Estrogen receptor (ER) α mutations in breast cancer: hidden in plain sight

Transfer Learning in Cancer Genetics, Mutation Detection, Gene Expression Analysis, and Syndrome Recognition

Artificial intelligence (AI), encompassing machine learning (ML) and deep learning (DL), has revolutionized medical research, facilitating advancements in drug discovery and cancer diagnosis. ML identifies patterns in data, while DL employs neural networks for intricate processing. Predictive modeling challenges, such as data labeling, are addressed by transfer learning (TL), leveraging pre-existing models for faster training. TL shows potential in genetic research, improving tasks like gene expression analysis, mutation detection, genetic syndrome recognition, and genotype-phenotype association. This review explores the role of TL in overcoming challenges in mutation detection, genetic syndrome detection, gene expression, or phenotype-genotype association. TL has shown effectiveness in various aspects of genetic research. TL enhances the accuracy and efficiency of mutation detection, aiding in the identification of genetic abnormalities. TL can improve the diagnostic accuracy of syndrome-related genetic patterns. Moreover, TL plays a crucial role in gene expression analysis in order to accurately predict gene expression levels and their interactions. Additionally, TL enhances phenotype-genotype association studies by leveraging pre-trained models. In conclusion, TL enhances AI efficiency by improving mutation prediction, gene expression analysis, and genetic syndrome detection. Future studies should focus on increasing domain similarities, expanding databases, and incorporating clinical data for better predictions.

The Biological Significance of AFF4: Promoting Transcription Elongation, Osteogenic Differentiation and Tumor Progression

As a member of the AF4/FMR2 (AFF) family, AFF4 is a scaffold protein in the superelongation complex (SEC). In this mini-view, we discuss the role of AFF4 as a transcription elongation factor that mediates HIV activation and replication and stem cell osteogenic differentiation. AFF4 also promotes the progression of head and neck squamous cell carcinoma, leukemia, breast cancer, bladder cancer and other malignant tumors. The biological function of AFF4 is largely achieved through SEC assembly, regulates SRY-box transcription factor 2 (SOX2), MYC, estrogen receptor alpha (ESR1), inhibitor of differentiation 1 (ID1), c-Jun and noncanonical nuclear factor-κB (NF-κB) transcription and combines with fusion in sarcoma (FUS), unique regulatory cyclins (CycT1), or mixed lineage leukemia (MLL). We explore the prospects of using AFF4 as a therapeutic in Acquired immunodeficiency syndrome (AIDS) and malignant tumors and its potential as a stemness regulator.

ESR1 Gene Mutations and Liquid Biopsy in ER-Positive Breast Cancers: A Small Step Forward, a Giant Leap for Personalization of Endocrine Therapy?

The predominant forms of breast cancer (BC) are hormone receptor-positive (HR+) tumors characterized by the expression of estrogen receptors (ERs) and/or progesterone receptors (PRs). Patients with HR+ tumors can benefit from endocrine therapy (ET). Three types of ET are approved for the treatment of HR+ BCs and include selective ER modulators, aromatase inhibitors, and selective ER downregulators. ET is the mainstay of adjuvant treatment in the early setting and the backbone of the first-line treatment in an advanced setting; however, the emergence of acquired resistance can lead to cancer recurrence or progression. The mechanisms of ET resistance are often related to the occurrence of mutations in the ESR1 gene, which encodes the ER-alpha protein. As ESR1 mutations are hardly detectable at diagnosis but are present in 30% to 40% of advanced BC (ABC) after treatment, the timeline of testing is crucial. To manage this resistance, ESR1 testing has recently been recommended; in ER+ HER2- ABC and circulating cell-free DNA, so-called liquid biopsy appears to be the most convenient way to detect the emergence of ESR1 mutations. Technically, several options exist, including Next Generation Sequencing and ultra-sensitive PCR-based techniques. In this context, personalization of ET through the surveillance of ESR1 mutations in the plasma of HR+ BC patients throughout the disease course represents an innovative way to improve the standard of care.

Exploring the mechanism of aloe-emodin in the treatment of liver cancer through network pharmacology and cell experiments

Objective: Aloe-emodin (AE) is an anthraquinone compound extracted from the rhizome of the natural plant rhubarb. Initially, it was shown that AE exerts an anti-inflammatory effect. Further studies revealed its antitumor activity against various types of cancer. However, the mechanisms underlying these properties remain unclear. Based on network pharmacology and molecular docking, this study investigated the molecular mechanism of AE in the treatment of hepatocellular carcinoma (HCC), and evaluated its therapeutic effect through in vitro experiments. Methods: CTD, Pharmmapper, SuperPred and TargetNet were the databases to obtain potential drug-related targets. DisGenet, GeneCards, OMIM and TTD were used to identify potential disease-related targets. Intersection genes for drugs and diseases were obtained through the Venn diagram. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of intersecting genes were conducted by the website of Bioinformatics. Intersection genes were introduced into STRING to construct a protein-protein interaction network, while the Cytoscape3.9.1 software was used to visualize and analyze the core targets. AutoDock4.2.6 was utilized to achieve molecular docking between drug and core targets. In vitro experiments investigated the therapeutic effects and related mechanisms of AE. Results: 63 overlapped genes were obtained and GO analysis generated 3,646 entries by these 63 intersecting genes. KEGG analysis mainly involved apoptosis, proteoglycans in cancer, TNF signaling pathway, TP53 signaling pathway, PI3K-AKT signaling pathway, etc. AKT1, EGFR, ESR1, TP53, and SRC have been identified as core targets because the binding energies of them between aloe-emodin were less than -5 kcal/Mol.The mRNA and protein expression, prognosis, mutation status, and immune infiltration related to core targets were further revealed. The involvement of AKT1 and EGFR, as well as the key target of the PI3K-AKT signaling pathway, indicated the importance of this signaling pathway in the treatment of HCC using AE. The results of the Cell Counting Kit-8 assay and flow analysis demonstrated the therapeutic effect of AE. The downregulation of EGFR, PI3KR1, AKT1, and BCL2 in mRNA expression and PI3KR1, AKT,p-AKT in protein expression confirmed our hypothesis. Conclusion: Based on network pharmacology and molecular docking, our study initially showed that AE exerted a therapeutic effect on HCC by modulating multiple signaling pathways. Various analyses confirmed the antiproliferative activity and pro-apoptotic effect of AE on HCC through the PI3K-AKT signaling pathway. This study revealed the therapeutic mechanism of AE in the treatment of HCC through a novel approach, providing a theoretical basis for the clinical application of AE.

Allele-Specific Gene Regulation, Phenotypes, and Therapeutic Vulnerabilities in Estrogen Receptor Alpha-Mutant Endometrial Cancer

Activating estrogen receptor alpha (ER; also known as ESR1) mutations are present in primary endometrial and metastatic breast cancers, promoting estrogen-independent activation of the receptor. Functional characterizations in breast cancer have established unique molecular and phenotypic consequences of the receptor, yet the impact of ER mutations in endometrial cancer has not been fully explored. In this study, we used CRISPR-Cas9 to model the clinically prevalent ER-Y537S mutation and compared results with ER-D538G to discover allele-specific differences between ER mutations in endometrial cancer. We found that constitutive activity of mutant ER resulted in changes in the expression of thousands of genes, stemming from combined alterations to ER binding and chromatin accessibility. The unique gene expression programs resulted in ER-mutant cells developing increased cancer-associated phenotypes, including migration, invasion, anchorage-independent growth, and growth in vivo. To uncover potential treatment strategies, we identified ER-associated proteins via Rapid Immunoprecipitation and Mass Spectrometry of Endogenous Proteins and interrogated two candidates, CDK9 and NCOA3. Inhibition of these regulatory proteins resulted in decreased growth and migration, representing potential novel treatment strategies for ER-mutant endometrial cancer.

