Down-regulation of epidermal growth factor receptor by selective expansion of a 5′-end regulatory dinucleotide repeat in colon cancer with microsatellite instability

Abstract

Purpose: The epidermal growth factor receptor (EGFR) is overexpressed in several tumor types, and its expression is influenced by the length of a 5′-end microsatellite repeat (CA)n: the longer the repeat, the lower the expression. Dinucleotide repeats accumulate insertion/deletion types of mutations in tumors with microsatellite instability.We designed this study to estimate the occurrence of thesemutations in EGFR(CA)n and their relevance in carcinogenesis of microsatellite instability - positive colon and gastric tumors. Experimental Design: We analyzed the frequency of EGFR(CA) n mutations in vivo in 55 colorectal and 14 gastric microsatellite instability - positive cancers, and in vitro in single-cell clone cultures of microsatellite instability - positive colon tumor cell line LS174. Single-cell clone cultures with different repeat lengths were analyzed by fluorescent-activated cell sorter for EGFR cell-surface expression. A correlation analysis was done between EGFR(CA)n mutations and mutations in KRAS, BRAF, and p53. Results: Unlike single-cell clone cultures,which exhibited higher rate of deletions compared with insertions, most of EGFR(CA)n mutations in colon and gastric tumors were insertions. Longer EGFR(CA)n correlated with lower EGFR cell-surface expression in single-cell clone cultures. In colon cancers, the elongation of the repeat was associated negatively with mutations in KRAS and BRAF, but not in p53. Conclusions: The EGFR(CA)n elongation observedin tumors cannot be explained by an intrinsic property of this repeat favoring insertions versus deletions. Instead, a selection for repeat elongation occurs in microsatellite instability - positive tumors, leading to EGFR down-regulation. These findings suggest that in microsatellite instability - positive tumors current therapies targeting EGFR overexpression may have either no effect or an opposite to the expected effect. © 2009 American Association for Cancer Research.