The human splice variant Δ16HER2 induces rapid tumor onset in a reporter transgenic mouse

Loss of the extracellular matrix glycoprotein EMILIN1 accelerates Δ16HER2-driven breast cancer initiation in mice

The extracellular matrix (ECM) is an important component of the tumor microenvironment and undergoes extensive remodeling during both initiation and progression of breast cancer (BC). EMILIN1 is an ECM glycoprotein, whose function has been linked to cancer and metastasis. However, EMILIN1 role during mammary gland and BC development has never been investigated. In silico and molecular analyses of human samples from normal mammary gland and BC showed that EMILIN1 expression was lower in tumors than in healthy mammary tissue and it predicted poor prognosis, particularly in HER2-positive BC. HER2+ BC accounts for 15-20% of all invasive BC and is characterized by high aggressiveness and poor prognosis. The Δ16HER2 isoform, a splice variant with very high oncogenic potential, is frequently expressed in HER2+ BC and correlates with metastatic disease. To elucidate the role of EMILIN1 in BC, we analyzed the phenotype of MMTV-Δ16HER2 transgenic mice, developing spontaneous multifocal mammary adenocarcinomas, crossed with EMILIN1 knock-out (KO) animals. We observed that Δ16HER2/EMILIN1 KO female mice exhibited an accelerated normal mammary gland development and a significantly anticipated appearance of palpable tumors (13.32 vs 15.28 weeks). This accelerated tumor initiation was corroborated by an increased number of tumor foci observed in mammary glands from Δ16HER2/EMILIN1 KO mice compared to the wild-type counterpart. Altogether our results underscore the centrality of ECM in the process of BC initiation and point to a role for EMILIN1 during normal mammary gland development and in protecting from HER2-driven breast tumorigenesis.

SIRT6 promotes metastasis and relapse in HER2-positive breast cancer

The histone deacetylase sirtuin 6 (SIRT6) has been endowed with anti-cancer capabilities in many tumor types. Here, we investigate the impact of SIRT6-overexpression (SIRT6-OE) in Delta16HER2 mice, which are a bona fide model of HER2-positive breast cancer. After an initial delay in the tumor onset, SIRT6-OE induces a more aggressive phenotype of Delta16HER2 tumors promoting the formation of higher number of tumor foci and metastases than controls. This phenotype of SIRT6-OE tumors is associated with cancer stem cell (CSC)-like features and tumor dormancy, and low senescence and oxidative DNA damage. Accordingly, a sub-set of HER2-positive breast cancer patients with concurrent SIRT6-OE has a significant poorer relapse-free survival (RFS) probability than patients with low expression of SIRT6. ChIP-seq, RNA-seq and RT-PCR experiments indicate that SIRT6-OE represses the expression of the T-box transcription factor 3 (Tbx3) by deacetylation of H3K9ac. Accordingly, loss-of-function mutations of TBX3 or low TBX3 expression levels are predictive of poor prognosis in HER2-positive breast cancer patients. Our work indicates that high levels of SIRT6 are indicative of poor prognosis and high risk of metastasis in HER2-positive breast cancer and suggests further investigation of TBX3 as a downstream target of SIRT6 and co-marker of poor-prognosis. Our results point to a breast cancer subtype-specific effect of SIRT6 and warrant future studies dissecting the mechanisms of SIRT6 regulation in different breast cancer subtypes.

Virus-like Particle (VLP) Vaccines for Cancer Immunotherapy

Cancer vaccines are increasingly being studied as a possible strategy to prevent and treat cancers. While several prophylactic vaccines for virus-caused cancers are approved and efficiently used worldwide, the development of therapeutic cancer vaccines needs to be further implemented. Virus-like particles (VLPs) are self-assembled protein structures that mimic native viruses or bacteriophages but lack the replicative material. VLP platforms are designed to display single or multiple antigens with a high-density pattern, which can trigger both cellular and humoral responses. The aim of this review is to provide a comprehensive overview of preventive VLP-based vaccines currently approved worldwide against HBV and HPV infections or under evaluation to prevent virus-caused cancers. Furthermore, preclinical and early clinical data on prophylactic and therapeutic VLP-based cancer vaccines were summarized with a focus on HER-2-positive breast cancer.

How Driver Oncogenes Shape and Are Shaped by Alternative Splicing Mechanisms in Tumors

The development of RNA sequencing methods has allowed us to study and better understand the landscape of aberrant pre-mRNA splicing in tumors. Altered splicing patterns are observed in many different tumors and affect all hallmarks of cancer: growth signal independence, avoidance of apoptosis, unlimited proliferation, invasiveness, angiogenesis, and metabolism. In this review, we focus on the interplay between driver oncogenes and alternative splicing in cancer. On one hand, oncogenic proteins-mutant p53, CMYC, KRAS, or PI3K-modify the alternative splicing landscape by regulating expression, phosphorylation, and interaction of splicing factors with spliceosome components. Some splicing factors-SRSF1 and hnRNPA1-are also driver oncogenes. At the same time, aberrant splicing activates key oncogenes and oncogenic pathways: p53 oncogenic isoforms, the RAS-RAF-MAPK pathway, the PI3K-mTOR pathway, the EGF and FGF receptor families, and SRSF1 splicing factor. The ultimate goal of cancer research is a better diagnosis and treatment of cancer patients. In the final part of this review, we discuss present therapeutic opportunities and possible directions of further studies aiming to design therapies targeting alternative splicing mechanisms in the context of driver oncogenes.

Investigation of the prevalence and clinical implications of ERBB2 exon 16 skipping mutations in Chinese pan-cancer patients

Background

Although rare, ERBB2 exon 16 skipping mutations (ERBB2ΔEx16) have been implicated in resistance to anti-HER2 and anti-EGFR targeted agents. Our study investigated the prevalence and clinical significance of ERBB2ΔEx16 in Chinese pan-cancer patients.

Methods

We retrospectively screened 40996 patients, spanning 19 cancer types, who had available genomic profiles acquired with DNA-based next-generation sequencing (NGS). We characterized the clinical and molecular features of the ERBB2ΔEx16-positive patients. Furthermore, we also analyzed a pan-cancer dataset from the Cancer Genome Atlas (TCGA; n=8705).

Results

A total of 22 patients were detected with ERBB2ΔEx16, resulting in an overall prevalence rate of 0.054% (22/40996). Of them, 16 patients had lung cancer (LC; 0.05%, 16/30890), five patients had gastric cancer (GC; 0.35%, 5/1448), and one patient had ovarian cancer (0.12%, 1/826). Among the 16 LC patients, ERBB2ΔEx16 was detected in four treatment-naïve EGFR/ALK-negative patients and 12 EGFR-positive patients after the onset of resistance to EGFR tyrosine kinase inhibitors (TKIs). The treatment-naïve patients harbored no LC-associated oncogenic drivers except ERBB2 amplification, suggesting a potential oncogenic role for ERBB2ΔEx16. Consistently, ERBB2ΔEx16+ patients from TCGA data also carried no known drivers despite various concurrent alterations. In the 12 EGFR TKI-resistant LC patients, relative variant frequencies for ERBB2ΔEx16 were lower than in untreated patients, suggesting ERBB2ΔEx16 as secondary alterations following TKI treatment and thereby implicating ERBB2ΔEx16 in mediating therapeutic resistance.

Conclusions

Our study identified an overall ERBB2ΔEx16 prevalence rate of 0.054% and provided insights into the clinical implications of ERBB2ΔEx16 in Chinese pan-cancer patients.

