Molecular Profiling of Multiple Primary Merkel Cell Carcinoma to Distinguish Genetically Distinct Tumors From Clonally Related Metastases

Squamoid Eccrine Ductal Carcinoma Displays Ultraviolet Mutations and Intermediate Gene Expression Relative to Squamous Cell Carcinoma, Microcystic Adnexal Carcinoma, and Porocarcinoma

Squamoid eccrine ductal carcinoma is a rare infiltrative tumor with morphologic features intermediate between squamous cell carcinoma (SCC) and sweat gland carcinomas such as microcystic adnexal carcinoma. Although currently classified as a sweat gland carcinoma, it has been debated whether squamoid eccrine ductal carcinoma is better classified as a variant of SCC. Furthermore, therapeutic options for patients with advanced disease are lacking. Here, we describe clinicopathologic features of a cohort of 15 squamoid eccrine ductal carcinomas from 14 unique patients, with next-generation sequencing DNA profiling for 12 cases. UV signature mutations were the dominant signature in the majority of cases. TP53 mutations were the most highly recurrent specific gene alteration, followed by mutations in NOTCH genes. Recurrent mutations in driver oncogenes were not identified. By unsupervised comparison of global transcriptome profiles in squamoid eccrine ductal carcinoma (n = 7) to SCC (n = 10), porocarcinoma (n = 4), and microcystic adnexal carcinoma (n = 4), squamoid eccrine ductal carcinomas displayed an intermediate phenotype between SCC and sweat gland tumors. Squamoid eccrine ductal carcinoma displayed significantly higher expression of 364 genes (including certain eccrine markers) and significantly lower expression of 525 genes compared with other groups. Our findings support the classification of squamoid eccrine ductal carcinoma as a carcinoma with intermediate features between SCC and sweat gland carcinoma.

Merkel cell carcinoma: updates in tumor biology, emerging therapies, and preclinical models

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma thought to arise via either viral (Merkel cell polyomavirus) or ultraviolet-associated pathways. Surgery and radiotherapy have historically been mainstays of management, and immunotherapy has improved outcomes for advanced disease. However, there remains a lack of effective therapy for those patients who fail to respond to these established approaches, underscoring a critical need to better understand MCC biology for more effective prognosis and treatment. Here, we review the fundamental aspects of MCC biology and the recent advances which have had profound impact on management. The first genetically-engineered mouse models for MCC tumorigenesis provide opportunities to understand the potential MCC cell of origin and may prove useful for preclinical investigation of novel therapeutics. The MCC cell of origin debate has also been advanced by recent observations of MCC arising in association with a clonally related hair follicle tumor or squamous cell carcinoma in situ. These studies also suggested a role for epigenetics in the origin of MCC, highlighting a potential utility for this therapeutic avenue in MCC. These and other therapeutic targets form the basis for a wealth of ongoing clinical trials to improve MCC management. Here, we review these recent advances in the context of the existing literature and implications for future investigations.

Integrative multi-region molecular profiling of primary prostate cancer in men with synchronous lymph node metastasis

Localized prostate cancer is frequently composed of multiple spatially distinct tumors with significant inter- and intra-tumoral molecular heterogeneity. This genomic diversity gives rise to many competing clones that may drive the biological trajectory of the disease. Previous large-scale sequencing efforts have focused on the evolutionary process in metastatic prostate cancer, revealing a potential clonal progression to castration resistance. However, the clonal origin of synchronous lymph node (LN) metastases in primary disease is still unknown. Here, we perform multi-region, targeted next generation sequencing and construct phylogenetic trees in men with prostate cancer with synchronous LN metastasis to better define the pathologic and molecular features of primary disease most likely to spread to the LNs. Collectively, we demonstrate that a combination of histopathologic and molecular factors, including tumor grade, presence of extra-prostatic extension, cellular morphology, and oncogenic genomic alterations are associated with synchronous LN metastasis.

