DNA-editing Enzyme Research Articles

A Nanobody Toolbox for Recognizing Distinct Epitopes on Cas9

Cas9s and fusions of Cas9s have emerged as powerful tools for genetic manipulations. Fusions of Cas9 with other DNA editing enzymes have led to variants capable of single base editing and catalytically dead Cas9s have emerged as tools to specifically target desired regions of a genome. Here we describe the generation of a panel of nanobodies directed against three unique epitopes on Streptococcus pyogenes Cas9. The nanobodies were identified from a nanobody library derived from an alpaca that had been immunized with Cas9. The most potent binders recognize Cas9 and RNA bound Cas9 equally well and do not inhibit Cas9 cleavage of target DNA. These nanobodies bind non-overlapping epitopes as determined by ELISA based epitope binning experiments and mass photometry. We present the sequences of these clones and supporting biochemical data so the broader scientific community can access these reagents.

Pathogen dynamics and discovery of novel viruses and enzymes by deep nucleic acid sequencing of wastewater

Wastewater contains an extensive reservoir of genetic information, yet largely unexplored. Here, we analyzed by high-throughput sequencing total nucleic acids extracted from wastewater samples collected during a 17 month-period in Berlin, Germany. By integrating global wastewater datasets and applying a novel computational approach to accurately identify viral strains within sewage RNA-sequencing data, we demonstrated the emergence and global dissemination of a specific astrovirus strain. Astrovirus abundance and sequence variation mirrored temporal and spatial patterns of infection, potentially serving as footprints of specific timeframes and geographical locations. Additionally, we revealed more than 100,000 sequence contigs likely originating from novel viral species, exhibiting distinct profiles in total RNA and DNA datasets and including undescribed bunyaviruses and parvoviruses. Finally, we identified thousands of new CRISPR-associated protein sequences, including Transposase B (TnpB), a class of compact, RNA-guided DNA editing enzymes. Collectively, our findings underscore the potential of high-throughput sequencing of total nucleic acids derived from wastewater for a broad range of applications.

Abstract 3401: Uncovering virus induced dysregulated processes in cancer and their impact on outcomes

Abstract The National Cancer Institute has designated several viruses such as EBV, HBV, HCV, HIV, HPV, and HTLV as agents that either cause cancer or increase the risk of developing cancer. However, extensive analyses of tumor genome and transcriptome datasets have identified numerous additional virus types, the functional implications of which remain largely unknown. Viral infection often dysregulates unique molecular processes within tumor cells and alters the composition of immune cells in the tumor microenvironment. For instance, HPV-positive tumors have been shown to exhibit a significantly elevated expression of the DNA editing enzyme APOBEC3B compared to HPV-negative tumors. Furthermore, HPV positive tumors display and increased number of C-to-T mutations induced by this protein. The genome and transcriptome of tumor tissue biopsies contain a wealth of information about these virus-associated signatures. However, advanced signal processing approaches are needed to extract these viral signatures from complex tumor datasets, and elucidate their specific roles in cancer. Here, we developed a quantitative approach to investigate the link between viral infection and dysregulation of a seven-membered A3 enzyme family (A3A/B/C/D/F/G/H). The primary role of these enzymes is to restrict viral infections by inducing C-to-U mutations in viral genomes. However, at least three members of this enzyme family (A3A/B/H) can also induce mutations in cellular DNA, and drive tumor formation and evolution. We used NMF (Nonnegative Matrix Factorization) to deconvolute the expression profiles of A3 enzymes in >10,000 bulk RNAseq datasets from diverse TCGA tumors. This analysis revealed the expression profiles of A3 enzymes within tumor and tumor microenvironment cells including various immune cells. Next, we conducted an association analysis to identify the link between the presence of viral infection and the proportion of each of these tumor and tumor microenvironment expression profiles. As expected, our analyses revealed strong associations between tumor-specific A3 expression profiles and infections by oncolytic viruses such as HPV. Notably, and somewhat unexpectedly, our analyses also revealed associations with viruses about which little is known in the field of cancer. For example, in certain tumors, we identified a robust association between bacteriophages and a tumor-specific A3 expression profile characterized by A3B upregulation. Taken together, our analyses provide evidence for the link between A3 dysregulation and infection by viruses including bacteriophages. Citation Format: Mohadeseh Soleimanpour moghadam, Azad Khosh, Diako Ebrahimi. Uncovering virus induced dysregulated processes in cancer and their impact on outcomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3401.

