Defining the cellular precursors to human breast cancer

Global changes in the mammary epigenome are induced by hormonal cues and coordinated by Ezh2

The mammary epithelium is a dynamic, highly hormone-responsive tissue. To explore chromatin modifications underlying its lineage specification and hormone responsiveness, we determined genome-wide histone methylation profiles of mammary epithelial subpopulations in different states. The marked differences in H3K27 trimethylation between subpopulations in the adult gland suggest that epithelial cell-fate decisions are orchestrated by polycomb-complex-mediated repression. Remarkably, the mammary epigenome underwent highly specific changes in different hormonal contexts, with a profound change being observed in the global H3K27me3 map of luminal cells during pregnancy. We therefore examined the role of the key H3K27 methyltransferase Ezh2 in mammary physiology. Its expression and phosphorylation coincided with H3K27me3 modifications and peaked during pregnancy, driven in part by progesterone. Targeted deletion of Ezh2 impaired alveologenesis during pregnancy, preventing lactation, and drastically reduced stem/progenitor cell numbers. Taken together, these findings reveal that Ezh2 couples hormonal stimuli to epigenetic changes that underpin progenitor activity, lineage specificity, and alveolar expansion in the mammary gland.

Is mammary not otherwise specified-type sarcoma with CD10 expression a distinct entity? A rare case report with immunohistochemical and ultrastructural study

Mammary sarcoma is extremely rare and the diagnosis is established only after metaplastic carcinomas and malignant phyllodes tumours are excluded. A rare case of not otherwise specified-type sarcoma with CD10 expression in the left breast in a 45-year-old female was presented. It was a high-grade tumour composed of spindle cells histologically. The immunohistochemical results showed that CD10, vimentin and EGFR were positive diffusely and SMA presented focally, whereas epithelial markers and other myoepithelial or myogenic markers were all negative. The electron microscope investigation demonstrated fibroblast-like features. The exact entity of the tumour remains to be studied because it resembles undifferentiated sarcoma or sarcomatoid metaplastic carcinoma to some degree, as well as high-grade malignant phyllodes tumour in particular.

VIRTUAL SLIDES

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/9019879588725702.

Does microenvironment contribute to the etiology of estrogen receptor-negative breast cancer?

What dictates the prevalence of certain types of breast cancer, which are classified by markers, particularly estrogen receptor (ER), expression profiles such as basal or luminal, and genetic alterations such as HER2 amplification, in particular populations is not well understood. It is increasingly evident that microenvironment disruption is highly intertwined with cancer progression. Here, the idea that microenvironment shapes the course of carcinogenesis, and hence breast cancer subtype, is discussed. Aggressive, basal-like, ER-negative breast tumors occur in younger women, African-American women, women who carry BRCA1 mutation, and women exposed to ionizing radiation. Recent experimental studies using ionizing radiation, a well-documented environmental exposure, suggest that certain processes in the microenvironment strongly favor the development of ER-negative tumors. Understanding the contribution of tissue microenvironment during carcinogenesis could lead to prevention strategies that are personalized to age, agent, and exposure to reduce the risk of aggressive breast cancer.

NF-κB non-cell-autonomously regulates cancer stem cell populations in the basal-like breast cancer subtype

Patients with triple-negative breast cancer display the highest rates of early relapse of all patients with breast cancer. The basal-like subtype, a subgroup of triple-negative breast cancer, exhibits high levels of constitutively active NF-κB signalling. Here we show that NF-κB activation, induced by inflammatory cytokines or by epigenetically dysregulated NIK expression, cell-autonomously upregulates JAG1 expression in non-cancer stem cells. This upregulation stimulates NOTCH signalling in cancer stem cells in trans, leading to an expansion of cancer stem cell populations. Among breast cancers, the NF-κB-dependent induction of JAG1 and the NOTCH-dependent expansion of the cancer stem cell population occur only in the basal-like subtype. Collectively, our results indicate that NF-κB has a non-cell-autonomous role in regulating cancer stem cell populations by forming intratumoural microenvironments composed of JAG1-expressing non-cancer stem cells with a basal-like subtype.

