Abstract
Overexpression of EGFR in Thyroid Tumor Progression
Landriscina et al. (1) used IHC to assess EGFR expression in well-differentiated papillary thyroid cancer (PTC), poorly differentiated thyroid cancer of follicular origin (PDTC), and anaplastic thyroid cancers (ATCs). EGFR1 is also symbolized by EGFR. The HUGO Gene Nomenclature Committee–approved symbol for epidermal growth factor receptor is EGFR (2), thus it will be used throughout. Landriscina et al. (1) showed that four out of nine PDTCs and 7 out of 10 ATCs overexpressed EGFR. By contrast, only 4 out of 30 PTCs overexpressed EGFR. This difference was highlighted in IHC of tumor samples, in which the well-differentiated component stained weakly for EFGR and the poorly differentiated component had strong staining for EGFR. The authors concluded from these results that increased expression of EGFR plays a role in progression of thyroid cancers. These data are consistent with results from a number of other groups (3 –6) (Table 1). The overall results from the five different groups show that 69 of 104 (66%) of ATCs overexpressed EGFR. Whereas Landriscina et al. did not explore the potential mechanism for increased EGFR expression in ATC, Liu et al. (7) used quantitative polymerase chain reaction to assess EGFR copy number in 41 ATCs. They found that 19 of 41 ATCs had an increase in copy number of the EGFR gene. Lee et al. (8) also found an increased copy number of EGFR in 14 of 23 (61%) ATCs. They further demonstrated that all ATCs found to overexpress EGFR by IHC had an increase in EGFR copy number, suggesting that the two were causally related. Rodrigues et al. (9) found high levels of amplification at 7p (location of EGFR) in cell lines derived from ATCs. Thus, the combined results of these groups indicate that EGFR is commonly overexpressed in advanced thyroid cancers, and in at least some cases this may be the result of an increase in genomic copies of EGFR.
EGFR, epidermal growth factor.
Role of EGFR in Thyroid Cancer Progression
An area requiring additional investigation is the role that increased EGFR levels may play in the progression of thyroid cancer. EGFR is a potent activator of both the mitogen activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways. Since both these pathways are important in thyroid tumor progression (10), EGFR may promote disease progression by further increasing their activity. Indeed, increased output of effectors downstream of Ras are required for progression of Kras-induced lung cancers (11), and in a knock-in model of Hras-induced papillomas, increased Hras activity was required for the transition from hyperplasia to papilloma (12). This possibility is also supported by the observation that activation of EGFR further accentuated RET/PTC activation and output of its downstream effectors (13). Similarly, the addition of epidermal growth factor (EGF) to a panel of thyroid cancer cell lines that express EGFR further increased activation of the PI3K/AKT and MAPK pathways (14 –17). This was independent of the oncogenic alteration since a subsequent report (10) found the cohort of lines had a mixture of oncogenic alterations including activating mutations of RAS and BRAF.
Recent studies suggest that epithelial-mesenchymal transition (EMT) is an important step in the progression to PDTC (18) and ATC (19 –22). EGF/EGFR signaling has been demonstrated to play an important role in EMT during development and in the progression of other cancers (reviewed by Hardy et al. [23]). Thus, it is possible that activation of EGFR and its downstream effectors may promote thyroid cancer progression by promoting EMT. In support of this, treatment of pig thyrocytes with a combination of EGF and transforming growth factor beta (TGFβ) was shown to induce EMT, which is dependent on activation of the MAPK pathway (24).
Clinical Implications for Increased EGFR in Advanced Thyroid Cancer
Is it possible that increased EGFR expression in ATCs might predict a response to EGFR-targeted therapies? The literature on the effects of EGFR inhibition in culture is confounded by the recent report that many of the commonly used cell lines in prior studies were not of thyroid origin (25). Nevertheless, if only the studies (14 –17,26) that used bona fide thyroid cancer cell lines are considered (i.e., Hth83, C643, 8505C, Hth74, SW1736, Hth7, Hth104, WRO, FRO, ACT1, OCUT), growth of most of them, except Hth104 and OCUT (14), was dampened by EGFR kinase inhibitors. All lines except OCUT were found to express EGFR, which likely explains its resistance. The reason for the lack of response in Hth104, which was subsequently shown to harbor a BRAFV600E alteration (10), is not clear.
As for human studies, a phase 1 clinical trial of gefitinib included one patient with ATC, who had a partial response. However, a phase 2 trial, which included 27 thyroid radioiodine-refractory, locally advanced, or metastatic thyroid cancer patients, found no partial or complete responses (27). Of note, these patients were not screened for mutations in EGFR, which predict sensitivity to EGFR kinase inhibitors in lung and colorectal cancers, or mutations in BRAF and RAF, which predict resistance (28 –31). There have been three case reports describing a clinical response to EGFR-targeted therapies in patients with thyroid cancers positive for an EGFR mutation (32 –34). However, the frequency of activating EGFR mutations in thyroid cancer is very low. Although Masago et al. (34) reported that EGFR mutations were common in PTCs (30%), a subsequent study (35) by the same group was not consistent with these findings, as only one EGFR alteration in 87 tumors samples was found. In other studies (Table 2), EGFR alterations in thyroid cancers were not found at all (6 –8,36) or were exceedingly rare (37,38). Of note, the mutations found in the latter two studies do not correspond to known oncogenic mutants and have not been verified to be gain-of-function substitutions.
Mutations found in papillary thyroid cancers (PTCs).
Mutation found in a poorly differentiated thyroid cancer of follicular origin.
Mutation found in bronchial and cervical metastases that was diagnosed as an ATC.
Mutation(s) found were in PTC and did not correspond to known oncogenic mutants which have been verified to be gain-of-function substitutions.
Mutation found in a oncocytic thyroid carcinoma.
In summary, it appears that overexpression of EGFR is a common event in thyroid tumor progression. As with lung cancer (39), the current clinical trial data (27), albeit somewhat limited, does not support the hypothesis that overexpression of EGFR predicts sensitivity to EGFR-targeted therapies. Moreover, advanced thyroid cancers commonly have alterations in BRAF and RAS, which in lung (30,31) and colorectal (28,29) cancer predict resistance to EGFR-targeted therapies.