CHROMATIN STRUCTURE OF THE EGFR GENE SUGGESTS A ROLE FOR INTRON-1 SEQUENCES IN ITS REGULATION IN BREAST-CANCER CELLS

The chromatin structure of the epidermal growth factor receptor gene (EGFR) has been analyzed in several human breast cancer cell lines exhibiting a wide range of EGFR expression. Using DNase I, structural differences were identified in the promoter, first exon, and intron 1 of the EGFR gene that correlate with its expression. Specifically, a DNase I hypersensitive site (DH site) around the exon 1/intron 1 boundary occurred preferentially in estrogen receptor positive breast cancer cell lines with low levels of EGFR expression, while a group of DH sites in intron 1 were observed in estrogen receptor negative, high EGFR expressors. Additionally, a region in the promoter was sensitive to DNase I in all breast cancer cells expressing EGFR, but showed differences in both the level of nuclease sensitivity and the extent of the area that was susceptible. Fine mapping by native genomic blotting revealed the presence of multiple protein footprints in both the promoter and first intron of the EGFR gene in MDA-MB-468 cells, a breast cancer cell line that overexpresses the EGFR gene. The appearance of DH sites in intron 1 associated with high levels of EGFR expression suggests that these regions of the gene contain potential enhancer elements, while the absence of a DH site at the exon 1/intron 1 boundary when the gene is up-regulated suggests the action of a repressor that may block transcriptional elongation.