Cell-type specific DNA methylation patterns define human breast cellular identity

DNA methylation plays a role in a variety of biological processes including embryonic development, imprinting, X-chromosome inactivation, and stem cell differentiation. Tissue specific differential methylation has also been well characterized. We sought to extend these studies to create a map of differential DNA methylation between different cell types derived from a single tissue. Using three pairs of isogenic human mammary epithelial and fibroblast cells, promoter region DNA methylation was characterized using MeDIP coupled to microarray analysis. Comparison of DNA methylation between these cell types revealed nearly three thousand cell-type specific differentially methylated regions (ctDMRs). MassARRAY was performed upon 87 ctDMRs to confirm and quantify differential DNA methylation. Each of the examined regions exhibited statistically significant differences ranging from 10-70%. Gene ontology analysis revealed the overrepresentation of many transcription factors involved in developmental processes. Additionally, we have shown that ctDMRs are associated with histone related epigenetic marks and are often aberrantly methylated in breast cancer. Overall, our data suggest that there are thousands of ctDMRs which consistently exhibit differential DNA methylation and may underlie cell type specificity in human breast tissue. In addition, we describe the pathways affected by these differences and provide insight into the molecular mechanisms and physiological overlap between normal cellular differentiation and breast carcinogenesis.

Keeping an open mind: highlights and controversies of the breast cancer stem cell theory

The discovery that breast cancers contain stem-like cells has fuelled exciting research in the last few years. These cells are referred to as breast cancer stem cells (BCSCs) and are thought to be involved in tumor initiation, progression, and metastasis. Being intrinsically resistant to chemo- and radiotherapy, they are also considered responsible for recurrence of the disease after treatment. BCSCs have been suggested to be at the basis of tumor complexity, as they have the ability to self-renew and give rise to highly proliferating and terminally differentiated cancer cells that comprise the heterogeneous bulk of the tumor. There has been much speculation on the BCSC model, and in this review we address some fundamental questions, such as the identity of BCSCs and their involvement in tumor intra- and interheterogeneity. As an alternative to the BCSC model, we discuss clonal evolution, as both theories show extensive evidence in support of their arguments. Finally, we discuss a unifying idea that reconciles both models, which is based on stem cell plasticity and epigenetic modifications induced by the tumor microenvironment. The implications of cancer stem cell plasticity for drug discovery and future therapeutic interventions are presented.

Mesenchymal traits are selected along with stem features in breast cancer cells grown as mammospheres

Increasing evidence indicates that invasive properties of breast cancers rely on gain of mesenchymal and stem features, which has suggested that the dual targeting of these phenotypes may represent an appealing therapeutic strategy. It is known that the fraction of stem cells can be enriched by culturing breast cancer cells as mammospheres (MS), but whether these pro-stem conditions favor also the expansion of cells provided of mesenchymal features is still undefined.

In the attempt to shed light on this issue, we compared the phenotypes of a panel of 10 breast cancer cell lines representative of distinct subtypes (luminal, HER2-positive, basal-like and claudin-low), grown in adherent conditions and as mammospheres. Under MS-proficient conditions, the increment in the fraction of stem-like cells was associated to upregulation of the mesenchymal marker Vimentin and downregulation of the epithelial markers expressed by luminal cells (E-cadherin, KRT18, KRT19, ESR1). Luminal cells tended also to upregulate the myoepithelial marker CD10. Taken together, our data indicate that MS-proficient conditions do favor mesenchymal/myoepithelial features, and indicate that the use of mammospheres as an in vitro tumor model may efficiently allow the exploitation of therapeutic approaches aimed at targeting aggressive tumors that have undergone epithelial-to-mesenchymal transition.

Phenotypic and functional characterisation of the luminal cell hierarchy of the mammary gland

Introduction

The organisation of the mammary epithelial hierarchy is poorly understood. Our hypothesis is that the luminal cell compartment is more complex than initially described, and that an understanding of the developmental relationships within this lineage will help in understanding the cellular context in which breast tumours occur.

Methods

We used fluorescence-activated cell sorting along with in vitro and in vivo functional assays to examine the growth and differentiation properties of distinct subsets of human and mouse mammary epithelial cells. We also examined how loss of steroid hormones influenced these populations in vivo. Gene expression profiles were also obtained for all the purified cell populations and correlated to those obtained from breast tumours.

Results

The luminal cell compartment of the mouse mammary gland can be resolved into nonclonogenic oestrogen receptor-positive (ER⁺) luminal cells, ER⁺ luminal progenitors and oestrogen receptor-negative (ER^-) luminal progenitors. The ER⁺ luminal progenitors are unique in regard to cell survival, as they are relatively insensitive to loss of oestrogen and progesterone when compared with the other types of mammary epithelial cells. Analysis of normal human breast tissue reveals a similar hierarchical organisation composed of nonclonogenic luminal cells, and relatively differentiated (EpCAM⁺CD49f⁺ALDH^-) and undifferentiated (EpCAM⁺CD49f⁺ALDH⁺) luminal progenitors. In addition, approximately one-quarter of human breast samples examined contained an additional population that had a distinct luminal progenitor phenotype, characterised by low expression of ERBB3 and low proliferative potential. Parent-progeny relationship experiments demonstrated that all luminal progenitor populations in both species are highly plastic and, at low frequencies, can generate progeny representing all mammary cell types. The ER^- luminal progenitors in the mouse and the ALDH⁺ luminal progenitors in the human appear to be analogous populations since they both have gene signatures that are associated with alveolar differentiation and resemble those obtained from basal-like breast tumours.