IMPLICATIONS

This study provides insight into mutant ER activity in endometrial cancer and identifies potential therapies for women with ER-mutant endometrial cancer.

Evolving perspectives on the treatment of HR+/HER2+ metastatic breast cancer

Breast cancer (BC) with expression of the estrogen receptor (ER) and/or progesterone receptor (PR) protein and with overexpression/amplification of the human epidermal growth factor receptor 2 (HER2), termed hormone receptor-positive (HR+)/HER2+ BC, represents ∼10% of all BCs in the United States. HR+/HER2+ BC includes HER2+ BCs that are ER+, PR+, or both ER+ and PR+ (triple-positive BC). Although the current guideline-recommended treatment combination of anti-HER2 monoclonal antibodies plus chemotherapy is an effective first-line therapy for many patients with HER2+ advanced disease, intratumoral heterogeneity within the HR+/HER2+ subtype and differences between the HR+/HER2+ subtype and the HR-/HER2+ subtype suggest that other targeted combinations could be investigated in randomized clinical trials for patients with HR+/HER2+ BC. In addition, published data indicate that crosstalk between HRs and HER2 can lead to treatment resistance. Dual HR and HER2 pathway targeting has been shown to be a rational approach to effective and well-tolerated therapy for patients with tumors driven by HER2 and HR, as it may prevent development of resistance by blocking receptor pathway crosstalk. However, clinical trial data for such approaches are limited. Treatments to attenuate other signaling pathways involved in receptor crosstalk are also under investigation for inclusion in dual receptor targeting regimens. These include cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, based on the rationale that association of CDK4/6 with cyclin D1 may play a role in resistance to HER2-directed therapies, and others such as phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway inhibitors. Herein, we will review the scientific and clinical rationale for combined receptor blockade targeting HER2 and ER for patients with advanced-stage HR+/HER2+ disease.

Estrogen Receptor Bio-Activities Determine Clinical Endocrine Treatment Options in Estrogen Receptor-Positive Breast Cancer

In estrogen receptor positive (ER+) breast cancer therapy, estrogen receptors (ERs) are the major targeting molecules. ER-targeted therapy has provided clinical benefits for approximately 70% of all breast cancer patients through targeting the ERα subtype. In recent years, mechanisms underlying breast cancer occurrence and progression have been extensively studied and largely clarified. The PI3K/AKT/mTOR pathway, microRNA regulation, and other ER downstream signaling pathways are found to be the effective therapeutic targets in ER+ BC therapy. A number of the ER+ (ER+) breast cancer biomarkers have been established for diagnosis and prognosis. The ESR1 gene mutations that lead to endocrine therapy resistance in ER+ breast cancer had been identified. Mutations in the ligand-binding domain of ERα which encoded by ESR1 gene occur in most cases. The targeted drugs combined with endocrine therapy have been developed to improve the therapeutic efficacy of ER+ breast cancer, particularly the endocrine therapy resistance ER+ breast cancer. The combination therapy has been demonstrated to be superior to monotherapy in overall clinical evaluation. In this review, we focus on recent progress in studies on ERs and related clinical applications for targeted therapy and provide a perspective view for therapy of ER+ breast cancer.

Type 1 Nuclear Receptor Activity in Breast Cancer: Translating Preclinical Insights to the Clinic

The nuclear receptor (NR) family of transcription factors is intimately associated with the development, progression and treatment of breast cancer. They are used diagnostically and prognostically, and crosstalk between nuclear receptor pathways and growth factor signalling has been demonstrated in all major subtypes of breast cancer. The majority of breast cancers are driven by estrogen receptor α (ER), and anti-estrogenic therapies remain the backbone of treatment, leading to clinically impactful improvements in patient outcomes. This serves as a blueprint for the development of therapies targeting other nuclear receptors. More recently, pivotal findings into modulating the progesterone (PR) and androgen receptors (AR), with accompanying mechanistic insights into NR crosstalk and interactions with other proliferative pathways, have led to clinical trials in all of the major breast cancer subtypes. A growing body of evidence now supports targeting other Type 1 nuclear receptors such as the glucocorticoid receptor (GR), as well as Type 2 NRs such as the vitamin D receptor (VDR). Here, we reviewed the existing preclinical insights into nuclear receptor activity in breast cancer, with a focus on Type 1 NRs. We also discussed the potential to translate these findings into improving patient outcomes.

Identification and Roles of miR-29b-1-3p and miR29a-3p-Regulated and Non-Regulated lncRNAs in Endocrine-Sensitive and Resistant Breast Cancer Cells

Despite improvements in the treatment of endocrine-resistant metastatic disease using combination therapies in patients with estrogen receptor α (ERα) primary tumors, the mechanisms underlying endocrine resistance remain to be elucidated. Non-coding RNAs (ncRNAs), including microRNAs (miRNA) and long non-coding RNAs (lncRNA), are targets and regulators of cell signaling pathways and their exosomal transport may contribute to metastasis. Previous studies have shown that a low expression of miR-29a-3p and miR-29b-3p is associated with lower overall breast cancer survival before 150 mos. Transient, modest overexpression of miR-29b1-3p or miR-29a-3p inhibited MCF-7 tamoxifen-sensitive and LCC9 tamoxifen-resistant cell proliferation. Here, we identify miR-29b-1/a-regulated and non-regulated differentially expressed lncRNAs in MCF-7 and LCC9 cells using next-generation RNA seq. More lncRNAs were miR-29b-1/a-regulated in LCC9 cells than in MCF-7 cells, including DANCR, GAS5, DSCAM-AS1, SNHG5, and CRND. We examined the roles of miR-29-regulated and differentially expressed lncRNAs in endocrine-resistant breast cancer, including putative and proven targets and expression patterns in survival analysis using the KM Plotter and TCGA databases. This study provides new insights into lncRNAs in endocrine-resistant breast cancer.

Integrated multi-omics profiling of high-grade estrogen receptor-positive, HER2-negative breast cancer

Estrogen receptor‐positive and human epidermal growth factor receptor 2‐negative (ER⁺HER2⁻) breast cancer accounts for ~ 60–70% of all cases of invasive breast carcinoma. High‐grade ER⁺HER2⁻ tumors respond poorly to endocrine therapy. In this study, we systematically analyzed clinical and multi‐omics data to find potential strategies for personalized therapy of patients with high‐grade ER⁺HER2⁻ disease. Six different cohorts were analyzed, for which multi‐omics data were available. Grade III ER⁺HER2⁻ cases harbored higher proportions of large tumor size (> 5 cm), lymph node metastasis, chemotherapy use, and luminal B subtypes defined by PAM50, as compared with grade I/II tumors. DNA methylation (HM450) data and methylation‐specific PCR indicated that the cg18629132 locus in the MKI67 promoter was hypermethylated in grade I/II cases and normal tissue, but hypomethylated in grade III cases or triple‐negative breast cancer, resulting in higher expression of MKI67. Mutations in ESR1 and TP53 were detected in post‐endocrine treatment metastatic samples at a higher rate than in treatment‐naive tumors in grade III cases. We identified 42 and 20 focal copy number events in nonmetastatic and metastatic high‐grade ER⁺HER2⁻ cases, respectively, with either MYC or MDM2 amplification representing an independent prognostic event in grade III cases. Transcriptional profiling within grade III tumors highlighted ER signaling downregulation and upregulation of immune‐related pathways in non‐luminal‐like tumors defined by PAM50. Recursive partitioning analysis was employed to construct a decision tree of an endocrine‐resistant subgroup (GATA3‐negative and AGR‐negative) of two genes that was validated by immunohistochemistry in a Chinese cohort. All together, these data suggest that grade III ER⁺HER2⁻ tumors have distinct clinical and molecular characteristics compared with low‐grade tumors, particularly in cases with non‐luminal‐like biology. Due to the dismal prognosis in this group, clinical trials are warranted to test the efficacy of potential novel therapies.

