Inhibition of RET increases the efficacy of antiestrogen and is a novel treatment strategy for luminal breast cancer

Novel developments in the study of estrogen in the pathogenesis and therapeutic intervention of lymphangioleiomyomatosis

OBJECTIVE

This study aimed to enhance the understanding of the role of estrogen in lymphangioleiomyomatosis(LAM) and to conclude the impact of estrogen-altering events on the condition and recent advances in estrogen-based treatments for LAM.

RESULTS

LAM development is strongly linked to mutations in the tuberous sclerosis gene (TSC1/2) and the presence of estrogen. Estrogen plays a significant role in the spread of TSC2-deficient uterine leiomyoma cells to the lungs and the production of pulmonary LAM. Menstruation, pregnancy, estrogen medication, and other events that cause an increase in estrogen levels can trigger the disorder, leading to a sudden worsening of symptoms. Current findings do not support using estrogen-blocking therapy regimens. However, Faslodex, which is an estrogen receptor antagonist, presents new possibilities for future therapeutic approaches in LAM.

CONCLUSION

Estrogen is crucial in the development and spread of LAM. The use of estrogen inhibitors or estrogen receptor antagonists alone does not provide good control of the disease or even poses a greater risk, and the use of a combination of mTOR receptor inhibitors, complete estrogen receptor antagonists, estrogen inhibitors, and autophagy inhibitors targeting important signaling pathways in LAM pathogenesis may be of greater benefit to the patient.

Targeting the crosstalk between estrogen receptors and membrane growth factor receptors in breast cancer treatment: Advances and opportunities

Estrogens play a critical role in the initiation and progression of breast cancer. Estrogen receptor (ER)α, ERβ, and G protein-coupled estrogen receptor are the primary receptors for estrogen in breast cancer. These receptors are mainly activated by binding with estrogens. The crosstalk between ERs and membrane growth factor receptors creates additional pathways that amplify the effects of their ligands and promote tumor growth. This crosstalk may cause endocrine therapy resistance in ERα-positive breast cancer. Furthermore, this may explain the resistance to anti-human epidermal growth factor receptor-2 (HER2) treatment in ERα-/HER2-positive breast cancer and chemotherapy resistance in triple-negative breast cancer. Accordingly, it is necessary to understand the complex crosstalk between ERs and growth factor receptors. In this review, we delineate the crosstalk between ERs and membrane growth factor receptors in breast cancer. Moreover, this review highlights the current progress in clinical treatment and discusses how pharmaceuticals target the crosstalk. Lastly, we discuss the current challenges and propose potential solutions regarding the implications of targeting crosstalk via pharmacological inhibition. Overall, the present review provides a landscape of the crosstalk between ERs and membrane growth factor receptors in breast cancer, along with valuable insights for future studies and clinical treatments using a chemotherapy-sparing regimen to improve patient quality of life.

Tamoxifen Response at Single-Cell Resolution in Estrogen Receptor-Positive Primary Human Breast Tumors

PURPOSE

In estrogen receptor-positive (ER+)/HER2- breast cancer, multiple measures of intratumor heterogeneity are associated with a worse response to endocrine therapy. We sought to develop a novel experimental model to measure heterogeneity in response to tamoxifen treatment in primary breast tumors.

EXPERIMENTAL DESIGN

To investigate heterogeneity in response to treatment, we developed an operating room-to-laboratory pipeline for the collection of live normal breast specimens and human tumors immediately after surgical resection for processing into single-cell workflows for experimentation and genomic analyses. Live primary cell suspensions were treated ex vivo with tamoxifen (10 μmol/L) or control media for 12 hours, and single-cell RNA libraries were generated using the 10X Genomics droplet-based kit.

RESULTS

In total, we obtained and processed normal breast tissue from two women undergoing reduction mammoplasty and tumor tissue from 10 women with ER+/HER2- invasive breast carcinoma. We demonstrate differences in tamoxifen response by cell type and identify distinctly responsive and resistant subpopulations within the malignant cell compartment of human tumors. Tamoxifen resistance signatures from resistant subpopulations predict poor outcomes in two large cohorts of ER+ breast cancer patients and are enriched in endocrine therapy-resistant tumors.

CONCLUSIONS

This novel ex vivo model system now provides the foundation to define responsive and resistant subpopulations within heterogeneous human tumors, which can be used to develop precise single cell-based predictors of response to therapy and to identify genes and pathways driving therapeutic resistance.

HER2-positive breast cancer progresses rapidly after pyrotinib resistance: acquired RET gene fusion and TP53 gene mutation are potential reasons

Approximately 15-20% of the patients with breast cancer overexpress human epidermal growth factor receptor 2 ( HER2 ). HER2 -positive breast cancer is highly aggressive and has a high relapse rate, suggesting that it is prone to and progresses rapidly after drug resistance. Pyrotinib resistance and changes in patients' conditions after drug resistance are challenging clinical issues and require medical attention. Recently, there are few clinical reports on changes in patients' conditions after pyrotinib resistance. We report a case of a 46-year-old patient with HER2 -positive breast cancer who developed resistance to pyrotinib and rapidly progressed to uncontrolled liver failure in less than a week. To elucidate the cause of the rapid progression, we collected samples of the patient's ascites and performed next-generation sequencing (NGS). On the basis of the NGS results, we speculated that the rapid progression after pyrotinib resistance might be due to RET gene fusion and TP53 gene mutations. Therefore, this case report aims to alert oncologists that patients with HER2 -positive breast cancer, who are resistant to pyrotinib or other targeted drugs, could experience rapid or even flare-up progression and that RET gene fusion and TP53 gene mutations might be potential causes.

