Abstract 4751: APOBEC3A inhibits tumor immune response independent of deamination in a novel genetic model of pancreatic cancer
Authors: Woermann Sonja , Cowan Robert , Ross Susan M. , Rhim Andrew D. 
Published: August 2018

Introduction: Apolipoprotein B mRNA Editing enzyme, Catalytic polypeptide 3A (A3A) is a C to U cytidine deaminase, preferentially deaminating the lagging strand of double-stranded DNA at specific sequence motifs. A3A is overexpressed across multiple tumor types, and in pancreatic ductal adenocarcinoma (PDA), high expression is associated with significantly decreased survival in early stage patients. Large scale sequencing studies of various human tumors (TCGA and ICGC), including PDA, have implicated A3A as a potential driver of tumor mutagenesis. However, the biological relevance of the deamination function of A3A has not yet been shown. Here, we utilize a novel genetically engineered mouse model to show that A3A supports the development of aggressive PDA independent of mutagenic capabilities.

Methods: As opposed to humans, mice only contain one APOBEC3 isoform which has limited to no deaminating activity on genomic DNA. Thus, to dissect the function of A3A on PDA initiation and progression, we made mice with a germline knockin for the coding sequence for human A3A and bred to a well-established GEMM of PDA to yield LSL-KrasG12D; p53fl/+; Pdx1-Cre; RosaLSL-YFP; A3A+/- (KPCY;A3A) mice and compared to KPCY mice. We genotyped tumors from both cohorts using VarScan after exome capture sequencing. Tumor and immune cells were FACS sorted comparative RNAseq performed. The immune contexture of murine and 155 untreated and treated human PDAs was analyzed by performing 7-color multiplex staining and immune cell spatial interaction analysis.

Results: All KPCY;A3A (n=11) mice developed tumors and expired significantly faster compared to KPCY (n=18) controls (3.9 v. 7.0 mo; p&lt;0.01). One third of the KPCY; A3A mice developed macrometastatic disease compared to 3/12 in the KPCY control. Whole exome sequencing of tumors revealed no significant difference in the number of point mutations or neoantigen load between KPCY;A3A and KPCY tumors. However, histologically, KPCY;A3A tumors contained significantly more desmoplasia and altered immune cell infiltrates. Specifically, comparative RNAseq and IHC analyses revealed dramatic differences in the number and distribution of various T-cell and B-cell subsets in KPCY;A3A mice. Moreover, A3A expression led to abrogation of CD8+ T cell activation and proliferation. Concomitantly, a panel of immune checkpoint-related genes were upregulated in A3A high expressing tumors and cell lines compared to non-A3A expressing controls. This was confirmed in a panel of resected human PDA.

Conclusions: Counter to our hypothesis, A3A supports the initiation and growth of PDA in vivo through effects independent of deamination or mutagenesis of cancer cell genomes but rather indirectly through suppression of the tumor immune response. Future studies will address the precise mechanisms underlying A3A-mediated tumor immune evasion.

Citation Format: Sonja Woermann, Robert Cowan, Susan M. Ross, Andrew D. Rhim. APOBEC3A inhibits tumor immune response independent of deamination in a novel genetic model of pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4751.