Hormonal modulation of ESR1 mutant metastasis

Estrogen receptor alpha gene (ESR1) mutations occur frequently in ER-positive metastatic breast cancer, and confer clinical resistance to aromatase inhibitors. Expression of the ESR1 Y537S mutation induced an epithelial-mesenchymal transition (EMT) with cells exhibiting enhanced migration and invasion potential in vitro. When small subpopulations of Y537S ESR1 mutant cells were injected along with WT parental cells, tumor growth was enhanced with mutant cells becoming the predominant population in distant metastases. Y537S mutant primary xenograft tumors were resistant to the antiestrogen tamoxifen (Tam) as well as to estradiol (E2) withdrawal. Y537S ESR1 mutant primary tumors metastasized efficiently in the absence of E2; however, Tam treatment significantly inhibited metastasis to distant sites. We identified a nine-gene expression signature, which predicted clinical outcomes of ER-positive breast cancer patients, as well as breast cancer metastasis to the lung. Androgen receptor (AR) protein levels were increased in mutant models, and the AR agonist dihydrotestosterone significantly inhibited estrogen-regulated gene expression, EMT, and distant metastasis in vivo, suggesting that AR may play a role in distant metastatic progression of ESR1 mutant tumors.

A Closer Look at Estrogen Receptor Mutations in Breast Cancer and Their Implications for Estrogen and Antiestrogen Responses

Breast cancer (BC) is the most common cancer among women worldwide. More than 70% of BC cases express estrogen receptor alpha (ERα), a central transcription factor that stimulates the proliferation of breast cancer cells, usually in the presence of estrogen. While most cases of ER-positive BC initially respond to antiestrogen therapies, a high percentage of cases develop resistance to treatment over time. The recent discovery of mutated forms of ERα that result in constitutively active forms of the receptor in the metastatic-resistance stage of BC has provided a strong rationale for the development of new antiestrogens. These molecules targeting clinically relevant ERα mutants and a combination with other pharmacological inhibitors of specific pathways may constitute alternative treatments to improve clinical practice in the fight against metastatic-resistant ER-positive BC. In this review, we summarize the latest advances regarding the particular involvement of point mutations of ERα in endocrine resistance. We also discuss the involvement of synonymous ERα mutations with respect to co-translational folding of the receptor and ribosome biogenesis in breast carcinogenesis.

Targeting Notch in oncology: the path forward

Notch signalling is involved in many aspects of cancer biology, including angiogenesis, tumour immunity and the maintenance of cancer stem-like cells. In addition, Notch can function as an oncogene and a tumour suppressor in different cancers and in different cell populations within the same tumour. Despite promising preclinical results and early-phase clinical trials, the goal of developing safe, effective, tumour-selective Notch-targeting agents for clinical use remains elusive. However, our continually improving understanding of Notch signalling in specific cancers, individual cancer cases and different cell populations, as well as crosstalk between pathways, is aiding the discovery and development of novel investigational Notch-targeted therapeutics.

Endocrine therapy resistance: new insights

The estrogen receptor positive (ER+) subset is the dominant contributor to global deaths from breast cancer which now exceeds 500,000 deaths annually. Lethality is driven by endocrine resistance, which has been shown to be associated with high mutational rates and extreme subclonal diversity. Treatment forces subclonal selection until the patient eventually succumbs to metastatic treatment-resistant disease. Recently, we have been addressing several questions related to this process: What is the cause of the increased mutation rate in lethal ER+ breast cancer? Why is endocrine therapy resistance related to mutational load? What are the functions of the somatic mutations that are eventually selected in the treatment resistant and metastatic clones? These questions have provoked new mechanistic hypotheses that link resistance to endocrine agents to: (1) Specific defects in single strand break repair are associated with increased mortality from ER+ breast cancer [1,2]; (2) Loss/mutations of certain single strand break repair proteins that disrupt estrogen-regulated cell cycle control through the ATM, CHK2, CDK4 axis [1,2] thereby directly coupling endocrine therapy resistance to specific DNA repair defects; (3) Acquired mutations that drive metastasis include the generation of in-frame ESR1 gene fusions that activate epithelial-to-mesenchymal transition (EMT) driven metastasis as well as endocrine drug-resistant proliferation [3].

Acetylation of histone H3K27 signals the transcriptional elongation for estrogen receptor alpha

As approximately 70% of human breast tumors are estrogen receptor α (ERα)-positive, estrogen and ERα play essential roles in breast cancer development. By interrupting the ERα signaling pathway, endocrine therapy has been proven to be an effective therapeutic strategy. In this study, we identified a mechanism by which Transcription Start Site (TSS)-associated histone H3K27 acetylation signals the Super Elongation Complex (SEC) to regulate transcriptional elongation of the ESR1 (ERα) gene. SEC interacts with H3K27ac on ESR1 TSS through its scaffold protein AFF4. Depletion of AFF4 by siRNA or CRISPR/Cas9 dramatically reduces expression of ESR1 and its target genes, consequently inhibiting breast cancer cell growth. More importantly, a AFF4 mutant which lacks H3K27ac interaction failed to rescue ESR1 gene expression, suggesting H3K27 acetylation at TSS region is a key mark bridging the transition from transcriptional initiation to elongation, and perturbing SEC function can be an alternative strategy for targeting ERα signaling pathway at chromatin level.

New Advances in Molecular Breast Cancer Pathology

Breast cancer (BC) comprises a diverse spectrum of diseases featuring distinct presentation, morphological, biological, and clinical phenotypes. BC behaviour and response to therapy also vary widely. Current evidence indicates that traditional prognostic and predictive classification systems are insufficient to reflect the biological and clinical heterogeneity of BC. Advancements in high-throughput molecular techniques and bioinformatics have contributed to the improved understanding of BC biology, refinement of molecular taxonomies and the development of novel prognostic and predictive molecular assays. Molecular testing has also become increasingly important in the diagnosis and treatment of BC in the era of precision medicine. Despite the enormous amount of research work to develop and refine BC molecular prognostic and predictive assays, it is still in evolution and proper incorporation of these molecular tests into clinical practice to guide patient's management remains a challenge. With the increasing use of more sophisticated high throughput molecular techniques, large amounts of data will continue to emerge, which could potentially lead to identification of novel therapeutic targets and allow more precise classification systems that can accurately predict outcome and response to therapy. In this review, we provide an update on the molecular classification of BC and molecular prognostic assays. Companion diagnostics, contribution of massive parallel sequencing and the use of liquid biopsy are also highlighted.

Synthesis and In Vitro Evaluation of Tetrahydroquinoline Derivatives as Antiproliferative Compounds of Breast Cancer via Targeting the GPER

BACKGROUND

Some reports have demonstrated the role of the G Protein-coupled Estrogen Receptor (GPER) in growth and proliferation of breast cancer cells.

OBJECTIVE

In an effort to develop new therapeutic strategies against breast cancer, we employed an in silico study to explore the binding modes of tetrahydroquinoline 2 and 4 to be compared with the reported ligands G1 and G1PABA.

METHODS

This study aimed to design and filter ligands by in silico studies determining their Lipinski's rule, toxicity and binding properties with GPER to achieve experimental assays as anti-proliferative compounds of breast cancer cell lines.

RESULTS

In silico studies suggest as promissory two tetrahydroquinoline 2 and 4 which contain a carboxyl group instead of the acetyl group (as is needed for G1 synthesis), which add low (2) and high hindrance (4) chemical moieties to explore the polar, hydrophobic and hindrance effects. Docking and molecular dynamics simulations of the target compounds were performed with GPER to explore their binding mode and free energy values. In addition, the target small molecules were synthesized and assayed in vitro using breast cancer cells (MCF-7 and MDA-MB-231). Experimental assays showed that compound 2 decreased cell proliferation, showing IC50 values of 50µM and 25µM after 72h of treatment of MCF-7 and MDA-MB-231 cell lines, respectively. Importantly, compound 2 showed a similar inhibitory effect on proliferation as G1 compound in MDA-MB-231 cells, suggesting that both ligands reach the GPER-binding site in a similar way, as was demonstrated through in silico studies.