Conclusion

The luminal cell compartment in the mammary epithelium is more heterogeneous than initially perceived since progenitors of varying levels of luminal cell differentiation and proliferative capacities can be identified. An understanding of these cells will be essential for understanding the origins and the cellular context of human breast tumours.

Cyclin D1 activity regulates autophagy and senescence in the mammary epithelium

Overexpression of cyclin D1 is believed to endow mammary epithelial cells (MEC) with a proliferative advantage by virtue of its contribution to pRB inactivation. Accordingly, abrogation of the kinase-dependent function of cyclin D1 is sufficient to render mice resistant to breast cancer initiated by ErbB2. Here, we report that mouse cyclin D1(KE/KE) MECs (deficient in cyclin D1 activity) upregulate an autophagy-like process but fail to implement ErbB2-induced senescence in vivo. In addition, immortalized cyclin D1(KE/KE) MECs retain high rates of autophagy and reduced ErbB2-mediated transformation in vitro. However, highlighting its dual role during tumorigenesis, downregulation of autophagy led to an increase in senescence in cyclin D1(KE/KE) MECs. Autophagy upregulation was also confirmed in human mammary epithelial cells (HMEC) subjected to genetic and pharmacologic inhibition of cyclin D1 activity and, similar to our murine system, simultaneous inhibition of Cdk4/6 and autophagy in HMECs enhanced the senescence response. Collectively, our findings suggest a previously unrecognized function of cyclin D1 in suppressing autophagy in the mammary epithelium.

Basal but not luminal mammary epithelial cells require PI3K/mTOR signaling for Ras-driven overgrowth

The mammary ducts of humans and mice are comprised of two main mammary epithelial cell (MEC) subtypes: a surrounding layer of basal MECs and an inner layer of luminal MECs. Breast cancer subtypes show divergent clinical behavior that may reflect properties inherent in their MEC compartment of origin. How the response to a cancer-initiating genetic event is shaped by MEC subtype remains largely unexplored. Using the mouse mammary gland, we designed organotypic three-dimensional culture models that permit challenge of discrete MEC compartments with the same oncogenic insult. Mammary organoids were prepared from mice engineered for compartment-restricted coexpression of oncogenic H-RAS(G12V) together with a nuclear fluorescent reporter. Monitoring of H-RAS(G12V)-expressing MECs during extended live cell imaging permitted visualization of Ras-driven phenotypes via video microscopy. Challenging either basal or luminal MECs with H-RAS(G12V) drove MEC proliferation and survival, culminating in aberrant organoid overgrowth. In each compartment, Ras activation triggered modes of collective MEC migration and invasion that contrasted with physiologic modes used during growth factor-initiated branching morphogenesis. Although basal and luminal Ras activation produced similar overgrowth phenotypes, inhibitor studies revealed divergent use of Ras effector pathways. Blocking either the phosphoinositide 3-kinase or the mammalian target of rapamycin pathway completely suppressed Ras-driven invasion and overgrowth of basal MECs, but only modestly attenuated Ras-driven phenotypes in luminal MECs. We show that MEC subtype defines signaling pathway dependencies downstream of Ras. Thus, cells-of-origin may critically determine the drug sensitivity profiles of mammary neoplasia.

EMT inducers catalyze malignant transformation of mammary epithelial cells and drive tumorigenesis towards claudin-low tumors in transgenic mice

