Polyomavirus T-Antigen InducesAPOBEC3BExpression using a LXCXE-Dependent and TP53-Independent Mechanism

Abstract

ABSTRACTAPOBEC3B is a single-stranded DNA cytosine deaminase with beneficial innate antiviral functions. However, misregulated APOBEC3B can also be detrimental by inflicting APOBEC signature C-to-T and C-to-G mutations in genomic DNA of multiple cancer types. Polyomaviruses and papillomaviruses use dominant oncoproteins to induce APOBEC3B overexpression, perhaps to their own benefit, but little is known about the cellular mechanisms hijacked by these viruses to do so. Here we investigate the molecular mechanism of APOBEC3B upregulation by the polyomavirus large T-antigen. First, truncated T-antigen (truncT) is sufficient for APOBEC3B upregulation and the RB interacting motif (LXCXE), but not the TP53 inhibition domain, is required. Second, upregulated APOBEC3B is strongly nuclear and partially localized to virus replication centers. Third, genetic knockdown of RB1 alone or in combination with RBL1 and/or RBL2 is insufficient to suppress truncT-mediated induction ofAPOBEC3B. Fourth, CDK4/6 inhibition by palbociclib is also insufficient to suppress truncT-mediated induction ofAPOBEC3B. Fifth, global gene expression analyses in a wide range of human cancers show significant associations between expression ofAPOBEC3Band other genes known to be regulated by the RB/E2F axis. These experiments combine to implicate the RB/E2F axis in promotingAPOBEC3Btranscription, yet they also suggest that the polyomavirus RB binding motif has in addition to RB inactivation at least one additional function for triggeringAPOBEC3Bupregulation in virus-infected cells.IMPORTANCEThe APOBEC3B DNA cytosine deaminase is overexpresssed in many different cancers and correlated with elevated frequencies of C-to-T and C-to-G mutations in 5’-TC motifs, oncogene activation, acquired drug resistance, and poor clinical outcomes. The mechanisms responsible for APOBEC3B overexpression are not fully understood. Here, we show that the polyomavirus truncated T-antigen (truncT) triggers APOBEC3B overexpression through its RB-interacting motif, LXCXE, which in turn likely enables one or more E2F family transcription factors to promoteAPOBEC3Bexpression. This work strengthens the mechanistic linkage between active cell cycling, APOBEC3B overexpression, and cancer mutagenesis. Although this mechanism damages cellular genomes, viruses may leverage it to promote evolution, immune escape, and pathogenesis. The cellular portion of the mechanism may also be relevant to non-viral cancers, where genetic mechanisms often activate the RB/E2F axis and APOBEC3B mutagenesis contributes to tumor evolution.