DNA-methylation patterns imply a common cellular origin of virus- and UV-associated Merkel cell carcinoma

Analyses of combined Merkel cell carcinomas with neuroblastic components suggests that loss of T antigen expression in Merkel cell carcinoma may result in cell cycle arrest and neuroblastic transdifferentiation

Merkel cell carcinoma (MCC) is an aggressive skin cancer frequently caused by genomic integration of the Merkel cell polyomavirus (MCPyV). MCPyV-negative cases often present as combined MCCs, which represent a distinctive subset of tumors characterized by association of an MCC with a second tumor component, mostly squamous cell carcinoma. Up to now, only exceptional cases of combined MCC with neuroblastic differentiation have been reported. Herein we describe two additional combined MCCs with neuroblastic differentiation and provide comprehensive morphologic, immunohistochemical, transcriptomic, genetic and epigenetic characterization of these tumors, which both arose in elderly men and appeared as an isolated inguinal adenopathy. Microscopic examination revealed biphasic tumors combining a poorly differentiated high-grade carcinoma with a poorly differentiated neuroblastic component lacking signs of proliferation. Immunohistochemical investigation revealed keratin 20 and MCPyV T antigen (TA) in the MCC parts, while neuroblastic differentiation was confirmed in the other component in both cases. A clonal relation of the two components can be deduced from 20 and 14 shared acquired point mutations detected by whole exome analysis in both combined tumors, respectively. Spatial transcriptomics demonstrated a lower expression of stem cell marker genes such as SOX2 and MCM2 in the neuroblastic component. Interestingly, although the neuroblastic part lacked TA expression, the same genomic MCPyV integration and the same large T-truncating mutations were observed in both tumor parts. Given that neuronal transdifferentiation upon TA repression has been reported for MCC cell lines, the most likely scenario for the two combined MCC/neuroblastic tumors is that neuroblastic transdifferentiation resulted from loss of TA expression in a subset of MCC cells. Indeed, DNA methylation profiling suggests an MCC-typical cellular origin for the combined MCC/neuroblastomas. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

DNA Damage Stress Control Is a Truncated Large T Antigen and Euchromatic Histone Lysine Methyltransferase 2-Dependent Central Feature of Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high mortality rate. Merkel cell polyomavirus causes 80% of MCCs, encoding the viral oncogenes small T and truncated large T (tLT) antigens. These proteins impair the RB1-dependent G1/S checkpoint blockade and subvert the host cell epigenome to promote cancer. Whole-proteome analysis and proximal interactomics identified a tLT-dependent deregulation of DNA damage response (DDR). Our investigation revealed, to our knowledge, a previously unreported interaction between tLT and the histone methyltransferase EHMT2. T antigen knockdown reduced DDR protein levels and increased the levels of the DNA damage marker γH2Ax. EHMT2 normally promotes H3K9 methylation and DDR signaling. Given that inhibition of EHMT2 did not significantly change the MCC cell proteome, tLT-EHMT2 interaction could affect the DDR. With tLT, we report that EHMT2 gained DNA damage repair proximal interactors. EHMT2 inhibition rescued proliferation in MCC cells depleted for their T antigens, suggesting impaired DDR and/or lack of checkpoint efficiency. Combined tLT and EHMT2 inhibition led to altered DDR, evidenced by multiple signaling alterations. In this study, we show that tLT hijacks multiple components of the DNA damage machinery to enhance tolerance to DNA damage in MCC cells, which could explain the genetic stability of these cancers.

The HDAC inhibitor domatinostat induces type I interferon α in Merkel cell carcinoma by HES1 repression

BACKGROUND

Class I selective histone deacetylase inhibitors (HDACi) have been previously demonstrated to not only increase major histocompatibility complex class I surface expression in Merkel cell carcinoma (MCC) cells by restoring the antigen processing and presentation machinery, but also exert anti-tumoral effect by inducing apoptosis. Both phenomena could be due to induction of type I interferons (IFN), as has been described for HDACi. However, the mechanism of IFN induction under HDACi is not fully understood because the expression of IFNs is regulated by both activating and inhibitory signaling pathways. Our own preliminary observations suggest that this may be caused by suppression of HES1.

METHODS

The effect of the class I selective HDACi domatinostat and IFNα on cell viability and the apoptosis of MCPyV-positive (WaGa, MKL-1) and -negative (UM-MCC 34) MCC cell lines, as well as, primary fibroblasts were assessed by colorimetric methods or measuring mitochondrial membrane potential and intracellular caspase-3/7, respectively. Next, the impact of domatinostat on IFNA and HES1 mRNA expression was measured by RT-qPCR; intracellular IFNα production was detected by flow cytometry. To confirm that the expression of IFNα induced by HDACi was due to the suppression of HES1, it was silenced by RNA interference and then mRNA expression of IFNA and IFN-stimulated genes was assessed.