Quantification of evolved DNA-editing enzymes at scale with DEQSeq

We introduce DEQSeq, a nanopore sequencing approach that rationalizes the selection of favorable genome editing enzymes from directed molecular evolution experiments. With the ability to capture full-length sequences, editing efficiencies, and specificities from thousands of evolved enzymes simultaneously, DEQSeq streamlines the process of identifying the most valuable variants for further study and application. We apply DEQSeq to evolved libraries of Cas12f-ABEs and designer-recombinases, identifying variants with improved properties for future applications. Our results demonstrate that DEQSeq is a powerful tool for accelerating enzyme discovery and advancing genome editing research.

The APOBEC3B c.783delG Truncating Mutation Is Not Associated with an Increased Risk of Breast Cancer in the Polish Population.

The APOBEC3B gene belongs to a cluster of DNA-editing enzymes on chromosome 22 and encodes an activation-induced cytidine deaminase. A large deletion of APOBEC3B was associated with increased breast cancer risk, but the evidence is inconclusive. To investigate whether or not APOBEC3B is a breast cancer susceptibility gene, we sequenced this gene in 617 Polish patients with hereditary breast cancer. We detected a single recurrent truncating mutation (c.783delG, p.Val262Phefs) in four of the 617 (0.65%) hereditary cases by sequencing. We then genotyped an additional 12,484 women with unselected breast cancer and 3740 cancer-free women for the c.783delG mutation. The APOBEC3B c.783delG allele was detected in 60 (0.48%) unselected cases and 19 (0.51%) controls (OR = 0.95, 95% CI 0.56-1.59, p = 0.94). The allele was present in 8 of 1968 (0.41%) familial breast cancer patients from unselected cases (OR = 0.80, 95% CI 0.35-1.83, p = 0.74). Clinical characteristics of breast tumors in carriers of the APOBEC3B mutation and non-carriers were similar. No cancer type was more frequent in the relatives of mutation carriers than in those of non-carriers. We conclude the APOBEC3B deleterious mutation p.Val262Phefs does not confer breast cancer risk. These data do not support the hypothesis that APOBEC3B is a breast cancer susceptibility gene.

Front Cover: The Wild‐Type tRNA Adenosine Deaminase Enzyme TadA Is Capable of Sequence‐Specific DNA Base Editing (ChemBioChem 16/2023)

The directed evolution of the adenosine deaminase enzyme TadA from an RNA-editing enzyme to an efficient DNA-editing enzyme was crucial for the development of the adenine base editor (ABE). Previous experiments showed no detectable DNA-editing activity by an ABE utilizing wild-type TadA (ABE0.1), but here we show ABE0.1 is able to edit DNA strictly within TACG motifs. This sequence requirement is partially governed by the C-terminal α5 helix of TadA, which accumulated many mutations during the directed evolution of TadA. The evolution of TadA to better access adenines within a diverse range of sequences is reflective of the Galapagos finches, which evolved different beak shapes in order to better access the food that was available to them. This cover art shows the protein structures of ecTadA (PDB: 1Z3A) and the evolved TadA8e (PDB: 6VCA), along with two species of Galapagos finches using the different α5 helix structures to access a target adenine within different sequence contexts. More information can be found in the Research Article by A. C. Komor et al.

The Design and Application of DNA-Editing Enzymes as Base Editors.

DNA-editing enzymes perform chemical reactions on DNA nucleobases. These reactions can change the genetic identity of the modified base or modulate gene expression. Interest in DNA-editing enzymes has burgeoned in recent years due to the advent of clustered regularly interspaced short palindromic repeat-associated (CRISPR-Cas) systems, which can be used to direct their DNA-editing activity to specific genomic loci of interest. In this review, we showcase DNA-editing enzymes that have been repurposed or redesigned and developed into programmable base editors. These include deaminases, glycosylases, methyltransferases, and demethylases. We highlight the astounding degree to which these enzymes have been redesigned, evolved, and refined and present these collective engineering efforts as a paragon for future efforts to repurpose and engineer other families of enzymes. Collectively, base editors derived from these DNA-editing enzymes facilitate programmable point mutation introduction and gene expression modulation by targeted chemical modification of nucleobases.

