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Merkel Cell Carcinoma: More Deaths but Still No Pathway to Blame

  • Bianca Lemos
    Affiliations
    University of Washington Dermatology/Medicine, Fred Hutchinson Cancer Research Center, Seattle Cancer Care Alliance, Seattle, Washington 98109, USA.
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  • Paul Nghiem
    Correspondence
    815 Mercer Street, Seattle, Washington 98109, USA
    Affiliations
    University of Washington Dermatology/Medicine, Fred Hutchinson Cancer Research Center, Seattle Cancer Care Alliance, Seattle, Washington 98109, USA.
    Search for articles by this author
      Merkel cell carcinoma (MCC) is a neuroendocrine skin cancer with a rising incidence (1500 U.S. cases per year) that now exceeds that of cutaneous T-cell lymphoma and a mortality (33%) exceeding that of melanoma. Despite this impact, little is known about its biology. Recent studies have shown that Ras/MAP kinase activity is absent and possibly detrimental to this cancer. This makes MCC distinct from other UV-induced skin cancers and highlights the question of what drives this malignancy.
      Merkel cell carcinoma (MCC) is an aggressive neuroendocrine carcinoma of the skin associated with ultraviolet exposure. Although uncommon, its incidence is increasing and has in fact tripled in the past 15 years (
      • Hodgson N.C.
      Merkel cell carcinoma: changing incidence trends.
      ). The rise in incidence is due in part to improved diagnosis through the routine use of cytokeratin-20 staining but is also due to an aging population with extensive sun exposure. The National Cancer Data Base, a registry that captures approximately 75% of all cancer cases in the United States, recorded 986 cases of MCC in 2004 (most recent data available). According to multiple national databases, there has been a 5–10% annual increase in MCC diagnoses beginning in the early 1990s (
      • Agelli M.
      • Clegg L.X.
      Epidemiology of primary Merkel cell carcinoma in the United States.
      ; National Cancer Data Base data provided by Jerri Linn Phillips). Taking these factors into account, there are now approximately 1,500 cases of MCC per year in the United States. This exceeds the incidence of cutaneous T-cell lymphoma, a well-known and frequently studied cancer, whose incidence has been stable since the early 1980s (
      • Weinstock M.A.
      • Gardstein B.
      Twenty-year trends in the reported incidence of mycosis fungoides and associated mortality.
      ). MCC is also one of the most lethal skin cancers, with a 33% mortality (
      • Hodgson N.C.
      Merkel cell carcinoma: changing incidence trends.
      )—it now kills more patients than cutaneous T-cell lymphoma and a number similar to that for chronic myelogenous leukemia (Figure 1) (;
      • Weinstock M.A.
      • Gardstein B.
      Twenty-year trends in the reported incidence of mycosis fungoides and associated mortality.
      ).
      Figure thumbnail gr1
      Figure 1Annual incidence and mortality for three cancers in the United States. MCC incidence is similar to that of CTCL, and MCC deaths are similar to those of CML. Abbreviations: MCC, Merkel cell carcinoma; CTCL, cutaneous T cell lymphoma; CML, chronic myelogenous leukemia.
      (Data from ;
      • Weinstock M.A.
      • Gardstein B.
      Twenty-year trends in the reported incidence of mycosis fungoides and associated mortality.
      .)
      Despite the mortality and increasing incidence, virtually nothing is known about the molecular basis of MCC. Several studies of oncogenic pathways have been carried out in MCC but have not elucidated a significant role for these pathways in this cancer (Table 1).
      • Van Gele M.
      • Kaghad M.
      • Leonard J.H.
      • Van Roy N.
      • Naeyaert J.M.
      • Geerts M.L.
      • et al.
      Mutation analysis of P73 and TP53 in Merkel cell carcinoma.
      revealed a lack of p53 mutations in 12 of 15 cases of MCC. Three of the five p53 mutations found in three tumors were of the signature UV type, consistent with MCC's known association with UV exposure. The lack of p53 mutations in 80% of MCCs, however, suggests that p53 is not typically involved in this cancer. The same group (
      • Van Gele M.
      • Leonard J.H.
      • Van Roy N.
      • Cook A.L.
      • De Paepe A.
