Ann Chambers, Ph.D., London Health Sciences Centre
Zena Werb, Ph.D., University of California San Francisco
Sara Courtneidge, Ph.D., Sanford-Burnham Medical Research Institute
Daniel Haber, MD, Ph.D., Massachusetts General Hospital
Christine Iacobuzio-Donahue, MD, Ph.D., Memorial Sloan-Kettering Cancer Center
Nada Jabado, MD, Ph.D., McGill University
Yibin Kang, Ph.D., Princeton University
Klaus Pantel, MD, Ph.D., University of Molecular Biology
Julie R. Park, MD, Children’s Hospital and Medical Center
Erik Sahai, Ph.D., London Research Institute
Gregg Semenza, MD, Ph.D., John Hopkins University
Patricia Steeg, Ph.D., National Cancer Institute
Karla Williams, Ph.D., London Health Science Centre
Most cancer deaths are due to metastasis – the spread of cancer from its site of origin to distant, vital organs, and the physiological damage caused by tumor growth in those organs. While the broad outlines of the process of metastatic spread are known, much of the details of the process remain poorly understood. To continue to improve cancer survival rates, we must face and tackle the problems inherent to metastatic disease. Cancers that are detected early, before they are believed to have spread to other organs, are generally treated with more success than cancers that are metastatic at diagnosis. However, even cancers that are detected early will recur in some patients, but our ability to predict which individuals will have recurrences is limited. Thus, adjuvant therapy is often given to patients with early stage disease who are believed as a group to be risk for recurrence, leading to over-treatment of some patients to benefit a subset of them. Some recurrences can occur years or even decades after apparently successful primary treatment, and research on tumor dormancy is providing insights into these delayed recurrences. Progress has been made in the basic biology of tumor invasion and metastasis, and in understanding some of the complexities of interactions of tumor cells with host cells in their microenvironment. Great advances have been made for many cancers, in terms of molecular markers/subtypes that are associated with favorable vs. poor outcome, as well as prediction of response to a growing list of molecularly targeted agents. However, we also recognize that tumors are not static entities, but instead evolve and change over time, and information from a primary tumor specimen may poorly characterize individual metastases that occur years later. Bioinformatic analyses of tumors and their metastases, over time, are providing a wealth of data to be interpreted. New models are being developed to address problems in metastasis. The challenge is to learn how to harness this growing body of information to help patients with cancer. Can we prevent metastasis? Can we delay appearance of metastases following primary treatment, either through information inherent to the primary tumor, or through life style or anti-metastatic chemoprevention strategies? Can we learn how to better treat metastases once they have developed?
Tumor progression and molecular genetics of metastatic disease
Zena Werb: Despite major advances in understanding the molecular and genetic basis of cancer, disease progression to metastasis remains the cause of >90% of cancer-related mortality. Dr. Werb posited that metastasis initiation is critical for the development of new therapeutic strategies to specifically treat and prevent progression to metastatic disease. Prevailing theories hypothesize that metastases are seeded by rare tumor cells with unique properties, which may function like stem cells in their ability to initiate and propagate new tumors in metastatic sites through self-renewal and differentiation. This is supported by studies in human colon and pancreatic cancer showing metastases arise from cancer stem cells (CSCs). However, the identity of metastasis-initiating cells in human breast cancer remains elusive, and specifically whether metastases are hierarchically organized is unclear. She showed at the single-cell level that early stage human disseminated tumor cells (DTCs) possess a distinct stem cell-like gene expression signature. To identify and isolate DTCs from patient-derived xenograft (PDX) models of human breast cancer, her group developed a highly sensitive FACS-based assay, which allowed them to compared gene signatures in DTCs at different stages of metastasis. She found that ‘early’ DTCs comprised a distinct population from ‘late’ DTCs and primary tumor cells due to their increased expression of stem cell, EMT, pro-survival, and dormancy-associated genes. These findings support a hierarchical model for metastatic cell initiation and progression, and open up new targets for the management of metastatic disease.
