Abstract
APOBEC3B (A3B)-catalyzed DNA cytosine deamination contributes to the overall mutational landscape in breast cancer. Molecular mechanisms responsible for A3B upregulation in cancer are poorly understood. Here we show that a single E2F cis-element mediates repression in normal cells and that expression is activated by its mutational disruption in a reporter construct or the endogenous A3B gene. The same E2F site is required for A3B induction by polyomavirus T antigen indicating a shared molecular mechanism. Proteomic and biochemical experiments demonstrate the binding of wildtype but not mutant E2F promoters by repressive PRC1.6/E2F6 and DREAM/E2F4 complexes. Knockdown and overexpression studies confirm the involvement of these repressive complexes in regulating A3B expression. Altogether, these studies demonstrate that A3B expression is suppressed in normal cells by repressive E2F complexes and that viral or mutational disruption of this regulatory network triggers overexpression in breast cancer and provides fuel for tumor evolution.
Highlights
Cancer is a collection of diseases characterized by a complex array of mutations ranging from gross chromosomal abnormalities to single-base substitution (SBS) mutations
The studies here are the first to demonstrate that two repressive E2F complexes, E2F4/DREAM and E2F6/PRC1.6, combine to suppress A3B transcription and thereby protect genomic integrity in normal cells
Site-directed mutation of either site caused full de-repression that could not be further enhanced by co-expression of BK-PyV truncated T antigen (tTAg)
Summary
Cancer is a collection of diseases characterized by a complex array of mutations ranging from gross chromosomal abnormalities to single-base substitution (SBS) mutations. HPV E7 has a LxCxE motif suggesting a shared mechanism in which these viral oncoproteins may activate A3B transcription by antagonizing the canonical retinoblastoma tumor suppressor protein RB1 and the related pocket proteins RB-like 1 (RBL1) and RBL2 (reviewed by An et al, 2012; Bellacchio and Paggi, 2013; DeCaprio, 2014; DeCaprio and Garcea, 2013; Rashid et al, 2015). The combined results demonstrate the functionality of a single E2F binding site in the A3B promoter and reveal overlapping roles for both E2F4-based DREAM and E2F6-based PRC1.6 complexes in repressing A3B transcription in non-tumorigenic cells. Loss of this A3B repression mechanism in tumor cells is likely to promote cancer mutagenesis
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