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Keywords:

  • Merkel cell carcinoma;
  • polyomavirus;
  • virus-like particles;
  • T antigen;
  • VP1

Abstract

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References

The Merkel cell polyomavirus (MCPyV), identified in humans in 2008, is associated with a relatively rare but aggressive neuroendocrine skin cancer, the Merkel cell carcinoma (MCC). MCC incidence is increasing due to the advancing age of the population, the increase in damaging sun exposure and in the number of immunocompromised individuals. MCPyV must be considered as the etiological agent of MCC and thus is the first example of a human oncogenic polyomavirus. MCPyV infection is common, and seroprevalence studies indicate that widespread exposure begins early in life. The majority of adults have anti-MCPyV antibodies and there is a growing body of evidence that healthy human skin harbors resident or transient MCPyV suggesting that MCPyV infection persists throughout life. However, the mode of transmission, the host cells, and the latency characteristics of this virus remain to be elucidated. In addition, it is still not clear whether MCPyV is associated with diseases or lesions other than Merkel cell carcinoma. The etiologic role of MCPyV in MCC opens up opportunities to improve the understanding of this cancer and to potentially improve its treatment.

Abbreviations
57k Tag

57 kilo Dalton tumor antigen

AJCC

American Joint Committee on Cancer

anti-LTag

Antibody to LTag

anti-sTag

Antibody to sTag

BKPyV

BK human polyomavirus

VP1, VP2 and VP3

Capsid proteins

CK20 and CK7

Cytokeratin 20 and cytokeratin 7

ELISA

Enzyme-linked immuno sorbent assay

HES staining

Hematoxylin erythrosine saffron staining

HPyV

Human polyomavirus

JCPyV

JC human polyomavirus

MCC

Merkel cell carcinoma

MCPyV

Merkel cell polyomavirus

LTag

Large Tumor antigen

sTag

Small Tumor antigen

Mab

Monoclonal antibody

NCCR

Non-coding control region

Ori

Origin of replication

PCR

Polymerase chain reaction

qPCR

Quantitative PCR

RCA

Rolling circle amplification

SNLB

Sentinel lymph node biopsy

IFN

Type I interferon

TTF1

Thyroid transcription factor 1

VLPs

Virus-like particles

The Merkel cell polyomavirus (MCPyV), a new human polyomavirus identified in 2008 by Feng et al. [1] is detected in the majority of Merkel cell carcinomas (MCCs) [1, 2] and was recently classified as a 2A carcinogen [3]. MCC is a relatively rare but aggressive form of cutaneous cancer, and the incidence of this cancer increases with age, immunodeficiency and sun exposure [4-6]. MCC tumor cells do not seem to produce viral particles [7, 8] and the specific cells in which MCPyV virions are produced have not been identified to date. It has also been shown that UV irradiation induces an increase in MCPyV small T antigen (sTag) transcripts that may explain the association between sun exposure, MCPyV infection, and the development of MCC [9]. As for other human polyomaviruses, primary MCPyV exposure occurs in early childhood and the majority of adults have anti-MCPyV antibodies, and there is a growing body of evidence that healthy human skin harbors resident or transient MCPyV.

MCPyV Virology and Genomic Organization

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References

MCPyV is a non-enveloped double stranded DNA virus belonging to the family Polyomaviridae, genera Orthopolyomavirus as other human polyomavirus (HPyV) species like BK polyomavirus (BKPyV) and JC polyomavirus (JCPyV). Its genome of about 5.4 kb is divided into early and late regions by a non-coding control region (NCCR) (Fig. 1). The early region encodes three proteins, large T (LTag), small T (sTag), and 57 kT (57 k Tag) antigens. Due to alternative splicing, these antigens share a common 78 amino acid sequence at their N-terminus that contains B-cell epitopes [10]. This has also been observed for BKPyV [11] and it must be noted that the expression of sT contributes to the development of antibodies against LTag. In MCC cells, mutations, insertions, and deletions within the Tag gene result in the production of truncated LTag and 57 k Tag proteins. The late region encodes three capsid proteins (VP1, VP2, and VP3). When expressed in mammalian or insect cells, VP1 proteins (or VP1 + VP2 proteins) self-assemble into 45–55 nm diameter virus-like particles (VLPs) (Fig. 2) that are used in serological assays [7, 12-14].

image

Figure 1. Genome organization of Merkel Cell Polyomavirus. The genome encodes early proteins including the LTag (purple), the sTag (blue) and the 57 k Tag (pink) and three late proteins (capsid proteins): VP1 (red), VP2 (orange) and VP3 (yellow). These two coding regions are separated by the promotor/replication origin region (Ori = replication origin).