Yibin Kang: Dr. Kang discussed the origin and evolution of metastatic traits in breast cancer. Metastasis represents the most devastating stage of cancer progression and is responsible for most of cancer-related death. How and when cancer cells acquire metastatic traits is a topic of intense investigation and debate in the field. It has become clear that the development of metastatic capability in cancer cells is a continuous process that is shaped by the tissue of origin of the primary tumor, early oncogenic events, as well as the stresses tumor cells endure when they encounter different microenvironments and therapeutic treatments. Many genes play multiple functions during primary tumorigenesis and metastatic progression, and may represent ideal targets for therapeutic intervention. In this lecture, Dr. Kang discussed some latest findings in our understanding of the origin and evolution of metastasis traits, with emphasis on the connection of metastasis genes to early events of tumor initiation, and speculated on the potential impact on improving therapeutic strategies against metastatic cancer.
Daniel Haber: Dr. Haber used circulating tumor cell (CTC) isolation technologies to study the process of blood borne metastasis. Using a series of microfluidic devices built by his MGH bioengineering collaborator, Dr. Mehmet Toner, Dr. Haber’s lab has focused on breast cancer metastasis, both in a mouse model and in blood specimens from women with breast cancer.
First, they compared the presence of single CTCs versus clustered CTCs in the blood of mammary orthotopic tumor-bearing mice. By color coding tumor cells within a single primary tumor or differentially coding tumor cells in two different primary tumors, they showed that CTC clusters originate from a single tumor site (i.e., they do not result from intravascular aggregation of CTCs), and they are oligoclonal (i.e., they are not the progeny of a single primary tumor cells). By comparing the very low frequency of CTC clusters in the blood with the high frequency of cluster-derived lung metastases, they calculated that CTC-clusters are ~50-fold more metastasis competent than single CTCs in the bloodstream. By analyzing the lung metastases at various time points, they showed that CTC-clusters undergo less early apoptosis, compared with single CTC-derived tumor deposits. Dr. Haber then described an analysis of blood samples from women with metastatic breast cancer. By separately selecting individual CTCs versus CTC clusters from individual blood specimens, and subjecting these to genome-wide RNAseq, they observed that the cell junction gene plakoglobin was ~200 fold more abundant in the CTC clusters. Plakoglobin is involved in tethering cell-cell junctions and may identify an important molecular that keeps CTCs clustered together as they circulate in the bloodstream. Returning to the mouse model, knocking down plakoglobin in human breast cancer xenografts did not affect the growth of the primary orthotopic tumor, bit it abrogated lung metastases. Dr. Haber finished his talk with a discussion of hypothetical ways in which these rare, but apparently lethal, CTC-clusters survive in the blood as they navigate through capillary beds. Reconstitution of capillary-sized channels showed that clustered cancer cells can pass through them under physiological pressures as they realign into single rows, only to regroup as clusters as they emerge on the other side. The discussion focused on the significance of CTC clusters versus single CTCs in cancer metastasis, the role of epithelial mesenchymal transition EMT in single cell migration versus clustered CTC migration, and the various insights that can be derived from RNA sequencing of individual CTCs from the blood of patients with cancer.
Karla Williams: Dr. Williams discussed cellular remodeling of the extracellular matrix (ECM), which facilitates tumor cell invasion. It is a critical step in metastasis and as such is an important area of research to enhance understanding of cancer biology. To move through the ECM, cells utilize integrins to bind to ECM proteins and proteases to degrade the matrix and in invasive tumor cells, these processes can occur through unique small subcellular structures called invadopodia. Trafficking of key proteins involved in alterations to cell-ECM interactions (including integrins and MMPs) is an important element in tumor cell invasion. Understanding the regulation of specific trafficking events is of interest to uncover how these structures are regulated. Research has identified key trafficking events regulated by SNAREs (soluble N-ethylmaleimide-sensitive factor activating protein receptors), which function to localize vesicles to target membranes transporting proteins such as membrane-type 1 matrix metalloproteinase (MT1-MMP) and epidermal growth factor receptor (EGFR). Her current research is investigating the in vivo function of invadopodia and their role in the metastatic cascade and has identified that inhibitors of regulators of invadopodia, such as PAK1, impair tumor cell extravasation. The discussion focused on the importance of proper invadopodium regulation since dysregulation enhancing ECM degradation did not correlate with increased tumor cell invasion or extravasation. While proper regulation of invadopodia was considered an important avenue of research, there was also a consensus that this regulated process was strongly similar to existing cellular processes such as podosome formation in immune cells. The co-opting of such existing cellular mechanisms by tumor cells to mediate metastasis remains an important consideration in ongoing research. The dialogue highlighted the potential of invadopodia research as there is still much to learn about how these structures function in vivo, their role in the metastatic process, and if they can be successfully targeted to impair metastasis.
