inhibition. Importantly, we show that HDAC3 loss limits growth of ATRi-resistant tumours in vivo. Moreover, pharmacological inhibition of HDAC3 or PRC2 phenocopies their genetic loss. Collectively, our findings highlight the therapeutic potential of targeting epigenetic regulators to induce transcriptional dysregulation and ensuing replication stress to overcome ATRi resistance.

The Ser/Thr kinase ATM orchestrates cellular responses to DNA double-strand breaks (DSBs) and promotes DSB repair by homologous recombination. In this process, ATM is activated by DNA and the MRN (MRE11, RAD50, and NBS1) complex. Here we show that mutations of the conserved PIKK regulatory domain (PRD) within ATM’s kinase domain can confer a maximally active state that no longer requires MRN/DNA. In ATM-knockout human cells, the PRD mutants display substantially higher phosphorylation of histone H2AX, KAP1, and CHK2 than wild-type ATM, with or without IR-induced DNA damage. Cryo-EM structures of two PRD mutants each revealed basal or activated conformations depending on bound ligands, suggesting that disrupting the ordered portion of the PRD results in an enzyme poised to transition to the active conformation. However, the identity of the active-site nucleotide is a key driver of the conformational switching. We speculate that this plasticity might be exploited to develop small-molecule ATM modulators for therapeutic applications.

Stimulation of the innate immune system by foreign RNA elicits a potent interferon response and can trigger cell death. The mechanisms by which cells balance a robust response with cell-intrinsic lethality are still being uncovered. Here, using genome-wide CRISPR–Cas9 genetic screens with triphosphorylated RNA stimulation, we discover that promyelocytic leukaemia (PML) nuclear body-localized speckled protein 110 (SP110) is a potent inhibitor of type 1 interferon-driven cell death. Death suppression by SP110 counteracts a toxic activity of SP100, a major constituent of PML bodies. Loss of SP110 leads to mitotic retention of SP100 and PML bodies, which associate with and perturb segregating chromosomes, leading to micronucleus formation, DNA damage and genotoxic cell death. A combination of cryo-electron microscopy, AlphaFold modelling and cellular biochemistry reveals that SP110 dissolves toxic SP100 oligomers via necessary and sufficient direct interactions between their caspase activation and recruitment domains. These data reveal the critical roles of SP100 and SP110 in governing the disassembly of PML bodies during mitosis, as well as the repercussions if this process is misregulated.

DNA double-strand breaks (DSBs) both pose threats to genome integrity and are commonly used for genome editing applications. Structural features of DSB ends play key roles in determining DNA repair pathway usage and outcomes during genome editing, but the cellular factors involved in these processes are only partially known. Through genome-wide CRISPRi screening, we identify ERCC6L2 as critical for repairing Cas12a-induced staggered DSBs but irrelevant for Cas9-induced blunt DSBs. We show that ERCC6L2 acts as a protection factor for staggered DSBs with either 5′ or 3′ polarity, preventing large deletions and translocations stemming from DNA damage induced by Cas12a, TALENs, or dual Cas9 nicks. Furthermore, ERCC6L2 loss hyper-sensitizes cells to multiple staggered DSBs induced by promiscuous Cas12a activity or etoposide-induced TOP2 trapping. By combining genetics and biochemical reconstitution, we find that ERCC6L2 counteracts MRE11-RAD50-NBS1 (MRN)-mediated resection by binding and melting staggered DNA ends, thereby promoting accurate end joining. Our data reveal a protective role of ERCC6L2 in staggered-end DSB repair, which suggests the molecular underpinnings of pathology in patients with ERCC6L2 mutations and cautions against using overhang-inducing genome editing tools for their treatment.