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image

Figure 2. Electron micrograph of virus-like particles (VLPs) obtained by expression of MCPyV VP1 gene in insect cells. Scale bars, 100 nm.

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Little is known concerning the viral MCPyV replication cycle as it has not been possible to cultivate this virus. Although infectious MCPyV molecular clones induce the production of low levels of virions in different cell lines [15-17], secondary transmission to uninfected cells has not been reported. There is an orderly gene expression cascade for MCPyV, LTag, and 57 k Tag proteins being expressed first, followed by expression of sTag and then VP1 proteins [15]. MCPyV replication and encapsidation are increased by overexpression of MCPyV sTag, consistent with sTag being a limiting factor during virus replication [15]. The findings also suggested that MCPyV had restrictive cell tropism and that the late phase of MCPyV replication may be governed by cell differentiation [15-17]. It is not known whether epidermal non-transformed Merkel cells or their progenitors [18] are susceptible to infection by MCPyV.

MCPyV Epidemiology

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References

The virus is ubiquitous and MCPyV is frequently established as a persistent skin infection in healthy subjects. Detection of the viral DNA on the skin of almost all adults has suggested that most MCPyV infections occur by direct skin to skin contact or from the environment, since MCPyV DNA seems to be frequently detected on environmental surfaces [19], but the exact mode(s) of MCPyV transmission, site(s) of initial infection, and the existence of a latent phase have not yet been characterized [20, 21]. In the absence of a cell culture system, detection of current MCPyV infection is based on detection of the viral DNA by PCR amplification, and current and past exposure by the detection of specific antibodies.

Serology

Anti-MCPyV antibodies are detected by ELISA, or luminex-based multiplex serological assays using VP1 or VP1 plus VP2 VLPs produced in insect cells (Fig. 2) or 299 TT cells, and GST-VP1 recombinant protein (capsomeres) [12-14, 22, 23]. Not all tests for the detection of anti-VP1 antibodies are equivalent. Assays using BKPyV VLPs have been shown to be more sensitive in detecting anti-VP1 antibodies than assays using GST-VP1 proteins [11], although no difference between GST-capsomers and VLPs has been demonstrated for the detection of monoclonal antibodies against human papillomaviruses [24]. In addition, assays using VP1 monomers have been shown to underestimate seroprevalence for both BKPyV and MCPyV in comparison with assays using VLPs [11, 23].

Neutralization assays using MCPyV pseudovirions produced in human embryonic kidney 293TT cells have also been used [7]. There is no evidence of cross-reactivity between MCPyV with JCPyV, BKPyV, or HPyV9 [12-14, 23] and neutralization assays using MCPyV pseudovirions also confirm the specificity of the MCPyV reactivity [7].

