Breast cancers are often treated with antioestrogens, response rates being highest in cancers expressing oestrogen receptors, responsive tumours will however eventually become resistant. This thesis examines the role of altered oestrogen receptors in the development of antioestrogen resistance. Oestiogen receptors are nuclear hormone receptors that on binding oestradiol bind to specific sequences on DNA and are able to modify the rate of transcription of target genes. Antioestrogens competitively bind to the oestrogen receptor displacing oestradiol but are unable to fully activate the receptor. Oestrogen receptors with aberrant properties may lead to antioestrogen resistance and could arise from mutation of the gene or through alternate splicing of exons. Variant oestrogen receptor MRNA arising from alternative splicing of coding exons has been reported in breast cancers. One splice variant lacking exon 5 encodes a truncated receptor, which has constitutive activity in yeast. To explore the role of this variant in antioestrogen resistant breast cancer, stable cell lines expressing the variant on a conditional promoter have been isolated. These clones do not appear to have altered proliferative response to oestrogen or antioestrogens and there is no constitutive induction of oestrogen responsive genes in the presence of splice variant receptor, however modest activity of some reporter constructs is seen. A short region of the hormone binding domain of the receptor has been identified as being important in determining the response of the receptor to ligand, specific mutations leading to loss of oestradiol activation but leading to stimulation of transcription by antioestrogens in HeLa cells. The activity of such mutant receptors has been examined in breast cancer cell lines. Modest agonist activity is seen in transient expression systems and attempts to establish stable expression of mutant receptors are described. A system for the identification of mutations in this region has been used to screen for mutations in human breast cancers. Mutations that alter the ligand binding properties of the receptor have been reported. In order to identify mutations that are unable to bind tamoxifen but are still activated by oestrogen, a yeast screening system was used to screen randomly mutated oestrogen receptors. The properties of mutations identified from this system are described