RESULTS

Our studies show that the previously reported reduction in viability of MCC cell lines after inhibition of HDAC by domatinostat is accompanied by an increase in IFNα expression, both of mRNA and at the protein level. We confirmed that treatment of MCC cells with external IFNα inhibited their proliferation and induced apoptosis. Re-analysis of existing single-cell RNA sequencing data indicated that induction of IFNα by domatinostat occurs through repression of HES1, a transcriptional inhibitor of IFNA; this was confirmed by RT-qPCR. Finally, siRNA-mediated silencing of HES1 in the MCC cell line WaGa not only increased mRNA expression of IFNA and IFN-stimulated genes but also decreased cell viability.

CONCLUSION

Our results demonstrate that the direct anti-tumor effect of HDACi domatinostat on MCC cells is at least in part mediated via decreased HES1 expression allowing the induction of IFNα, which in turn causes apoptosis.

Merkel Cell Polyomavirus: Infection, Genome, Transcripts and Its Role in Development of Merkel Cell Carcinoma

The best characterized polyomavirus family member, i.e., simian virus 40 (SV40), can cause different tumors in hamsters and can transform murine and human cells in vitro. Hence, the SV40 contamination of millions of polio vaccine doses administered from 1955-1963 raised fears that this may cause increased tumor incidence in the vaccinated population. This is, however, not the case. Indeed, up to now, the only polyomavirus family member known to be the most important cause of a specific human tumor entity is Merkel cell polyomavirus (MCPyV) in Merkel cell carcinoma (MCC). MCC is a highly deadly form of skin cancer for which the cellular origin is still uncertain, and which appears as two clinically very similar but molecularly highly different variants. While approximately 80% of cases are found to be associated with MCPyV the remaining MCCs carry a high mutational load. Here, we present an overview of the multitude of molecular functions described for the MCPyV encoded oncoproteins and non-coding RNAs, present the available MCC mouse models and discuss the increasing evidence that both, virus-negative and -positive MCC constitute epithelial tumors.

Epidemiology of Merkel Cell Polyomavirus Infection and Merkel Cell Carcinoma

Merkel cell polyomavirus (MCPyV) is a ubiquitous virus replicating in human dermal fibroblasts. MCPyV DNA can be detected on healthy skin in 67−90% of various body sites, and intact virions are regularly shed from the skin. Infection occurs early in life, and seropositivity increases from 37 to 42% in 1- to 6-year-olds to 92% in adults. Merkel cell carcinoma (MCC) is a rare but very aggressive neuroendocrine tumor of the skin. It develops mainly on sun-exposed areas as a fast-growing, reddish nodule. Two MCC entities exist: about 80% of MCC are MCPyV-associated. Tumorigenesis is driven by viral integration into the host genome and MCPyV oncogene expression. In MCPyV-negative MCC, UV radiation causes extensive DNA damage leading to the deregulation of the cell cycle. In recent decades, MCC incidence rates have increased worldwide, e.g., in the United States, from 0.15 in 1986 to 0.7/100,000 in 2016. Risk factors for the development of MCC include male sex, older age (>75 years), fair skin, intense UV exposure, and immunosuppression. Projections suggest that due to aging populations, an increase in immunosuppressed patients, and enhanced UV exposure, MCC incidence rates will continue to rise. Early diagnosis and prompt treatment are crucial to reducing high MCC morbidity and mortality.

Merkel Cell Polyomavirus (MCPyV) and Cancers: Emergency Bell or False Alarm?

Merkel cell polyomavirus (MCPyV), the sole member of Polyomavirus associated with oncogenesis in humans, is the major causative factor of Merkel cell carcinoma (MCC), a rare, neuroendocrine neoplasia of the skin. Many aspects of MCPyV biology and oncogenic mechanisms remain poorly understood. However, it has been established that oncogenic transformation is the outcome of the integration of the viral genome into the host DNA. The high prevalence of MCPyV in the population, along with the detection of the virus in various human tissue samples and the strong association of MCPyV with the emergence of MCC, have prompted researchers to further investigate the role of MCPyV in malignancies other than MCC. MCPyV DNA has been detected in several different non-MCC tumour tissues but with significantly lower prevalence, viral load and protein expression. Moreover, the two hallmarks of MCPyV MCC have rarely been investigated and the studies have produced generally inconsistent results. Therefore, the outcomes of the studies are inadequate and unable to clearly demonstrate a direct correlation between cellular transformation and MCPyV. This review aims to present a comprehensive recapitulation of the available literature regarding the association of MCPyV with oncogenesis (MCC and non-MCC tumours).

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.