Rapid detection of nucleic acids is integral for clinical diagnostics and biotechnological applications. We recently developed a platform termed SHERLOCK (Specific High Sensitivity Enzymatic Reporter UnLOCKing) that combines isothermal pre-amplification with Cas13 to detect single molecules of RNA or DNA. Through characterization of CRISPR enzymology and application development, we report here four advances integrated into SHERLOCKv2: 1) 4-channel single reaction multiplexing using orthogonal CRISPR enzymes; 2) quantitative measurement of input down to 2 aM; 3) 3.5-fold increase in signal sensitivity by combining Cas13 with Csm6, an auxilary CRISPR-associated enzyme; and 4) lateral flow read-out. SHERLOCKv2 can detect Dengue or Zika virus ssRNA as well as mutations in patient liquid biopsy samples via lateral flow, highlighting its potential as a multiplexable, portable, rapid, and quantitative detection platform of nucleic acids.

RNA plays important and diverse roles in biology, but molecular tools to manipulate and measure RNA are limited. For example, RNA interference (RNAi)1-3 can efficiently knockdown RNAs, but it is prone to off-target effects4, and visualizing RNAs typically relies on the introduction of exogenous tags5. Here, we demonstrate that the class 2 type VI6,7 RNA-guided RNA-targeting CRISPR-Cas effector Cas13a8 (previously known as C2c2) can be engineered for mammalian cell RNA knockdown and binding. After initial screening of fifteen orthologs in E. coli, we identified Cas13a from Leptotrichia wadei (LwaCas13a) as the most effective. LwaCas13a can be heterologously expressed in mammalian and plant cells for targeted knockdown of either reporter or endogenous transcripts. We demonstrate that LwaCas13a is capable of providing comparable levels of knockdown as RNAi, but with dramatically improved specificity. Moreover, catalytically inactive LwaCas13a maintains targeted RNA binding, allowing for programmable tracking of transcripts in live cells. Our results establish CRISPR-Cas13a as a flexible platform for RNA targeting with wide applicability for studying RNA in mammalian cells.

Nucleic acid editing holds promise for treating genetic disease, particularly at the RNA level, where disease-relevant sequences can be rescued to yield functional protein products. Type VI CRISPR-Cas systems contain the programmable single-effector RNA-guided RNases Cas13. Here, we profile Type VI systems to engineer a Cas13 ortholog capable of robust knockdown and demonstrate RNA editing by using catalytically-inactive Cas13 (dCas13) to direct adenosine to inosine deaminase activity by ADAR2 to transcripts in mammalian cells. This system, referred to as RNA Editing for Programmable A to I Replacement (REPAIR), has no strict sequence constraints, can be used to edit full-length transcripts containing pathogenic mutations. We further engineer this system to create a high specificity variant, REPAIRv2, that is 919 times more specific than REPAIRv1 as well as minimize the system to ease viral delivery. REPAIR presents a promising RNA editing platform with broad applicability for research, therapeutics, and biotechnology.