A Rare Pulmonary Myoepithelial Tumor With Novel TRPS1-PLAG1 Fusions

Background: Little is known about the morphological and molecular features of intra-thoracic myoepithelial tumors. Here, we describe a rare pulmonary myoepithelial tumor with novel TRPS1-PLAG1 fusions. Case presentation: A 30-year-old male presented with multiple lung nodules that detected by routine computed tomography (CT) scan 3 years ago. Positron emission tomography-computed tomography (PET-CT) demonstrated a well-circumscribed mass with a diameter of 3cm in the left pulmonary hilum and multiple nodules in bilateral lungs. No other extra-thoracic primary lesion and distant metastasis was found. The clinical history was not other specied. Microscopically, these lesions were well-dened, showed nodular growth pattern lling the bronchioles and alveoli. The tumor cells were epithelioid to plasmacytoid with slightly eosinophilic cytoplasm, arranged in aggregates, nests and cords. Myxoid background presented in some foci. Nuclear pleomorphism was mild and mitotic activity was not prominent. No distinct gland formation was identied. Immunostains demonstrated that the tumor cells were diffusely positive for pan-cytokeratin (AE1/AE3), CK5/6, S100, SOX10, and negative for calponin. Noticeably, a TRPS1-PLAG1 gene fusion was identied in this tumor by targeted RNA sequencing involved TRPS1 exon1 and PLAG1 exon2, this rare rearrangement was validated by uorescence in situ hybridization (FISH) for PLAG1 break-apart probe, and 30% tumor cells showed break apart signals. Conclusions: Here we present the rst case of a disseminated thoracic myoepithelial tumor with novel TRPS1-PLAG1 fusions. Targeted RNA sequencing strongly facilitates precise classication and providing opportunities for unknown fusion partner discovery, which is signicant for risk stratication and development of potential therapeutic targets.


Introduction
Primary salivary gland-type tumors of the lungs are very rare, most of which occur in the central large bronchus, among them mucoepidermoid carcinoma (MEC) and adenoid cystic carcinoma (AdCC) are relatively common. The incidence of pulmonary myoepithelial neoplasms is extremely low and little is known about its morphological and genetic pro le. Here, we present a rare pulmonary myoepithelial tumor with novel TRPS1-PLAG1 fusions that detected by targeted RNA sequencing.

Case Presentation
A 30-year-old male presented with multiple lung nodules that detected by routine CT scan 3 years ago.
During this period, regular CT follow-up showed no obvious progression of these lesions. PET-CT demonstrated a well-circumscribed mass with a diameter of 3 cm in the left pulmonary hilum and multiple nodules in bilateral lungs, F 18 -FDG metabolism was increased in some lesions (Fig. 1). No other extra-thoracic primary lesion and distant metastasis was found by PET-CT. The clinical history was not other speci ed. Due to the di culty of complete removal of hilar mass, a diagnostic wedge resection of nodules in right middle and lower lobes was performed in May 2020.

Pathologic ndings
Microscopically, these lesions were well-de ned, showed nodular growth pattern lling the bronchioles and alveoli. Spreading into surrounding air spaces was occasionally seen. The tumor cells were epithelioid to plasmacytoid with slightly eosinophilic cytoplasm, arranged in aggregates, nests and cords.
Myxoid background presented in some foci. Prominent lymphoplasmacytic in ltration was found in some nodules. Nuclear pleomorphism was mild and mitotic activity was not prominent. No distinct gland formation was identi ed (Fig. 2). Immunostains demonstrated that the tumor cells were diffusely positive for pan-cytokeratin (AE1/AE3), CK5/6, S100, SOX10, and negative for calponin. INI-1 (SMARCB1) nuclear expression in the tumor cells was intact.

Molecular ndings
Noticeably, a TRPS1-PLAG1 gene fusion was identi ed in this tumor by targeted RNA sequencing involved TRPS1 exon1 and PLAG1 exon2 (Fig. 3), this rare rearrangement was further validated by uorescence in situ hybridization (FISH) for PLAG1 break-apart probe, and 30% tumor cells showed break apart signals.
3. Discussion TRPS1-PLAG1 gene fusion is a recently found novel fusion type in soft tissue myoepithelioma 1 , uterine myxoid leiomyosarcoma 2, 3 , and chondroid syringoma 4 . Leduc et al summarized 8 primary thoracic myoepithelial tumor, EWSR1-PBX1, EWSR1-ZNF444, and FUS-KLF17 fusions were found in half of the cases, while no cases were found to harbor HMGA2 or PLAG1 abnormalities 5 . To the best of our knowledge, this case may represent the rst thoracic myoepithelial tumor with TRPS1-PLAG1 fusions.
As is known to us, PLAG1 is one of the most common partners involved in pleomorphic adenoma/mixed tumor arising in salivary glands and skin. The detection of TRPS1-PLAG1 fusions may be more favor the diagnosis of pleomorphic adenoma rather myepithelioma. While in this case, the young patient lack of the history of previous salivary pleomorphic adenoma or cutaneous chondroid syringoma. What is more, the resected specimen showed purely myoepithelial components without gland formation. Thus, this lesion may be pulmonary primary with intrathoracic dissemination, though metastasis of unknown primary origin could not be totally excluded. Clinically, this lesion disseminated into surrounding lung parenchyma, which showed more aggressive behavior than common myoepithelioma. The de nitive relationship between the biology and this novel fusion partner is needed to be further investigated in additional cases.
In summary, here we present the rst case of a disseminated thoracic myoepithelial tumor with novel TRPS1-PLAG1 fusions. Integration of morphological, immunohistochemical and molecular investigations is necessary for the precise diagnosis and optimal clinical management of rare lung tumors. Targeted RNA sequencing strongly facilitates precise classi cation and providing opportunities for unknown fusion partner discovery, which is signi cant for risk strati cation and development of potential therapeutic targets. This study has been approved by institutional board of fudan university shanghai cancer center.

Consent for publication
Consent for publication has been obtained from the patient.

Availability of data and materials
All data generated or analysed during this study are included in this published article.

Competing interests
The authors declare that they have no competing interests  Multiple nodules were also found scattered in bilateral lungs with increased F18 -FDG metabolism (B). Microscopically, the tumors showed nodular growth pattern extended into the bronchioles and alveoli (A).
Spreading into surrounding air spaces was occasionally seen (B). The tumor cells were epithelioid to plasmacytoid with slightly eosinophilic cytoplasm, myxoid background presented in some foci(C and D). The tumor cells were positive for S100 (E) and SOX10 (F).