M-MuLV reverse transcriptase: Selected properties and improved mutants

Structural basis for pegRNA-guided reverse transcription by a prime editor

The prime editor system composed of Streptococcus pyogenes Cas9 nickase (nSpCas9) and engineered Moloney murine leukaemia virus reverse transcriptase (M-MLV RT) collaborates with a prime editing guide RNA (pegRNA) to facilitate a wide variety of precise genome edits in living cells1. However, owing to a lack of structural information, the molecular mechanism of pegRNA-guided reverse transcription by the prime editor remains poorly understood. Here we present cryo-electron microscopy structures of the SpCas9-M-MLV RTΔRNaseH-pegRNA-target DNA complex in multiple states. The termination structure, along with our functional analysis, reveals that M-MLV RT extends reverse transcription beyond the expected site, resulting in scaffold-derived incorporations that cause undesired edits at the target loci. Furthermore, structural comparisons among the pre-initiation, initiation and elongation states show that M-MLV RT remains in a consistent position relative to SpCas9 during reverse transcription, whereas the pegRNA-synthesized DNA heteroduplex builds up along the surface of SpCas9. On the basis of our structural insights, we rationally engineered pegRNA variants and prime-editor variants in which M-MLV RT is fused within SpCas9. Collectively, our findings provide structural insights into the stepwise mechanism of prime editing, and will pave the way for the development of a versatile prime editing toolbox.

Reverse transcription of plasma-derived HIV-1 RNA generates multiple artifacts through tRNA(Lys-3)-priming

In vitro reverse transcription of full-length HIV-1 RNA extracted from the blood plasma of people living with HIV-1 remains challenging. Here, we describe the initiation of reverse transcription of plasma-derived viral RNA in the absence of an exogenous primer. Real-time PCR and Sanger sequencing were applied to identify the source and to monitor the outcome of this reaction. Results demonstrated that during purification of viral RNA from plasma, tRNA(Lys-3) is co-extracted in a complex with the viral RNA. In the presence of a reverse transcription enzyme, this tRNA(Lys-3) can induce reverse transcription, a reaction that is not confined to transcription of the 5' end of the viral RNA. A range of cDNA products is generated, most of them indicative for the occurrence of in vitro strand transfer events that involve translocation of cDNA from the 5' end to random positions on the viral RNA. This process results in the formation of cDNAs with large internal deletions. However, near full-length cDNA and cDNA with sequence patterns resembling multiple spliced HIV-1 RNA were also detected. Despite its potential to introduce significant bias in the interpretation of results across various applications, tRNA(Lys-3)-driven reverse transcription has been overlooked thus far. A more in-depth study of this tRNA-driven in vitro reaction may provide new insight into the complex process of in vivo HIV-1 replication.IMPORTANCEThe use of silica-based extraction methods for purifying HIV-1 RNA from viral particles is a common practice, but it involves co-extraction of human tRNA(Lys-3) due to the strong interactions between these molecules. This co-extraction becomes particularly significant when the extracted RNA is used in reverse transcription reactions, as the tRNA(Lys-3) then serves as a primer. Reverse transcription from tRNA(Lys-3) is not confined to cDNA synthesis of the 5' end of the viral RNA but extends across various regions of the viral genome through in vitro strand transfer events. Co-extraction of tRNA(Lys-3) has been overlooked thus far, despite its potential to introduce bias in downstream, reverse transcription-related applications. The observed events in the tRNA(Lys-3)-induced in vitro reverse transcription resemble in vivo replication processes. Therefore, these reactions may offer a unique model to better understand the replication dynamics of HIV-1.

Critical view on oligo(dT)-based RNA-seq: bias arising, modeling, and mitigating

The precise biological interpretation of oligo(dT)-based RNA sequencing (RNA-seq) datasets, particularly in single-cell RNA-seq (scRNA-seq), is invaluable for understanding complex biological systems. However, the presence of biases can lead to misleading results in downstream analysis. This study has now identified two additional biases that are not accounted for in established bias models: poly(A)-tail length bias and fixed-position GC-content bias. These biases have a significant negative impact on the overall quality of oligo(dT)-based RNA-seq data. To address these biases, we have developed a universal bias-mitigating method based on the lower-affinity binding of short and nonanchored oligo(dT) primers to poly(A) tails. This method significantly reduces poly(A) length bias and completely eliminates fixed-position GC bias. Furthermore, the use of short oligo(dT) with impartial binding behavior toward the diverse poly(A) tails renders RNA-seq with more reliable measurements. The findings of this study are particularly beneficial for scRNA-seq datasets, where accurate benchmarking is critical.

Profiling of RNA-binding protein binding sites by in situ reverse transcription-based sequencing

RNA-binding proteins (RBPs) regulate diverse cellular processes by dynamically interacting with RNA targets. However, effective methods to capture both stable and transient interactions between RBPs and their RNA targets are still lacking, especially when the interaction is dynamic or samples are limited. Here we present an assay of reverse transcription-based RBP binding site sequencing (ARTR-seq), which relies on in situ reverse transcription of RBP-bound RNAs guided by antibodies to identify RBP binding sites. ARTR-seq avoids ultraviolet crosslinking and immunoprecipitation, allowing for efficient and specific identification of RBP binding sites from as few as 20 cells or a tissue section. Taking advantage of rapid formaldehyde fixation, ARTR-seq enables capturing the dynamic RNA binding by RBPs over a short period of time, as demonstrated by the profiling of dynamic RNA binding of G3BP1 during stress granule assembly on a timescale as short as 10 minutes.

