Sensitive analysis of single nucleotide variation by Cas13d orthologs, EsCas13d and RspCas13d

Rapid, sensitive, and visual detection of swine Japanese encephalitis virus with a one-pot RPA-CRISPR/EsCas13d-based dual readout portable platform

Japanese encephalitis (JE), a mosquito-borne zoonotic disease caused by the Japanese encephalitis virus (JEV), poses a serious threat to global public health. The low viremia levels typical in JEV infections make RNA detection challenging, necessitating early and rapid diagnostic methods for effective control and prevention. This study introduces a novel one-pot detection method that combines recombinant enzyme polymerase isothermal amplification (RPA) with CRISPR/EsCas13d targeting, providing visual fluorescence and lateral flow assay (LFA) results. Our portable one-pot RPA-EsCas13d platform can detect as few as two copies of JEV nucleic acid within 1 h, without cross-reactivity with other pathogens. Validation against clinical samples showed 100 % concordance with real-time PCR results, underscoring the method's simplicity, sensitivity, and specificity. This efficacy confirms the platform's suitability as a novel point-of-care testing (POCT) solution for detecting and monitoring the JE virus in clinical and vector samples, especially valuable in remote and resource-limited settings.

Massively parallel profiling of RNA-targeting CRISPR-Cas13d

CRISPR-Cas13d cleaves RNA and is used in vivo and for diagnostics. However, a systematic understanding of its RNA binding and cleavage specificity is lacking. Here, we describe an RNA Chip-Hybridized Association-Mapping Platform (RNA-CHAMP) for measuring the binding affinity for > 10,000 RNAs containing structural perturbations and other alterations relative to the CRISPR RNA (crRNA). Deep profiling of Cas13d reveals that it does not require a protospacer flanking sequence but is exquisitely sensitive to secondary structure within the target RNA. Cas13d binding is penalized by mismatches in the distal crRNA-target RNA region, while alterations in the proximal region inhibit nuclease activity. A biophysical model built from these data reveals that target recognition initiates in the distal end of the target RNA. Using this model, we design crRNAs that can differentiate between SARS-CoV-2 variants by modulating nuclease activation. This work describes the key determinants of RNA targeting by a type VI CRISPR enzyme.

Development of an RT-PCR-based RspCas13d system to detect porcine deltacoronavirus

Porcine deltacoronavirus (PDCoV) is an enteropathogen that causes diarrhea in piglets and may undergo cross-species transmission. The prevention and control of PDCoV are complicated, and a sensitive, specific, and accessible method of diagnosis would be advantageous. Whereas qPCR is a standard approach for detecting PDCoV, it is not effectively sensitive. In the present study, we report such a strategy using an RT-PCR-based RspCas13d detection system and its efficacy in clinical sample diagnosis. The detection limit of this method was 4 copies/μL and no cross-reaction with other viruses such as the porcine epidemic diarrhea virus, classical swine fever virus, pseudorabies virus, porcine reproductive and respiratory syndrome virus, transmissible gastroenteritis virus and porcine rotavirus. The method was also effective in clinical samples. In summary, we demonstrate that RT-PCR-based RspCas13d detection system is an extremely sensitive and specific nucleic acid-based approach for detecting PDCoV. KEY POINTS: • RspCas13d can be used as a candidate molecular diagnostic tool to diagnose viral genomes. • A novel method is proposed using an RT-PCR-based RspCas13d detection system and its effectiveness in the detection of PDCoV. • The RT-PCR-based RspCas13d detection system has excellent sensitivity and specificity.

Engineering highly thermostable Cas12b via de novo structural analyses for one-pot detection of nucleic acids

CRISPR-Cas-based diagnostics have the potential to elevate nucleic acid detection. CRISPR-Cas systems can be combined with a pre-amplification step in a one-pot reaction to simplify the workflow and reduce carryover contamination. Here, we report an engineered Cas12b with improved thermostability that falls within the optimal temperature range (60°C-65°C) of reverse transcription-loop-mediated isothermal amplification (RT-LAMP). Using de novo structural analyses, we introduce mutations to wild-type BrCas12b to tighten its hydrophobic cores, thereby enhancing thermostability. The one-pot detection assay utilizing the engineered BrCas12b, called SPLENDID (single-pot LAMP-mediated engineered BrCas12b for nucleic acid detection of infectious diseases), exhibits robust trans-cleavage activity up to 67°C in a one-pot setting. We validate SPLENDID clinically in 80 serum samples for hepatitis C virus (HCV) and 66 saliva samples for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with high specificity and accuracy. We obtain results in as little as 20 min, and with the extraction process, the entire assay can be performed within an hour.

