Letter to the Editor
Diffuse TTF-1 expression in a case of Merkel cell carcinoma

Dear Editor,

We herein report an unusual case of a cytokeratin-20 positive/cytokeratin-7 negative (CK20+/CK7–) Merkel cell carcinoma (MCC) that also showed immunoreactivity to two different thyroid transcription factor-1 (TTF-1) clones. An 88-year-old woman with no known history of a non-cutaneous neuroendocrine neoplasm presented with a 1.1 cm tumour located in the upper lip, allegedly having noticed it 2 months previously. The lesion was excised with negative margins. Lymph nodes were negative according to clinical examination (no pathological node examination was performed).
Histologically, the presence of a fairly circumscribed neoplasm was observed with neuroendocrine cell tumour morphology, compatible with MCC. It consisted of intermediate size, roundish cells, in groups or trabeculae, with minimal cytoplasm, a coarse chromatin pattern and increased mitotic activity (Fig. 1). In between neoplastic buds extending to the subcutaneous fatty tissue, multiple vessels and areas of necrosis were observed, but no lymphovascular invasion and no skin ulceration. Immunohistochemically, nuclear TTF-1 positivity was observed in a substantial amount of tumour cells. Two different clones raised against TTF-1 were used, namely SPT24 [Novocastra, UK, 1/80 dilution] (Fig. 1B) and 8G7G1/1 [DAKO, Denmark, 1/50 dilution] (Fig. 1C). The rest of the marker panel immunophenotype, namely chromogranin A+, CD56+, NF+, CK20+ (dot-like/crescentic) (Fig. 1D), and CK7- confirmed initial histological diagnosis of MCC. MCC polyomavirus antigen presence could not be investigated.
The neoplasm was staged as IB (T1cN0M0). The patient was clinically evaluated with no other relevant finding. The presence of a primary extra-cutaneous neuroendocrine carcinoma was excluded. During a 2-year follow-up period the patient did not show recurrence.
Merkel cell carcinoma is a rare, clinically aggressive, primary neuroendocrine skin neoplasm with variable prognosis. Adverse histopathological prognostic factors include lymphovascular invasion, large tumour size, small cell size and high mitotic rate [1]. Merkel cell carcinoma should be distinguished from a metastatic small cell carcinoma (SCC), mostly of the lung. Expression of TTF-1, especially when combined with immunostaining with CK20 and certain neuroendocrine markers, holds great differential-diagnostic value in the distinction between an MCC and a skin metastasis of an SCC (pulmonary or extra-pulmonary). Although TTF-1 is considered a sensitive and specific marker for SCCs of the lung, positivity should not exclude neuroendocrine tumours of other sites, not even an MCC, as of late. Thyroid transcription factor-1 positivity in MCCs is highly unusual, and there are, to our knowledge, only eight reported cases in the English medical literature [1-6]. Its significance remains to be elucidated through further studies. Only two of the reports referred to the clones used [5, 6], and those were different. It is of note that Buresh et al. described weak positivity to clone SPT24.
In our case TTF-1 nuclear immunopositivity was observed for both available and widely used antibody clones (SPT24 and 8G7G1/1). This is of certain significance, since it has been observed that SPT24 has a higher affinity but lower specificity in comparison to 8G7G1/1, due to a shorter amino acid chain, amongst other findings [7, 8].
Even though the unexpected TTF-1 positivity in carcinomas arising in organs other than lung and thyroid has been reported more frequently with the recently available SPT24 anti-TTF-1 monoclonal antibody, it has also been shown to occur with the commonly used 8G7G3/1 clone, albeit in a lower percentage of cases [9]. In our opinion, if it is possible, both clones should be used when investigating TTF-1 expression in an MCC, in view of a possible cross-reaction manifested in positive staining with the one, after a negative result with the other, an issue mentioned in other tumours as well. In addition, it could be of some interest for future studies to examine whether TTF-1 positive MCC cases would show a statistically significant preference either for one clone or the other and investigate the clinicopathological correlation.

References

1. Shalin SC, Cifarelli CP, Suen JY, Gao L. Loss of cytokeratin 20 and acquisition of thyroid transcription factor-1 expression in a merkel cell carcinoma metastasis to the brain. Am J Dermatopathol 2014; 36: 904-906.
2. Reddi DM, Puri PK. Expression of focal TTF-1 expression in a case of CK7/CK20-positive Merkel cell carcinoma. J Cutan Pathol 2013; 40: 431-433.
3. Koba S, Inoue T, Okawa T, et al. Merkel cell carcinoma with cytokeratin 20-negative and thyroid transcription factor-1-positive immunostaining admixed with squamous cell carcinoma. J Dermatol Sci 2011; 64: 77-79.
4. Sierakowski A, Al-Janabi K, Dam Hv, Sood M. Metastatic Merkel cell carcinoma with positive expression of thyroid transcription factor-1 – a case report. Am J Dermatopathol 2009; 31: 384-386.
5. Ralston J, Chiriboga L, Nonaka D. MASH 1: a useful marker in differentiating pulmonary small cell carcinoma from Merkel cell carcinoma. Mod Pathol 2008; 21: 1357-1362.
6. Buresh CJ, Oliai BR, Miller RT. Reactivity with TdT in Merkel cell carcinoma: a potential diagnostic pitfall. Am J Clin Pathol 2008; 129: 894-898.
7. Compérat E, Zhang F, Perrotin C, et al. Variable sensitivity and specificity of TTF-1 antibodies in lung metastatic adenocarcinoma of colorectal origin. Mod Pathol 2005; 18: 1371-1376.
8. Galloway M, Sim R. TTF-1 staining in glioblastoma multiforme. Virchows Arch 2007; 451: 109-111.
9. Ordó¼ez NG. Value of thyroid transcription factor-1 immunostaining in tumor diagnosis: a review and update. Appl Immunohistochem Mol Morphol 2012; 20: 429-444.

Address for correspondence

Alexandros Iliadis, MD
Pathology Department Faculty of Medicine
Aristotle University of Thessaloniki, University Campus
54124 Thessaloniki, Greece
tel. +302310999225
fax +302310999229 e-mail: iliadis.alexander@gmail.com