Merkel cell carcinoma: an update

Merkel cell carcinoma (MCC) is an uncommon primary cutaneous neuroendocrine carcinoma associated with an adverse prognosis. In recent years, our understanding of MCC biology has markedly progressed. Since the discovery of the Merkel cell polyomavirus, it has become clear that MCC represents an ontogenetically dichotomous group of neoplasms with overlapping histopathology. Specifically, most MCCs arise secondary to viral oncogenesis, while a smaller subset is the direct result of UV-associated mutations. The distinction of these groups bears relevance in their immunohistochemical and molecular characterization, as well as in disease prognosis. Further recent developments relate to the landmark utilization of immunotherapeutics in MCC, providing optimistic options for the management of this aggressive disease. In this review, we discuss both fundamental and emerging concepts in MCC, with a particular focus on topics of practical relevance to the surgical or dermatopathologist.

BRCA1/2 Pathogenic Variants Are Not Common in Merkel Cell Carcinoma: Comprehensive Molecular Study of 30 Cases and Meta-Analysis of the Literature

Merkel cell carcinoma (MCC) is a rare and aggressive cutaneous neuroendocrine cancer. Management of advanced MCC is mainly based on immune-checkpoint inhibitors. The high failure rate warrants an investigation of new therapeutic targets. The recent identification of BRCA1 or BRCA2 (BRCA1/2) mutations in some MCC raises the issue of the use of poly-(ADP-Ribose)-polymerase inhibitors in selected advanced cases. The main objective of our study is to determine the accurate frequency of BRCA1/2 pathogenic variants. We studied a series of 30 MCC and performed a meta-analysis of BRCA1/2 variants of published cases in the literature. In our series, we detected only one BRCA2 pathogenic variant. The low frequency of BRCA1/2 pathogenic variants in our series of MCC (3%) was confirmed by the meta-analysis of BRCA1/2 variants in the literature. Among the 915 MCC from 13 published series studied for molecular alterations of BRCA1/2, only 12 BRCA1/2 pathogenic mutations were identified (1-2% of MCC), whereas many other BRCA1/2 variants were variants of unknown significance or benign. BRCA1/2 pathogenic variants are uncommon in MCC. However, in BRCA-mutated MCC, poly-(ADP-Ribose)-polymerase inhibitors might be a valuable therapeutic option requiring validation by clinical trials.

Chromosomal Aberrations Accumulate during Metastasis of Virus-Negative Merkel Cell Carcinoma

Merkel cell carcinoma is a rare, aggressive skin tumor initiated by polyomavirus integration or UV light DNA damage. In New Zealand, there is a propensity toward the UV-driven form (31 of 107, 29% virus positive). Using archival formalin-fixed, paraffin-embedded tissues, we report targeted DNA sequencing covering 246 cancer genes on 71 tumor tissues and 38 nonmalignant tissues from 37 individuals, with 33 of 37 being negative for the virus. Somatic variants of New Zealand virus-negative Merkel cell carcinomas partially overlapped with those reported overseas, including TP53 variants in all tumors and RB1, LRP1B, NOTCH1, and EPHA3/7 variants each found in over half of the cohort. Variants in genes not analyzed or reported in previous studies were also found. Cataloging variants in TP53 and RB1 from published datasets revealed a broad distribution across these genes. Chr 1p gain and Chr 3p loss were identified in around 50% of New Zealand virus-negative Merkel cell carcinomas, and RB1 loss of heterozygosity was found in 90% of cases. Copy number variants accumulate in most metastases. Virus-negative Merkel cell carcinomas have complex combinations of somatic DNA-sequence variants and copy number variants. They likely carry the small genomic changes permissive for metastasis from early tumor development; however, chromosomal alterations may contribute to driving metastatic progression.

