Genomic adenine

This target refers to a specific adenine nucleotide within a double-stranded DNA sequence that is designated for therapeutic modification. The selection of this adenine is dictated by its position within a protospacer sequence defined by a single-guide RNA (sgRNA) and its proximity to an NRCH protospacer adjacent motif (PAM). This configuration is typically utilized by engineered CRISPR-Cas9 systems, such as NRCH-Cas9 or SpRY, which have been modified to recognize non-canonical PAM sequences, thereby expanding the range of targetable genomic sites (Walton et al., 2020). In a therapeutic context, this adenine is the substrate for adenine base editors (ABEs), which catalyze the conversion of adenine to inosine (Gaudelli et al., 2017). Because inosine is interpreted as guanine by DNA polymerases, the process results in a precise A•T to G•C transition, allowing for the correction of pathogenic point mutations associated with various genetic disorders (Miller et al., 2020). This approach is highly specific and avoids the double-strand breaks associated with traditional CRISPR-Cas9 nucleases, potentially reducing the risk of large deletions or chromosomal translocations.

Other names

Targeted adenineA•T base pairProtospacer adenineGenomic deoxyadenosineTherapeutic genomic adenine within double-stranded DNA specified by the sgRNA protospacer and an NRCH PAM

The target adenine is converted to inosine via hydrolytic deamination by a deoxyadenosine deaminase (e.g., TadA variant) tethered to a Cas9 nickase. The inosine is read as guanine during DNA replication, resulting in a permanent A-to-G transition at the specified genomic locus.