Seroprevalence of 50–95% is reported in adults [7, 12-14, 25-29]. The age-specific seroprevalence of MCPyV (Fig. 3) indicates that exposure occurs early in life [13, 14, 22] with seroprevalence of 20–40% in children aged 1–5 years [23, 29]. In addition, MCPyV antibody titers are correlated with the viral load on the skin surface [26, 30]. Merkel cell polyomavirus is the first polyomavirus to be clearly implicated as a causal agent underlying a human cancer, Merkel cell carcinoma. Infection with MCPyV is common in the general population, and a majority of adults shed MCPyV from the surface of their skin [21]. In this study, MCPyV DNA was quantitated in skin swab specimens from healthy volunteers sampled at different anatomical sites over time periods ranging from 3 months to 4 years. The volunteers were also tested using a serological assay that detects antibodies specific for the MCPyV pseudovirion. There was a positive correlation between MCPyV virion-specific antibody titers and viral load at all anatomical sites tested (dorsal portion of the hands, forehead, and buttocks) (Spearman r = 0.644, p < 0.0001). The study results are consistent with previous findings suggesting that the skin is primary site of chronic MCPyV infection in healthy adults and suggest that the magnitude of an individual's seroresponsiveness against the MCPyV virion generally reflects the overall MCPyV DNA load across wide areas of the skin. In light of previous reports indicating a correlation between MCC and strong MCPyV-specific seroresponsiveness, this model suggests that poorly controlled chronic MCPyV infection might be a risk factor in the development of MCC. To validate whether Merkel cell polyomavirus serology correlates with MCPyV infection, MCV viral load from various skin lesions and healthy skin from 434 patients has been compared to MCV serology results using virus-like particle based ELISA and neutralization assays. Sixty-five percent of participants were MCPyV seropositive and 18% were MCPyV DNA positive [26]. The presence of antibodies was correlated with the presence of virus DNA [odds ratio, 27.85 (95% confidence interval, 6.6–166.5)], with 97% of patients who tested positive for MCPyV DNA being MCPyV seropositive [26]. High antibody levels correlated with high MCPyV load (p < 0.01) [26, 30], indicating that individuals with persistent and active skin shedding of MCPyV virions have high levels of anti-VP1 antibodies. This also suggests that the skin is the primary site where MCPyV virions are produced and the reservoir of the virus, although this does not exclude the possibility that additional sites of virus replication exist [26, 30].

image

Figure 3. Age-specific seroprevalence of MCPyV [adapted from Touzé et al. [13] and Nicol et al. [29]].

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Detection of the viral genome

Viral DNA has been detected by PCR, nested PCR, quantitative PCR (qPCR), multiplex PCR, and Rolling circle amplification (RCA) in MCC tumor tissues collected around the world. MCPyV genome sequences have been detected using different primers specific to the MCPyV LTag, sTag, and VP1 genes. These methodological differences between studies may explain the reported variations in prevalence. It is possible that the primers used may affect the detection of variant MCPyV strains. It has also been reported that formalin fixation may fragment DNA and result in differences in DNA amplification according to the size of the amplicons. This is particularly important for samples containing very low copy numbers per cell [30, 31]. Comparison of results regarding MCPyV DNA prevalence and viral load must therefore be undertaken with caution.

MCPyV DNA has been detected at varying levels of frequency (0–100%) in skin samples, and in anal and penile swabs [1, 9, 20, 21, 26, 32-38]. The MCPyV viral load on the skin surface varies from less than 1 copy per 1000 cells to 1000 copies per cell [9, 26, 30, 39-41]. The mean DNA copy per cell in normal skin has been reported to be 0.02–0.07 [42]. It has also been reported that low levels of MCPyV sequence can be amplified from many human tissues where it may undergo low-level replication or latency [1, 39, 43, 44]. MCPyV DNA has also been detected in the oral cavity, with prevalence ranging from 8% to as high as 60% [34, 35, 39]. The sensitivity of the detection of MCPyV DNA is affected by variations in the number of cells present in different samples. Numbers are generally lower in swabs compared with biopsies, but there is also variation in the percentage of tumor cells between different biopsies (10–100% of cells). However, variation in the number of cells per sample had limited impact on the determination of the number of DNA copies per cell (viral load). Although viral load is depending on the primers and housekeeping gene used, comparison of data is preferably achieved by comparing viral loads. The presence of MCPyV DNA has also been reported in monocytes of MCC patients, in the buffy coat of healthy subjects and in normal lymph nodes [36, 45, 46]. These findings suggest that MCPyV could persist in peripheral monocytes and leukocytes and that this may serve as a tissue reservoir.

MCPyV Pathology

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References

Merkel cell carcinoma (MCC) is a rare skin neoplasm whose incidence has been estimated at 0.35 per 100 000 person-years in the Netherlands [47] and 0.24 per 100 000 person-years in the USA [4], where approximately 1500 cases are identified each year [48]. Due to its rarity, few epidemiological studies are available on MCC, and data from cancer registries indicate that MCC preferentially affects whites, males, and those older than 65 years [4]. Increased incidence of MCC in immunosuppressed patients has been reported in organ transplant recipients [49] and in HIV patients [5], leading to the hypothesis of an infectious etiology for MCC [1]. MCC is a rare tumor caused by a very common viral skin infection as, in addition to loss of immune surveillance, viral integration and LTag deletion and/or mutations eliminating both T antigen replication capacity and expression of highly antigenic major capsid protein are required for MCC cell survival [50].