An update on Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare cancer of the skin characterized by a neuroendocrine phenotype and an aggressive clinical behavior. It frequently originates in sun-exposed body areas, and its incidence has steadily increased in the last three decades. Merkel cell polyomavirus (MCPyV) and ultraviolet (UV) radiation exposure are the main causative agents of MCC, and distinct molecular features have been documented in virus-positive and virus-negative malignancies. Surgery remains the cornerstone of treatment for localized tumors, but even when integrated with adjuvant radiotherapy is able to definitively cure only a fraction of MCC patients. While characterized by a high objective response rate, chemotherapy is associated with a short-lasting benefit of approximately 3 months. On the other hand, immune checkpoint inhibitors including avelumab and pembrolizumab have demonstrated durable antitumor activity in patients with stage IV MCC, and investigations on their use in the neoadjuvant or adjuvant setting are currently underway. Addressing the needs of those patients who do not persistently benefit from immunotherapy is currently one of the most compelling unmet needs in the field, and multiple clinical trials of new tyrosine kinase inhibitors (TKIs), peptide receptor radionuclide therapy (PRRT), therapeutic vaccines, immunocytokines as well as innovative forms of adoptive cellular immunotherapies are under clinical scrutiny at present.

Genomic Alterations and Tumor Mutation Burden in Merkel Cell Carcinoma

IMPORTANCE

Merkel cell carcinoma (MCC) is a rare and highly aggressive cutaneous neuroendocrine carcinoma with increasing incidence. Cytotoxic chemotherapy and checkpoint inhibitors provide treatment options in the metastatic setting; however, there are no approved or standard of care targeted therapy treatment options.

OBJECTIVE

To identify actionable alterations annotated by the OncoKB database therapeutic evidence level in association with tumor mutation burden (TMB).

DESIGN, SETTING, AND PARTICIPANTS

This is a retrospective, cross-sectional study using data from the American Association for Cancer Research Genomics Evidence Neoplasia Information Exchange, a multicenter international cancer consortium database. Patients with MCC were enrolled in participating institutions between 2017 and 2022. Data from version 11.0 of the database were released in January 2022 and analyzed from April to June 2022.

MAIN OUTCOMES AND MEASURES

The main outcome was the percentage of patients with high TMB and OncoKB level 3B and 4 alterations.

RESULTS

A total of 324 tumor samples from 313 patients with MCC (107 women [34.2%]; 287 White patients [91.7%]; 7 Black patients [2.2%]) were cataloged in the database. The median (range) number of alterations was 4.0 (0.0-178.0), with a mean (SD) of 13.6 (21.2) alterations. Oncogenic alterations represented 20.2% of all alterations (862 of 4259 alterations). Tissue originated from primary tumor in 55.0% of patients (172 patients) vs metastasis in 39.6% (124 patients). TMB-high (≥10 mutations per megabase) was present in 26.2% of cases (82 patients). Next-generation sequencing identified 55 patients (17.6%) with a level 3B variation for a Food and Drug Administration-approved drug for use in a biomarker-approved indication or approved drug in another indication. An additional 8.6% of patients (27 patients) had a level 4 variation. Actionable alterations were more common among high TMB cases, with 37 of 82 patients (45.1%) harboring level 3 alterations compared with only 18 of 231 patients (7.8%) with low TMB. The most common level 3B gene variants included PIK3CA (12 patients [3.8%]), BRCA1/2 (13 patients [4.2%]), ATM (7 patients [2.2%]), HRAS (5 patients [1.6%]), and TSC1/2 (6 patients [1.9%]). The most common level 4 variants include PTEN (13 patients [4.1%]), ARID1A (9 patients [2.9%]), NF1 (7 patients [2.2%]), and CDKN2A (7 patients [2.2%]). Copy number alterations and fusions were infrequent. In 61.0% of cases (191 cases), a PanCancer pathway was altered, and 39.9% (125 cases) had alterations in multiple pathways. Commonly altered pathways were RTK-RAS (119 patients [38.0%]), TP53 (103 patients [32.9%]), cell cycle (104 patients [33.2%]), PI3K (99 patients [31.6%]), and NOTCH (93 patients [29.7%]). In addition, oncogenic DNA mismatch repair gene alterations were present in 8.0% of cases (25 patients).

CONCLUSIONS AND RELEVANCE

In this cross-sectional retrospective study of alterations and TMB in MCC, a minority of patients had potentially actionable alterations. These findings support the investigation of targeted therapies as single agent or in combination with immunotherapy or cytotoxic chemotherapy in selected MCC populations.

