Merkel cell polyomavirus DNA replication induces senescence in human dermal fibroblasts in a Kap1/Trim28 dependent manner

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus associated with tumorigenesis. Similar to other polyomaviruses, MCPyV expresses a Large Tumor Antigen (LT) that together with Small Tumor Antigen (sT) contributes to cellular transformation and is of critical importance for the initiation of the viral DNA replication. Understanding the cellular protein network regulated by MCPyV early proteins will significantly contribute to our knowledge how MCPyV ensures its own replication and how the virus contributes to cellular transformation. We here describe the KRAB-associated protein 1 (Kap1), a chromatin remodeling factor involved in co-transcriptional regulation, as a novel protein interaction partner of both MCPyV T-Antigens, sT and LT. Kap1 knockout results in a significant increase in viral DNA replication highly suggestive of Kap1 being an important host restriction factor during MCPyV infection. Interestingly, different to other DNA viruses, MCPyV gene expression is unaffected in the absence of Kap1 and Kap1 does not associate with the viral genome. Instead, we show that MCPyV DNA replication but not the T protein expression alone induce ATM dependent Kap1 S824 phosphorylation, a mechanism that typically facilitates repair of double-strand breaks in heterochromatin by arresting the cells in G2. By inserting specific mutations within LT in the context of the viral genome we show that MCPyV induced inhibition of cell proliferation is mainly conferred by residues within the origin binding domain and thus by viral DNA replication while a previously suggested phosphorylation site S816A has no significant effect on cell proliferation of primary nHDF cells.