Suppression of malignancy by cell fusion

When DNA Mutations Interplay with Cellular Proliferation: A Narrative History of Theories of Carcinogenesis

While cancer is one of the most documented diseases, how normal cells become cancerous is still debated. To address this question, in the first part of this review, we investigated the long succession of theories of carcinogenesis since antiquity. Initiated by Hippocrates, Aristotle, and Galen, the humoral theory interpreted cancer as an excess of acid, the black bile. The discovery of the circulation of blood by Harvey in 1628 destroyed the basis of the humoral theory but revived the spontaneous generation hypothesis which was also promoted by Aristotle. In 1859, the theory of microbes promoted by Pasteur demonstrated the irrelevance of this last theory and contributed to the emergence of the germ cancer theory, opposed to the cellular theory of cancer, in which cancer was supposed to be caused by microbes or transformed cells, respectively. These theories were progressively refined by the notions of initiation, promotion, and progression thanks to advances in mutagenesis and cellular proliferation. In the second part of this review, recent discoveries and paradigms in carcinogenesis, notably the role of the protein ATM, a major actor of the stress response involved in both mutagenesis and cellular proliferation, were discussed to better understand the current state of the art of carcinogenesis.

Haploinsufficient Transcription Factors in Myeloid Neoplasms

Many transcription factors (TFs) function as tumor suppressor genes with heterozygous phenotypes, yet haploinsufficiency generally has an underappreciated role in neoplasia. This is no less true in myeloid cells, which are normally regulated by a delicately balanced and interconnected transcriptional network. Detailed understanding of TF dose in this circuitry sheds light on the leukemic transcriptome. In this review, we discuss the emerging features of haploinsufficient transcription factors (HITFs). We posit that: (a) monoallelic and biallelic losses can have distinct cellular outcomes; (b) the activity of a TF exists in a greater range than the traditional Mendelian genetic doses; and (c) how a TF is deleted or mutated impacts the cellular phenotype. The net effect of a HITF is a myeloid differentiation block and increased intercellular heterogeneity in the course of myeloid neoplasia.

Therapeutic Implications of PTEN in Non-Small Cell Lung Cancer

Lung cancer remains one of the major human malignancies affecting both men and women worldwide, with non-small cell lung cancer (NSCLC) being the most prevalent type. Multiple mechanisms have been identified that favor tumor growth as well as impede the efficacy of therapeutic regimens in lung cancer patients. Among tumor suppressor genes that play critical roles in regulating cancer growth, the phosphatase and tensin homolog (PTEN) constitutes one of the important family members implicated in controlling various functional activities of tumor cells, including cell proliferation, apoptosis, angiogenesis, and metastasis. Notably, clinical studies have also documented that lung tumors having an impaired, mutated, or loss of PTEN are associated with low survival or high tumor recurrence rates. To that end, PTEN has been explored as a promising target for anti-cancer agents. Importantly, the ability of PTEN to crosstalk with several signaling pathways provides new approaches to devise effective treatment options for lung cancer treatment. The current review highlights the significance of PTEN and its implications in therapeutic approaches against NSCLC.

AF9 sustains glycolysis in colorectal cancer via H3K9ac-mediated PCK2 and FBP1 transcription

BACKGROUND

The tumourigenesis of various cancers is influenced by epigenetic deregulation. Among 591 epigenetic regulator factors (ERFs) examined, AF9 showed significant inhibition of malignancy in colorectal cancer (CRC) based on our wound healing assays. However, the precise role of AF9 in CRC remains to be explored.

METHODS

To investigate the function of AF9 in CRC, we utilised small interfering RNAs (siRNAs) to knock down the expression of 591 ERFs. Subsequently, we performed wound healing assays to evaluate cell proliferation and migration. In vitro and in vivo assays were conducted to elucidate the potential impact of AF9 in CRC. Clinical samples were analysed to assess the association between AF9 expression and CRC prognosis. Additionally, an Azoxymethane-Dextran Sodium Sulfate (AOM/DSS) induced CRC AF9IEC-/- mouse model was employed to confirm the role of AF9 in CRC. To identify the target gene of AF9, RNA-seq and coimmunoprecipitation analyses were performed. Furthermore, bioinformatics prediction was applied to identify potential miRNAs that target AF9.

RESULTS

Among the 591 ERFs examined, AF9 exhibited downregulation in CRC and showed a positive correlation with prolonged survival in CRC patients. In vitro and in vivo assays proved that depletion of AF9 could promote cell proliferation, migration as well as glycolysis. Specifically, knockout of MLLT3 (AF9) in intestinal epithelial cells significantly increased tumour formation induced by AOM/DSS. We also identified miR-145 could target 3'untranslated region of AF9 to suppress AF9 expression. Loss of AF9 led to decreased expression of gluconeogenic genes, including phosphoenolpyruvate carboxykinase 2 (PCK2) and fructose 1,6-bisphosphatase 1 (FBP1), subsequently promoting glucose consumption and tumourigenesis.

CONCLUSIONS

AF9 is essential for the upregulation of PCK2 and FBP1, and the disruption of the miR-145/AF9 axis may serve as a potential target for the development of CRC therapeutics.

Restoring tumor immunogenicity with dendritic cell reprogramming

Decreased antigen presentation contributes to the ability of cancer cells to evade the immune system. We used the minimal gene regulatory network of type 1 conventional dendritic cells (cDC1) to reprogram cancer cells into professional antigen-presenting cells (tumor-APCs). Enforced expression of the transcription factors PU.1, IRF8, and BATF3 (PIB) was sufficient to induce the cDC1 phenotype in 36 cell lines derived from human and mouse hematological and solid tumors. Within 9 days of reprogramming, tumor-APCs acquired transcriptional and epigenetic programs associated with cDC1 cells. Reprogramming restored the expression of antigen presentation complexes and costimulatory molecules on the surfaces of tumor cells, allowing the presentation of endogenous tumor antigens on MHC-I and facilitating targeted killing by CD8⁺ T cells. Functionally, tumor-APCs engulfed and processed proteins and dead cells, secreted inflammatory cytokines, and cross-presented antigens to naïve CD8⁺ T cells. Human primary tumor cells could also be reprogrammed to increase their capability to present antigen and to activate patient-specific tumor-infiltrating lymphocytes. In addition to acquiring improved antigen presentation, tumor-APCs had impaired tumorigenicity in vitro and in vivo. Injection of in vitro generated melanoma-derived tumor-APCs into subcutaneous melanoma tumors delayed tumor growth and increased survival in mice. Antitumor immunity elicited by tumor-APCs was synergistic with immune checkpoint inhibitors. Our approach serves as a platform for the development of immunotherapies that endow cancer cells with the capability to process and present endogenous tumor antigens.