Tamoxifen Response at Single Cell Resolution in Estrogen Receptor-Positive Primary Human Breast Tumors

In ER+/HER2- breast cancer, multiple measures of intra-tumor heterogeneity are associated with worse response to endocrine therapy. To investigate heterogeneity in response to treatment, we developed an operating room-to-laboratory pipeline for the collection of live human tumors and normal breast specimens immediately after surgical resection for processing into single-cell workflows for experimentation and genomic analyses. We demonstrate differences in tamoxifen response by cell type and identify distinctly responsive and resistant subpopulations within the malignant cell compartment of human tumors. Tamoxifen resistance signatures from 3 distinct resistant subpopulations are prognostic in large cohorts of ER+ breast cancer patients and enriched in endocrine therapy resistant tumors. This novel ex vivo model system now provides a foundation to define responsive and resistant sub-populations within heterogeneous tumors, to develop precise single cell-based predictors of response to therapy, and to identify genes and pathways driving resistance to therapy.

High expression of the RET receptor tyrosine kinase and its ligand GDNF identifies a high-risk subset of estrogen receptor positive breast cancer

PURPOSE

Resistance to endocrine therapy is the primary cause of treatment failure and death in patients with ER-positive (ER +)/luminal breast cancer. Expression and activation of the RET receptor tyrosine kinase may be driving poor outcomes. We aim to identify high-risk patients and druggable pathways for biomarker-based clinical trials.

METHODS

We obtained batch-normalized mRNA expression data from Breast Invasive Carcinoma-The Cancer Genome Atlas, PanCancer Atlas (BRCA-TCGA). To determine clinically significant cutoffs for RET expression, patients were grouped at different thresholds for Kaplan-Meier plotting. Differential gene expression (DGE) analysis and enrichment for gene sets was performed. transcriptomic dataset of antiestrogen-treated ER + tumors stratified by clinical response was then analyzed.

RESULTS

High RET expression was associated with worse outcomes in patients with ER + tumors, and stratification was enhanced by incorporating GDNF expression. High RET/GDNF patients had significantly lower overall survival (HR = 2.04, p = 0.012), progression-free survival (HR = 2.87, p < 0.001), disease-free survival (HR = 2.67, p < 0.001), and disease-specific survival (HR = 3.53, p < 0.001) than all other ER + patients. High RET/GDNF tumors were enriched for estrogen-independent signaling and targetable pathways including NTRK, PI3K, and KRAS. Tumors with adaptive resistance to endocrine therapy were enriched for gene expression signatures of high RET/GDNF primary tumors.

CONCLUSION

Expression and activation of the RET receptor tyrosine kinase may be driving poor outcomes in some patients with ER + breast cancer. ER + patients above the 75th percentile may benefit from clinical trials with tyrosine kinase inhibitors.

RET signaling in breast cancer therapeutic resistance and metastasis

RET, a single-pass receptor tyrosine kinase encoded on human chromosome 10, is well known to the field of developmental biology for its role in the ontogenesis of the central and enteric nervous systems and the kidney. In adults, RET alterations have been characterized as drivers of non-small cell lung cancer and multiple neuroendocrine neoplasms. In breast cancer, RET signaling networks have been shown to influence diverse functions including tumor development, metastasis, and therapeutic resistance. While RET is known to drive the development and progression of multiple solid tumors, therapeutic agents selectively targeting RET are relatively new, though multiple multi-kinase inhibitors have shown promise as RET inhibitors in the past; further, RET has been historically neglected as a potential therapeutic co-target in endocrine-refractory breast cancers despite mounting evidence for a key pathologic role and repeated description of a bi-directional relationship with the estrogen receptor, the principal driver of most breast tumors. Additionally, the recent discovery of RET enrichment in breast cancer brain metastases suggests a role for RET inhibition specific to advanced disease. This review assesses the status of research on RET in breast cancer and evaluates the therapeutic potential of RET-selective kinase inhibitors across major breast cancer subtypes.

Advances in the Diagnosis and Treatment of a Driving Target: RET Rearrangements in non-Small-Cell Lung Cancer (NSCLC) Especially in China

In the era of precision medicine, with the deepening of the research on malignant tumor driving genes, clinical oncology has fully entered the era of targeted therapy. For non-small-cell lung cancer (NSCLC), the development of targeted drugs targeting driver genes, such as epidermal growth factor receptor (EGFR) and anaplastic lymphoma kinase (ALK), has successfully opened up a new model of targeted therapy. At present, proto-oncogene rearranged during transfection (RET) fusion gene is an important novel oncogenic driving target, and specific receptor tyrosine kinase inhibitors (TKIs) targeting RET fusion have been approved. This article will review the latest research about the molecular characteristics, pathogenesis, detection, and clinical treatment strategies of RET rearrangements especially in China.

Neoadjuvant endocrine therapy for luminal breast tumors: State of the art, challenges and future perspectives

Neoadjuvant endocrine treatment (NET) associates to satisfactory rates of breast conservative surgery and conversions from inoperable to operable hormone receptor-positive (HR+)/HER2-negative breast cancer (BC), with less toxicities than neoadjuvant chemotherapy (NACT) and similar outcomes. Hence, it has been proposed as a logical alternative to NACT in patients with HR+/HER2- BC candidate to a neoadjuvant approach. Nevertheless, potential barriers to the widespread use of NET include the heterogeneous nature of patient response coupled with the long duration needed to achieve a clinical response. However, interest in NET has significantly increased in the last decade, owing to more in-depth investigation of several biomarkers for a more adequate patient selection and on-treatment benefit monitoring, such as PEPI score, Ki67 and genomic assays. This review is intended to describe the state-of-the-art regarding NET, its future perspectives and potential integration with molecular biomarkers for the optimal selection of patients, regimen and duration of (neo)adjuvant treatments.

