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Sickle cell disease (SCD) is caused by a mutation in the β-globin gene HBB1. We used a custom adenine base editor (ABE8e-NRCH)2,3 to convert the SCD allele (HBBS) into Makassar β-globin (HBBG), a non-pathogenic variant4,5. Ex vivo delivery of mRNA encoding the base editor with a targeting guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD resulted in 80% conversion of HBBS to HBBG. Sixteen weeks after transplantation of edited human HSPCs into immunodeficient mice, the frequency of HBBG was 68% and hypoxia-induced sickling of bone marrow reticulocytes had decreased fivefold, indicating durable gene editing. To assess the physiological effects of HBBS base editing, we delivered ABE8e-NRCH and guide RNA into HSPCs from a humanized SCD mouse6 and then transplanted these cells into irradiated mice. After sixteen weeks, Makassar β-globin represented 79% of β-globin protein in blood, and hypoxia-induced sickling was reduced threefold. Mice that received base-edited HSPCs showed near-normal haematological parameters and reduced splenic pathology compared to mice that received unedited cells. Secondary transplantation of edited bone marrow confirmed that the gene editing was durable in long-term haematopoietic stem cells and showed that HBBS-to-HBBG editing of 20% or more is sufficient for phenotypic rescue. Base editing of human HSPCs avoided the p53 activation and larger deletions that have been observed following Cas9 nuclease treatment. These findings point towards a one-time autologous treatment for SCD that eliminates pathogenic HBBS, generates benign HBBG, and minimizes the undesired consequences of double-strand DNA breaks. A custom adenine base editor can edit the variant of the β-globin gene that causes sickle cell disease into a non-pathogenic variant in human and mouse cells, and transplantation of the edited cells rescues sickle cell disease in mice.

Applications of adenine base editors (ABEs) have been constrained by the limited compatibility of the deoxyadenosine deaminase component with Cas homologs other than SpCas9. We evolved the deaminase component of ABE7.10 using phage-assisted non-continuous and continuous evolution (PANCE and PACE), resulting in ABE8e. ABE8e contains eight additional mutations that increase activity (kapp) 590-fold compared with ABE7.10. ABE8e offers substantially improved editing efficiencies when paired with a variety of Cas9 or Cas12 homologs. ABE8e is more processive than ABE7.10, which could benefit screening, disrupting regulatory regions and multiplex base editing applications. A modest increase in Cas9-dependent and -independent DNA off-target editing, and in transcriptome-wide RNA off-target editing can be ameliorated by introducing additional mutations in the TadA-8e domain. Finally, we show that ABE8e can efficiently edit natural mutations in a GATA1 binding site in the BCL11A enhancer or the HBG promoter in human cells, targets which were poorly edited with ABE7.10. ABE8e broadens the effectiveness and applicability of adenine base editing., Editorial Summary A continuously evolved adenine base editor is compatible with various Cas proteins and mediates efficient A•T-to-G•C base conversions at a wide variety of PAM sites.

In contrast to traditional CRISPR–Cas9 homology-directed repair, base editing can correct point mutations without supplying a DNA-repair template. Here we show in a mouse model of tyrosinaemia that hydrodynamic tail-vein injection of plasmid DNA encoding the adenine base editor (ABE) and a single-guide RNA (sgRNA) can correct an A>G splice-site mutation. ABE treatment partially restored splicing, generated fumarylacetoacetate hydrolase (FAH)-positive hepatocytes in the liver, and rescued weight loss in mice. We also generated FAH+ hepatocytes in the liver via lipid-nanoparticle-mediated delivery of a chemically modified sgRNA and an mRNA of a codon-optimized base editor that displayed higher base-editing efficiency than the standard ABEs. Our findings suggest that adenine base editing can be used for the correction of genetic diseases in adult animals. Intravenous delivery of an adenine base editor and a single-guide RNA for the Fah gene can correct an A>G splice-site mutation in an adult mouse model of tyrosinaemia.

