Eric Jackson Aird

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.

Base editors create precise genomic edits by directing nucleobase deamination or removal without inducing double-stranded DNA breaks. However, a vast chemical space of other DNA modifications remains to be explored for genome editing. Here we harness the bacterial antiphage toxin DarT2 to append ADP-ribosyl moieties to DNA, unlocking distinct editing outcomes in bacteria versus eukaryotes. Fusing an attenuated DarT2 to a Cas9 nickase, we program site-specific ADP-ribosylation of thymines within a target DNA sequence. In tested bacteria, targeting drives homologous recombination, offering flexible and scar-free genome editing without base replacement or counterselection. In tested yeast, plant and human cells, targeting drives substitution of the modified thymine to adenine or a mixture of adenine and cytosine with limited insertions or deletions, offering edits inaccessible to current base editors. Altogether, our approach, called append editing, leverages the addition of chemical moieties to DNA to expand current modalities for precision gene editing.

The DNA-PKcs inhibitor AZD7648 enhances CRISPR–Cas9-directed homology-directed repair efficiencies, with potential for clinical utility, but its possible on-target consequences are unknown. We found that genome editing with AZD7648 causes frequent kilobase-scale and megabase-scale deletions, chromosome arm loss and translocations. These large-scale chromosomal alterations evade detection through typical genome editing assays, prompting caution in deploying AZD7648 and reinforcing the need to investigate multiple types of potential editing outcomes.

Stimulation of the innate immune system by foreign RNA elicits a potent response against invading pathogens and can trigger cell death. The mechanisms by which cells balance a robust response with cell-intrinsic lethality are still being uncovered. Employing genome-wide CRISPR-Cas9 genetic screens with triphosphorylated RNA stimulation, we identify speckled protein 110 (SP110) as a potent negative regulator of type 1 interferon-driven cell death. Death suppression by SP110 counteracts a death-promoting activity of another speckled protein, SP100. Both SP110 suppression and SP100 toxicity are mediated by direct interactions between the caspase activation and recruitment domains (CARDs) in each protein. SP100-induced death is mediated by homomeric CARD filaments that are disassembled by a heteromeric CARD interaction with SP110. Overexpression of SP100 is sufficient to overwhelm normal levels of SP110, leading to genotoxicity. Using cryo-EM and AlphaFold modeling, we develop and validate an atomic description of SP100 CARD filament formation and filament breaking by SP110. Genome-wide binding studies reveal that SP110 and SP100 normally associate at active promoters, but disruption of the CARD interaction releases SP100 to form toxic filaments. Overall, we uncover a novel regulatory partnership in human innate immunity that balances signal potency with cell intrinsic lethality.

CRISPR-Cas induced homology-directed repair (HDR) enables the installation of a broad range of precise genomic modifications from an exogenous donor template. However, applications of HDR in human cells are often hampered by poor efficiency, stemming from a preference for error-prone end joining pathways that yield short insertions and deletions. Here, we describe Recursive Editing, an HDR improvement strategy that selectively retargets undesired indel outcomes to create additional opportunities to produce the desired HDR allele. We introduce a software tool, named REtarget, that enables the rational design of Recursive Editing experiments. Using REtarget-designed guide RNAs in single editing reactions, Recursive Editing can simultaneously boost HDR efficiencies and reduce undesired indels. We also harness REtarget to generate databases for particularly effective Recursive Editing sites across the genome, to endogenously tag proteins, and to target pathogenic mutations. Recursive Editing constitutes an easy-to-use approach without potentially deleterious cell manipulations and little added experimental burden.

Prime editing brings immense promise to correct a large number of human pathogenic mutations and enact diverse edit types without introducing widespread undesired editing events. Delivery of prime editors in vivo would enable such edits to be introduced in a clinical setting. The coding sequence for prime editor, however, is too large to fit within the size-constrained adeno-associated virus (AAV) genome. Herein, we describe a split Staphylococcus aureus prime editor capable of being delivered by dual AAVs. We characterize the editing ability of plasmid-based versions of an S. aureus prime editor in vitro at a variety of loci with diverse edit types. We investigate various split prime editor architectures and alternative dimerization domains. Finally, we demonstrate the capacity of prime editor to be co-delivered by dual AAVs in vitro. While editing rates are lower than desired, this approach presents an important step to translate prime editing for in vivo delivery.

Replication initiator proteins (Reps) from the HUH-endonuclease superfamily process specific single-stranded DNA (ssDNA) sequences to initiate rolling circle/hairpin replication in viruses, such as crop ravaging geminiviruses and human disease causing parvoviruses. In biotechnology contexts, Reps are the basis for HUH-tag bioconjugation and a critical adeno-associated virus genome integration tool. We solved the first co-crystal structures of Reps complexed to ssDNA, revealing a key motif for conferring sequence specificity and for anchoring a bent DNA architecture. In combination, we developed a deep sequencing cleavage assay, termed HUH-seq, to interrogate subtleties in Rep specificity and demonstrate how differences can be exploited for multiplexed HUH-tagging. Together, our insights allowed engineering of only four amino acids in a Rep chimera to predictably alter sequence specificity. These results have important implications for modulating viral infections, developing Rep-based genomic integration tools, and enabling massively parallel HUH-tag barcoding and bioconjugation applications.