Cellular Senescence, a Novel Area of Investigation for Metastatic Diseases

Metastasis is a systemic condition and the major challenge among cancer types, as it can lead to multiorgan vulnerability. Recently, attention has been drawn to cellular senescence, a complex stress response condition, as a factor implicated in metastatic dissemination and outgrowth. Here, we examine the current knowledge of the features required for cells to invade and colonize secondary organs and how senescent cells can contribute to this process. First, we describe the role of senescence in placentation, itself an invasive process which has been linked to higher rates of invasive cancers. Second, we describe how senescent cells can contribute to metastatic dissemination and colonization. Third, we discuss several metabolic adaptations by which senescent cells could promote cancer survival along the metastatic journey. In conclusion, we posit that targeting cellular senescence may have a potential therapeutic efficacy to limit metastasis formation.

Hybrid/Atypical Forms of Circulating Tumor Cells: Current State of the Art

Cancer is one of the most common diseases worldwide, and its treatment is associated with many challenges such as drug and radioresistance and formation of metastases. These difficulties are due to tumor heterogeneity, which has many causes. One may be the cell fusion, a process that is relevant to both physiological (e.g., wound healing) and pathophysiological (cancer and viral infection) processes. This literature review aimed to summarize the existing data on the hybrid/atypical forms of circulating cancer cells and their role in tumor progression. For that, the bioinformatics search in universal databases, such as PubMed, NCBI, and Google Scholar was conducted by using the keywords “hybrid cancer cells”, “cancer cell fusion”, etc. In this review the latest information related to the hybrid tumor cells, theories of their genesis, characteristics of different variants with data from our own researches are presented. Many aspects of the hybrid cell research are still in their infancy. However, with the level of knowledge already accumulated, circulating hybrids such as CAML and CHC could be considered as promising biomarkers of cancerous tumors, and even more as a new approach to cancer treatment.

Cell Fusion-Related Proteins and Signaling Pathways, and Their Roles in the Development and Progression of Cancer

Cell fusion is involved in many physiological and pathological processes, including gamete binding, and cancer development. The basic processes of cell fusion include membrane fusion, cytoplasmic mixing, and nuclear fusion. Cell fusion is regulated by different proteins and signaling pathways. Syncytin-1, syncytin-2, glial cell missing 1, galectin-1 and other proteins (annexins, myomaker, myomerger etc.) involved in cell fusion via the cyclic adenosine-dependent protein kinase A, mitogen-activated protein kinase, wingless/integrase-1, and c-Jun N-terminal kinase signaling pathways. In the progression of malignant tumors, cell fusion is essential during the organ-specific metastasis, epithelial-mesenchymal transformation, the formation of cancer stem cells (CSCs), cancer angiogenesis and cancer immunity. In addition, diploid cells can be induced to form polyploid giant cancer cells (PGCCs) via cell fusion under many kinds of stimuli, including cobalt chloride, chemotherapy, radiotherapy, and traditional Chinese medicine. PGCCs have CSC-like properties, and the daughter cells derived from PGCCs have a mesenchymal phenotype and exhibit strong migration, invasion, and proliferation abilities. Therefore, exploring the molecular mechanisms of cell fusion can enable us better understand the development of malignant tumors. In this review, the basic process of cell fusion and its significance in cancer is discussed.

The two-hit theory hits 50

Few ideas in cancer genetics have been as influential as the “two-hit” theory of tumor suppressors. This idea was introduced in 1971 by Al Knudson in a paper in the Proceedings of the National Academy of Science and forms the basis for our current understanding of the role of mutations in cancer. In this theoretical discussion proposing a genetic basis for retinoblastoma, a childhood cancer of the retina, Knudson posited that these tumors arise from two inactivating mutations, targeting both alleles of a putative tumor suppressor gene. While this work built on earlier proposals that cancers are the result of mutations in more than one gene, it was the first to propose a plausible mechanism by which single genes that are affected by germ-line mutations in heritable cancers could also cause spontaneous, nonheritable tumors when mutated in somatic tissues. Remarkably, Knudson described the existence and properties of a retinoblastoma tumor suppressor gene a full 15 years before the gene was cloned.

Cell Fate Reprogramming in the Era of Cancer Immunotherapy

Advances in understanding how cancer cells interact with the immune system allowed the development of immunotherapeutic strategies, harnessing patients' immune system to fight cancer. Dendritic cell-based vaccines are being explored to reactivate anti-tumor adaptive immunity. Immune checkpoint inhibitors and chimeric antigen receptor T-cells (CAR T) were however the main approaches that catapulted the therapeutic success of immunotherapy. Despite their success across a broad range of human cancers, many challenges remain for basic understanding and clinical progress as only a minority of patients benefit from immunotherapy. In addition, cellular immunotherapies face important limitations imposed by the availability and quality of immune cells isolated from donors. Cell fate reprogramming is offering interesting alternatives to meet these challenges. Induced pluripotent stem cell (iPSC) technology not only enables studying immune cell specification but also serves as a platform for the differentiation of a myriad of clinically useful immune cells including T-cells, NK cells, or monocytes at scale. Moreover, the utilization of iPSCs allows introduction of genetic modifications and generation of T/NK cells with enhanced anti-tumor properties. Immune cells, such as macrophages and dendritic cells, can also be generated by direct cellular reprogramming employing lineage-specific master regulators bypassing the pluripotent stage. Thus, the cellular reprogramming toolbox is now providing the means to address the potential of patient-tailored immune cell types for cancer immunotherapy. In parallel, development of viral vectors for gene delivery has opened the door for in vivo reprogramming in regenerative medicine, an elegant strategy circumventing the current limitations of in vitro cell manipulation. An analogous paradigm has been recently developed in cancer immunotherapy by the generation of CAR T-cells in vivo. These new ideas on endogenous reprogramming, cross-fertilized from the fields of regenerative medicine and gene therapy, are opening exciting avenues for direct modulation of immune or tumor cells in situ, widening our strategies to remove cancer immunotherapy roadblocks. Here, we review current strategies for cancer immunotherapy, summarize technologies for generation of immune cells by cell fate reprogramming as well as highlight the future potential of inducing these unique cell identities in vivo, providing new and exciting tools for the fast-paced field of cancer immunotherapy.

