Introduction
Thyroid follicular cells are found in the thyroid gland, specifically in the epithelial monolayer. In total, >95% of thyroid tumors are derived from these follicular cells (1). In 2016, the incidence of thyroid tumors rose globally, largely due to technological and diagnostic advances (2). However, it remains difficult to distinguish whether a thyroid nodule is benign or malignant. Follicular thyroid tumors may be divided into malignant follicular thyroid carcinoma (FTC) and benign follicular thyroid adenoma (FA). Only 5–10% of thyroid nodules are malignant (3). Patients with follicular tumors usually must undergo thyroid lobectomy for diagnosis, which is often an unnecessary surgery, as the disease is usually benign. Fine-needle aspiration cytology is considered the most accurate method for the diagnosis of FTC and FA (4).
Previously, microRNAs (miRNAs/miRs) have been demonstrated to be involved in the pathogenesis of various diseases, like cancer, diabetes and osteoarthritis (5–7). miRNAs are small (18–25 nucleotides) non-coding, single-stranded RNA molecules that bind to targets in a base pair-mediated manner, resulting in the degradation or inhibition of the expression and function of protein-coding mRNAs. miRNAs often bind to the 3′-untranslated region (3′UTR) of target genes (8), although they are usually only partially complementary to the target (9). miRNAs regulate ~30% of the human genes associated with proliferation, apoptosis, metastasis, cell immunity and differentiation (10). Each miRNA is able to regulate several hundred mRNAs, and each mRNA may be the target of several miRNAs. Therefore, a regulatory control network exists between miRNAs and mRNAs (11). Furthermore, miRNAs have been associated with several types of tumors, including non-small cell lung cancer, colon and esophageal cancer, and FTC (12–14). However, there are few studies of specific miRNA and mRNA analyses of follicular thyroid tumors.
Several microarray studies have already described the differentially expressed genes (DEGs) between malignant and benign thyroid nodules. However, these studies have several restrictions, including the fact that the samples are limited, they contain significant false-negatives, and they require external analysis at an offsite company laboratory (15–17). Certain studies have aimed to reveal the potential miRNAs associated with follicular thyroid tumors (18).
In the present study, an integrated analysis of differentially expressed miRNAs (DEMs) and DEGs between FTC and FA was performed. A Gene Ontology (GO) analysis of the DEGs was performed. A total of 36 miRNA-gene pairs were identified between the DEGs and the target genes of the DEMs. A miRNA-mRNA network analysis was then performed to additionally investigate the pathogenesis of FTC.
Discussion
The important roles of miRNAs in the pathogenesis of FTC have been identified previously (28). miRNAs exhibit different expression patterns within different tumor types, and are closely associated with the diagnosis, treatment and prognosis of tumors (29–31). Ak et al (21) observed that DEMs and differentially expressed mRNAs vary between benign and malignant tumors, which may suggest the different roles of these miRNAs and mRNAs. miR-197 and miR-346 have been indicated to be overexpressed in FTC, resulting in the dysregulation of their target genes (32). However, studies regarding DEMs and DEGs in FTC are rare. In the present study, the difference between miRNA-mRNA regulatory networks from FTC and FA samples were compared in order to investigate the mechanism of FTC. It was identified that miR-7, miR-1179, miR-7-2, miR-486-5p and miR-130b were the top downregulated miRNAs, and that miR-663b, miR-137, miR-30c-1, miR-767-5p and miR-603 were the top upregulated miRNAs. For the DEGs, the top downregulated genes were FABP4, CMAHP, ITM2A, CA4 and FAM189A2, and the top upregulated genes were EPB41L3, SCG5, PAX1, MTHFD2 and CDH2. In addition, miR-7, miR-296-5p, miR-10a, miR-144, miR-139-5p, miR-452 and miR-145 were downregulated, and miR-137 and miR-493 were upregulated in the FTC miRNA-mRNA regulatory network compared with those in FA. The gene arrays identified DEGs, in which leucine rich repeat neuronal 3, chromodomain helicase DNA binding protein 9, PKIA, zinc finger protein 148 (ZNF148), TGFB induced factor homeobox 1, transforming growth factor β receptor 2, gap junction protein α1 and CDH2 were observed to be target genes inversely correlated with miR-7, miR-144, miR-139-5p, miR-145 and miR-137. In other studies, FTC or FA have been compared with normal tissue, and differences in miRNA expression were observed to occur in the range between 1.2- and 2-fold, which was similar to the data of the present study (33–35).