CONCLUSION

A concentration-dependent inhibition of cell proliferation occurred with compound 2 in the two cell lines regardless of GPER.

Identification of a novel C-terminally truncated estrogen receptor α variant (ERαi34) with constitutive transactivation and estrogen receptor antagonist resistance

Constitutively active estrogen receptor α (ERα) variants with C-terminal truncation are candidate molecules for gain of both endocrine- and chemotherapy-resistance in estrogen-sensitive tumors. Our previous reports using artificially truncated ERα constructs demonstrated that ERα variants encoded in 1-2-3-cryptic exon- and 1-2-3-4-cryptic exon-types of transcripts have potentials to display constitutive transactivation of an estrogen response element reporter. However, naturally occurring 1-2-3-cryptic exon-type ERα variants have not been cloned yet. Therefore, the present study was designed to identify naturally occurring ERα variants encoded in 1-2-3-cryptic exon-type ERα transcripts. We cloned a novel C-terminally truncated ERα variant (ERαi34) encoded in a 1-2-3-i34 transcript from MCF-7 cells and characterized its constitutive and ER antagonist-resistant transactivation in transfected cells. Stable transfection of the variant into MCF-7 cells augmented basal cell proliferation insensitive to fulvestrant. Collectively, we validated the structure-based mechanisms underlying constitutive and ER antagonist-resistant transactivation by C-terminally truncated ERα variants.

Ventral prostate and mammary gland phenotype in mice with complete deletion of the ERβ gene

Disagreements about the phenotype of estrogen receptor β (ERβ) knockout mouse, created by removing the DNA-binding domain of the ERβ gene or interruption of the gene with a neocassette (Oliver Smithies ERβ knockout mice [ERβ^OS-/-]), prompted us to create an ERβ knockout mouse by deleting the ERβ gene with the use of CRISPR/Cas9 technology. We confirmed that the ERβ gene was eliminated from the mouse genome and that no ERβ mRNA or protein was detectable in tissues of this mouse. Overall the phenotype of the ventral prostate (VP) and mammary gland (MG) in ERβ^crispr-/- mice was similar to, but more severe than, that in the ERβ^OS-/-mice. In the VP of 6-mo-old ERβ^crispr-/- mice there was epithelial hyperplasia, fibroplasia, inflammation, stromal overgrowth, and intraductal cancer-like lesions. This was accompanied by an increase in Ki67 and P63 and loss in DACH1 and PURα, two androgen receptor (AR) repressors. In the MG there was overexpression of estrogen receptor α and progesterone receptor, loss of collagen, increase in proliferation and expression of metalloproteases, and invasive epithelium. Surprisingly, by 18 mo of age, the number of hyperplastic foci was reduced, the ducts of the VP and MG became atrophic, and, in the VP, there was massive immune infiltration and massive desquamation of the luminal epithelial cells. These changes were coincident with reduced levels of androgens in males and estrogens in females. We conclude that ERβ is a tumor suppressor gene in the VP and MG where its loss increases the activity AR and ERα, respectively.

ESR1 mutations are frequent in newly diagnosed metastatic and loco-regional recurrence of endocrine-treated breast cancer and carry worse prognosis

BACKGROUND

Emerging mutations in the ESR1 gene that encodes for the estrogen receptor (ER) are associated with resistance to endocrine therapy. ESR1 mutations rarely exist in primary tumors (~ 1%) but are relatively common (10-50%) in metastatic, endocrine therapy-resistant cancers and are associated with a shorter progression-free survival. Little is known about the incidence and clinical implication of these mutations in early recurrence events, such as local recurrences or newly diagnosed metastatic disease.

METHODS

We collected 130 archival tumor samples from 103 breast cancer patients treated with endocrine therapy prior to their local/metastatic recurrence. The cohort consisted of 41 patients having at least 1 sample from local/loco-regional recurrence and 62 patients with metastatic disease (of whom 41 newly diagnosed and 28 with advanced disease). The 5 most common ESR1 hotspot mutations (D538G, L536R, Y537S/N/C) were analyzed either by targeted sequencing or by droplet digital PCR. Progression-free survival (PFS), disease-free survival (DFS), and distant recurrence-free survival (DRFS) were statistically tested by Kaplan-Meier analysis.

RESULTS

The prevalence of ESR1 mutations was 5/41 (12%) in newly diagnosed metastatic patients and 5/28 (18%) for advanced metastases, detected at allele frequency > 1%. All mutations in advanced metastases were detected in patients previously treated with both tamoxifen (TAM) and aromatase inhibitors (AI). However, in newly diagnosed metastatic patients, 4/5 mutations occurred in patients treated with TAM alone. PFS on AI treatment in metastatic patients was significantly shorter for ESR1 mutation carriers (p = 0.017). In the local recurrence cohort, ESR1 mutations were identified in 15/41 (36%) patients but only 4/41 (10%) were detected at allele frequency > 1%. Again, most mutations (3/4) were detected under TAM monotherapy. Notably, 1 patient developed ESR1 mutation while on neoadjuvant endocrine therapy. DFS and DRFS were significantly shorter (p = 0.04 and p = 0.017, respectively) in patients that had ESR1 mutations (> 1%) in their loco-regional recurrence tumor.

CONCLUSIONS

Clinically relevant ESR1 mutations are prevalent in newly diagnosed metastatic and local recurrence of endocrine-treated breast cancer. Since local recurrences are amenable to curative therapy, these mutations may inform the selection of subsequent endocrine therapies.

Estrogen-independent molecular actions of mutant estrogen receptor 1 in endometrial cancer

Estrogen receptor 1 (ESR1) mutations have been identified in hormone therapy-resistant breast cancer and primary endometrial cancer. Analyses in breast cancer suggest that mutant ESR1 exhibits estrogen-independent activity. In endometrial cancer, ESR1 mutations are associated with worse outcomes and less obesity, however, experimental investigation of these mutations has not been performed. Using a unique CRISPR/Cas9 strategy, we introduced the D538G mutation, a common endometrial cancer mutation that alters the ligand binding domain of ESR1, while epitope tagging the endogenous locus. We discovered estrogen-independent mutant ESR1 genomic binding that is significantly altered from wild-type ESR1. The D538G mutation impacted expression, including a large set of nonestrogen-regulated genes, and chromatin accessibility, with most affected loci bound by mutant ESR1. Mutant ESR1 is distinct from constitutive ESR1 activity because mutant-specific changes are not recapitulated with prolonged estrogen exposure. Overall, the D538G mutant ESR1 confers estrogen-independent activity while causing additional regulatory changes in endometrial cancer cells that are distinct from breast cancer cells.

ESR1 mutations in breast cancer

The acquisition of ligand-independent ESR1 mutations during aromatase inhibitor therapy in metastatic estrogen receptor (ER)-positive breast cancer is a common mechanism of hormonal therapy resistance. Preclinical and clinical studies have demonstrated that ESR1 mutations can preexist in primary tumors and can be enriched during metastasis. Furthermore, ESR1 mutations express a unique transcriptional profile that favors tumor progression, suggesting that selected ESR1 mutations may influence metastasis. Several groups have used sensitive detection methods using patient liquid biopsies to track ESR1 or truncal somatic mutations to predict treatment outcome and tumor progression, and some of these techniques may eventually be used to guide sequential treatment options in patients. Further development and standardization of mutation tracking in circulating tumor DNA is ongoing. Clinically, patients with ESR1 mutations derive clinical benefit when treated with fulvestrant and CDK4/6-targeted therapies, but the development of more potent selective ER degraders and/or new targeted biotherapies are needed to overcome the endocrine-resistant phenotype of ESR1 mutant-bearing tumors. In this review, we discuss the mechanisms of resistance and dissemination of ESR1 mutations as well as the detection methods for ESR1 mutation tracking, newly discovered potential therapeutic targets, and the clinical implications and treatment options for treating patients with ESR1 mutant-bearing tumors.