Autosomal Dominant Immune Dysfunction in Five Family Members with Truncating Variant in the Activation Induced Cytidine Deaminase (AICDA) Gene

Activation induced cytidine deaminase (AICDA) is an DNA editing enzyme specifically expressed in germinal center-like B-cells that is involved in somatic hypermutation and class-switch recombination. Bi-allelic defects in AICDA are a recognized cause of autosomal recessive hyper-IgM syndrome. However, truncating variants in AICDA have been rarely described as an autosomal dominant cause of hyper-IgM syndrome.

Abstract 1483: Examining the role of genome editing enzymes in anti tumor immunity

Abstract Background: Tumor antigens (TA) on cancer cells can trigger immune response TA are peptides that are either not present, or are present to a much lesser extent, on normal cells compared to tumor cells. CD8+ Cytotoxic T-cells (CTL) recognize TAs bound to class I major histocompatibility complex (MHC) molecules on the surface of tumor cells. We and others have shown that viruses subvert the mutational activity of the endogenous DNA-editing APOBEC enzymes in human host cells towards immune escape. Viruses do so by adapting their genomic DNA code to position APOBEC's favored mutational “hotspot” DNA sequences such that upon being mutated by the host APOBEC enzymes, the mutated viral protein is less immunogenic for CTL recognition. Like viruses, cancer cells have genomes that are highly plastic and adaptable through mutation; therefore, we hypothesize that the APOBEC DNA-editing enzymes are exploited by cancer cells for modulating TA immunogenicity. Aims of the Study: Aim1: To elucidate the impact of A3-mediated mutations on modulating TA immunogenicity for CTL recognition. Aim 2: To examine whether genomic sequences encoding TAs in the human genome have co-evolved to either attract or avoid APOBEC-driven mutations. Methods: Aim 1: Using the human reference proteome from EMBL/EBI, we located APOBEC-mutable hotspots in genomic DNA encoding the immunopeptidome. We simulated APOBEC-mediated mutations and translated to peptide TAs. We used NetMHCpan 4 to measure the impact of TA mutations on MHC class I binding affinity. In parallel, we are searching for already-identified APOBEC-mediated mutations in sequenced tumor genomes in cancer genome databases such as The Cancer Genome Atlas (TCGA), Catalogue of Observed Somatic Mutations in Cancer (COSMIC) and International Cancer Genome Consortium (ICGC). Aim 2: To investigate the enrichment of APOBEC-hotspots in genomic sequences encoding TAs restricted to each HLA, we are tabulating APOBEC hotspots in genomic DNA sequencing encoding TAs vs. non-TA-encoding sequences. At the sequence level, the ratio of hotspot frequency, in combination with scores of predicted impacts on immunogenicity as obtained in aim 1, will be used to glean whether and how the human genomic sequence has co-evolved with APOBECs from the standpoint of TAs. Results and Significance: Based on the preliminary data we observe that APOBEC-mediated mutations can diminish or enhance immunogenicity. Understanding the balance between these two effects is key in informing how APOBEC expression and activity ought to be regarded in the context of cancer immunotherapy. Citation Format: Faezeh Borzooee. Examining the role of genome editing enzymes in anti tumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1483.

Evolutionary Comparative Analyses of DNA-Editing Enzymes of the Immune System: From 5-Dimensional Description of Protein Structures to Immunological Insights and Applications to Protein Engineering.