Christine Iacobuzio: Dr. Iacobuzio discussed the observations of different patterns of failure in patients with pancreatic cancer, oligometastatic and widely metastatic, and the genetic features that underlie each. She then went on to show new data of the role of TGFbeta signaling in these two phenotypes. For example, loss of expression of TGFB1 and TGFBR2 in human tissues was more frequent in oligometastatic pancreatic cancers, and conditional loss of one TGFBR2 allele in the KPC mouse model led to an oligometastatic phenotype. Moreover, she demonstrated that loss of TGFbeta signaling reduced distant metastasis in experimental metastasis models that included a reduction in extravasation from the vasculature in the liver. She also showed data regarding the phylogenetic relationships of coexistent primary and metastatic tumors in patients at autopsy based on whole genome and targeted whole exome sequencing, with the interpretation that metastases are derived from more than subclone in the primary tumor. Finally, she presented data of the mutational spectra and epigenetic alterations that occurs in pancreatic cancers and how these alterations have spatially distinct patterns.
Issues and progress in pediatric brain tumors
Julie Park: Neuroblastoma is a heterogeneous cancer arising from primordial neural crest cells that give rise to sympathetic neural ganglia and adrenal medulla. It has a widely divergent clinical spectrum ranging from spontaneous tumor regression to widely metastatic, aggressive disease (high-risk neuroblastoma). Despite multi-modal, dose intensive therapy; approximately 50% of high-risk neuroblastoma disease will not be cured by dose intensive multi-modal therapy. In contrast to adult carcinomas, there is a striking lack of recurrent neuroblastoma somatic mutations. Alternatively, pre-clinical studies have identified altered molecular pathway signaling for cellular differentiation, metastasis, angiogenesis and inflammation that are associated with aggressive tumor behavior. Multi-gene expression profiles have identified cohorts of tumors with aggressive behavior including a 14-gene classifier that includes genes important in inflammation and immune response. Moreover, recent clinical trials using immunologic therapeutic approaches have demonstrated encouraging anti-tumor activity. Unfortunately, despite the addition of antibody-directed immunotherapy in the clinical setting of non-detectable disease greater than 30% of patients will experience tumor recurrence. Novel immunotherapeutic approaches that better harness the anti-tumor activity of cellular therapy approaches are underway. However, results from both pre-clinical and clinical trials highlight the need for a combination of both immunological approaches with those that target the microenvironment to better treat the most aggressive forms of the neuroblastoma.
Nada Jabado: Dr. Jabado’s group was one of two to first identify a histone mutation in human disease. High-frequency recurrent somatic mutations at specific residues in histone 3 (H3) variants occur in a particularly lethal form of brain tumor, high-grade astrocytomas affecting children and young adults. H3 mutants, or oncohistones as we label them, were later identified by another group in two bone cancers affecting children and young adults and in very rare leukemia samples. Cancers associated with oncohistone mutations thus mainly affect pediatric and young adults where they lead to significant morbidities and/or mortality. These groundbreaking discoveries of oncohistones implicate a direct effect of epigenetic misregulation in oncogenesis, tumor mirco-environment and potential invasion and metastatic spread that we discussed at the meeting. There is limited knowledge on how oncohistones act in tumor formation and current understanding of this new paradigm of cancer is limited, impeding the design of effective therapies. These aspects as well as the cross-talk between tumor and microenvironment which may be regulated by epigenetic alterations that favor implementation and growth of tumor cells at distant sites are the subject of our ongoing investigations.