Atropisomerism in the Pharmaceutically Relevant Realm

Atropisomerism is a conformational chirality that occurs when there is hindered rotation about a σ-bond. While atropisomerism is exemplified by biaryls, it is observed in many other pharmaceutically relevant scaffolds including heterobiaryls, benzamides, diarylamines, and anilides. As bond rotation leads to racemization, atropisomers span the gamut of stereochemical stability. LaPlante has classified atropisomers based on their half-life of racemization at 37 °C: class 1 (t_1/2 < 60 s), class 2 (60 s < t_1/2 < 4.5 years), and class 3 (t_1/2 > 4.5 years). In general, class-3 atropisomers are considered to be suitable for drug development. There are currently four FDA-approved drugs that exist as stable atropisomers, and many others are in clinical trials or have recently appeared in the drug discovery literature. Class-1 atropisomers are more prevalent, with ∼30% of recent FDA-approved small molecules possessing at least one class-1 axis. While class-1 atropisomers do not possess the requisite stereochemical stability to meet the classical definition of atropisomerism, they often bind a given target in a specific set of chiral conformations.Over the past decade, our laboratory has embarked on a research program aimed at leveraging atropisomerism as a design feature to improve the target selectivity of promiscuous lead compounds. Our studies initially focused on introducing class-3 atropisomerism into promiscuous kinase inhibitors, resulting in a proof of principle in which the different atropisomers of a compound can have different selectivity profiles with potentially improved target selectivity. This inspired a careful analysis of the binding conformations of diverse ligands bound to different target proteins, resulting in the realization that the sampled dihedral conformations about a prospective atropisomeric axis played a key role in target binding and that preorganizing the prospective atropisomeric axis into a desired target's preferred conformational range can lead to large gains in target selectivity.As atropisomerism is becoming more prevalent in modern drug discovery, there is an increasing need for strategies for atropisomerically pure samples of pharmaceutical compounds. This has led us and other groups to develop catalytic atroposelective methodologies toward pharmaceutically privileged scaffolds. Our laboratory has contributed examples of atroposelective methodologies toward heterobiaryl systems while also exploring the chirality of less-studied atropisomers such as diarylamines and related scaffolds.This Account will detail recent encounters with atropisomerism in medicinal chemistry and how atropisomerism has transitioned from a "lurking menace" into a leverageable design strategy in order to modulate various properties of biologically active small molecules. This Account will also discuss recent advances in atroposelective synthesis, with a focus on methodologies toward pharmaceutically privileged scaffolds. We predict that a better understanding of the effects of conformational restriction about a prospective atropisomeric axis on target binding will empower chemists to rapidly "program" the selectivity of a lead molecule toward a desired target.

Phase Ib/II dose expansion study of lenvatinib combined with letrozole in post-menopausal women with hormone receptor positive breast cancer

BACKGROUND

RET is an estrogen response gene with preclinical studies demonstrating cross talk between RET and estrogen receptor (ER) pathways. We investigate the role of lenvatinib, a multi-kinase inhibitor with potent activity against RET, in patients with metastatic breast cancer.

PATIENTS AND METHODS

Patients with advanced ER+/HER2- breast cancer were treated with lenvatinib plus letrozole in a phase Ib/II trial. Primary objectives included safety and recommended phase II dose (RP2D) determination in phase Ib, and objective response rates (ORR) in phase II dose expansion.

RESULTS

Sixteen patients were recruited in dose finding, where de-escalating doses of lenvatinib from 20mg to 14mg were investigated. Lenvatinib 14mg plus letrozole 2.5mg daily was determined as RP2D. Thirty-one patients with 5 median lines of prior therapy in the metastatic setting (range 0-11) were recruited in dose expansion. In this cohort, ORR was 23.3% (95% CI 9.9 to 42.3%), with median duration of response (DoR) of 6.9 months (Interquartile range(IQR) 5.9 to 13.1). Clinical benefit rate {greater than or equal to}6 months (CBR) was 50.0% (95% CI 31.3 to 68.7%). Similar efficacy was observed in the subgroup of 25 patients who progressed on prior CDK4/6 inhibitor therapy (ORR 20.0% (95% CI 6.8 to 40.7%), median DoR 6.9 months (IQR 5.9 to 13.1) and CBR 52.0% (95% CI 31.3 to 72.2%). Pharmacodynamic studies showed target modulation, with paired tumor biopsies indicating downregulation of RET/pERK and improved vascular normalisation index.

CONCLUSION

Lenvatinib plus letrozole had manageable toxicity, with target engagement and preliminary antitumor activity observed, supporting further assessment in randomized studies.

Aberrant RET expression impacts on normal mammary gland post-lactation transition enhancing cancer potential

RET is a receptor tyrosine kinase with oncogenic potential in the mammary epithelium. Several receptors with oncogenic activity in the breast are known to participate in specific developmental stages. We found that RET is differentially expressed during mouse mammary gland development: RET is present in lactation and its expression dramatically decreases in involution, the period during which the lactating gland returns to a quiescent state after weaning. Based on epidemiological and pre-clinical findings, involution has been described as tumor promoting. Using the Ret/MTB doxycycline-inducible mouse transgenic system, we show that sustained expression of RET in the mammary epithelium during the post-lactation transition to involution is accompanied by alterations in tissue remodeling and an enhancement of cancer potential. Following constitutive Ret expression, we observed a significant increase in neoplastic lesions in the post-involuting versus the virgin mammary gland. Furthermore, we show that abnormal RET overexpression during lactation promotes factors that prime involution, including premature activation of Stat3 signaling and, using RNA sequencing, an acute-phase inflammatory signature. Our results demonstrate that RET overexpression negatively affects the normal post-lactation transition.

Summary: We show that RET activation stimulates Stat3 signaling in mammary epithelial cell culture and in vivo during post-lactation transition, demonstrating that the RET receptor participates in the post-lactation transition priming tumorigenesis.

