Estrogen-independent Myc overexpression confers endocrine therapy resistance on breast cancer cells expressing ERαY537S and ERαD538G mutations

Approximately 30% of metastatic breast cancers harbor estrogen receptor α (ERα) mutations associated with resistance to endocrine therapy and reduced survival. Consistent with their constitutive proliferation, T47D and MCF7 cells in which wild-type ERα is replaced by the most common mutations, ERαY537S and ERαD538G, exhibit partially estrogen-independent gene expression. A novel invasion/dissociation/rebinding assay demonstrated that the mutant cells have a higher tendency to dissociate from invasion sites and rebind to a second site. Compared to ERαD538G breast tumors, ERαY537S tumors exhibited a dramatic increase in lung metastasis. Transcriptome analysis showed that the ERαY537S and ERαD538G mutations each elicit a unique gene expression profile. Gene set enrichment analysis showed Myc target pathways are highly induced in mutant cells. Moreover, chromatin immunoprecipitation showed constitutive, fulvestrant-resistant, recruitment of ERα mutants to the Myc enhancer region, resulting in estrogen-independent Myc overexpression in mutant cells and tumors. Knockdown and virus transduction showed Myc is necessary and sufficient for ligand-independent proliferation of the mutant cells but had no effect on metastasis-related phenotypes. Thus, Myc plays a key role in aggressive proliferation-related phenotypes exhibited by breast cancer cells expressing ERα mutations.