Rosandra Kaplan: Dr. Kaplan was elected as a Forbeck Scholar and presented her innovative research on the pre-metastatic niche and its role in metastasis. In order to understand the process of metastasis, the Kaplan lab, using both orthotopic mouse models and blood samples from patients with localized and metastatic disease, is able to investigate early systemic changes in response to cancer that promote metastatic progression. Dr. Kaplan’s lab has identified the early events leading to formation of the pre-metastatic niche, which is a metastasis-promoting microenvironment composed of bone marrow-derived cells and stromal cells that enhance disseminated tumor cell survival and proliferation. Her team has identified that the bone marrow, which is the site of definitive hematopoiesis and the essential building blocks of the immune system, is altered early on in tumor progression. Using both mouse models and validation in patients with different types of cancer, her lab has demonstrated that the bone marrow hematopoietic stem cell niche is activated with expansion of hematopoietic stem and progenitor cells that proliferate and mobilize into the circulation. This activation of the bone marrow microenvironment leads to homing of the hematopoietic stem/progenitor cells to distant tissues sites such as the lung where they differentiate into myeloid derived suppressor cells thereby contributing to pre-metastatic niche formation. This hematopoietic stem cell niche expansion can be marked by increased circulating hematopoietic stem/progenitor cells in patients and may serve as a biomarker to predict which patients are at highest risk for metastatic progression. In addition to the hematopoietic component of the pre-metastatic niche, the Kaplan lab has also demonstrated stromal cell activation contributing to the niche environment. As cellular niches are functional microenvironments that govern cellular proliferation, differentiation, and quiescence within tissues, the Kaplan lab, by better understanding the role of niche biology in cancer, may be able to determine how cancers create these niches and thereby develop approaches to target these conducive microenvironments and prevent metastasis.
Mario Shields: Dr. Shields was elected as a Forbeck Scholar and presented his innovative research on the role of the stroma in invasion of pancreatic cancer using live animal imaging. A hallmark of pancreatic cancer is the rich fibrotic stroma, consisting of an extensive deposit of fibrillar type I collagen and infiltration of myeloid cells. To understand the role of the stroma in pancreatic cancer cell invasion and metastasis, Dr. Shields developed a live animal imaging platform to examine the dynamics of the interaction between component of the tumor microenvironment and cancer cells. Using a novel genetically engineered mouse model of pancreatic cancer, where KrasG12D oncogene is expressed with tet-regulated shRNA against PTEN, he showed that cancer cell invasion occurs in early stage tumors. Upon restoration of PTEN expression, tumors regressed with a marked increase in cell death and recruitment of stromal cells. To examine the contribution of myeloid cells or type I collagen to cancer cell invasion, he imaged tumors derived from orthotopically implanted pancreatic cancer cells in transgenic mice expressing fluorescent reporter specific to myeloid cells or GFPtopaz fused to the α2 chain of type I collagen. Dr. Shields showed that cancer cells invaded independently of myeloid cells, but were more migratory in the vicinity of linear collagen. Further, perturbing the cross-linking of type I collagen, by inhibiting the activity or expression of lysyl oxidases, increased collagen thickness and alignment and promoted the spread of cancer cells to the liver. He then presented preliminary data showing the inverse relationship between collagen deposition and TGF-beta transcriptional activity in cancer cells. The data highlighted the regulatory role of stromally derived fibrillar collagen on cancer cells invasion that may be mediated through a cross-talk with TGF-beta signaling. Further studies to understand the mechanism(s) of stroma-regulated cancer cell invasion may provide avenues for limiting metastatic spread.
Tumor cell and host/microenvironmental interactions
Sara Courtneidge: Dr. Courtneidge discussed recent research from her laboratory on membrane protrusions known as invadopodia, which are associated with invasive behavior of cancer cells. By studying the obligate invadopodia scaffold protein Tks5, her laboratory has been able to determine what role these structures play in cancer progression. She reported that expression of high levels of Tks5 mRNA correlates with a worse outcome for breast cancer patients, particularly those with early stage disease. In keeping with this, reduction in Tks5 expression not only reduces the invasiveness of breast cancer cells, but also inhibits their growth in 3-dimensional tissue culture systems, and in xenograft assays. This growth inhibitory phenotype is accompanied by specific changes in gene expression. Her current research seeks to exploit these gene expression changes to define the mechanisms by which invadopodia control growth, to develop a signature that would define which tumors elaborate invadopodia in vivo, and to use genetically engineered models of cancer to study the role of Tks5 in more detail.