Merkel cell carcinoma (MCC) usually presents as an asymptomatic, rapidly growing, solitary, erythematous or violaceous, dome-shaped lump or plaque, most commonly under a smooth, sometime telangiectatic skin surface (Fig. 4). It is often located on sun exposed areas, most commonly the head and neck, followed in frequency by the limbs and the trunk [4] but it can affect any part of the body including the mucosae [51]. The acronym AEIOU summarizes the most characteristic features of MCCs: Asymptomatic, Expanding rapidly (≤3 months), Immunosuppression, Older than age 50, UV-exposed site [6]. Interestingly, skin areas that predominantly contain Merkel cells (lips, nipple, volae, genital skin) are not especially prone to develop MCC, thus raising concerns about the origin of MCC. MCPyV-positive MCCs do not seem to harbor specific clinical features, though it has occasionally been suggested that they are more often associated with female sex and located on a limb, in comparison with cases of MCPyV-negative MCC [52-54]. On diagnosis, the majority of patients have local disease (66%) followed by nodal disease (27%), and distant metastatic disease (7%) [55]. MCC can also be revealed by nodal or distant metastasis of unknown origin, with no primary skin or mucosal MCC on diagnosis [56].

image

Figure 4. Clinical appearance of Merkel cell carcinoma lesion: A 2 cm, dome-shaped Merkel cell carcinoma on the buttock of an 84-year-old woman.

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This aggressive skin neoplasm evidences a high propensity to metastasize both to lymph nodes and as distant metastases. In the majority of studies, relative five-year survival ranges between 50 and 60% [4, 55, 57-59]. Prognosis is related to the MCC staging according to the 7th American Joint Committee on Cancer (AJCC), mainly based on primary tumor size, microscopic and macroscopic lymph node involvement and distant metastases [60]. Five-year overall survival rates for AJCC stage I, II, III, and IV are 81%, 67%, 52%, and 11%, respectively [57, 61-63] (Table 1). Other factors associated with poor prognosis of MCC are summarized in Table 2. It must be noted that the prognostic value of these factors is either controversial or need to be confirmed on large prospective studies at the exception of immunosuppression.

Table 1. Prognosis and treatment for Merkel cell carcinoma (MCC) according to American Joint Committee on Cancer (AJCC) stage (based on references [57], [61-63])
AJCC stagePrognosisTreatment
 OS 2 yearsaOS 5 yearsbWide local excision (2 cm)Lymph node dissectionRadiotherapy (tumoral site)Radiotherapy (lymph node site)Chemotherapy
  1. a

    Overall survival at 2 years.

  2. b

    Overall survival at 5 years.

Size < 2 cm (stage I)67%81%++
Size > 2 cm (stage II)59%67%++±
Nodal disease (stage III)49%52%++++
Systemic metastases (stage IV)23%11%±±±±+
Table 2. (A) Factors associated with poor outcome (recognized factors are in bold), (B) controversial factors associated with poor outcome. Most factors have to be confirmed on larger prospective studies as being independently associated with poor prognosis
Clinical factors Histological and immunohistochemistry factors Biological and Virological factors
(A)
-AJCC stage based on tumor size and presence of metastases [4, 47, 55, 64-68]Lymphovascular infiltration [58, 71, 72]No or low anti-VP1 titers [8]
-Immunosuppression [65, 69, 70]

Low tumor infiltrating lymphocytes [64, 71, 73]

Low tumor infiltrating CD8 + T cells [64]

Seroconversion or persistance of Anti-LTag antibodies [76]
 

P53 expression and TP53 gene mutations [54, 68]

P63 expression and TP63 gene mutations [68, 74]

Ki67 index > 50% [68, 75]