The epithelial-mesenchymal transition (EMT) is an embryonic transdifferentiation process consisting of conversion of polarized epithelial cells to motile mesenchymal ones. EMT–inducing transcription factors are aberrantly expressed in multiple tumor types and are known to favor the metastatic dissemination process. Supporting oncogenic activity within primary lesions, the TWIST and ZEB proteins can prevent cells from undergoing oncogene-induced senescence and apoptosis by abolishing both p53- and RB-dependent pathways. Here we show that they also downregulate PP2A phosphatase activity and efficiently cooperate with an oncogenic version of H-RAS in malignant transformation of human mammary epithelial cells. Thus, by down-regulating crucial tumor suppressor functions, EMT inducers make cells particularly prone to malignant conversion. Importantly, by analyzing transformed cells generated in vitro and by characterizing novel transgenic mouse models, we further demonstrate that cooperation between an EMT inducer and an active form of RAS is sufficient to trigger transformation of mammary epithelial cells into malignant cells exhibiting all the characteristic features of claudin-low tumors, including low expression of tight and adherens junction genes, EMT traits, and stem cell–like characteristics. Claudin-low tumors are believed to be the most primitive breast malignancies, having arisen through transformation of an early epithelial precursor with inherent stemness properties and metaplastic features. Challenging this prevailing view, we propose that these aggressive tumors arise from cells committed to luminal differentiation, through a process driven by EMT inducers and combining malignant transformation and transdifferentiation.

The epithelial-mesenchymal transition (EMT) is essential to germ layer formation and cell migration in the early vertebrate embryo. EMT is aberrantly reactivated under pathological conditions, including fibrotic disease and cancer progression. In the latter process, EMT is known to promote invasion and metastatic dissemination of tumor cells. EMT is orchestrated by a variety of embryonic transcription factors called EMT inducers. Among these, the TWIST and ZEB proteins are known to be frequently reactivated during tumor development. We here report in vitro and in vivo observations demonstrating that activation of these factors fosters cell transformation and primary tumor growth by alleviating key oncosuppressive mechanisms, thereby minimizing the number of events required for acquisition of malignant properties. In a model of breast cancer, cooperation between a single EMT inducer and a single mitogenic oncoprotein is sufficient to transform mammary epithelial cells into malignant cells and to drive the development of aggressive and undifferentiated tumors. Overall, these data underscore the oncogenic role of embryonic transcription factors in initiating the development of poor-prognosis neoplasms by promoting both cell transformation and dedifferentiation.

EMT in breast cancer stem cell generation

The concept of cancer stem cells (CSCs) has been proposed to explain the ability of single disseminated cancer cells to reconstitute tumours with heterogeneity similar to that of the primary tumour they arise from. Although this concept is now commonly accepted, the origin of these CSCs remains a source of debate. First proposed to arise through stem/progenitor cell transformation, CSCs might also or alternatively arise from differentiated cancer cells through epithelial to mesenchymal transition (EMT), an embryonic transdifferentiation process. Using breast carcinomas as a study model, I propose revisiting the role of EMT in generating CSCs and the debate on potential underlying mechanisms and biological significance.

Derivation of myoepithelial progenitor cells from bipotent mammary stem/progenitor cells

There is increasing evidence that breast and other cancers originate from and are maintained by a small fraction of stem/progenitor cells with self-renewal properties. Recent molecular profiling has identified six major subtypes of breast cancer: basal-like, ErbB2-overexpressing, normal breast epithelial-like, luminal A and B, and claudin-low subtypes. To help understand the relationship among mammary stem/progenitor cells and breast cancer subtypes, we have recently derived distinct hTERT-immortalized human mammary stem/progenitor cell lines: a K5(+)/K19(-) type, and a K5(+)/K19(+) type. Under specific culture conditions, bipotent K5(+)/K19(-) stem/progenitor cells differentiated into stable clonal populations that were K5(-)/K19(-) and exhibit self-renewal and unipotent myoepithelial differentiation potential in contrast to the parental K5(+)/K19(-) cells which are bipotent. These K5(-)/K19(-) cells function as myoepithelial progenitor cells and constitutively express markers of an epithelial to mesenchymal transition (EMT) and show high invasive and migratory abilities. In addition, these cells express a microarray signature of claudin-low breast cancers. The EMT characteristics of an un-transformed unipotent mammary myoepithelial progenitor cells together with claudin-low signature suggests that the claudin-low breast cancer subtype may arise from myoepithelial lineage committed progenitors. Availability of immortal MPCs should allow a more definitive analysis of their potential to give rise to claudin-low breast cancer subtype and facilitate biological and molecular/biochemical studies of this disease.