Gregg Semenza: Triple negative breast cancers (TNBCs) are defined by the lack of estrogen receptor (ER), progesterone receptor (PR), and HER2 expression, and are treated with cytotoxic chemotherapy such as paclitaxel or gemcitabine, with a durable response rate of less than 20%. TNBCs are enriched for the Basal subtype gene expression profile and the presence of breast cancer stem cells, which are endowed with self-renewing and tumor-initiating properties and resistance to chemotherapy. Dr. Semenza showed that hypoxia-inducible factors (HIFs) and their target gene products are highly active in TNBCs. He demonstrated that HIF expression and transcriptional activity are induced by treatment of MDA-MB-231, SUM-149, and SUM-159 TNBC cells, as well as ER+/PR+ MCF-7 cells, with paclitaxel or gemcitabine. Chemotherapy-induced HIF activity enriched the breast cancer stem cell population through interleukin-6 and interleukin-8 signaling and increased expression of multidrug resistance. Co-administration of HIF inhibitors overcame the resistance of breast cancer stem cells to paclitaxel or gemcitabine, both in vitro and in vivo, leading to tumor eradication. Increased expression of HIF-1α or HIF target genes in breast cancer biopsies was associated with decreased overall survival, particularly in patients with Basal subtype tumors and those treated with chemotherapy alone. Based on these results, clinical trials are warranted to test whether treatment of TNBC patients with a combination of cytotoxic chemotherapy and HIF inhibitors will improve patient survival.
Erik Sahai: Dr. Sakai’s presentation focused on insights from imaging invasion and metastasis. The problem of different modes of cell migration and invasion was introduced. Depending on how cells move they have different responses to potential ‘anti-invasive’ drugs. This problem is further exacerbated by the ability of cancer cells to switch between different modes of migration. To understand these complex issues the Sahai group has been collaborating with modelers to develop computational models of cell migration. These models can then be used to explore the plasticity of cancer migration strategy and likely response to anti-invasive drugs. Erik then discussed how tumor invasion is influenced by stromal cells, in particular stromal fibroblasts. These cells play a key role in remodeling the extra-cellular matrix in tumors and thereby guiding patterns of migration. The possibility of targeting them therapeutically was discussed. Further, their role in modulating the response of melanoma cells to targeted therapy was presented. This work further emphasizes how it is crucial to consider the effects of kinase targeted therapies the tumor stroma, and not just the cancer cells. Given the importance of the tumor stroma in determining the response to therapies; Erik then proposed that varying stromal environments at metastatic locations might explain the differential responses of metastases.
Klaus Pantel: Improved early detection and adjuvant therapy have facilitated progress in diagnosis and therapy for patients with solid tumors; however, the prognosis of cancer patients is still limited by the occurrence of distant metastases. In patients with completely resectable primary tumors this relapse is largely due to clinically occult micrometastasis present in secondary organs at primary diagnosis but not detectable even with high resolution imaging procedures. Dr. Pantel has found that sensitive and specific immunocytochemical and molecular assays enable now the detection and characterization of disseminated tumor cells (DTC) at the single cell level in bone marrow (BM) as a common homing site of carcinoma-derived DTC. Because of the high variability of results in DTC detection, there is an urgent need for standardized methods. While the prognostic impact of DTC in BM has clearly been shown for primary breast cancer patients, less is known about the clinical relevance of DTC in patients with other carcinomas. Current findings suggest that DTC are capable to survive chemotherapy and persist in a dormant non-proliferating state over many years. To what extent these DTC have stem cell properties is subject of ongoing investigations. Further characterization is required to understand the biology of DTC and to identify new target structures for improved risk prevention and tailoring of therapy. Since BM sampling is invasive, detection of circulating tumor cells (CTCs) in the peripheral blood of cancer patients has received great attention. CTCs are usually detected by immunostaining or RT-PCR assays, and more recently by the EPISPOT assay which measures the number of cells releasing/secreting tumor-associated marker proteins. Interestingly, detection of cell-free nucleic acids released by tumor cells such as tumor-associated DNA or microRNAs into the blood might become an indirect way to detect micrometastatic disease. At present, most CTC assays rely on epithelial markers and miss CTCs undergoing an epithelial-mesenchymal transition (EMT). New markers such as the actin bundling protein plastin-3 are not downregulated during EMT and not expressed in normal blood cells might overcome this important limitation and, therefore, increase the sensitivity of CTC assays. Recently, in vivo capture of CTCs with an antibody-coated wire placed into the peripheral arm vein has become feasible and allows now the capture for CTCs from approx. 1.5 liters of blood within 30 minutes. CTC enumeration and characterization with certified systems provides reliable information on prognosis and may serve as liquid biopsy. Moreover, monitoring of CTCs before, during and after systemic therapy (e.g., chemotherapy, hormonal therapy, antibody therapy) might provide unique information for the future clinical management of the individual cancer patient and might serve as surrogate marker for response to therapy. Besides CTCs the analysis of ctDNA and circulating microRNAs may provide complementary information as “liquid biopsy”. This information can be used as companion diagnostics to improve the stratification of therapies and to obtain insights into therapy-induced selection of cancer cells.