Harnessing the CRISPR-Cas13d System for Protein Detection by Dual-Aptamer-Based Transcription Amplification

CRISPR-based biosensing technology has been emerging as a revolutionary diagnostic tool for many disease-related biomarkers. In particular, RspCas13d, a newly identified RNA-guided Cas13d ribonuclease derived from Ruminococcus sp., has shown great promise for accurate and sensitive detection of RNA due to its RNA sequence-specific recognition and robust collateral trans-cleavage activity. However, its diagnostic utility is limited to detecting nucleic-acid-related biomarkers. To address this limitation, herein we present a proof-of-concept demonstration of a target-responsive CRISPR-Cas13d sensing system for protein biomarkers. This system was rationally designed by integrating a dual-aptamer-based transcription amplification strategy with CRISPR-Cas13d (DATAS-Cas13d), in which the protein binding initiates in-vitro RNA transcription followed by the activation of RspCas13d. Using a short fluorescent ssRNA as the signal reporter and cardiac troponin I (cTnI) as the model analyte, the DATAS-Cas13d system showed a wide linear range, low detection limit, and high specificity for the detection of cTnI in buffer and human serum. Thanks to the facile integration of various bioreceptors into the DATAS-Cas13d system, the method could be adapted to detecting a broad range of clinically relevant protein biomarkers, and thus broaden the medical applications of Cas13d-based diagnostics.

Cas13d: A New Molecular Scissor for Transcriptome Engineering

The discovery of Clustered Regularly Interspaced Palindromic Repeats (CRISPR) and its associated Cas endonucleases in bacterial and archaeal species allowed scientists to modify, utilized, and revolutionize this tool for genetic alterations in any species. Especially the type II CRISPR-Cas9 system has been extensively studied and utilized for precise and efficient DNA manipulation in plant and mammalian systems over the past few decades. Further, the discovery of the type V CRISPR-Cas12 (Cpf1) system provides more flexibility and precision in DNA manipulation in prokaryotes, plants, and animals. However, much effort has been made to employ and utilize the above CRISPR tools for RNA manipulation but the ability of Cas9 and Cas12 to cut DNA involves the nuisance of off-target effects on genes and thus may not be employed in all RNA-targeting applications. Therefore, the search for new and diverse Cas effectors which can precisely detect and manipulate the targeted RNA begins and this led to the discovery of a novel RNA targeting class 2, type VI CRISPR-Cas13 system. The CRISPR-Cas13 system consists of single RNA-guided Cas13 effector nucleases that solely target single-stranded RNA (ssRNA) in a programmable way without altering the DNA. The Cas13 effectors family comprises four subtypes (a-d) and each subtype has distinctive primary sequence divergence except the two consensuses Higher eukaryotes and prokaryotes nucleotide-binding domain (HEPN) that includes RNase motifs i.e. R-X4-6-H. These two HEPN domains are solely responsible for executing targetable RNA cleavage activity with high efficiency. Further, recent studies have shown that Cas13d exhibits higher efficiency and specificity in cleaving targeted RNA in the mammalian system compared to other Cas13 endonucleases of the Cas13 enzyme family. In addition to that, Cas13d has shown additional advantages over other Cas13 variants, structurally as well as functionally which makes it a prominent and superlative tool for RNA engineering and editing. Therefore considering the advantages of Cas13d over previously characterized Cas13 subtypes, in this review, we encompass the structural and mechanistic properties of type VI CRISPR-Cas13d systems, an overview of the current reported various applications of Cas13d, and the prospects to improve Cas13d based tools for diagnostic and therapeutic purposes.

A field-deployable method for single and multiplex detection of DNA or RNA from pathogens using Cas12 and Cas13

For some Cas nucleases, trans-cleavage activity triggered by CRISPR/Cas-mediated cis-cleavage upon target nucleic acid recognition has been explored for diagnostic detection. Portable single and multiplex nucleic acid-based detection is needed for crop pathogen management in agriculture. Here, we harnessed and characterized RfxCas13d as an additional CRISPR/Cas nucleic acid detection tool. We systematically characterized AsCas12a, LbCas12a, LwaCas13a, and RfxCas13d combined with isothermal amplification to develop a CRISPR/Cas nucleic acid-based tool for single or multiplex pathogen detection. Our data indicated that sufficient detection sensitivity was achieved with just a few copies of DNA/RNA targets as input. Using this tool, we successfully detected DNA from Fusarium graminearum and Fusarium verticillioides and RNA from rice black-streaked dwarf virus in crude extracts prepared in the field. Our method, from sample preparation to result readout, could be rapidly and easily deployed in the field. This system could be extended to other crop pathogens, including those that currently lack a detection method and have metabolite profiles that make detection challenging. This nucleic acid detection system could also be used for single-nucleotide polymorphism genotyping, transgene detection, and qualitative detection of gene expression in the field.