Thyroid cancers (TCs) are the most common malignancies of endocrine system and comprise several subtypes with remarkably different biological characteristics. Among these subtypes, papillary and follicular thyroid cancers (PTC and FTC) are referred to well-differentiated thyroid cancers (WDTCs) accounting for more than 90% of all thyroid malignancies (1–3). WDTCs typically are well managed and represent favorable prognosis, however, sometimes, patients progress toward poorly-differentiated and more aggressive subtypes (4).
Nowadays, the molecular pathogenesis of TCs is not completely clear. However, environmental factors alongside the genetic alterations have been shown to play a putative role in the development of thyroid cancers (5, 6). The effect of environmental factors exerts on the regulation of gene expression which are known as epigenetic alterations (5). These alterations are specific to each histological subtype of TCs and define their gene expression patterns and phenotypic characteristics. Therefore, understanding the details of epigenetic alterations in TCs can provide effective therapeutic strategies.
Aberrant DNA methylation has been introduced as a major epigenetic event in tumorigenesis (7, 8). Gene inactivation through hypermethylation of CpG islands located in promoter regions were described in many types of cancers (9, 10). Promoter methylation of tumor suppressor genes which are thyroid specific have been widely studied in WDTCs, however, to date, the majority of these reports are relied on the non-quantitative analysis of methylation specific PCR (MSP) methodology (9, 11–14).
Solute carrier family 5 member 5 (NIS also called SLC5A5) gene is located on chromosome 19 (19p13.11) and encodes an 80–90 kDa transmembrane glycoprotein that actively transports iodide from the bloodstream into the thyroid follicular cells (15). Thyroid-stimulating hormone (TSH) regulates NIS protein activity which consequently participates in the regulation of iodine uptake by thyroid follicular cells. In this process, a Na+/K+-ATPase pump generates a sodium gradient, transporting two Na+ to an I− as the first step in the thyroid hormones biosynthesis (16).
Tissue inhibitor of metalloproteinase-3 (TIMP-3), as a member of tissue inhibitors of matrix metalloproteinases (MMPs), is involved in different cellular processes like cell growth, angiogenesis, invasion, and metastasis in human cancers. TIMP-3 plays an important role in the regulation of vascular endothelial growth factor (VEGF)-mediated angiogenesis by direct binding to VEGF receptor-2 (VEGFR-2). This process prevents downstream essential signaling pathways for cell stimulation. Therefore, according to the potential angiogenesis inhibitory role of TIMP-3, it has drawn particular attention for therapeutic purposes. It has been demonstrated that hypermethylation of TIMP-3 gene was associated with clinicopathological characteristics of PTC, including extrathyroid invasion, lymph node metastasis, and advanced disease stages (III and IV) (17). TIMP-3 blocks the binding of VEGF to the receptor, and inhibit downstream signaling pathways and angiogenesis (18). Beside this, the promoter hyper-methylation of TIMP-3 gene and subsequently its down-regulation, have been observed in thyroid cancers and they are in association with aggressive pathological features (11, 19).
According to the pivotal role of NIS in the process of WDTCs pathogenesis and also in the effectiveness of the current therapeutic strategies by radioactive iodine, the objective of this paper is an investigation on NIS promoter methylation and its expression in PTC and FTC patients. Furthermore, TIMP-3 is the other studied gene in the present study owing to its targeting can be one of the essential therapeutic strategies. In the present study, DNA methylation in the promoter of these two genes including has been assessed in PTC and FTC thyroid tissues compared to matched non-tumoral as well as other benign tissues, in Iranian population.