Understanding the structural details of APOBEC3-DNA interactions using graph-based representations

Exploring ABOBEC3A and APOBEC3B substrate specificity and their role in HPV positive head and neck cancer

APOBEC3 family members are cytidine deaminases catalyzing conversion of cytidine to uracil. Many studies have established a link between APOBEC3 expression and cancer development and progression, especially APOBEC3A (A3A) and APOBEC3B (A3B). Preclinical studies with human papillomavirus positive (HPV+) head and neck squamous cell carcinoma (HNSCC) and clinical trial specimens revealed induction of A3B, but not A3A expression after demethylation. We examined the kinetic features of the cytidine deaminase activity for full length A3B and found that longer substrates and a purine at −2 position favored by A3B, whereas A3A prefers shorter substrates and an adenine or thymine at −2 position. The importance and biological significance of A3B catalytic activity rather than A3A and a preference for purine at the −2 position was also established in HPV+ HNSCCs. Our study explored factors influencing formation of A3A and A3B-related cancer mutations that are essential for understanding APOBEC3-related carcinogenesis and facilitating drug discovery.

•

A3B is upregulated after 5-AzaC treatment and related to 5-AzaC sensitivity in HPV+ HNSCC

•

Full-length A3B prefers longer substrates and a purine at −2 site biochemically

•

A3B also prefers a purine at −2 site in both HPV+ and HPV− HNSCC cells

•

A3B signature at -2 site linked to poor patient survival in HPV+ HNSCC low smokers

Structural basis of substrate specificity in human cytidine deaminase family APOBEC3s

The human cytidine deaminase family of APOBEC3s (A3s) plays critical roles in both innate immunity and the development of cancers. A3s comprise seven functionally overlapping but distinct members that can be exploited as nucleotide base editors for treating genetic diseases. Although overall structurally similar, A3s have vastly varying deamination activity and substrate preferences. Recent crystal structures of ssDNA-bound A3s together with experimental studies have provided some insights into distinct substrate specificities among the family members. However, the molecular interactions responsible for their distinct biological functions and how structure regulates substrate specificity are not clear. In this study, we identified the structural basis of substrate specificities in three catalytically active A3 domains whose crystal structures have been previously characterized: A3A, A3B- CTD, and A3G-CTD. Through molecular modeling and dynamic simulations, we found an interdependency between ssDNA substrate binding conformation and nucleotide sequence specificity. In addition to the U-shaped conformation seen in the crystal structure with the CTC0 motif, A3A can accommodate the CCC0 motif when ssDNA is in a more linear (L) conformation. A3B can also bind both U- and L-shaped ssDNA, unlike A3G, which can stably recognize only linear ssDNA. These varied conformations are stabilized by sequence-specific interactions with active site loops 1 and 7, which are highly variable among A3s. Our results explain the molecular basis of previously observed substrate specificities in A3s and have implications for designing A3-specific inhibitors for cancer therapy as well as engineering base-editing systems for gene therapy.