Protein-mediated viral latency is a novel mechanism for Merkel cell polyomavirus persistence

Abstract

Viral latency, in which a virus genome does not replicate independently of the host cell genome and produces no infectious particles, is required for long-term virus persistence. There is no known latency mechanism for chronic small DNA virus infections. Merkel cell polyomavirus (MCV) causes an aggressive skin cancer after prolonged infection and requires an active large T (LT) phosphoprotein helicase to replicate. We show that evolutionarily conserved MCV LT phosphorylation sites are constitutively recognized by cellular Fbw7, βTrCP, and Skp2 Skp-F-box-cullin (SCF) E3 ubiquitin ligases, which degrade and suppress steady-state LT protein levels. Knockdown of each of these E3 ligases enhances LT stability and promotes MCV genome replication. Mutations at two of these phosphoreceptor sites [serine (S)220 and S239] in the full viral genome increase LT levels and promote MCV virion production and transmission, which can be neutralized with anti-capsid antibody. Virus activation is not mediated by viral gene transactivation, given that these mutations do not increase late gene transcription in the absence of genome replication. Mechanistic target of rapamycin inhibition by either nutrient starvation or use of an active site inhibitor reduces Skp2 levels and stabilizes LT, leading to enhanced MCV replication and transmission. MCV can sense stresses in its intracellular environment, such as nutrient loss, through SCF E3 ligase activities, and responds by initiating active viral transmission. Protein-mediated viral latency through cellular SCF E3 ligase targeting of viral replication proteins is a unique form of latency that may promote chronic viral persistence for some small DNA and RNA viruses.Merkel cell polyomavirus | large T | latency | E3 ligase | transmission

Promoter activity of Merkel cell Polyomavirus variants in human dermal fibroblasts and a Merkel cell carcinoma cell line

“…However, 500 ng of LT expression plasmid reduced early and late promoter activities in HDF and early promoter activity in MCC13 cells, but had only a slight or no effect, but significantly stimulated the HUN and MS1 (a 20 and 40% increase, respectively) late promoters. Kwun et al found that MCPyV LT repressed early and late promoter activity of MCPyV isolate MCC339 in HEK293 cells, but they did not examine the effect of LT in MCC13 or HDF cells [42]. Yet, in another study by Ajuh and co-workers, the authors showed that LT trans-activated the MCPyV R17b (= consensus) early and late promoter in HEK293T cells [29].…”

“…In these cells, truncated MKL-1 and MS-1 LT inhibited the corresponding early promoter. The reason for the cell-specific effect of truncated LT on the MCPyV promoter is not known, but MCPyV LT has been shown to interact with several cellular factors [42]. Different interaction partners in distinct cell types may determine the effect of LT on the MCPyV promoter activity.…”

Detection of Merkel Cell Polyomavirus DNA in Serum Samples of Healthy Blood Donors

“…After primary infection, which occurs very early in life, MCPyV seems to remain in a latent/persistent state lifelong in immune competent hosts ( 4 ). The cellular protein SCF E3 ligase, targeting MCPyV LT oncoprotein seems to allow this small DNA tumor virus to remain in the latency state ( 30 ). Our data indicate that MCPyV may persist/reactivate in the host, whereas it can be detected at low prevalence and at low-DNA viral copy in the sera of immunocompetent healthy blood donors.…”

The biology and treatment of Merkel cell carcinoma: current understanding and research priorities

“…Peripheral blood monocytes have been proposed to act as a reservoir of infected cells 24 . MCPyV reporter pseudovirus can enter many cell types, including keratinocytes 28 , 29 , although dermal fibroblasts and HEK 293 cells are the only cells in which productive viral infection has been demonstrated in vitro 28 , 30 .…”