RB1-deficient squamous cell carcinoma: the proposed source of combined Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine (NE) carcinoma arising from integration of Merkel cell polyomavirus (MCPyV) DNA into a host cell or from ultraviolet light-induced genetic damage (proportions vary geographically). Tumors in the latter group include those with "pure" NE phenotype and those "combined" with other elements, most often squamous cell carcinoma (SCC). We performed comprehensive genomic profiling (CGP) of MCPyV+ and MCPyV- (pure and combined) tumors, to better understand their mutational profiles and shed light on their pathogenesis. Supplemental immunohistochemistry for Rb expression was also undertaken. After eliminating low quality samples, 37 tumors were successfully analyzed (14 MCPyV+, 8 pure MCPyV- and 15 combined MCPyV-). The SCC and NE components were sequenced separately in 5 combined tumors. Tumor mutational burden was lower in MCPyV+ tumors (mean 1.66 vs. 29.9/Mb, P < 0.0001). MCPyV- tumors featured frequent mutations in TP53 (95.6%), RB1 (87%), and NOTCH family genes (95.6%). No recurrently mutated genes were identified in MCPyV+ tumors. Mutational overlap in the NE and SCC components of combined tumors was substantial ('similarity index' >24% in 4/5 cases). Loss of Rb expression correlated with RB1 mutational (P < 0.0001) and MCPyV- status (P < 0.0001) in MCCs and it was observed more frequently in the SCC component of combined MCC than in a control group of conventional cutaneous SCC (P = 0.0002). Our results (i) support existing evidence that MCPyV+ and MCPyV- MCCs are pathogenetically distinct entities (ii) concur with earlier studies linking the NE and SCC components of combined MCCs via shared genetic profiles and (iii) lend credence to the proposal that an Rb-deficient subset of SCC's is the source of phenotypically divergent combined MCCs.

Serial monitoring of genomic alterations in circulating tumor cells of ER-positive/HER2-negative advanced breast cancer: feasibility of precision oncology biomarker detection

Nearly all estrogen receptor (ER)-positive (POS) metastatic breast cancers become refractory to endocrine (ET) and other therapies, leading to lethal disease presumably due to evolving genomic alterations. Timely monitoring of the molecular events associated with response/progression by serial tissue biopsies is logistically difficult. Use of liquid biopsies, including circulating tumor cells (CTC) and circulating tumor DNA (ctDNA), might provide highly informative, yet easily obtainable, evidence for better precision oncology care. Although ctDNA profiling has been well investigated, the CTC precision oncology genomic landscape and the advantages it may offer over ctDNA in ER-POS breast cancer remain largely unexplored. Whole-blood (WB) specimens were collected at serial time points from patients with advanced ER-POS/HER2-negative (NEG) advanced breast cancer in a phase I trial of AZD9496, an oral selective ER degrader (SERD) ET. Individual CTC were isolated from WB using tandem CellSearch® /DEPArray™ technologies and genomically profiled by targeted single-cell DNA next-generation sequencing (scNGS). High-quality CTC (n = 123) from 12 patients profiled by scNGS showed 100% concordance with ctDNA detection of driver estrogen receptor α (ESR1) mutations. We developed a novel CTC-based framework for precision medicine actionability reporting (MI-CTCseq) that incorporates novel features, such as clonal predominance and zygosity of targetable alterations, both unambiguously identifiable in CTC compared to ctDNA. Thus, we nominated opportunities for targeted therapies in 73% of patients, directed at alterations in phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), fibroblast growth factor receptor 2 (FGFR2), and KIT proto-oncogene, receptor tyrosine kinase (KIT). Intrapatient, inter-CTC genomic heterogeneity was observed, at times between time points, in subclonal alterations. Our analysis suggests that serial monitoring of the CTC genome is feasible and should enable real-time tracking of tumor evolution during progression, permitting more combination precision medicine interventions.

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.

