Introduction: Thymic carcinomas represent the most aggressive histotype of thymic epithelial tumors (TETs). The 2004 World Health Organization classification has assigned a subgroup of thymic carcinomas as t(15;19) carcinomas based on the presence of t(15;19), a translocation found in poorly differentiated and highly aggressive NUT midline carcinomas. These tumors are characterized byrearrangement of the NUT (nuclear protein in testis) gene on chromosome 15q14, which in most cases fuses to the bromodomain containing 4 (BRD4) gene on chromosome 19 p13.1 through reciprocal t(15;19) translocation, resulting in constitutive BRD4-NUTfusion protein expression. To our knowledge, NUT translocation has been reported only in four thymic carcinomas. Due to the rarity of TETs, the prevalence of NUT rearrangement in TETs has however never been systematically explored.
Methods: Formalin-fixed paraffin-embedded samples of histologically confirmed TETs were evaluated for NUT expression and rearrangement by immunohistochemistry and fluorescence in situ hybridization, respectively.
Results: A series of 148 TETs (37 carcinomas and 111 thymomas) were examined for NUT expression and rearrangement. Only one thymic carcinoma (2.7% of thymic carcinomas or 0.68% of TETs) was found positive for NUT expression and rearrangement.
Conclusions: Rearrangement of NUT is infrequent in TETs. We propose that caution should be taken to distinguish t(15;19) thymic carcinoma from other mediastinal carcinomas, as NUT midline carcinomas are often associated with dreadful prognosis or overt lethality.