Holly A. Rees

Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage

The spontaneous deamination of cytosine is a major source of transitions from C•G to T•A base pairs, which account for half of known pathogenic point mutations in humans. The ability to efficiently convert targeted A•T base pairs to G•C could therefore advance the study and treatment of genetic diseases. The deamination of adenine yields inosine, which is treated as guanine by polymerases, but no enzymes are known to deaminate adenine in DNA. Here we describe adenine base editors (ABEs) that mediate the conversion of A•T to G•C in genomic DNA. We evolved a transfer RNA adenosine deaminase to operate on DNA when fused to a catalytically impaired CRISPR–Cas9 mutant. Extensive directed evolution and protein engineering resulted in seventh-generation ABEs that convert targeted A•T base pairs efficiently to G•C (approximately 50% efficiency in human cells) with high product purity (typically at least 99.9%) and low rates of indels (typically no more than 0.1%). ABEs introduce point mutations more efficiently and cleanly, and with less off-target genome modification, than a current Cas9 nuclease-based method, and can install disease-correcting or disease-suppressing mutations in human cells. Together with previous base editors, ABEs enable the direct, programmable introduction of all four transition mutations without double-stranded DNA cleavage.

Efficient delivery of genome-editing proteins using bioreducible lipid nanoparticles

Significance The therapeutic potential of protein-based genome editing is dependent on the delivery of proteins to appropriate intracellular targets. Here we report that combining bioreducible lipid nanoparticles and negatively supercharged Cre recombinase or anionic Cas9:single-guide (sg)RNA complexes drives the self-assembly of nanoparticles for potent protein delivery and genome editing. The design of bioreducible lipids facilitates the degradation of nanoparticles inside cells in response to the reductive intracellular environment, enhancing the endosome escape of protein. In addition, modulation of protein charge through either genetic fusion of supercharged protein or complexation of Cas9 with its inherently anionic sgRNA allows highly efficient protein delivery and effective genome editing in mammalian cells and functional recombinase delivery in the rodent brain. A central challenge to the development of protein-based therapeutics is the inefficiency of delivery of protein cargo across the mammalian cell membrane, including escape from endosomes. Here we report that combining bioreducible lipid nanoparticles with negatively supercharged Cre recombinase or anionic Cas9:single-guide (sg)RNA complexes drives the electrostatic assembly of nanoparticles that mediate potent protein delivery and genome editing. These bioreducible lipids efficiently deliver protein cargo into cells, facilitate the escape of protein from endosomes in response to the reductive intracellular environment, and direct protein to its intracellular target sites. The delivery of supercharged Cre protein and Cas9:sgRNA complexed with bioreducible lipids into cultured human cells enables gene recombination and genome editing with efficiencies greater than 70%. In addition, we demonstrate that these lipids are effective for functional protein delivery into mouse brain for gene recombination in vivo. Therefore, the integration of this bioreducible lipid platform with protein engineering has the potential to advance the therapeutic relevance of protein-based genome editing.

Evolved Cas9 variants with broad PAM compatibility and high DNA specificity

A key limitation of the use of the CRISPR–Cas9 system for genome editing and other applications is the requirement that a protospacer adjacent motif (PAM) be present at the target site. For the most commonly used Cas9 from Streptococcus pyogenes (SpCas9), the required PAM sequence is NGG. No natural or engineered Cas9 variants that have been shown to function efficiently in mammalian cells offer a PAM less restrictive than NGG. Here we use phage-assisted continuous evolution to evolve an expanded PAM SpCas9 variant (xCas9) that can recognize a broad range of PAM sequences including NG, GAA and GAT. The PAM compatibility of xCas9 is the broadest reported, to our knowledge, among Cas9 proteins that are active in mammalian cells, and supports applications in human cells including targeted transcriptional activation, nuclease-mediated gene disruption, and cytidine and adenine base editing. Notably, despite its broadened PAM compatibility, xCas9 has much greater DNA specificity than SpCas9, with substantially lower genome-wide off-target activity at all NGG target sites tested, as well as minimal off-target activity when targeting genomic sites with non-NGG PAMs. These findings expand the DNA targeting scope of CRISPR systems and establish that there is no necessary trade-off between Cas9 editing efficiency, PAM compatibility and DNA specificity.

Improving the DNA specificity and applicability of base editing through protein engineering and protein delivery.