Virus-positive Merkel Cell Carcinoma Is an Independent Prognostic Group with Distinct Predictive Biomarkers

PURPOSE

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma that can be divided into two classes: virus-positive (VP) MCC, associated with oncogenic Merkel cell polyomavirus (MCPyV); and virus-negative (VN) MCC, associated with photodamage.

EXPERIMENTAL DESIGN

We classified 346 MCC tumors from 300 patients for MCPyV using a combination of IHC, ISH, and qPCR assays. In a subset of tumors, we profiled mutation status and expression of cancer-relevant genes. MCPyV and molecular profiling results were correlated with disease-specific outcomes. Potential prognostic biomarkers were further validated by IHC.

RESULTS

A total of 177 tumors were classified as VP-MCC, 151 tumors were VN-MCC, and 17 tumors were indeterminate. MCPyV positivity in primary tumors was associated with longer disease-specific and recurrence-free survival in univariate analysis, and in multivariate analysis incorporating age, sex, immune status, and stage at presentation. Prioritized oncogene or tumor suppressor mutations were frequent in VN-MCC but rare in VP-MCC. TP53 mutation developed with recurrence in one VP-MCC case. Importantly, for the first time we find that VP-MCC and VN-MCC display distinct sets of prognostic molecular biomarkers. For VP-MCC, shorter survival was associated with decreased expression of immune markers including granzyme and IDO1. For VN-MCC, shorter survival correlated with high expression of several genes including UBE2C.

CONCLUSIONS

MCPyV status is an independent prognostic factor for MCC. Features of the tumor genome, transcriptome, and microenvironment may modify prognosis in a manner specific to viral status. MCPyV status has clinicopathologic significance and allows for identification of additional prognostic subgroups.

Molecular Profiling of Merkel Cell Polyomavirus-Associated Merkel Cell Carcinoma and Cutaneous Melanoma

Merkel cell carcinoma (MCC) is a rare, high-grade, aggressive cutaneous neuroendocrine malignancy most commonly associated with sun-exposed areas of older individuals. A relatively newly identified human virus, the Merkel cell polyomavirus (MCPyV) has been implicated in the pathogenesis of MCC. Our study aimed to examine nine MCC cases and randomly selected 60 melanoma cases to identify MCPyV status and to elucidate genetic differences between virus-positive and -negative cases. Altogether, seven MCPyV-positive MCC samples and four melanoma samples were analyzed. In MCPyV-positive MCC RB1, TP53, FBXW7, CTNNB1, and HNF1A pathogenic variants were identified, while in virus-negative cases only benign variants were found. In MCPyV-positive melanoma cases, besides BRAF mutations the following genes were also affected: PIK3CA, STK11, CDKN2A, SMAD4, and APC. In contrast to studies found in the literature, a higher tumor burden was detected in virus-associated MCC compared to MCPyV-negative cases. No association was identified between virus infection and tumor burden in melanoma samples. We concluded that analyzing the key morphologic and immunohistological features of MCC is critical to avoid confusion with other cutaneous malignancies. Molecular genetic investigations such as next-generation sequencing (NGS) enable molecular stratification, which may have future clinical impact.

High-resolution analysis of Merkel Cell Polyomavirus in Merkel Cell Carcinoma reveals distinct integration patterns and suggests NHEJ and MMBIR as underlying mechanisms

Merkel Cell Polyomavirus (MCPyV) is the etiological agent of the majority of Merkel Cell Carcinomas (MCC). MCPyV positive MCCs harbor integrated, defective viral genomes that constitutively express viral oncogenes. Which molecular mechanisms promote viral integration, if distinct integration patterns exist, and if integration occurs preferentially at loci with specific chromatin states is unknown. We here combined short and long-read (nanopore) next-generation sequencing and present the first high-resolution analysis of integration site structure in MCC cell lines as well as primary tumor material. We find two main types of integration site structure: Linear patterns with chromosomal breakpoints that map closely together, and complex integration loci that exhibit local amplification of genomic sequences flanking the viral DNA. Sequence analysis suggests that linear patterns are produced during viral replication by integration of defective/linear genomes into host DNA double strand breaks via non-homologous end joining, NHEJ. In contrast, our data strongly suggest that complex integration patterns are mediated by microhomology-mediated break-induced replication, MMBIR. Furthermore, we show by ChIP-Seq and RNA-Seq analysis that MCPyV preferably integrates in open chromatin and provide evidence that viral oncogene expression is driven by the viral promoter region, rather than transcription from juxtaposed host promoters. Taken together, our data explain the characteristics of MCPyV integration and may also provide a model for integration of other oncogenic DNA viruses such as papillomaviruses.