Interspecies cell fusion between mouse embryonic stem cell and porcine pluripotent cell

In the area of stem cell research, fusion of somatic cells into pluripotent cells such as mouse embryonic stem (ES) cells induces reprogramming of the somatic nucleus and can be used to study the effect of trans-acting factors from the pluripotent cell on the pluripotent state of somatic nucleus. As many other groups, we previously established a porcine pluripotent cell line at a low potential. Therefore, here, we performed experiments to investigate if the fusion with mouse ES cell could improve the pluripotent state of porcine pluripotent cell. Our data showed that resultant mouse-porcine interspecies fused cells are AP positive, and could be passaged up to 20 passages. Different degrees of increases in expression of porcine pluripotent genes proved that pig-origin gene network can be programmed by mouse ES. Further differentiation study also confirmed these fused cells' potential to form three germ layers. However, unexpectedly, we found that chromosome loss and aberrant (especially in porcine chromosomes) is severe after the cell fusion, implying that interspecies cell fusion may be not suitable to study porcine pluripotency without additional supportive conditions for genome stabilization.

Tumor Suppressors Having Oncogenic Functions: The Double Agents

Cancer progression involves multiple genetic and epigenetic events, which involve gain-of-functions of oncogenes and loss-of-functions of tumor suppressor genes. Classical tumor suppressor genes are recessive in nature, anti-proliferative, and frequently found inactivated or mutated in cancers. However, extensive research over the last few years have elucidated that certain tumor suppressor genes do not conform to these standard definitions and might act as “double agents”, playing contrasting roles in vivo in cells, where either due to haploinsufficiency, epigenetic hypermethylation, or due to involvement with multiple genetic and oncogenic events, they play an enhanced proliferative role and facilitate the pathogenesis of cancer. This review discusses and highlights some of these exceptions; the genetic events, cellular contexts, and mechanisms by which four important tumor suppressors—pRb, PTEN, FOXO, and PML display their oncogenic potentials and pro-survival traits in cancer.

Discovery of the PTEN Tumor Suppressor and Its Connection to the PI3K and AKT Oncogenes

PTEN (phosphatase and tensin homolog on chromosome 10) was discovered over 20 years ago in 1997 and linked to the phosphatidylinositol 3-kinase (PI3K) and AKT oncogenes the following year. The discovery of PTEN emerged from the linked concepts of oncogenes and tumor suppressor genes that cause and prevent cancer and the fields of tumor viruses and human cancer genetics from which these two concepts arose. While much has been learned since, the initial discovery and characterization, including the discovery that PTEN is a regulator of PI3K and AKT, provide the foundation on which we continue to build our knowledge. To provide the context in which these cancer genes were discovered, background information that led to their discovery will also be discussed, which will hopefully be a useful guide for readers seeking to build on the work of others.

The Case of the Scribble Polarity Module in Asymmetric Neuroblast Division in Development and Tumorigenesis

The Scribble polarity module is composed by Scribble (Scrib), Discs large 1 (Dlg1) and Lethal (2) giant larvae (L(2)gl), a group of highly conserved neoplastic tumor suppressor genes (TSGs) from flies to humans. Even though the Scribble module has been profusely studied in epithelial cell polarity, the number of tissues and processes in which it is involved is increasingly growing. Here we discuss the role of the Scribble module in the asymmetric division of Drosophila neuroblasts (NBs), as well as the underlying mechanisms by which those TSGs act in this process. Finally, we also describe what we know about the consequences of mutating these genes in impairing the process of asymmetric NB division and promoting tumor-like overgrowth.

Understanding and Modulating Immunity With Cell Reprogramming

Cell reprogramming concepts have been classically developed in the fields of developmental and stem cell biology and are currently being explored for regenerative medicine, given its potential to generate desired cell types for replacement therapy. Cell fate can be experimentally reversed or modified by enforced expression of lineage specific transcription factors leading to pluripotency or attainment of another somatic cell type identity. The possibility to reprogram fibroblasts into induced dendritic cells (DC) competent for antigen presentation creates a paradigm shift for understanding and modulating the immune system with direct cell reprogramming. PU.1, IRF8, and BATF3 were identified as sufficient and necessary to impose DC fate in unrelated cell types, taking advantage of Clec9a, a C-type lectin receptor with restricted expression in conventional DC type 1. The identification of such minimal gene regulatory networks helps to elucidate the molecular mechanisms governing development and lineage heterogeneity along the hematopoietic hierarchy. Furthermore, the generation of patient-tailored reprogrammed immune cells provides new and exciting tools for the expanding field of cancer immunotherapy. Here, we summarize cell reprogramming concepts and experimental approaches, review current knowledge at the intersection of cell reprogramming with hematopoiesis, and propose how cell fate engineering can be merged to immunology, opening new opportunities to understand the immune system in health and disease.

Induction of pluripotency in mammalian fibroblasts by cell fusion with mouse embryonic stem cells

BACKGROUND

Cell fusion is a phenomenon that is observed in various tissues in vivo, resulting in acquisition of physiological functions such as liver regeneration. Fused cells such as hybridomas have also been produced artificially in vitro. Furthermore, it has been reported that cellular reprogramming can be induced by cell fusion with stem cells.

METHODS

Fused cells between mammalian fibroblasts and mouse embryonic stem cells were produced by electrofusion methods. The phenotypes of each cell lines were analyzed after purifying the fused cells.

RESULTS

Colonies which are morphologically similar to mouse embryonic stem cells were observed in fused cells of rabbit, bovine, and zebra fibroblasts. RT-PCR analysis revealed that specific pluripotent marker genes that were never expressed in each mammalian fibroblast were strongly induced in the fused cells, which indicated that fusion with mouse embryonic stem cells can trigger reprogramming and acquisition of pluripotency in various mammalian somatic cells.