Clinical implication of genetic composition and molecular mechanism on treatment strategies of HER2-positive breast cancers

The current clinical management model of HER2-positive breast cancers is commonly based on guidelines, which in turn are based on the design and outcome of clinical trials. While this model is useful to most practicing clinicians, the treatment outcome of individual patient is not certain at the start of treatment. As the understanding of the translational research of carcinogenesis and the related changes in cancer genetics and tumor microenvironment during treatment is critical in the selection of right choice of treatment to maximize the successful clinical outcome for the patient, this review article intends to discuss the latest developments in the genetic and molecular mechanisms of cancer progression and treatment resistance, and how they influence the planning of the treatment strategies of HER2-positive breast cancers.

Prevention and Therapy of Metastatic HER-2+ Mammary Carcinoma with a Human Candidate HER-2 Virus-like Particle Vaccine

Vaccines are a promising therapeutic alternative to monoclonal antibodies against HER-2+ breast cancer. We present the preclinical activity of an ES2B-C001, a VLP-based vaccine being developed for human breast cancer therapy. FVB mice challenged with HER-2+ mammary carcinoma cells QD developed progressive tumors, whereas all mice vaccinated with ES2B-C001+Montanide ISA 51, and 70% of mice vaccinated without adjuvant, remained tumor-free. ES2B-C001 completely inhibited lung metastases in mice challenged intravenously. HER-2 transgenic Delta16 mice developed mammary carcinomas by 4−8 months of age; two administrations of ES2B-C001+Montanide prevented tumor onset for >1 year. Young Delta16 mice challenged intravenously with QD cells developed a mean of 68 lung nodules in 13 weeks, whereas all mice vaccinated with ES2B-C001+Montanide, and 73% of mice vaccinated without adjuvant, remained metastasis-free. ES2B-C001 in adjuvant elicited strong anti-HER-2 antibody responses comprising all Ig isotypes; titers ranging from 1−10 mg/mL persisted for many months. Antibodies inhibited the 3D growth of human HER-2+ trastuzumab-sensitive and -resistant breast cancer cells. Vaccination did not induce cytokine storms; however, it increased the ELISpot frequency of IFN-γ secreting HER-2-specific splenocytes. ES2B-C001 is a promising candidate vaccine for the therapy of tumors expressing HER-2. Preclinical results warrant further development towards human clinical studies.

Resistance to Trastuzumab

Simple Summary

Trastuzumab is a humanized antibody that has significantly improved the management and treatment outcomes of patients with cancers that overexpress HER2. Many research groups, both in academia and industry, have contributed towards understanding the various mechanisms engaged by trastuzumab to mediate its anti-tumor effects. Nevertheless, data from several clinical studies have indicated that a significant proportion of patients exhibit primary or acquired resistance to trastuzumab therapy. In this article, we discuss underlying mechanisms that contribute towards to resistance. Furthermore, we discuss the potential strategies to overcome some of the mechanisms of resistance to enhance the therapeutic efficacy of trastuzumab and other therapies based on it.

Abstract

One of the most impactful biologics for the treatment of breast cancer is the humanized monoclonal antibody, trastuzumab, which specifically recognizes the HER2/neu (HER2) protein encoded by the ERBB2 gene. Useful for both advanced and early breast cancers, trastuzumab has multiple mechanisms of action. Classical mechanisms attributed to trastuzumab action include cell cycle arrest, induction of apoptosis, and antibody-dependent cell-mediated cytotoxicity (ADCC). Recent studies have identified the role of the adaptive immune system in the clinical actions of trastuzumab. Despite the multiple mechanisms of action, many patients demonstrate resistance, primary or adaptive. Newly identified molecular and cellular mechanisms of trastuzumab resistance include induction of immune suppression, vascular mimicry, generation of breast cancer stem cells, deregulation of long non-coding RNAs, and metabolic escape. These newly identified mechanisms of resistance are discussed in detail in this review, particularly considering how they may lead to the development of well-rationalized, patient-tailored combinations that improve patient survival.

HER2-Displaying M13 Bacteriophages induce Therapeutic Immunity against Breast Cancer

Simple Summary

The high incidence and death rates of breast cancer make the development of new therapies an urgent need. The introduction into the clinic of the anti-HER2 monoclonal antibody trastuzumab considerably improved the overall survival and time-to-disease progression of patients with HER2-positive breast cancer. However, many patients do not benefit from it because of resistance to therapy. Cancer vaccines, by inducing into the patient an anti-cancer specific immunity, might represent an alternative immunotherapeutic approach, but despite promises, so far no anti-HER2 cancer vaccine has been approved for human use. In this study, we propose therapeutic phage-based vaccines, against HER2 and its aggressive isoform Δ16HER2, able to elicit a protective immunity and potentially capable of preventing relapse in HER2-positive breast cancer patients, even in those who develop trastuzumab resistance.

Abstract

The advent of trastuzumab has significantly improved the prognosis of HER2-positive (HER2+) breast cancer patients; nevertheless, drug resistance limits its clinical benefit. Anti-HER2 active immunotherapy represents an attractive alternative strategy, but effective immunization needs to overcome the patient’s immune tolerance against the self-HER2. Phage display technology, taking advantage of phage intrinsic immunogenicity, permits one to generate effective cancer vaccines able to break immune tolerance to self-antigens. In this study, we demonstrate that both preventive and therapeutic vaccination with M13 bacteriophages, displaying the extracellular (EC) and transmembrane (TM) domains of human HER2 or its Δ16HER2 splice variant on their surface (ECTM and Δ16ECTM phages), delayed mammary tumor onset and reduced tumor growth rate and multiplicity in ∆16HER2 transgenic mice, which are tolerant to human ∆16HER2. This antitumor protection correlated with anti-HER2 antibody production. The molecular mechanisms underlying the anticancer effect of vaccine-elicited anti-HER2 antibodies were analyzed in vitro against BT-474 human breast cancer cells, sensitive or resistant to trastuzumab. Immunoglobulins (IgG) purified from immune sera reduced cell viability mainly by impairing ERK phosphorylation and reactivating retinoblastoma protein function in both trastuzumab-sensitive and -resistant BT-474 cells. In conclusion, we demonstrated that phage-based HER2 vaccines impair mammary cancer onset and progression, opening new perspectives for HER2+ breast cancer treatment.

Inactivating amplified HER2: challenges, dilemmas, and future directions

The pharmaceutical inactivation of driver oncogenes has revolutionized the treatment of cancer replacing cytotoxic chemotherapeutic approaches with kinase inhibitor therapies for many types of cancers. This approach has not yet been realized for the treatment of HER2-amplified cancers. The monotherapy activities associated with HER2-targeting antibodies and kinase inhibitors are modest, and their clinical use has been in combination with, and not in replacement of cytotoxic chemotherapies. This stands in sharp contrast to achievements in the treatment of many other oncogene-driven cancers. The mechanism-based treatment hypothesis regarding the inactivation of HER2 justifies expectations far beyond what is currently realized. Overcoming this barrier requires mechanistic insights that can fuel new directions for pursuit, but scientific investigation of this treatment hypothesis, particularly with regards to trastuzumab, has been complicated by conflicting and confusing data sets, ironclad dogma, and mechanistic conclusions that have repeatedly failed to translate clinically. We are now approaching a point of convergence regarding the challenges and resiliency in this tumor driver, and I will provide here a review and opinion to inform where we currently stand with this treatment hypothesis and where the future potential lies.

