Foulds' dangerous idea revisited: the multistep development of tumors 40 years later

Functional Genomic Analysis of Breast Cancer Metastasis: Implications for Diagnosis and Therapy

Simple Summary

Metastasis remains the greatest cause of fatalities in breast cancer patients world-wide. The process of metastases is highly complex, and the current research efforts in this area are still rather fragmented. The revolution of genomic profiling methods to analyze samples from human and animal models dramatically improved our understanding of breast cancer metastasis. This article summarizes the recent breakthroughs in genomic analyses of breast cancer metastasis and discusses their implications for prognostic and therapeutic applications.

Abstract

Breast cancer (BC) is one of the most diagnosed cancers worldwide and is the second cause of cancer related death in women. The most frequent cause of BC-related deaths, like many cancers, is metastasis. However, metastasis is a complicated and poorly understood process for which there is a shortage of accurate prognostic indicators and effective treatments. With the rapid and ever-evolving development and application of genomic sequencing technologies, many novel molecules were identified that play previously unappreciated and important roles in the various stages of metastasis. In this review, we summarize current advancements in the functional genomic analysis of BC metastasis and discuss about the potential prognostic and therapeutic implications from the recent genomic findings.

Understanding the Progression of Bone Metastases to Identify Novel Therapeutic Targets

Bone is one of the most preferential target site for cancer metastases, particularly for prostate, breast, kidney, lung and thyroid primary tumours. Indeed, numerous chemical signals and growth factors produced by the bone microenvironment constitute factors promoting cancer cell invasion and aggression. After reviewing the different theories proposed to provide mechanism for metastatic progression, we report on the gene expression profile of bone-seeking cancer cells. We also discuss the cross-talk between the bone microenvironment and invading cells, which impacts on the tumour actions on surrounding bone tissue. Lastly, we detail therapies for bone metastases. Due to poor prognosis for patients, the strategies mainly aim at reducing the impact of skeletal-related events on patients' quality of life. However, recent advances have led to a better understanding of molecular mechanisms underlying bone metastases progression, and therefore of novel therapeutic targets.

Anti-CSF-1 treatment is effective to prevent carcinoma invasion induced by monocyte-derived cells but scarcely by microglia

The mononuclear phagocytic system is categorized in three major groups: monocyte-derived cells (MCs), dendritic cells and resident macrophages. During breast cancer progression the colony stimulating factor 1 (CSF-1) can reprogram MCs into tumor-promoting macrophages in the primary tumor. However, the effect of CSF-1 during colonization of the brain parenchyma is largely unknown.

Thus, we analyzed the outcome of anti-CSF-1 treatment on the resident macrophage population of the brain, the microglia, in comparison to MCs, alone and in different in vitro co-culture models. Our results underline the addiction of MCs to CSF-1 while surprisingly, microglia were not affected. Furthermore, in contrast to the brain, the bone marrow did not express the alternative ligand, IL-34. Yet treatment with IL-34 and co-culture with carcinoma cells partially rescued the anti-CSF-1 effects on MCs. Further, MC-induced invasion was significantly reduced by anti-CSF-1 treatment while microglia-induced invasion was reduced to a lower extend. Moreover, analysis of lung and breast cancer brain metastasis revealed significant differences of CSF-1 and CSF-1R expression.

Taken together, our findings demonstrate not only differences of anti-CSF-1 treatment on MCs and microglia but also in the CSF-1 receptor and ligand expression in brain and bone marrow as well as in brain metastasis.

Epigenetics and colorectal cancer pathogenesis

Colorectal cancer (CRC) develops through a multistage process that results from the progressive accumulation of genetic mutations, and frequently as a result of mutations in the Wnt signaling pathway. However, it has become evident over the past two decades that epigenetic alterations of the chromatin, particularly the chromatin components in the promoter regions of tumor suppressors and oncogenes, play key roles in CRC pathogenesis. Epigenetic regulation is organized at multiple levels, involving primarily DNA methylation and selective histone modifications in cancer cells. Assessment of the CRC epigenome has revealed that virtually all CRCs have aberrantly methylated genes and that the average CRC methylome has thousands of abnormally methylated genes. Although relatively less is known about the patterns of specific histone modifications in CRC, selective histone modifications and resultant chromatin conformation have been shown to act, in concert with DNA methylation, to regulate gene expression to mediate CRC pathogenesis. Moreover, it is now clear that not only DNA methylation but also histone modifications are reversible processes. The increased understanding of epigenetic regulation of gene expression in the context of CRC pathogenesis has led to development of epigenetic biomarkers for CRC diagnosis and epigenetic drugs for CRC therapy.