CONCLUSIONS

Our results can help elucidate the mechanism of pluripotency maintenance and the establishment of highly reprogrammed pluripotent stem cells in various mammalian species.

Loss of p53 function at late stages of tumorigenesis confers ARF-dependent vulnerability to p53 reactivation therapy

Mouse studies demonstrating regression of p53-null tumors following reinstatement of functional p53 have fueled the development of p53 reactivating drugs. However, successful p53 reactivation responses have only been formally demonstrated in tumor models where p53 inactivation served as the initiating event. Our study provides the first proof-of-principle evidence that p53 inactivation at late stages of tumorigenesis can also generate a vulnerability to p53 reactivation. However, this is dependent on intact ARF function highlighting ARF as a potential biomarker for p53 reactivation responses in tumors with late-stage p53 inactivation. It furthermore suggests the use of Mdm2 inhibitors as ARF mimetics for sensitizing ARF-deficient tumors to p53-reactivating drugs.

Cancer development is driven by activated oncogenes and loss of tumor suppressors. While oncogene inhibitors have entered routine clinical practice, tumor suppressor reactivation therapy remains to be established. For the most frequently inactivated tumor suppressor p53, genetic mouse models have demonstrated regression of p53-null tumors upon p53 reactivation. While this was shown in tumor models driven by p53 loss as the initiating lesion, many human tumors initially develop in the presence of wild-type p53, acquire aberrations in the p53 pathway to bypass p53-mediated tumor suppression, and inactivate p53 itself only at later stages during metastatic progression or therapy. To explore the efficacy of p53 reactivation in this scenario, we used a reversibly switchable p53 (p53ER^TAM) mouse allele to generate Eµ-Myc–driven lymphomas in the presence of active p53 and, after full lymphoma establishment, switched off p53 to model late-stage p53 inactivation. Although these lymphomas had evolved in the presence of active p53, later loss and subsequent p53 reactivation surprisingly activated p53 target genes triggering massive apoptosis, tumor regression, and long-term cure of the majority of animals. Mechanistically, the reactivation response was dependent on Cdkn2a/p19Arf, which is commonly silenced in p53 wild-type lymphomas, but became reexpressed upon late-stage p53 inactivation. Likewise, human p53 wild-type tumor cells with CRISPR-engineered switchable p53ER^TAM alleles responded to p53 reactivation when CDKN2A/p14ARF function was restored or mimicked with Mdm2 inhibitors. Together, these experiments provide genetic proof of concept that tumors can respond, in an ARF-dependent manner, to p53 reactivation even if p53 inactivation has occurred late during tumor evolution.

Cellular reprogramming by single-cell fusion with mouse embryonic stem cells in pig

Fusion of differentiated somatic cells with pluripotent stem cells can be used for cellular reprogramming, but the efficiency to obtain hybrid cells is extremely low. Here, we explored a novel cell fusion system, termed single-cell fusion, the efficiency was significantly improved verified by fusion of mouse embryonic stem cells (mESCs), comparing to traditional polyethylene glycol fusion. Then, we employed the optimized system to perform cell fusion of porcine embryonic fibroblasts (PEFs) and porcine pluripotent stem cells (pPSCs) with mESCs. The hybrid cells showed both red and green fluorescence and expressed species-specific genes of mouse and pig to evidence that the fusion was successful. The hybrid cells displayed characteristics similar with mESCs, including colony morphology, alkaline phosphatase positive and formation of embryoid body, and the expressions of core pluripotent factors OCT4, NANOG, and SOX2 of the pig were induced in the mESC/PEF hybrid cells. The results indicate PEFs and pPSCs could be reprogrammed by mESCs via the single-cell fusion. Taking advantage of the hybrid cells to investigate the signaling pathways depended on the pluripotency of pig, we suggest the transforming growth factor-β signaling pathways may play important roles. In summary, the single-cell fusion is highly efficient, and we believe in the future it will be widely used in the application and fundamental research.

Cell Fusion in Malignancy: A Cause or Consequence? A Provocateur or Cure?

Cell fusion has been observed in malignancy, and cancer cells have been found especially apt to fuse with other cells. Investigation of human and experimental malignancies suggests spontaneous fusion of normal cells can induce manifold genetic changes and manifestations of malignant transformation. Fusion of transformed cells with other cells can promote the progression of cancer to more malignant forms. However, observations in various fields suggest cell fusion also potentially contributes to natural defenses against cancer. Thus, cell fusion potentially corrects genetic and/or phenotypic changes underlying malignant transformation. Cell fusion also might help nonmalignant cells in tumors thwart tumor growth. Perhaps most importantly, cell fusion may generate genetic changes that lead to the expression of neoantigens, provide the mass of neoantigen expression needed to elicit immunity, and promote the function of antigen-presenting cells in a way that favors protective immunity as a defense against malignancy. To the extent that cell fusion promotes cellular, tissue, and/or systemic resistance to malignancy, the propensity of tumor cells to fuse with other cells might constitute a natural defense against malignancy.

Drosophila Model in Cancer: An Introduction

Cancer is a cumulative manifestation of several complicated disease states that affect multiple organs. Over the last few decades, the fruit fly Drosophila melanogaster, has become a successful model for studying human cancers. The genetic simplicity and vast arsenal of genetic tools available in Drosophila provides a unique opportunity to address questions regarding cancer initiation and progression that would be extremely challenging in other model systems. In this chapter we provide a historical overview of Drosophila as a model organism for cancer research, summarize the multitude of genetic tools available, offer a brief comparison between different model organisms and cell culture platforms used in cancer studies and briefly discuss some of the latest models and concepts in recent Drosophila cancer research.

Common signalling pathways in macrophage and osteoclast multinucleation

Macrophage cell fusion and multinucleation are fundamental processes in the formation of multinucleated giant cells (MGCs) in chronic inflammatory disease and osteoclasts in the regulation of bone mass. However, this basic cell phenomenon is poorly understood despite its pathophysiological relevance. Granulomas containing multinucleated giant cells are seen in a wide variety of complex inflammatory disorders, as well as in infectious diseases. Dysregulation of osteoclastic bone resorption underlies the pathogenesis of osteoporosis and malignant osteolytic bone disease. Recent reports have shown that the formation of multinucleated giant cells and osteoclast fusion display a common molecular signature, suggesting shared genetic determinants. In this Review, we describe the background of cell-cell fusion and the similar origin of macrophages and osteoclasts. We specifically focus on the common pathways involved in osteoclast and MGC fusion. We also highlight potential approaches that could help to unravel the core mechanisms underlying bone and granulomatous disorders in humans.

