Although the majority of thyroid nodules are benign, 7% are diagnosed as thyroid cancer (TC) [15]. It is highly important to distinguish between malignant and benign thyroid nodule. Overcoming the challenges of accurate assessments of the risk for individual patients is important to establish appropriate therapeutic strategies and to optimize clinical outcomes.
Currently, tissue diagnosis is the gold standard method. However, other methods could predict the nature of the nodule in an individual patient.
Distinct molecular changes are associated with tumorigenic process. Numerous studies have demonstrated that there is a potential application of miRNA in thyroid cancer diagnosis.[15, 23–26]. In contrast to mRNAs, mature miRNAs are comparatively stable and remain largely intact in routinely collected, formalin-fixed paraffin embedded (FFPE) Tissue specimens.[23]. Previous research revealed that dysregulated miRNAs could have diagnostic role in PTC cell line and even in FNA samples. [27].
In our study, using 282 patients with stage I PTC, and 57 normal thyroid tissue samples from TCGA, differentially expressed miRNAs were identified. Since diagnostic accuracy of a combination of miRNAs alteration is greater value compared with a single miRNA [28], we decided to choose a panel of miRNAs (miR-221, miR-222, miR-9 and miR-20b) based on the bioinformatics analysis of TCGA data sets and also a comprehensive literature review in which all selected miRNAs could play a biological role in tumor genesis pathways. Our miRNA- target prediction result showed that, although miR-9 and miR-20b target several shared proteins involved in cancer signaling pathways, there is limited shared results for miR-221 and miR-222 (involved exclusively in cellular senescence pathway). Next, we sought to evaluate the diagnostic accuracy of selected miRNAs in PTC Compared with benign tissue samples. Interestingly miR-20b and miR-9 indicated an up-regulated pattern in benign samples, while miR-221 and miR-222 were highly expressed in PTC.
Since we observed down-regulation of miR-9 and miR-20b in thyroid cancer samples, we can expect the up-regulation of their target genes. Based on bioinformatics analysis, MAPK and PI3K-Akt are the two signaling pathways highly influenced by these two miRNAs, with 14 and 12 target genes, respectively. In HIPPO signaling pathway, four membrane receptors including TGFBR1, TGFBR2, BMPR2 and FZD3 are simultaneously targeted by these two miRNAs, indicating that the down-regulation of miR-9 and miR-22 plays a significant role in the activation of this pathway. TGFBR1 and TGFBR2 are also involved in MAPK, P53, Relaxin, and FOXO signaling pathways. Inspecting the activation or inhibition role of target genes in different pathways showed that the up-regulation of most genes results in cell survival, cell cycle progression and angiogenesis.
Given that the up-regulation of miR-9 and miR-20b targets may result in down-regulation of MAPK, PI3K, Hippo and other cancer related pathway, they may have tumor suppressive role in thyroid cells and could be utilized as a new therapeutic target, strategy for patient with PTC.
In line with our finding, several studies determined that levels of miR-221 and − 222 are up-regulated in PTC. Some researchers claim that the overexpression of miR-222 is associated with high-risk features such as lymph node involvement, extra thyroidal extension, invasiveness, and recurrence in PTC [29–31]. These studies indicate that miR-222 had a great sensitivity in identifying thyroid malignancies [32], as well as a potential biomarker for PTC stratification.[33] furthermore, miR-221 and miR-222 are associated with poor prognostic features [34, 35]. It has been determined that miR-222, as one of the most typically overexpressed miRNAs in PTC, is associated with important prognostic features such as vascular invasion, capsular invasion, lymph node metastasis and larger tumor size in PTCs[33].
Additionally, miR-137, -222 and 181b levels seem to be associated with malignancy in PTC [27]. Yip et al showed that a panel of miR-146b, miR-222, miR34b and miR-130b were differentially expressed in aggressive BRAF-positive PTC[36].
Experimental studies showed that miR-221 and miR-222 are involved in thyroid cell proliferation and cell transformation via the suppression of cell cycle regulator (p27kip1) and long non-coding RNA Growth Arrest-Specific 5 (Gas5) as a target of miR-222-3p [37] ,[38] ,[39]. Our bioinformatics analysis revealed that up-regulation of miR-222 and miR − 221 may result in the down-regulation of cellular senesces through targeting more than ten components in the signaling pathway and therefore potentially may help cancer cells to be immortal.
On the other hand, correlations between the miRNA expression levels and the Bethesda reporting of cytology specimens, showed that selected miRNAs could be measured on cytology specimens and might have a good potential to predict the nature of thyroid nodules (Table 3).
It should be noted that a panel of miRNAs could be used to differentiate malignant nodule from a benign nodule.
Understanding the molecular basis of thyroid nodule development will be useful to identify novel diagnostic, prognostic, and therapeutic targets. Although the molecular mechanisms of the miRNAs in oncogenesis are remained to be fully elucidated, evaluating the miRNA expression panel in cytology and tissue specimens provides a diagnostic tool to differentiate PTC form non-PTC it may also serve as a novel therapeutic target for PTC in the future.