Breast cancer-derived M543V mutation in helix 12 of estrogen receptor α inverts response to estrogen and SERMs

We have isolated from human breast cancers several mutations in the Helix 12 component of activation function 2 (AF-2) in the estrogen receptor alpha (ERα). We used a novel approach to detect changes in the hormone-binding domain of ERα, based on the evidence that antiestrogens, such as 4-hydroxytamoxifen (ZOHT) and ICI 182,780, block the function of ERα by binding and folding the AF-2 transcriptional domain in a way that inhibits its association with coactivator proteins. We have identified a Helix 12 mutation, M543V, which leads to greater ERα transcription with ZOHT and other antiestrogens (including 1,1-dichloro-2,2,3-triarylcyclopropanes, DTACs) than with 17-β estradiol (E2). We also found an independent mutation at the same position, M543I, which did not show this inverted ligand phenotype. In comparison to further Helix 12 mutations made in vitro, it appears that relative hydrophobicity of the amino acid side chains on the inner face of Helix 12 is key to maintaining the transcriptionally active, agonist conformation with bound E2. This active conformation can be induced, resulting in increased transcription, by adding excess p160 coactivator AIB1 in transcriptional assays with E2-bound receptors, while the ZOHT-bound receptors were not further activated by AIB1. Other experiments show that the cross talk between ERα and AP-1 protein from AP-1-binding sites is not dependent on Helix 12 integrity. We show that two alleles containing a proline substitution in Helix 12 that inactivate AF-2 function of ERα at EREs have little negative effect on function through AP-1 elements, supporting a prominent role for the N-terminal AF-1 of ERα in AP-1/ERα transcriptional cross talk.