In vivo CRISPR base editing of PCSK9 durably lowers cholesterol in primates.

Gene-editing technologies, which include the CRISPR-Cas nucleases1-3 and CRISPR base editors4,5, have the potential to permanently modify disease-causing genes in patients6. The demonstration of durable editing in target organs of nonhuman primates is a key step before in vivo administration of gene editors to patients in clinical trials. Here we demonstrate that CRISPR base editors that are delivered in vivo using lipid nanoparticles can efficiently and precisely modify disease-related genes in living cynomolgus monkeys (Macaca fascicularis). We observed a near-complete knockdown of PCSK9 in the liver after a single infusion of lipid nanoparticles, with concomitant reductions in blood levels of PCSK9 and low-density lipoprotein cholesterol of approximately 90% and about 60%, respectively; all of these changes remained stable for at least 8 months after a single-dose treatment. In addition to supporting a 'once-and-done' approach to the reduction of low-density lipoprotein cholesterol and the treatment of atherosclerotic cardiovascular disease (the leading cause of death worldwide7), our results provide a proof-of-concept for how CRISPR base editors can be productively applied to make precise single-nucleotide changes in therapeutic target genes in the liver, and potentially in other organs.

A Modular RNA Domain That Confers Differential Ligand Specificity.

The modularity of protein domains is well-known, but the existence of independent domains that confer function in RNA is less established. Recently, a family of RNA aptamers termed ykkC was discovered; they bind at least four ligands of very different chemical composition, including guanidine, phosphoribosyl pyrophosphate (PRPP), and guanosine tetraphosphate (ppGpp) (graphical abstract). Structures of these aptamers revealed an architecture characterized by two coaxial helical stacks. The first helix appears to be a generic scaffold, while the second helix forms the most contacts to the ligands. To determine if these two regions within the aptamer are modular units for ligand recognition, we swapped the ligand-binding coaxial stacks of a guanidine aptamer and a PRPP aptamer. This operation, in combination with a single mutation in the scaffold domain, achieved full switching of ligand specificity. This finding suggests that the ligand-binding helix largely dictates the ligand specificity of ykkC RNAs and that the scaffold coaxial stack is generally compatible with various ykkC ligand-binding modules. This work presents an example of RNA domain modularity comparable to that of a ligand-binding protein, showcasing the versatility of RNA as an entity capable of molecular evolution through adaptation of existing motifs.

Structural basis for ligand binding to the guanidine-II riboswitch.

The guanidine-II riboswitch, also known as mini-ykkC, is a conserved mRNA element with more than 800 examples in bacteria. It consists of two stem-loops capped by identical, conserved tetraloops that are separated by a linker region of variable length and sequence. Like the guanidine-I riboswitch, it controls the expression of guanidine carboxylases and SugE-like genes. The guanidine-II riboswitch specifically binds free guanidinium cations and functions as a translationally controlled on-switch. Here we report the structure of a P2 stem-loop from the Pseudomonas aeruginosa guanidine-II riboswitch aptamer bound to guanidine at 1.57 Å resolution. The hairpins dimerize via the conserved tetraloop, which also contains the binding pocket. Two guanidinium molecules bind near the dimerization interface, one in each tetraloop. The guanidinium cation is engaged in extensive hydrogen bonding to the RNA. Contacts include the Hoogsteen face of a guanine base and three nonbridging phosphate oxygens. Cation-π interactions and ionic interactions also stabilize ligand binding. The guanidine-II riboswitch utilizes the same recognition strategies as the guanidine-I riboswitch while adopting an entirely different and much smaller RNA fold.

Structural Basis for Ligand Binding to the Guanidine-I Riboswitch.

The guanidine-I riboswitch is a conserved RNA element with approximately 2,000 known examples across four phyla of bacteria. It exists upstream of nitrogen metabolism and multidrug resistance transporter genes and alters expression through the specific recognition of a free guanidinium cation. Here we report the structure of a guanidine riboswitch aptamer from Sulfobacillus acidophilus at 2.7 Å resolution. Helices P1, P1a, P1b, and P2 form a coaxial stack that acts as a scaffold for ligand binding. A previously unidentified P3 helix docks into P1a to form the guanidinium binding pocket, which is completely enclosed. Every functional group of the ligand is recognized through hydrogen bonding to guanine bases and phosphate oxygens. Guanidinium binding is further stabilized through cation-π interactions with guanine bases. This allows the riboswitch to recognize guanidinium while excluding other bacterial metabolites with a guanidino group, including the amino acid arginine.