The immune system is unique among all biological sub-systems in its usage of DNA-editing enzymes to introduce targeted gene mutations and double-strand DNA breaks to diversify antigen receptor genes and combat viral infections. These processes, initiated by specific DNA-editing enzymes, often result in mistargeted induction of genome lesions that initiate and drive cancers. Like other molecules involved in human health and disease, the DNA-editing enzymes of the immune system have been intensively studied in humans and mice, with little attention paid (< 1% of published studies) to the same enzymes in evolutionarily distant species. Here, we present a systematic review of the literature on the characterization of one such DNA-editing enzyme, activation-induced cytidine deaminase (AID), from an evolutionary comparative perspective. The central thesis of this review is that although the evolutionary comparative approach represents a minuscule fraction of published works on this and other DNA-editing enzymes, this approach has made significant impacts across the fields of structural biology, immunology, and cancer research. Using AID as an example, we highlight the value of the evolutionary comparative approach in discoveries already made, and in the context of emerging directions in immunology and protein engineering. We introduce the concept of 5-dimensional (5D) description of protein structures, a more nuanced view of a structure that is made possible by evolutionary comparative studies. In this higher dimensional view of a protein’s structure, the classical 3-dimensional (3D) structure is integrated in the context of real-time conformations and evolutionary time shifts (4th dimension) and the relevance of these dynamics to its biological function (5th dimension).

Mitochondrial targeted meganuclease as a platform to eliminate mutant mtDNA in vivo

Diseases caused by heteroplasmic mitochondrial DNA mutations have no effective treatment or cure. In recent years, DNA editing enzymes were tested as tools to eliminate mutant mtDNA in heteroplasmic cells and tissues. Mitochondrial-targeted restriction endonucleases, ZFNs, and TALENs have been successful in shifting mtDNA heteroplasmy, but they all have drawbacks as gene therapy reagents, including: large size, heterodimeric nature, inability to distinguish single base changes, or low flexibility and effectiveness. Here we report the adaptation of a gene editing platform based on the I-CreI meganuclease known as ARCUS®. These mitochondrial-targeted meganucleases (mitoARCUS) have a relatively small size, are monomeric, and can recognize sequences differing by as little as one base pair. We show the development of a mitoARCUS specific for the mouse m.5024C>T mutation in the mt-tRNAAla gene and its delivery to mice intravenously using AAV9 as a vector. Liver and skeletal muscle show robust elimination of mutant mtDNA with concomitant restoration of mt-tRNAAla levels. We conclude that mitoARCUS is a potential powerful tool for the elimination of mutant mtDNA.

Elimination of Mutant mtDNA by an Optimized mpTALEN Restores Differentiation Capacities of Heteroplasmic MELAS-iPSCs

Various mitochondrial diseases, including mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), are associated with heteroplasmic mutations in mitochondrial DNA (mtDNA). Herein, we refined a previously generated G13513A mtDNA-targeted platinum transcription activator-like effector nuclease (G13513A-mpTALEN) to more efficiently manipulate mtDNA heteroplasmy in MELAS-induced pluripotent stem cells (iPSCs). Introduction of a nonconventional TALE array at position 6 in the mpTALEN monomer, which recognizes the sequence around the m.13513G>A position, improved the mpTALEN effect on the heteroplasmic shift. Furthermore, the reduced expression of the new Lv-mpTALEN(PKLB)/R-mpTALEN(PKR6C) pair by modifying codons in their expression vectors could suppress the reduction in the mtDNA copy number, which contributed to the rapid recovery of mtDNA in mpTALEN-applied iPSCs during subsequent culturing. Moreover, MELAS-iPSCs with a high proportion of G13513A mutant mtDNA showed unusual properties of spontaneous, embryoid body-mediated differentiation in vitro, which was relieved by decreasing the heteroplasmy level with G13513A-mpTALEN. Additionally, drug-inducible, myogenic differentiation 1 (MYOD)-transfected MELAS-iPSCs (MyoD-iPSCs) efficiently differentiated into myosin heavy chain-positive myocytes, with or without mutant mtDNA. Hence, heteroplasmic MyoD-iPSCs controlled by fine-tuned mpTALENs may contribute to a detailed analysis of the relationship between mutation load and cellular phenotypes in disease modeling.

Reduced VIP Expression Affects Circadian Clock Function in VIP-IRES-CRE Mice (JAX 010908).

