Pulmonary epithelial–myoepithelial carcinoma without AKT1, HRAS or PIK3CA mutations: a case report

Background Pulmonary epithelial–myoepithelial carcinoma is a rare subtype of lung cancer. Because of its rarity, the molecular information on this carcinoma is insufficient. Case presentation We report a case of pulmonary epithelial–myoepithelial carcinoma without AKT1, HRAS or PIK3CA mutations in a 76-year-old woman. Computed tomography revealed a tumor located in the left lower lung. Thoracoscopic left lower lobectomy was performed. Histopathologically, the tumor consisted of duct-like structures and polygonal and spindle cell features. The duct-like structures were composed of two distinct cell layers. The inner layer consisted of cuboidal cells that were positive for pan-cytokeratin and negative for p63, whereas the outer layer consisted of polygonal and spindle cells that were positive for p63 and weakly positive for pan-cytokeratin. We evaluated mutations in AKT1, BRAF, CTNNB1, HRAS, KRAS and PIK3CA but did not detect any mutations. Conclusion Pulmonary epithelial–myoepithelial carcinoma is a rare subtype of lung cancer, with only 56 previous cases reported in the English literature. The genetic alterations in pulmonary epithelial–myoepithelial carcinoma are still unknown. We examined the 6 genes mutation analysis, however no mutation was detected.


Introduction
Epithelial-myoepithelial carcinoma (EMC) is a malignant tumor that occurs mainly in the salivary glands [1]. This tumor also arises in other locations such as the respiratory tract, minor salivary glands and lacrimal glands [1][2][3]. Primary salivary gland-type tumors of the lung account for 0.1-1% of all primary lung carcinomas, and the majority are mucoepidermoid or adenoid cystic carcinomas [4,5]. EMC is a rare subset of salivary glandtype tumor of the lung, with only 56 cases previously reported in the English literature [6]. Because of its rarity, molecular information on this tumor type is not sufficient.

Case presentation
A 76-year-old woman was diagnosed with colon carcinoma and underwent preoperative examinations. Computed tomography coincidently revealed a welldemarcated tumor, 1.8 × 1.3 cm in size, located in the left lower lung (Fig. 1). She did not have any respiratory symptoms. Laboratory data revealed no significantly abnormal findings. Endoscopic submucosal dissection of the colon carcinoma was performed. Bronchoscopy revealed an endobronchial mass, and transbronchial biopsy was performed. She was diagnosed with an adenocarcinoma, and thoracoscopic left lower lobectomy with hilar and mediastinal lymph node dissection was performed.

Pathological findings Macroscopic findings
A specimen containing the tumor was obtained at surgery. Macroscopically, the tumor size was 2.7 × 1.9 × 1.8 cm, and the cut surface of the tumor was whitish-yellow to gray, shiny, and well-demarcated (Fig. 2a).

Histopathological and immunohistochemical findings
The tumor consisted of duct-like structures and polygonal and spindle cell features (Fig. 2b). The duct-like structures were composed of two distinct cell layers. The inner layer comprised cuboidal cells with eosinophilic cytoplasm and round nuclei, and the outer layer comprised cells with clear cytoplasm and oval nuclei (Fig. 2c). The duct-like structures contained eosinophilic material in the luminal spaces. The polygonal and spindle cells (black arrowhead) were similar to the outerlayer cells (black arrow) (Fig. 2d). There was no necrosis or hemorrhage. Mitotic activity was found (2 mitoses/10 high-power fields). Immunohistochemically, the innerlayer cuboidal cells were positive for pan-cytokeratin  (Fig. 3a), suggesting a myoepithelial phenotype. Overexpression of p53 protein was not found.

Mutation analysis
We conducted polymerase chain reaction (PCR) followed by Sanger sequencing and pyrosequencing to investigate the mutation status of the oncogenes associated with EMC of the salivary gland [7]. Briefly, DNA from formalin-fixed paraffin-embedded tissues was The PCR products were purified using the NucleoSpin Gel and PCR Clean-up, Mini kit (Marcherey-Nagel, Duren, Germany). Each purified product was directly sequenced using a forward primer and the BigDye Terminator version 3.1 cycle sequencing kit on the ABI 3730 instrument (Applied Biosystems Inc., Foster City, CA). Mutation analyses of AKT1 (exon 2), CTNNB1 (exon 3), HRAS (exons 2 and 3) and PIK3CA (exons 9 and 20) were performed based on the method described by Urano et al. [7]. The primer sequences are listed in Table 1. In addition, mutation analyses of BRAF (exon 15) and KRAS (exons 2 and 3) were performed using the BRAF Pyro Kit and KRAS Pyro Kit (Qiagen, Venlo, Netherlands), respectively, in real-time using pyrosequencing technology on the PyroMark Q24 System (Qiagen). No mutation in any of the six genes was detected.

Discussion
EMC is a rare malignant salivary gland tumor that accounts for < 1% of all salivary gland epithelial neoplasms and nearly 2% of malignant salivary gland tumors [1,8]. EMC of the salivary gland was first described by Donath et al. in 1972 [9]. EMC is characterized by a biphasic morphology, with an inner layer of duct-like structures composed of epithelial cells and a surrounding layer of myoepithelial cells immunoreactive for S-100 and smooth muscle actin [1,2,6]. The tracheobronchial glands are considered counterparts of the minor salivary glands in the respiratory tract and can develop similar tumors. Within this type of neoplasia, EMC of the respiratory tract is very rare, and the diagnosis is often difficult [10,11]. Salivary gland-type tumors of the lung account for 0.1-1% of all primary lung carcinomas, among which mucoepidermoid carcinoma is the most frequently observed histological subtype, followed by adenoid cystic carcinoma and then EMC [4,5]. Recently, Nakashima et al. conducted a literature review of 56 patients (32 females and 24 males; average age [range], 56  years) with pulmonary EMC [6]. Of these, 45   [15]. Although our case was diagnosed as adenocarcinoma in the small biopsy, it proved to be typical EMC featuring two ductal components and immunohistochemically. Several genetic mutations have been detected in EMC of the salivary glands [7,16,17]; HRAS mutations are the most frequently detected, followed by PIK3CA and AKT1 mutations. The genetic alterations associated with pulmonary EMC are still unknown because of the rarity of this tumor. Urano et al. are the only ones who described HRAS mutations in all three pulmonary EMC that they analyzed [7]. However, our case did not show any mutations in AKT1, HRAS or PIK3CA. Although the number of reported cases is very small, the frequency of HRAS mutations in pulmonary EMC is 75% (3/4 cases). At this time, we cannot conclude whether the genetic alterations in pulmonary EMC are similar to those of other EMC types. Typical cases of pulmonary EMC are easy to diagnose, whereas atypical cases can be difficult to distinguish. Furthermore, because the biopsy specimen is small, the duct-like structure may be misdiagnosed as adenocarcinoma. Determination of the characteristic gene mutations will be useful for differentiating pulmonary EMC from other salivary gland tumors of the lung.
In conclusion, we report a case of pulmonary EMC containing no AKT1, HRAS, or PIK3CA mutations. Further examinations will be needed.