The expression of terminal deoxynucleotidyl transferase and paired box gene 5 in Merkel cell carcinomas and its relation to the presence of Merkel cell polyomavirus DNA

Background Merkel cell carcinoma (MCC) tumor samples frequently express B‐lymphoid lineage markers. However, the reasons for expression of specific B‐lymphoid lineage markers are still unclear. We studied the expression of TdT and PAX5 (two B‐cell lymphoid lineage markers) in a large pool of MCC tissue microarray samples. Methods Immunoexpression and staining intensities of TdT and Pax‐5 were statistically correlated with patient, tumor, Merkel cell polyomavirus (MCV), and disease‐specific parameters. Results In a cohort of 117 MCC patients and their corresponding tumor samples, TdT was expressed in 37 (31.6%) samples and PAX5 in 26 (22.2%). Simultaneous immunostaining for TdT and PAX5 was observed in 13 (11.1%) samples. A statistically significant relationship was observed between MCV virus copy number and positive TdT expression (P = 0.0056). Similarly, a significant relationship was also observed between positive TdT and tumor MCV virus positivity (P = 0.000495). Conclusion We observed frequent TdT and PAX5 immunoexpression in MCC tumor samples. However, simultaneous immunoexpression of these markers was scarce. TdT expression was statistically significantly associated with MCV positivity. The absence of a statistically significant association between tumor parameters and disease progression markers undermines the systemic use of these markers in clinical practice.