Origin Of Merkel Cell Carcinoma Research Articles

Genomic evidence suggests that cutaneous neuroendocrine carcinomas can arise from squamous dysplastic precursors

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma without a known dysplastic precursor. In some cases, MCC is associated with SCCIS in the overlying epidermis; however, the MCC and SCCIS populations display strikingly different morphologies, and thus far a relationship between these components has not been demonstrated. To better understand the relationship between these distinct tumor cell populations, we evaluated 7 pairs of MCC-SCCIS for overlapping genomic alterations by cancer profiling panel. A subset was further characterized by transcriptional profiling and immunohistochemistry. In 6 of 7 MCC-SCCIS pairs there was highly significant mutational overlap including shared TP53 and/or RB1 mutations. In some cases, oncogenic events previously implicated in MCC (MYCL gain, MDM4 gain, HRAS mutation) were detected in both components. Although FBXW7 mutations were enriched in MCC, no gene mutation was unique to the MCC component across all cases. Transcriptome analysis identified 2736 differentially expressed genes between MCC and SCCIS. Genes upregulated in the MCC component included Polycomb repressive complex targets; downregulated transcripts included epidermal markers, and immune genes such as HLA-A. Immunohistochemical studies revealed increased expression of SOX2 in the MCC component, with diminished H3K27Me3, Rb, and HLA-A expression. In summary, MCC-SCCIS pairs demonstrate clonal relatedness. The shift to neuroendocrine phenotype is associated with loss of Rb protein expression, decrease in global H3K27Me3, and increased expression of Merkel cell genes such as SOX2. Our findings suggest an epidermal origin of MCC in this setting, and to our knowledge provide the first molecular evidence that intraepithelial squamous dysplasia may represent a direct precursor for small cell carcinoma.

Concurrence of Merkel Cell Carcinoma and Squamous Cell Carcinoma in A Patient with Generalized Actinic Keratosis: A Case Report

Abstract Introduction: Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous malignancy, and its pathogenesis might relate to ultraviolet light and Merkel cell polyomavirus infection. MCC in the Chinese population is uncommon. Here, we present a case of MCC that occurred based on widespread actinic keratosis (AK) in a Chinese female. Case report: An 82-year-old woman presented with two rapidly enlarging and rupture lesions on the face for 1 year. Biopsy was suggestive of squamous cell carcinoma (SCC) on the forehead and MCC on the left cheek. The patient had a history of generalized AK for 3years. The lesion on the left cheek was also revealed as an AK by histopathological examination 1 year ago. Complete surgical resection was performed to remove the two malignancies. Discussion: The co-occurrence of AK, SCC, and MCC in a Chinese woman is unusual. Immunohistopathological examination is vital for correct diagnosis. The three tumors, in this case, may originate from two different precursor cells and are affected by the same carcinogen. Alternatively, they may come from the same pluripotent epidermal stem cells, and chronic exposure to ultraviolet light and Merkel cell polyomavirus lead to the formation of different types of tumors. The coexist of MCC with other cutaneous tumors provided a train of thought for exploring the origin of MCC. Conclusion: We reported a rare co-existence phenomenon of MCC associated with AK and SCC. Hence, long-term follow-up and early treatment are imperative for patients with premalignant lesions, such as widespread AK.

Merkel Cell Polyomavirus: Oncogenesis in a Stable Genome.

Merkel cell polyomavirus (MCV) is the causative agent for the majority of Merkel cell carcinoma (MCC) cases. Polyomavirus-associated MCC (MCCP) is characterized by the integration of MCV DNA into the tumor genome and a low tumor mutational burden. In contrast, nonviral MCC (MCCN) is characterized by a high tumor mutational burden induced by UV damage. Since the discovery of MCV, much work in the field has focused on understanding the molecular mechanisms of oncogenesis driven by the MCV tumor (T) antigens. Here, we review our current understanding of how the activities of large T (LT) and small T (ST) promote MCC oncogenesis in the absence of genomic instability. We highlight how both LT and ST inhibit tumor suppressors to evade growth suppression, an important cancer hallmark. We discuss ST interactions with cellular proteins, with an emphasis on those that contribute to sustaining proliferative signaling. Finally, we examine active areas of research into open questions in the field, including the origin of MCC and mechanisms of viral integration.