Non-Melanoma Skin Cancers: Biological and Clinical Features

Non-melanoma skin cancers (NMSCs) include basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and Merkel cell carcinoma (MCC). These neoplasms are highly diverse in their clinical presentation, as well as in their biological evolution. While the deregulation of the Hedgehog pathway is commonly observed in BCC, SCC and MCC are characterized by a strikingly elevated mutational and neoantigen burden. As result of our improved understanding of the biology of non-melanoma skin cancers, innovative treatment options including inhibitors of the Hedgehog pathway and immunotherapeutic agents have been recently investigated against these malignancies, leading to their approval by regulatory authorities. Herein, we review the most relevant biological and clinical features of NMSC, focusing on innovative treatment approaches.

Next-generation sequencing implicates oncogenic roles for p53 and JAK/STAT signaling in microcystic adnexal carcinomas

Microcystic adnexal carcinoma is a locally aggressive sweat gland carcinoma characterized by its infiltrative growth and histopathologic overlap with benign adnexal tumors, often posing challenges to both diagnosis and management. Understanding the molecular underpinnings of microcystic adnexal carcinoma may allow for more accurate diagnosis and identify potential targetable oncogenic drivers. We characterized 18 microcystic adnexal carcinomas by targeted, multiplexed PCR-based DNA next-generation sequencing of the coding sequence of over 400 cancer-relevant genes. The majority of cases had relatively few (<8) prioritized somatic mutations, and lacked an ultraviolet (UV) signature. The most recurrent mutation was TP53 inactivation in four (22%) tumors. Frame-preserving insertions affecting the kinase domain of JAK1 were detected in three (17%) cases, and were nonoverlapping with TP53 mutations. Seven (39%) cases demonstrated copy number gain of at least one oncogene. By immunohistochemistry, p53 expression was significantly higher in microcystic adnexal carcinomas with TP53 mutations compared with those without such mutations and syringomas. Similarly, phospho-STAT3 expression was significantly higher in microcystic adnexal carcinomas harboring JAK1 kinase insertions compared with those with wild-type JAK1 and syringomas. In conclusion, microcystic adnexal carcinomas are molecularly heterogeneous tumors, with inactivated p53 or activated JAK/STAT signaling in a subset. Unlike most other nonmelanoma skin cancers involving sun-exposed areas, most microcystic adnexal carcinomas lack evidence of UV damage, and hence likely originate from a relatively photo-protected progenitor population in the dermis. These findings have implications for the biology, diagnosis, and treatment of microcystic adnexal carcinomas, including potential for therapeutic targeting of p53 or the JAK/STAT pathway in advanced tumors.

New perspectives in Merkel cell carcinoma

PURPOSE OF REVIEW

Merkel cell carcinoma (MCC), a rapidly progressing skin cancer, has poor prognosis. We reviewed the epidemiology, pathogenesis, diagnosis and treatment of MCC, with a focus on recent therapeutic advancements.

RECENT FINDINGS

Risk factors for MCC, such as old age, immunosuppression, polyomavirus infection and exposure to UV radiation have already been identified, but the underlying mechanisms leading to carcinogenesis still need clarification. On the basis of recent advances, immunotherapy - in particular, inhibition targeting the programmed cell death protein 1/programmed death-ligand 1 (PD1)/PDL1) immune checkpoint blockade - is currently being investigated in the treatment of metastatic MCC. Avelumab, an anti-PDL1 antibody, was the first drug to be approved internationally as second-line monotherapy for patients with advanced MCC, based on results from the JAVELIN Merkel 200 clinical trial. Avelumab has also recently been approved as first-line treatment for advanced MCC in Europe. Pembrolizumab (anti-PD1) in first-line and nivolumab (anti-PD1) in first-line and second-line treatments are two other checkpoint inhibitors that are under investigation, and showing promising results. New innovative therapies are also in development.

