Merkel Cell Tumor in a Trichilemmal Cyst: Collision or Association?

Detection of wildtype Merkel cell polyomavirus genomic sequence and VP1 transcription in a subset of Merkel cell carcinoma

AIMS

Merkel cell carcinoma (MCC) is frequently caused by the Merkel cell polyomavirus (MCPyV). Characteristic for these virus-positive (VP) MCC is MCPyV integration into the host genome and truncation of the viral oncogene Large T antigen (LT), with full-length LT expression considered as incompatible with MCC growth. Genetic analysis of a VP-MCC/trichoblastoma combined tumour demonstrated that virus-driven MCC can arise from an epithelial cell. Here we describe two further cases of VP-MCC combined with an adnexal tumour, i.e. one trichoblastoma and one poroma.

METHODS AND RESULTS

Whole-genome sequencing of MCC/trichoblastoma again provided evidence of a trichoblastoma-derived MCC. Although an MCC-typical LT-truncating mutation was detected, we could not determine an integration site and we additionally detected a wildtype sequence encoding full-length LT. Similarly, Sanger sequencing of the combined MCC/poroma revealed coding sequences for both truncated and full-length LT. Moreover, in situ RNA hybridization demonstrated expression of a late region mRNA encoding the viral capsid protein VP1 in both combined as well as in a few cases of pure MCC.

CONCLUSION

The data presented here suggest the presence of wildtype MCPyV genomes and VP1 transcription in a subset of MCC.

Morphologic Diversity of Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare and highly aggressive neuroendocrine carcinoma of unknown origin. We performed a retrospective histologic review of primary cutaneous MCCs diagnosed from 1997 to 2018 in several clinical institutions and literature review to determine the frequency of various unusual morphologic appearances of MCC. Of the 136 primary MCCs identified, intraepidermal carcinoma or epidermotropism was noted in 11/136 (8%) cases. An association with pilar cyst in 1/136 (0.7%) case, with actinic keratosis in 2/136 (1.5%) cases, with either invasive or in situ squamous cell carcinoma (SCC) in 14/136 (10%) cases, with poroma in 1/136 (0.7%), and with basal cell carcinoma in 1/136 (0.7%) case was noted. Trabecular pattern and rosettes were noted in 7/136 (5%) and 3/136 (2%) cases, respectively. There was one case of metastatic MCC in a lymph node with chronic lymphocytic leukemia and one rare case of metastatic MCC and SCC in a lymph node. Although uncommon, differentiation toward other cell lineage can be observed in both primary and metastatic MCCs. The tumor can assume a variety of histologic appearances including association with SCC, basal cell carcinoma, melanocytic neoplasm, and follicular cyst; as well as exhibit glandular, sarcomatous, and mesenchymal differentiation. This diversity of morphologic appearance of MCC reflects the complexity of its underlying pathogenesis.

Incidental Finding of Benign Trichilemmal Cysts on 99mTc-Octreotide Scan

A 63-year-old woman with neuroendocrine carcinoma of the colon was referred to our department for staging with Tc-octreotide scan. Two focal, highly intense, superficial lesions were detected in patient's scalp and paralumbar region, medial to the right kidney. Pathology correlation was recommended, revealing them both to be benign trichilemmal cysts. This unexpected finding strengthens the hypothesis of Merkel cell carcinoma and trichilemmal cysts originating from pluripotent stem cells of adnexal epithelium, making a neuroendocrine differentiation for the latter plausible.

Trichilemmal cyst with carcinoma in situ within an atypical fibroxanthoma

A 91-year-old lady presented with an 8-month history of a slow growing keratotic nodule on the forehead, which was excised. Histological examination showed a well-defined lesion with marked cellular pleomorphism. Numerous mitoses were present. The cells stained positively with CD10 and CD68 and weakly with smooth muscle actin. However, the tumor was negative for cytokeratins (AE1/AE3), epithelial membrane antigen (EMA), S100, Melan A, HMB45, leukocyte common antigen (LCA), desmin, and CD31. Therefore, the lesion was designated as atypical fibroxanthoma (AFX) by exclusion. Within the AFX, there was also a cyst lined by squamous epithelium showing pilar keratinization. The epithelium showed full-thickness dysplasia with increased mitotic activity and was stained positively with AE1/AE3, thus supporting our view of carcinoma in situ within the wall of the cyst. To the best of our knowledge, this article describes the first case of trichilemmal cyst with carcinoma in situ arising within an AFX.

Merkel cell carcinoma: review

Merkel cell carcinoma (MCC) is synonymous with primary cutaneous neuroendocrine carcinoma. It tends to affects elderly whites, but there is also an increased incidence among immunosuppressed patients. The recent identification of a novel polyomavirus associated with the tumor has stimulated renewed interest in its pathogenesis. MCC tends to show classic histologic features of a neuroendocrine carcinoma and is often positive for CK20, but nonclassic cytologic findings and unusual immunophenotypes may be observed and can lead to a diagnostic confusion. MCC needs to be distinguished from other primary cutaneous tumors with a small cell appearance and metastatic tumors. Surgical excision is the treatment of choice, but radiation therapy has also found to be effective. Sentinel lymph node biopsy has become an integral part of the staging of patients with MCC.

Merkel cell carcinoma within follicular cysts: report of two cases

Merkel cell carcinoma is a rare cutaneous neoplasm of unknown histogenesis. Several reports have described the association of Merkel cell carcinoma of the skin with other cutaneous neoplasms within the same lesion, and there are also reports describing three examples of Merkel cell carcinoma within follicular cysts. We describe two examples of Merkel cell carcinoma developed within epithelial cysts. Neoplastic cells of Merkel cell tumor expressed immunoreactivity for chromogranin, synaptophysin, neuron-specific enolase, CAM 5.2 and cytokeratin 20, the last two markers showing the characteristic paranuclear dot-like pattern. In contrast, the epithelial wall lining the cyst and surrounding Merkel cell tumor only expressed immunoreactivity for cytokeratin MNF116. The description of five cases of Merkel cell carcinoma within follicular cysts, including the two cases of this report, support some relationship between Merkel cell tumor and the hair follicle.