Vitamin D deficiency [70]
(B)
Male sex [4, 8, 47, 59, 64-67, 70]Infiltrative growth pattern (vs nodular) [68, 71, 77]Low intra-tumoral viral load [53, 54, 67, 79-84]
Older age (>70 years) [4, 64-66]Depth of invasion [54, 67, 68, 71, 77, 78]No or low levels of LTag intra-tumoral expression [54, 79, 85]
Head and neck location [4, 66, 86]  
History of secondary malignancy [65, 66, 86]  

The origin of MCC is still controversial [87]. It has been mainly thought to derive from the resident Merkel cells distributed in the basal layer of the epidermis and associated with nerve fiber endings involved in skin mechanoreception properties. Indeed, MCC and Merkel cells share common ultrastructural features, namely intracytoplasmic neurosecretory dense core granules reflecting their neuroendocrine differentiation. A more recent theory is that MCC might derive from epidermal stem cells located in the basal layer of the epidermis or in hair follicles displaying the capacity for both neuroendocrine and epidermoid differentiation [88]. However, this hypothesis is not supported by the dermal location of MCC with no connection with the epidermis. Identifying the natural tropism of the MCPyV would probably assist the understanding of the natural history and the cell origin of MCC.

MCPyV viral markers in MCC patients

MCPyV DNA has been detected in 59–100% of MCCs [1, 8, 21, 25, 32, 53, 82, 89-94]. Viral loads of 0.1 to thousands of copies per cell have been observed in about 80% of MCCs. The mean viral load has been reported to be 1–5 copies per cell, in agreement with integration of the viral genome into chromosomal DNA [1, 90, 93, 94].

Anti-VP1 antibodies have been detected in 88–100% of MCC cases [7, 8, 14, 25]. In addition, the capsid antibody titer in many of these MCC cases is much higher than observed in control populations. The anti-VP1 geometric mean titer in MCC patients is 14–59 times higher than in controls [7, 8]. One explanation is that loss of immune surveillance in some patients enhances active replication of the virus and, as a consequence, the recombination of virus genome fragments with the host genome. [50]. This could also explain why most MCC patients have high anti-VP1 antibody titers as it was shown that individuals with persistent and active skin shedding of MCPyV have high levels of anti-VP1 antibodies [26, 30].

An assay for the detection of antibodies against LTag has also been developed using GST recombinant proteins as antigens [76]. Anti-MCPyV LTag antibodies are rarely detected in the general population, but are detected in around 40% of MCC patients. The detection of anti-LTag appears to be a useful marker to confirm the involvement of MCV in MCC tumors and, more importantly, in monitoring tumor progression, as anti-LTag antibodies have been found to disappear or decrease after tumor removal, and then reappear or increase in cases of recurrence or progression of the disease [76].

MCC histopathology and detection of MCPyV antigens in MCC

Merkel cell carcinoma (MCC) appears on pathology sections as a small, round, tumor usually located in the dermis with various depth extensions into the underlying tissues, and separated from the epidermis by a ‘grenz’ zone. On hematoxylin erythrosine saffron (HES) staining, the tumor cells had monomorphous morphology (Fig. 5), they were hyperbasophilic with a scant cytoplasm and round hyperchromatic nuclei, and can display a solid, trabecular or diffuse pattern of growth [95]. Positive diagnosis of MCC is based on immunohistochemistry, as MCC expresses both neuroendocrine and epithelial markers. MCC tumor cells display paranuclear dot-like staining with cytokeratin 20 and absence of staining with cytokeratin 7 (CK20+/CK7−), although very rare variants with CK20−/CK7+ expression have been reported [96]. Positive staining with a neuroendocrine marker (synaptophysin or chromogranin A) is therefore a clue for diagnosis. Negativity for thyroid transcription factor-1 (TTF-1) allows differential diagnosis from cutaneous metastases of neuroendocrine carcinomas of the lung [95].

image

Figure 5. Histological features of Merkel cell carcinoma. Hematoxylin erythrosine saffron (HES) staining: Dermal proliferation of blue cells separated from the epidermis by a ‘grenz’ zone (panel A), monomorphous hyperbasophilic cells with high mitotic rates (panel B). Positive immunostaining with CK20 antibody (panel C) with a paranuclear dot like pattern (panel D). Positive nuclear immunostaining for MCPyV LTag in a MCPyV -positive Merkel cell carcinoma (MCC) (panel E).

