Targeting APOBEC3-induced squamous differentiation in bladder cancer

ABSTRACT In 2023, there will be an estimated 81,180 new cases of bladder cancer in the United States, resulting in an estimated 17,100 deaths. Over 90% of bladder cancers are urothelial carcinomas (UC) and the mortality associated with metastatic disease remains particularly grim. APOBEC3 (apolipoprotein B mRNA editing enzyme catalytic polypeptide like) is a family of enzymes that catalyze the deamination of cytosine nucleotides and has a physiologic role in both B cell receptor diversification as well as antiviral defense by restricting viral replication through cytidine deamination of the viral genome. There are 7 APOBEC3 family members in human and 1 Apobec3 in mouse. Aberrant expression of APOBEC3A (A3A) and APOBEC3B (A3B) has been shown to be upregulated in many cancers, but the APOBEC mutational signature is most highly enriched in bladder cancer. Indeed, nearly 70% of single nucleotide variants (SNVs) in bladder cancer are attributable to APOBEC induced mutagenesis. To address the role of APOBEC3 enzymes on bladder cancer initiation and progression, we generated a novel mouse strain: Rosa26LSL-Apobec3 and crossed them with our previously reported UPP mice that inactivate Trp53 and Pten in Upk3a expressing intermediate/umbrella cells through Cre-mediated recombination. These UPPA mice develop tumors with shorter latency than control UPP mice and have a striking phenotype of squamous differentiation. Our preliminary work has shown that Apobec3 is sufficient to promote squamous differentiation and that IL1A is both necessary and sufficient for squamous transdifferentiation of bladder cancer cell lines and normal urothelial organoids. Analysis of primary bladder tumors demonstrates that human APOBEC3A and IL1A, but not APOBEC3B, is associated with squamous differentiation in human bladder tumors. Moreover, APOBEC3A is significantly upregulated in chemotherapy resistant tumors. Lineage plasticity is a known mechanism of treatment resistance and squamous transdifferentiation has been recognized as an important manifestation of lineage plasticity and as a documented mechanism of therapy resistance to kinase inhibitors in lung adenocarcinomas as well as KRAS G12C inhibitors. Successful completion of this proposal will give us a better understanding of APOBEC3’s role in bladder tumorigenesis and in particular an in depth understanding of how it promotes squamous differentiation, to what extent APOBEC3 mediates resistance to currently approved therapy for bladder cancer, and whether co-inhibition of the IL1A pathway can be leveraged in the future for reversal of APOBEC-induced treatment resistance.