Abstract 919: APOBEC3 promotes tumor progression, squamous differentiation and metastasis in bladder cancer mouse model

Bladder cancer is the sixth most common type of cancer in the United States. In 2021, there are an estimated 83,730 new cases and 17,200 deaths from bladder cancer. Only about 15% of patients with metastatic disease survive past five years. The apolipoprotein B mRNA editing enzyme catalytic subunit 3B (APOBEC3B) is overexpressed in bladder tumors compared to corresponding healthy tissues. This enzyme catalyzes the conversion of cytidine to uracil playing an important physiologic role in restricting viral replication. When aberrantly over-expressed, APOBEC3B can damage single-stranded DNA present during DNA replication and transcription, resulting in an increased tumor mutational burden. Strikingly, APOBEC mutagenesis accounts for 70% of all single-nucleotide variants found in bladder cancer. Despite the strong evidence of APOBEC mutagenesis in bladder cancer, the effects of APOBEC3B on the disease progression remain unclear. To address this question, we generated the UPPA (urothelial specific over-expression of mouse Apobec3 along with Pten and p53 knock-out) and control UPP mouse models to interrogate the role Apobec3 in bladder cancer progression. Apobec3 over-expression shortened tumor latency in the UPPA model with the median tumor free survival time of 43.3 weeks compared to 53.6 weeks in the UPP model. Histological analysis showed that bladder epithelial cells in the UPPA mice underwent squamous differentiation which is associated with aggressive disease in human. Additionally, immunofluorescence analysis showed up-regulation of krt6a (a squamous marker) in UPPA urothelium compared to a complete lack of krt6a signal in the urothelium of UPP mice. Necropsy revealed the presence of metastatic nodules in lung, liver and peritoneum in UPPA mice. Cell lines derived from a matched pair of UPPA primary tumor and peritoneal metastatic tumor formed lung and liver nodules in wild-type C57BL/6J mice via tail vein injection. Our results so far suggest that Apobec3 promotes tumor progression, squamous differentiation and metastasis. Additional work utilizing bladder organoids and scRNAseq are underway to unravel the mechanism by which Apobec3 promotes these observed phenotypes. Currently, single-cell RNAseq analysis on primary UPPA tumor demonstrated high degree to T cell infiltration. Approximately 60% of cells sequenced were annotated as T-cells and natural killer T-cells by SingleR. We treated mice bearing UPPA primary bladder tumors with anti-PD-1 antibody an