      • Speleman F.
      Frequent allelic loss at 10q23 but low incidence of PTEN mutations in Merkel cell carcinoma.
      ) also looked at the role of the tumor suppressor PTEN. Although loss of heterozygosity (loss of one allele) was seen in 9 of 21 cases (mostly involving loss of the entire arm of chromosome 10), only a single mutation was seen in the remaining allele of one MCC case. Thus, inactivation of PTEN (by loss of one allele and disruption of the second) does not play a frequent role in MCC oncogenesis. It is possible, however, that loss of a second PTEN allele would be via an epigenetic silencing mechanism such as methylation, which has not been examined.
      Table 1Cancer-associated pathways and genes studied in MCC oncogenesis
      Cancer-associatedpathway/geneLikely relevantSummary of findingsReference
      p53No mutations found in 12 of 15 samples
      • Van Gele M.
      • Kaghad M.
      • Leonard J.H.
      • Van Roy N.
      • Naeyaert J.M.
      • Geerts M.L.
      • et al.
      Mutation analysis of P73 and TP53 in Merkel cell carcinoma.
      RasNo activating mutations in H-ras, K-ras, or N-ras found in six MCC cell lines
      • Popp S.
      • Waltering S.
      • Herbst C.
      • Moll I.
      • Boukamp P.
      UV-B-type mutations and chromosomal imbalances indicate common pathways for the development of Merkel and skin squamous cell carcinomas.
      B-RafV600ENo mutations in 46 MCCs
      • Houben R.
      • Michel B.
      • Vetter-Kauczok C.S.
      • Pfohler C.
      • Laetsch B.
      • Wolter M.D.
      • et al.
      Absence of classical MAP kinase pathway signalling in Merkel cell carcinoma.
      MAP kinase activityMAP kinase silenced in 42/44 MCCs
      • Houben R.
      • Michel B.
      • Vetter-Kauczok C.S.
      • Pfohler C.
      • Laetsch B.
      • Wolter M.D.
      • et al.
      Absence of classical MAP kinase pathway signalling in Merkel cell carcinoma.
      WntNo mutations in β-catenin, APC, AXIN1, or AXIN2 in 12 MCC tumors
      • Liu S.
      • Daa T.
      • Kashima K.
      • Kondoh Y.
      • Yokoyama S.
      The Wnt-signaling pathway is not implicated in tumorigenesis of Merkel cell carcinoma.
      c-KitNo activating mutations in nine MCC tumors
      • Swick B.L.
      • Ravdel L.
      • Fitzpatrick J.E.
      • Robinson W.A.
      Merkel cell carcinoma: evaluation of KIT (CD117) expression and failure to demonstrate activating mutations in the C-KIT proto-oncogene–implications for treatment with imatinib mesylate.
      PTEN?No mutations in 20 of 21 samples but loss of heterozygosity for region in 43%
      • Van Gele M.
      • Leonard J.H.
      • Van Roy N.
      • Cook A.L.
      • De Paepe A.
      • Speleman F.
      Frequent allelic loss at 10q23 but low incidence of PTEN mutations in Merkel cell carcinoma.
      bcl-2+High expression in 15 of 20 MCC tumors; bcl-2 antisense decreases tumor size in xenograft model
      • Kennedy M.M.
      • Blessing K.
      • King G.
      • Kerr K.M.
      Expression of bcl-2 and p53 in Merkel cell carcinoma. An immunohistochemical study.
      ;
      • Plettenberg A.
      • Pammer J.
      • Tschachler E.
      Merkel cells and Merkel cell carcinoma express the BCL-2 proto-oncogene.
      ;
      • Schlagbauer-Wadl H.
      • Klosner G.
      • Heere-Ress E.
      • Waltering S.
      • Moll I.
      • Wolff K.
      Bcl-2 antisense oligonucleotides (G3139) inhibit Merkel cell carcinoma growth in SCID mice.
      The Wnt pathway and its role in MCC have also been evaluated by looking for nuclear accumulation of β-catenin and for mutations in β-catenin and three other related genes. In a series of 12 MCC cases,
      • Liu S.
      • Daa T.
      • Kashima K.
      • Kondoh Y.
      • Yokoyama S.
