An estrogen receptor mutant with strong hormone-independent activity from a metastatic breast cancer

Thirty tumors from metastatic breast cancer patients were screened for mutations in the estrogen receptor (ER) gene using single-strand conformation polymorphism and sequence analysis. Three missense mutations, Ser47Thr, Lys531Glu, and Tyr537Asn, were identified in these lesions. To investigate these mutated ERs or altered transcriptional activation function, expression vectors containing wild-type (wt) and mutant ERs were constructed and cotransfected with different estrogen response element reporter gene constructs into HeLa cells and MDA-MB-231 human breast cancer cells. The first two ER mutants were similar to wt ER. However, the Tyr537Asn ER mutant possessed a potent, estradiol-independent transcriptional activity, as compared to wt ER. Moreover, the constitutive activity of the Tyr537Asn ER mutant was virtually unaffected by estradiol, tamoxifen, or the pure antiestrogen ICI 164,384. Tyr537 is located at the beginning of exon 8 in the COOH-terminal portion of the hormone-binding domain of the ER, to which dimerization and transcription activation functions have also been ascribed. It has been identified as a phosphorylation site implicated in hormone binding, dimerization, and hormone-dependent transcriptional activity. Our results suggest that the Tyr537Asn substitution induces conformational changes in the ER that might mimic hormone binding, not affecting the ability of the receptor to dimerize, but conferring a constitutive transactivation function to the receptor. If present in other metastatic breast tumors, this naturally occurring ER mutant may contribute to breast cancer progression and/or hormone resistance.