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Merkel cell carcinoma (MCC) is an aggressiveskin cancer occurring in about three thousand persons per year in the United States.[1] It is also known as cutaneous APUDoma,primaryneuroendocrine carcinoma of the skin, primary small cell carcinoma of the skin, and trabecular carcinoma of the skin.[2] Factors involved in the development of MCC include theMerkel cell polyomavirus (MCPyV or MCV), a weakened immune system, or exposure toultraviolet radiation.[3] Merkel cell carcinoma usually arises on the head, neck, and extremities, as well as in the perianal region and on the eyelid.[4] It ‘most often presents in an older white population with a male predominance'; 'median age of presentation falls within the late seventh decade, despite a broad age range (fourth to tenth decades).’[5] MCC is less common in children.[1][4]
Small spot on the left arm is Merkel cell cancer.Merkel cell carcinoma. Gross pathology specimen.
Merkel cell carcinoma (MCC) usually presents as a firmnodule (up to 2 cm diameter) or mass (>2 cm diameter). These flesh-colored, red, or bluetumors typically vary in size from 0.5 cm (less than one-quarter of an inch) to more than 5 cm (2 inches) in diameter and may enlarge rapidly. Tumors can present as painless, tender or itchy, and other MCC manifestations aspapules or plaques have also been reported.[6] Although MCC may arise almost anywhere on the body, it is most commonly found in sun-exposed areas such as the head, neck, or extremities.[7] Five key attributes of MCC were summarized in 2008 in theacronym AEIOU (Asymptomatic/lack of tenderness, Expanding rapidly, Immune suppression, Older than 50 years, and Ultraviolet-exposed site on a person with fair skin).[8] Ninety percent of MCC's have three or more of those features.[9]MCC is sometimes mistaken for otherhistological types of cancer, includingbasal cell carcinoma,squamous cell carcinoma, melanoma,lymphoma, andsmall cell carcinoma, or as a benigncyst.[10] Merkel cell carcinomas have been described in children, but pediatric cases are very rare.[11]
Merkel cell cancers tend to invade locally, infiltrating the underlyingsubcutaneous fat,fascia, and muscle, and typicallymetastasize early in their natural history, most often to the regional lymph nodes. MCCs also spread aggressively through the blood vessels to many organs, particularly to the liver, lung, brain, and bone.[12]
Although MCC was initially named for theMerkel cell due to histologic and physiologic similarities between MCC and Merkel cells, the cellular progenitor of MCC has been a heavily debated question. Merkel cells are highly specialized cells that act as pressure receptors in the epidermis. The origin of Merkel cells themselves is debated and proposed to be derived from neural crest cells or epidermal progenitors.[14] MCC is similar to Merkel cells in its histological appearance (see below: Diagnosis) and shares manyimmunohistochemical markers with Merkel cells, including epidermal markercytokeratin 20 and neuroendocrine markerssynaptophysin andchromogranin A. Furthermore, the ion channelPiezo2 and transcription factorAtoh1, both specific to Merkel cells, are also expressed by MCC.[3] However, Merkel cells are post-mitotic cells with a low probability of cancerous transformation.[14] Additionally, they have not been shown to support Merkel cell polyoma virus infection, which is believed to drive oncogenesis in approximately 80% of MCC.[15]
Instead, it has been proposed the MCC may originate from a Merkel cell precursor, at which point it gains features similar to those of Merkel cells. One such precursor is the human fibroblast. Evidence for a fibroblast precursor includes its location in thedermis, which is thought to be the primary site of origin for MCC. Additionally,in vitro experiments have demonstrated that fibroblasts not only supportMerkel cell polyomavirus (MCV) infection but can be induced into having a MCC phenotype by the expression of viral proteins.[15][16]
However, others have argued that MCC likely derives from an epithelial precursor cell due to its frequent presence in mixed tumors including epithelial neoplasms such as squamous cell carcinoma. While epithelial cells are not typically found in the dermis, hair follicles include epithelial cells that have been shown to have oncogenic potential, and have therefore been proposed as a possible site for a MCC precursor.[3][17]