Therapeutic strategies to combat metastasis – prevent, delay or treat?
Ann Chambers: Dr. Chambers discussed issues that need to be addressed to better understand the biology underlying tumor dormancy, information that will be needed if tumor dormancy is to be a target for therapeutic intervention and if late recurrences are to be prevented. Three questions were discussed. First, are dormant tumor cells a therapeutic target in cancer? The answer to this is a tentative “Yes”, based on the results from the MA.14 clinical trial, which showed benefit for very long-term hormone therapy in women with hormone responsive breast cancer, as well as numerous experimental studies. For breast and prostate cancer, and likely other tumor types as well, these diseases appear to be chronic, relapsing diseases. Even in cancers diagnosed early, recurrences can happen, sometimes years or decades after apparently successful primary treatment. However, we cannot predict well which individual patients will recur and which will not, nor do we know whether this information lies within the primary tumor or is affected by post-treatment lifestyle or other influences. Second, do we know how to target dormant tumor cells therapeutically? The answer to this question is a clear “No” at our current stage of understanding of the biology of dormancy. While many groups worldwide are studying molecular aspects of dormancy and recurrence, translating this limited molecular information to individual patients is not yet feasible. Third, If and when we do develop answers to this question, would we know which patients to treat? Again, the answer to this question is, at present “No”. Our problem is that we simply do not know the extent of micrometastatic burden in patients for whom there is no current evidence of disease. We know that some of these patients must indeed do harbor dormant cancer, and based on comparisons with other patients with cancer of similar type and stage, we can predict what percentage of these patients are likely to recur. But on an individual basis, we are not now able to accurately predict which patients will vs. will not have recurrences. Currently we are not able to tell with certainty which patients are cured of their cancer and which patients harbor undetected “minimal residual disease”. The possibility of dormant cancer cells persisting introduces years of uncertainty for patients and their physicians, since we do not know if an individual patient is cured of their tumor, or in a state of tumor dormancy with the possibility of recurrence of the tumor. We need improved methods to assess for occult, micrometastatic burden in patients who are at risk for later recurrences. This information could come from autopsy studies on people who had cancer, to determine the burden of undetected metastatic disease, as well as of circulating biomarkers and novel imaging methods to detect microscopic disease, in patients at risk for recurrence. Only when we understand which patients harbor undetected, dormant disease will we be in position to consider individualized treatment to prevent recurrence of dormant disease or attack and destroy dormant cells. Until then, we will continue to over-treat some patients in a group at risk for recurrence, only some of whom would otherwise develop recurrence, as well as undertreat some patients in groups of good prognosis who nonetheless develop recurrence. We have a need to better understand the biology and prevalence of micrometastatic cancer, in order to improve personalize therapy to prevent recurrences.
