1. ¹Division of Molecular and Genomic Medicine, National Health Research Institutes, Zhunan, Taiwan
2. ²Institute of Genetics and Genome Research Center, National Yang-Ming University, Taipei, Taiwan
3. ³Department of Pathology, Chang Gung Memorial Hospital, Taipei, Taiwan
4. ⁴Department of Pathology, Municipal Wan-Fang Hospital, Taipei, Taiwan
5. ⁵Department of Pathology, School of Medicine, Taipei Medical University, Taipei, Taiwan

Correspondence: Dr S-F Huang, Division of Molecular and Genomic Medicine, National Health Research Institutes, 35, Keyan Road, Zhunan Town, Miaoli County 350, Taiwan. E-mail: sfhuang@nhri.org.tw

Received 8 June 2005; Revised 29 August 2005; Accepted 29 August 2005; Published online 3 October 2005.

Abstract

Mutations in the kinase domain of epidermal growth factor receptor (EGFR) are associated with clinical responsiveness to gefitinib in patients with non-small-cell lung cancers (NSCLC). Recently, we have identified many novel EGFR mutations in NSCLC tissues. In this study, we found that gefitinib could suppress the tyrosine phosphorylation of most EGFR mutants better than the wild-type receptor. However, gefitinib had quite variable growth-suppressive effects on different EGFR mutant-expressing cells. All tested EGFR mutants have high basal phosphorylation at multiple tyrosine residues. Upon EGF stimulation, the mutated EGFRs did not have apparently stronger phosphorylation at tyrosines 845, 992, 1068, and 1173 than the wild-type receptor. However, stronger phosphorylation at tyrosine 1045 was observed in the S768I, L861Q, E709G, and G719S mutants. The E746-A750 deletion mutant was less responsive to EGF than the wild-type and other mutant receptors. The S768I, L861Q, E709G, and G719S mutants were refractory to EGF-induced ubiquitination and had more sustained tyrosine phosphorylation. E709G and G719S also lacked EGF-induced receptor downregulation. Our results indicate that, in addition to sensitivity to gefitinib, EGFR mutations also caused various changes in EGFR's regulatory mechanisms, which may contribute to the constitutive activation of EGFR mutants and oncogenesis in NSCLC.