Alternative RNA splicing in tumour heterogeneity, plasticity and therapy

Alternative splicing is a process by which a single gene is able to encode multiple different protein isoforms. It is regulated by the inclusion or exclusion of introns and exons that are joined in different patterns prior to protein translation, thus enabling transcriptomic and proteomic diversity. It is now widely accepted that alternative splicing is dysregulated across nearly all cancer types. This widespread dysregulation means that nearly all cellular processes are affected - these include processes synonymous with the hallmarks of cancer - evasion of apoptosis, tissue invasion and metastasis, altered cellular metabolism, genome instability and drug resistance. Emerging evidence indicates that the dysregulation of alternative splicing also promotes a permissive environment for increased tumour heterogeneity and cellular plasticity. These are fundamental regulators of a patient's response to therapy. In this Review, we introduce the mechanisms of alternative splicing and the role of aberrant splicing in cancer, with particular focus on newfound evidence of alternative splicing promoting tumour heterogeneity, cellular plasticity and altered metabolism. We discuss recent in vivo models generated to study alternative splicing and the importance of these for understanding complex tumourigenic processes. Finally, we review the effects of alternative splicing on immune evasion, cell death and genome instability, and how targeting these might enhance therapeutic efficacy.

Antibiotic-induced disturbances of the gut microbiota result in accelerated breast tumor growth

The gut microbiota's function in regulating health has seen it linked to disease progression in several cancers. However, there is limited research detailing its influence in breast cancer (BrCa). This study found that antibiotic-induced perturbation of the gut microbiota significantly increases tumor progression in multiple BrCa mouse models. Metagenomics highlights the common loss of several bacterial species following antibiotic administration. One such bacteria, Faecalibaculum rodentium, rescued this increased tumor growth. Single-cell transcriptomics identified an increased number of cells with a stromal signature in tumors, and subsequent histology revealed an increased abundance of mast cells in the tumor stromal regions. We show that administration of a mast cell stabilizer, cromolyn, rescues increased tumor growth in antibiotic treated animals but has no influence on tumors from control cohorts. These findings highlight that BrCa-microbiota interactions are different from other cancers studied to date and suggest new research avenues for therapy development.

HER2 Signaling and Breast Cancer Stem Cells: The Bridge behind HER2-Positive Breast Cancer Aggressiveness and Therapy Refractoriness

Simple Summary

Breast cancer (BC) is not a single disease, but a group of different tumors, and altered HER2 expression defines a particularly aggressive subtype. Although HER2 pharmacological inhibition has dramatically improved the prognosis of HER2-positive BC patients, there is still an urgent need for improved knowledge of HER2 biology and mechanisms underlying HER2-driven aggressiveness and drug susceptibility. Emerging data suggest that the clinical efficacy of molecularly targeted therapies is related to their ability to target breast cancer stem cells (BCSCs), a population that is not only self-sustaining and able to differentiate into distinct lineages, but also contributes to tumor growth, aggressiveness, metastasis and treatment resistance. The aim of this review is to provide an overview of how the full-length HER2 receptor, the d16HER2 splice variant and the truncated p95HER2 variants are involved in the regulation and maintenance of BCSCs.

Abstract

HER2 overexpression/amplification occurs in 15–20% of breast cancers (BCs) and identifies a highly aggressive BC subtype. Recent clinical progress has increased the cure rates of limited-stage HER2-positive BC and significantly prolonged overall survival in patients with advanced disease; however, drug resistance and tumor recurrence remain major concerns. Therefore, there is an urgent need to increase knowledge regarding HER2 biology and implement available treatments. Cancer stem cells (CSCs) represent a subset of malignant cells capable of unlimited self-renewal and differentiation and are mainly considered to contribute to tumor onset, aggressiveness, metastasis, and treatment resistance. Seminal studies have highlighted the key role of altered HER2 signaling in the maintenance/enrichment of breast CSCs (BCSCs) and elucidated its bidirectional communication with stemness-related pathways, such as the Notch and Wingless/β-catenin cascades. d16HER2, a splice variant of full-length HER2 mRNA, has been identified as one of the most oncogenic HER2 isoform significantly implicated in tumorigenesis, epithelial-mesenchymal transition (EMT)/stemness and the response to targeted therapy. In addition, expression of a heterogeneous collection of HER2 truncated carboxy-terminal fragments (CTFs), collectively known as p95HER2, identifies a peculiar subgroup of HER2-positive BC with poor prognosis, with the p95HER2 variants being able to regulate CSC features. This review provides a comprehensive overview of the current evidence regarding HER2-/d16HER2-/p95HER2-positive BCSCs in the context of the signaling pathways governing their properties and describes the future prospects for targeting these components to achieve long-lasting tumor control.

Systematic Analysis of Survival-Associated Alternative Splicing Signatures in Thyroid Carcinoma

Alternative splicing (AS) is a key mechanism involved in regulating gene expression and is closely related to tumorigenesis. The incidence of thyroid cancer (THCA) has increased during the past decade, and the role of AS in THCA is still unclear. Here, we used TCGA and to generate AS maps in patients with THCA. Univariate analysis revealed 825 AS events related to the survival of THCA. Five prognostic models of AA, AD, AT, ES, and ME events were obtained through lasso and multivariate analyses, and the final prediction model was established by integrating all the AS events in the five prediction models. Kaplan–Meier survival analysis revealed that the overall survival rate of patients in the high-risk group was significantly shorter than that of patients in the low-risk group. The ROC results revealed that the prognostic capabilities of each model at 3, 5, and 8 years were all greater than 0.7, and the final prognostic capabilities of the models were all greater than 0.9. By reviewing other databases and utilizing qPCR, we verified the established THCA gene model. In addition, gene set enrichment analysis showed that abnormal AS events might play key roles in tumor development and progression of THCA by participating in changes in molecular structure, homeostasis of the cell environment and in cell energy. Finally, a splicing correlation network was established to reveal the potential regulatory patterns between the predicted splicing factors and AS event candidates. In summary, AS should be considered an important prognostic indicator of THCA. Our results will help to elucidate the underlying mechanism of AS in the process of THCA tumorigenesis and broaden the prognostic and clinical application of molecular targeted therapy for THCA.

HER2 Isoforms Uniquely Program Intratumor Heterogeneity and Predetermine Breast Cancer Trajectories During the Occult Tumorigenic Phase

HER2-positive breast cancers are among the most heterogeneous breast cancer subtypes. The early amplification of HER2 and its known oncogenic isoforms provide a plausible mechanism in which distinct programs of tumor heterogeneity could be traced to the initial oncogenic event. Here a Cancer rainbow mouse simultaneously expressing fluorescently barcoded wildtype (^WTHER2), exon-16 null (^d16HER2), and N-terminally truncated (^p95HER2) HER2 isoforms is used to trace tumorigenesis from initiation to invasion. Tumorigenesis was visualized using whole-gland fluorescent lineage tracing and single-cell molecular pathology. We demonstrate that within weeks of expression, morphologic aberrations were already present and unique to each HER2 isoform. Although ^WTHER2 cells were abundant throughout the mammary ducts, detectable lesions were exceptionally rare. In contrast, ^d16HER2 and ^p95HER2 induced rapid tumor development. ^d16HER2 incited homogenous and proliferative luminal-like lesions which infrequently progressed to invasive phenotypes whereas ^p95HER2 lesions were heterogenous and invasive at the smallest detectable stage. Distinct cancer trajectories were observed for ^d16HER2 and ^p95HER2 tumors as evidenced by oncogene-dependent changes in epithelial specification and the tumor microenvironment. These data provide direct experimental evidence that intratumor heterogeneity programs begin very early and well in advance of screen or clinically detectable breast cancer. IMPLICATIONS: Although all HER2 breast cancers are treated equally, we show a mechanism by which clinically undetected HER2 isoforms program heterogenous cancer phenotypes through biased epithelial specification and adaptations within the tumor microenvironment.