Personalized medicine in breast cancer: pharmacogenomics approaches

Abstract: Breast cancer is the fifth cause of cancer death among women worldwide and represents a global health concern due to the lack of effective therapeutic regimens that could be applied to all disease groups. Nowadays, strategies based on pharmacogenomics constitute novel approaches that minimize toxicity while maximizing drug efficacy; this being of high importance in the oncology setting. Besides, genetic profiling of malignant tumors can lead to the development of targeted therapies to be included in effective drug regimens. Advances in molecular diagnostics have revealed that breast cancer is a multifaceted disease, characterized by inter-tumoral and intra-tumoral heterogeneity and, unlike the past, molecular classifications based on the expression of individual biomarkers have led to devising novel therapeutic strategies that improve patient survival. In this review, we report and discuss the molecular classification of breast cancer subtypes, the heterogeneity resource, and the advantages and disadvantages of current drug regimens with consideration of pharmacogenomics in response and resistance to treatment.

History of Estrogen: Its Purification, Structure, Synthesis, Biologic Actions, and Clinical Implications

This mini-review summarizes key points from the Clark Sawin Memorial Lecture on the History of Estrogen delivered at Endo 2018 and focuses on the rationales and motivation leading to various discoveries and their clinical applications. During the classical period of antiquity, incisive clinical observations uncovered important findings; however, extensive anatomical dissections to solidify proof were generally lacking. Initiation of the experimental approach followed later, influenced by Claude Bernard's treatise "An Introduction to the Study of Experimental Medicine." With this approach, investigators began to explore the function of the ovaries and their "internal secretions" and, after intensive investigations for several years, purified various estrogens. Clinical therapies for hot flashes, osteoporosis, and dysmenorrhea were quickly developed and, later, methods of hormonal contraception. Sophisticated biochemical methods revealed the mechanisms of estrogen synthesis through the enzyme aromatase and, after discovery of the estrogen receptors, their specific biologic actions. Molecular techniques facilitated understanding of the specific transcriptional and translational events requiring estrogen. This body of knowledge led to methods to prevent and treat hormone-dependent neoplasms as well as a variety of other estrogen-related conditions. More recently, the role of estrogen in men was uncovered by prismatic examples of estrogen deficiency in male patients and by knockout of the estrogen receptor and aromatase in animals. As studies became more extensive, the effects of estrogen on nearly every organ were described. We conclude that the history of estrogen illustrates the role of intellectual reasoning, motivation, and serendipity in advancing knowledge about this important sex steroid.

Estrogen Signaling in Endometrial Cancer: a Key Oncogenic Pathway with Several Open Questions

Endometrial cancer is the most common gynecological cancer in the developed world, and it is one of the few cancer types that is becoming more prevalent and leading to more deaths in the USA each year. The majority of endometrial tumors are considered to be hormonally driven, where estrogen signaling through estrogen receptor α (ER) acts as an oncogenic signal. The major risk factors and some treatment options for endometrial cancer patients emphasize a key role for estrogen signaling in the disease. Despite the strong connections between estrogen signaling and endometrial cancer, important molecular aspects of ER function remain poorly understood; however, progress is being made in our understanding of estrogen signaling in endometrial cancer. Here, we discuss the evidence for the importance of estrogen signaling in endometrial cancer, details of the endometrial cancer-specific actions of ER, and open questions surrounding estrogen signaling in endometrial cancer.

Strategies to degrade estrogen receptor α in primary and ESR1 mutant-expressing metastatic breast cancer

With the advent of omic technologies, our understanding of the molecular mechanisms underlying estrogen receptor α (ERα)-expressing breast cancer (BC) progression has grown exponentially. Nevertheless, the most widely used therapy for inhibiting this disease is endocrine therapy (ET) (i.e., aromatase inhibitors, tamoxifen - Tam, faslodex/fulvestrant - FUL). However, in a considerable number of cases, prolonged patient treatment with ET generates the development of resistant tumor cells and, consequently, tumor relapse, which manifests as metastatic disease that is extremely difficult to manage, especially because such metastatic BCs (MBCs) often express ERα mutations (e.g., Y537S, D538G) that confer pronounced growth advantages to tumor cells. Interestingly, ET continues to be the therapy of choice for this neoplasia, which underscores the need to identify novel drugs that could work in primary and MBCs. In this study, we review the approaches that have been undertaken to discover these new anti-ERα compounds, especially considering those focused on evaluating ERα degradation. A literature analysis demonstrated that current strategies for discovering new anti-BC drugs are focusing on the identification either of novel ERα inhibitors, of compounds that inhibit ERα-related pathways or of drugs that influence ERα-unrelated cellular pathways. Several lines of evidence suggest that all of these molecules alter the ERα content and block the proliferation of both primary and MBCs. In turn, we propose to rationalize all these discoveries into the definition of e.m.eral.d.s (i.e., selective modulators of ERα levels and degradation) as a novel supercategory of anti-ERα drugs that function both as modulators of ERα levels and inhibitors of BC cell proliferation.

Moving Breast Cancer Therapy up a Notch

Breast cancer is the second most common malignancy, worldwide. Treatment decisions are based on tumor stage, histological subtype, and receptor expression and include combinations of surgery, radiotherapy, and systemic treatment. These, together with earlier diagnosis, have resulted in increased survival. However, initial treatment efficacy cannot be guaranteed upfront, and these treatments may come with (long-term) serious adverse effects, negatively affecting a patient's quality of life. Gene expression-based tests can accurately estimate the risk of recurrence in early stage breast cancers. Disease recurrence correlates with treatment resistance, creating a major need to resensitize tumors to treatment. Notch signaling is frequently deregulated in cancer and is involved in treatment resistance. Preclinical research has already identified many combinatory therapeutic options where Notch involvement enhances the effectiveness of radiotherapy, chemotherapy or targeted therapies for breast cancer. However, the benefit of targeting Notch has remained clinically inconclusive. In this review, we summarize the current knowledge on targeting the Notch pathway to enhance current treatments for breast cancer and to combat treatment resistance. Furthermore, we propose mechanisms to further exploit Notch-based therapeutics in the treatment of breast cancer.

Fulvestrant in management of hormone receptor-positive metastatic breast cancer

Fulvestrant is a steroidal selective estrogen receptor degrader that was approved by the US FDA in 2002 for treatment of ER-positive metastatic breast cancer (ER + MBC) post-progression on aromatase inhibitors. In 2017, the label was updated to include endocrine therapy naive ER + MBC. While initially fulvestrant was thought to be equivalent to aromatase inhibitors with monthly dose of 250 mg intramuscular injection, several postmarketing trials challenged this understanding. Subsequently, the recommended dose changed to 500 mg monthly plus loading dose, and this was proven to be superior to anastrozole in efficacy. Most recently the results of FALCON trial have further challenged the way fulvestrant is viewed and used. This report provides a historical perspective on this compound, its evolving role in management of ER + MBC and what the future may hold for this drug.

Structural underpinnings of oestrogen receptor mutations in endocrine therapy resistance

Oestrogen receptor-α (ERα), a key driver of breast cancer, normally requires oestrogen for activation. Mutations that constitutively activate ERα without the need for hormone binding are frequently found in endocrine-therapy-resistant breast cancer metastases and are associated with poor patient outcomes. The location of these mutations in the ER ligand-binding domain and their impact on receptor conformation suggest that they subvert distinct mechanisms that normally maintain the low basal state of wild-type ERα in the absence of hormone. Such mutations provide opportunities to probe fundamental issues underlying ligand-mediated control of ERα activity. Instructive contrasts between these ERα mutations and those that arise in the androgen receptor (AR) during anti-androgen treatment of prostate cancer highlight differences in how activation functions in ERs and AR control receptor activity, how hormonal pressures (deprivation versus antagonism) drive the selection of phenotypically different mutants, how altered protein conformations can reduce antagonist potency and how altered ligand-receptor contacts can invert the response that a receptor has to an agonist ligand versus an antagonist ligand. A deeper understanding of how ligand regulation of receptor conformation is linked to receptor function offers a conceptual framework for developing new anti-oestrogens that might be more effective in preventing and treating breast cancer.

