An A13 Repeat within the 3′-Untranslated Region of Epidermal Growth Factor Receptor (EGFR) Is Frequently Mutated in Microsatellite Instability Colon Cancers and Is Associated with Increased EGFR Expression

Colorectal cancers with microsatellite instability (MSI) have clinical, pathological, genetic, and epigenetic features distinct from microsatellite stable (MSS) CRC. Examination of EGFR mRNA and protein expression levels in a panel of colon cancer cell lines identified strong expression of EGFR in multiple cell lines with microsatellite instability. While no relationship between EGFR overexpression and the length of a CA dinucleotide repeat in intron 1 was observed, a variant A13/A14 repeat sequence within the 3’-UTR of the EGFR gene was identified which was mutated by either mono or dinucleotide adenosine deletions in 64% of MSI cell lines, and 69% of MSI colon tumors. Utilizing a Tet-Off system we demonstrate that this mutation increases EGFR mRNA stability in colon cancer cells, providing a mechanistic basis for EGFR over-expression in MSI colon cancer cell lines. To determine whether this mutation is a driver or a bystander event in MSI colon cancer, we examined the effect of pharmacological and molecular inhibition of EGFR in EGFR 3’UTR mutant MSI cell lines. Cell lines with an EGFR 3’UTR mutation and which were wild type for downstream signaling mediators in the Ras/BRAF and PIK3CA/PTEN pathways were sensitive to EGFR inhibition, while those harboring mutations in these signaling mediators were not. Furthermore, in cell lines wild type for downstream signaling mediators, those with EGFR 3’UTR mutations were more sensitive to EGFR inhibition than EGFR 3’UTR WT cells, suggesting this mutation provides a growth advantage to this subset of MSI colon tumors.