Role of microRNAs in the regulation of breast cancer stem cells

There is increasing evidence that many human cancers, including breast cancer, are driven and maintained by cancer stem cells (CSCs) which mediate tumor metastasis and contribute to treatment resistance and relapse. Our group was the first to describe "breast cancer stem cells" (BCSCs) characterized by expression of the cell surface markers ESA and CD44 and the absence of expression of the marker CD24. More recently, we have demonstrated that breast cancer cells contain subpopulations with stem cell properties that can be isolated by virtue of their expression of Aldehyde dehydrogenase (ALDH) as assessed by the Aldefluor assay. Interestingly, these markers identify overlapping, but not identical cell populations. Recent studies have suggested similarities between cancer stem cells and the epithelial mesenchymal transition (EMT) state. Our studies suggest that both normal and malignant breast stem cells exist in distinct, inter-convertible states (EMT and MET), the inter-conversion of which is regulated by microRNAs. EMT-like CSCs have a mesenchymal morphology, are largely quiescent, invasive and characterized by expression of the CSC markers CD24(-)CD44(+) and are EpCAM(-)CD49f(+). In contrast, the MET (mesenchymal epithelial transition) state of CSCs is characterized by active self-renewal and expression of the CSC markers ALDH and EpCAM(+)CD49f(+). A subpopulation of cells expressing both CD24(-)CD44(+) and ALDH may represent cells in transition between these states. This transition is regulated by signals originating in the microenvironment which in turn modulate microRNA networks in the CSC populations. The existence of multiple stem cell states suggests the necessity of developing therapeutic strategies capable of effectively targeting CSCs in all of these states. In addition, since CSC states are regulated by miRNAs, these small non-coding RNAs may be useful therapeutic agents to target CSCs.

Basal/HER2 breast carcinomas: integrating molecular taxonomy with cancer stem cell dynamics to predict primary resistance to trastuzumab (Herceptin)

High rates of inherent primary resistance to the humanized monoclonal antibody trastuzumab (Herceptin) are frequent among HER2 gene-amplified breast carcinomas in both metastatic and adjuvant settings. The clinical efficacy of trastuzumab is highly correlated with its ability to specifically and efficiently target HER2-driven populations of breast cancer stem cells (CSCs). Intriguingly, many of the possible mechanisms by which cancer cells escape trastuzumab involve many of the same biomarkers that have been implicated in the biology of CS-like tumor-initiating cells. In the traditional, one-way hierarchy of CSCs in which all cancer cells descend from special self-renewing CSCs, HER2-positive CSCs can occur solely by self-renewal. Therefore, by targeting CSC self-renewal and resistance, trastuzumab is expected to induce tumor shrinkage and further reduce breast cancer recurrence rates when used alongside traditional therapies. In a new, alternate model, more differentiated non-stem cancer cells can revert to trastuzumab-refractory, CS-like cells via the activation of intrinsic or microenvironmental paths-to-stemness, such as the epithelial-to-mesenchymal transition (EMT). Alternatively, stochastic transitions of trastuzumab-responsive CSCs might also give rise to non-CSC cellular states that lack major attributes of CSCs and, therefore, can remain "hidden" from trastuzumab activity. Here, we hypothesize that a better understanding of the CSC/non-CSC social structure within HER2-overexpressing breast carcinomas is critical for trastuzumab-based treatment decisions in the clinic. First, we decipher the biological significance of CSC features and the EMT on the molecular effects and efficacy of trastuzumab in HER2-positive breast cancer cells. Second, we reinterpret the genetic heterogeneity that differentiates trastuzumab-responders from non-responders in terms of CSC cellular states. Finally, we propose that novel predictive approaches aimed at better forecasting early tumor responses to trastuzumab should identify biological determinants that causally underlie the intrinsic flexibility of HER2-positive CSCs to "enter" into or "exit" from trastuzumab-sensitive states. An accurate integration of CSC cellular states and EMT-related biomarkers with the currently available breast cancer molecular taxonomy may significantly improve our ability to make a priori decisions about whether patients belonging to HER2 subtypes differentially enriched with a "mesenchymal transition signature" (e.g., luminal/HER2 vs. basal/HER2) would distinctly benefit from trastuzumab-based therapy ab initio.

Conservative surgery in the Zollinger-Ellison syndrome

Breast cancer stem cells have been known to contribute immensely to the carcinogenesis of the breast and therapeutic resistance in the clinic. Current studies show that the population of breast cancer stem cells is heterogeneous, involving various cellular markers and regulatory signaling pathways. In addition, different subtypes of breast cancer exhibit distinct subtypes and frequencies of breast cancer stem cells. In this review, we provide an overview of the characteristics of breast cancer stem cells, including their various molecular markers, prominent regulatory signaling, and complex microenvironment. The cellular origins of breast cancer are discussed to understand the heterogeneity and diverse differentiations of stem cells. Importantly, we also outline the recent advances and controversies in the therapeutic implications of breast cancer stem cells in different subtypes of breast cancer.