Challenging diagnosis in pulmonary nut carcinoma: A report of two cases with different histopathologic and molecular features and a novel NUTM1::SPECC1 gene fusion.

Background: NUT carcinoma (NC), formerly known as NUT midline carcinoma, is a rare but highly aggressive cancer. It is a poorly differentiated carcinoma characterized by rearrangements of the NUTM1 (nuclear protein in Testis) gene with a member of the bromodomain-containing protein (BRD) family gene, usually BRD4. There is limited knowledge about primary pulmonary NC till now. It is probably underestimated or underdiagnosed because of its poorly differentiated character, misleading immunophenotype, and wide range of differential diagnoses.

Method: We report here two cases of pulmonary NC with different clinicopathological and molecular presentations to draw attention to some atypical clinicopathologic features that can help clinicians and pathologists consider this rare entity.

Results: The first case shows a nested pattern with small, uniform, blue epithelioid cells and aberrant expression of neuroendocrine markers, which has a known BRD3::NUTM1 fusion accompanied by a novel IGR (downstream ROR2)::NUTM1 fusion. The second case demonstrates solid sheets and cords of eosinophilic epithelioid-polygonal cells with a mucoid stroma and TTF1 expression, which has a novel SPECC1::NUTM1 gene fusion accompanied by TP53 and JAK1 gene oncogenic variants.

Conclusion: As a result, our study contributes to expanding the variant spectrum of the NUTM1 gene. NUT carcinoma with different fusion partners seems to have unique clinicopathological characteristics, yet more cases need to accumulate experience.