Patricia S. Steeg: The translation of metastasis experiments to the clinic remains problematic. Dr. Steeg discussed the fact that basically, standard phase I-III trials in the metastatic setting quantify the shrinkage of metastatic lesions, not the prevention of their occurrence. Two types of new trial designs were discussed, primary metastasis prevention and secondary metastasis prevention. Primary prevention may enroll patients at very high risk of metastases, for instance those who underwent neoadjuvant therapy and did not obtain a pathological complete response, those with multiple positive lymph nodes, those with chest wall recurrences. Secondary metastasis prevention trials could enroll patients with limited, treated metastatic disease at high risk of relapse; the endpoint would be time until the development of a new metastasis.
Louise van der Weyden: Dr. van der Weyden was elected as a Forbeck Scholar and used Dr. Stephen Paget's “seed and soil” hypothesis of metastasis as the foundation of her studies on understanding melanoma metastasis. The incidence of melanoma is less than 2% of all skin cancers, yet it is responsible for 75% of skin cancer-related deaths - this is due to its inherent ability to metastasize early on. To understand the nature of the “seed”, whole-genome DNA and RNA sequencing of mouse melanoma cell lines with differing metastatic capacities is being performed, to look for genes whose loss or gain of expression, or mutation, correlates with enhanced metastatic ability. Similarly, series of canine oral melanomas (normal tissue, primary melanoma and metastatic lesion from the same dog) are being exome sequenced. To ensure relevance of the findings, cross-species comparison will be used to identify which differentially expressed or mutated genes in the animal model datasets are also found in human datasets, and more importantly, that correlate with survival. To understand the “soil” Dr. van der Weyden is using a mouse melanoma cell line to perform an ‘experimental metastasis assay’ on mutant mouse lines coming through the Mouse Genetics Program at the Wellcome Trust Sanger Institute. This provides a unique opportunity to interrogate the genome in an unbiased manner and identify host genes that are able to regulate the ability of melanoma cells to successfully metastasize to the lung. In the 500 mutant lines screened to date, there have been those that show significantly increased levels (such as Irf1-/- mice) or decreased levels (such as Cd39-/- and Hsp90aa1-/- mice) of pulmonary metastasis relative to controls, and the key is now to investigate of the mechanism of actions of these genes. Understanding genes that are altered in metastasis or host genes that can regulate metastasis will hopefully pave the way for identifying potential new drug targets.
Chad Pecot: Dr. Pecot was elected as a Forbeck Scholar and presented his work studying lymphatic metastasis in lung cancer. On average, lung cancer patients die within a year of presentation due to development of distant metastasis. Even if caught in the early stages, patients often die following surgical resection due to rapid recurrence, suggesting primary tumors are very ‘fit’ for dissemination. Intriguingly, patients with micro-metastases in surgically resected lymph nodes are known to have significantly higher chance of distant relapse. This suggests that the lymphatic route of spread may not be a ‘dead end’, and in fact may represent a parallel process to hematogenous metastases with unique mechanisms. To address this question, Dr. Pecot presented data from lung cancer sub-clones he’s developed using in vivo selective pressure which have markedly enhanced capacity to metastasize to lymph nodes. With this approach, Dr. Pecot’s team found these sub-clones have dramatic changes in their miRNA profiles. Future directions will be to characterize the molecular pathways regulated by select miRNAs that may have important roles in lymphatic metastases.
The Forbeck Symposium offered a unique environment to discuss the latest issues in metastasis and new approached that may advance our knowledge and give rise to new therapies in the clinic. The discussions were wide ranging and exiting. Importantly, several new collaborations arose immediately as a result of these interactions.
The conclusions at the end of the meeting were:
- We are beginning to define new mechanisms that contribute to tumor invasion and metastasis. We are recognizing that cancer is a heterogeneous and dynamic disease that can evolve in patients, and new therapies need to address this concept.
- Progress has been made in advancing our understanding of metastatic disease but much more is needed. We appreciate the willingness of patients to participate in clinical trials and tissue and clinical data banking as a partnership to further our understanding and lead to improved treatments.
- There is slow progress in developing new approaches to prevent, treat or delay metastatic disease. However, we are learning that cancer can affect the whole body, and that the body in some cases can limit metastatic spread. We need to learn better how this occurs, to be able to harness this ability.
- There is a need for comprehensive basic research to learn more about metastasis and there is a great need to identify strategies to improve response to targeted agents. The first of the new ideas are beginning to be translated to the clinic to benefit patients, but it is early days.