Elsevier

Lung Cancer

Volume 63, Issue 3, March 2009, Pages 315-321
Lung Cancer

Review
Impact of EGFR mutation analysis in non-small cell lung cancer

https://doi.org/10.1016/j.lungcan.2008.06.021Get rights and content

Abstract

The discovery of mutations in the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) gene in non-small cell lung cancer (NSCLC) accelerated the research of molecular-targeted therapy by EGFR-tyrosine kinase inhibitors (TKIs), such as gefitinib and erlotinib. About 90% of EGFR mutations are clustered in exons 19 (deletion) and 21 (point mutation at codon 858) and patients with these mutations have great response to EGFR-TKIs. However, tumors that initially respond to EGFR-TKIs almost inevitably become resistant later and T790M secondary mutation in the EGFR gene and MET amplification are reported to account for the mechanism of this acquired resistance. In this review, we summarize the recent findings about EGFR mutations, amplification, alterations of other related genes and sensitivity and acquired resistance to EGFR-TKIs. We also discuss from our studies the relationship between EGFR mutations and other molecular alterations such as aberrant methylation in tumor suppressor genes (TSGs), which indicates that they are related to the mechanism of the pathogenesis of lung cancer. The accumulated important data confer further insights on translational research, providing us with the new strategies for the treatment of NSCLCs.

Introduction

Lung cancer is the leading cause of cancer-related death in many countries around the world including Japan [1], [2]. Non-small cell lung cancers (NSCLCs) account for approximately 80% of lung cancers and the most effective treatment for them is surgery when the tumor is confined to primary site with minimal regional lymph node involvement. However, only small portion of total NSCLC cases are operable and even patients with early stage of lung cancer who undergo curative surgery develop recurrence. Platinum-based chemotherapy is the standard therapy for advanced or recurrent NSCLCs, but the results of the treatment are still unsatisfactory [3]. Therefore, more effective treatments are urgently needed for advanced or recurrent NSCLCs.

As one of the new therapeutic strategies to improve the outcome of treatment for NSCLCs, targeting epidermal growth factor receptor (EGFR) was considered because EGFR is frequently overexpressed and aberrantly activated in NSCLCs [4]. The presence of activating mutations in EGFR was initially reported in 2004 [5], [6], [7]. Following these reports, various groups confirmed and extended these findings [8], [9], [10], [11], [12], [13], [14], [15]. The finding of these mutations within kinase domain of EGFR gene in lung adenocarcinomas is of great clinical interest, because many of these tumors are highly sensitive to EGFR-tyrosine kinase inhibitors (TKIs) [5], [6], [7]. While most of these tumors initially respond to EGFR-TKIs, vast majority of them finally become resistant to drug treatment. Second-site point mutation in EGFR (T790M) was reported to lead the resistance to TKIs in EGFR-mutant NSCLCs [16], [17]. More recently, the amplification of MET proto-oncogene was also reported to contribute to the acquired resistance [18], [19] and MET amplification was shown to occur independently of EGFR T790M mutations [18], [19]. MET is a heterodimeric transmembrane receptor tyrosine kinase (RTK) for the hepatocyte growth factor [20], [21]. Recently Guo et al. [22] reported that multiple RTKs are activated in lung cancer cell lines with EGFR mutation or MET amplification.

From the point of view of the molecular pathogenesis, it is also important to elucidate the relationship between EGFR mutations and other molecular alterations such as other EGFR-related gene mutations or aberrant methylation in tumor suppressor genes (TSGs), because a specific interaction of genetic and epigenetic alterations has been reported in colorectal cancers, suggesting the specific interaction between mutations and aberrant methylation [23].

In this review, we discuss the significance of EGFR mutation analysis in NSCLCs, focusing on the relationship among EGFR mutations, response to TKIs, the activation of multiple RTKs, other molecular alterations such as methylation and acquired resistance including EGFR T790M mutation and MET amplification.

