Are Estrogen Receptor Genomic Aberrations Predictive of Hormone Therapy Response in Breast Cancer?

Abstract

Context: Breast cancer is the most common cancer in women worldwide. Estrogen receptor (ER) positive breast cancer constitutes the majority of these cancers. Hormone therapy has significantly improved clinical outcomes for early- and late-stage hormone receptor positive breast cancer. Although most patients with early stage breast cancer are treated with curative intent, approximately 20% - 30% of patients eventually experience a recurrence. During the last two decades, there have been tremendous efforts to understand the biological mechanisms of hormone therapy resistance, with the ultimate goal of implementing new therapeutic strategies to improve the current treatments for ER positive breast cancer. Several mechanisms of hormone therapy resistance have been proposed, including genetic alterations that lead to altered ER expression or ERs with changed protein sequence.

Evidence acquisition: A Pubmed search was performed utilizing various related terms. Articles over the past 20 years were analyzed and selected for review.

Results: On the basis of published studies, the frequencies of ESR1 (the gene encoding ER) mutations in ER positive metastatic breast cancer range from 11% to 55%. Future larger prospective studies with standardized mutation detection methods may be necessary to determine the true incidence of ESR1 mutations. ESR1 amplification in breast cancer remains a controversial issue, with numerous studies either confirmed or challenged the reports of ESR1 amplification. The combination of intra-tumor heterogeneity regarding ESR1 copy number alterations and low level ESR1 copy number increase may account for these discrepancies.

Conclusions: While numerous unknown issues on the role of ESR1 mutations in advanced breast cancer remain, these new findings will certainly deepen current knowledge on molecular evolution of breast cancer and acquired resistance to hormone therapy.

Keywords: Breast Cancer; ESR1 Amplification; ESR1 Mutation; Estrogen Receptor; Hormone Therapy.

Figure 1.. Estrogen Signaling Pathway

Pathways of estrogen regulation of gene expression have been shown. In the classical estrogen signaling, estrogen (E) binds estrogen receptors (ERs), induces dimerization of the protein, and activates gene expression through binding to estrogen response elements (EREs) in the promoter of target genes, in complex with co-activators (CoAs) and histone acetyl transferases (HATs). Activation of signaling events downstream of receptor tyrosine kinases (RTKs) can also lead to ER phosphorylation (P) through Erk or Akt serine/threonine kinases and subsequent ligand-independent activation of ER. Estrogen signaling can also be mediated through non-genomic mechanisms by ER in the cytoplasm or membrane. Ligand binding leads to the formation of functional protein complexes that involve other signaling pathways, resulting in transcription factor (TF) activation. For instance, ligand binding leads to methylation (M) of ER, and subsequent formation of the ER-PI3K-Src-focal adhesion kinase (FAK) complex which activates Akt, and ultimately alters gene expression. GF: Growth factor; RE: Response element; VEGF: Vascular endothelial growth factor; IGFR1: Insulin-like growth factor receptor 1; TGFα: Transforming growth factor α.

Figure 2.. Evolution of ESR1 Mutations in ER + Breast Cancer with Hormone Therapy

ESR1 mutations are rare in primary ER + breast cancers, and are considerably enriched in metastatic, hormone therapy resistant breast tumors. Almost all of the mutations in the estrogen receptor are localized in the ligand binding domain (LBD) of the estrogen receptor and leads to constitutive activation of the protein. AF1: Activation Function 1; DBD: DNA binding domain; AF2: Activation function 2; LBD: Ligand binding domain.