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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 (
). 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 (
; 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 (
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).
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 (
) 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
) 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,
) 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).
) 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 (
). 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 (
). 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,
) 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 (
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
) 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.
) 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 (
). 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.
This work was supported by Harvard/NCI Skin Cancer SPORE, the University of Washington Merkel cell carcinoma patient fund, and NIH K02-AR050993.
Epidemiology of primary Merkel cell carcinoma in the United States.