Modulation of EGFR gene transcription by secondary structures, a polymorphic repetitive sequence and mutations - a link between genetics and epigenetics

The epidermal growth factor receptor (EGFR) plays a crucial role in growth, differentiation and motility of normal as well as tumor cells. The transdu ction of extracellular signals to the cytoplasm via the receptor not only d epends on ligand binding, but is also determined by the receptor density on the cell surface. Therefore, in terms of cancer diagnosis and therapeutic approaches targeting EGFR it is decisive to know how the expression level o f EGFR is controlled. We found that transcription activity declines with in creasing numbers of CA dinucleotides of a highly polymorphic CA repeat in t he first intron epidermal growth factor receptor gene. In vivo data from cu ltured cell lines support these findings, although other regulation mechani sms can compensate this effect. In addition, we showed that RNA elongation terminates at a site closely downstream of the simple sequence repeat (SSR) and that there are two separate major transcription start sites. Model cal culations for the helical DNA conformation revealed a high bendability in t he EGFR polymorphic region, especially if the CA stretch is extended. These data suggest that the CA-SSR can act like a joint bringing the promoter in proximity to a putative repressor protein bound downstream of the CA-SSR. The data suggest that this polymorphism is a marker for cancer linking gene tic and epigenetic risk. Furthermore in breast cancer, heterozygous tumours with short CA-SSR showed an elevated EGFR-expression in contrast to tumour s with longer CA-SSR. Tumours with loss of heterozygosity in intron 1 of eg fr revealed an increased EGFR expression if the longer allele was lost. Mor eover, deceased egfr gene dosages were significantly correlated to poor pro gnosis in breast cancer.