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

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine malignancy of the skin. The cell of origin of MCC is thus far unknown and proposed cells of origin include Merkel cells, pro-/pre- or pre-B cells, epithelial stem cells, and dermal stem cells. In this study, we aimed to shed further light on the possibility that a subset of MCC tumors arise from epithelial stem cells of the skin by examining the expression of hair follicle and epidermal stem cell markers in MCC and normal human skin. We also aimed to elucidate any correlation between the expression of these markers and tumor Merkel cell polyomavirus (MCPyV) status or other clinicopathological characteristics or patient survival. Expression of CK19, SOX9, LGR5, and LRIG1 in MCC and normal human skin was studied by immunohistochemistry, and the staining patterns or intensities were statistically correlated with patient, tumor, MCPyV, and survival parameters. In a cohort of 137 cases of MCC, we observed dot-like immunoexpression of CK19 in 30 cases (22.1%) and homogeneous expression in 103 cases (75.7%). We also observed positive immunoexpression of SOX9 in 21 cases (15.3%), LGR5 in 118 cases (86.1%), and LRIG1 in 117 cases (86.0%). Immunoexpression of LRIG1 was found to correlate with better overall and MCC-specific survival. We observed frequent immunoexpression of several hair follicle and epidermal stem cell markers in MCC and found LRIG1 to be a positive prognostic marker in MCC.

Merkel cell carcinoma.

Merkel cell carcinoma (MCC) is a rare neuroendocrine carcinoma. The cellular origin of MCC may include Merkel cell precursors. The incidence of MCC has increased significantly however trends may have been confounded by evolving diagnostic criteria. The two key aetiologies of MCC are ultraviolet radiation and Merkel cell polyoma virus (MCPyV). Both have unique mechanisms of carcinogenesis. MCC presents non‐specifically as a rapidly growing, red‐to‐violet nodule on sun‐exposed areas. Diagnostic accuracy has improved through immunohistochemical markers such as CK‐20. Lymph nodes should be evaluated in MCC through examination and sentinel biopsy. USS, CT, MRI and CT‐PET may be useful in staging. Management depends on tumour location, stage and comorbidities. MCPyV status may guide treatment strategy in the future. Treatment for the primary MCC is commonly wide local excision followed by radiotherapy, guided by anatomical constraints. There is uncertainty about surgical margins. Treatments for nodal disease have not been determined through trials. They include nodal dissection or radiotherapy for clinically or radiologically apparent disease, and adjuvant nodal irradiation for negative nodes, microscopic disease or following nodal dissection for definite disease. Patients with loco‐regional advanced inoperable disease should be considered for combination therapy including chemotherapy, radiotherapy, surgery and immunotherapy. Systemic therapy for advanced disease includes immune checkpoint inhibitors targeting the PD‐1/PD‐L1 pathway. Avelumab can improve survival in metastatic MCC. Immunotherapy may result in longer disease control. Various other immunotherapeutic and molecular agents are undergoing trials. MCC continues to have a high mortality characterized by high recurrence and early metastases.

Merkel-cell carcinoma

Merkel-cell carcinoma (MCC) is arare and aggressive neuroendocrine carcinoma named for its Merkel-cell-like ultrastructure. The neuroendocrine Merkel cell was previously believed to be the cell of origin. However, Merkel cells are postmitotic and thus probably not the cell of origin of MCC. It is derived from an epidermal stem cell, which also might represent the cell of origin of MCC. Further putative cells of origin are dermal stem cells and pre/pro‑B cells, the latter showing some similar markers (e.g. PAX5).About 80% of MCCs are induced by the integration of DNA of the Merkel cell polyoma virus (MCPyV) into the genome. On the other hand, about 20% of MCCs show UV-induced mutations in numerous genes (e.g. TP53, RB1). In routine histology, MCC appears monomorphic and the diagnosis is confirmed by immunohistochemistry showing CK20 arranged in typical paranuclear plaques, together with the presence of neurofilaments and chromograninA. Virus-positive and virus-negative MCC are not different histologically.UV-induced and viral neoantigens cause the strong immunogenicity of MCC. Moreover, over the last few years, the presence of PD-1 and PD-L1 has been demonstrated within tumor and immune cells. For the checkpoint inhibitors pembrolizumab and avelumab, responses of about 50% have been shown, independent of virus state. Circulating tumor cells (CTCs) seem to be helpful in tumor tracking. Further immunological and molecular studies are necessary for future individual therapies, also concerning immunocompromised patients.

