NUT midline carcinoma

Abstract

NUT midline carcinoma (NMC) is a rare, aggressive human cancer, genetically defined by rearrangements of the gene NUT (HUGO symbol: C15orf55). In the majority (∼75%) of NMCs, most of the coding sequence of NUT on chromosome 15q14 is fused with BRD4 creating chimeric genes that encode BRD-NUT fusion proteins. In the remaining cases, NUT is fused to BRD3 or an unknown partner gene; these tumors are termed NUT-variant. Diagnosis of NMC is made by demonstration of expression of the NUT-fusion protein using a monoclonal antibody to NUT for immunohistochemistry, and confirmation of the fusion (BRD-NUT or NUT-variant) by fluorescent in situ hybridization or reverse transcriptase-polymerase chain reaction. BRD-NUT functions to block cellular differentiation and promote uncontrolled growth of carcinoma cells. Because the reagents and expertise required to diagnose NMC are not available in most laboratories, and because of incomplete awareness of this disease, NMC is frequently undiagnosed or misdiagnosed, and its actual prevalence is unknown. NUT midline carcinoma does not arise from any specific tissue type or organ. It presents as a poorly differentiated carcinoma originating from midline locations such as the head, neck or mediastinum. Although rare, NMCs occur throughout life, and advanced local disease is frequently accompanied by distant hematogenous metastases. There still is no effective treatment for NMC, there are no guidelines, and current approaches to treatment are based on discussions among a few oncologists who each have had a single experience treating this disease.

FIG 1

Cartoon of BRD4, BRD3, NUT, and BRD-NUT fusion proteins. ET, extraterminal domain. NLS, nuclear localization signal. NES, nuclear export signal.

FIG 2

Knockdown of BRD4-NUT results in squamous differentiation and growth arrest (from [20]). (A-C) Control siRNA (ct) has no effect on the BRD4-NUT-expressing cell lines TC797 and PER-403 [10], or the BRD3-NUT-expressing cell line 10326, resp. (hematoxylin and eosin stain, 400x, 96h post-transfection). Morphologic changes induced by NUT siRNA-induced knockdown (kd) in TC797, PER-403, and 10326 cells (hematoxylin and eosin stain, 400x, 96h post-transfection). (A-C, insets) BRD-NUT knockdown induces keratin expression, a biomarker of epithelial differentiation, as evidenced by increased staining for pan-keratin monoclonal antibody (clone MNF116, DAKO). (D) Knockdown of BRD-NUT decreases proliferation. Staining for Ki-67+ (a marker of cell cycle progression) was performed on sections of cell blocks prepared 96h post-siRNA transfection. Bars = 50 µm, A-C. (E) Effects of a NUT-specific siRNA on BRD-NUT. Protein extracts obtained 24h after siRNA transfection were analyzed on a Western blot stained with a polyclonal NUT antibody.