Abstract 1035: Direct evidence for a pro-tumor role of APOBEC3A in cancer initiation and progression in vivo: enhanced mutagenesis and immune suppression in a novel humanized autochthonous model of pancreatic cancer
Authors: Woermann Sonja , Cowan Robert , Ross Susan M. , Rhim Andrew D. 
Published: July 2017

Abstract
Introduction: The apolipoprotein B editing complex 3 (APOBEC3) family of enzymes are possible candidates for inducing mutations across a number of tumors, including pancreatic ductal adenocarcinoma (PDA; Alexandrov et al, 2013; Roberts et al, 2013). APOBEC3A (hA3A) is one of eight identified isoforms in humans and known to deaminate cytidines in genomic ssDNA at a specific sequence motif. Large scale sequencing studies demonstrate significant enrichment of this signature in a number of human cancers. Furthermore, hA3A is overexpressed in a variety of solid tumors, including PDA. Taken together, these data suggest that hA3A may catalyze point mutations in cancer and drive cancer initiation and progression. However, direct evidence to support this hypothesis in vivo are lacking. Here, we utilize a novel genetically engineered mouse model to determine the precise effects of hA3A on PDA initiation and progression.
Methods: As opposed to humans, mice only contain one APOBEC3 isoform which has limited to no deaminating activity on genomic DNA. Thus, to delineate the function of hA3A on PDA initiation and progression, we exchanged the murine APOBEC3 protein coding sequence for hA3A, leaving the murine 5’ and 3’ endogenous regulatory sequences intact to ensure physiologic expression (murine APOBEC3 and hA3A reflect similar expression kinetics during PDA development). We then bred these mice to a well-established GEMM of PC (Rhim et al., Cell 2012) to yield LSL-KrasG12D; p53fl/+; Pdx1-Cre; RosaLSL-YFP; hA3A+/- (KPCY;hA3A) mice. We compared these mice to KPCY mice. We genotyped tumors from both cohorts with Ilumina exome capture sequencing with variant calling by a custom caller based on VarScan.
Results: All KPCY; hA3A (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; hA3A mice developed macrometastatic disease compared to 3/12 in the KPCY control. Interestingly, KPCY; hA3A tumors contained dramatic desmoplasia, far surpassing the KPCY controls. Moreover, comparative RNAseq and histologic analyses revealed dramatic differences in the number and distribution of various T-cell and B-cell subsets. Finally, we found that KPCY; hA3A mice contained significantly more single nucleotide variants compared to KPCY controls (51.3 v. 8.4; p&lt;0.05).
Conclusions: These data show for the first time that physiologic expression of hA3A in vivo leads to increased mutations, altered immune response and more aggressive cancer. Future studies will address the precise mechanisms by which hA3A catalyzes tumorigenesis using our novel gain of function model. Furthermore, our data suggest that incorporating our humanized A3A germline allele may provide a more genomically recapitulative model of cancer.
Citation Format: Sonja Woermann, Robert Cowan, Susan M. Ross, Andrew D. Rhim. Direct evidence for a pro-tumor role of APOBEC3A in cancer initiation and progression in vivo: enhanced mutagenesis and immune suppression in a novel humanized autochthonous model of pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1035. doi:10.1158/1538-7445.AM2017-1035