Comprehensive immunohistochemical analysis of RET, BCAR1, and BCAR3 expression in patients with Luminal A and B breast cancer subtypes

PURPOSE

Breast cancer (BC) is considered a heterogeneous disease composed of distinct subtypes with diverse clinical outcomes. Luminal subtype tumors have the best prognosis, and patients benefit from endocrine therapy. However, resistance to endocrine therapies in BC is an obstacle to successful treatment, and novel biomarkers are needed to understand and overcome this mechanism. The RET, BCAR1, and BCAR3 genes may be associated with BC progression and endocrine resistance.

METHODS

Aiming to evaluate the expression profile and prognostic value of RET, BCAR1, and BCAR3, we performed immunohistochemistry on tissue microarrays (TMAs) containing a cohort of 361 Luminal subtype BC.

RESULTS

Low expression levels of these three proteins were predominantly observed. BCAR1 expression was correlated with nuclear grade (p = 0.057), and BCAR3 expression was correlated with lymph node status (p = 0.011) and response to hormonal therapy (p = 0.021). Further, low expression of either BCAR1 or BCAR3 was significantly associated with poor prognosis (p = 0.005; p = 0.042). Pairwise analysis showed that patients with tumors with low BCAR1/low BCAR3 expression had a poorer overall survival (p = 0.013), and the low BCAR3 expression had the worst prognosis with RET high expression stratifying these patients into two different groups. Regarding the response to hormonal therapy, non-responder patients presented lower expression of RET in comparison to the responder group (p = 0.035). Additionally, the low BCAR1 expression patients had poorer outcomes than BCAR1 high (p = 0.015).

CONCLUSION

Our findings suggest RET, BCAR1, and BCAR3 as potential candidate markers for endocrine therapy resistance in Luminal BC.

The Emerging Portrait of Glial Cell Line-derived Neurotrophic Factor Family Receptor Alpha (GFRα) in Cancers

Glial cell line-derived neurotrophic factor family receptor alpha (GFRα) members have been widely connected to the mechanisms contributing to cell growth, differentiation, cell migration and tissue maturation. Here we review GFRα biological functions and discussed the evidence indicating whether GFRα signaling complex present novel opportunities for oncogenic intervention and treatment resistance. Thus, our work systematically reviewed the emerging role of GFRα family members in cancers, and provided novel insights for further researches.

JMJD6 orchestrates a transcriptional program in favor of endocrine resistance in ER+ breast cancer cells

High expression of Jumonji domain containing protein 6 (JMJD6) is strongly associated with poor prognosis in estrogen receptor positive (ER+) breast cancer. We overexpressed JMJD6 in MCF7 cells (JOE cells) and performed RNA-seq analysis. 76% of differentially expressed genes (DEGs) overlapped with ER target genes. Pathway analysis revealed that JMJD6 upregulated a larger subset of genes related to cell proliferation as compared to ER. Interestingly, JOE cells showed a decrease in ER target gene expression prompting us to check ER levels. Indeed, JOE cells showed a significant decrease in both ESR1 and ER levels and JMJD6 siRNA transfection increased the expression of both. Additionally, JOE cells showed increased RET and ERK1 expression, events associated with resistance to endocrine therapy. Accordingly, JOE cells displayed lower sensitivity and survived better at higher doses of 4-hydroxy tamoxifen (Tam) as compared to parental MCF-7 cells. Conversely, LTED-I and TAM R that resist Tam induced death, showed high expression of JMJD6. Further, JMJD6 siRNA treatment decreased growth and improved Tam sensitivity in TAM R. Comparison of JOE DEGs with known Tam signature genes showed a substantial overlap. Overall, these data suggest that blocking ER alone in patients may not eradicate proliferation of JMJD6 expressing ER+ cells and JMJD6 may predispose and sustain endocrine therapy resistance. We propose that immunostaining for JMJD6 could be developed as a potential marker for predicting endocrine therapy resistance. Further, antagonizing JMJD6 action in women expressing higher amounts of this protein, may offer a greater clinical benefit than endocrine therapy.

A Pilot Study of Preoperative Vandetanib on Markers of Proliferation and Apoptosis in Breast Cancer

INTRODUCTION

Preclinical data supports antitumor activity of tyrosine kinase inhibitor vandetanib with Ret as the therapeutic target in breast cancer. We investigated the effect of preoperative vandetanib on markers of proliferation and apoptosis in breast cancer.

METHODS

Patients with invasive breast cancer were randomly assigned vandetanib 300 mg or placebo PO daily for 2 weeks before operative resection from January 2014 to June 2017. Pretreatment and posttreatment specimens were analyzed by immunohistochemistry for Ki-67, TUNEL, and p-ERK with stratification by Ret expression by immunohistochemistry.

RESULTS

Ten patients were enrolled. There was no statistically significant difference in ERK activation compared with placebo (P=0.45); however, ERK activation was reduced 74% compared with pretreatment biopsy with vandetinib treatment (P=0.005) without a significant reduction in the placebo group (-29%, P=0.55). Mean change in Ki-67 after vandetanib treatment was +0.3% compared with +2.0% in placebo treated patients, P=0.72. Mean change in TUNEL was +0.48 apoptotic nuclei per HPF in the vandetanib arm compared with +1.02 in the placebo arm, P=0.32. In vandetanib treated patients, Ki-67 was reduced 0.3% in RET-positive tumors compared with increased 1.0% in RET-negative tumors, P=0.43 and TUNEL was increased 0.77 in RET-positive tumors and 0.2 in RET-negative tumors, P=0.21.

CONCLUSIONS

In this pilot study, no statistically significant differences on prespecified markers were seen with vandetanib compared with placebo. In accordance with the investigational hypothesis, there was a nonsignificant trend with vandetanib treatment of reduction in p-ERK and increased effects in Ret expressing tumors.

CATCH: A Prospective Precision Oncology Trial in Metastatic Breast Cancer

PURPOSE

CATCH (Comprehensive Assessment of clinical feaTures and biomarkers to identify patients with advanced or metastatic breast Cancer for marker driven trials in Humans) is a prospective precision oncology program that uses genomics and transcriptomics to guide therapeutic decisions in the clinical management of metastatic breast cancer. Herein, we report our single-center experience and results on the basis of the first 200 enrolled patients of an ongoing trial.