In the present study, it was identified that miR-7, miR-296-5p, miR-10a, miR-144, miR-139-5p, miR-452, miR-145, miR-137 and miR-493 are important miRNAs that are differentially expressed between carcinoma and adenoma samples. Certain studies have suggested that miR-7 is not only a tumor promoter, but also a tumor suppressor. As a tumor suppressor, miR-7 is downregulated in tumors, such as thyroid cancer, breast cancer and castration-resistant prostate cancer, leading to a derepression of the oncogenes epidermal growth factor receptor, insulin receptor substrate 1, Raf-1 proto-oncogene, serine/threonine kinase, tyrosine kinase non-receptor 2, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit δ, mechanistic target of rapamycin kinase, Ribosomal protein S6 kinase β-1 and phosphatidylinositol-4,5-bisphosphate 3-kinase (36–38). miR-296-5p has been revealed to be significantly inversely correlated with post-contrast T1 values for diffuse myocardial fibrosis in patients with hypertrophic cardiomyopathy, and is a downstream effector under conditions that promote glioblastoma stem cell stemness, and inhibit glioblastoma cell stemness and their capacity to self-renew as spheres and propagate glioma xenografts in vivo (39,40). miR-10a has been identified as a downregulated miRNA associated with human metastatic medullary thyroid carcinoma, and it may be important for tumor development and/or reflect C-cell lineage (41,42). miR-144 may suppress the invasion and migration capability of thyroid cancer and suppress the expression of zinc finger E-box-binding homeobox (ZEB)1 and ZEB2, the two E-cadherin suppressors, by directly binding their 3′UTRs (43). miR-137 was indicated to participate in hematopoiesis, particularly in the efficacy of warfarin, wherein miR-137 may cause aberrant vitamin K epoxide reductase complex subunit 1 expression (44). miR-139-5p is an oncogenic molecule in the process of tumorigenesis, and has been demonstrated to be a sensitive and specific biomarker for the diagnosis of thyroid tumors and others tumor types (45). Furthermore, it may be of use as a tractable therapeutic target to decrease the mortality rate and increase the survival rate (46). miR-145 has primarily been indicated as being downregulated in colorectal tumors. Previously, certain studies have identified that miR-145 is highly expressed in mesenchymal cells such as fibroblasts and smooth muscle cells (47). The miRNA was demonstrated to directly regulate the expression of thyroid hormone receptor TRβ1 in renal cancer cells and to correlate with intracellular triiodothyronine concentrations in renal tumors (48). miR-493 also promoted the invasion and chemoresistance of gastric cancer cells. However, dickkopf related-protein 1 overexpression reversed its effects on proliferation, invasion and chemo-sensitivity (49). Based on these data, we hypothesize that these miRNAs serve important roles in FTC with different pathways.
In the present study, several genes that were overlapping were identified between the DEGs and the target genes of the DEMs. These may be upregulated or downregulated. However, they all contributed to the development of FTC. Certain functions of these genes in cancer have been studied. For example, ZNF148 is a member of the human zinc finger Krüppel family and it maps to regions implicated in recurrent chromosomal rearrangements in hematological malignancies (50). The present study identified spalt-like transcription factor 1 (SALL1), which is one of the four human family members of the Spalt family. Members of the Spalt family are highly conserved zinc-finger transcription factors that are conserved from Caenorhabditis elegans to vertebrates, with regulatory functions in organogenesis, limb formation and cell fate assignment during neural development. SALL1 expression has been identified to correlate with the expression of CDH1, which is consistent with its tumor suppressive function and suggests its potential involvement in epithelial-to-mesenchymal transition (51,52). Cell division cycle 27 (CDC27) is a core component of the anaphase-promoting complex and is involved in the regulation of mitotic checkpoints to ensure chromosomal integrity (53). CDC27 may significantly affect the function of the polymeric protein complex and is also a target of certain anticancer drugs (54,55). Nuclear receptor subfamily 2 group F member 2 (NR2F2), also known as chicken ovalbumin upstream promoter transcription factor, is highly prioritized as a candidate gene associated with hypertension (56). Certain studies have demonstrated that NR2F2 is nuclear receptor transcription factor vital for angiogenesis and heart development (57). These data suggest that several genes have functions in numerous pathways involved in tumorigenesis and progression.
Each miRNA is able to regulate several hundred mRNAs. In addition, each mRNA may be targeted by several miRNAs and each mRNA participates in several biological functions in the human body. Therefore, each miRNA may affect different biological processes and pathways through a miRNA-mRNA network (58,59). It is important to understand the pathogenesis and treatment of tumors by investigating the specific miRNA-mRNA co-regulation effects. In the present study, mRNAs and their functions were described with GO enrichment analysis. There were 86 mRNAs and 8 biological functions involved. In total, ~80% of follicular carcinomas contain Ras gene mutations or a paired box gene 8/peroxisome proliferator-activated receptor γ gene rearrangement, which leads to uncontrolled proliferation. Mutations in the phosphatase and tensin homologue suppressor gene and the phosphatidylinositol 3-kinase pathway may be an important factor in the development of more aggressive thyroid cancer types and may be more common in follicular cancer, which is responsible for cell motility, locomotion and response to external stimulus (60–62). Other factors that have been implicated in the pathogenesis of FTC include gene mutations in p53, c-myc, c-fos and the thyrotropin receptor (63–66). These molecules serve functions in cell proliferation, apoptosis, cytoskeleton rearrangement and responses to drugs. Additionally, FTC, but not adenoma, recruits tumor-associated macrophages by releasing Chemokine (C-C motif) ligand 5; therefore, an abnormal immune response, including T cell activation, may be involved in follicular cancer. Other GO terms may be validated in future studies (3,67).
In conclusion, the present study identified 86 DEGs and 32 DEMs between FTC and FA. A total of 24 overlapping genes were identified between the DEGs and the target genes of the DEMs. Network analysis indicated a co-regulatory association between miR-296-5p, miR-10a, miR-139-5p, miR-452, miR-493, miR-7, miR-137, miR-144, miR-145 and corresponding targeted mRNAs in FTC. However, the present study has limitations, such as the small sample size, although attention was paid to ensure the use of two genetically homogenous populations to avoid population stratification. The mechanism of the miRNA-mRNA network and the roles of these genes in FTC require additional study and validation in vitro and in vivo.