DOI: 10.1111/exd.12490
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Merkel cell carcinoma: is this a true carcinoma? Marek Jankowski1, Piotr Kopinski2, Robert Schwartz3 and Rafal Czajkowski1 1
Faculty of Medicine, Department of Dermatology, Sexually Transmitted Diseases and Immunodermatology, Nicolaus Copernicus University, Bydgoszcz, Poland; 2Faculty of Medicine, Department of Gene Therapy, Nicolaus Copernicus University, Bydgoszcz, Poland; 3Department of Dermatology and Pathology, Rutgers University New Jersey Medical School, Newark, NJ, USA Correspondence: Marek Jankowski, Sklodowskiej-Curie 9, 85-094 Bydgoszcz, Poland, Tel.: +48 52 585 45 68, Fax: +48 52 585 38 67, e-mail:
[email protected] Abstract: 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. Key words: Merkel cell carcinoma – Merkel cell polyoma virus – precursor B-cell lymphoma – primary cutaneous lymphoma – skin cancer
Accepted for publication 29 June 2014
Background
Premises
Merkel cell carcinoma (MCC) is a rare cutaneous neoplasm, the incidence of which has risen three-fold in recent decades (1,2). Neurosecretory Merkel cells were suggested as a source of MCC based on the presence of dense-core neuroendocrine granules in the cytoplasm of tumor cells similar to those observed in Merkel cells (3). The relationship between the normal epidermal Merkel cell and those of this tumor remains controversial. Merkel cells are most abundant in touch-sensitive areas, such as palmar aspects of the hands, plantar aspects of the feet, lips and buccal mucosa, all uncommon locations for MCC. Moreover, normal Merkel cells reside in epidermis, while MCC originates in the dermis and only rarely demonstrates epidermal involvement. If epidermis is involved, presence of both MCC and squamous cell carcinoma tissue is often reported. MCCs may occasionally have follicular, porocarcinoma-like, sarcomatous, glandular and neuroblastic components (4). Clonally integrated Merkel cell polyomavirus (MCPyV) has been found in approximately 80% of MCC tumors (5) and is considered as the aetiological agent of MCC. Rodig et al. (6) suggest that all MCC harbour MCPyV; however, none of more recent reports confirms these findings (7). The virus is ubiquitous; non-integrated MCPyV DNA is frequently isolated from healthy subjects. In MCPyV-related MCC tumors, MCPyV is integrated into host genome. Expression of MCPyV-derived antigens is crucial for MCC growth; knockdown of MCPyV T antigen causes cell death and cell cycle arrest in MCV-positive MCC cell lines (8). UV-radiation, thermal injury clinically evident as erythema ab igne and arsenide exposure, known to be associated with pathogenesis of skin cancers, have also been linked with pathogenesis of MCC (9,10).
Histologically, MCC appears as a so-called small round blue cell tumor consisting of densely packed sheets or nests of small to moderately sized, monomorphic cells and growing without capsule formation, frequently infiltrating the surrounding stroma. MCC is often restricted to the reticulated dermis and subcutis usually sparing papillary dermis and the epidermis. Routine haematoxylin-eosin staining may be insufficient for a definite diagnosis of MCC, as cutaneous metastases of small cell carcinoma of the lung, melanoma and lymphoma may have a similar appearance. The established markers for routine diagnosis include cytokeratin 20 (CK20) and neuroendocrine markers such as chromogranin A, neuron-specific enolase (NSE) and synaptophysin. The presence of chromogranin A, a neuropeptide precursor, as well as numerous neuropeptides themselves has been confirmed both in primary and metastatic MCC cells (11–14) supporting the hypothesis of true Merkel cell origin of MCC. However, the expression of chromogranin A, although common, is almost exclusively focal and highly variable ranging from 5% to 80% of positive cells compared with the whole tumor’s cellularity (12). While chromogranin A is used as an index of neuroendocrine cells, its production by fibroblasts has recently been reported (15). Expression of synaptophysin, considered as a highly specific marker of neuroendocrine cells, has also been reported in precursor leukaemia/lymphoma cells (16). This suggests that the expression of synaptophysin may not be as specific to neuroendocrine cells as it was initially thought. According to Godlewski et al., (11) only single cells in primary MCC stain positively for neuroendocrine marker PGP 9.5 and such neuropeptides as GAL, VIP, PACAP, NPY and CGRP, while
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ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 792–794
MCC - a lymphoma hypothesis
a high percentage of neuropeptide-positive cells is linked with MCC metastases. Expression of PGP 9.5, initially regarded as a specific marker for neural and neuroendocrine cell, has recently been shown in a subset of non-human and human lymphomas (17,18). Expression of NPY has been shown in paediatric B-cell precursor leukaemia (19). Apart from controversies on MCC origin, reported differences are evident between MCPyV-positive and MCPyV-negative MCC in morphology (20), gene expression (21–24) and miRNA profiles (25) (Table 1). Immunoglobulin heavy chains are among genes differentially expressed between MCPyV-positive and MCPyVnegative MCC (21,24). As reported by Iwasaki et al., (26) MCPyV-positive and MCPyV-negative MCC have clearly distinct morphology with high nuclear polymorphism and relatively abundant cytoplasm. Uncommon admixture of a squamous cell carcinoma is restricted exclusively to MCPyV-negative MCC (27). In the established MCC cell lines, morphological features are independent from MCPyV presence. The distinguishing features may, however, have become independent of MCPyV presence in the course of cell line establishment (28). Survival analysis revealed that the presence of MCPyV was associated with favourable MCCspecific survival and overall survival (20,29), although not all reports support this observation.