METHODS

From June 2017 to March 2019, 200 patients who had either primary metastatic or progressive disease, with any number of previous treatment lines and at least one metastatic site accessible to biopsy, were enrolled. DNA and RNA from tumor tissue and corresponding blood-derived nontumor DNA were profiled using whole-genome and transcriptome sequencing. Identified actionable alterations were brought into clinical context in a multidisciplinary molecular tumor board (MTB) with the aim of prioritizing personalized treatment recommendations.

RESULTS

Among the first 200 enrolled patients, 128 (64%) were discussed in the MTB, of which 64 (50%) were subsequently treated according to MTB recommendation. Of 53 evaluable patients, 21 (40%) achieved either stable disease (n = 13, 25%) or partial response (n = 8, 15%). Furthermore, 16 (30%) of those patients showed improvement in progression-free survival of at least 30% while on MTB-recommended treatment compared with the progression-free survival of the previous treatment line.

CONCLUSION

The initial phase of this study demonstrates that precision oncology on the basis of whole-genome and RNA sequencing is feasible when applied in the clinical management of patients with metastatic breast cancer and provides clinical benefit to a substantial proportion of patients.

Targeted therapies for RET-fusion cancer: Dilemmas and breakthrough

Genomic profiling has revolutionized treatment options for patients with oncogene-driven cancers, such as epidermal growth factor receptor (EGFR) mutant carcinoma. Rearranged during transfection (RET) rearrangement, as one of the main activated oncogenes, has been well studied and found to be involved in the malignant behavior of carcinogenesis, resulting in acquired resistance to EGFR tyrosine kinase inhibitors and inducing an intrinsic resistance to immunotherapy. Thus, targeted therapies have been investigated against RET arrangement cancers, including several multi-kinase inhibitors and selective RET inhibitors. However, modest efficacy, a relatively high rate of toxicity, and poor effectiveness against brain metastasis are common limitations of multi-targeted novel molecular inhibitors. A promising prospect was shown recently in selective RET inhibitors in several ongoing clinical trials. In this review, we reviewed the concurrent dilemmas of targeted therapies against RET arrangement cancer from preclinical and clinical studies and proposed several clinical considerations for clinical practice prospectively.

Targeting the Extra-Cellular Matrix-Tumor Cell Crosstalk for Anti-Cancer Therapy: Emerging Alternatives to Integrin Inhibitors

The extracellular matrix (ECM) is a complex network composed of a multitude of different macromolecules. ECM components typically provide a supportive structure to the tissue and engender positional information and crosstalk with neighboring cells in a dynamic reciprocal manner, thereby regulating tissue development and homeostasis. During tumor progression, tumor cells commonly modify and hijack the surrounding ECM to sustain anchorage-dependent growth and survival, guide migration, store pro-tumorigenic cell-derived molecules and present them to enhance receptor activation. Thereby, ECM potentially supports tumor progression at various steps from initiation, to local growth, invasion, and systemic dissemination and ECM-tumor cells interactions have long been considered promising targets for cancer therapy. Integrins represent key surface receptors for the tumor cell to sense and interact with the ECM. Yet, attempts to therapeutically impinge on these interactions using integrin inhibitors have failed to deliver anticipated results, and integrin inhibitors are still missing in the emerging arsenal of drugs for targeted therapies. This paradox situation should urge the field to reconsider the role of integrins in cancer and their targeting, but also to envisage alternative strategies. Here, we review the therapeutic targets implicated in tumor cell adhesion to the ECM, whose inhibitors are currently in clinical trials and may offer alternatives to integrin inhibition.

Ret Receptor Has Distinct Alterations and Functions in Breast Cancer

Ret receptor tyrosine kinase is a proto-oncogene that participates in development of various cancers. Several independent studies have recently identified Ret as a key player in breast cancer. Although Ret overexpression and function have been under investigation, mainly in estrogen receptor positive breast cancer, a more comprehensive analysis of the impact of recurring Ret alterations in breast cancer is needed. This review consolidates the current knowledge of Ret alterations and their potential effects in breast cancer. We discuss and integrate data on Ret changes in different breast cancer subtypes and potential function in progression, as well as the participation of distinct Ret network signaling partners in these processes. We propose that it will be essential to define a shared molecular feature of tumors with alteration in Ret receptor, be this at the genetic level or via overexpression in order to design effective therapies to target the Ret pathway. Here we review experimental evidence from basic research and pre-clinical studies concentrating on Ret alterations as potential biomarkers for recurrence, and we discuss the possibility that targeting the Ret pathway might in the future become a treatment for breast cancer.

RET fusions in solid tumors

The RET proto-oncogene has been well-studied. RET is involved in many different physiological and developmental functions. When altered, RET mutations influence disease in a variety of organ systems from Hirschsprung's disease and multiple endocrine neoplasia 2 (MEN2) to papillary thyroid carcinoma (PTC) and non-small cell lung cancer (NSCLC). Changes in RET expression have been discovered in 30-70% of invasive breast cancers and 50-60% of pancreatic ductal adenocarcinomas in addition to colorectal adenocarcinoma, melanoma, small cell lung cancer, neuroblastoma, and small intestine neuroendocrine tumors. RET mutations have been associated with tumor proliferation, invasion, and migration. RET fusions or rearrangements are somatic juxtapositions of 5' sequences from other genes with 3' RET sequences encoding tyrosine kinase. RET rearrangements occur in approximately 2.5-73% of sporadic PTC and 1-3% of NSCLC patients. The most common RET fusions are CDCC6-RET and NCOA4-RET in PTC and KIF5B-RET in NSCLC. Tyrosine kinase inhibitors are drugs that target kinases such as RET in RET-driven (RET-mutation or RET-fusion-positive) disease. Multikinase inhibitors (MKI) target various kinases and other receptors. Several MKIs are FDA-approved for cancer therapy (sunitinib, sorafenib, vandetanib, cabozantinib, regorafenib, ponatinib, lenvatinib, alectinib) and non-oncologic disease (nintedanib). Selective RET inhibitor drugs LOXO-292 (selpercatinib) and BLU-667 (pralsetinib) are also undergoing phase I/II and I clinical trials, respectively, with preliminary results demonstrating partial response and low incidence of serious adverse events. RET fusions provide a viable therapeutic target for oncologic treatment, and further study is warranted into the prevalence and pathogenesis of RET fusions as well as development of current and new tyrosine kinase inhibitors.