Histogenesis of Merkel Cell Carcinoma: A Comprehensive Review.

Merkel cell carcinoma (MCC) is a primary neuroendocrine carcinoma of the skin. This neoplasia features aggressive behavior, resulting in a 5-year overall survival rate of 40%. In 2008, Feng et al. identified Merkel cell polyomavirus (MCPyV) integration into the host genome as the main event leading to MCC oncogenesis. However, despite identification of this crucial viral oncogenic trigger, the nature of the cell in which MCC oncogenesis occurs is actually unknown. In fact, several hypotheses have been proposed. Despite the large similarity in phenotype features between MCC tumor cells and physiological Merkel cells (MCs), a specialized subpopulation of the epidermis acting as mechanoreceptor of the skin, several points argue against the hypothesis that MCC derives directly from MCs. Alternatively, MCPyV integration could occur in another cell type and induce acquisition of an MC-like phenotype. Accordingly, an epithelial as well as a fibroblastic or B-cell origin of MCC has been proposed mainly based on phenotype similarities shared by MCC and these potential ancestries. The aim of this present review is to provide a comprehensive review of the current knowledge of the histogenesis of MCC.

Evidence of proliferative activity in human Merkel cells: implications in the histogenesis of Merkel cell carcinoma.

The cellular origin of Merkel cell carcinoma (MCC) is controversial. We previously hypothesized that MCC originates from hair follicle stem cells or Merkel cell (MC) progenitors residing within the hair follicle bulge. Examination of three cases of combined MCC led to the unexpected discovery that numerous keratin 20 (CK20)-positive MCs within the squamous cell carcinoma (SCC) component of combined MCC appeared morphologically normal with dendritic and oval shapes. Moreover, one extremely rare case of combined SCC and MCC showed both intra-epidermal and dermal MCCs. These three cases represent the first documentation of MC hyperplasia in MCC, besides various benign follicular neoplasms associated with MC hyperplasia. Therefore, to elucidate the proliferating potential of MCs and their histogenetic relationship with MCCs, we further investigated these cases based on pathological observations. We identified numerous cells co-expressing CK20 and the proliferation marker Ki-67, identical to the morphological and immunohistochemical features of normal MCs. This finding indicated that MCs can no longer be considered as pure post-mitotic cells. Instead, they have proliferative potential under specific conditions in the diseased or wounded skin, or adjacent to various skin tumors, including MCC. Intimate co-existence of two malignant cell components composed of intradermal and intra-epidermal MCCs, with the proliferation of normal-appearing MCs in the same lesion, lends support to the hypothesis that MCs and MCC cells are derived from MC progenitors residing within the hair follicle bulge. Specifically, MCCs are derived from transformed MC progenitors with potential for dual-directional differentiation towards neuroendocrine and epithelial lineages.

Neuroendocrine Key Regulator Gene Expression in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a highly aggressive non-melanoma skin cancer of the elderly which is associated with the Merkel cell polyomavirus (MCPyV). MCC reveals a trilinear differentiation characterized by neuroendocrine, epithelial and pre/pro B-cell lymphocytic gene expression disguising the cellular origin of MCC. Here we investigated the expression of the neuroendocrine key regulators RE1 silencing transcription factor (REST), neurogenic differentiation 1 (NeuroD1) and the Achaete-scute homolog 1 (ASCL1) in MCC. All MCCs were devoid of REST and were positive for NeuroD1 expression. Only one MCC tissue revealed focal ASCL1 expression. This was confirmed in MCPyV-positive MCC cell lines. Of interest, MCPyV-negative cell lines did express REST. The introduction of REST expression in REST-negative, MCPyV-positive MCC cells downregulated the neuroendocrine gene expression. The lack of the neuroendocrine master regulator ASCL1 in almost all tested MCCs points to an important role of the absence of the negative regulator REST towards the MCC neuroendocrine phenotype. This is underlined by the expression of the REST-regulated microRNAs miR-9/9* in REST-negative MCC cell lines. These data might provide the basis for the understanding of neuroendocrine gene expression profile which is expected to help to elucidate the cellular origin of MCC.

Are there multiple cells of origin of Merkel cell carcinoma?

