Engineered LwaCas13a with enhanced collateral activity for nucleic acid detection.

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 13 (Cas13) has been rapidly developed for nucleic-acid-based diagnostics by using its characteristic collateral activity. Despite the recent progress in optimizing the Cas13 system for the detection of nucleic acids, engineering Cas13 protein with enhanced collateral activity has been challenging, mostly because of its complex structural dynamics. Here we successfully employed a novel strategy to engineer the Leptotrichia wadei (Lwa)Cas13a by inserting different RNA-binding domains into a unique active-site-proximal loop within its higher eukaryotes and prokaryotes nucleotide-binding domain. Two LwaCas13a variants showed enhanced collateral activity and improved sensitivity over the wild type in various buffer conditions. By combining with an electrochemical method, our variants detected the SARS-CoV-2 genome at attomolar concentrations from both inactive viral and unextracted clinical samples, without target preamplification. Our engineered LwaCas13a enzymes with enhanced collateral activity are ready to be integrated into other Cas13a-based platforms for ultrasensitive detection of nucleic acids.

Multiplex Base-Editing Enables Combinatorial Epigenetic Regulation for Genome Mining of Fungal Natural Products.

Genome mining of cryptic natural products (NPs) remains challenging, especially in filamentous fungi, owing to their complex genetic regulation. Increasing evidence indicates that several epigenetic modifications often act cooperatively to control fungal gene transcription, yet the ability to predictably manipulate multiple genes simultaneously is still largely limited. Here, we developed a multiplex base-editing (MBE) platform that significantly improves the capability and throughput of fungal genome manipulation, leading to the simultaneous inactivation of up to eight genes using a single transformation. We then employed MBE to inactivate three negative epigenetic regulators combinatorially in Aspergillus nidulans, enabling the activation of eight cryptic gene clusters compared to the wild-type strains. A group of novel NPs harboring unique cichorine and polyamine hybrid chemical scaffolds were identified, which were not reported previously. We envision that our scalable and efficient MBE platform can be readily applied in other filamentous fungi for the genome mining of novel NPs, providing a powerful approach for the exploitation of fungal chemical diversity.

Amplification-Free Detection of SARS-CoV-2 and Respiratory Syncytial Virus Using CRISPR Cas13a and Graphene Field-Effect Transistors.

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems have recently received notable attention for their applications in nucleic acid detection. Despite many attempts, the majority of current CRISPR-based biosensors in infectious respiratory disease diagnostic applications still require target preamplifications. This study reports a new biosensor for amplification-free nucleic acid detection via harnessing the trans-cleavage mechanism of Cas13a and ultrasensitive graphene field-effect transistors (gFETs). CRISPR Cas13a-gFET achieves the detection of SARS-CoV-2 and respiratory syncytial virus (RSV) genome down to 1 attomolar without target preamplifications. Additionally, we validate the detection performance using clinical SARS-CoV-2 samples, including those with low viral loads (Ct value >30). Overall, these findings establish our CRISPR Cas13a-gFET among the most sensitive amplification-free nucleic acid diagnostic platforms to date.

A general theoretical framework to design base editors with reduced bystander effects.

Base editors (BEs) hold great potential for medical applications of gene therapy. However, high precision base editing requires BEs that can discriminate between the target base and multiple bystander bases within a narrow active window (4 - 10 nucleotides). Here, to assist in the design of these optimized editors, we propose a discrete-state stochastic approach to build an analytical model that explicitly evaluates the probabilities of editing the target base and bystanders. Combined with all-atom molecular dynamic simulations, our model reproduces the experimental data of A3A-BE3 and its variants for targeting the "TC" motif and bystander editing. Analyzing this approach, we propose several general principles that can guide the design of BEs with a reduced bystander effect. These principles are then applied to design a series of point mutations at T218 position of A3G-BEs to further reduce its bystander editing. We verify experimentally that the new mutations provide different levels of stringency on reducing the bystander editing at different genomic loci, which is consistent with our theoretical model. Thus, our study provides a computational-aided platform to assist in the scientifically-based design of BEs with reduced bystander effects.