Expression of RET is associated with Oestrogen receptor expression but lacks prognostic significance in breast cancer

BACKGROUND

The Rearranged during Transfection (RET) protein is overexpressed in a subset of Estrogen Receptor (ER) positive breast cancer, with both signalling pathways functionally interacting. This cross-talk plays a pivotal role in the resistance of breast cancer cells to anti-endocrine therapies, and RET expression is assumed to correlate with poor prognosis based on findings in small patient cohorts. The aim of our study was to investigate the impact of RET expression on patient outcome in human breast cancer.

METHODS

We performed an immunohistochemical analysis of RET protein expression on a tissue microarray encompassing 990 breast cancer patients and correlated its expression with clinicopathological parameters and survival data.

RESULTS

Expression of RET was detected in 409 out of 990 cases (41.3%). RET and ER expression significantly correlated (p < 0.0001). The Luminal B HER2-positive subtype showed the highest expression rate (48.9%). In univariate and multivariate survival analyses, RET expression had no impact on overall survival.

CONCLUSION

We confirmed the co-expression of RET and ER, but we did not find RET expression to be an independent prognostic factor in human breast cancer. Clinical trials with newly developed RET inhibitors are needed to evaluate if RET inhibition has a beneficial impact on patient survival in ER positive breast cancer.

RET rearrangements are actionable alterations in breast cancer

Fusions involving the oncogenic gene RET have been observed in thyroid and lung cancers. Here we report RET gene alterations, including amplification, missense mutations, known fusions, novel fusions, and rearrangements in breast cancer. Their frequency, oncogenic potential, and actionability in breast cancer are described. Two out of eight RET fusions (NCOA4-RET and a novel RASGEF1A-RET fusion) and RET amplification were functionally characterized and shown to activate RET kinase and drive signaling through MAPK and PI3K pathways. These fusions and RET amplification can induce transformation of non-tumorigenic cells, support xenograft tumor formation, and render sensitivity to RET inhibition. An index case of metastatic breast cancer progressing on HER2-targeted therapy was found to have the NCOA4-RET fusion. Subsequent treatment with the RET inhibitor cabozantinib led to a rapid clinical and radiographic response. RET alterations, identified by genomic profiling, are promising therapeutic targets and are present in a subset of breast cancers.

Fusions of the gene RET have been described in thyroid and lung cancers. Here, the AUs identify RET gene alterations, including known fusions, novel fusions, and rearrangements in breast cancer (BC) that are involved in the tumorigenic process and show the benefit of RET therapy in a recurrent BC patient carrying the NCOA4-RET fusion.

Reciprocal feedback regulation of ST3GAL1 and GFRA1 signaling in breast cancer cells

GFRA1 and RET are overexpressed in estrogen receptor (ER)-positive breast cancers. Binding of GDNF to GFRA1 triggers RET signaling leading to ER phosphorylation and estrogen-independent transcriptional activation of ER-dependent genes. Both GFRA1 and RET are membrane proteins which are N-glycosylated but no O-linked sialylation site on GFRA1 or RET has been reported. We found GFRA1 to be a substrate of ST3GAL1-mediated O-linked sialylation, which is crucial to GDNF-induced signaling in ER-positive breast cancer cells. Silencing ST3GAL1 in breast cancer cells reduced GDNF-induced phosphorylation of RET, AKT and ERα, as well as GDNF-mediated cell proliferation. Moreover, GDNF induced transcription of ST3GAL1, revealing a positive feedback loop regulating ST3GAL1 and GDNF/GFRA1/RET signaling in breast cancers. Finally, we demonstrated ST3GAL1 knockdown augments anti-cancer efficacy of inhibitors of RET and/or ER. Moreover, high expression of ST3GAL1 was associated with poor clinical outcome in patients with late stage breast cancer and high expression of both ST3GAL1 and GFRA1 adversely impacted outcome in those with high grade tumors.

Chronic expression of wild-type Ret receptor in the mammary gland induces luminal tumors that are sensitive to Ret inhibition

The receptor tyrosine kinase Ret, a key gain-of-function mutated oncoprotein in thyroid carcinomas, has recently been implicated in other cancer types. While Ret copy number gains and mutations have been reported at low frequencies in breast tumors, we and others have reported that Ret is overexpressed in about 40% of human tumors and this correlates with poor patient prognosis. Ret activation regulates numerous intracellular pathways related to proliferation and inflammation, but it is not known whether abnormal Ret expression is sufficient to induce mammary carcinomas. Using a novel doxycycline-inducible transgenic mouse model with the MMTV promoter controlling Ret expression, we show that overexpression of wild-type Ret in the mammary epithelium produces mammary tumors, displaying a morphology that recapitulates characteristics of human luminal breast tumors. Ret-evoked tumors are estrogen receptor positive and negative for progesterone receptor. Moreover, tumors rapidly regress after doxycycline withdrawal, indicating that Ret is the driving oncoprotein. Using next-generation sequencing, we examined the levels of transcripts in these tumors, confirming a luminal signature. Ret-evoked tumors have been passaged in mice and used to test novel therapeutic approaches. Importantly, we have determined that tumors are resistant to endocrine therapy, but respond successfully to treatment with a Ret kinase inhibitor. Our data provide the first compelling evidence for an oncogenic role of non-mutated Ret in the mammary gland and are an incentive for clinical development of Ret as a cancer biomarker and therapeutic target.

