Proteomic profiling identifies key coactivators utilized by mutant ERα proteins as potential new therapeutic targets

Approximately 75% of breast cancers are estrogen receptor alpha(ERα)-positive and are treatable with endocrine therapies, but often patients develop lethal resistant disease. Frequent mutations (10–40%) in the ligand-binding domain (LBD) codons in the gene encoding ERα (ESR1) have been identified, resulting in ligand-independent, constitutively active receptors. In addition, ESR1 chromosomal translocationscan occur, resulting in fusion proteins that lack the LBD and are entirely unresponsive to all endocrine treatments. Thus, identifying coactivatorsthat bind to these mutant ERα proteins may offer new therapeutic targets for endocrine- resistant cancer. To define coactivatorcandidate targets, a proteomics approach was performed profiling proteins recruited to the two most common ERα LBD mutants, Y537S and D538G, and an ESR1- YAP1fusion protein. These mutants displayed enhanced coactivatorinteractions as compared to unliganded wild-type ERα. Inhibition of these coactivatorsdecreased the ability of ESR1 mutants to activate transcription and promote breast cancer growth in vitro and in vivo. Thus, we have identified specific coactivatorsthat may be useful as targets for endocrine-resistant breast cancers.