Finally, the presence of B-cell surface markers on MCC in addition to the high correlation between MCC and B-cell lymphomatous cancers have also led to suggestions that MCC may share a progenitor with B-cells.[3][18] Because of the differences in physiology and prognosis between MCV+ and MCV- MCC (see below), however, some have suggested that these two subtypes of MCC may derive from different progenitor cells.[19]
Several factors are involved in the pathophysiology of MCC, including MCV, ultraviolet radiation (UV) exposure, and weakened immune function.[20]
TheMCV is a small double-stranded DNA virus that is believed to contribute to the development of the majority of MCC.[21] About 80% of MCC tumors are infected with MCV, with the virus integrated into the host genome in amonoclonal pattern.[21] However, the majority of people with MCV infection do not develop MCC: MCV is a ubiquitous virus and infection commonly occurs during childhood but remains asymptomatic throughout an individual's lifetime.[16]
MCC was first believed to be associated with MCV when it was observed to occur at a much higher rate in HIV patients during the 1980s.[22] Since then, studies have demonstrated integration of the MCV genome into the genome of MCC tumor cells. Central to the understanding of the pathogenicity of MCV are two viral proteins expressed in infected cells known as thelarge tumor antigen (LT) andsmall tumor antigen (sT).[23] Normally, patients infected with MCV show low levels of antibodies to the LT protein, perhaps due to a nuclear localization domain in its C-terminal that limits its cellular dispersion. However, integration of the viral genome into the host genome can result in truncation of the LT protein proximal to this domain. This serves two oncogenic purposes: first, it prevents successful viral replication that would culminate in the lysis of the infected cell. Second, it redistributes the LT protein to the cytoplasm, where it can interact with cytoplasmic signaling.[24] The N-terminal LXCXE motif of the LT protein has been shown to interact with known oncogeneRb and is conserved in other cancer-causing viruses.[24] Studies suggest that LT may also preserve cell proliferation signals such as c-Myc and cyclin E and cause DNA injury to thep53 tumor suppressor.[15][16]
Meanwhile, sT has been shown to induce cell proliferation through hyper-phosphorylation of the translation initiator4EBP1 as well as the inhibition of a ubiquitin ligase complex responsible for the degradation of cellular proliferation signals. sT also contains a region known as the LT stabilization domain (LSD), which potentiates the LT protein's oncogenic function. Unlike LT, MCC samples have been identified that express sT alone, and sT expression in fibroblasts has been shown to cause MCC phenotype development.[15][16]
About 20% of MCC tumors are MCV negative.[7] In contrast to MCV-induced MCC, these tumors tend to have much higher mutational burdens with mutational signatures characteristic of UV damage.[15] Genes frequently mutated in MCV-negative MCC include p53 and Rb, among others.[23] The link between MCC and UV exposure has been demonstrated through various epidemiological studies indicating a higher incidence of MCC in fair-skinned people in areas of high UV exposure, as well as among those receiving UV phototherapy.[6] The typical distribution of MCC in sun-exposed regions and its co-occurrence with otherskin cancers also indicate that UV exposure is a contributing factor to MCC development. It is unclear whether this is through direct mutational impact, immune down-regulation, or some combination of the two.[6][10]
The incidence of MCC is increased in conditions with defective immune functions such as malignancy,HIV infection, and organ transplant patients, etc.[6] Conversely, patients with brisk immune response have been shown to have improved prognoses.[25] This is suspected to be due to the inability of the body to defend itself from infection by or reactivation of MVC.[26] The body of data indicating the importance of immune function in MCC pathogenesis has been exploited for the development of immunotherapies discussed below.