How mutations shape p53 interactions with the genome to promote tumorigenesis and drug resistance

The tumor suppressive transcription factor p53 regulates a wide array of cellular processes that confer upon cells an essential protection against cancer development. Wild-type p53 regulates gene expression by directly binding to DNA in a sequence-specific manner. p53 missense mutations are the most common mutations in malignant cells and can be regarded as synonymous with anticancer drug resistance and poor prognosis. The current review provides an overview of how the extraordinary variety of more than 2000 different mutant p53 proteins, known as the p53 mutome, affect the interaction of p53 with DNA. We discuss how the classification of p53 mutations to loss of function (LOF), gain of function (GOF), and dominant-negative (DN) inhibition of a remaining wild-type allele, hides a complex p53 mutation spectrum that depends on the distinctive nature of each mutant protein, requiring different therapeutic strategies for each mutant p53 protein. We propose to regard the different mutant p53 categories as continuous variables, that may not be independent of each other. In particular, we suggest here to consider GOF mutations as a special subset of LOF mutations, especially when mutant p53 binds to DNA through cooperation with other transcription factors, and we present a model for GOF mechanism that consolidates many observations on the GOF phenomenon. We review how novel mutant p53 targeting approaches aim to restore a wild-type-like DNA interaction and to overcome resistance to cancer therapy.

Onco-Suppressor Genes in Human Cancer

The analysis of the molecular mechanisms governing multistep carcinogenesis became experimentally approachable since the identification and characterization in tumor cells of altered or activated versions of cellular genes (oncogenes) that normally control cell growth and differentiation. The activating mutations confer new properties to the oncogene products and should therefore be considered as gain of function mutations. In addition, the oncogenes appear to act as dominant genetic traits since they act also in the presence of the homologous wild-type allele. However, the concept of a dominance of the transformed phenotype has been challenged by early experiments with somatic cell hybrids which showed that the fusion of normal and malignant cells may suppress the tumorigenic phenotype. The suppression or reversion of the malignant phenotype by the introduction of a normal chromosome into a tumor cell line has lent support to the idea that a family of cellular genes are coding for factors capable to interact with the cell-growth control machinery. These genes seem to reconstitute the normal control of cell growth even in the presence of an activated oncogene. In addition, a two-mutation model has been proposed to explain the epidemiological and clinical features of childhood cancers. According to the model, the development of these malignancies can be caused by the loss or inactivation of both alleles of cellular genes, as suggested by the somatic cell hybrid experiments where the function of the inactivated genes is restored by the contribution of those derived from the normal parental cells. This family of genes is designated as onco-suppressor genes since their product is necessary for the normal regulated cell growth and is lacking or inactivated in malignant cells. At gene level they should be considered as recessive genetic traits, since the tumor phenotype appears when both alleles of an oncosuppressor gene are inactivated. The mutations affecting their normal functions belong to the type « loss of function ». The molecular analysis of retinoblastoma has led to the cloning and sequencing of the related onco-suppressor gene (RB gene) whose product displays the features of a gene-regulatory protein. In addition, a binding between the RB product and various viral onco-proteins (E1A, large T, E7) has been demonstrated, thus suggesting a mechanism of RB inactivation by which some DNA viruses can transform the host cell. Finally, the increasing availability of DNA markers, defining restriction fragment length polymorphisms, has led to the mapping of the loci of inherited predisposition for familial cancer syndromes such as MEN-1, VHL and NF-2 and to the extension to common cancers of the allele losses analysis that can reveal onco-suppressor gene inactivation. This indirect approach has suggested the occurrence of different onco-suppressor genes for sporadic breast, colonic and lung cancers, bladder carcinoma, germinal tumors of the testis and malignant melanoma. In particular, colonic cancer provides a significant example of a possible multistep scenario for carcinogenesis in humans in which activated oncogenes (e.g. ras) and inactivated putative onco-suppressor genes (on chromosome 17 and 18) coexist in the same cell.

CRISPR/Cas9 and cancer targets: future possibilities and present challenges

All cancers have multiple mutations that can largely be grouped into certain classes depending on the function of the gene in which they lie and these include oncogenic changes that enhance cellular proliferation, loss of function of tumor suppressors that regulate cell growth potential and induction of metabolic enzymes that confer resistance to chemotherapeutic agents. Thus the ability to correct such mutations is an important goal in cancer treatment. Recent research has led to the developments of reagents which specifically target nucleotide sequences within the cellular genome and these have a huge potential for expanding our anticancer armamentarium. One such a reagent is the clustered regulatory interspaced short palindromic repeat (CRISPR)-associated 9 (Cas9) system, a powerful, highly specific and adaptable tool that provides unparalleled control for editing the cellular genome. In this short review, we discuss the potential of CRISPR/Cas9 against human cancers and the current difficulties in translating this for novel therapeutic approaches.

The CGL1 (HeLa × Normal Skin Fibroblast) Human Hybrid Cell Line: A History of Ionizing Radiation Induced Effects on Neoplastic Transformation and Novel Future Directions in SNOLAB

Cellular transformation assays have been utilized for many years as powerful in vitro methods for examining neoplastic transformation potential/frequency and mechanisms of carcinogenesis for both chemical and radiological carcinogens. These mouse and human cell based assays are labor intensive but do provide quantitative information on the numbers of neoplastically transformed foci produced after carcinogenic exposure and potential molecular mechanisms involved. Several mouse and human cell systems have been generated to undertake these studies, and they vary in experimental length and endpoint assessment. The CGL1 human cell hybrid neoplastic model is a non-tumorigenic pre-neoplastic cell that was derived from the fusion of HeLa cervical cancer cells and a normal human skin fibroblast. It has been utilized for the several decades to study the carcinogenic/neoplastic transformation potential of a variety of ionizing radiation doses, dose rates and radiation types, including UV, X ray, gamma ray, neutrons, protons and alpha particles. It is unique in that the CGL1 assay has a relatively short assay time of 18-21 days, and rather than relying on morphological endpoints to detect neoplastic transformation utilizes a simple staining method that detects the tumorigenic marker alkaline phosphatase on the neoplastically transformed cells cell surface. In addition to being of human origin, the CGL1 assay is able to detect and quantify the carcinogenic potential of very low doses of ionizing radiation (in the mGy range), and utilizes a neoplastic endpoint (re-expression of alkaline phosphatase) that can be detected on both viable and paraformaldehyde fixed cells. In this article, we review the history of the CGL1 neoplastic transformation model system from its initial development through the wide variety of studies examining the effects of all types of ionizing radiation on neoplastic transformation. In addition, we discuss the potential of the CGL1 model system to investigate the effects of near zero background radiation levels available within the radiation biology lab we have established in SNOLAB.

