Project description:The intrinsic collateral cleavage activity of Cas13d,which refers to the gRNA-independent degradation of bystander RNA, significantly limits its therapeutic potential. To extensively evaluate the collateral effects of hpCas13d genome-wide, we conducted RNA-seq analysis of the transcriptome in HEK293T cells after the transfections with dCas13d, wtCas13d, hpCas13d, and hfCas13d . Compared to dCas13d, wtCas13d with a PPIA gRNA (reported to induce the significant collateral cleavage) resulted in significant downregulations of thousands genes. In sharp contrast, hpCas13d and hfCas13d showed much less off-target genes, respectively. These findings demonstrate that hpCas13d exhibits reduced collateral activity, rendering it suitable for in vivo applications.
Project description:We found that several variants with 2-4 mutations on Higher Eukaryotes and Prokaryotes Nucleotide-binding (HEPN) domains showed undiminished on-target activity but markedly reduced collateral effects. Furthermore, transcriptome-wide off-target effects and cell growth arrest induced by wild-type Cas13d were found to be absent for a Cas13d variant. Thus, high-fidelity Cas13 variants with minimal collateral effects are now available for the targeted degradation of RNAs in basic research and therapeutic applications.
Project description:Cas13 is a unique family of CRISPR endonucleases exhibiting programmable binding and cleavage of RNAs and is a strong candidate for eukaryotic RNA knockdown in the laboratory and the clinic. However, sequence-specific binding of Cas13 to the target RNA unleashes non-specific bystander RNA cleavage, or collateral activity, which may confound knockdown experiments and raises concerns for therapeutic applications. Although conserved across orthologs and robust in cell-free and bacterial environments, the extent of collateral activity in mammalian cells remains disputed. Here, we investigate Cas13d collateral activity in the context of an RNA-targeting therapy for myotonic dystrophy type 1, a disease caused by a transcribed long CTG repeat expansion. We find that when targeting CUGn RNA in HeLa and other cell lines, Cas13d depletes endogenous and transgenic RNAs, interferes with critical cellular processes, and activates stress response and apoptosis pathways. We also observe collateral effects when targeting other repetitive and unique transgenic sequences, and we provide evidence for collateral activity when targeting highly expressed endogenous transcripts. To minimize collateral activity for repeat-targeting Cas13d therapeutics, we introduce gRNA excision for negative-autoregulatory optimization (GENO), a simple strategy that leverages crRNA processing to control Cas13d expression and is easily integrated into an AAV gene therapy. We argue that thorough assessment of collateral activity is necessary when applying Cas13d in mammalian cells and that implementation of GENO illustrates the advantages of compact and universally robust regulatory systems for Cas-based gene therapies.
Project description:We performed an experimental Cas13d-SARScov2 genome-wide screen to identify gRNAs that would allow Cas13d to degrade the viral RNA. We built mCherry reporter plasmids that express mCherry with a 3kb 3'UTR deriving from the SARScov2 genome. In total we designed 11 reporters covering the entire plus strand of the viral genome and 11 other reporters covering the entire minus strand. Each of the 22 mCherry reporter plasmids carries a U6 expression cassette containing a Cas13d gRNA that targets the 3'UTR of the mCherry reporter. Each reporter is represented by a pool of reporters each containing a different gRNA that targets mCherry 3'UTR for a total average of ~300 gRNA per 3'UTR. The entire pool of 22 reporters, each with a pool of ~300 different gRNAs constitutes a comprehensive set ~6,500 reporters (~ 6,500 different gRNAs) that allowed us to interrogate the entire SARScov2 plus and minus strand viral RNA for regions of vulnerability and targetability. In order to specifically interrogate Cas13d activity an remove the biases that would be introduced in the reporter expression by the presence of a large 3kb 3'UTR we used a case (presence of Cas13d) control (absence of Cas13d) design. Briefly, the ~6,500 reporters were lentiviral transduced in RKO cells, the cells were split in 2 populations, 1 population was transduced with Cas13d and the other serving as control did not. The population expressing Cas13d was FACS sorted in low mCherry (efficient gRNAs) and high mCherry (un-efficient gRNAs) in 2 biological replicates and the genomic DNA of these populations was extracted, gRNAs were PCR amplified and sequenced. For the population that did not express Cas13d, a low mCherry, one high mCherry and unsorted population were sequenced as control libraries.
Project description:We conducted a two-vector CRISPR/Cas13d proliferation screening experiment. The screening library contains 10,830 sgRNAs targeting 192 protein-coding genes and 234 lncRNAs, and the screening experiment was performed using a melanoma cell line A375. It provides a unique dataset to model Cas13d sgRNA efficiency and specificity. We designed a deep learning model, named DeepCas13, to predict the sgRNA on-target activity with high accuracy from sgRNA sequences and RNA secondary structures.