Estrogen-regulated transcription: Mammary gland and uterus

Estrogen (E2) plays a central role in the developmental, metabolic and reproductive functions of both males and females. E2 acts via the estrogen receptor alpha (ERα) to regulate the transcription of genes involved in numerous cellular functions. The E2-dependent engagement of ERα across regulatory regions of the genome, termed "enhancers", exhibits a high degree of complexity and plasticity. The E2-transcriptional response is defined by pioneer factors, transcription co-factors, posttranslational modifications of ERα, the chromatin environment, and cross talk with other signaling pathways. These inputs collectively define the normal functions of tissues like the mammary gland and the uterus. This mechanism can also provide a selective and aberrant growth advantage in pathological conditions, such as cancer. E2-regulated transcription continues to be an area of great interest in the fields of reproduction and cancer. The goal of these field is to decipher the molecular mechanisms governing ERα transcription to design effective therapeutic strategies for the improvement of clinical care and control of fertility.

Discovery of naturally occurring ESR1 mutations in breast cancer cell lines modelling endocrine resistance

Resistance to endocrine therapy remains a major clinical problem in breast cancer. Genetic studies highlight the potential role of estrogen receptor-α (ESR1) mutations, which show increased prevalence in the metastatic, endocrine-resistant setting. No naturally occurring ESR1 mutations have been reported in in vitro models of BC either before or after the acquisition of endocrine resistance making functional consequences difficult to study. We report the first discovery of naturally occurring ESR1 Y537C and ESR1 Y537S mutations in MCF7 and SUM44 ESR1-positive cell lines after acquisition of resistance to long-term-estrogen-deprivation (LTED) and subsequent resistance to fulvestrant (ICIR). Mutations were enriched with time, impacted on ESR1 binding to the genome and altered the ESR1 interactome. The results highlight the importance and functional consequence of these mutations and provide an important resource for studying endocrine resistance.

Effects of Atrial Natriuretic Peptide on p53 and Estrogen Receptor in Breast Cancer Cells

The atrial natriuretic peptide (ANP) hormone is secreted by cardiac atrial myocytes and acts to regulate blood pressure homeostasis in humans. Previous research indicates ANP treatment significantly decreases the proliferation of human prostate cancer cells, pancreatic adenocarcinoma, and breast cancer cells. Minimal studies have been conducted with regard to ANP regulating tumor suppressor genes and steroid hormone receptors in breast cancer cells. Our study analyzed the effects of ANP in combination with 17β-estradiol (E₂) and antiestrogen treatments on p53 and ERα levels in T-47D breast cancer cells. Preliminary studies through Western blot analysis showed that ANP treatment decreases p53 and ERα expression levels in a concentration-dependent (10-100 nM) manner. Treatment with ANP alone, at a 100 nM concentration, causes a decrease of p53 and ERα expression compared with Cs (control stripped), but with E₂ and antiestrogen combinations, expression of both protein levels decreased compared with treatments without ANP. Combined treatment with E₂, an estrogen antagonist, and ANP decreased cellular proliferation compared with treatments without ANP, except in the case of raloxifene (RAL). Our studies indicate that ANP has potential as a therapeutic breast cancer treatment and should inspire further studies on the molecular mechanism of ANP in T-47D breast cancer cells.

Estrogen receptors in breast and bone: from virtue of remodeling to vileness of metastasis

Bone metastasis is a prominent cause of morbidity and mortality in cancer. High rates of bone colonization in breast cancer, especially in the subtype expressing estrogen receptors (ERs), suggest tissue-specific proclivities for metastatic tumor formation. The mechanisms behind this subtype-specific organ-tropism remains largely elusive. Interestingly, as the major driver of ER+ breast cancer, ERs also have important roles in bone development and homeostasis. Thus, any agents targeting ER will also inevitably affect the microenvironment, which involves the osteoblasts and osteoclasts. Yet, how such microenvironmental effects are integrated with direct therapeutic responses of cancer cells remain poorly understood. Recent findings on ER mutations, especially their enrichment in bone metastasis, raised even more provocative questions on the role of ER in cancer-bone interaction. In this review, we evaluate the importance of ERs in bone metastasis and discuss new avenues of investigation for bone metastasis treatment based on current knowledge.

Characterization of rodent constitutively active estrogen receptor α variants and their constitutive transactivation mechanisms

Estrogen receptor α (ERα) mRNAs exhibit remarkable heterogeneity owing to complicated alternative splicing. Some encode C-terminally-truncated ERα proteins, which display ligand-independent transactivation or dominant-negative activity. We previously characterized C-terminally-truncated ERα mRNA variants with cryptic sequences in humans and mice, and demonstrated that helices in the ligand-binding domains (LBDs) of ERα variants contribute to ligand-independent transcriptional activity. However, existence of non-conventional coding exons and generation of constitutively active ERα variants have remained to be examined in rats. To comparatively analyze modular organization and splicing profiles of the ERα genes, the range of C-terminally-truncated ERα variants was explored in rats and mice using rapid amplification of cDNA ends and RT-PCR cloning. Furthermore, their functions were determined in transiently transfected cells using expression constructs and luciferase reporter assays. Multiple cryptic exons and C-terminally-truncated ERα variant mRNAs were identified in rats and mice. Naturally occurring and artificially truncated variants/constructs lacking helix 5 potentially exhibited gain-of-function in transfected cells. Although cryptic exons and splicing profiles were poorly conserved among humans, mice, and rats, constitutively active variants were generated from the ERα genes. Moreover, the primary mechanism of ligand-independent activation by C-terminally-truncated ERα variants is C-terminus to helix 5 deletion in the LBD. This comparative study documented the complexity of ERα genes, mRNAs, and proteins, and further determined the underlying structural basis of ligand-independent activation by C-terminally-truncated ERα variants.

Mutation site and context dependent effects of ESR1 mutation in genome-edited breast cancer cell models

Background

Mutations in the estrogen receptor alpha (ERα) 1 gene (ESR1) are frequently detected in ER+ metastatic breast cancer, and there is increasing evidence that these mutations confer endocrine resistance in breast cancer patients with advanced disease. However, their functional role is not well-understood, at least in part due to a lack of ESR1 mutant models. Here, we describe the generation and characterization of genome-edited T47D and MCF7 breast cancer cell lines with the two most common ESR1 mutations, Y537S and D538G.

Methods

Genome editing was performed using CRISPR and adeno-associated virus (AAV) technologies to knock-in ESR1 mutations into T47D and MCF7 cell lines, respectively. Various techniques were utilized to assess the activity of mutant ER, including transactivation, growth and chromatin-immunoprecipitation (ChIP) assays. The level of endocrine resistance was tested in mutant cells using a number of selective estrogen receptor modulators (SERMs) and degraders (SERDs). RNA sequencing (RNA-seq) was employed to study gene targets of mutant ER.

Results

Cells with ESR1 mutations displayed ligand-independent ER activity, and were resistant to several SERMs and SERDs, with cell line and mutation-specific differences with respect to magnitude of effect. The SERD AZ9496 showed increased efficacy compared to other drugs tested. Wild-type and mutant cell co-cultures demonstrated a unique evolution of mutant cells under estrogen deprivation and tamoxifen treatment. Transcriptome analysis confirmed ligand-independent regulation of ERα target genes by mutant ERα, but also identified novel target genes, some of which are involved in metastasis-associated phenotypes. Despite significant overlap in the ligand-independent genes between Y537S and D538G, the number of mutant ERα-target genes shared between the two cell lines was limited, suggesting context-dependent activity of the mutant receptor. Some genes and phenotypes were unique to one mutation within a given cell line, suggesting a mutation-specific effect.

