OBJECTIVETo investigate the importance of expression of the NUT gene and its rearrangement in diagnosing NUT midline carcinoma (NMC) of the upper respiratory tract; and to evaluate the prevalence, histological features and differential diagnosis of NMC of the upper respiratory tract.

METHODSOne-hundred and sixty-three small cell malignant tumors of the upper respiratory tract were reviewed at the Beijing Tongren Hospital, Capital Medical University over a 20-year period. These cases included poorly-differentiated squamous cell carcinomas (n = 31), undifferentiated carcinoma (n = 1), non-keratizing undifferentiated nasopharyneal carcinomas (n = 60), small cell neuroendocrine carcinomas (n = 6) and non-epithelial small round cell malignant tumors (n = 65). The clinical and pathologic features were investigated. All cases were subjected to Epstein-Barr virus encoded RNA (EBER) in situ hybridization and NUT monoclonal antibody immunohistochemical staining. Cases positive for NUT immunohistochemistry and negative for EBER in situ hybridization were submitted for fluorescent in situ hybridization (FISH) for rearrangements in both BRD4 and NUT genes, and immunohistochemical staining for a set of cytokeratins (AE1/AE3, CK7, CK8), p63,and neuroendocrine markers (NSE, Syn, CgA, S-100 protein, CD56).

RESULTSThree cases of poorly-differentiated squamous cell carcinomas and one case of undifferentiated carcinoma showed diffuse nuclear immunohistochemical staining with antibody against NUT. These positive cases approximately accounted for 12.5% (4/32) of this group, 4.1% (4/98) of the malignant epithelial carcinomas and 2.5% (4/163) of all small round cell malignant tumors in the study. The age of these patients were 42 - 59 years. Other groups were all negative for NUT immunohistochemistry. These four cases also stained for antibodies against cytokeratins and p63, but were negative for neuroendocrine markers and not associated with EBV infection. Only two of these four cases showed rearrangements of the NUT and BRD4 genes by FISH. These two patients died within one year. The other two patients that did not demonstrate NUT rearrangement by FISH were alive and did not have an aggressive clinical course, surviving 40 and 12 months respectively.

CONCLUSIONSNMC is a rare small round cell malignant tumor in the upper respiratory tract. Only in the groups of primary poorly differentiated squamous cell carcinoma and undifferentiated carcinoma were positive for NUT immunohistochemical staining and NUT rearrangement by FISH. NMC typically occurs in midline organs, and affects the sinonasal tract. It is not associated with EBV infection. There is difference in the clinical course and prognosis among NMC patients. NUT immunohistochemical staining and NUT gene rearrangement analysis can differentiate NMC from other small cell tumors in the upper respiratory tract.