SUMMARY

New insights concerning advances in MCC diagnosis and treatment have been highlighted. Immunotherapy for metastatic MCC constitutes a recent breakthrough in an unmet medical need, but alternative therapies should continue to be investigated.

The Ubiquitin-Specific Protease Usp7, a Novel Merkel Cell Polyomavirus Large T-Antigen Interaction Partner, Modulates Viral DNA Replication

Merkel cell polyomavirus (MCPyV) is the major cause for Merkel cell carcinoma (MCC), a rare but highly aggressive skin cancer predominantly found in elderly and immunosuppressed patients. The early viral gene products large T-antigen (LT) and small T-antigen (sT) are important for efficient viral DNA replication, and both contribute to transformation processes. These functions are executed mainly through interactions with host factors. Here, we identify the cellular ubiquitin-specific processing protease 7 (Usp7) as a new interaction partner of the MCPyV LT. Using glutathione S-transferase pulldown experiments, we show that MCPyV LT directly binds to Usp7 and that N- as well as C-terminal regions of LT bind to the TRAF (tumor necrosis factor receptor-associated) domain of Usp7. We demonstrate that endogenous Usp7 coprecipitates with MCPyV T-antigens and relocalizes to viral DNA replication centers in cells actively replicating MCPyV genomes. We show that Usp7 does not alter ubiquitination levels of the T-antigens; however, Usp7 binding increases the binding affinity of LT to the origin of replication, thereby negatively regulating viral DNA replication. Together, these data identify Usp7 as a restriction factor of MCPyV replication. In contrast to other DNA viruses, Usp7 does not affect MCPyV gene expression via its ubiquitination activity but influences MCPyV DNA replication solely via a novel mechanism that modulates binding of LT to viral DNA.IMPORTANCE MCPyV is the only human polyomavirus that is associated with cancer; the majority of Merkel cell cancers have a viral etiology. While much emphasis was placed on investigations to understand the transformation process by MCPyV oncoproteins and cellular factors, we have only limited knowledge of cellular factors participating in the MCPyV life cycle. Here, we describe Usp7, a cellular deubiquitination enzyme, as a new factor involved in MCPyV replication. Usp7 is known in the context of large DNA tumor viruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus, to restrict viral replication. Similar to EBV, where Usp7 binding to EBNA1 increases EBNA1 binding affinity to viral DNA, we find MCPyV LT binding to the origin of replication to be increased in the presence of Usp7, resulting in restriction of viral DNA replication. However, Usp7-induced restriction of MCPyV replication is independent of its enzymatic activity, thereby constituting a novel mechanism of Usp7-induced restriction of viral replication.

Two separate nodules of Merkel cell carcinoma occurring concomitantly on one cheek

Merkel cell carcinoma (MCC) is a rare and highly aggressive neuroectodermal carcinoma arising from mechanoreceptor Merkel cells. Multiple MCCs are even rarer. We report a case of two independent MCCs simultaneously present in the cheek of a patient, which were effectively and esthetically treated using a cheek flap. Punch biopsy performed in a 60-year-old woman admitted with a chief complaint of two skin-colored hard nodules in her left cheek, accompanied by an itching sensation, was suggestive of MCC. Accordingly, we performed sentinel lymph node biopsy through the modified Blair incision under general anesthesia, in cooperation with the head and neck surgery department. The defect was covered with a cheek flap by slightly extending the existing incision following wide excision with a safety margin of 1 cm. This paper is significant in that it introduces an effective reconstruction technique that maintains function using a cheek flap for the management of this rare case. In addition, this paper is the first to classify multiple MCCs according to the time of onset. We believe that this paper presents an effective alternative reconstruction technique with sentinel node biopsy through the modified Blair incision.

