**1. Introduction**

The incidence of thyroid cancer (TC) has increased from about five new cases per 100,000 persons observed in the early 90s to 15 new cases per 100,000 persons recorded in 2012 [1]. This increase is mainly due to the improved ability to detect malignancy in small thyroid nodules [2, 3]. TC represents about 96% of all endocrine malignancies and one of the most frequent cancers in women [1]. Based on histological and clinical criteria, TC are classified as well-differentiated TC (DTC), which includes the papillary (PTC) and follicular (FTC) histotypes, poorly differentiated TC (PDTC), and anaplastic TC (ATC). The PTC accounts for about 86% of all epithelial TC. It appears as a mass of branching papillae covered by cells with eosinophilic cytoplasm and enlarged nuclei and typically metastasizes via lymphatic vessels to local lymph nodes [4]. The FTC accounts for approximately 9% of all TC. It resembles the normal microscopic pattern of the thyroid and is characterized by hematogenous spread producing lung and bone metastases [4]. The less differentiated and more aggressive PDTC and ATC, each of which accounts for 1–2% of all TC, are thought to develop from the dedifferentiation of DTC, according to the multistep model of thyroid carcinogenesis [4–8]. The PDTC was included as a separate entity in the WHO classification of TC in 2004. PDTC retains sufficient differentiation to produce scattered small follicular structures and some thyroglobulin but generally lacks the usual morphologic characteristics of DTC, showing an intermediate clinical behavior between DTC and ATC. In addition, it is characterized by highgrade features such as widely infiltrative growth, necrosis, vascular invasion, and numerous mitotic figures [6, 9]. The ATC is composed of disseminated fleshy masses with areas of necrosis and hemorrhage. The cells have an undifferentiated phenotype with marked cytological atypia and high mitotic activity, and they are negative for thyroglobulin [4].

Established risk factors for TC include radiation exposure, family history of TC, lymphocytic thyroiditis, reduced iodine intake, and female gender [10]. All of them are thought to induce chromosome instability (CIN) in thyrocytes through still poorly defined direct and indirect mechanisms [10–13]. Actually, number and frequency of chromosomal abnormalities increase from DTC to PDTC and ATC [13]. CIN is also sustained by alterations in cell cycle regulators, frequently encountered in TC [10]. In particular, a deregulated control of the G1/S transition, following either an increased expression of promoting factors (cyclin D1 and E2F) or the downregulation or presence of loss-of-function mutations of factors inhibiting the G1/S transition (retinoblastoma, p16INK4A, p21CIP1, p27KIP1, and p53), has been documented in TC [10]. In addition, the aberrant expression of mitotic kinases, such as the polo-like kinase and the three members of the Aurora kinase family, is held co-responsible for abnormal cell divisions and the establishment of aneuploid TC cells [14, 15].

About 80% of PTC are characterized by mutually exclusive activating somatic mutations of genes encoding for proteins involved in the mitogen-activated protein kinase (MAPK) signaling pathway [4, 16]. These include rearrangements of the *RET* (rearranged during transformation) (RET/PTC) and neurotrophic tyrosine kinase receptor 1 (*NTRK1*) genes, and activating point mutations of the three *RAS* oncogenes (*HRAS*, *KRAS*, and *NRAS*) and *BRAF* [16]. In addition, mutations of genes encoding key players of the phosphoinositide 3-kinase (PI3K) pathway, such as *PTEN*, *PIK3CA*, and *AKT1*, have been reported in PTC at lower frequencies [16]. Genetic alterations encountered in FTC include activating point mutations of *RAS*, present in about 45% of FTC; rearrangement of the paired-box gene 8 (*PAX-8*) with the peroxisome proliferator-activator receptor-γ (PAX8-PPARγ), observed in 35% of FTC; loss-offunction mutations of the tumor suppressor *PTEN* gene, encountered in about 10% of FTC; activating mutations or amplification of the *PI3KCA* gene, present in about 10% of FTC [17, 18].

Progression of DTC to PDTC and ATC implies tumor acquisition of novel genetic alterations, which are absent or present with low frequency in DTC tissues. Among these are mutations of the tumor suppressor gene *p53*, thought to be a gatekeeper of TC progression from the indolent DTC to the aggressive PDTC and lethal ATC [19]. In fact, *p53* mutations are rarely encountered in DTC (5–9% of cases), while increase in the PDTC (17–38% of cases) and ATC (67–88% of cases) [10, 20, 21]. A similar trend regards the *CTNNB1* gene, encoding the βcatenin, involved in cell adhesion and in the wingless (Wnt) signaling pathway [10]. In particular, *CTNNB1* gene mutations are not found in DTC, while they are present in PDTC (25% of cases) and ATC (66% of cases) [22, 23]. The conversion of early-stage TC to more aggressive and invasive malignancies occurs through an epithelial-to-mesenchymal transition (EMT), which implies the loss of cell-cell contacts, remodeling of cytoskeleton, and the acquisition of a migratory phenotype [24, 25]. Reduced expression of E-cadherin and abnormal expression of integrins, Notch, MET, TGFβ, NF-kB, PI3K, TWIST1, matrix metalloproteinases, components of the urokinase plasminogen-activating system and p21-activated kinase, all of them involved in the EMT, have been identified in TC progression [24–29].

Total thyroidectomy followed by adjuvant therapy with radioactive iodide (131I) is the treatment of choice for the majority of patients affected by DTC [30]. Although the prognosis of these patients is favorable, with 10-year survival rate around 90%, about one-third of them face the morbidity of disease recurrence and TC-related deaths [30]. The worst outcomes are observed in patients with PDTC and ATC, in which the reduced expression of the thyroidspecific gene natrium/iodide symporter (NIS) renders 131I treatment useless [31–33]. In particular, patients affected by ATC have a dismal prognosis with a mean survival time of few months from the diagnosis [32]. Outcome of ATC patients is not influenced by current anticancer treatments (i.e., palliative surgery when possible, chemotherapy, and radiotherapy), and in the majority of cases, death occurs following tumor airway obstruction [34]. Thus, the identification of novel therapeutic approaches capable of improving PDTC and ATC patients' outcome is very much needed.