Nectin-4 and p95-ErbB2 cooperatively regulate Hippo signaling-dependent SOX2 gene expression, enhancing anchorage-independent T47D cell proliferation

Nectin-4, upregulated in various cancer cells, cis-interacts with ErbB2 and its trastuzumab-resistant splice variants, p95-ErbB2 and ErbB2∆Ex16, enhancing DNA synthesis through the PI3K-AKT signaling in human breast cancer T47D cells in an adherent culture. We found here that nectin-4 and p95-ErbB2, but not nectin-4 and either ErbB2 or ErbB2∆Ex16, cooperatively enhanced SOX2 gene expression and cell proliferation in a suspension culture. This enhancement of T47D cell proliferation in a suspension culture by nectin-4 and p95-ErbB2 was dependent on the SOX2 gene expression. In T47D cells, nectin-4 and any one of p95-ErbB2, ErbB2, or ErbB2∆Ex16 cooperatively activated the PI3K-AKT signaling, known to induce the SOX2 gene expression, to similar extents. However, only a combination of nectin-4 and p95-ErbB2, but not that of nectin-4 and either ErbB2 or ErbB2∆Ex16, cooperatively enhanced the SOX2 gene expression. Detailed studies revealed that only nectin-4 and p95-ErbB2 cooperatively activated the Hippo signaling. YAP inhibited the SOX2 gene expression in this cell line and thus the MST1/2-LATS1/2 signaling-mediated YAP inactivation increased the SOX2 gene expression. These results indicate that only the combination of nectin-4 and p95-ErbB2, but not that of nectin-4 and either ErbB2 or ErbB2∆Ex16, cooperatively regulates the Hippo signaling-dependent SOX2 gene expression, enhancing anchorage-independent T47D cell proliferation.

HER2 splice variants in breast cancer: investigating their impact on diagnosis and treatment outcomes

Overexpression of the HER2 receptor occurs in approximately 20% of breast cancer patients. HER2 positivity is associated with poor prognosis and aggressive tumour phenotypes, which led to rapid progress in HER2 targeted therapeutics and diagnostic testing. Whilst these advances have greatly increased patients' chances of survival, resistance to HER2 targeted therapies, be that intrinsic or acquired, remains a problem. Different forms of the HER2 protein exist within tumours in tandem and can display altered biological activities. Interest in HER2 variants in breast cancer increased when links between resistance to anti-HER2 therapies and a particular variant, Δ16-HER2, were identified. Moreover, the P100 variant potentially reduces the efficacy of the anti-HER2 therapy trastuzumab. Another variant, Herstatin, exhibits 'auto-inhibitory' behaviour. More recently, new HER2 variants have been identified and are currently being assessed for their pro- and anti-cancer properties. It is important when directing the care of patients to consider HER2 variants collectively. This review considers HER2 variants in the context of the tumour environment where multiple variants are co-expressed at altered ratios. This study also provides an up to date account of the landscape of HER2 variants and links this to patterns of resistance against HER2 therapies and treatment plans.

RNA splicing alteration in the response to platinum chemotherapy in ovarian cancer: A possible biomarker and therapeutic target

Since its discovery, alternative splicing has been recognized as a powerful way for a cell to amplify the genetic information and for a living organism to adapt, evolve, and survive. We now know that a very high number of genes are regulated by alternative splicing and that alterations of splicing have been observed in different types of human diseases, including cancer. Here, we review the accumulating knowledge that links the regulation of alternative splicing to the response to chemotherapy, focusing our attention on ovarian cancer and platinum-based treatments. Moreover, we discuss how expanding information could be exploited to identify new possible biomarkers of platinum response, to better select patients, and/or to design new therapies able to overcome platinum resistance.

Acetylshikonin isolated from Lithospermum erythrorhizon roots inhibits dihydrofolate reductase and hampers autochthonous mammary carcinogenesis in Δ16HER2 transgenic mice

Breast cancer (BC) is the most common cancer in women and, among different BC subtypes, triple negative (TN) and human epidermal growth factor receptor 2 (HER2)-positive BCs have the worst prognosis. In this study, we investigated the anticancer activity of the root ethanolic and hexane extracts from Lithospermum erythrorhizon, a traditional Chinese herbal medicine known also as tzu ts'ao or tzu-ken, against in vitro and in vivo models of TNBC and HER2-positive BC. Treatment with L. erythrorhizon root extracts resulted in a dose-dependent inhibition of BC cell viability and in a significant reduction of the growth of TNBC cells transplanted in syngeneic mice. Acetylshikonin, a naphthoquinone, was identified as the main bioactive component in extracts and was responsible for the observed antitumor activity, being able to decrease BC cell viability and to interfere with autochthonous mammary carcinogenesis in Δ16HER2 transgenic mice. Acetylshikonin anticancer effect depends on its ability to act as a potent inhibitor of dihydrofolate reductase (DHFR), to down-regulate key mediators governing cancer growth and progression, such as HER2, Src and STAT3, and to induce apoptosis by caspase-3 activation. The accumulation of acetylshikonin in blood samples as well as in brain, kidney, liver and tumor tissues was also investigated by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) highlighting that L. erythrorhizon treatment is effective in delivering the active compound into the target tissues. These results provide evidence that L. erythrorhizon extract and in particular its main component acetylshikonin are effective against aggressive BC subtypes and reveal new acetylshikonin mechanisms of action.

In vivo modeling of the EGFR family in breast cancer progression and therapeutic approaches

Modeling breast cancer through the generation of genetically engineered mouse models (GEMMs) has become the gold standard in the study of human breast cancer. Notably, the in vivo modeling of the epidermal growth factor receptor (EGFR) family has been key to the development of therapeutics and has helped better understand the signaling pathways involved in cancer initiation, progression and metastasis. The HER2/ErbB2 receptor is a member of the EGFR family and 20% of breast cancers are found to belong in the HER2-positive histological subtype. Historical and more recent advances in the field have shaped our understanding of HER2-positive breast cancer signaling and therapeutic approaches.

T-cell Activating Tribodies as a Novel Approach for Efficient Killing of ErbB2-positive Cancer Cells

The Tyrosine Kinase Receptor ErbB2 (HER2) when overexpressed in breast cancer (BC) is associated with poor prognosis. The monoclonal antibody Trastuzumab has become a standard treatment of ErbB2+BC. The antibody treatment has limited efficacy, often meets resistance and induces cardiotoxicity. T-cell recruiting bispecific antibody derivatives (TRBA) offer a more effective alternative to standard antibody therapy. We evaluated a panel of TRBAs targeting 3 different epitopes on the ErbB2 receptor either in a bivalent targeting tribody structure or as a monovalent scFv-fusion (BiTE format) for binding, cytotoxicity on Trastuzumab-resistant cell lines, and induction of cardiotoxicity. All three TRBAs bind with high affinity to the ErbB2 extracellular domain and a large panel of ErbB2-positive tumor cells. Tribodies had an increased in vitro cytotoxic potency as compared to BiTEs. It is interesting to note that, Tribodies targeting the epitopes on ErbB2 receptor domains I and II bind and activate lysis of mammary and gastric tumor cells more efficiently than a Tribody targeting the Trastuzumab epitope on domain IV. The first 2 are also active on Trastuzumab-resistant cancer cells lacking or masking the epitope recognized by Trastuzumab. None of the Tribodies studied showed significant toxicity on human cardiomyocytes. Altogether these results make these novel anti-ErbB2 bispecific Tribodies candidates for therapeutic development for treating ErbB2-positive Trastuzumab-resistant cancer patients.