Merkel cell carcinoma (MCC) is a rare but lethal cancer with the highest case-by-case fatality rate among all skin cancers. 80% of cancers are associated with the Merkel cell polyomavirus (MCPyV). 20% of MCCs are virus negative. Recent epidemiological data suggest that there are important, clinically relevant differences between these two subtypes of MCC. Recent studies in cancer genomics, mouse genetics, and virology experiments have transformed our understanding of MCC pathophysiology. Importantly, dramatic differences in the genetics of these two MCC subtypes suggest fundamental differences in their pathophysiology. We review these recent works and find that they provocatively suggest that MCPyV-positive and MCPyV-negative MCCs arise from two different cells of origin: the MCPyV-negative MCC from epidermal keratinocytes and the MCPyV-positive MCC from dermal fibroblasts. If true, this would represent the first cancer that we are aware of that evolves from cells of origin from two distinct germ layers: MCPyV-negative MCCs from ectodermal keratinocytes and MCPyV-positive MCCs from mesodermal fibroblasts. Future epigenetic experiments may prove valuable in confirming these distinct lineages for these MCC subtypes, especially for the clinical importance the cell of origin has on MCC treatment and prevention.

Reviewing the current evidence supporting early B-cells as the cellular origin of Merkel cell carcinoma.

Merkel cell carcinoma (MCC) is a highly malignant skin cancer characterized by early metastases and poor survival. Although MCC is a rare malignancy, its incidence is rapidly increasing in the U.S. and Europe. The discovery of the Merkel cell polyomavirus (MCPyV) has enormously impacted our understanding of its etiopathogenesis and biology. MCCs are characterized by trilinear differentiation, comprising the expression of neuroendocrine, epithelial and B-lymphoid lineage markers. To date, it is generally accepted that the initial assumption of MCC originating from Merkel cells (MCs) is unlikely. This is owed to their post-mitotic character, absence of MCPyV in MCs and discrepant protein expression pattern in comparison to MCC. Evidence from mouse models suggests that epidermal/dermal stem cells might be of cellular origin in MCC. The recently formulated hypothesis of MCC originating from early B-cells is based on morphology, the consistent expression of early B-cell lineage markers and the finding of clonal immunoglobulin chain rearrangement in MCC cells. In this review we elaborate on the cellular ancestry of MCC, the identification of which could pave the way for novel and more effective therapeutic regimens.

Merkel cell carcinoma: cutaneous manifestation of a highly malignant pre-/pro-Bcell neoplasia? : Novel concept about the cellular origin of Merkel cell carcinoma

Merkel cell carcinoma (MCC) is arelatively rare but highly malignant non-melanoma skin cancer of the elderly and immunosuppressed patients. The discovery of the Merkel cell polyomavirus (MCPyV) in 2008 significantly impacted the understanding of the etiopathogenesis of MCC. MCPyV is clonally integrated into the MCC genome and approximately 80% of MCC are MCPyV-positive. Recent results of clinical trials using blockade of the PD-1 immune modulatory pathway are promising for the future treatment of MCC. Despite this major progress of the past few years, the cellular origin of MCC still remains obscure. Based on histomorphology, gene expression profiling, and molecular analyses, we have recently hypothesized that MCC originates from pre‑/pro-Bcells. Here we review putative cells of MCC, including Merkel cells, (epi‑)dermal stem cells, and pro‑/pre-Bcells. In the present work, the focus is on the concept of pre‑/pro-Bcells as the cellular origin of MCC, which might also impact the understanding of other human small cell malignancies of unknown cellular origin, such as small cell carcinomas of the lung and other anatomical locations. In addition, this concept might pave the way for novel treatment options, especially for advanced MCC.

Roles for miR-375 in Neuroendocrine Differentiation and Tumor Suppression via Notch Pathway Suppression in Merkel Cell Carcinoma

Dysfunction of key miRNA pathways regulating basic cellular processes is a common driver of many cancers. However, the biological roles and/or clinical applications of such pathways in Merkel cell carcinoma (MCC), a rare but lethal cutaneous neuroendocrine (NE) malignancy, have yet to be determined. Previous work has established that miR-375 is highly expressed in MCC tumors, but its biological role in MCC remains unknown. Herein, we show that elevated miR-375 expression is a specific feature of well-differentiated MCC cell lines that express NE markers. In contrast, miR-375 is strikingly down-regulated in highly aggressive, undifferentiated MCC cell lines. Enforced miR-375 expression in these cells induced NE differentiation, and opposed cancer cell viability, migration, invasion, and survival, pointing to tumor-suppressive roles for miR-375. Mechanistically, miR-375-driven phenotypes were caused by the direct post-transcriptional repression of multiple Notch pathway proteins (Notch2 and RBPJ) linked to cancer and regulation of cell fate. Thus, we detail a novel molecular axis linking tumor-suppressive miR-375 and Notch with NE differentiation and cancer cell behavior in MCC. Our findings identify miR-375 as a putative regulator of NE differentiation, provide insight into the cell of origin of MCC, and suggest that miR-375 silencing may promote aggressive cancer cell behavior through Notch disinhibition.