The involvement of a Nanog, Klf4 and c-Myc transcriptional circuitry in the intertwining between neoplastic progression and reprogramming

One undisputed milestone of traditional oncology is neoplastic progression, which consists of a progressive selection of dedifferentiated cells driven by a chance sequence of genetic mutations. Recently it has been demonstrated that the overexpression of well-defined transcription factors reprograms somatic cells to the pluripotent stem status. The demonstration raises crucial questions as to whether and to what extent this reprogramming contributes to tumorigenesis, and whether the epigenetic changes involved in it are reversible. Here, we show for the first time that a tumor produced in vivo by a chemical carcinogen is the product of the interaction between neoplastic progression and reprogramming. The experimental model employed the prototype of ascites tumors, the Yoshida AH130 hepatoma and other neoplasias, including human melanoma. AH130 hepatoma was started in the liver by the carcinogen o-aminoazotoluene. This compound binds to and abolishes the p53 protein, producing a genomic instability that promotes both the neoplastic progression and the hepatoma reprogramming. Eventually this tumor contained 100% CD133(+) elements and pO(2)-dependent percentages of the three embryonic transcription factors Nanog, Klf4 and c-Myc. Once transferred into aerobic cultures, the minor cellular fraction expressing this triad generates various types of adherent cells, which are progressively substituted by non-tumorigenic elements committed to fibromuscular, neuronal and glial differentiation. This reprogramming appears to be accomplished stepwise, with the assembly of the triad into a sophisticated transcriptional, oxygen-dependent circuitry, in which Nanog and Klf4 antagonistically regulate c-Myc, and hence, cell hypoxia survival and cell cycle activation.

Combined experimental and mathematical approach for development of microfabrication-based cancer migration assay

Migration of cancer cells is a key determinant of metastasis, which is correlated with poor prognosis in patients. Evidence shows that cancer cell motility is regulated by stromal cell interactions. To quantify the role of homotypic and heterotypic cell-cell interaction in migration, a two-dimensional migration assay has been developed by microfabrication techniques. Two breast cancer cell lines, MDA-MB-231 and MDA-MB-453, were used to develop micropatterns of cancer cells (cell islands) that revealed distinct migration profiles in this assay. Although the individual migration rates of these cells showed only a sevenfold difference, MDA-MB-453 islands migrated significantly lower than MDA-MB-231 islands, indicating differential regulation of migration in isolated cells vs. islands. Island size had the greatest impact on migration, primarily for MDA-MB-231 cells. Migration of MDA-MB-231 islands was decreased by interaction with homotypic cells, and significantly more by heterotypic non-cancer-associated fibroblasts. In addition, a mathematical model of island migration in multi-cellular population has been developed using Stefan-Maxwell's equation. The model showed qualitative agreement with experimental results and predicted a biphasic relation between cell densities and island sizes. The combined experimental and mathematical model can be used to quantitatively study the impact of cell-cell interactions on migration.

Parallel progression of primary tumours and metastases

Systemic cancer progression is accounted for in two basic models. The prevailing archetype places the engine of cancer progression within the primary tumour before metastatic dissemination of fully malignant cells. The second posits parallel, independent progression of metastases arising from early disseminated tumour cells. This Perspective draws together data from disease courses, tumour growth rates, autopsy studies, clinical trials and molecular genetic analyses of primary and disseminated tumour cells in support of the parallel progression model. Consideration of this model urges review of current diagnostic and therapeutic routines.

The effect of cell-matrix interactions and aging on the malignant process

The malignant process, transformation of normal cells, proliferation, and metastasis formation, was considered as if originating from one single cell. Although the intrinsic mechanisms of transformation from the normal to the malignant state were both confirmed, an increasing body of evidence points to the surrounding matrix and cell-matrix interactions as major players in this process. Some of the most important arguments in favor of this contention are cited and commented in this chapter. Another important question concerns the relationship between the aging process and malignant transformation. A few decades ago, the frequency of clinically manifest tumors of several organs and tissues appeared to increase with age. As, however, average life expectancy increased rapidly over the last decades, clinical frequency of malignant tumors did not follow this tendency. It was argued that late in life the malignant process appears to decline. This justly inspired several teams to study the relationship between cellular senescence and malignant transformation. This is now an actively growing field which deserves special attention. Some of the pertinent experimental and theoretical arguments in favor of an antioncogene-mediated switch between these two processes are also reviewed with the caveat that this important and new subject of basic and clinical research on the malignant process is just at its beginning. It will certainly take an increasing importance during the coming years and decades with the hope to contribute to answer one of the most burning questions concerning the aging process: will life expectancy continue to increase linearly as predicted by some gerontologists, or will life expectancy level off or even decline as predicted by other epidemiologists. The relationship between cellular senescence and malignant transformation will play in this respect an important role.