Phage display as a tool for vaccine and immunotherapy development

Bacteriophages, or phages, are viruses that specifically infect bacteria and coopt the cellular machinery to create more phage proteins, eventually resulting in the release of new phage particles. Phages are heavily utilized in bioengineering for applications ranging from tissue engineering scaffolds to immune signal delivery. Of specific interest to vaccines and immunotherapies, phages have demonstrated an ability to activate both the innate and adaptive immune systems. The genome of these viral particles can be harnessed for DNA vaccination, or the surface proteins can be exploited for antigen display. More specifically, genes that encode an antigen of interest can be spliced into the phage genome, allowing antigenic proteins or peptides to be displayed by fusion to phage capsid proteins. Phages therefore present antigens to immune cells in a highly ordered and repetitive manner. This review discusses the use of phage with adjuvanting activity as antigen delivery vehicles for vaccination against infectious disease and cancer.

Downregulation of miR-223 Expression Is an Early Event during Mammary Transformation and Confers Resistance to CDK4/6 Inhibitors in Luminal Breast Cancer

miR-223 is an anti-inflammatory miRNA that in cancer acts either as an oncosuppressor or oncopromoter, in a context-dependent manner. In breast cancer, we demonstrated that it dampens the activation of the EGF pathway. However, little is known on the role of miR-223 during breast cancer onset and progression. miR-223 expression was decreased in breast cancer of luminal and HER2 subtypes and inversely correlated with patients' prognosis. In normal luminal mammary epithelial cells, miR-223 acted cell autonomously in the control of their growth and morphology in three-dimensional context. In the MMTV-Δ16HER2 transgenic mouse model, oncogene transformation resulted in a timely abrogation of miR-223 expression, likely due to activation of E2F1, a known repressor of miR-223 transcription. Accordingly, treatment with CDK4/6 inhibitors, which eventually results in restraining E2F1 activity, restored miR-223 expression and miR-223 ablation induced luminal breast cancer resistance to CDK4/6 inhibition, both in vitro and in vivo. Notably, miR-223 expression was lost in microdissected ductal carcinoma in situ (DCIS) from patients with luminal and HER2-positive breast cancer. Altogether, these results identify downmodulation of miR-223 as an early step in luminal breast cancer onset and suggest that it could be used to identify aggressive DCIS and predict the response to targeted therapy. SIGNIFICANCE: miR-223 may represent a predictive biomarker of response to CDK4/6 inhibitors and its loss could identify DCIS lesions that are likely to progress into invasive breast cancer.

Anticancer innovative therapy: Highlights from the ninth annual meeting

The Ninth Annual Conference of "Anticancer Innovative Therapy", organized by Fondazione IRCCS Istituto Nazionale dei Tumori di Milano (Fondazione IRCCS INT) and hosted by Hotel Michelangelo, was held in Milan on 25 January 2019. Cutting-edge science was presented in two main scientific sessions: i) pre-clinical evidences and new targets, and ii) clinical translation. The Keynote lecture entitled "Cancer stem cells (CSCs): metabolic strategies for their identification and eradication" presented by M. Lisanti, was one of the highlights of the conference. One key concept of the meeting was how the continuous advances in our knowledge about molecular mechanisms in various fields of research (cancer metabolism reprogramming, epigenetic regulation, transformation/invasiveness, and immunology, among others) are driving cancer research towards more effective personalized antineoplastic strategies. Specifically, recent preclinical data on the following topics were discussed: 1. Polycomb group proteins in cancer; 2. A d16HER2 splice variant is a flag of HER2 addiction across HER2-positive cancers; 3. Studying chromatin as a nexus between translational and basic research; 4. Metabolomic analysis in cancer patients; 5. CDK4-6 cyclin inhibitors: clinical activity and future perspectives as immunotherapy adjuvant; and 6. Cancer stem cells (CSCs): metabolic strategies for their identification and eradication. In terms of clinical translation, several novel approaches were presented: 1. Developing CAR-T cell therapies: an update of preclinical and clinical development at University of North Carolina; 2. Vγ9Vδ2 T-cell activation and immune suppression in multiple myeloma; 3. Predictive biomarkers for real-world immunotherapy: the cancer immunogram model in the clinical arena; and 4. Mechanisms of resistance to immune checkpoint blockade in solid tumors. Overall, the pre-clinical and clinical findings presented could pave the way to identify novel actionable therapeutic targets to significantly enhance the care of persons with cancer.

Nectin-4 cis-interacts with ErbB2 and its trastuzumab-resistant splice variants, enhancing their activation and DNA synthesis

Nectin-4 cell adhesion molecule and ErbB2 tyrosine kinase receptor are upregulated in many cancers, including breast cancer, and promote cancer cell proliferation and metastasis. Using human breast cancer cell lines T47D and SUM190-PT, in which both nectin-4 and ErbB2 were upregulated, we showed here that nectin-4 cis-interacted with ErB2 and enhanced its dimerization and activation, followed by the activation of the phosphoinositide 3-kinase-AKT signalling pathway for DNA synthesis. The third immunoglobulin-like domain of nectin-4 cis-interacted with domain IV of ErbB2. This region differs from the trastuzumab-interacting region but is included in the trastuzumab-resistant splice variants of ErbB2, p95-ErbB2 and ErbB2ΔEx16. Nectin-4 also cis-interacted with these trastuzumab-resistant splice variants and enhanced the activation of the phosphoinositide 3-kinase-AKT signalling pathway for DNA synthesis. In addition, nectin-4 enhanced the activation of the p95-ErbB2-induced JAK-STAT3 signalling pathway, but not the ErbB2- or ErbB2ΔEx16-induced JAK-STAT3 signalling pathway. These results indicate that nectin-4 cis-interacts with ErbB2 and its trastuzumab-resistant splice variants and enhances the activation of these receptors and downstream signalling pathways in a novel mechanism.

A novel 3'-tRNAGlu-derived fragment acts as a tumor suppressor in breast cancer by targeting nucleolin

tRNA-derived fragments (tRFs) have been defined as a novel class of small noncoding RNAs. tRFs have been reported to be deregulated in cancer, but their biologic function remains to be fully understood. We have identified a new tRF (named tRF3E), derived from mature tRNAGlu, that is specifically expressed in healthy mammary glands but not in breast cancer (BC). Consistently, tRF3E levels significantly decrease in the blood of patients with epidermal growth factor receptor 2 (HER2)-positive BC reflecting tumor status (control > early cancer > metastatic cancer). tRF3E down-regulation was recapitulated in Δ16HER2 transgenic mice, representing a BC preclinical model. Pulldown assays, used to search for proteins capable to selectively bind tRF3E, have shown that this tRF specifically interacts with nucleolin (NCL), an RNA-binding protein overexpressed in BC and able to repress the translation of p53 mRNA. The binding properties of NCL-tRF3E complex, predicted in silico and analyzed by EMSA assays, are congruent with a competitive displacement of p53 mRNA by tRF3E, leading to an increased p53 expression and consequently to a modulation of cancer cell growth. Here, we provide evidence that tRF3E plays an important role in the pathogenesis of BC displaying tumor-suppressor functions through a NCL-mediated mechanism.-Falconi, M., Giangrossi, M., Elexpuru Zabaleta, M., Wang, J., Gambini, V., Tilio, M., Bencardino, D., Occhipinti, S., Belletti, B., Laudadio, E., Galeazzi, R., Marchini, C., Amici, A. A novel 3'-tRNAGlu-derived fragment acts as a tumor suppressor in breast cancer by targeting nucleolin.