We recently developed base editing, a genome-editing approach that enables the programmable conversion of one base pair into another without double-stranded DNA cleavage, excess stochastic insertions and deletions, or dependence on homology-directed repair. The application of base editing is limited by off-target activity and reliance on intracellular DNA delivery. Here we describe two advances that address these limitations. First, we greatly reduce off-target base editing by installing mutations into our third-generation base editor (BE3) to generate a high-fidelity base editor (HF-BE3). Next, we purify and deliver BE3 and HF-BE3 as ribonucleoprotein (RNP) complexes into mammalian cells, establishing DNA-free base editing. RNP delivery of BE3 confers higher specificity even than plasmid transfection of HF-BE3, while maintaining comparable on-target editing levels. Finally, we apply these advances to deliver BE3 RNPs into both zebrafish embryos and the inner ear of live mice to achieve specific, DNA-free base editing in vivo.

Structural and evolutionary versatility in protein complexes with uneven stoichiometry

Proteins assemble into complexes with diverse quaternary structures. Although most heteromeric complexes of known structure have even stoichiometry, a significant minority have uneven stoichiometry--that is, differing numbers of each subunit type. To adopt this uneven stoichiometry, sequence-identical subunits must be asymmetric with respect to each other, forming different interactions within the complex. Here we first investigate the occurrence of uneven stoichiometry, demonstrating that it is common in vitro and is likely to be common in vivo. Next, we elucidate the structural determinants of uneven stoichiometry, identifying six different mechanisms by which it can be achieved. Finally, we study the frequency of uneven stoichiometry across evolution, observing a significant enrichment in bacteria compared with eukaryotes. We show that this arises due to a general increased tendency for bacterial proteins to self-assemble and form homomeric interactions, even within the context of a heteromeric complex.

Phage-assisted continuous evolution of proteases with altered substrate specificity

Here we perform phage-assisted continuous evolution (PACE) of TEV protease, which canonically cleaves ENLYFQS, to cleave a very different target sequence, HPLVGHM, that is present in human IL-23. A protease emerging from ∼2500 generations of PACE contains 20 non-silent mutations, cleaves human IL-23 at the target peptide bond, and when pre-mixed with IL-23 in primary cultures of murine splenocytes inhibits IL-23-mediated immune signaling. We characterize the substrate specificity of this evolved enzyme, revealing shifted and broadened specificity changes at the six positions in which the target amino acid sequence differed. Mutational dissection and additional protease specificity profiling reveal the molecular basis of some of these changes. This work establishes the capability of changing the substrate specificity of a protease at many positions in a practical time scale and provides a foundation for the development of custom proteases that catalytically alter or destroy target proteins for biotechnological and therapeutic applications.Proteases are promising therapeutics to treat diseases such as hemophilia which are due to endogenous protease deficiency. Here the authors use phage-assisted continuous evolution to evolve a variant TEV protease with altered target peptide sequence specificities.

In vivo base editing of post-mitotic sensory cells

Programmable nucleases can introduce precise changes to genomic DNA through homology-directed repair (HDR). Unfortunately, HDR is largely restricted to mitotic cells, and is typically accompanied by an excess of stochastic insertions and deletions (indels). Here we present an in vivo base editing strategy that addresses these limitations. We use nuclease-free base editing to install a S33F mutation in β-catenin that blocks β-catenin phosphorylation, impedes β-catenin degradation, and upregulates Wnt signaling. In vitro, base editing installs the S33F mutation with a 200-fold higher editing:indel ratio than HDR. In post-mitotic cells in mouse inner ear, injection of base editor protein:RNA:lipid installs this mutation, resulting in Wnt activation that induces mitosis of cochlear supporting cells and cellular reprogramming. In contrast, injection of HDR agents does not induce Wnt upregulation. These results establish a strategy for modifying posttranslational states in signaling pathways, and an approach to precision editing in post-mitotic tissues.Base editing allows the precise introduction of point mutations into cellular DNA without requiring double-stranded DNA breaks or homology-directed repair, which is inefficient in postmitotic cells. Here the authors demonstrate in vivo base editing of post-mitotic somatic cells in the postnatal mouse inner ear with physiological outcomes.

Publisher Correction: Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage

In this Article, owing to an error during the production process, in Fig. 1a, the dark blue and light blue wedges were incorrectly labelled as ‘G•C → T•A’ and ‘G•C → A•T’, instead of ‘C•G → T•A’ and ‘C•G → A•T’, respectively. Fig. 1 has been corrected online.

Publisher Correction: Base editing: precision chemistry on the genome and transcriptome of living cells

The originally published article contained errors in reference numbering throughout table 1 (DNA base editors and their approximate editing windows) due to the unintended propagation of reference numbering from an earlier version of the table. The article has now been corrected online. The editors apologize for this error.