Targeting of RET oncogene by naphthalene diimide-mediated gene promoter G-quadruplex stabilization exerts anti-tumor activity in oncogene-addicted human medullary thyroid cancer

Medullary thyroid cancer (MTC) relies on the aberrant activation of RET proto-oncogene. Though targeted approaches (i.e., tyrosine kinase inhibitors) are available, the absence of complete responses and the onset of resistance mechanisms indicate the need for novel therapeutic interventions. Due to their role in regulation of gene expression, G-quadruplexes (G4) represent attractive targets amenable to be recognized or stabilized by small molecules. Here, we report that exposure of MTC cells to a tri-substituted naphthalene diimide (NDI) resulted in a significant antiproliferative activity paralleled by inhibition of RET expression. Biophysical analysis and gene reporter assays showed that impairment of RET expression was consequent to the NDI-mediated stabilization of the G4 forming within the gene promoter. We also showed for the first time that systemic administration of the NDI in mice xenotransplanted with MTC cells resulted in a remarkable inhibition of tumor growth in vivo. Overall, our findings indicate that NDI-dependent RET G4 stabilization represents a suitable approach to control RET transcription and delineate the rationale for the development of G4 stabilizing-based treatments for MTC as well as for other tumors in which RET may have functional and therapeutic implications.

Targeting the receptor tyrosine kinase RET in combination with aromatase inhibitors in ER positive breast cancer xenografts

The majority of breast cancers are estrogen receptor positive (ER+). Blockade of estrogen biosynthesis by aromatase inhibitors (AIs) is the first-line endocrine therapy for post-menopausal women with ER+ breast cancers. However, AI resistance remains a major challenge. We have demonstrated previously that increased GDNF/RET signaling in ER+ breast cancers promotes AI resistance. Here we investigated the efficacy of different small molecule RET kinase inhibitors, sunitinib, cabozantinib, NVP-BBT594 and NVP-AST487, and the potential of combining a RET inhibitor with the AI letrozole in ER+ breast cancers. The most effective inhibitor identified, NVP-AST487, suppressed GDNF-stimulated RET downstream signaling and 3D tumor spheroid growth. Ovariectomized mice were inoculated with ER+ aromatase-overexpressing MCF7-AROM1 cells and treated with letrozole, NVP-AST487 or the two drugs in combination. Surprisingly, the three treatment regimens showed similar efficacy in impairing MCF7-AROM1 tumor growth in vivo. However in vitro, NVP-AST487 was superior to letrozole in inhibiting the GDNF-induced motility and tumor spheroid growth of MCF7-AROM1 cells and required in combination with letrozole to inhibit GDNF-induced motility in BT474-AROM3 aromatase expressing cells. These data indicate that inhibiting RET is as effective as the current therapeutic regimen of AI therapy but that a combination treatment may delay cancer cell dissemination and metastasis.

Targeting RET-driven cancers: lessons from evolving preclinical and clinical landscapes

The gene encoding the receptor-tyrosine kinase RET was first discovered more than three decades ago, and activating RET rearrangements and mutations have since been identified as actionable drivers of oncogenesis. Several multikinase inhibitors with activity against RET have been explored in the clinic, and confirmed responses to targeted therapy with these agents have been observed in patients with RET-rearranged lung cancers or RET-mutant thyroid cancers. Nevertheless, response rates to RET-directed therapy are modest compared with those achieved using targeted therapies matched to other oncogenic drivers of solid tumours, such as sensitizing EGFR or BRAF^V600E mutations, or ALK or ROS1 rearrangements. To date, no RET-directed targeted therapeutic has received regulatory approval for the treatment of molecularly defined populations of patients with RET-mutant or RET-rearranged solid tumours. In this Review, we discuss how emerging data have informed the debate over whether the limited success of multikinase inhibitors with activity against RET can be attributed to the tractability of RET as a drug target or to the lack, until 2017, of highly specific inhibitors of this oncoprotein in the clinic. We emphasize that novel approaches to targeting RET-dependent tumours are necessary to improve the clinical efficacy of single-agent multikinase inhibition and, thus, hasten approvals of RET-directed targeted therapies.

Identifying subtype-specific associations between gene expression and DNA methylation profiles in breast cancer

BACKGROUND

Breast cancer is a complex disease in which different genomic patterns exists depending on different subtypes. Recent researches present that multiple subtypes of breast cancer occur at different rates, and play a crucial role in planning treatment. To better understand underlying biological mechanisms on breast cancer subtypes, investigating the specific gene regulatory system via different subtypes is desirable.

METHODS

Gene expression, as an intermediate phenotype, is estimated based on methylation profiles to identify the impact of epigenomic features on transcriptomic changes in breast cancer. We propose a kernel weighted l1-regularized regression model to incorporate tumor subtype information and further reveal gene regulations affected by different breast cancer subtypes. For the proper control of subtype-specific estimation, samples from different breast cancer subtype are learned at different rate based on target estimates. Kolmogorov Smirnov test is conducted to determine learning rate of each sample from different subtype.

RESULTS

It is observed that genes that might be sensitive to breast cancer subtype show prediction improvement when estimated using our proposed method. Comparing to a standard method, overall performance is also enhanced by incorporating tumor subtypes. In addition, we identified subtype-specific network structures based on the associations between gene expression and DNA methylation.

CONCLUSIONS

In this study, kernel weighted lasso model is proposed for identifying subtype-specific associations between gene expressions and DNA methylation profiles. Identification of subtype-specific gene expression associated with epigenomic changes might be helpful for better planning treatment and developing new therapies.