      The Wnt-signaling pathway is not implicated in tumorigenesis of Merkel cell carcinoma.
      ) found elevated β-catenin accumulation in only one tumor and no mutations in any tumors and concluded that the Wnt pathway is not implicated in MCC. Several studies have reported on c-Kit expression in MCC tumor samples. Most recently,
      • Swick B.L.
      • Ravdel L.
      • Fitzpatrick J.E.
      • Robinson W.A.
      Merkel cell carcinoma: evaluation of KIT (CD117) expression and failure to demonstrate activating mutations in the C-KIT proto-oncogene–implications for treatment with imatinib mesylate.
      ) determined that although eight of nine MCC tumors were positive for c-Kit by immunohistochemical staining, no activating mutations were present in the four exons commonly found to have mutations in this gene.
      MAP kinase signaling, a common feature of many epithelial cancers, is the most studied oncogenic pathway in MCC (Figure 2).
      • Popp S.
      • Waltering S.
      • Herbst C.
      • Moll I.
      • Boukamp P.
      UV-B-type mutations and chromosomal imbalances indicate common pathways for the development of Merkel and skin squamous cell carcinomas.
      ) looked for activating mutations in exons 1 and 2 of the H-, K-, and N-ras genes in six MCC cell lines but found none. Recently,
      • Houben R.
      • Michel B.
      • Vetter-Kauczok C.S.
      • Pfohler C.
      • Laetsch B.
      • Wolter M.D.
      • et al.
      Absence of classical MAP kinase pathway signalling in Merkel cell carcinoma.
      ) evaluated 46 MCC tumors for the characteristic activating B-Raf mutation (V600E) found in 43% of melanomas, and all tumors were negative. Further, the authors showed that the MAP kinase pathway was silent (no phospho-ERK staining) in 42 of 44 cases, and Raf kinase inhibitor protein was increased in all 20 tumors evaluated. Taken together, the studies strongly suggest no role for MAP kinase pathway activation in MCC, a pathway heavily implicated in the etiology of melanoma, squamous cell carcinoma, and many other cancers (
      • Schubbert S.
      • Shannon K.
      • Bollag G.
      Hyperactive Ras in developmental disorders and cancer.
      ).
      Figure thumbnail gr2
      Figure 2The MAP kinase pathway. Text boxes annotate study findings of relevance in MCC. Double box indicates findings from study in this issue.
      (Adapted from
      • Houben R.
      • Michel B.
      • Vetter-Kauczok C.S.
      • Pfohler C.
      • Laetsch B.
      • Wolter M.D.
      • et al.
      Absence of classical MAP kinase pathway signalling in Merkel cell carcinoma.
      .)
      The sole potentially positive finding in MCC is that of bcl-2. Its expression was seen in 15 of 20 MCC tumors in two studies (
      • Kennedy M.M.
      • Blessing K.
      • King G.
      • Kerr K.M.
      Expression of bcl-2 and p53 in Merkel cell carcinoma. An immunohistochemical study.
      ;
      • Plettenberg A.
      • Pammer J.
      • Tschachler E.
      Merkel cells and Merkel cell carcinoma express the BCL-2 proto-oncogene.
      ). A separate study determined that decreasing bcl-2 expression in vivo by systemic antisense oligonucleotide (oblimersen/Genasense) administration in a SCID mouse/human tumor xenograft model resulted in tumor shrinkage (
      • Schlagbauer-Wadl H.
      • Klosner G.
      • Heere-Ress E.
      • Waltering S.
      • Moll I.
      • Wolff K.
      Bcl-2 antisense oligonucleotides (G3139) inhibit Merkel cell carcinoma growth in SCID mice.
      ). The expression of this anti-apoptosis protein is a common finding in many cancers and suggests one of its mechanisms to avoid cell death; however, it does not illuminate the promitotic pathways that drive MCC.
      In this issue of the Journal of Investigative Dermatology,
      • Houben R.
      • Ortmann S.
      • Schrama D.
      • Herold M.J.
      • Berberich I.
      • Reichardt H.M.
      • et al.
      Activation of the MAP kinase pathway induces apoptosis in the Merkel cell carcinoma cell line UISO.