Photomicrographs of a typical Merkel cell carcinoma at a) 4×, b) 40×, and c–d 100× magnification. Hematoxylin and eosin staining demonstrates small, undifferentiated cells with a high N/C ratio and scanty cytoplasm. Typical immunopanel demonstrates positive staining with e cytokeratin AE1/AE3 (100× oil immersion), f CK 20 (100× oil immersion), and neuroendocrine markers such as g chromogranin (100× oil immersion).[27]
Diagnosis of MCC begins with a clinical examination of the skin andlymph nodes to determine suspicious areas for further investigation.[28] Definitive diagnosis requires examination ofbiopsy tissue to identify itshistopathologic features.[6][28] An ideal biopsy specimen is either apunch biopsy or a full-thickness incisional biopsy of the skin including full-thicknessdermis andsubcutaneous fat. Onlight microscopy, MCC shows basaloid tumor nests with neuroendocrine features ("salt and pepper" chromatin, scarce cytoplasm, and brisk mitotic activity).[6][28] In addition to standard examination under light microscopy,immunohistochemistry (IHC) is also generally required to differentiate MCC from other morphologically similar tumors such as skin metastases ofsmall cell lung cancer, the small cell variant ofmelanoma, various cutaneous leukemic/lymphoid neoplasms, andEwing's sarcoma. Neuroendocrine molecular markers such assynaptophysin orchromogranin A are characteristic of MCC and other neuroendocrine tumors, while other markers such asPAX5 orcytokeratin 20 can distinguish MCC from these tumors.[3][7] Longitudinal imaging may also help in ruling out a diagnosis of metastatic small cell lung cancer. Once an MCC diagnosis is made, asentinel lymph node biopsy as well as imaging are recommended as a part of the staging process, which determinesprognosis and subsequent treatment options.[6][28] Imaging may includepositron emission tomography or CT scan.[29]
Merkel cell carcinoma is typically treated with surgery and radiation; immunotherapy is used in advanced disease. If an affected person is being treated withimmunosuppressive drugs, they are reduced as much as possible.[29]
Surgical resection is typically performed before other treatments, if possible. The type of surgery used may be a simple wide excision or a more specialized technique likeMohs surgery, depending on the individual. Part of the staging process –sentinel lymph node biopsy – is often performed at the same time.[29]
Radiation therapy is the primary management of Merkel cell carcinoma (MCC). The largest studies to date, from Australia, demonstrated that radiotherapy alone achieves equal outcomes with upfront or neoadjuvant surgery followed by radiation therapy.[33][34] The role of surgery is largely historical and relegated to biopsy. There have been no head-to-head trials comparing the two treatment strategies, and in the absence of this, surgeons tend to most commonly perform excisional biopsy prior to referring for radiation therapy to eradicate MCC. MCC is exquisitely radiosensitive.[35] The conclusion amongst published studies in the Radiation Oncology community is that MCC should be managed ideally with radiation therapy alone.[34]
Because of its significant adverse effects, traditional chemotherapy has been saved for late-stage highly metastasized cases of MCC. While some chemotherapeutic regimens have been shown to have transient effects, studies have not found any significant long-term effect on recurrence rate or life expectancy.[15] As of 2015, there were no FDA-approved standard chemotherapy regimens for MCC treatment.[25] The most recent American guidelines do not recommend adjuvant chemotherapy, citing a lack of evidence to suggest improved outcomes. Instead, consideration of the need for chemotherapy on a case-by-case basis is recommended.[36]
Immunotherapies, namelyinhibitors of the PD1-PDL1 checkpoint signaling pathway, are novel anticancer agents that have shown benefit in advanced-stage MCC or chemotherapy-resistant MCC.[37] ThePD-1 pathway is responsible for regulating the balance betweenT-cell activation and over-activation leading toT-cell exhaustion or autoimmunity.[38] However, over-expression of PD-1 ligands (PDL1) have been observed in tumors as a method of evading immune attack.[39] PD-1 inhibition therefore enhances the body's immune response, enabling it to target cancer cells for destruction.[40] Due to their side effects, however,National Comprehensive Cancer Network guidelines recommendPD-1 inhibitors for people with disseminated rather than early-stage MCC.[9]
PD1/PDL1 pathway inhibitors approved or in clinical trials for use in MCC treatment include:
In March 2017, the U.S. Food and Drug Administration granted accelerated approval toavelumab,a PDL1 inhibitor, to treat adults and children above 12 years with metastatic MCC.[41]
In December 2018, the U.S. Food and Drug Administration granted accelerated approval topembrolizumab (KEYTRUDA, Merck & Co. Inc.) for all ages (adults and pediatrics) with recurrent locally advanced or metastatic Merkel cell carcinoma[42]
Nivolumab (brand name Opdivo, Bristol-Myers Squibb) is in phase III/IV clinical trials[23][43]
