Multiple DNA damage signaling and repair pathways deregulated by simian virus 40 large T antigen

We demonstrated previously that expression of simian virus 40 (SV40) large T antigen (LT), without a viral origin, is sufficient to induce the hallmarks of a cellular DNA damage response (DDR), such as focal accumulation of gamma-H2AX and 53BP1, via Bub1 binding. Here we expand our characterization of LT effects on the DDR. Using comet assays, we demonstrate that LT induces overt DNA damage. The Fanconi anemia pathway, associated with replication stress, becomes activated, since FancD2 accumulates in foci, and monoubiquitinated FancD2 is detected on chromatin. LT also induces a distinct set of foci of the homologous recombination repair protein Rad51 that are colocalized with Nbs1 and PML. The FancD2 and Rad51 foci require neither Bub1 nor retinoblastoma protein binding. Strikingly, wild-type LT is localized on chromatin at, or near, the Rad51/PML foci, but the LT mutant in Bub1 binding is not localized there. SV40 infection was previously shown to trigger ATM activation, which facilitates viral replication. We demonstrate that productive infection also triggers ATR-dependent Chk1 activation and that Rad51 and FancD2 colocalize with LT in viral replication centers. Using small interfering RNA (siRNA)-mediated knockdown, we demonstrate that Rad51 and, to a lesser extent, FancD2 are required for efficient viral replication in vivo, suggesting that homologous recombination is important for high-level extrachromosomal replication. Taken together, the interplay of LT with the DDR is more complex than anticipated, with individual domains of LT being connected to different subcomponents of the DDR and repair machinery.