      ) expand on their findings noted above, that the MAP kinase pathway is silent in 42 of 44 MCC tumors. Using an MCC cell line (UISO) that, like primary tumors, shows no evidence of MAP kinase activity, they tested whether activating this pathway could result in MCC cell death. Indeed, inducible expression of c-Raf-1 kinase in the UISO MCC cell line resulted in morphologic changes and apoptosis. Rounding up and detachment of cells, development of long cytoplasmic extensions, and loss of actin stress fibers were seen beginning at 10 hours after activation of the pathway. Further, they demonstrated that active Raf kinase induced apoptosis in UISO cells in a time- and dose-dependent manner. This appears to be via a classic apoptotic pathway because it was blocked by a caspase inhibitor (Z-VAD-FMK). The fact that a MEK kinase inbibitor (U0126) also blocked Raf's ability to induce apoptosis suggests that the Raf effect is indeed mediated via the predicted MAP kinase pathway (
      • Houben R.
      • Ortmann S.
      • Schrama D.
      • Herold M.J.
      • Berberich I.
      • Reichardt H.M.
      • et al.
      Activation of the MAP kinase pathway induces apoptosis in the Merkel cell carcinoma cell line UISO.
      ).
      A limitation of the study is that it was performed in a single MCC cell line. Few reagents for MCC are available to study, and of the available cell lines, only one (UISO) had preservation of the MCC in vivo characteristic of silencing of the MAP kinase pathway. The lack of cell death observed in the study from Raf overexpression in other cell lines positive for MAP kinase activity (three melanoma lines and two other MCC lines) argues for some level of specificity for the cell-death effect to cells that are silent for MAP kinase activity.
      The striking lack of knowledge of what drives MCC oncogenesis is an important impediment because no effective targeted therapies exist. Molecular therapies used in other malignancies are now being extended to clinical trials in MCC. Phase II trials of imatinib (Gleevec) and oblimersen (Genasense) are currently under way in MCC. The tyrosine kinase inhibitor imatinib, initially designed to target the Bcr–Abl translocation in chronic myelogenous leukemia, is being explored because of the c-Kit positivity of MCC. Recent data from
      • Swick B.L.
      • Ravdel L.
      • Fitzpatrick J.E.
      • Robinson W.A.
      Merkel cell carcinoma: evaluation of KIT (CD117) expression and failure to demonstrate activating mutations in the C-KIT proto-oncogene–implications for treatment with imatinib mesylate.
      ) suggest that its effectiveness may be limited, however, given that no c-Kit-activating mutations were seen in their series. The bcl-2 inhibitor oblimersen is also in a phase II trial for MCC, but no results of its efficacy in humans with MCC have been published.
      The present study by
      • Houben R.
      • Ortmann S.
      • Schrama D.
      • Herold M.J.
      • Berberich I.
      • Reichardt H.M.
      • et al.
      Activation of the MAP kinase pathway induces apoptosis in the Merkel cell carcinoma cell line UISO.
      ) provides data for the possible use of MAP kinase pathway–activating agents in MCC. Use of the Raf-1 activator ZM336372 did indeed induce ERK phosphylation in UISO MCC cells that are normally silent for MAP kinase signaling. A separate study using carcinoid tumor cells (another neuroendocrine carcinoma) demonstrated that in vitro treatment with ZM336372 inhibited cellular proliferation (
      • Van Gompel J.J.
      • Kunnimalaiyaan M.
      • Holen K.
      • Chen H.
      ZM336372, a Raf-1 activator, suppresses growth and neuroendocrine hormone levels in carcinoid tumor cells.
      ). The data suggest that pharmacologic activation of this normally silent pathway is a potential target for treatment of certain neuroendocrine tumors.
      Even though we know essentially nothing about what promitotic signaling mechanisms drive MCC, there is a glimmer of hope that recent knowledge about what does not drive it may be brought to bear in the fight against MCC and, potentially, other neuroendocrine cancers.

      Conflict of Interest

      The authors state no conflict of interest.

      ACKNOWLEDGMENTS

      This work was supported by Harvard/NCI Skin Cancer SPORE, the University of Washington Merkel cell carcinoma patient fund, and NIH K02-AR050993.

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