The evolutionary origin of genetic instability in cancer development

The standard model of carcinogenesis is currently being questioned. The main controversy concerns genetic instability and has links to fundamental questions in evolutionary biology. This paper aims to clarify the underlying conflict between the linear configuration of the standard model and the non-linear dynamics of Darwinian evolution. It addresses the problem of applying the concept of clonal selection to genetically unstable cells and presents an alternative perspective based on the principles of molecular evolution. This model explains genetic instability in terms of competition between genetic strategies and draws lines to basic aspects of evolutionary biology.

The Systemic Progression of Human Cancer: A Focus on the Individual Disseminated Cancer Cell—The Unit of Selection

The metastatic progression of solid tumors is discussed controversially. Because metastasis is usually lethal, it appears as an end point of successive cellular changes. This has led to the prevailing interpretation that genetic changes, in addition to those present in the most advanced clone of the primary tumor, are required to initiate invasion, dissemination, and growth at anatomically distant sites. It has become possible to detect and analyze single disseminated cancer cells at ectopic sites long before metastasis can be diagnosed by standard clinical techniques. Because the finding of single disseminated cancer cells correlates with the subsequent development of distant metastasis, these cells have been identified as the precursors of metastasis. Their direct molecular-genetic characterization, however, shows that dissemination occurs very early in the process of accumulation of genetic changes and suggests that metastases may seldom be derived from the dominant clone of the primary tumor. In contrast, it appears that cancer cell evolution explores a multitude of variant cells from which systemic cancer can develop independently. This review integrates data derived by different approaches into a model of systemic cancer progression.

Early cancer cell dissemination and late metastatic relapse: clinical reflections and biological approaches to the dormancy problem in patients

Two clinical entities, unknown-primary cancer and inadvertent transmission of cancer with organ transplants are reviewed and discussed in the context of early and occult tumor cell dissemination. Both entities are taken as chief witnesses for cell dissemination being an early event in tumor progression. The involuntary transmission of tumor by organ grafts directly supports the notion that very few quiescent cells lodging at improbable sites such as kidney or heart suffice to generate de novo metastatic disease in the organ recipient. As to the nature of the cells and their biological and clinical significance a short review is given on the detection of disseminated cells in bone marrow and their prognostic significance for a metastatic relapse in patients with resected primary tumors. A novel single-cell genomic analysis is described, that allows the detection of multiple chromosomal aberration in single tumor cells.

Radiation-induced genomic instability: a paradigm-breaking phenomenon and its relevance to environmentally induced cancer

The existing paradigm governing radiobiology which is fundamental to the estimation of environmental radiation risk, cannot explain the phenomena of radiation induced genomic instability and the bystander effect. Both effects can, however, be understood in terms of the dynamical genome concept, qualitatively described herein. The dynamical genome concept may find further application in better understanding other aspects of biology, most notably the cancer process in general and the consequences of genetic modification.

Risk assessment for possible carcinogens: a critical look

Our overall understanding of mechanisms of toxicology in relation to human disease, with prevention of disease as a major objective, depends in part on the development of an adequate number of ways to assess risks, both short term and long term. Despite the cost, the long duration of the test, and some pitfalls, the long-term animal tests remain, to date, the only reliable assay for possible carcinogens. Recent work has concentrated increasingly on the development of short-term tests to replace the long-term tests. Such a development would be most welcome from several points of view. To date, a variety of approaches have been or are being used. These include (1) activation to an alkylating agent with DNA as the most important target, generating possible mutations in DNA and DNA damage with or without repair, (2) induction of cell proliferation, at least a few cycles, with DNA synthesis as the major target, again favoring mutations, and (3) decrease in cell-to-cell communication (gap-junctional intercellular communication) as a supposed test for promotion. None of these proposed assays are reliable indices for possible carcinogenic effects of chemicals or other agents; the scientific basis for this negative conclusion is discussed.

Comparative genomic hybridization, loss of heterozygosity, and DNA sequence analysis of single cells

A PCR strategy is described for global amplification of DNA from a single eukaryotic cell that enables the comprehensive analysis of the whole genome. By comparative genomic hybridization, not only gross DNA copy number variations, such as monosomic X and trisomic 21 in single male cells and cells from Down's syndrome patients, respectively, but multiple deletions and amplifications characteristic for human tumor cells are reliably retrieved. As a model of heterogeneous cell populations exposed to selective pressure, we have studied single micrometastatic cells isolated from bone marrow of cancer patients. The observed congruent pattern of comparative genomic hybridization data, loss of heterozygosity, and mutations as detected by sequencing attests to the technique's fidelity and demonstrates its usefulness for assessing clonal evolution of genetic variants in complex populations.