Background The osmolyte and antioxidant taurine plays an important role in regulation of cellular volume, oxidative status and Ca2+-homeostasis. Taurine uptake in human cells is regulated by the Na+- and Cl−-dependent taurine transporter TauT. In order to gain deeper structural insights about the substrate binding pocket of TauT, a HEK293 cell line producing a GFP-TauT fusion protein was generated. Methods Transport activity was validated using cell-based [3H]-taurine transport assays. We determined the Km and IC50 values of taurine, β-alanine and γ-aminobutyrate. Additionally we were able to identify structurally similar compounds as potential new substrates or inhibitors of the TauT transporter. Substrate induced cytotoxicity was analyzed using a cell viability assay. Results In this study we show competitive effects of the 3-pyridinesulfonate, 2-aminoethylhydrogen sulfate, 5-aminovalerate, β-aminobutyrate, piperidine-4-sulfonate, 2-aminoethylphosphate and homotaurine. We demonstrate that taurine uptake can be inhibited by a phosphate. Furthermore our studies revealed that piperidine-4-sulfonate interacts with TauT with a higher affinity than γ-aminobutyrate and imidazole-4-acetate. Conclusion We propose that piperidine-4-sulfonate may serve as a potential lead structure for the design of novel drug candidates required for specific modulation of the TauT transporter in therapy of neurodegenerative diseases.

Moving targets of neurotoxins Proteases that cleave protein targets at specific sequences control many biological functions. The ability to reprogram proteases to cleave new sequences of our choosing would enable new therapeutic and biotechnological applications. Blum et al. report a laboratory evolution method to rapidly evolve proteases that cut new protein sequences and lose their ability to cut nontarget sequences (see the Perspective by Stenmark). Using this method, they evolved botulinum neurotoxin proteases, an important class of enzymes used in patients, to selectively cleave new targets, including a protein unrelated to those natively cleaved by these proteases. This work establishes a powerful approach to generate proteases with tailor-made specificities. Science , this issue p. 803 ; see also p. 782

Secrets of a fast base editor CRISPR-Cas9 base editors comprise RNA-guided Cas proteins fused to an enzyme that can deaminate a DNA nucleoside. No natural enzyme deaminates adenine in DNA, and so a breakthrough came when a natural transfer RNA deaminase was fused to Cas9 and evolved to give an adenine base editor (ABE) that works on DNA. Further evolution provided the enzyme ABE8e, which catalyzes deamination more than 1000 times faster than early ABEs. Lapinaite et al. now present a 3.2-angstrom resolution structure of ABE8e bound to DNA in which the target adenine is replaced with an analog designed to trap the catalytic conformation. The structure, together with kinetic data comparing ABE8e to earlier ABEs, explains how ABE8e edits DNA bases and could inform future base-editor design. Science , this issue p. 566

In contrast to traditional CRISPR-Cas9 homology-directed repair, base editing can correct point mutations without supplying a DNA-repair template. Here we show in a mouse model of tyrosinaemia that hydrodynamic tail-vein injection of plasmid DNA encoding the adenine base editor (ABE) and a single-guide RNA (sgRNA) can correct an AG splice-site mutation. ABE treatment partially restored splicing, generated fumarylacetoacetate hydrolase (FAH)-positive hepatocytes in the liver, and rescued weight loss in mice. We also generated FAH

Pau d'arco is a plant-derived traditional medicine that acts by poorly understood molecular mechanisms. Here, we studied the effect of pau d'arco on the cytoprotective transcription factor Nrf2. An aqueous extract of pau d'arco stimulated Nrf2-dependent gene expression and led to nuclear localization of Nrf2 in vitro. Chromatographic separation and mass spectrometry of the extract identified benzene trioles or benzene tetraoles within the active fractions. The extract stimulated the mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase (MEK)/extracellular-signal-regulated kinase (ERK1/2) pathway. The pharmacological inhibition of MEK, but not of p38 mitogen-activated protein kinase, glycogen synthase kinase-3 or phosphoinositide 3-kinase was required for the activation of Nrf2-dependent gene expression by pau d'arco, but not for the nuclear translocation of Nrf2. In vivo pau d'arco increased the expression of Nrf2-target genes in the intestine. The results suggest that the activation of Nrf2 could mediate beneficial effects of pau d'arco, in particular in the intestine.