Merkel cell carcinoma: is this a true carcinoma?

Recent years have brought an enhanced understanding of Merkel cell carcinoma (MCC) biology, especially with regard to the Merkel cell polyoma virus as a causative agent. Differences between Merkel cell polyomavirus-positive and Merkel cell polyomavirus-negative MCC in morphology; gene expression, miRNA profiles and prognosis have been reported. Origin of MCC is controversial. Presence of neurosecretory granules has suggested that these carcinomas originate from one of the neurocrest derivatives, most probably Merkel cells; the name Merkel cell carcinoma is now widely accepted. Expression of PGP 9.5, chromogranin A and several neuropeptides, initially regarded as specific markers for neural and neuroendocrine cells, has recently been shown in a subset of lymphomas. MCC commonly expresses terminal deoxynucleotidyl transferase and PAX5. Their co-expression under physiologic circumstances is restricted to pro/pre-B cells and pre-B cells. These findings lead to the hypothesis by zur Hausen et al. that MCC originates from early B cells. This review was intended to critically appraise zur Hausen's hypothesis and discuss the possibility that MCC is a heterogenous entity with distinct subtypes.

Merkel cell carcinoma versus metastatic small cell primary bronchogenic carcinoma

Introduction: Merkel cell carcinoma (MCC) of the skin is a rare, aggressive, malignant neuroendocrine neoplasm. The tumor classically demonstrates positive immunohistochemistry (IHC) staining for chromogranin A(ChrA), cytokeratin 20 (CK20), neuron specific enolase (NSE) and/or achaete-acute complex-like 1 (MASH1). The newly identified Merkel cell polyomavirus (MCPyV) has been found to be associated with most MCC cases. The primary histologic differential diagnoses of cutaneous MCC is small cell primary bronchogenic carcinoma (SCLC); moreover, both are of neuroendocrine origin. SCLC accounts for approximately 10-15% of all primary lung cancer cases; this histologic subtype is a distinct entity with biological and oncological features distinct from non-small cell lung cancer (NSCLC). In contradistinction to MCC, SCLC is classically IHC positive for cytokeratin 7 (CK7) and transcription factor (TTF-1). Similar to SCLC, MCC cell lines may be classified into two different biochemical subgroups designated as Classic and Variant. In our review and case report, we aim to emphasize the importance of a multidisciplinary approach to the approach to this difficult differential diagnosis. We also aim to comment about features of the cells of origin of MCC and SCLC; to summarize the microscopic features of both tumors; and to review their respective epidemiologic, clinical, prognostic and treatment features. We want to emphasize the initial workup study of the differential diagnosis patient, including evaluating clinical lymph nodes, a clinical history of any respiratory abnormality, and chest radiogram. If a diagnosis of primary cutaneous MCC is confirmed, classic treatment includes excision of the primary tumor with wide margins, excision of a sentinel lymph node, and computed tomography, positron emission tomography and/or Fluorine-18fluorodeoxyglucose positron emission tomography scan studies.

Which Are the Cells of Origin in Merkel Cell Carcinoma?

Merkel cell carcinoma (MCC), a highly aggressive skin tumour with increasing incidence, is associated with the newly discovered Merkel cell polyomavirus (MCPyV). Studies on MCC and MCPyV as well as other risk factors have significantly increased our knowledge of MCC pathogenesis, but the cells of origin, which could be important targets in future therapies, are still unknown. Merkel cells (MCs), the neuroendocrine cells of the skin, were believed to be at the origin of MCC due to their phenotypic similarities. However, for several reasons, for example, heterogeneous differentiation of MCCs and postmitotic character of MCs, it is not very likely that MCC develops from differentiated MCs. Skin stem cells, probably from the epidermal lineage, are more likely to be cells of origin in MCC. Future studies will have to address these questions more directly in order to identify the physiological cells which are transformed to MCC cells.

