Abstract 5156: The mutagenic effects of APOBEC3A and APOBEC3B in urothelial carcinoma
Authors: Sturdivant Michael S. , Truong Andrew S. , Zhou Mi , Damrauer Jeffrey S. , Kim William Y. 
Published: April 2023

APOBEC3A (A3A) and APOBEC3B (A3B) are members of a family of cytidine deaminase enzymes that catalyze the removal of an amino group from cytosine nucleotides generating an uracil in its place that serve as a source of mutations. The resulting C→U transition has been linked to various oncogenic mutations, most predominately in bladder cancer. While both A3A and A3B activity can account for widespread changes to the genomic landscape, prior studies have shown tumor specific effects. In hepatocellular carcinoma, A3A has been shown to be the only APOBEC3 family member capable of driving tumor formation in the presence of p53 downregulation, while studies in breast cancer have suggested A3B as a driver of tamoxifen resistance. Currently, it is unclear if both A3A and A3B drive APOBEC-induced mutagenesis in bladder cancer, or if one enzyme plays a larger mutagenic role than the other. In urothelial carcinoma, there has not been a direct comparison of A3A and A3B mutagenic activity, highlighting the need to determine the effects of each respective enzyme. To explore the effects of A3A and A3B enzymes, we accessed publicly available single cell RNA (scRNA seq) data of tumor samples from 8 bladder cancer patients. Analysis from scRNA seq data revealed an enrichment of A3A expression in both monocyte and urothelial cell populations, while A3B expression is primarily restricted to urothelial cells, suggesting a cell type specific functional difference between A3A and A3B in bladder cancer. Immunogenomic analysis of TCGA bladder cancer tumors stratified by APOBEC mutational load (High, Low, No) showed that tumors with APOBEC high mutational load had high levels of interferon gamma response, lymphocyte infiltration signature score, M1 macrophages, intratumor heterogeneity, and stromal fraction compared to tumors with low APOBEC mutational load. In addition, increases in features of genomic instability was observed in tumors with high APOBEC mutational load. Interestingly, we saw unique correlations of A3A and A3B in TCGA bladder tumors. A3A expression correlated with characteristics of an inflamed tumor microenvironment while A3B expression correlated with features of genomic instability. To elucidate the divergent effects of A3A and A3B independently, we have generated isogenic mouse bladder cancer cell lines that overexpress HA tagged A3A and A3B, to directly compare their mutagenic effects through in-vitro and in-vivo experiments. We have validated through deamination assays, the functionality of A3B in these cell lines. In silico data has shown differential roles for A3A and A3B as it relates to the tumor/immune environment; we hypothesize that this might result in subunit specific response and resistance to anti-PD-1 immunotherapy. Elucidation of the A3 specific driver of APOBEC mutagenesis in urothelial carcinoma will help identity a predictive and/or prognostic biomarker to anti-PD-1 checkpoint immune inhibition.

Citation Format: Michael S. Sturdivant, Andrew S. Truong, Mi Zhou, Jeffrey S. Damrauer, William Y. Kim. The mutagenic effects of APOBEC3A and APOBEC3B in urothelial carcinoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5156.