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The foundational adenine base editors (for example, ABE7.10) enable programmable A•T to G•C point mutations but editing efficiencies can be low at challenging loci in primary human cells. Here we further evolve ABE7.10 using a library of adenosine deaminase variants to create ABE8s. At NGG protospacer adjacent motif (PAM) sites, ABE8s result in ~1.5× higher editing at protospacer positions A5-A7 and ~3.2× higher editing at positions A3-A4 and A8-A10 compared with ABE7.10. Non-NGG PAM variants have a ~4.2-fold overall higher on-target editing efficiency than ABE7.10. In human CD34+ cells, ABE8 can recreate a natural allele at the promoter of the γ-globin genes HBG1 and HBG2 with up to 60% efficiency, causing persistence of fetal hemoglobin. In primary human T cells, ABE8s achieve 98-99% target modification, which is maintained when multiplexed across three loci. Delivered as messenger RNA, ABE8s induce no significant levels of single guide RNA (sgRNA)-independent off-target adenine deamination in genomic DNA and very low levels of adenine deamination in cellular mRNA.

Conversion of the pathogenic sickle allele to a naturally occurring, non-pathogenic hemoglobin variant, Hb G-Makassar, represents a long-term and durable treatment strategy for sickle cell disease (SCD). Using our engineered adenine base editor, we achieved highly efficient base editing in mobilized sickle trait (HbAS) and non-mobilized homozygous sickle (HbSS) CD34 + cells that led to >70% conversion of sickle allele to Makassar allele in in vitro erythroid differentiated (IVED) cells derived from ex vivo edited CD34 + cells. At this level of editing, >70% bi-allelic Makassar editing could be achieved in HbSS IVED cells, with ~20% of cells being mono-allelically Makassar edited. These mono-allelically edited cells behaved similarly to sickle trait (HbAS) cells, when exposed to hypoxic conditions in vitro. In vivo proof of concept xenotransplantation studies demonstrated that Makassar edited HbAS CD34 + cells achieved long-term, multi-lineage hematopoietic engraftment as well as Makassar globin protein expression in human erythroid glycophorin A + cells in thebone marrow of immunocompromised mice. Although the Makassar variant is naturally occurring in human genetics and present in individuals in Southeast Asia with normal hematologic parameters in both heterozygous and homozygous states, we sought to further characterize Makassar hemoglobin and assess its biophysical and biochemical properties. Recombinant Makassar globin was co-expressed with alpha globin in E. coli and tetramers were purified to homogeneity. Recombinant tetramers were assessed for identity, purity, globin content, and heme content demonstrating comparability to hemoglobin tetramers isolated from primary sources (whole blood). Several characterization methods were employed, to assess size, molecular weight, oligomerization state, tetramer composition, and oxygen binding properties. These studies indicated Makassar globin could properly assemble into hemoglobin tetramers, displaying biochemical properties characteristic of hemoglobins. Furthermore, we assessed polymerization potential using a temperature jump method previously employed for kinetic measurements of sickle-fiber formation and found Makassar hemoglobin did not polymerize in vitro under conditions where sickle hemoglobin (HbS) readily polymerizes, consistent with behavior observed previously by others. Finally, a crystal structure of Hb G-Makassar has been determined at the 2.2 Å resolution and showed high similarity to the HbA (wildtype hemoglobin) structure with a RMSD of 0.385 Å for all the Cα atoms, which indicates that the glutamic acid to alanine (E6A) substitution in beta-hemoglobin does not seem to induce any significant conformational change in hemoglobin structures. Altogether, our biophysical and biochemical characterization shows that the Makassar variant behaves as a functional hemoglobin. By replacing the pathogenic sickle globin with a benign hemoglobin variant with normal function, our base editing approach provides a promising autologous investigational cell therapy for the treatment of SCD. Disclosures Chu: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Ortega: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Feliciano: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Winton: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Xu: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Haupt: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company. McDonald: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Martinez: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Liquori: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Marshall: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Lam: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Olins: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Rinaldi: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Rehberger: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Lazarra: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Decker: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Gantzer: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Bohnuud: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Born: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Barrera: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Yan: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Slaymaker: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties. Packer: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Smith: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Zambonelli: Beam Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Lee: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Gaudelli: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Hartigan: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company. Ciaramella: Beam Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees.