Transcriptional Profiles and Stromal Changes Reveal Bone Marrow Adaptation to Early Breast Cancer in Association with Deregulated Circulating microRNAs

The presence of a growing tumor establishes a chronic state of inflammation that acts locally and systemically. Bone marrow responds to stress signals by expanding myeloid cells endowed with immunosuppressive functions, further fostering tumor growth and dissemination. How early in transformation the cross-talk with the bone marrow begins and becomes detectable in blood is unknown. Here, gene expression profiling of the bone marrow along disease progression in a spontaneous model of mammary carcinogenesis demonstrates that transcriptional modifications in the hematopoietic compartment occurred as early as preinvasive disease stages. The transcriptional profile showed downregulation of adaptive immunity and induction of programs related to innate immunity and response to danger signals triggered by activating transcription factor 3. Transcriptional reprogramming was paralleled by the expansion of myeloid populations at the expense of erythroid and B lymphoid fractions. Hematopoietic changes were associated with modifications of the bone marrow stromal architecture through relocalization and increased density in the interstitial area of Nestin⁺ mesenchymal cells expressing CXCL12 and myeloid cells expressing CXCL12 receptor CXCR4. These early events were concomitant with deregulation of circulating miRNAs, which were predicted regulators of transcripts downregulated in the bone marrow and involved in lymphoid differentiation and activation. These data provide a link between sensing of peripheral cancer initiation by the bone marrow and hematopoietic adaptation to distant noxia through transcriptional rewiring toward innate/inflammatory response programs. SIGNIFICANCE: The bone marrow senses distant tissue transformation at premalignant/preinvasive stages, suggesting that circulating messengers, intercepted in the blood, could serve as early diagnostic markers.

The d16HER2 Splice Variant: A Friend or Foe of HER2-Positive Cancers?

Human epidermal growth factor receptor 2 (ERBB2 or HER2) amplification/overexpression is associated with a particularly aggressive molecular subtype of breast cancer (BC), characterized by a poor prognosis, increased metastatic potential, and disease recurrence. As only approximately 50% of HER2-positive patients respond to HER2-targeted therapies, greater knowledge of the biology of HER2 and the mechanisms that underlie drug susceptibility is needed to improve cure rates. Evidence suggests that the coexistence of full-length, wild-type HER2 (wtHER2) and altered forms of HER2—such as carboxy-terminus-truncated fragments, activating mutations, and splice variants—significantly increases the heterogeneity of HER2-positive disease, affecting its biology, clinical course, and treatment response. In particular, expression of the d16HER2 splice variant in human HER2-positive BC has a crucial pathobiological function, wherein the absence of sixteen amino acids from the extracellular domain induces the formation of stable and constitutively active HER2 homodimers on the tumor cell surface. Notably, the d16HER2 variant significantly influences the initiation and aggressiveness of tumors, cancer stem cell properties, epithelial–mesenchymal transition (EMT), and the susceptibility of HER2-positive BC cells to trastuzumab compared with its wtHER2 counterpart, thus constituting a novel and potentially clinically useful biomarker. The aims of this review are to summarize the existing evidence regarding the pathobiological functions of the d16HER2 variant and discuss its current and future value with regard to risk assessment and treatment choices in HER2-positive disease.

HER2-L755S mutation induces hyperactive MAPK and PI3K-mTOR signaling, leading to resistance to HER2 tyrosine kinase inhibitor treatment

L755S, a HER2 kinase domain mutation, is the most common HER2 mutation in breast cancer associated with resistance to anti-HER2 trastuzumab treatment. Here, we showed that HER2-L755S confers hyperactivation of MAPK and PI3K/AKT/mTOR pathways and resistance to both reversible and irreversible HER2 tyrosine kinase inhibitors. We further demonstrated that the HER2 TKIs in combination with MEK inhibitor, AZD6244, or PI3K inhibitor, GDC0941, yield robust killing in HER2-L755S cancer cells, indicating a novel targeted strategy to overcome HER2-L755S resistance to anti-HER2 treatment.

Transgenic Animal Models to Visualize Cancer-Related Cellular Processes by Bioluminescence Imaging

Preclinical animal models are valuable tools to improve treatments of malignant diseases, being an intermediate step of experimentation between cell culture and human clinical trials. Among different animal models frequently used in cancer research are mouse and, more recently, zebrafish models. Indeed, most of the cellular pathways are highly conserved between human, mouse and zebrafish, thus rendering these models very attractive. Recently, several transgenic reporter mice and zebrafishes have been generated in which the luciferase reporter gene are placed under the control of a promoter whose activity is strictly related to specific cancer cellular processes. Other mouse models have been generated by the cDNA luciferase knockin in the locus of a gene whose expression/activity has increased in cancer. Using BioLuminescence Imaging (BLI), we have now the opportunity to spatiotemporal visualize cell behaviors, among which proliferation, apoptosis, migration and immune responses, in any body district in living animal in a time frame process. We provide here a review of the available models to visualized cancer and cancer-associated events in living animals by BLI and as they have been successful in identifying new stages of early tumor progression, new interactions between different tissues and new therapeutic responsiveness.

The landscape of d16HER2 splice variant expression across HER2-positive cancers

The HER2 splice variant characterized by the deletion of exon 16 and denominated as d16HER2, is associated with HER2-positive breast cancer (BC) aggressiveness, stemness, and trastuzumab susceptibility and is considered to be a “flag” of HER2 dependence. However, with the exception of quantitative real-time PCR analysis, easily reproducible assays are still lacking to clinically detect and quantify the d16HER2 expression. Further, no data on d16HER2 expression and its potential role are available in HER2-positive gastrointestinal malignancies. Here, we used a novel RNA in situ hybridization technique (BaseScope) to discriminate d16HER2 variant expression from the wild type isoform (WTHER2) and to assess their levels across different HER2-positive histological samples. Our results demonstrate the existence of outliers, with d16HER2 mRNA high scores restricted to HER2-positive gastric cancer (GC) and colorectal cancer (CRC) coupled with increased d16HER2 expression compared with BC. Consistent with previously reported data on BC, experiments performed in HER2-positive GC patient-derived xenografts suggest that increased d16HER2 expression is associated with a clinical benefit/response to single-agent trastuzumab. Therefore, d16HER2 may be considered as a “flag” of HER2 dependence in GC and can be clinically investigated as a marker of trastuzumab susceptibility in several other HER2-driven cancers, including CRC. As a clinical proof-of-concept, we indicate that high d16HER2 mRNA scores are exclusively found in patients with a long-term benefit from trastuzumab exceeding 12 months (clinical “outliers”), and that d16HER2 expression is also increased in circulating tumor-released exosomes obtained from baseline plasma samples of long-term responders.