Merkel cell carcinoma: Our experience in this rare pathology

Merkel cell carcinoma (MCC) was first described in 1972 by Toker, who described five cases of the so-called "trabecular carcinoma of the skin ". MCC is a rare, aggressive skin cancer that affects mainly the elderly. Sun exposed areas are mainly affected, specially the head and neck. Immunohistochemical analysis is essential to reach a correct diagnosis. According to the origin of MCC, the tumor expresses both epithelial and neuroendocrine markers. MCC has a propensity for recurrence, regional and distant metastases. Several treatment options are available, such as surgical excision or Moh's surgery, accompanied by neck dissection or radio and chemotherapy in advanced cases. The present study aims to evaluate the clinical behaviour and the evolution of five cases of this disease that were treated at our institution (reference to 1100000 inhabitants) in the last five years. It is stressed on the bad prognosis of this pathology, which presents high rates of locoregional recurrence, even though the recent advances in radio and chemotherapy.

Primary merkel cell carcinoma clinically presenting as deep oedematous mass of the groin

Merkel cell carcinoma (MCC) is a relatively rare, polyomavirus associated, primary neuroendocrine carcinoma of the skin which is usually arising from dermal skin layers. However, the origin of MCC in the subcutaneous tissue is debatable. We report a 58-yearold female patient with an oedematous mass on her left groin that was firm in consistency and had no discoloration or other visible abnormality of the overlying skin. On histology and immunohistology the tumour was consistent with the diagnosis of MCC showing a predominant subcutanous growth pattern. Pelvic magnetic resonance tomography revealed a tumour conglomerate reaching from the subcutis of the left groin to the left paraaortal and parailiacal region indicating widespread lymphogenic metastisation. Despite complete medical work-up no other MCC primary could be detected. In conclusion, predominant subcutaneous growth pattern as well as tumour localization in the groin are uncommon features of MCC. MCC showing the aforementioned features may be associated with significant delay of diagnosis and therefore represents an unfavourable prognostic factor.

Merkel cell carcinoma: case report and literature review, from a remote region of France

Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer. The first case was described by Toker in 1972 as a trabecular carcinoma of the skin. The most accepted origin of MCC is the Merkel cell (touch receptor). Merkel cell carcinoma occurs mostly in Caucasians aged over 65 years as a dome-shaped purple node on the head, neck or extremities. This report describes a case of MCC of 0.5 cm size, located on the left upper leg. The patient was Caucasian, female and aged 69 years with the specific risk factor of a history of sun exposure on the beach for 50 years. The excision biopsy returned a diagnosis of MCC. The excision of the biopsy scar, with clear margins of 3 cm and to a depth of 3 cm was performed 2 days later and returned a result of no MCC cells present. Biological tests were normal. No lesions were detected on CT scan of thorax and abdomen. No dissemination was detected on PET scan. It was ranked as stage I MCC. National Cancer Institute guidelines (2007) were used for follow up: physical examination every 3 months, PET scan every 6 months for 1 year; after this physical examination every 6 months and PET scan every year for 5 years. The role of the primary care physician operating in remote location is crucial for the early detection of skin cancers. Early detection is the only chance for a patient to survive an MCC. Every primary care physician operating in a remote location should master the surgical technique for excision biopsy, the gold standard for diagnosis of skin cancers.

Merkel cell polyomavirus and Merkel cell carcinoma

A new polyomavirus, Merkel cell polyomavirus, was identified from Merkel cell carcinoma, a rare skin cancer. Origin of Merkel cell carcinoma is Merkel cell, a neuroendocrine cell in the skin. Merkel cell carcinoma occurs in the skin of head and face of white elders. Like other polyomaviruses, a 5.4 kbp-virus genome encodes VP1, VP2, VP3, small T antigen, and large T (LT) antigen. MCV has been frequently detected in Merkel cell carcinoma in the world by PCR. It was demonstrated that MCV genome integrated into the host genome of Merkel cell carcinoma. LT plays an important role in replication of a circular MCV. However, mutations with stop codons were identified in the LT genes derived from Merkel cell carcinoma cases. Such mutations cause a truncation of the LT gene, resulting in defect of LTs helicase activity and in induction of Rb binding function in the LT. MCV is a new member of human oncovirus belonging to the human polyomavirus.