NKX3‐1‐Expressing Mucous Gland Adenoma With <i>LMNA::NTRK1</i> Fusion of the Lung: Toward a Spectrum of NKX3‐1‐Expressing <i>NTRK</i> ‐Rearranged Mucinous Neoplasms

Mucous gland adenoma (MGA) is a rare benign pulmonary tumor derived from bronchial seromucinous glands, accounting for < 1% of primary lung tumors [1]. Although recognized as a distinct entity in the current World Health Organization classification, its diagnosis relies on subtle morphological clues, making it difficult to distinguish MGA from other mucin-producing lesions such as invasive mucinous adenocarcinoma (IMA), low-grade mucoepidermoid carcinoma, and bronchiolar adenoma/ciliated muconodular papillary tumor (CMPT), particularly when only small biopsies or limited resections are available [2]. Sasaki et al. [3] reported that samples of two MGAs stained positive for the transcription factor NKX3-1, once thought to be prostate-specific, and speculated that NKX3-1 is sensitive and specific for bronchial seromucinous differentiation. However, no other independent studies have yet validated NKX3-1 positivity in additional MGA cases, so its diagnostic value remains unconfirmed. We report the case of a 67-year-old woman who was a lifelong nonsmoker, in whom routine imaging incidentally detected a 10 mm nodule in the right lower lobe of the lung. She had no respiratory symptoms or relevant medical history. Because malignancy could not be ruled out, she elected to undergo a right lower lobectomy for both diagnosis and treatment. Gross examination revealed a well-demarcated, whitish nodule (10 mm) within a segmental bronchiole (Supporting Information S1: Figure S1A). The lesion was confined to the bronchiolar lumen. Histology showed a complex tubulopapillary architecture with bland columnar to cuboidal cells containing abundant mucin (Figure 1A, Supporting Information S1: Figures S1B,C and S2A,C). The lesion appeared to be continuous with the adjacent submucosal bronchial glands (Supporting Information S1: Figure S2E). Immunohistochemically, the tumor showed strong nuclear NKX3-1 staining (Figure 1B). The Ki-67 labeling index was < 5% (Figure 1C). The tumor was negative for TTF1 and HNF4A, diffusely positive for CK7, and focally positive for MUC5AC (Figure 1D, Supporting Information S1: Figure S3A,B). The tumor contained a few p40-positive cells and sparse abluminal CF5/6-positive cells (Supporting Information S1: Figure S3C,D). The differential diagnosis included bronchiolar adenoma/CMPT, IMA, and low-grade mucoepidermoid carcinoma. Bronchiolar adenoma/CMPT is typically peripherally located, arises from bronchiolar epithelium, and often expresses TTF-1 and Napsin A, which were absent in this case. IMA may resemble MGA histologically but shows infiltrative growth, more pronounced cytologic atypia, a higher Ki-67 labeling index, and HNF4A positivity. Mucoepidermoid carcinoma can have mucin-producing elements but typically shows a mixture of mucinous, squamous, and intermediate cells, along with higher mitotic activity and MAML2 rearrangements. Based on these bland morphological and immunohistochemical features, the tumor was initially diagnosed as MGA. However, given recent literature linking similar tumors to NTRK fusions [4], we conducted further molecular analysis. Pan-TRK immunohistochemistry showed strong cytoplasmic and nuclear membrane staining (Figure 1E). Fluorescence in situ hybridization (FISH) analysis revealed the presence of an LMNA::NTRK1 fusion gene (Figure 1F). Bronchial seromucinous glands are histological counterparts of minor salivary glands, and classic MGA is a duct‑centric/intraductal tumor composed of bland mucinous epithelium with endobronchial confinement and a patchy abluminal basal layer. MGA is typically TTF-1-negative and NKX3-1-positive [3]. These features parallel salivary gland intraductal papillary mucinous neoplasm (SG-IPMN), although SG-IPMN usually lacks basal cells. SG-IPMN and salivary mucinous adenocarcinoma can express NKX3-1 but generally lack NTRK fusions, unlike salivary secretory carcinoma, which characteristically harbors ETV6::NTRK3 fusion. In the airway, a case of low‑grade tracheal adenocarcinoma with ETV6::NTRK3 has been reported [4]. The tumor was characterized by tubulopapillary/cribriform architecture, resembling that of sinonasal low-grade non-intestinal-type adenocarcinomas, which often express NKX3-1. Another case of aggressive NKX3-1-expressing tracheal mucinous adenocarcinoma of bronchial gland cell origin [5] was subsequently shown to harbor an NTRK fusion (data not published). Taken together, these observations suggest a biological continuum of NKX3-1-expressing NTRK-rearranged mucinous neoplasms of tracheobronchial seromucinous origin ranging from classic MGA, through low‑grade adenocarcinoma with features overlapping sinonasal low-grade non-intestinal-type adenocarcinoma [4], to aggressive mucinous adenocarcinoma [5]. In conclusion, NKX3-1 is a practical lineage marker of tracheobronchial seromucinous differentiation. In centrally located, TTF-1-negative mucinous tumors, NKX3-1 positivity should prompt consideration of a tracheobronchial mucinous neoplasm (including MGA) and context-dependent NTRK fusion testing. We recommend a stepwise approach—pan-TRK immunohistochemistry for screening, followed by confirmatory fusion testing (RNA-based sequencing or targeted FISH)—in diagnostically challenging cases or when the results would alter management. Our LMNA::NTRK1-positive case shows that tumors within this spectrum may harbor targetable alterations; however, in a completely resected, non‑infiltrative tumor, this finding is primarily diagnostic and does not, considered on its own, require malignant reclassification. Larger, multicenter case series with clinical follow-up are needed to define the prevalence of NTRK fusions and test whether NKX3-1-expressing NTRK-rearranged mucinous neoplasms constitute a clinicopathological continuum—from classic MGA, through low-grade adenocarcinoma [4], to aggressive mucinous adenocarcinoma [5]. Recognizing this spectrum would enable more precise classification, consistent molecular triage, and rational application of TRK-directed therapy. Mahmut Amori performed the pathological evaluation and wrote the manuscript. Gulanbar Amori and Hirotoshi Kawata contributed to the diagnostic process and discussion. Yoshihiko Kanai performed the operation. Seiji Sakata, Keisuke Sugita, Satoko Baba, and Hironori Ninomiya performed the molecular analysis. Noriyoshi Fukushima contributed to the discussion. Kentaro Inamura was responsible for the study conception, performed the pathological evaluation, and contributed to writing the manuscript. All authors participated in revising the manuscript and read and approved the final manuscript. We thank the pathology laboratory staff at Jichi Medical University for their technical assistance. We thank Ms. Tomoko Kamiakito for her secretarial expertise. Ethical approval was not required for single-case presentation. Written informed consent was obtained, and all investigations were conducted in accordance with the Declaration of Helsinki (1975). Written informed consent for publication of this case report and the accompanying images was obtained from the patient. The authors declare no conflicts of interest. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.