LRIG1 is a positive prognostic marker in Merkel cell carcinoma and Merkel cell carcinoma expresses epithelial stem cell markers

Current status of Merkel cell carcinoma: Epidemiology, pathogenesis and prognostic factors

Merkel cell carcinoma (MCC) is an extremely rare cutaneous neuroendocrine cancer, with an incidence approximately 40 times lower than that of malignant melanoma; however, its significantly inferior survival rate compared to melanoma establishes MCC as the most lethal form of skin cancer. In recent years, a substantial body of literature has demonstrated a gradual increase in the incidence of MCC. Although the two factors that contribute to MCC, ultraviolet radiation and Merkel cell polyomavirus infection, have been well established, the specific pathogenesis of this disease remains unclear. Additionally, considering the high lethality and recurrence rates of MCC, as well as the absence of specific antitumor drugs, it is crucial to elucidate the factors that can accurately predict patients' outcomes. In this review, we summarized the significant advancements in the epidemiological characteristics, pathogenesis, and the factors that influence patient prognosis of MCC to enhance clinical practices and public health efforts.

Clinical and Molecular Features of Morpheaform Basal Cell Carcinoma: A Systematic Review

Basal cell carcinoma (BCC) is the most common skin cancer, with a lifetime risk currently approaching up to 40% in Caucasians. Among these, some clinical and pathological BCC variants pose a higher risk due to their more aggressive biological behavior. Morpheaform BCC (morBCC), also known as sclerosing, fibrosing, or morpheic BCC, represents up to 5-10% of all BCC. Overall, morBCC carries a poorer prognosis due to late presentation, local tissue destruction, tumor recurrence, and higher frequency of metastasis. In this systematic review, we review the epidemiological, clinical, morphological, dermatoscopical, and molecular features of morBCC. After the title and abstract screening of 222 studies and the full-text review of 84 studies, a total of 54 studies met the inclusion criteria and were thus included in this review.

Merkel Cell Polyomavirus T Antigen-Mediated Reprogramming in Adult Merkel Cell Progenitors

Whether Merkel cells regenerate in adult skin and from which progenitor cells they regenerate are a subject of debate. Understanding Merkel cell regeneration is of interest to the study of Merkel cell carcinoma, a rare neuroendocrine skin cancer hypothesized to originate in a Merkel cell progenitor transformed by Merkel cell polyomavirus small and large T antigens. We sought to understand what the adult Merkel cell progenitors are and whether they can give rise to Merkel cell carcinoma. We used lineage tracing to identify SOX9-expressing cells (SOX9+ cells) as Merkel cell progenitors in postnatal murine skin. Merkel cell regeneration from SOX9+ progenitors occurs rarely in mature skin unless in response to minor mechanical injury. Merkel cell polyomavirus small T antigen and functional imitation of large T antigen in SOX9+ cells enforced neuroendocrine and Merkel cell lineage reprogramming in a subset of cells. These results identify SOX9+ cells as postnatal Merkel cell progenitors that can be reprogrammed by Merkel cell polyomavirus T antigens to express neuroendocrine markers.

Somatostatin receptor expression in Merkel cell carcinoma: correlation with clinical data

BACKGROUND

Merkel cell carcinoma (MCC) is a rare, high-grade neuroendocrine neoplasm (NEN) of the skin. Somatostatin receptors (SSTRs) are G protein-linked receptors that regulate cell proliferation and growth. SSTRs are expressed in many NENs; however, scant information is available on their expression in MCCs or their association with clinical parameters and patient outcomes.

MATERIAL AND METHODS

This retrospective study was conducted at Helsinki University Hospital and the University of Helsinki. Using a tissue microarray, we investigated SSTR1-5 expression by immunohistochemistry in 99 MCC tissue samples. Samples were collected between 1983 and 2017 and coupled with the patients' clinical data.

RESULTS

SSTR2-SSTR5 were detected in 69%, 6%, 4%, and 1% of the tumours, respectively. However, SSTR1 expression was not observed. Cytoplasmic SSTR2 positivity was associated with metastatic disease at the time of diagnosis (p = 0.009), but it did not correlate with disease-specificity or overall survival.

CONCLUSION

SSTR2-5 expression was observed in MCCs. In particular, SSTR2 expression is clinically valid because it is associated with metastatic disease at the time of diagnosis and can thus serve as a prognostic marker. Moreover, SSTR2 overexpression provides a molecular basis for tumour imaging and treatment with somatostatin analogues.

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.

T-Cell Mediated Immunity in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare and frequently lethal skin cancer with neuroendocrine characteristics. MCC can originate from either the presence of MCC polyomavirus (MCPyV) DNA or chronic ultraviolet (UV) exposure that can cause DNA mutations. MCC is predominant in sun-exposed regions of the body and can metastasize to regional lymph nodes, liver, lungs, bone, and brain. Older, light-skinned individuals with a history of significant sun exposure are at the highest risk. Previous studies have shown that tumors containing a high number of tumor-infiltrating T-cells have favorable survival, even in the absence of MCPyV DNA, suggesting that MCPyV infection enhances T-cell infiltration. However, other factors may also play a role in the host antitumor response. Herein, we review the impact of tumor infiltrating lymphocytes (TILs), mainly the CD4⁺, CD8⁺, and regulatory T-cell (Tregs) responses on the course of MCC, including their role in initiating MCPyV-specific immune responses. Furthermore, potential research avenues related to T-cell biology in MCC, as well as relevant immunotherapies are discussed.

Oncogenic Merkel Cell Polyomavirus T Antigen Truncating Mutations are Mediated by APOBEC3 Activity in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer, which is frequently caused by Merkel cell polyomavirus (MCPyV). Mutations of MCPyV tumor (T) antigens are major pathologic events of virus-positive (MCPyV+) MCCs, but their source is unclear. Activation-induced cytidine deaminase (AID)/APOBEC family cytidine deaminases contribute to antiviral immunity by mutating viral genomes and are potential carcinogenic mutators. We studied the contribution of AID/APOBEC cytidine deaminases to MCPyV large T (LT) truncation events. The MCPyV LT area in MCCs was enriched with cytosine-targeting mutations, and a strong APOBEC3 mutation signature was observed in MCC sequences. AICDA and APOBEC3 expression were detected in the Finnish MCC sample cohort, and LT expression correlated with APOBEC3H and APOBEC3G. Marginal but statistically significant somatic hypermutation targeting activity was detected in the MCPyV regulatory region. Our results suggest that APOBEC3 cytidine deaminases are a plausible cause of the LT truncating mutations in MCPyV+ MCC, while the role of AID in MCC carcinogenesis is unlikely.

Significance:

We uncover APOBEC3 mutation signature in MCPyV LT that reveals the likely cause of mutations underlying MCPyV+ MCC. We further reveal an expression pattern of APOBECs in a large Finnish MCC sample cohort. Thus, the findings presented here suggest a molecular mechanism underlying an aggressive carcinoma with poor prognosis.