Mitochondrial base editor induces substantial nuclear off-target mutations.

DddA-derived cytosine base editors (DdCBEs)-which are fusions of split DddA halves and transcription activator-like effector (TALE) array proteins from bacteria-enable targeted C•G-to-T•A conversions in mitochondrial DNA1. However, their genome-wide specificity is poorly understood. Here we show that the mitochondrial base editor induces extensive off-target editing in the nuclear genome. Genome-wide, unbiased analysis of its editome reveals hundreds of off-target sites that are TALE array sequence (TAS)-dependent or TAS-independent. TAS-dependent off-target sites in the nuclear DNA are often specified by only one of the two TALE repeats, challenging the principle that DdCBEs are guided by paired TALE proteins positioned in close proximity. TAS-independent off-target sites on nuclear DNA are frequently shared among DdCBEs with distinct TALE arrays. Notably, they co-localize strongly with binding sites for the transcription factor CTCF and are enriched in topologically associating domain boundaries. We engineered DdCBE to alleviate such off-target effects. Collectively, our results have implications for the use of DdCBEs in basic research and therapeutic applications, and suggest the need to thoroughly define and evaluate the off-target effects of base-editing tools.

Detect-seq reveals out-of-protospacer editing and target-strand editing by cytosine base editors.

Cytosine base editors (CBEs) have the potential to correct human pathogenic point mutations. However, their genome-wide specificity remains poorly understood. Here we report Detect-seq for the evaluation of CBE specificity. It enables sensitive detection of CBE-induced off-target sites at the genome-wide level. Detect-seq leverages chemical labeling and biotin pulldown to trace the editing intermediate deoxyuridine, thereby revealing the editome of CBE. In addition to Cas9-independent and typical Cas9-dependent off-target sites, we discovered edits outside the protospacer sequence (that is, out-of-protospacer) and on the target strand (which pairs with the single-guide RNA). Such unexpected off-target edits are prevalent and can exhibit a high editing ratio, while their occurrences exhibit cell-type dependency and cannot be predicted based on the sgRNA sequence. Moreover, we found out-of-protospacer and target-strand edits nearby the on-target sites tested, challenging the general knowledge that CBEs do not induce proximal off-target mutations. Collectively, our approaches allow unbiased analysis of the CBE editome and provide a widely applicable tool for specificity evaluation of various emerging genome editing tools.

Increasing the efficiency and precision of prime editing with guide RNA pairs.

The recently reported prime editor (PE) can produce all types of base substitution, insertion and deletion, greatly expanding the scope of genome editing. However, improving the editing efficiency and precision of PE represents a major challenge. Here, we report an approach termed the homologous 3' extension mediated prime editor (HOPE). HOPE uses paired prime editing guide RNAs (pegRNAs) encoding the same edits in both sense and antisense DNA strands to achieve high editing efficiency in human embryonic kidney 293T cells as well as mismatch repair-deficient human colorectal carcinoma 116 cells. In addition, we found that HOPE shows greatly improved product purity compared to the original PE3 system. We envision that this enhanced tool could broaden both fundamental research and therapeutic applications of prime editing.

Re-engineering the adenine deaminase TadA-8e for efficient and specific CRISPR-based cytosine base editing.

Cytosine base editors (CBEs) efficiently generate precise C·G-to-T·A base conversions, but the activation-induced cytidine deaminase/apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (AID/APOBEC) protein family deaminase component induces considerable off-target effects and indels. To explore unnatural cytosine deaminases, we repurpose the adenine deaminase TadA-8e for cytosine conversion. The introduction of an N46L variant in TadA-8e eliminates its adenine deaminase activity and results in a TadA-8e-derived C-to-G base editor (Td-CGBE) capable of highly efficient and precise C·G-to-G·C editing. Through fusion with uracil glycosylase inhibitors and further introduction of additional variants, a series of Td-CBEs was obtained either with a high activity similar to that of BE4max or with higher precision compared to other reported accurate CBEs. Td-CGBE/Td-CBEs show very low indel effects and a background level of Cas9-dependent or Cas9-independent DNA/RNA off-target editing. Moreover, Td-CGBE/Td-CBEs are more efficient in generating accurate edits in homopolymeric cytosine sites in cells or mouse embryos, suggesting their accuracy and safety for gene therapy and other applications.

DNA repair glycosylase hNEIL1 triages damaged bases via competing interaction modes.

DNA glycosylases must distinguish the sparse damaged sites from the vast expanse of normal DNA bases. However, our understanding of the nature of nucleobase interrogation is still limited. Here, we show that hNEIL1 (human endonuclease VIII-like 1) captures base lesions via two competing states of interaction: an activated state that commits catalysis and base excision repair, and a quarantine state that temporarily separates and protects the flipped base via auto-inhibition. The relative dominance of the two states depends on key residues of hNEIL1 and chemical properties (e.g. aromaticity and hydrophilicity) of flipped bases. Such a DNA repair mechanism allows hNEIL1 to recognize a broad spectrum of DNA damage while keeps potential gratuitous repair in check. We further reveal the molecular basis of hNEIL1 activity regulation mediated by post-transcriptional modifications and provide an example of how exquisite structural dynamics serves for orchestrated enzyme functions.