Downregulation of EGFR gene expression in colon cancer with microsatellite instability

LB-154

The epidermal growth factor receptor (EGFR), a prominent member of the erbB gene family that encodes a tyrosine kinase receptor, regulates cell growth and survival by binding specific growth factors. Overexpression of EGFR occurs in a variety of cancers, contributing to neoplastic transformation, and is linked to a poor cancer prognosis. The basal EGFRgene expression is influenced by the length of a microsatellite repeat sequence (CA)_n that is located in the proximity of an enhancer in intron 1: the longer the repeat length, the lower the gene expression. Due to inactivation of DNA mismatch repair mechanism, genes with repeat sequences accumulate many mutations in tumors with microsatellite instability (MSI). In this study, we show that the EGFR (CA)_n microsatellite is frequently mutated in MSI-positive colon tumors. The vast majority of the observed mutations results in selective elongation of the repeat length, which should lead to a decrease of EGFR expression in tumor cells. The elongation of the dinucleotide repeat was negatively associated with mutations in KRAS and BRAF, but not in p53. This elongation tendency is not an intrinsic property of the repeat because it is not observed in MSI-positive colon tumor cells cultured in vitro, under the presumed absence of the selective pressure for down-regulation of the EGFR expression. It appears, therefore, that the length expansion of the EGFR (CA)_n is selected for during tumorigenesis of MSI-positive colorectal tumors. FACS analysis of the single cell clones with different CA numbers shows that cell surface EGFR expression decreases with (CA)_n elongation and depends exclusively on the length of the repeat in vitro. Although, EGFR is considered a protooncogene, the results of our study suggest the existence of a strong selection for EGFR downregulation in vivo, which may be an early event contributing to tumorigenesis of MSI-positive colon cancer.