Ubiquitin E3 ligases play crucial roles in the DNA damage response (DDR) by modulating the turnover, localization, activation, and interactions of DDR and DNA replication proteins. We performed a CRISPR-Cas9 knockout screen focused on ubiquitin E3 ligases and related proteins with the DNA topoisomerase I inhibitor camptothecin. This led us to establish that MAEA, a core subunit of the CTLH E3 ligase complex, is a critical regulator of homologous recombination and the replication stress response. In tandem, we identified eight patients with variants in MAEA who present with a neurodevelopmental disorder that we term DIADEM ( D evelopmental delay and I ntellectual disability A ssociated with DE fects in M AEA). Analysis of patient-derived cell lines and mutation modeling reveal an underlying defect in HR-dependent DNA repair and replication fork restart and protection as a likely cause of disease. Mechanistically, we find that MAEA dysfunction hinders DNA repair by reducing the efficiency of RAD51 loading at sites of DNA damage, which we propose may contribute to the presentation of DIADEM by compromising genome integrity and cell division during development.

MDC1 is a key protein in DNA damage signaling. When DNA double-strand breaks (DSBs) occur, MDC1 localizes to the sites of DNA damage to promote the recruitment of other factors, including the 53BP1-mediated DSB repair pathway. By studying mechanisms of poly (ADP-ribose) polymerase inhibitor (PARPi) resistance in BRCA2; p53-deficient mouse mammary tumors, we identified a thus far unknown role of MDC1 in replication fork biology. Our results show that MDC1 localizes at active replication forks during normal DNA replication and regulates replication fork progression. It suppresses spontaneous fork reversal and regulates fork nucleolytic processing thereby promoting sensitivity to PARPi and cisplatin. In this way, MDC1 loss improves DNA damage tolerance and causes chemoresistance in BRCA1/2-deficient cells. We demonstrate that limiting MRE11 activity abolishes the reduced fork speed while MRE11 inhibition/depletion overcomes PARPi resistance in these cells. Overall, our data provides new insights into the role of MDC1 in replication fork progression that mediates PARPi- and cisplatin-induced DNA damage, in addition to its role in DSB repair.

Inhibitors of the protein kinase WEE1 have emerged as promising agents for cancer therapy. In this study, we uncover synergistic interactions between WEE1 small-molecule inhibitors and defects in mRNA translation, mediated by activation of the integrated stress response (ISR) through the kinase GCN2. Using a pooled CRISPRi screen, we identify GSPT1 and ALKBH8 as factors whose depletion confer hypersensitivity to the WEE1 inhibitor, AZD1775. We demonstrate that this synergy depends on ISR activation, which is induced by the off-target activity of WEE1 inhibitors. Furthermore, PROTAC-based WEE1 inhibitors and molecular glues show reduced or no ISR activation, suggesting potential strategies to minimise off-target toxicity. Our findings reveal that certain WEE1 inhibitors elicit dual toxicity via ISR activation and genotoxic stress, with ISR activation being independent of WEE1 itself or cell-cycle status. This dual mechanism highlights opportunities for combination therapies, such as pairing WEE1 inhibitors with agents targeting the mRNA translation machinery. This study also underscores the need for more precise WEE1 targeting strategies to mitigate off-target effects, with implications for optimising the therapeutic potential of WEE1 inhibitors.

The eukaryotic replisome, which consists of the CDC45-MCM2-7-GINS (CMG) helicase, replicative polymerases, and several accessory factors, sometimes encounters proteinaceous obstacles that threaten genome integrity. These obstacles are targeted for removal or proteolysis by the E3 ubiquitin ligase TRAIP, which associates with the replisome. However, TRAIP must be carefully regulated to avoid inappropriate ubiquitylation and disassembly of the replisome. Here, we demonstrate that human cells lacking the de-ubiquitylating enzyme USP37 are hypersensitive to topoisomerase poisons and other replication stress-inducing agents. Furthermore, TRAIP loss rescues the hypersensitivity of USP37 knockout cells to topoisomerase inhibitors. In Xenopus egg extracts depleted of USP37, TRAIP promotes premature CMG ubiquitylation and disassembly when converging replisomes stall. Finally, guided by AlphaFold-Multimer, we discovered that binding to CDC45 mediates USP37’s response to topological stress. We propose that USP37 protects genome stability by preventing TRAIP-dependent CMG unloading when replication stress impedes timely termination.