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Case ReportsAbstract
We encountered a rare case of Merkel cell carcinosarcoma and performed whole exome sequencing, yielding intriguing findings suggesting an association between the Wnt pathway and sarcomatoid transformation. Microscopic examination of the resected specimen revealed a sarcomatoid component contiguous with Merkel cell carcinoma. This component exhibited cellular proliferation with atypical spindle cells alongside cells bearing pleomorphic nuclei and abundant eosinophilic cytoplasm. Immunohistochemical analysis demonstrated complete loss of p53 and Rb expression in both Merkel cell carcinoma and sarcomatoid components, negativity for MCPyV, and retained expression of SMARCA4, SMARCB1, and ARID1A. Notably, nuclear/cytoplasmic accumulation of β-catenin was observed exclusively in the sarcomatoid component. To elucidate the genetic alterations underlying the nuclear/cytoplasmic accumulation of β-catenin in the sarcomatoid component, we conducted whole exome sequencing of both the conventional Merkel cell carcinoma and the sarcomatoid component. Gene signature analysis revealed UV signatures in both components. Commonly detected mutations included a TP53 stop gained mutation and an RB1 splicing site mutation. Additionally, APC stop gained mutations involved in Wnt pathway activation were specifically identified in the sarcomatous component. Our experience with this rare case of Merkel cell carcinosarcoma provided insights into sarcomatoid transformation, immunostaining expression patterns, and genetic alterations. This valuable case suggested that a subclone of Merkel cell carcinoma that acquired a higher malignancy potential through activation of the Wnt pathway may have contributed to the sarcomatoid transformation.