Hypothesis Based on these premises, we hypothesise that MCC may be a heterogenous entity with separate MCC types: pluripotent epidermal stem cell-derived MCPyV-negative MCC with epidermal tropism, ‘dermal’ epidermal stem cell-derived MCPyV-negative MCC and MCPyV-positive MCC possibly originating from precursor B cell, as proposed by zur Hausen et al. (24) MCC express early B-cell lineage markers such as terminal deoxynucleotidyl transferase (TdT) and B-cell-specific activating protein (PAX5) in about 80% of cases (24,30,31). Coexpression of TdT and PAX5 under physiologic circumstances is restricted to B-cell progenitors. PAX-5 expression is known for several malignancies, expression of both markers is found in those in which MCPyV presence has also been reported: MCC, SCLC (32) as well as extrapulmonary small cell cancer (33). PAX5 promoter activity is influenced in B-cell precursors by SOX2 (34). SOX2 can bind to DNA on its own, but interactions with partners such as Oct4 in other tissues have been shown to be required for activation of enhancers (35). Both SOX2 and Oct4 are expressed in MCC (36,37). Expression in MCC of a set of transcription factors (PAX-5, TdT, SOX2 and Oct4) known to act in a coordinated manner in B-cell devel-
Table 1. Features of Merkel cell carcinomas with regard to MCPyV status
Feature Morphology Coexistence with SCC Peritumoral CD8+ infiltrate IgG, IgA, IgM expression IgH locus rearrangement Mir-203 Survivin RB1 b5-Integrin expression BMI1
MCPyVpositive MCC
MCPyVnegative MCC
Uniform
Nuclear polymorphism +
+ + + Low High Normal
High Low Downregulated + +
References 20 4,20,26 21 24 24 25 51 21 23 22
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 792–794
opment points strongly to common trancsriptional phenomena in early B cell and MCC. This does not constitute a direct proof of haematopoetic lineage, and an aberrant expression of transcription factors in a cell of epithelial origin could be an alternative explanation. Pre-B cells exhibit immunoglobulins and, in addition, rearrangement of the IgH and Igk locus. Expression of immunoglobulins is common in MCPyV-positive MCC (21,24). Although only a minority of the MCCs show rearrangement of the IgH and Igk locus, this still strongly points to B-cell precursors as the cells of origin (24). The reported higher risk of developing non-Hodgkin lymphoma in MCC patients and a remarkably strong and reciprocal association between MCC and chronic lymphocytic leukaemia/small lymphocytic lymphoma (38,39) also corroborate the hypothesis of B-cell lineage origin of MCC. According to Pantulu et al. (40), in up to 27% of CLL cases, MCPyV DNA can be demonstrated. In up to 8.6% of CLL cases, MCPyV DNA harbours large T antigen truncating mutations which indicate that MCPyV is not a passenger virus in the CLL cell. Noticeably, SV40, a polyoma virus closely related to MCPyV, can induce murine CLL-like disease (41). However, causative role of immunosuppression cannot be ruled out. It is noteworthy that several cases classified of adult extranodal precursor B lymphoma mimicking MCC have been reported. Adult precursor lymphoma has little propensity for bone marrow involvement and leukaemic transformation and can present as indolent cutaneous tumors with PAX5+, TdT+, bcl-2+ and CD20immunophenotype common in MCC (42,43). Expression of cytokeratins in MCC is considered as a proof of non-hematopoietic origin of MCC. Presence of PAX-5 binding sites in CK20 promoter (44) could explain expression of CK20 in non-epithelial cells. An ability to induce cytokeratins in cytokeratin-negative cells has previously been shown in case of closely related SV40 polyomavirus (45). Positive cytokeratin staining has been reported in several types of lymphomas, also with dot-like pattern (46–49).
Conclusions As MCC simultaneously express markers typical for neuroendocrine cells and B-cell progenitors and there is high variability in intensity, pattern and percentage of marker-positive cells and a high variability in published reports with regards to antigens tested, a large cohort study is necessary to discern the exact phenotype of cells present within the MCC. Study design should include a representative number of MCC samples with SCC admixture. Use of the laser capture microdissection technique would facilitate verification of possible differences between (i) MCPyV-positive versus MCPyV-negative MCC cells with further distinction between ‘dermal’ MCC vs ‘epidermal’ MCC, (ii) chromogranin A-positive vs chromogranin A-negative and (iii) CK20positive versus CK20-negative cells. Markers of very early stages of B-cell development, before acquisition of CD19, should also be included (50). It is also conceivable that MCPyV transformation leads to a partial acquisition of B-cell transcriptional phenotype by epidermal cells. Confirmation of such a phenomenon could lead to novel therapeutic approach to MCC.
Acknowledgement MJ, PK, RS and RC analysed the data and wrote the manuscript.
Conflict of interest The authors have declared no conflicting interests.
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