Prime editing: Mechanism insight and recent applications in plants

Prime editing (PE) technology utilizes an extended prime editing guide RNA (pegRNA) to direct a fusion peptide consisting of nCas9 (H840) and reverse transcriptase (RT) to a specific location in the genome. This enables the installation of base changes at the targeted site using the extended portion of the pegRNA through RT activity. The resulting product of the RT reaction forms a 3' flap, which can be incorporated into the genomic site through a series of biochemical steps involving DNA repair and synthesis pathways. PE has demonstrated its effectiveness in achieving almost all forms of precise gene editing, such as base conversions (all types), DNA sequence insertions and deletions, chromosomal translocation and inversion and long DNA sequence insertion at safe harbour sites within the genome. In plant science, PE could serve as a groundbreaking tool for precise gene editing, allowing the creation of desired alleles to improve crop varieties. Nevertheless, its application has encountered limitations due to efficiency constraints, particularly in dicotyledonous plants. In this review, we discuss the step-by-step mechanism of PE, shedding light on the critical aspects of each step while suggesting possible solutions to enhance its efficiency. Additionally, we present an overview of recent advancements and future perspectives in PE research specifically focused on plants, examining the key technical considerations of its applications.

Addressing the dNTP bottleneck restricting prime editing activity

Prime editing efficiency is modest in cells that are quiescent or slowly proliferating where intracellular dNTP levels are tightly regulated. MMLV-reverse transcriptase - the prime editor polymerase subunit - requires high intracellular dNTPs levels for efficient polymerization. We report that prime editing efficiency in primary cells and in vivo is increased by mutations that enhance the enzymatic properties of MMLV-reverse transcriptase and can be further complemented by targeting SAMHD1 for degradation.

Charge Engineering of the Nucleic Acid Binding Cleft of a Thermostable HIV-1 Reverse Transcriptase Reveals Key Interactions and a Novel Mechanism of RNase H Inactivation

Coupled with PCR, reverse transcriptases (RTs) have been widely used for RNA detection and gene expression analysis. Increased thermostability and nucleic acid binding affinity are desirable RT properties to improve yields and sensitivity of these applications. The effects of amino acid substitutions in the RT RNase H domain were tested in an engineered HIV-1 group O RT, containing mutations K358R/A359G/S360A and devoid of RNase H activity due to the presence of E478Q (O3MQ RT). Twenty mutant RTs with Lys or Arg at positions interacting with the template-primer (i.e., at positions 473-477, 499-502 and 505) were obtained and characterized. Most of them produced significant amounts of cDNA at 37, 50 and 65 °C, as determined in RT-PCR reactions. However, a big loss of activity was observed with mutants A477K/R, S499K/R, V502K/R and Y505K/R, particularly at 65 °C. Binding affinity experiments confirmed that residues 477, 502 and 505 were less tolerant to mutations. Amino acid substitutions Q500K and Q500R produced a slight increase of cDNA synthesis efficiency at 50 and 65 °C, without altering the KD for model DNA/DNA and RNA/DNA heteroduplexes. Interestingly, molecular dynamics simulations predicted that those mutations inactivate the RNase H activity by altering the geometry of the catalytic site. Proof of this unexpected effect was obtained after introducing Q500K or Q500R in the wild-type HIV-1BH10 RT and mutant K358R/A359G/S360A RT. Our results reveal a novel mechanism of RNase H inactivation that preserves RT DNA binding and polymerization efficiency without substituting RNase H active site residues.

Efficient and versatile multiplex prime editing in hexaploid wheat

Prime editing is limited by low efficiency in plants. Here, we develop an upgraded engineered plant prime editor in hexaploid wheat, ePPEplus, by introducing a V223A substitution into reverse transcriptase in the ePPEmax* architecture. ePPEplus enhances the efficiency by an average 33.0-fold and 6.4-fold compared to the original PPE and ePPE, respectively. Importantly, a robust multiplex prime editing platform is established for simultaneous editing of four to ten genes in protoplasts and up to eight genes in regenerated wheat plants at frequencies up to 74.5%, thus expanding the applicability of prime editors for stacking of multiple agronomic traits.

New CRISPR Tools to Correct Pathogenic Mutations in Usher Syndrome

Inherited retinal degenerations are a leading cause of blindness in the UK. Significant advances have been made to tackle this issue in recent years, with a pioneering FDA approved gene therapy treatment (Luxturna^®), which targets a loss of function mutation in the RPE65 gene. However, there remain notable shortcomings to this form of gene replacement therapy. In particular, the lack of viability for gene sequences exceeding the 4.7 kb adeno-associated virus (AAV) packaging limit or for toxic gain of function mutations. The USH2A gene at ~15.7 kb for instance is too large for AAV delivery: a safe and effective vehicle capable of transducing photoreceptor cells for gene replacement therapy. Usher Syndrome is a clinically and genetically heterogenous deaf-blindness syndrome with autosomal recessive inheritance. The USH2A gene encodes the protein usherin, which localises to the photoreceptor cilium and cochlear hair cells. Mutations in the USH2A gene cause Usher Syndrome type II (USH2), which is the most common subtype of Usher Syndrome and the focus of this review. To date, researchers have been unable to create an efficient, safe editing tool that is small enough to fit inside a single AAV vector for delivery into human cells. This article reviews the potential of CRISPR technology, derived from bacterial defence mechanisms, to overcome these challenges; delivering tools to precisely edit and correct small insertions, deletions and base transitions in USH2A without the need to deliver the full-length gene. Such an ultra-compact therapy could make strides in combating a significant cause of blindness in young people.