Circadian rhythms are programmed by the suprachiasmatic nucleus (SCN), which relies on neuropeptide signaling to maintain daily timekeeping. Vasoactive intestinal polypeptide (VIP) is critical for SCN function, but the precise role of VIP neurons in SCN circuits is not fully established. To interrogate their contribution to SCN circuits, VIP neurons can be manipulated specifically using the DNA-editing enzyme Cre recombinase. Although the Cre transgene is assumed to be inert by itself, we find that VIP expression is reduced in both heterozygous and homozygous adult VIP-IRES-Cre mice (JAX 010908). Compared with wild-type mice, homozygous VIP-Cre mice display faster reentrainment and shorter free-running period but do not become arrhythmic in constant darkness. Consistent with this phenotype, homozygous VIP-Cre mice display intact SCN PER2::LUC rhythms, albeit with altered period and network organization. We present evidence that the ability to sustain molecular rhythms in the VIP-Cre SCN is not due to residual VIP signaling; rather, arginine vasopressin signaling helps to sustain SCN function at both intracellular and intercellular levels in this model. This work establishes that the VIP-IRES-Cre transgene interferes with VIP expression but that loss of VIP can be mitigated by other neuropeptide signals to help sustain SCN function. Our findings have implications for studies employing this transgenic model and provide novel insight into neuropeptide signals that sustain daily timekeeping in the master clock.

DNA-editing enzymes as potential treatments for heteroplasmic mtDNA diseases.

Mutations in the mitochondrial genome are the cause of many debilitating neuromuscular disorders. Currently, there is no cure or treatment for these diseases, and symptom management is the only relief doctors can provide. Although supplements and vitamins are commonly used in treatment, they provide little benefit to the patient and are only palliative. This is why gene therapy is a promising research topic to potentially treat and, in theory, even cure diseases caused by mutations in the mitochondrial DNA (mtDNA). Mammalian cells contain approximately a thousand copies of mtDNA, which can lead to a phenomenon called heteroplasmy, where both wild-type and mutant mtDNA molecules co-exist within the cell. Disease only manifests once the per cent of mutant mtDNA reaches a high threshold (usually >80%), which causes mitochondrial dysfunction and reduced ATP production. This is a useful feature to take advantage of for gene therapy applications, as not every mutant copy of mtDNA needs to be eliminated, but only enough to shift the heteroplasmic ratio below the disease threshold. Several DNA-editing enzymes have been used to shift heteroplasmy in cell culture and mice. This review provides an overview of these enzymes and discusses roadblocks of applying these to gene therapy in humans.

Perturbing proteomes at single residue resolution using base editing

Base editors derived from CRISPR-Cas9 systems and DNA editing enzymes offer an unprecedented opportunity for the precise modification of genes, but have yet to be used at a genome-scale throughput. Here, we test the ability of the Target-AID base editor to systematically modify genes genome-wide by targeting yeast essential genes. We mutate around 17,000 individual sites in parallel across more than 1500 genes. We identify over 700 sites at which mutations have a significant impact on fitness. Using previously determined and preferred Target-AID mutational outcomes, we find that gRNAs with significant effects on fitness are enriched in variants predicted to be deleterious based on residue conservation and predicted protein destabilization. We identify key features influencing effective gRNAs in the context of base editing. Our results show that base editing is a powerful tool to identify key amino acid residues at the scale of proteomes.

Human PCNA Structure, Function and Interactions.

Proliferating cell nuclear antigen (PCNA) is an essential factor in DNA replication and repair. It forms a homotrimeric ring that embraces the DNA and slides along it, anchoring DNA polymerases and other DNA editing enzymes. It also interacts with regulatory proteins through a sequence motif known as PCNA Interacting Protein box (PIP-box). We here review the latest contributions to knowledge regarding the structure-function relationships in human PCNA, particularly the mechanism of sliding, and of the molecular recognition of canonical and non-canonical PIP motifs. The unique binding mode of the oncogene p15 is described in detail, and the implications of the recently discovered structure of PCNA bound to polymerase δ are discussed. The study of the post-translational modifications of PCNA and its partners may yield therapeutic opportunities in cancer treatment, in addition to illuminating the way PCNA coordinates the dynamic exchange of its many partners in DNA replication and repair.

MagnEdit-interacting factors that recruit DNA-editing enzymes to single base targets.