The role of syncytins in human reproduction and reproductive organ cancers

Human life begins with sperm and oocyte fusion. After fertilization, various fusion events occur during human embryogenesis and morphogenesis. For example, the fusion of trophoblastic cells constitutes a key process for normal placental development. Fusion in the placenta is facilitated by syncytin 1 and syncytin 2. These syncytins arose from retroviral sequences that entered the primate genome 25 million and more than 40 million years ago respectively. About 8% of the human genome consists of similar human endogenous retroviral (HERVs) sequences. Many are inactive because of mutations or deletions. However, the role of the few that remain transcriptionally active has not been fully elucidated. Syncytin proteins maintain cell-cell fusogenic activity based on ENV: gene-mediated viral cell entry. In this review, we summarize how syncytins and their receptors are involved in fusion events during human reproduction. The significance of syncytins in tumorigenesis is also discussed.

Cell Fusion in the War on Cancer: A Perspective on the Inception of Malignancy

Cell fusion occurs in development and in physiology and rarely in those settings is it associated with malignancy. However, deliberate fusion of cells and possibly untoward fusion of cells not suitably poised can eventuate in aneuploidy, DNA damage and malignant transformation. How often cell fusion may initiate malignancy is unknown. However, cell fusion could explain the high frequency of cancers in tissues with low underlying rates of cell proliferation and mutation. On the other hand, cell fusion might also engage innate and adaptive immune surveillance, thus helping to eliminate or retard malignancies. Here we consider whether and how cell fusion might weigh on the overall burden of cancer in modern societies.

Targeted toxins

Objective. Current modalities used in the treatment of cancer often cause unacceptable damage to normal tissue. Toxins targeted toward tumor cells by antibodies or growth factors have the potential to selectively kill tumor cells while leaving normal tissue intact. The purpose of this review is to provide background information on targeted toxins and current clinical studies for this new class of anti-cancer compounds.

Data sources. A MEDLINE search was conducted using the term “immunotoxins.” Relevant articles were also obtained by the systematic examination of article references.

Data synthesis. The toxins Pseudomonas exotoxin, diphtheria toxin, and ricin toxin are often used as targeted toxins. Deletion or mutation of the binding domains of these toxins decreased binding of the toxins to normal tissues. Antibodies or growth factors can be used as targeting moiety, and the resulting agents are called immunotoxins or fusion proteins, respectively. DNA technology and chemical modifications of the toxin as well as the antibody moiety led to smaller and less immunogenic targeted toxins. Smaller targeted toxins are less toxic and penetrate further into the tumor. The summary of several targeted toxins elicited during clinical trials in this review makes it clear that several targeted toxins are potential agents for the treatment of various cancers, although some problems still need to be overcome. These problems include toxicity, immunogenicity, cross-reactivity of the targeted toxin with life-sustaining tissue, heterogenicity of tumor cells, and limited tumor penetration.

CRISPR/Cas9 and cancer targets: future possibilities and present challenges

All cancers have multiple mutations that can largely be grouped into certain classes depending on the function of the gene in which they lie and these include oncogenic changes that enhance cellular proliferation, loss of function of tumor suppressors that regulate cell growth potential and induction of metabolic enzymes that confer resistance to chemotherapeutic agents. Thus the ability to correct such mutations is an important goal in cancer treatment. Recent research has led to the developments of reagents which specifically target nucleotide sequences within the cellular genome and these have a huge potential for expanding our anticancer armamentarium. One such a reagent is the clustered regulatory interspaced short palindromic repeat (CRISPR)-associated 9 (Cas9) system, a powerful, highly specific and adaptable tool that provides unparalleled control for editing the cellular genome. In this short review, we discuss the potential of CRISPR/Cas9 against human cancers and the current difficulties in translating this for novel therapeutic approaches.

PTEN: History of a Tumor Suppressor

Starting from the discovery of "inhibitory chromosomes" by Theodor Boveri to the finding by Henry Harris that fusing a normal cell to a cancer cell reduced tumorigenic potential, the notion of tumor suppression was recognized well before any tumor-suppressor genes were discovered. Although not the first to be revealed, PTEN has been demonstrated to be one of the most frequently altered tumor suppressors in cancer. This introductory chapter provides a historical perspective on our current understanding of PTEN including some of the seminal discoveries in the tumor suppressor field, the events leading to PTEN's discovery, and an introduction to some of the most important researchers and their studies which have shed light on PTEN biology and function as we know it today.

The SWI/SNF ATPases Are Required for Triple Negative Breast Cancer Cell Proliferation

The Brahma (BRM) and Brahma-related Gene 1 (BRG1) ATPases are highly conserved homologs that catalyze the chromatin remodeling functions of the multi-subunit human SWI/SNF chromatin remodeling enzymes in a mutually exclusive manner. SWI/SNF enzyme subunits are mutated or missing in many cancer types, but are overexpressed without apparent mutation in other cancers. Here, we report that both BRG1 and BRM are overexpressed in most primary breast cancers independent of the tumor's receptor status. Knockdown of either ATPase in a triple negative breast cancer cell line reduced tumor formation in vivo and cell proliferation in vitro. Fewer cells in S phase and an extended cell cycle progression time were observed without any indication of apoptosis, senescence, or alterations in migration or attachment properties. Combined knockdown of BRM and BRG1 showed additive effects in the reduction of cell proliferation and time required for completion of cell cycle, suggesting that these enzymes promote cell cycle progression through independent mechanisms. Knockout of BRG1 or BRM using CRISPR/Cas9 technology resulted in the loss of viability, consistent with a requirement for both enzymes in triple negative breast cancer cells.