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Immunodetection of MCPyV antigens on MCC tissues could be used as diagnostic markers. Interestingly, the VP1 capsid protein is not detected in MCC tumor cells [7, 8], in agreement with the absence of viral replication in MCC [10]. On the other hand, using the CM2B4 monoclonal antibody [91], MCPyV LTag is detected (Fig. 5) in the nuclei of 58–96% of MCCs [82, 91, 97-103]. According to these studies, immunodetection of LTag expression is found to be less sensitive but more specific than detection of MCPyV DNA by PCR. Absence of LTag expression seems to be correlated with the absence of detection of MCPyV DNA [101], supporting the hypothesis of the existence of a subset of MCPyV -negative MCCs. CM2B4 immunostaining also appears to be a reliable tool for the positive diagnosis of MCC in daily practice when the tumor displays an unusual differentiation pattern [104] and might be helpful for pathologists in the diagnosis of MCC in the future. One case of basal cell carcinoma positive for both moderate levels of MCPyV DNA and strong LTag expression was recently reported, raising the question of specificity of these markers or the role of MCPyV in the pathogenesis of at least certain other tumors [42]. Another monoclonal antibody generated against a specific epitope of the sTag antigen has been found to be more sensitive than CM2B4 antibody, with 92% of MCCs positive for sTag expression compared with 75% positive for LTag [105]. However, using another Mab directed against the same LTag sequence as CM2B4 Mab, MCPyV LTag expression was recently reported in 96% of 58 tumors [106].

MCPyV status and MCC prognosis

Factors recognized as associated with poor prognosis are size of the tumor and presence of metastases according to the AJCC staging [4, 47, 55, 64-68], and immunosuppression [65, 69, 70]. Recently, histological and immunochemistry markers have been described as potentially associated with poor outcome like low tumor infiltrating CD8 + T cells [101] and P53 and P63 expression and gen mutations [54, 68, 74] (Table 2). However, these factors have to be confirmed as independently associated with poor outcome in larger prospective studies. In addition, other controversial clinical factors such as male sex, head and neck location, and history of secondary malignancy, and controversial histology factors such as depth of invasion and infiltrative growth pattern have also been advanced. Recently, Vitamin D deficiency has also been associated with a poor prognosis [70].

Whether the MCPyV status influences MCC outcome is still a matter of debate, although a range of findings argues for a more favorable prognosis of MCPyV-positive MCCs (Table 2). Detection of MCPyV DNA in MCC tumor samples was not found to be associated with a better MCC outcome in three studies [82-84], whereas high viral levels of DNA copies in MCC tumors seemed to be a favorable prognostic factor in three others [53, 80, 81]. Such differences might reflect differences in DNA detection techniques, tissue heterogeneity, and type of material investigated (frozen vs paraffin embedded). Similarly, there is a controversy regarding the prognostic value of LTag detection using CM2B4 antibody. Detection of LTag expression together with high MCPyV DNA levels has been found to be associated with better prognosis in two studies [54, 79], whereas LTag expression did not influence MCC outcome in another [85]. Serological investigations have also suggested that patients with higher anti-VP1 antibody titers have a better prognosis [8]. On the contrary, persistence or reappearance of anti-LTag antibodies has been associated with poor prognosis [76].

MCPyV and B-cell proliferative disorders

An epidemiological association between MCC and B-cell lymphoproliferative disorders has been reported [107, 108], leading to the hypothesis that immunosuppression induced by these hematological malignancies promotes the development of MCC. Another hypothesis for this association is the possible involvement of MCPyV in the pathogenesis of B-cell lymphoproliferative disorders. Lymphocytes may be a reservoir for MCPyV [46]. Detection of various levels of MCPyV DNA in B-cell lymphoproliferative disorders [46, 91] is not a valid argument for an etiological link, as MCPyV is also detected in a wide range of control tissues. However, clonal integration and truncated mutation of MCPyV DNA, a molecular signature of MCPyV oncogenesis, were evidenced in a subset of patients with chronic lymphocytic leukemia [109]. Whether MCPyV has a role in the pathogenesis of a subset of B-cell lymphoproliferative disorders remains to be demonstrated.