Section snippets

EGFR mutational subtypes

EGFR mutations mainly exist in exons 18–21, the first four exons encoding tyrosine kinase domain (Fig. 1A). The majority of EGFR mutations consist of deletions in exon 19 and point mutation (L858R) in exon 21 [24]. In exon 19, there are over 20 variant types of deletion. As minor EGFR mutations, we also see the mutation at codon 719 (exon 18) and in-frame insertion mutations in exon 20. According to the data of compiled publications, 569 mutations were found in 2880 lung cancer patients and

Biological significance of EGFR mutations and the activation of multiple RTKs

Mutations in exons 19 and 21 lead to the constitutive autophosphorylation, thus activation of EGFR [26], [27]. Mutant EGFR is reported to selectively activate Akt and signal transducer and activator of transcription protein (STAT) signaling pathways that promote cell survival but less effect on the mitogen-activated protein kinase (MAPK) pathway that induces proliferation [28]. Distinct mutational patterns result in different tumorigenicity and response to TKI according to in vitro analysis [26]

EGFR mutations and sensitivity to EGFR-TKIs

The reason why the finding of EGFR mutations conferred a great impact on the clinicians and researchers is that lung cancers harboring this feature showed a dramatic response to EGFR-TKIs [5], [6], [7]. High response rates (around 70%) in patients with EGFR mutations to gefitinib were confirmed in the recently published prospective phase II studies ([33], [34], [35], [36] and Fig. 2). Besides, several reports indicate that patients with EGFR mutations have a significantly longer survival than

Increased EGFR gene copy number

In addition to mutations, increased gene copy number is another mechanism of oncogene activation. Cappuzzo et al. [51] first reported that an increase in the EGFR gene copy number, as measured by fluorescence in situ hybridization (FISH), is more predictive of patient survival after gefitinib treatment than EGFR mutations. However, this report does not necessarily refute the role of EGFR mutations as predictive factors, because EGFR mutations failed only to significantly affect overall survival

Relationship between EGFR mutations and alterations of other related genes

In addition to EGFR, other related gene alterations are important issues regarding tumorigenesis of lung. Mutations of EGFR and KRAS are reported to be mutually exclusive [8], [10]. Besides, there are a few reports that any of EGFR, HER2, KRAS or BRAF mutations occur simultaneously in individual patients, indicating that at least one activating mutation in the EGFR–RAS–RAF signaling pathway may be sufficient for the pathogenesis of many lung cancers. By contrast, PIK3CA mutations, which are

EGFR mutations and other molecular alterations

Epigenetic alteration of TSGs including aberrant methylation is an important mechanism in human carcinogenesis [61]. We previously reported that methylation of p16 and CDH13 genes is less methylated in EGFR-mutant adenocarcinomas than in EGFR-wild-type adenocarcinomas. In addition, the degree of methylation represented with methylation index is lower in EGFR-mutant tumors than in EGFR-wild-type tumors. As for prognosis, Brock et al. [62] recently reported that the methylation of p16 and CDH13

Acquired resistance to TKIs and MET amplification

Even the patients with EGFR-activating mutations initially respond to TKIs dramatically, the vast majority of them develop the acquired resistance to TKIs. In approximately half of these cases, the acquired resistance to TKIs is due to the second-site point mutation of the EGFR gene at codon 790 in exon 20 (T790M) [16], [17], [65], [66]. We reported that T790M is present in a very small fraction (0.5%) of patients without history of gefitinib treatment [67]. We developed an assay for the

Conclusions

The development of drug therapies that inhibit receptor tyrosine kinases, especially EGFR-TK, and the discovery of EGFR gene mutations have provided a great opportunity to develop individualized therapies for NSCLCs. Understanding the molecular backgrounds of EGFR and other targeted genes that determine sensitivity and resistance to a certain drug will develop the strategies for the treatment of lung cancer. Further understanding of EGFR and its related genes will be essential so as to decrease

Conflict of interest

TM received honoraria of less than $10,000 per year from AstraZeneca, Chugai Pharmaceuticals, Bristol-Meyers Squibb, Sanofi-Aventis, Taiho Pharmaceuticals, Kyowa Hakko.

Acknowledgment

This work was supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan 18390386 (to TM) and 18790993 (to ST).

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