Abstract 598: APOBEC mutagenesis: a link between innate immunity and cancer

Introduction: Cytidine deaminase activity of APOBEC3 enzymes generates mutations that restrict viral infection and eliminate tumor cells but also contribute to viral and tumor evolution. For example, human immunodeficiency virus (HIV) is hypermutated by APOBEC3G (A3G), and human papilloma virus (HPV) can be hypermutated by APOBEC3s, including A3A and A3B. In many human tumor types explored by The Cancer Genome Atlas (TCGA), C to T or G substitutions in the TCA or TCT motifs are predominantly attributed to the activity of A3A and A3B enzymes. In bladder tumors more than 60% of all exonic mutations are of the APOBEC-signature type, with nearly 100% of all bladder tumors carrying these mutations. Previously, we reported that two germline genetic variants – a single nucleotide polymorphism (SNP) rs1014971 and a 30Kb germline deletion that eliminates A3B and creates an A3AB chimera, are the strongest factors associated with bladder and breast cancer risk, expression of A3A and A3B, and APOBEC mutagenesis in human tumors. Other factors affecting the source of APOBEC mutagenesis in human tissues are not well understood. Because some APOBEC3s can be induced in the course of antiviral response, we hypothesized that APOBEC mutagenesis might be a misfired innate immune response. Methods: We used TaqMan-based expression analysis for selected APOBECs (A3A, A3B, and A3G) to explore expression in bladder (HT-1376, HTB-9, and RT-4) and breast cancer (MCF-7, MDA-MB-231, and T-47D) cell lines infected with Sendai virus (SeV). SeV is a murine RNA virus that results in self-limiting infection in a wide range of human cells, efficiently controlled by innate immune response of the host. The strong induction of APOBEC3s in RT-4 and MDA-MB-231 was followed-up by global transcriptome analysis with RNA-seq and pathway analysis. Cell lines were also treated with IFN-α, IFN-γ, IFN-λ3 and IFN-λ4 and tested for induction of A3A, A3B and A3G by TaqMan assays. Results: A3A, A3B and A3G are interferon-stimulated genes (ISGs) because they were induced in the course of innate antiviral response to SeV infection and by IFNs in some conditions. Specifically, A3A was most strongly induced by SeV, with 32, 51 and 12,000-fold induction in 3 breast cancer cell lines, and 4, 5 and 167-fold induction in 3 bladder cancer cell lines. A3B was also induced by SeV but only in the range of 1-5 fold in all cell lines tested.  Conclusion: APOBEC3s can be strongly induced in the course of innate immune response even to transient and self-limiting viral infections. In the presence of single-stranded DNA (ssDNA), the APOBEC3 substrate generated by different cellular and environmental conditions, strong induction of some nuclear APOBEC3s, such as A3A and A3B, may result in mutation of human genomic DNA. In turn, accumulation of these mutations can lead to tumor initiation and evolution, particularly in individuals with germline APOBEC3 risk variants. Citation Format: A Rouf Banday, Olusegun O. Onabajo, Krizia-Ivana Udquim, Adeola Obajemu, Ludmila Prokunina-Olsson. APOBEC mutagenesis: a link between innate immunity and 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 598. doi:10.1158/1538-7445.AM2017-598