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 alpha (ER alpha) mutations associated with resistance to endocrine therapy and reduced survival. Consistent with their constitutive proliferation, T47D and MCF7 cells in which wild-type ER alpha is replaced by the most common mutations, ER alpha Y537S and ER alpha D538G, 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 ER alpha D538G breast tumors, ER alpha Y537S tumors exhibited a dramatic increase in lung metastasis. Transcriptome analysis showed that the ER alpha Y537S and ER alpha D538G 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 ER alpha 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 ER alpha mutations.