In-frame deletion in the EGF receptor alters kinase inhibition by gefitinib.

The existence of an in-frame deletion mutant correlates with the sensitivity of lung cancers to EGFR (epidermal growth factor receptor)-targeted tyrosine kinase inhibitors. We reported previously that the in-frame 15-bp deletional mutation (delE746-A750 type deletion) was constitutively active in cells. Kinetic parameters are important for characterizing an enzyme; however, it remains unclear whether the kinetic parameters of deletion mutant EGFR are similar to those of wild-type EGFR. We analysed autophosphorylation in response to ATP and inhibition of gefitinib for deletion mutant EGFR and wild-type EGFR. Kinetic studies, examining autophosphorylation, were carried out using EGFR fractions extracted from 293-pDelta15 and 293-pEGFR cells transfected with deletion mutant EGFR and wild-type EGFR respectively. We demonstrated the difference in activities between unstimulated wild-type (K(m) for ATP=4.0+/-0.3 microM) and mutant EGFR (K(m) for ATP=2.5+/-0.2 microM). There was no difference in K(m) values between EGF-stimulated wild-type EGFR (K(m) for ATP=1.9+/-0.1 microM) and deletion mutant EGFR (K(m) for ATP=2.2+/-0.2 microM). These results suggest that mutant EGFR is active without ligand stimulation. The K(i) value for gefitinib of the deletion mutant EGFR was much lower than that of wild-type EGFR. These results suggest that the deletion mutant EGFR has a higher affinity for gefitinib than wild-type EGFR.