Pleural effusion is a common disease that is caused by pulmonary carcinomas and other malignant tumors, such as breast cancer and ovarian cancer, and even some nonmalignant diseases including tuberculosis and pneumonia [1, 2]. Malignant pleural effusion (MPE) is usually associated with cancer-related mortality and morbidity. Thus, it is important to diagnose MPEs and to treat and evaluate prognosis.
Cytology detection is the conventional method used to distinguish tumor cells in pleural effusions, as described in the International Union Against Cancer/American Joint Committee on Cancer’s tumor-node metastasis (TNM) classification system . However, cytology detection is imperfect in diagnosing MPEs. Moreover, when pleural effusion cytology cannot establish a patient’s diagnosis, additional invasive procedures must be performed to sample pleura for histological examination to enhance the diagnostic rate . However, there are high risks associated with these procedures, and many hospitals do not have these technologies, which limits their clinical application. Therefore, the diagnosis of MPE presents challenges to clinicians, and it is urgent to search for an effective diagnostic biomarker for this disease.
Lung cancer markers, including carcinoembryonic antigen (CEA), neuron-specific enolase (NSE), squamous cell carcinoma (SCC) antigen, and cytokeratin 19 (CK19), have been generally utilized to identify malignant and nonmalignant pleural effusions [4–7]. However, the diagnostic utility of these markers is unsatisfactory. Lung-specific X protein (Lunx), which was isolated by Yoshiyuki and colleagues through differential-display mRNA analysis, is a 206 bp cDNA fragment specifically amplified in the lung . The Lunx gene consists of 1,015 nucleotides, including an open reading frame of 768 nucleotides that encodes 256 amino acids . Lunx has served as a useful molecular marker for the detection of pulmonary carcinoma in bronchial brushing specimens and for the detection of peripheral blood and lymph node metastases [9–12]. Cheng et al.  reported that Lunx mRNA was the most specific biomarker with the highest sensitivity when compared with CK19, CEA, vascular endothelial growth factor-C (VEGF-C), and heterogeneous ribonuclear protein (hnRNP) for the differential diagnosis of non-small cell lung cancer from pleural effusion. However, it is still unclear whether Lunx mRNA expression in pleural effusions can predict the source of tumor cells and the responses of patients to chemotherapy.
Reverse transcriptase polymerase chain reaction (RT-PCR) is the most sensitive method for the detection of micrometastatic diseases, allowing for the detection of one cancer cell in 106 to 107 mononuclear cells [14, 15], but it is not effective in evaluating therapeutic effect and prognosis. Quantitative real-time RT-PCR can be used to assess gene expression levels and further evaluate the relationship between genes and disease. Currently, very little information is available on the relationship between the expression of Lunx mRNA and MPE.
The main purpose of this study was to evaluate Lunx mRNA expression in lung cancer cells using quantitative real-time RT-PCR, and to assess the diagnostic usefulness of Lunx mRNA expression as a tumor marker in pleural effusion. Furthermore, the correlation of Lunx mRNA expression in pulmonary carcinoma patients with pleural effusion and clinical factors was investigated.