Merkel cell polyomavirus T-antigens regulate DICER1 mRNA stability and translation through HSC70

Kit-mediated autophagy suppression driven by a viral oncoprotein emerges as a crucial survival mechanism in Merkel cell carcinoma

The KIT/c-KIT proto-oncogene is frequently over-expressed in Merkel cell carcinoma (MCC), an aggressive skin cancer commonly caused by Merkel cell polyomavirus (MCPyV). Here, we demonstrated that truncated MCPyV-encoded large T-antigen (LT) suppressed macroautophagy/autophagy by stabilizing and sequestering KIT in the paranuclear compartment via binding VPS39. KIT engaged with phosphorylated BECN1, thereby enhancing its association with BCL2 while diminishing its interaction with the PIK3C3 complex. This process ultimately resulted in the suppression of autophagy. Depletion of KIT triggered both autophagy and apoptosis, and decreased LT expression. Conversely, blocking autophagy in KIT-depleted cells restored LT levels and rescued apoptosis. Additionally, stimulating autophagy efficiently increased cell death and inhibited tumor growth of MCC xenografts in mice. These insights into the interplay between MCPyV LT and autophagy regulation reveal important mechanisms by which viral oncoproteins are essential for MCC cell viability. Thus, autophagy-inducing agents represent a therapeutic strategy in advanced MCPyV-associated MCC.Abbreviation: 3-MA, 3-methyladenine; AL, autolysosome; AP, autophagosome; Baf-A1, bafilomycin A1; BARA, β-α repeated autophagy specific domain; BH3, BCL2 homology 3 domain; CCD, coiled-coil domain; CHX, cycloheximide; Co-IP, co-immunoprecipitation; CQ, chloroquine; CTR, control; DAPI, 4',6-diamidino-2-phenylindole; EBSS, Earle's balanced salt solution; ECD, evolutionarily conserved domain; EEE, three-tyrosine phosphomimetic mutations Y229E Y233E Y352E; ER, endoplasmic reticulum; FFF, three-tyrosine non-phosphomimetic mutations; FFPE, formalin-fixed paraffin-embedded; FL, full-length; GIST, gastrointestinal stromal tumor; IB, immunoblotting; IHC, immunohistochemistry; KIT-HEK293, KIT stably expressing HEK293 cells; KRT20/CK20, keratin 20; LT, large T-antigen; LT339, MCPyV truncated LT antigen; LTco, codon-optimized MCPyV LT antigen; MCC, Merkel cell carcinoma; MCPyV-, MCPyV-negative; MCPyV, Merkel cell polyomavirus; MCPyV+, MCPyV-positive; PARP1, poly(ADP-ribose) polymerase 1; PCI, pan-caspase inhibitor; PI, propidium iodide; PtdIns3K, class III phosphatidylinositol 3-kinase; PtdIns3P, phosphatidylinositol-3-phosphate; RB1, RB transcriptional corepressor 1; RTKs, receptor tyrosine kinases; KITLG/SCF, KIT ligand; sT, small T-antigen; sTco, codon-optimized MCPyV sT antigen; T-B, Tat-BECN1; T-S, Tat-scrambled; TEM, transmission electron microscopy.

Merkel Cell Polyomavirus-Pathophysiology and Treatment in the Era of Gene-Targeted Therapies

Merkel cell polyomavirus (MCPyV) is a significant contributor to the development of Merkel cell carcinoma (MCC), an aggressive skin cancer with high recurrence and a low survival rate. In fact, it is the deadliest skin cancer. The precise routes of transmission for MCPyV-positive MCC remain unclear, but several factors may trigger its development. Conventional treatments for MCC are not highly effective, especially in patients with metastasis, with a clear need for new treatment options. Gene-targeted therapies hold great promise for the treatment of MCC, including the use of siRNA and CRISPR/Cas (C/Cas) but critically none have yet been translated into clinical trials. Validating this approach is the fact that several siRNA products are already FDA licenced, while C/Cas has entered clinical trial, albeit for conditions other than MCC. There are many challenges that must be overcome to move from preclinical research to the clinic. In this review, we provide a comprehensive summary of the current understanding of MCC, with a particular focus on MCPyV-positive MCC, and the status of gene-targeted therapies. Additionally, we discuss the major obstacles that impede MCC research and explore future prospects.

On the Chopping Block: Overview of DICER1 Mutations in Endocrine and Neuroendocrine Neoplasms

Mutational inactivation of the DICER1 gene causes aberrant micro-RNA maturation, which in turn may have consequences for the posttranscriptional regulation of gene expression, thereby contributing to tumor formation in various organs. Germline DICER1 mutations cause DICER1 syndrome, a pleiotropic condition with an increased risk of various neoplastic conditions in the pleura, ovaries, thyroid, pituitary, pineal gland, and mesenchymal tissues. Somatic DICER1 mutations are also frequently observed in a wide variety of solid tumors, thereby highlighting the importance of this gene in tumor development. In this review, the importance of DICER1 inactivation in endocrine tumors is discussed.

Functional Domains of the Early Proteins and Experimental and Epidemiological Studies Suggest a Role for the Novel Human Polyomaviruses in Cancer

As their name indicates, polyomaviruses (PyVs) can induce tumors. Mouse PyV, hamster PyV and raccoon PyV have been shown to cause tumors in their natural host. During the last 30 years, 15 PyVs have been isolated from humans. From these, Merkel cell PyV is classified as a Group 2A carcinogenic pathogen (probably carcinogenic to humans), whereas BKPyV and JCPyV are class 2B (possibly carcinogenic to humans) by the International Agency for Research on Cancer. Although the other PyVs recently detected in humans (referred to here as novel HPyV; nHPyV) share many common features with PyVs, including the viral oncoproteins large tumor antigen and small tumor antigen, as their role in cancer is questioned. This review discusses whether the nHPyVs may play a role in cancer based on predicted and experimentally proven functions of their early proteins in oncogenic processes. The functional domains that mediate the oncogenic properties of early proteins of known PyVs, that can cause cancer in their natural host or animal models, have been well characterized and we examined whether these functional domains are conserved in the early proteins of the nHPyVs and presented experimental evidence that these conserved domains are functional. Furthermore, we reviewed the literature describing the detection of nHPyV in human tumors.