TP53: an oncogene in disguise

The standard classification used to define the various cancer genes confines tumor protein p53 (TP53) to the role of a tumor suppressor gene. However, it is now an indisputable fact that many p53 mutants act as oncogenic proteins. This statement is based on multiple arguments including the mutation signature of the TP53 gene in human cancer, the various gains-of-function (GOFs) of the different p53 mutants and the heterogeneous phenotypes developed by knock-in mouse strains modeling several human TP53 mutations. In this review, we will shatter the classical and traditional image of tumor protein p53 (TP53) as a tumor suppressor gene by emphasizing its multiple oncogenic properties that make it a potential therapeutic target that should not be underestimated. Analysis of the data generated by the various cancer genome projects highlights the high frequency of TP53 mutations and reveals that several p53 hotspot mutants are the most common oncoprotein variants expressed in several types of tumors. The use of Muller's classical definition of mutations based on quantitative and qualitative consequences on the protein product, such as ‘amorph', ‘hypomorph', ‘hypermorph' ‘neomorph' or ‘antimorph', allows a more meaningful assessment of the consequences of cancer gene modifications, their potential clinical significance, and clearly demonstrates that the TP53 gene is an atypical cancer gene.

A note from history: Landmarks in history of cancer, part 7

In the 2 and half decades reviewed (1970-1995), research established that chromosomal translocation, deletion, and DNA amplification are prerequisites to cancerogenesis and that oncogenes, tumor-suppressor genes, growth factors, and cytokines play crucial roles in the pathomechanism of cancer. Human papillomavirus, human immunodeficiency virus, herpes virus, and hepatitis B virus were identified as cancer-causing viruses. Several laboratory tests were developed for the detection of primary and recurrent cancers, and cancer prevention by screening methods was popularized. Sonography, computerized tomography, magnetic resonance imaging, positron emission tomography, excision of sentinel lymph nodes, and immunohistochemical techniques became routine procedures. Clinicopathologic staging and classification of tumors were standardized. Limited surgery, adjuvant and neoadjuvant chemoradiation, and the therapeutic use of monoclonal antibodies, tumor vaccines, and targeted chemotherapy became routine practice. The decline in cancer incidence and mortality demonstrated that cancer prevention and advancement in oncology are pivotal to success in the crusade against cancer. Above all, it was clearly established that the care of patients with cancer can be accomplished best in a multidisciplinary setting involving surgical oncologists, radiologists, radiation therapists, medical oncologists, surgical pathologists, and laboratory scientists. In conclusion, the 25 years from 1970 and 1995 are the high-water mark in clinical oncology, and this is the period when oncology turned from art to science.

Viruses and human cancers: a long road of discovery of molecular paradigms

About a fifth of all human cancers worldwide are caused by infectious agents. In 12% of cancers, seven different viruses have been causally linked to human oncogenesis: Epstein-Barr virus, hepatitis B virus, human papillomavirus, human T-cell lymphotropic virus, hepatitis C virus, Kaposi's sarcoma herpesvirus, and Merkel cell polyomavirus. Here, we review the many molecular mechanisms of oncogenesis that have been discovered over the decades of study of these viruses. We discuss how viruses can act at different stages in the complex multistep process of carcinogenesis. Early events include their involvement in mutagenic events associated with tumor initiation such as viral integration and insertional mutagenesis as well as viral promotion of DNA damage. Also involved in tumor progression is the dysregulation of cellular processes by viral proteins, and we describe how this has been investigated by studies in cell culture and in experimental animals and by molecular cellular approaches. Also important are the molecular mechanisms whereby viruses interact with the immune system and the immune evasion strategies that have evolved.

Targeting tumor suppressor networks for cancer therapeutics

Cancer is a consequence of mutations in genes that control cell proliferation, differentiation and cellular homeostasis. These genes are classified into two categories: oncogenes and tumor suppressor genes. Together, overexpression of oncogenes and loss of tumor suppressors are the dominant driving forces for tumorigenesis. Hence, targeting oncogenes and tumor suppressors hold tremendous therapeutic potential for cancer treatment. In the last decade, the predominant cancer drug discovery strategy has relied on a traditional reductionist approach of dissecting molecular signaling pathways and designing inhibitors for the selected oncogenic targets. Remarkable therapies have been developed using this approach; however, targeting oncogenes is only part of the picture. Our understanding of the importance of tumor suppressors in preventing tumorigenesis has also advanced significantly and provides a new therapeutic window of opportunity. Given that tumor suppressors are frequently mutated, deleted, or silenced with loss-of-function, restoring their normal functions to treat cancer holds tremendous therapeutic potential. With the rapid expansion in our knowledge of cancer over the last several decades, developing effective anticancer regimens against tumor suppressor pathways has never been more promising. In this article, we will review the concept of tumor suppression, and outline the major therapeutic strategies and challenges of targeting tumor suppressor networks for cancer therapeutics.

Sir John Gurdon: father of nuclear reprogramming

Sir John Gurdon founded the field of nuclear reprogramming. His work set the stage for the ever burgeoning area of stem cell biology and regenerative medicine. Here I provide personal reflections on times I shared with John Gurdon and professional reflections of the impact of his ground-breaking research on my own development as a scientist and on the field in general. His paradigm-shifting experiments will continue to provoke scientists to think outside the box for many years to come.

Cell fusion induced by ERVWE1 or measles virus causes cellular senescence

Cellular senescence limits proliferation of potentially detrimental cells, preventing tumorigenesis and restricting tissue damage. However, the function of senescence in nonpathological conditions is unknown. Here, Krizhanovsky and colleagues discover a new pathway to activate senescence cell fusion. The authors find that fusion-induced senescence occurs during embryonic development in the placenta. A counterpart of this process is also observed after infection by the measles virus. The results suggest that fusion-induced senescence is essential during development, and reuse of this program later in life protects agains viral infections.