Although CRISPR/Cas9 technology has created a renaissance in genome engineering, particularly for gene knockout generation, methods to introduce precise single base changes are also highly desirable. The covalent fusion of a DNA-editing enzyme such as APOBEC to a Cas9 nickase complex has heightened hopes for such precision genome engineering. However, current cytosine base editors are prone to undesirable off-target mutations, including, most frequently, target-adjacent mutations. Here, we report a method to "attract" the DNA deaminase, APOBEC3B, to a target cytosine base for specific editing with minimal damage to adjacent cytosine bases. The key to this system is fusing an APOBEC-interacting protein (not APOBEC itself) to Cas9n, which attracts nuclear APOBEC3B transiently to the target site for editing. Several APOBEC3B interactors were tested and one, hnRNPUL1, demonstrated proof-of-concept with successful C-to-T editing of episomal and chromosomal substrates and lower frequencies of target-adjacent events.

Rationalizing the Effect of Mutations on the Editing Efficiency of Adenine Base Editors

Base editing technologies enable the precise conversion of one base pair in genomic DNA into another. The most recent class of base editors, adenine base editors (ABEs), facilitate the efficient modification of A:T base pairs to G⋮C at targeted genomic loci via the deamination of adenine to inosine. ABEs were developed by engineering and evolving a tRNA adenosine deaminase enzyme (TadA) into a single-stranded DNA (ssDNA) editing enzyme. A mechanistic understanding of the contributions of the individual mutations to ssDNA editing efficiency of ABEs remains unknown.To reduce the experimental effort associated with directed evolution and to explore the sequence space encoded by mutating multiple positions simultaneously, we employ computational design strategies. All-atom, explicit-solvent, molecular dynamics (MD) simulations of the various TadA-ssDNA complexes were performed on GPUs using the CUDA version of PMEMD in AMBER. These simulations help us to explore the structural and functional role played by the initial mutations in the onset of ssDNA modifying activity of the TadA enzyme. Atomistic insights into the system reveal that these early mutations lead to intricate conformational changes in the structure of the protein. These mutations are associated with an enhancement in the binding free energy of the TadA∗ to ssDNA, which we confirm using umbrella sampling calculations. Simulations and experimental reversion analyses verify the importance of these mutations in imparting functional promiscuity to the TadA enzyme. We plan on generalizing this engineering approach and aiding the discovery of more genome editors in conjunction with canonical directed evolution.

APOBEC3B reporter myeloma cell lines identify DNA damage response pathways leading to APOBEC3B expression.

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) DNA cytosine deaminase 3B (A3B) is a DNA editing enzyme which induces genomic DNA mutations in multiple myeloma and in various other cancers. APOBEC family proteins are highly homologous so it is especially difficult to investigate the biology of specifically A3B in cancer cells. To easily and comprehensively investigate A3B function in myeloma cells, we used CRISPR/Cas9 to generate A3B reporter cells that contain 3×FLAG tag and IRES-EGFP sequences integrated at the end of the A3B gene. These reporter cells stably express 3xFLAG tagged A3B and the reporter EGFP and this expression is enhanced by known stimuli, such as PMA. Conversely, shRNA knockdown of A3B decreased EGFP fluorescence and 3xFLAG tagged A3B protein levels. We screened a series of anticancer treatments using these cell lines and identified that most conventional therapies, such as antimetabolites or radiation, exacerbated endogenous A3B expression, but recent molecular targeted therapeutics, including bortezomib, lenalidomide and elotuzumab, did not. Furthermore, chemical inhibition of ATM, ATR and DNA-PK suppressed EGFP expression upon treatment with antimetabolites. These results suggest that DNA damage triggers A3B expression through ATM, ATR and DNA-PK signaling.

APOBEC3s: DNA‐editing human cytidine deaminases

Nucleic acid editing enzymes are essential components of the human immune system that lethally mutate viral pathogens and somatically mutate immunoglobulins. Among these enzymes are cytidine deaminases of the apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) super family, each with unique target sequence specificity and subcellular localization. We focus on the DNA-editing APOBEC3 enzymes that have recently attracted attention because of their involvement in cancer and potential in gene-editing applications. We review and compare the crystal structures of APOBEC3 (A3) domains, binding interactions with DNA, substrate specificity, and activity. Recent crystal structures of A3A and A3G bound to ssDNA have provided insights into substrate binding and specificity determinants of these enzymes. Still many unknowns remain regarding potential cooperativity, nucleic acid interactions, and systematic quantification of substrate preference of many APOBEC3s, which are needed to better characterize the biological functions and consequences of misregulation of these gene editors.