Effect of loss of a chromosome 7 containing multiple copies of EGFR gene on acquired resistance to EGFR-TKIs in EGFR mutated NSCLC.

Abstract

e18091 Background: The epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) show notable effects for non-small cell lung cancers (NSCLC) harboring EGFR activating mutations. However, almost all patients eventually acquire resistance to EGFR-TKIs within several years. The major mechanisms of acquired resistance are acquisition of a secondary EGFR mutation T790M and amplification of MET. However, the mechanisms of other resistance remain unclear. In this study, we established novel erlotinib-resistant NSCLC cells and examined their resistant mechanism. Methods: Resistant cells were established by continuously exposing EGFR mutated NSCLC cells HCC827 to 0.1, 1 or 10 μM of erlotinib in 96 well plates for three months. The resistant mechanisms were determined by direct sequence analysis and EGFR FISH analysis. Results: Resistant cells were emerged from 14/96 and 3/96 wells by 0.1 μM and 1 μM of erlotinib exposure, respectively. No resistant cells appeared in the wells of 10 μM of erlotinib. The IC50 values of these resistant cells were more than 100-fold higher than that of parental cells. No secondary mutation of T790M was detected in any of the resistant cells. Instead, we found that 13/17 resistant cells were dominated by EGFR not amplified cells and one of resistant cells B10 consisted of more than 99.9% of them, while the parental cells consisted of 2.5% of EGFR not amplified cells. Then, we isolated EGFR not amplified clone 4D8 from parental cells and found that this clone also had a resistance to erlotinib comparable to the resistant cells B10. Metaphase FISH analysis showed that EGFR amplified cells in parental cells had a chromosome 7 containing multiple copies of EGFR, while EGFR not amplified cells in B10 did not have it. Furthermore, we found that EGFR not amplified cells were constantly emerged from EGFR amplified clone isolated from parental cells under normal cell culture condition. Conclusions: Loss of a chromosome 7 containing multiple copies of EGFR causes acquired resistance in HCC827 cells when exposed to relatively low concentration of erlotinib, while high concentration of erlotinib deprives HCC827 cells of the chance of emergence of resistant cells.