Conclusions

Taken together, ESR1 mutations in genome-edited breast cancer cell lines confer ligand-independent growth and endocrine resistance. These biologically relevant models can be used for further mechanistic and translational studies, including context-specific and mutation site-specific analysis of the ESR1 mutations.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0851-4) contains supplementary material, which is available to authorized users.

Detection of ESR1 mutations in plasma and tumors from metastatic breast cancer patients using next-generation sequencing

PURPOSE

Liquid biopsy using digital PCR (dPCR) has been widely used for the screening of ESR1 mutations, since they are frequently identified in the hotspot. However, dPCR is limited to the known mutations. Therefore, we aimed to analyze the utility of next-generation sequencing (NGS) to discover novel ESR1 mutations.

METHODS

Whole exon sequencing of the ESR1 gene using NGS was performed in 16 primary and 47 recurrent tumor samples and 38 plasma samples from hormone receptor-positive metastatic breast cancer patients. Functional analyses were then performed for the novel mutations we detected.

RESULTS

We identified no mutations in primary tumors and six mutations in five recurrent tumors, including three types of known mutations (Y537C, Y537N, and D538G) and two novel mutations (E279V and G557R). We also identified seven mutations in five plasma samples, including three types of known mutations (S463P, Y537S, and D538G) and one mutation not reported in COSMIC database (L536H). All nine patients with ESR1 mutations were treated with aromatase inhibitors (AIs) prior to sampling, and the mutations were frequently detected in patients who received AI treatments in the metastatic setting. Among the three novel mutations (E279V, L536H, and G557R), L536H, but not E279V and G557R, showed ligand-independent activity. All three mutant proteins showed nuclear localization and had no relation with non-genomic ER pathways.

CONCLUSIONS

Although the molecular mechanisms of the E279V and G557R mutations remain unclear, our data suggest the utility of NGS as a liquid biopsy for metastatic breast cancer patients and the potential to identify novel ESR1 mutations.

Different epigenetic mechanisms of ERα implicated in the fate of fulvestrant-resistant breast cancer

Approximately 70% of breast cancers express estrogen receptor α (ERα), which plays critical roles in breast cancer development. Fulvestrant has been effectively used to treat ERα-positive breast cancer, although resistance remains a critical problem. To elucidate the mechanism of resistance to fulvestrant, we established fulvestrant-resistant cell-lines named MFR (MCF-7 derived fulvestrant resistance) and TFR (T-47D derived fulvestrant resistance) from the ERα-positive luminal breast cancer cell lines MCF-7 and T-47D, respectively. Both fulvestrant-resistant cell lines lost sensitivity to estrogen and anti-estrogens. We observed diminished ERα expression at both the protein and mRNA levels. To address the mechanism of gene expression regulation, we examined epigenetic alteration, especially the DNA methylation level of ERα gene promoters. MFR cells displayed high methylation levels upstream of the ERα gene, whereas no change in DNA methylation was observed in TFR cells. Hence, we examined the gene expression plasticity of ERα, as there are differences in its reversibility following fulvestrant withdrawal. ERα gene expression was not restored in MFR cells, and alternative intracellular phosphorylation signals were activated. By contrast, TFR cells exhibited plasticity of ERα gene expression and ERα-dependent growth; moreover, these cells were resensitized to estrogen and anti-estrogens. The difference in epigenetic regulation among individual cells might explain the difference in the plasticity of ERα expression. We also identified an MFR cell-activating HER/Src-Akt/MAPK pathway; thus, the specific inhibitors effectively blocked MFR cell growth. This finding implies the presence of multiple fulvestrant resistance mechanisms and suggests that the optimal therapies differ among individual tumors as a result of differing epigenetic mechanisms regulating ERα gene expression.

Pushing estrogen receptor around in breast cancer

The estrogen receptor-α (herein called ER) is a nuclear sex steroid receptor (SSR) that is expressed in approximately 75% of breast cancers. Therapies that modulate ER action have substantially improved the survival of patients with ER-positive breast cancer, but resistance to treatment still remains a major clinical problem. Treating resistant breast cancer requires co-targeting of ER and alternate signalling pathways that contribute to resistance to improve the efficacy and benefit of currently available treatments. Emerging data have shown that other SSRs may regulate the sites at which ER binds to DNA in ways that can powerfully suppress the oncogenic activity of ER in breast cancer. This includes the progesterone receptor (PR) that was recently shown to reprogram the ER DNA binding landscape towards genes associated with a favourable outcome. Another attractive candidate is the androgen receptor (AR), which is expressed in the majority of breast cancers and inhibits growth of the normal breast and ER-positive tumours when activated by ligand. These findings have led to the initiation of breast cancer clinical trials evaluating therapies that selectively harness the ability of SSRs to 'push' ER towards anti-tumorigenic activity. Our review will focus on the established and emerging clinical evidence for activating PR or AR in ER-positive breast cancer to inhibit the tumour growth-promoting functions of ER.

Targeted therapy for breast cancer and molecular mechanisms of resistance to treatment

In recent years, clinical trials investigating new drugs and therapeutic combinations have led to promising advances in breast cancer therapy. Subtyping breast cancers into hormone receptor (HR) positive, epidermal growth factor receptor (HER2) positive, and triple negative breast cancer (TNBC) is currently the basis of diagnosing and treating this disease. In addition to endocrine and HER2-targeted therapies in their respective subtypes, evidence from recent preclinical studies have shown several targetable pathways that overcome resistance in the clinical setting. The mTOR inhibitor everolimus and the CDK4/6 inhibitor palbociclib have been approved in HR-positive metastatic breast cancer (MBC) due to improved disease-free survival (DFS). Adding pertuzumab to trastuzumab in combination with taxanes further improves DFS in HER2-positive breast cancer. Targeted therapy to the heterogeneous group of TNBC is needed in combination with chemotherapy. However, patient selection and predictive biomarker development remains a big challenge for targeted therapy development in TNBC.

Steroid Hormone Receptor Positive Breast Cancer Patient-Derived Xenografts

The vast majority of breast cancers are positive for estrogen receptor (ER) and depend on estrogens for growth. These tumors are treated with a variety of ER-targeted endocrine therapies, although eventual resistance remains a major clinical problem. Other steroid hormone receptors such as progesterone receptor (PR) and androgen receptor (AR) are emerging as additional prospective targets in breast cancer. The fundamental mechanism of action of these steroid receptors in gene regulation has been defined mainly by several breast cancer cell lines that were established in the late 1970s. More recently, breast cancer patient-derived xenografts (PDX) have been developed by multiple groups at institutions in several countries. These new models capture the large degree of heterogeneity between patients and within tumors and promise to advance our understanding of steroid hormone receptor positive breast cancer and endocrine resistance. Unfortunately, steroid hormone receptor positive breast cancers are much more difficult than their receptor negative counterparts to establish into sustainable PDX. Herein we discuss the derivation of steroid hormone receptor positive breast cancer PDX, several pitfalls in their genesis, and their utility in preclinical and translational steroid hormone receptor research.

