SUBPROGRAMA: ATENCIÓN A LA INFANCIA DEL SECTOR BIENESTAR

EGFR over expression and prolonged activation of its signalling pathway are characteristic of many solid tumours including that of the head and neck, cervical,

bladder, brain, breast, lung stomach, ovary and prostate (29). In these instances EGFR becomes an oncogene. A summary of studies investigating the expression of EGFR in HNSCC is shown in Table 1.2.1.

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Study Main findings

Grandis and colleagues, 1993 (41)

EGFR mRNA levels were elevated by an average of 69 fold in 92% of SCCHN tumours compared to mRNA levels in healthy normal mucosa.

Chung and colleagues, 2006 (42)

58% of HNSCC tissue samples showed EGFR gene amplification as determined by FISH.

Ryott and colleagues, 2008 (43)

EGFR protein expression was found to be high in 72% in tumours of the oral tongue squamous cell carcinoma. Furthermore 54% of these tumours had a high EGFR gene copy numbers (≥ four gene copy numbers).

Sarkis and colleagues, 2010 (44)

EGFR expression was found in all epithelial layers on oral squamous cell carcinoma specimens where as in normal oral epithelia, EGFR could only be detected in the basal cell layer.

Table 1.2.1 Summary of studies indicating that EGFR is over expressed in HNSCC.

Ryott and colleagues in 2008 showed that EGFR protein was over expressed in 72% of oral tongue squamous cell carcinoma (OTSCC) and that EGFR gene amplification was

correlated with EGFR over-expression, (p=0.004) (43). Figure 1.2.7 shows low and high expression of EGFR protein as detected by IHC and normal and amplified levels of EGFR

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Figure 1.2.7 Levels of EGFR protein and gene copy number as determined by IHC and FISH respectively in OTSCC.

(A) EGFR IHC weak staining. (B) EGFR strong staining. (C) EGFR FISH analysis showing normal disomy copy of genes. (D) EGFR gene amplification. Figure reproduced from reference (43).

As well as gene amplification, EGFR protein may be overexpressed in tumours due to reduced rate of ubiquitin mediated proteasomal degradation. Shortly after EGF binding to wild-type EGFR, EGFR is endocytosed, internalized, ubiquitinated and then

undergoes proteasomal degradation (45). This process allows for the termination of growth signal and provides a refractory period before the next growth signal is

generated. In cells expressing endogenous levels of EGFR (<200,000 receptor molecules per cell), receptor half life is typically between 6-10h, whereas in cells over expressing

This text box is where the original thesis contained the diagram showing ‘levels of EGFR protein and gene copy number as determined by IHC and FISH respectively in OTSCC’ which was reproduced from reference (43).

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EGFR, such as in the epidermoid carcinoma cell line A-431, EGFR half life can exceed 24h (46).

Degradation of EGFR once internalised, is rapid. Following EGFR

transphosphorylation, members of the Cbl family of proteins, which contain ring finger E3 ubiquitin ligases, are able to tag EGFR with ubiquitin thereby directing its

proteasomal degradation (46). Cbl proteins contain a tyrosine kinase binding domain (TKB) that can bind directly to Tyr1045 of EGFR. In this way EGFR, once

phosphorylated, is quickly degraded via Cbl.

EGFR tyrosine kinase domain mutations such as L858R/T790M can impair Cbl-EGFR association and significantly slow endocytosis and degradation compared to cells expressing wild type EGFR (47). Cells expressing these mutant receptors have shown persistently longer mitogenic and antiapoptotic signalling compared to wild type cells (47).

In addition to mutations of the tyrosine kinase domain, EGFRvIII mutants lack the ectodomain (ligand binding portion) of EGFR (29) and are constitutively active, independent of EGF (29). EGFRvIII mutations are commonly found in gliomas, non small cell lung cancer and SCCHN. (29). Furthermore, a study by Sok and colleagues in 2006, found 42% of SCCHN tumours expressing EGFRvIII (48). In addition, EGFRvIII

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transfected cells showed reduced rates of apoptosis when treated with cisplatin and decreased growth inhibition with cetuximab treatment when compared to parental cells (48).

Normally growth factors exert their effects via paracrine stimulation (36). However, it has been observed that SCCHN cells may be able to self induce EGFR receptor

activation via secretion of such ligands as TGF-α (transforming growth factor) (29). i.e. autocrine stimulation.

Increased EGFR gene copy number, over-expression (or reduced degradation) of EGFR and autocrine TGF-α stimulation, have all been correlated with poor survival outcome and increased metastatic potential (29, 49, 50). Grandis and colleagues found 2 year survival rates for SCCHN patients to be 31% for patients with tumours expressing high EGFR levels compared with 90% for those with tumours expressing low EGFR

levels.(p=0.001) (50). In the same study they also observed that the 2 year overall survival for patients with tumours secreting high levels of TGF-α as compared to those with tumours secreting low levels was 50% and 89% (p=0.001) respectively (50).

Several studies have suggested that EGFR signalling pathway may be important in causing resistance to radiotherapy treatment. A correlative study was performed, by Ang and colleagues, which examined EGFR levels in patients with advanced head and neck

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cancer who had undergone radiotherapy (51). Patients who had tumours expressing relatively low levels of EGFR had significantly greater overall survival, disease free survival, and local regional control rates (51) Thereby inferring that tumours expressing relatively higher levels of EGFR expression are more radioresistant. Similarly, studies involving patients with glioblastoma multiforme or cervical cancer, also demonstrated that EGFR over-expression was a significant predictor for poor response to radiation (52, 53).