Ipilimumab (brand name Yervoy, Bristol-Myers Squibb) is in phase II clinical trials for use in adults with metastatic MCC.[44][43]
Studies to date have shown a clinical response rate between 50 and 65% for MCC treated with PD-1 pathway inhibitors. Suggestions for further immunotherapy research areas have includedtherapeutic vaccines or epigenetic modification ofHLA-receptors.[23][24][25][37]
According to theAmerican Joint Committee on Cancer (AJCC), the natural course of MCC is "variable and heavily dependent on the stage at diagnosis".[45] Staging of MCC is classified according to theTNM staging system, a notation system that describes thestage of cancer according to the size of theprimary tumor (T), the degree of spread to regionallymph nodes (N), and the presence of distantmetastasis (M).[45] A combination of T, N, and M stages dictate the final clinical stage group (0, I, IIA, IIB, IIIA, IIIB, IV).[46] Advanced stage (i.e. increased size of the tumor, spreading of the tumor into surrounding and/or distanttissue, and involvement of lymph nodes) is associated with lower survival rates.[7]
The National Cancer Data Base has survival rates collected from nearly 3000 MCC patients from year 1996–2000 with 5-year survival rates listed as follows:[47]Stage IA: 80%.Stage IB: 60%.Stage IIA: 60%.Stage IIB: 50%.Stage IIC: 50%.Stage IIIA: 45%.Stage IIIB: 25%.Stage IV: 20%.Five-year survival may be 51% among people with localized disease, 35% for those with nodal disease, and 14% with metastases to a distant site.[9]
Several other features may also affect prognosis, independent of tumor stage. They include MCV viral status,histological features, and immune status. In viral status, MCV large tumor antigen (LT antigen) andretinoblastoma protein (RB protein) expression correlates with a more favorable prognosis, whilep63 expression correlates with a poorer prognosis.[48][49] Histological features such as intratumoralCD8+ T lymphocyte infiltration may be associated with a favorable prognosis, while lymphovascular infiltrative pattern may be associated with a poorer prognosis.[50][51] Immunosuppressed status, especially T cell immunosuppression (e.g., organ transplant, HIV infection, certain malignancy) predicts poorer prognosis and higher mortality.[52] Immunosuppression also raises the risk of recurrence.[29]
Theantibody titer in the blood to the Merkel cell polyomavirus oncoprotein can be used as a treatment responsebiomarker in people who have detectable antibodies at the time of diagnosis.[53][54]
Merkel cell carcinoma occurs most often inwhite men between 60 and 80 years of age. Itsincidence is about twice as high in males as in females.[55] It is a rare type of skin cancer, with a 2013 incidence of only 0.7 per 100,000 persons in the U.S.[56] As of 2005, roughly 2,500 new cases of MCC are diagnosed each year in theUnited States,[56] as compared to around 60,000 new cases ofmelanoma and over 1 million new cases ofnonmelanoma skin cancer.[57] Similar to melanoma, the incidence of MCC in the US is increasing rapidly.[10] Worldwide, MCC is most commonly found in regions with increased sun exposure. Australia is the country with the highest incidence of MCC[55] but has a lower incidence of MCV-positive MCC than observed in other countries.[58]
Since 2006, it has been known that other primary cancers increase the risk of MCC significantly, especially in those with prior multiple myeloma,chronic lymphocytic leukemia, and melanoma.[59]Immunosuppression including HIV infection or immunosuppressant therapy followingorgan transplant or forautoimmune disease can also increase the odds of developing MCC.[20]
Friedrich Sigmund Merkel (1845–1919) was a German anatomist and histopathologist who first described theTastzellen (touch cells) in the skin in 1875.[60] In 1878 the termMerkel cell was coined by the anatomist Robert Bonnet (1851–1921).
Merkel cell carcinoma was first described in 1972 by Cyril Toker.[61] He reported five cases of 'trabecular carcinoma of the skin'.
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^abcdefMacDonald, Margo; You, Jianxin (2017). "Merkel Cell Polyomavirus: A New DNA Virus Associated with Human Cancer".Infectious Agents Associated Cancers: Epidemiology and Molecular Biology. Advances in Experimental Medicine and Biology. Vol. 1018. pp. 35–56.doi:10.1007/978-981-10-5765-6_4.ISBN978-981-10-5764-9.PMID29052131.
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^Bichakjian, Christopher K.; Olencki, Thomas; Aasi, Sumaira Z.; Alam, Murad; Andersen, James S.; Blitzblau, Rachel; Bowen, Glen M.; Contreras, Carlo M.; Daniels, Gregory A.; Decker, Roy; Farma, Jeffrey M.; Fisher, Kris; Gastman, Brian; Ghosh, Karthik; Grekin, Roy C.; Grossman, Kenneth; Ho, Alan L.; Lewis, Karl D.; Loss, Manisha; Lydiatt, Daniel D.; Messina, Jane; Nehal, Kishwer S.; Nghiem, Paul; Puzanov, Igor; Schmults, Chrysalyne D.; Shaha, Ashok R.; Thomas, Valencia; Xu, Yaohui G.; Zic, John A.; Hoffmann, Karin G.; Engh, Anita M. (11 June 2018)."Merkel Cell Carcinoma, Version 1.2018, NCCN Clinical Practice Guidelines in Oncology".Journal of the National Comprehensive Cancer Network.16 (6):742–774.doi:10.6004/jnccn.2018.0055.PMC9440150.PMID29891526.
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