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

Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer associated with advanced age and immunosuppression. Over the past decade, an association has been discovered between MCC and either integration of the Merkel cell polyomavirus, which likely drives tumorigenesis, or somatic mutations owing to ultraviolet-induced DNA damage. Both virus-positive and virus-negative MCCs are immunogenic, and inhibition of the programmed cell death protein 1 (PD-1)–programmed cell death 1 ligand 1 (PD-L1) immune checkpoint has proved to be highly effective in treating patients with metastatic MCC; however, not all patients have a durable response to immunotherapy. Despite these rapid advances in the understanding and management of patients with MCC, many basic, translational and clinical research questions remain unanswered. In March 2018, an International Workshop on Merkel Cell Carcinoma Research was held at the US National Cancer Institute, at which academic, government and industry experts met to identify the highest-priority research questions. Here, we review the biology and treatment of MCC and report the consensus-based recommendations agreed upon during the workshop.

Merkel cell carcinoma (MCC) is a rare and aggressive form of nonmelanoma skin cancer. The availability of immune checkpoint inhibition has improved the outcomes of a subset of patients with MCC, although many unmet needs continue to exist. In this Consensus Statement, the authors summarize developments in our understanding of MCC while also providing consensus recommendations for future research.

Characterization of a Merkel Cell Polyomavirus-Positive Merkel Cell Carcinoma Cell Line CVG-1

Merkel cell polyomavirus (MCV) plays a causal role in ∼80% of Merkel cell carcinomas (MCC). MCV is clonally integrated into the MCC tumor genome, which results in persistent expression of large T (LT) and small T (sT) antigen oncoproteins encoded by the early locus. In MCV-positive MCC tumors, LT is truncated by premature stop codons or deletions that lead to loss of the C-terminal origin binding (OBD) and helicase domains important for replication. The N-terminal Rb binding domain remains intact. MCV-positive cell lines derived from MCC explants have been valuable tools to study the molecular mechanism of MCV-induced Merkel cell carcinogenesis. Although all cell lines have integrated MCV and express truncated LT antigens, the molecular sizes of the LT proteins differ between cell lines. The copy number of integrated viral genome also varies across cell lines, leading to significantly different levels of viral protein expression. Nevertheless, these cell lines share phenotypic similarities in cell morphology, growth characteristics, and neuroendocrine marker expression. Several low-passage MCV-positive MCC cell lines have been established since the identification of MCV. We describe a new MCV-positive MCV cell line, CVG-1, with features distinct from previously reported cell lines. CVG-1 tumor cells grow in more discohesive clusters in loose round cell suspension, and individual cells show dramatic size heterogeneity. It is the first cell line to encode an MCV sT polymorphism resulting in a unique leucine (L) to proline (P) substitution mutation at amino acid 144. CVG-1 possesses a LT truncation pattern near identical to that of MKL-1 cells differing by the last two C-terminal amino acids and also shows an LT protein expression level similar to MKL-1. Viral T antigen knockdown reveals that, like other MCV-positive MCC cell lines, CVG-1 requires T antigen expression for cell proliferation.

Epidemiology, biology and therapy of Merkel cell carcinoma: conclusions from the EU project IMMOMEC

Merkel cell carcinoma (MCC) is a highly aggressive, often lethal neuroendocrine cancer. Its carcinogenesis may be either caused by the clonal integration of the Merkel cell polyomavirus into the host genome or by UV-induced mutations. Notably, virally-encoded oncoproteins and UV-induced mutations affect comparable signaling pathways such as RB restriction of cell cycle progression or p53 inactivation. Despite its low incidence, MCC recently received much attention based on its exquisite immunogenicity and the resulting major success of immune modulating therapies. Here, we summarize current knowledge on epidemiology, biology and therapy of MCC as conclusion of the project 'Immune Modulating strategies for treatment of Merkel Cell Carcinoma', which was funded over a 5-year period by the European Commission to investigate innovative immunotherapies for MCC.