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Received Sep 8, 2017; Revised Nov 29, 2017; Accepted Jan 10, 2018
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1. Introduction
Lung cancer is the leading cause of cancer related death and has become an increasingly serious public health burden worldwide [1]. Small cell lung carcinoma (SCLC) and non-small-cell lung carcinoma (NSCLC) are the two main types of lung cancer in terms of clinical and pathological classification. NSCLC accounts for about 80% of all lung cancers and has three primary subtypes: adenocarcinoma (AC), squamous cell carcinoma (SC), and large cell carcinoma. Radiation therapy, a key therapeutic option for thoracic malignancies including lung cancers [2], damages the DNA of tumor cells to induce cellular death. Radiation therapy has been shown to be synergistic with surgical resection and chemotherapy in lung cancer [3]. However, increasing clinical data demonstrate that lung cancer patients show heterogeneous responses to radiation therapy. Recent studies suggest that gene expression is a significant contributor to the radiosensitivity of lung cancer cells [4, 5]. Therefore, with the advancements of precision medicine and next-generation sequencing, it is important to identify the differentially expressed genes in order to meet the trend of personalized lung cancer treatment.
MicroRNAs (miRNAs or miRs) are a class of short endogenous noncoding RNAs containing 20–22 nucleotides, which play an important role in tumor development and progression, including tumor cell proliferation and metastasis, cancer stem cell differentiation, and cell apoptosis [6]. miRNAs recognize and bind to the 3′-untranslated region (UTR) of the target mRNA, leading to degradation of the mRNA and/or suppression of protein translation [7–9]. In recent years, growing evidence has shown that miRNAs also play a pivotal role in the radiation sensitivity of various types of cancers, including lung cancer. For instance, high expression levels of miR-98-5p, miR-302e, miR-495-3p, and miR-613 are significantly correlated with the radiosensitivity of NSCLC patients [10]. In contrast, silencing miR-21 expression in A549 cells has been reported to significantly sensitize cells to radiation through inhibition of the PI3K/AKT pathway [11]. Additionally, miR-21 upregulation has been found associated with an attenuated radiation efficacy and a shorter median survival time of NSCLC patients, whereas...