Genetic and epigenetic factors affect RET gene expression in breast cancer cell lines and influence survival in patients

Germline and somatic mutations play a crucial role in breast cancer (BC), driving the initiation, progression, response to therapy and outcome of the disease. Hormonal therapy is limited to patients with tumors expressing steroid hormone receptors, such as estrogen receptor (ER), nevertheless resistance often limits its success.The RET gene is known to be involved in neurocristopathies such as Hirschsprung disease and Multiple Endocrine Neoplasia type 2, in the presence of loss-of-function and gain-of-function mutations, respectively. More recently, RET over-expression has emerged as a new player in ER-positive (ER+) BC, and as a potential target to enhance sensitivity and avoid resistance to tamoxifen therapy.Therefore, targeting the RET pathway may lead to new therapies in ER+ BC. To this end, we have investigated the molecular mechanisms which underlie RET overexpression and its possible modulation in two BC cell lines, MCF7 and T47D, showing different RET expression levels. Moreover, we have carried out a pilot association study in 93 ER+ BC patients. Consistent with the adverse role of RET over-expression in BC, increased overall survival was observed in carriers of the variant allele of SNP rs2435357, a RET polymorphism already known to be associated with reduced RET expression.

Germline ESR2 mutation predisposes to medullary thyroid carcinoma and causes up-regulation of RET expression

Familial medullary thyroid cancer (MTC) and its precursor, C cell hyperplasia (CCH), is associated with germline RET mutations causing multiple endocrine neoplasia type 2. However, some rare families with apparent MTC/CCH predisposition do not have a detectable RET mutation. To identify novel MTC/CCH predisposition genes we undertook exome resequencing studies in a family with apparent predisposition to MTC/CCH and no identifiable RET mutation. We identified a novel ESR2 frameshift mutation, c.948delT, which segregated with histological diagnosis following thyroid surgery in family members and demonstrated loss of ESR2-encoded ERβ expression in the MTC tumour. ERα and ERβ form heterodimers binding DNA at specific oestrogen-responsive elements (EREs) to regulate gene transcription. ERβ represses ERα-mediated activation of the ERE and the RET promoter contains three EREs. In vitro, we showed that ESR2 c.948delT results in unopposed ERα mediated increased cellular proliferation, activation of the ERE and increased RET expression. In vivo, immunostaining of CCH and MTC using an anti-RET antibody demonstrated increased RET expression. Together these findings identify germline ESR2 mutation as a novel cause of familial MTC/CCH and provide important insights into a novel mechanism causing increased RET expression in tumourigenesis.

EGFR Is Regulated by TFAP2C in Luminal Breast Cancer and Is a Target for Vandetanib

Expression of TFAP2C in luminal breast cancer is associated with reduced survival and hormone resistance, partially explained through regulation of RET. TFAP2C also regulates EGFR in HER2 breast cancer. We sought to elucidate the regulation and functional role of EGFR in luminal breast cancer. We used gene knockdown (KD) and treatment with a tyrosine kinase inhibitor (TKI) in cell lines and primary cancer isolates to determine the role of RET and EGFR in regulation of p-ERK and tumorigenesis. KD of TFAP2C decreased expression of EGFR in a panel of luminal breast cancers, and chromatin immunoprecipitation sequencing (ChIP-seq) confirmed that TFAP2C targets the EGFR gene. Stable KD of TFAP2C significantly decreased cell proliferation and tumor growth, mediated in part through EGFR. While KD of RET or EGFR reduced proliferation (31% and 34%, P < 0.01), combined KD reduced proliferation greater than either alone (52% reduction, P < 0.01). The effect of the TKI vandetanib on proliferation and tumor growth response of MCF-7 cells was dependent upon expression of TFAP2C, and dual KD of RET and EGFR eliminated the effects of vandetanib. The response of primary luminal breast cancers to TKIs assessed by ERK activation established a correlation with expression of RET and EGFR. We conclude that TFAP2C regulates EGFR in luminal breast cancer. Response to vandetanib was mediated through the TFAP2C target genes EGFR and RET. Vandetanib may provide a therapeutic effect in luminal breast cancer, and RET and EGFR can serve as molecular markers for response.

Receptor Tyrosine Kinase Expression Predicts Response to Sunitinib in Breast Cancer

BACKGROUND

Preliminary data indicate that tyrosine kinase inhibitors (TKIs) function through rearranged during transfection (RET) in breast cancer. However, TKIs are not specific and can block several receptor tyrosine kinases (RTKs). This study used cell lines and primary breast cancer specimens to determine factors associated with TKI response.

METHODS

Proliferation was assessed after short interfering RNA knockdown with or without sunitinib in breast cancer cell lines by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide). Breast cancer tissue and matched normal breast was obtained from 30 women with invasive breast carcinoma. Gene expression was assessed by reverse transcriptase-polymerase chain reaction. Fresh tissue was treated in vitro with sunitinib or control media for 30 min, and response was assessed by phosphorylation-specific western blot.

RESULTS

The RTKs including epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR1-3), platelet-derived growth factor receptor (PDGFRa/b), and Kit were overexpressed in triple-negative breast tumors relative to HER2- and estrogen receptor-alpha (ERα)-positive tumors and normal breast tissue. Knockdown of EGFR reduced in vitro proliferation in MCF-7 and MDA-MB-231 but not in SKBR-3 or ZR-75-1 breast cancer cells. With the exception of RET, response to sunitinib was independent of RTK expression in all four cell lines. Both ERα-positive and low-EGFR-expressing tumors had an increased in vitro sunitinib response, as determined by alteration of Erk activation. Expression of other RTKs and additional clinical factors were not associated with response.

CONCLUSION

Triple-negative breast cancers overexpress RTKs but have decreased in vitro response to the TKI sunitinib. In addition to RET, TKIs that block EGFR may increase the therapeutic efficacy of TKIs in breast cancer.