Non-equilibrium structural dynamics of supercoiled DNA plasmids exhibits asymmetrical relaxation

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.

Structural basis of Cas9 DNA interrogation with a 5' truncated sgRNA

The efficiency and accuracy of CRISPR-Cas9 targeting varies considerably across genomic targets and remains a persistent issue for using this system in cells. Studies have shown that the use of 5' truncated single guide RNAs (sgRNAs) can reduce the rate of unwanted off-target recognition while still maintaining on-target specificity. However, it is not well-understood how reducing target complementarity enhances specificity or how truncation past 15 nucleotides (nts) prevents full Cas9 activation without compromising on-target binding. Here, we use biochemistry and cryogenic electron microscopy to investigate Cas9 structure and activity when bound to a 14-nt sgRNA. Our structures reveal that the shortened path of the displaced non-target strand (NTS) sterically occludes docking of the HNH L1 linker and prevents proper positioning of the nuclease domains. We show that cleavage inhibition can be alleviated by either artificially melting the protospacer adjacent motif (PAM)-distal duplex or providing a supercoiled substrate. Even though Cas9 forms a stable complex with its target, we find that plasmid cleavage is ∼1000-fold slower with a 14-nt sgRNA than with a full-length 20-nt sgRNA. Our results provide a structural basis for Cas9 target binding with 5' truncated sgRNAs and underline the importance of PAM-distal NTS availability in promoting Cas9 activation.

Quantitative Visualization of Lipid Nanoparticle Fusion as a Function of Formulation and Process Parameters

Lipid nanoparticles (LNPs) have proven to be promising delivery vehicles for RNA-based vaccines and therapeutics, particularly in LNP formulations containing ionizable cationic lipids that undergo protonation/deprotonation in response to buffer pH changes. These nanoparticles are typically formulated using a rapid mixing technique at low pH, followed by a return to physiological pH that triggers LNP-LNP fusion. A detailed understanding of these dynamic processes is crucial to optimize the overall performance and efficiency of LNPs. However, knowledge gaps persist regarding how particle formation mechanisms impact drug loading and delivery functions. In this work, we employ single-molecule Convex Lens-induced Confinement (CLiC) microscopy in combination with Förster resonance energy transfer (FRET) measurements to study LNP fusion dynamics in relation to various formulation parameters, including lipid concentration, buffer conditions, drug loading ratio, PEG-lipid concentrations, and ionizable lipid selection. Our results reveal a strong correlation between the measured fusion dynamics and the formulation parameters used; these findings are consistent with DLS and Cryo-TEM-based assays. These measurements offer a cost-effective method for characterizing and screening potential drug candidates and can provide additional insights into their design, with opportunities for optimization.

DNA superhelicity

Closing each strand of a DNA duplex upon itself fixes its linking number L. This topological condition couples together the secondary and tertiary structures of the resulting ccDNA topoisomer, a constraint that is not present in otherwise identical nicked or linear DNAs. Fixing L has a range of structural, energetic and functional consequences. Here we consider how L having different integer values (that is, different superhelicities) affects ccDNA molecules. The approaches used are primarily theoretical, and are developed from a historical perspective. In brief, processes that either relax or increase superhelicity, or repartition what is there, may either release or require free energy. The energies involved can be substantial, sufficient to influence many events, directly or indirectly. Here two examples are developed. The changes of unconstrained superhelicity that occur during nucleosome attachment and release are examined. And a simple theoretical model of superhelically driven DNA structural transitions is described that calculates equilibrium distributions for populations of identical topoisomers. This model is used to examine how these distributions change with superhelicity and other factors, and applied to analyze several situations of biological interest.

Interaction of Proteins with Inverted Repeats and Cruciform Structures in Nucleic Acids

Cruciforms occur when inverted repeat sequences in double-stranded DNA adopt intra-strand hairpins on opposing strands. Biophysical and molecular studies of these structures confirm their characterization as four-way junctions and have demonstrated that several factors influence their stability, including overall chromatin structure and DNA supercoiling. Here, we review our understanding of processes that influence the formation and stability of cruciforms in genomes, covering the range of sequences shown to have biological significance. It is challenging to accurately sequence repetitive DNA sequences, but recent advances in sequencing methods have deepened understanding about the amounts of inverted repeats in genomes from all forms of life. We highlight that, in the majority of genomes, inverted repeats are present in higher numbers than is expected from a random occurrence. It is, therefore, becoming clear that inverted repeats play important roles in regulating many aspects of DNA metabolism, including replication, gene expression, and recombination. Cruciforms are targets for many architectural and regulatory proteins, including topoisomerases, p53, Rif1, and others. Notably, some of these proteins can induce the formation of cruciform structures when they bind to DNA. Inverted repeat sequences also influence the evolution of genomes, and growing evidence highlights their significance in several human diseases, suggesting that the inverted repeat sequences and/or DNA cruciforms could be useful therapeutic targets in some cases.

Biophysical Reviews ‘Meet the Editors Series’ — a profile of Sabrina Leslie

It is my pleasure to introduce myself to the readers of Biophysical Reviews as part of the ‘Meet the Editors Series’.