RNA Splicing and Disease: Animal Models to Therapies

Alternative splicing of pre-mRNA increases genetic diversity, and recent studies estimate that most human multiexon genes are alternatively spliced. If this process is not highly regulated and accurate, it leads to mis-splicing events, which may result in proteins with altered function. A growing body of work has implicated mis-splicing events in a range of diseases, including cancer, neurodegenerative diseases, and muscular dystrophies. Understanding the mechanisms that cause aberrant splicing events and how this leads to disease is vital for designing effective therapeutic strategies. In this review, we focus on advances in therapies targeting splicing, and highlight the animal models developed to recapitulate disease phenotypes as a model for testing these therapies.

Stathmin Is Required for Normal Mouse Mammary Gland Development and Δ16HER2-Driven Tumorigenesis

Postnatal development of the mammary gland relies on the maintenance of oriented cell division and apicobasal polarity, both of which are often deregulated in cancer. The microtubule (MT) network contributes to control these processes; however, very little is known about the impact of altered MT dynamics in the development of a complex organ and on the role played by MT-interacting proteins such as stathmin. In this study, we report that female stathmin knock-out (STM KO) mice are unable to nurse their litters due to frank impairment of mammary gland development. In mouse mammary epithelial cells, loss of stathmin compromised the trafficking of polarized proteins and the achievement of proper apicobasal polarity. In particular, prolactin receptor internalization and localization was altered in STM KO mammary epithelial cells, leading to decreased protein stability and downmodulation of the Prl/PrlR/STAT5 signaling pathway. Absence of stathmin induced alterations in mitotic spindle orientation, accumulation of mitotic defects, and apoptosis, overall contributing to tissue disorganization and further decreasing the expansion of the mammary epithelial compartment. Loss of stathmin in MMTV-Δ16HER2 transgenic mice decreased the incidence and increased the latency of these very aggressive mammary carcinomas. Collectively, these data identify the essential mammary protein stathmin as protumorigenic and suggest it may serve as a potential therapeutic target in breast cancer. SIGNIFICANCE: Stathmin expression is critical to maintain oriented cell division and apicobasal polarity in normal mammary glands and to establish a protumorigenic program that eventually sustains HER2-positive breast cancer formation in mice.

Splicing dysregulation as a driver of breast cancer

Breast cancer is known to be a heterogeneous disease driven by a large repertoire of molecular abnormalities, which contribute to its diverse clinical behaviour. Despite the success of targeted therapy approaches for breast cancer patient management, there is still a lack of the molecular understanding of aggressive forms of the disease and clinical management of these patients remains difficult. The advent of high-throughput sequencing technologies has paved the way for a more complete understanding of the molecular make-up of the breast cancer genome. As such, it is becoming apparent that disruption of canonical splicing within breast cancer governs its clinical progression. In this review, we discuss the role of dysregulation of spliceosomal component genes and associated factors in the progression of breast cancer, their role in therapy resistance and the use of quantitative isoform expression as potential prognostic and predictive biomarkers with a particular focus on oestrogen receptor-positive breast cancer.

Targeting HER2 in colorectal cancer: The landscape of amplification and short variant mutations in ERBB2 and ERBB3

BACKGROUND

In contrast to lung cancer, few precision treatments are available for colorectal cancer (CRC). One rapidly emerging treatment target in CRC is ERBB2 (human epidermal growth factor receptor 2 [HER2]). Oncogenic alterations in HER2, or its dimerization partner HER3, can underlie sensitivity to HER2-targeted therapies.

METHODS

In this study, 8887 CRC cases were evaluated by comprehensive genomic profiling for genomic alterations in 315 cancer-related genes, tumor mutational burden, and microsatellite instability. This cohort included both colonic (7599 cases; 85.5%) and rectal (1288 cases; 14.5%) adenocarcinomas.

RESULTS

A total of 569 mCRCs were positive for ERBB2 (429 cases; 4.8%) and/or ERBB3 (148 cases; 1.7%) and featured ERBB amplification, short variant alterations, or a combination of the 2. High tumor mutational burden (≥20 mutations/Mb) was significantly more common in ERBB-mutated samples, and ERBB3-mutated CRCs were significantly more likely to have high microsatellite instability (P<.002). Alterations affecting KRAS (27.3%) were significantly underrepresented in ERBB2-amplified samples compared with wild-type CRC samples (51.8%), and ERBB2- or ERBB3-mutated samples (49.0% and 60.8%, respectively) (P<.01). Other significant differences in mutation frequency were observed for genes in the PI3K/MTOR and mismatch repair pathways.

CONCLUSIONS

Although observed less often than in breast or upper gastrointestinal carcinomas, indications for which anti-HER2 therapies are approved, the percentage of CRC with ERBB genomic alterations is significant. Importantly, 32% of ERBB2-positive CRCs harbor short variant alterations that are undetectable by routine immunohistochemistry or fluorescence in situ hybridization testing. The success of anti-HER2 therapies in ongoing clinical trials is a promising development for patients with CRC. Cancer 2018;124:1358-73. © 2018 Foundation Medicine, Inc. Cancer published by Wiley Periodicals, Inc. on behalf of American Cancer Society.

Virus-like particle display of HER2 induces potent anti-cancer responses

Overexpression of human epidermal growth factor receptor-2 (HER2) occurs in 20–30% of invasive breast cancers. Monoclonal antibody therapy is effective in treating HER2-driven mammary carcinomas, but its utility is limited by high costs, side effects and development of resistance. Active vaccination may represent a safer, more effective and cheaper alternative, although the induction of strong and durable autoantibody responses is hampered by immune-tolerogenic mechanisms. Using a novel virus-like particle (VLP) based vaccine platform we show that directional, high-density display of human HER2 on the surface of VLPs, allows induction of therapeutically potent anti-HER2 autoantibody responses. Prophylactic vaccination reduced spontaneous development of mammary carcinomas by 50%-100% in human HER2 transgenic mice and inhibited the growth of HER2-positive tumors implanted in wild-type mice. The HER2-VLP vaccine shows promise as a new cost-effective modality for prevention and treatment of HER2-positive cancer. The VLP platform may represent an effective tool for development of vaccines against other non-communicable diseases.

Alternative Splicing in Breast Cancer and the Potential Development of Therapeutic Tools

Alternative splicing is a key molecular mechanism now considered as a hallmark of cancer that has been associated with the expression of distinct isoforms during the onset and progression of the disease. The leading cause of cancer-related deaths in women worldwide is breast cancer, and even when the role of alternative splicing in this type of cancer has been established, the function of this mechanism in breast cancer biology is not completely decoded. In order to gain a comprehensive view of the role of alternative splicing in breast cancer biology and development, we summarize here recent findings regarding alternative splicing events that have been well documented for breast cancer evolution, considering its prognostic and therapeutic value. Moreover, we analyze how the response to endocrine and chemical therapies could be affected due to alternative splicing and differential expression of variant isoforms. With all this knowledge, it becomes clear that targeting alternative splicing represents an innovative approach for breast cancer therapeutics and the information derived from current studies could guide clinical decisions with a direct impact in the clinical advances for breast cancer patients nowadays.