Dysregulation of Aromatase in Breast, Endometrial, and Ovarian Cancers: An Overview of Therapeutic Strategies

Aromatase is the rate-limiting enzyme in the biosynthesis of estrogens, which play crucial roles on a spectrum of developmental and physiological processes. The biological actions of estrogens are classically mediated by binding to two estrogen receptors (ERs), ERα and ERβ. Encoded by the cytochrome P450, family 19, subfamily A, polypeptide 1 (CYP19A1) gene, aromatase is expressed in a wide variety of tissues, as well as benign and malignant tumors, and is regulated in a pathway- and tissue-specific manner. Overexpression of aromatase, leading to elevated systemic levels of estrogen, is unequivocally linked to the pathogenesis and growth of a number malignancies, including breast, endometrium, and ovarian cancers. Aromatase inhibitors (AIs) are routinely used to treat estrogen-dependent breast cancers in postmenopausal women; however, their roles in endometrial and ovarian cancers remain obscure. While AI therapy is effective in hormone sensitive cancers, they diminish estrogen production throughout the body and, thus, generate undesirable side effects. Despite the effectiveness of AI therapy, resistance to endocrine therapy remains a major concern and is the leading cause of cancer death. Considerable advances, toward mitigating these issues, have evolved in conjunction with a number of histone deacetylase (HDAC) inhibitors for countering an assortment of diseases and cancers, including the aforesaid malignancies. HDACs are a family of enzymes that are frequently dysregulated in human tumors. This chapter will discuss the current understanding of aberrant regulation and expression of aromatase in breast, endometrial, and ovarian cancers, and potential therapeutic strategies for prevention and treatment of these life-threatening diseases.

Anticipatory UPR Activation: A Protective Pathway and Target in Cancer

The endoplasmic reticulum (EnR) stress sensor, the unfolded protein response (UPR), plays a key role in regulating intracellular protein homeostasis. The extensively studied reactive mode of UPR activation is characterized by unfolded protein, or other EnR stress, triggering UPR activation. Here we focus on the emerging anticipatory mode of UPR activation in which mitogenic steroid and peptide hormones and other effectors preactivate the UPR and anticipate a future need for increased protein folding capacity. Mild UPR activation in breast cancer can be protective and contributes to antiestrogen resistance. Hyperactivation of the anticipatory UPR pathway in cancer cells with a small molecule converts it from cytoprotective to cytotoxic, highlighting its potential as a therapeutic target in estrogen receptor-positive breast cancer.

ESR1 mutations affect anti-proliferative responses to tamoxifen through enhanced cross-talk with IGF signaling

The purpose of this study was to address the role of ESR1 hormone-binding mutations in breast cancer. Soft agar anchorage-independent growth assay, Western blot, ERE reporter transactivation assay, proximity ligation assay (PLA), coimmunoprecipitation assay, silencing assay, digital droplet PCR (ddPCR), Kaplan-Meier analysis, and statistical analysis. It is now generally accepted that estrogen receptor (ESR1) mutations occur frequently in metastatic breast cancers; however, we do not yet know how to best treat these patients. We have modeled the three most frequent hormone-binding ESR1 (HBD-ESR1) mutations (Y537N, Y537S, and D538G) using stable lentiviral transduction in human breast cancer cell lines. Effects on growth were examined in response to hormonal and targeted agents, and mutation-specific changes were studied using microarray and Western blot analysis. We determined that the HBD-ESR1 mutations alter anti-proliferative effects to tamoxifen (Tam), due to cell-intrinsic changes in activation of the insulin-like growth factor receptor (IGF1R) signaling pathway and levels of PIK3R1/PIK3R3. The selective estrogen receptor degrader, fulvestrant, significantly reduced the anchorage-independent growth of ESR1 mutant-expressing cells, while combination treatments with the mTOR inhibitor everolimus, or an inhibitor blocking IGF1R, and the insulin receptor significantly enhanced anti-proliferative responses. Using digital drop (dd) PCR, we identified mutations at high frequencies ranging from 12 % for Y537N, 5 % for Y537S, and 2 % for D538G in archived primary breast tumors from women treated with adjuvant mono-tamoxifen therapy. The HBD-ESR1 mutations were not associated with recurrence-free or overall survival in response in this patient cohort and suggest that knowledge of other cell-intrinsic factors in combination with ESR1 mutation status will be needed determine anti-proliferative responses to Tam.

Recurrent hormone-binding domain truncated ESR1 amplifications in primary endometrial cancers suggest their implication in hormone independent growth

The estrogen receptor alpha (ERα) is highly expressed in both endometrial and breast cancers, and represents the most prevalent therapeutic target in breast cancer. However, anti-estrogen therapy has not been shown to be effective in endometrial cancer. Recently it has been shown that hormone-binding domain alterations of ERα in breast cancer contribute to acquired resistance to anti-estrogen therapy. In analyses of genomic data from The Cancer Genome Atlas (TCGA), we observe that endometrial carcinomas manifest recurrent ESR1 gene amplifications that truncate the hormone-binding domain encoding region of ESR1 and are associated with reduced mRNA expression of exons encoding the hormone-binding domain. These findings support a role for hormone-binding alterations of ERα in primary endometrial cancer, with potentially important therapeutic implications.

Endocrine resistance in breast cancer--An overview and update

Tumors that express detectable levels of the product of the ESR1 gene (estrogen receptor-α; ERα) represent the single largest molecular subtype of breast cancer. More women eventually die from ERα+ breast cancer than from either HER2+ disease (almost half of which also express ERα) and/or from triple negative breast cancer (ERα-negative, progesterone receptor-negative, and HER2-negative). Antiestrogens and aromatase inhibitors are largely indistinguishable from each other in their abilities to improve overall survival and almost 50% of ERα+ breast cancers will eventually fail one or more of these endocrine interventions. The precise reasons why these therapies fail in ERα+ breast cancer remain largely unknown. Pharmacogenetic explanations for Tamoxifen resistance are controversial. The role of ERα mutations in endocrine resistance remains unclear. Targeting the growth factors and oncogenes most strongly correlated with endocrine resistance has proven mostly disappointing in their abilities to improve overall survival substantially, particularly in the metastatic setting. Nonetheless, there are new concepts in endocrine resistance that integrate molecular signaling, cellular metabolism, and stress responses including endoplasmic reticulum stress and the unfolded protein response (UPR) that provide novel insights and suggest innovative therapeutic targets. Encouraging evidence that drug combinations with CDK4/CDK6 inhibitors can extend recurrence free survival may yet translate to improvements in overall survival. Whether the improvements seen with immunotherapy in other cancers can be achieved in breast cancer remains to be determined, particularly for ERα+ breast cancers. This review explores the basic mechanisms of resistance to endocrine therapies, concluding with some new insights from systems biology approaches further implicating autophagy and the UPR in detail, and a brief discussion of exciting new avenues and future prospects.

Directly targeting transcriptional dysregulation in cancer

Drugs that target intracellular signalling pathways have markedly improved progression-free survival of patients with cancers who were previously regarded as untreatable. However, the rapid emergence of therapeutic resistance, as a result of bypass signalling or downstream mutation within kinase-mediated signalling cascades, has curtailed the benefit gained from these therapies. Such resistance mechanisms are facilitated by the linearity and redundancy of kinase signalling pathways. We argue that, in each cancer, the dysregulation of key transcriptional regulators not only defines the cancer phenotype but is essential for its development and maintenance. Furthermore, we propose that, as therapeutic targets, these transcriptional regulators are less prone to bypass by alternative mutational events or clonal heterogeneity, and therefore we must rekindle our efforts to directly target transcriptional regulation across a broad range of cancers.

Estrogen receptor mutations and functional consequences for breast cancer

A significant number of estrogen receptor α (ERα)-positive breast tumors develop resistance to endocrine therapy and recur with metastatic disease. Several mechanisms of endocrine resistance have been proposed, including genetic alterations that lead to ERs with altered protein sequence. By altering the conformation of the protein and increasing the interaction with coactivators, point mutations in ESR1, the gene encoding ERα, promote an active form of the receptor in the absence of hormone that assists tumor cells to evade hormonal treatments. Recent studies have confirmed that ESR1 point mutations frequently occur in metastatic breast tumors that are refractory to endocrine therapy, and suggest the development of novel strategies that may be more effective in controlling ER signaling and benefit patients with recurrent and metastatic disease.