Therapeutic implications

Conventional treatment of MCC depends on disease stage (Table 1). Appropriate staging is based on clinical examination, especially looking for satellite cutaneous metastases, palpable lymph nodes, and any sign of distant metastases. Appropriate imaging, including regional lymph node sonography or whole body CT scan is performed based on clinical findings. When there are no palpable regional lymph nodes, a sentinel lymph node biopsy is necessary as the presence of micrometastases has been reported to be associated with poorer outcome [55]. Overall, these findings allow appropriate disease staging according to the American Joint Committee on Cancer (AJCC classification for therapeutic management). In addition, it has been suggested that detection of MCPyV DNA in a histologically normal sentinel lymph node could be considered a risk factor for regional relapse [110], but this remains to be demonstrated.

Surgical removal with wide margins (2–3 cm) remains the gold standard for treatment of localized tumors. Microscopic as well as macroscopic involvement of regional lymph nodes requires lymph node dissection. Adjuvant radiation therapy of the tumor site and, if involved, of the regional lymph node site after surgery has been found to be associated with decreased risk of locoregional relapse, with no benefit demonstrated on overall survival [51, 111-113]. For unresectable tumors, radiation alone is the recommended option, as MCC appears to be a relatively radiosensitive tumor [114]. For metastatic disease, various regimens of chemotherapy including anthracyclines, cyclophosphamide, etoposide, and platinum derivatives, alone or in combination have been reported. Response to chemotherapy is observed in about 60% of MCC patients, but prognosis remains poor, with a median overall survival of 21.5 months [115].

Before the discovery of MCPyV, the molecular pathogenesis of MCC was unclear and this has resulted in a lack of MCC-specific targeted therapies. Identification of MCPyV and progressive understanding of its involvement in the pathogenesis of MCC, like the inhibition of apoptosis regulated by proteins such as bcl-2 and survivin, has led to new therapeutic possibilities.

Drugs inhibiting MCPyV-positive MCC cell lines have been identified in vitro. For instance, type I interferon (IFN) was found to reduce expression of MCPyV LTag, in association with apoptosis of MCPyV-positive MCC cell lines [116]. However, no clinical response was reported after IFN treatment of two patients with advanced MCC [117]. Another drug, YM155, a selective inhibitor of survivin, a protein reported to play a critical role in the survival of MCPyV -positive MCC cell lines, was found to induce cell death in MCPyV -positive cells and at least a cytostatic effect on MCC xenograft tumors in mice [118]. As expression of MCPyV T antigens has been found to be necessary for survival of MCPyV -positive MCC cell lines [2, 105], T antigens also appear to be a potential therapeutic target for MCPyV -positive MCC management. Specific T-cell lymphocytes recognizing various epitopes of MCPyV have been identified, especially within the tumor, and also in the blood of some MCC patients [100]. The persistence of tumor growth despite development of these specific T cell responses has suggested that immune evasion mechanisms, namely inhibition of the T cell cytotoxic effects or promotion of T-regulatory lymphocytes, remain to be identified. Nevertheless, using a mouse model of MCC, it was recently reported that DNA vaccine encoding the first 258 aminoacids of LT antigen generates antitumor effects mainly mediated by CD4+ T cells against LT expressing tumors [119], and this may represent an additional approach for the control of MCC.

Conclusions

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References

MCPyV infection is common, and age-specific seroprevalence studies indicate that widespread exposure begins early in life. The detection of MCPyV DNA on the skin surface of most adults suggests that MCPyV infection persists throughout life. However, the mode of transmission, the host cells, and the latency characteristics of this virus remain to be elucidated.

MCPyV is the etiological agent of MCC and thus is the first example of a human oncogenic polyomavirus. MCC is a rare neuroendocrine skin cancer with increasing incidence due to the advancing age of the population, and the increase in damaging sun exposure and in the number of immunocompromised individuals. In addition, it is still not clear whether MCPyV is associated with diseases or lesions other than Merkel cell carcinoma. The etiologic role of MCPyV in MCC opens up opportunities to improve the understanding of this cancer and to potentially improve its treatment.

References

  1. Top of page
  2. Abstract
  3. MCPyV Virology and Genomic Organization
  4. MCPyV Epidemiology
  5. MCPyV Pathology
  6. Conclusions
  7. References
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