HER2 isoforms co-expression differently tunes mammary tumor phenotypes affecting onset, vasculature and therapeutic response

Full-length HER2 oncoprotein and splice variant Delta16 are co-expressed in human breast cancer. We studied their interaction in hybrid transgenic mice bearing human full-length HER2 and Delta16 (F1 HER2/Delta16) in comparison to parental HER2 and Delta16 transgenic mice. Mammary carcinomas onset was faster in F1 HER2/Delta16 and Delta16 than in HER2 mice, however tumor growth was slower, and metastatic spread was comparable in all transgenic mice. Full-length HER2 tumors contained few large vessels or vascular lacunae, whereas Delta16 tumors presented a more regular vascularization with numerous endothelium-lined small vessels. Delta16-expressing tumors showed a higher accumulation of i.v. injected doxorubicin than tumors expressing full-length HER2. F1 HER2/Delta16 tumors with high full-length HER2 expression made few large vessels, whereas tumors with low full-length HER2 and high Delta16 contained numerous small vessels and expressed higher levels of VEGF and VEGFR2. Trastuzumab strongly inhibited tumor onset in F1 HER2/Delta16 and Delta16 mice, but not in full-length HER2 mice. Addiction of F1 tumors to Delta16 was also shown by long-term stability of Delta16 levels during serial transplants, in contrast full-length HER2 levels underwent wide fluctuations. In conclusion, full-length HER2 leads to a faster tumor growth and to an irregular vascularization, whereas Delta16 leads to a faster tumor onset, with more regular vessels, which in turn could better transport cytotoxic drugs within the tumor, and to a higher sensitivity to targeted therapeutic agents. F1 HER2/Delta16 mice are a new immunocompetent mouse model, complementary to patient-derived xenografts, for studies of mammary carcinoma onset, prevention and therapy.

Resveratrol fuels HER2 and ERα-positive breast cancer behaving as proteasome inhibitor

The phytoestrogen resveratrol has been reported to possess cancer chemo-preventive activity on the basis of its effects on tumor cell lines and xenograft or carcinogen-inducible in vivo models. Here we investigated the effects of resveratrol on spontaneous mammary carcinogenesis using Δ16HER2 mice as HER2+/ERα+ breast cancer model. Instead of inhibiting tumor growth, resveratrol treatment (0.0001% in drinking water; daily intake of 4μg/mouse) shortened tumor latency and enhanced tumor multiplicity in Δ16HER2 mice. This in vivo tumor-promoting effect of resveratrol was associated with up-regulation of Δ16HER2 and down-regulation of ERα protein levels and was recapitulated in vitro by murine (CAM6) and human (BT474) tumor cell lines. Our results demonstrate that resveratrol, acting as a proteasome inhibitor, leads to Δ16HER2 accumulation which favors the formation of Δ16HER2/HER3 heterodimers. The consequential activation of downstream mTORC1/p70S6K/4EBP1 pathway triggers cancer growth and proliferation. This study provides evidence that resveratrol mechanism of action (and hence its effects) depends on the intrinsic molecular properties of the cancer model under investigation, exerting a tumor-promoting effect in luminal B breast cancer subtype models.

mRNA expression of delta-HER2 and its clinicopathological correlation in HER2-overexpressing breast cancer

Delta-human epidermal growth factor receptor 2 (HER2) is a splice variant of HER2, which lacks 16 amino acids in the extracellular domain. The aim of the present study was to elucidate the expression of delta-HER2 and its clinicopathological correlation in Japanese patients with HER2-overexpressing breast cancer. A total of 40 cases of HER2-overexpressing breast cancer were investigated. The mRNA expression levels of wild type (wt)-HER2 and delta-HER2 were quantitated by quantitative polymerase chain reaction using RNA extracted from formalin-fixed paraffin-embedded specimens. In addition, the activated form of phosphorylated-SRC (pSRC) and Ki-67 were detected by immunostaining. The mRNA expression levels of wt-HER2 varied largely, and the mRNA levels of delta-HER2 were correlated with those of wt-HER2. Furthermore, the enhanced immunostaining of pSRC was detected in 15 cases. Unsupervised hierarchal clustering of wt-HER2, delta-HER2 and pSRC was used to separate the cases into three clusters. In the first cluster, the levels of wt-HER2 and delta-HER2 were low, and pSRC immunostaining was low. Two other clusters were characterized by moderately and highly increased levels of wt-HER2 and delta-HER2 mRNA expression and enhanced pSRC. The delta/wt ratio was higher in the first cluster. Positive lymph node status and recurrence were more frequent in the first cluster compared with those in the two other clusters. Furthermore, the delta/wt ratio was significantly increased in lymph node-positive and recurrent cases, compared with in lymph node-negative and non-recurrent cases. The present study demonstrated that delta-HER2 was expressed in Japanese patients with HER2-overexpressing breast cancer. High mRNA expression levels of delta-HER2 were associated with pSRC and increased proliferation of tumor cells. A poor prognosis may be predicted by the increase in delta/wt ratio in HER2-overexpressing breast cancer.

Pathobiological implications of the d16HER2 splice variant for stemness and aggressiveness of HER2-positive breast cancer

We have previously shown that the d16HER2 splice variant is linked to HER2-positive breast cancer (BC) tumorigenesis, progression and response to Trastuzumab. However, the mechanisms by which d16HER2 contributes to HER2-driven aggressiveness and targeted therapy susceptibility remain uncertain. Here, we report that the d16HER2-positive mammary tumor cell lines MI6 and MI7, derived from spontaneous lesions of d16HER2 transgenic (tg) mice and resembling the aggressive features of primary lesions, are enriched in the expression of Wnt, Notch and epithelial–mesenchymal transition pathways related genes compared with full-length wild-type (WT) HER2-positive cells (WTHER2_1 and WTHER2_2) derived from spontaneous tumors arising in WTHER2 tg mice. MI6 cells exhibited increased resistance to anoikis and significantly higher mammosphere-forming efficiency (MFE) and self-renewal capability than the WTHER2-positive counterpart. Furthermore, d16HER2-positive tumor cells expressed a higher fraction of CD29^High/CD24⁺/SCA1^Low cells and displayed greater in vivo tumor engraftment in serial dilution conditions than WTHER2_1 cells. Accordingly, NOTCH inhibitors impaired mammosphere formation only in MI6 cells. A comparative analysis of stemness-related features driven by d16HER2 and WTHER2 in ad hoc engineered human BC cells (MCF7 and T47D) revealed a higher MFE and aldehyde dehydrogenase-positive staining in d16HER2- vs WTHER2-infected cells, sustaining consistent BC-initiating cell enrichment in the human setting. Moreover, marked CD44 expression was found in MCF7_d16 and T47D_d16 cells vs their WTHER2 and Mock counterparts. Clinically, BC cases from two distinct HER2-positive cohorts characterized by high levels of expression of the activated-d16HER2 metagene were significantly enriched in the Notch family and signal transducer genes vs those with low levels of the metagene.

Therapeutic siRNA for drug-resistant HER2-positive breast cancer

HER2 is overexpressed in about 20% of breast cancers and contributes to poor prognosis. Unfortunately, a large fraction of patients have primary or acquired resistance to the HER2-targeted therapy trastuzumab, thus a multi-drug combination is utilized in the clinic, putting significant burden on patients. We systematically identified an optimal HER2 siRNA from 76 potential sequences and demonstrated its utility in overcoming intrinsic and acquired resistance to trastuzumab and lapatinib in 18 HER2-positive cancer cell lines. We provided evidence that the drug-resistant cancer maintains dependence on HER2 for survival. Importantly, cell lines did not readily develop resistance following extended treatment with HER2 siRNA. Using our recently developed nanoparticle platform, systemic delivery of HER2 siRNA to trastuzumab-resistant tumors resulted in significant growth inhibition. Moreover, the optimal HER2 siRNA could also silence an exon 16 skipped HER2 splice variant reported to be highly oncogenic and linked to trastuzumab resistance.