The foundational adenine base editors (for example, ABE7.10) enable programmable A•T to G•C point mutations but editing efficiencies can be low at challenging loci in primary human cells. Here we further evolve ABE7.10 using a library of adenosine deaminase variants to create ABE8s. At NGG protospacer adjacent motif (PAM) sites, ABE8s result in ~1.5× higher editing at protospacer positions A5-A7 and ~3.2× higher editing at positions A3-A4 and A8-A10 compared with ABE7.10. Non-NGG PAM variants have a ~4.2-fold overall higher on-target editing efficiency than ABE7.10. In human CD34

SIRT1 is an NAD(+)-dependent lysine deacetylase that promotes healthy aging and longevity in diverse organisms. Small molecule allosteric activators of SIRT1 such as resveratrol and SRT2104 directly bind to the N-terminus of SIRT1 and lower the K(m) for the protein substrate. In rodents, sirtuin-activating compounds (STACs) protect from age-related diseases and extend life span. In human clinical trials, STACs have a high safety profile and anti-inflammatory activities. Here, we describe methods for identifying and characterizing STACs, including production of recombinant protein, in vitro assays with recombinant protein, and cellular assays based on mitochondrial dynamics. The methods described in this chapter will facilitate this discovery of improved STACs, natural and synthetic, in the pursuit of interventions to treat age-related diseases.

Sickle cell disease (SCD) and Beta thalassemia are disorders of beta globin production and function that lead to severe anemia and significant disease complications across a multitude of organ systems. Autologous transplantation of hematopoietic stem cells engineered through the upregulation of fetal hemoglobin (HbF) or correction of the beta globin gene have the potential to reduce disease burden in patients with beta hemoglobinopathies. Base editing is a recently developed technology that enables precise modification of the genome without the introduction of double strand DNA breaks. Gamma globin gene promoters were comprehensively screened with cytosine and adenine base editors (ABE) for the identification of alterations that would derepress HbF. Three regions were identified that significantly upregulated HbF, and the most effective nucleotide residue conversions are supported by natural variation seen in patients with hereditary persistence of fetal hemoglobin (HPFH). ABEs have been developed that significantly increase the level of HbF following nucleotide conversion at key regulatory motifs within the HBG1 and HBG2 promoters. CD34+ hematopoietic stem and progenitor cells (HSPC) were purified at clinical scale and edited using a process designed to preserve self-renewal capacity. Editing at two independent sites with different ABEs reached 94 percent and resulted in up to 63 percent gamma globin by UPLC. The levels of HbF observed should afford protection to the majority of SCD and Beta thalassemia patients based on clinical observations of HPFH and non-interventional therapy that links higher HbF dosage with milder disease (Ngo et al, 2011 Brit J Hem; Musallam et al, 2012 Blood). Directly correcting the Glu6Val mutation of SCD has been a recent goal of genetic therapies designed for the SCD population. Current base editing technology cannot yet convert mutations like those that result from the A-T transversion in sickle beta globin; however, ABE variants have been designed to recognize and edit the opposite stranded adenine residue of valine. This results in the conversion of valine to alanine and the production of a naturally occurring variant known as Hb G-Makassar. Beta globin with alanine at this position does not contribute to polymer formation, and patients with Hb G-Makassar present with normal hematological parameters and red blood cell morphology. SCD patient fibroblasts edited with these ABE variants achieve up to 70 percent conversion of the target adenine. CD34 cells from healthy donors were then edited with a lead ABE variant, targeting a synonymous mutation in an adjacent proline that resides within the editing window and serves as a proxy for editing the SCD mutation. The average editing frequency was 40 percent. Donor myeloid chimerism documented at these levels in the allogeneic transplant setting exceeds the 20 percent that is required for reversing the sickle phenotype (Fitzhugh et al, 2017 Blood). These next generation editing approaches provide a promising new modality for treating patients with Beta thalassemia and SCD. Disclosures No relevant conflicts of interest to declare.

The evolution of antibiotic resistance in bacteria has become one of the defining problems in modern biology. Bacterial resistance to antimicrobial therapy threatens to eliminate one of the pillars of the practice of modern medicine. Yet, in spite of the importance of this problem, only recently have the dynamics of the shift from antibiotic sensitivity to resistance in a bacterial population been studied. In this study, a novel chemostat method was used to observe the evolution of resistance to streptomycin in a sensitive population of Escherichia coli , which grew while the concentration of antibiotic was constantly increasing. The results indicate that resistant mutants remain at a low frequency for longer than expected and do not begin to rise to a high frequency until the antibiotic concentrations are above the measured MIC, creating a “lull period” in which there were few bacterial cells growing in the chemostats. Overall, mutants resistant to streptomycin were found in >60% of the experimental trial replicates. All of the mutants detected were found to have MICs far above the maximum levels of streptomycin to which they were exposed and reached a high frequency within 96 h.