Cellular senescence limits proliferation of potentially detrimental cells, preventing tumorigenesis and restricting tissue damage. However, the function of senescence in nonpathological conditions is unknown. We found that the human placental syncytiotrophoblast exhibited the phenotype and expressed molecular markers of cellular senescence. During embryonic development, ERVWE1-mediated cell fusion results in formation of the syncytiotrophoblast, which serves as the maternal/fetal interface at the placenta. Expression of ERVWE1 caused cell fusion in normal and cancer cells, leading to formation of hyperploid syncytia exhibiting features of cellular senescence. Infection by the measles virus, which leads to cell fusion, also induced cellular senescence in normal and cancer cells. The fused cells activated the main molecular pathways of senescence, the p53- and p16–pRb-dependent pathways; the senescence-associated secretory phenotype; and immune surveillance-related proteins. Thus, fusion-induced senescence might be needed for proper syncytiotrophoblast function during embryonic development, and reuse of this senescence program later in life protects against pathological expression of endogenous fusogens and fusogenic viral infections.

MAPK kinase 3 is a tumor suppressor with reduced copy number in breast cancer

Cancers are initiated as a result of changes that occur in the genome. Identification of gains and losses in the structure and expression of tumor-suppressor genes and oncogenes lies at the root of the understanding of cancer cell biology. Here, we show that the mitogen-activated protein kinase (MAPK) MKK3 suppresses the growth of breast cancer, in which it varies in copy number. A pervasive loss of MKK3 gene copy number in patients with breast cancer is associated with an impairment of MKK3 expression and protein level in malignant tissues. To assess the functional role of MKK3 in breast cancer, we showed in an animal model that MKK3 activity is required for suppression of tumor growth. Active MKK3 enhanced expression of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1), leading to increased cell-cycle arrest in G1 phase of the cell cycle. Our results reveal the functional significance of MKK3 as a tumor suppressor and improve understanding of the dynamic role of the MAPK pathway in tumor progression.

Cellular reprogramming in pursuit of immortality

The discovery that phenotypic diversity among differentiated cells results from epigenetic and not genetic differences, and can be reset to restore pluripotency, promises revolutionary advances in medicine. I discuss how this and related seminal discoveries have brought us to an exciting future.

Is PML a Tumor Suppressor?

The role of the promyelocytic leukemia (PML) protein has been widely tested in many different contexts, as attested by the hundreds of papers present in the literature. In most of these studies, PML is regarded as a tumor suppressor, a notion on the whole accepted by the scientific community. In this review, we examine how the concept of tumor-suppressor gene has evolved until now and then systematically assess whether this assumption for PML is supported by unambiguous experimental evidence.

Drosophila melanogaster: a model and a tool to investigate malignancy and identify new therapeutics

For decades, lower-model organisms such as Drosophila melanogaster have often provided the first glimpse into the mechanism of action of human cancer-related proteins, thus making a substantial contribution to elucidating the molecular basis of the disease. More recently, D. melanogaster strains that are engineered to recapitulate key aspects of specific types of human cancer have been paving the way for the future role of this 'workhorse' of biomedical research, helping to further investigate the process of malignancy, and serving as platforms for therapeutic drug discovery.

Complexity in cancer biology: is systems biology the answer?

Complex phenotypes emerge from the interactions of thousands of macromolecules that are organized in multimolecular complexes and interacting functional modules. In turn, modules form functional networks in health and disease. Omics approaches collect data on changes for all genes and proteins and statistical analysis attempts to uncover the functional modules that perform the functions that characterize higher levels of biological organization. Systems biology attempts to transcend the study of individual genes/proteins and to integrate them into higher order information. Cancer cells exhibit defective genetic and epigenetic networks formed by altered complexes and network modules arising in different parts of tumor tissues that sustain autonomous cell behavior which ultimately lead tumor growth. We suggest that an understanding of tumor behavior must address not only molecular but also, and more importantly, tumor cell heterogeneity, by considering cancer tissue genetic and epigenetic networks, by characterizing changes in the types, composition, and interactions of complexes and networks in the different parts of tumor tissues, and by identifying critical hubs that connect them in time and space.

Reprogramming cell fates: insights from combinatorial approaches

Epigenetic reprogramming can be achieved in different ways, including nuclear transfer, cell fusion, or the expression of transcription factors (TFs). Combinatorial overexpression provides an opportunity to define the minimal core network of TFs that instructs specific cell fates. This approach has been employed to induce mouse and human pluripotency and differentiated cell types from cells that can be also as distant as cells from different germ layers. This suggests the possibility that any specific cell type may be directly converted into another if the appropriate reprogramming TF core is determined. Herein, we review the factors used for reprogramming multiple cell identities and raise the question of whether there is a common underlying blueprint for reprogramming factors. In addition to the generation of human cell types of interest for cell-replacement therapies, we propose that the TF-mediated conversion of differentiated cell types, especially somatic stem cells, will have an impact on our understanding of their biological development.

TCTP in development and cancer

The translationally controlled tumor protein (TCTP) is highly conserved among animal species. It is widely expressed in many different tissues. It is involved in regulating many fundamental processes, such as cell proliferation and growth, apoptosis, pluripotency, and the cell cycle. Hence, it is not surprising that it is essential for normal development and, if misregulated, can lead to cancer. Provided herein is an overview of the diverse functions of TCTP, with a focus on development. Furthermore, we discuss possible ways by which TCTP misregulation or mutation could result in cancer.

An analysis of substitution, deletion and insertion mutations in cancer genes

Cancer-associated mutations in cancer genes constitute a diverse set of mutations associated with the disease. To gain insight into features of the set, substitution, deletion and insertion mutations were analysed at the nucleotide level, from the COSMIC database. The most frequent substitutions were c→t, g→a, g→t, and the most frequent codon changes were to termination codons. Deletions more than insertions, FS (frameshift) indels more than I-F (in-frame) ones, and single-nucleotide indels, were frequent. FS indels cause loss of significant fractions of proteins. The 5′-cut in FS deletions, and 5′-ligation in FS insertions, often occur between pairs of identical bases. Interestingly, the cut-site and 3′-ligation in insertions, and 3′-cut and join-pair in deletions, were each found to be the same significantly often (p < 0.001). It is suggested that these features aid the incorporation of indel mutations. Tumor suppressors undergo larger numbers of mutations, especially disruptive ones, over the entire protein length, to inactivate two alleles. Proto-oncogenes undergo fewer, less-disruptive mutations, in selected protein regions, to activate a single allele. Finally, catalogues, in ranked order, of genes mutated in each cancer, and cancers in which each gene is mutated, were created. The study highlights the nucleotide level preferences and disruptive nature of cancer mutations.