Local regulation of homeostasis favors chromosomal instability

Tissues of long-lived multicellular organisms have to maintain a constant number of functioning cells for many years. This process is called homeostasis. Homeostasis breaks down when cells emerge with mutations in tumor suppressor genes or oncogenes. Such mutated cells can have increased net rates of proliferation, which is increased somatic fitness. We show that the best protection against such mutations is achieved when homeostasis is regulated locally via small compartments. Small compartments, on the other hand, allow the accumulation of cells with reduced somatic fitness. Cells with mutations conferring genetic instability normally have a reduced somatic fitness because they have an increased probability of producing deleterious mutations or triggering apoptosis. Thus, small compartments protect against mutations in tumor suppressor genes or oncogenes but promote the emergence of genetic instability.     

Mutations and epimutations in the origin of cancer

Cancer is traditionally viewed as a disease of abnormal cell proliferation controlled by a series of mutations. Mutations typically affect oncogenes or tumor suppressor genes thereby conferring growth advantage. Genomic instability facilitates mutation accumulation. Recent findings demonstrate that activation of oncogenes and inactivation of tumor suppressor genes, as well as genomic instability, can be achieved by epigenetic mechanisms as well. Unlike genetic mutations, epimutations do not change the base sequence of DNA and are potentially reversible. Similar to genetic mutations, epimutations are associated with specific patterns of gene expression that are heritable through cell divisions. Knudson's hypothesis postulates that inactivation of tumor suppressor genes requires two hits, with the first hit occurring either in somatic cells (sporadic cancer) or in the germline (hereditary cancer) and the second one always being somatic. Studies on hereditary and sporadic forms of colorectal carcinoma have made it evident that, apart from genetic mutations, epimutations may serve as either hit or both. Furthermore, recent next-generation sequencing studies show that epigenetic genes, such as those encoding histone modifying enzymes and subunits for chromatin remodeling systems, are themselves frequent targets of somatic mutations in cancer and can act like tumor suppressor genes or oncogenes. This review discusses genetic vs. epigenetic origin of cancer, including cancer susceptibility, in light of recent discoveries. Situations in which mutations and epimutations occur to serve analogous purposes are highlighted.     

Oncogene and tumor suppressor gene alterations in nasal tumors

The development of nasal tumors in humans and rodents is likely mediated through the accumulation of genetic alterations in genes that regulate cell proliferation, cell death and differentiation (oncogenes and tumor suppressor genes). By examination of the relationship between genetic alterations that are known to occur in human cancers with those induced in rodent tumors with defined carcinogenic exposures, biologically relevant mechanistic linkages of molecular events leading to tumors in rodents and humans can be established. Molecular genetic studies on nasal squamous cell carcinomas (SCC) in rats thus far have indicated the presence of oncogenes unrelated to the ras oncogene family and that p53 mutation occurs at a high frequency among the nasal SCC examined. The finding of p53 mutations in rat nasal SCC and the high prevalence of p53 mutations among human SCC, indicates that a common molecular alteration is shared between rodent and human SCC.     

Malignant cell transformation and oncogenes

In genome of all transforming retroviruses special genes (oncogenes) have been identified which play a key role in malignant conversion of the cells, infected with these viruses. The homologues of these genes (protooncogenes) are persist in all normal cells. During transformation protooncogenes can be activated as a result of one of following processes: insertion of promotor-like elements, mutations, translocations, amplifications or rearrangements. Using transfection technique the transforming genes were isolated from different human tumors. The activation of one of the cellular oncogenes may switch on the other genes and malignant cell transformation may be characterized as a multifactor and multistage process.     

Transformation effector and suppressor genes.

Much has been learned about the molecular basis of cancer from the study of the dominantly acting viral and cellular oncogenes and their normal progenitors, the proto-oncogenes. More recent studies have resulted in the isolation and characterization of several genes prototypic of a second class of cancer genes. Whereas oncogenes act to promote the growth of cells, members of this latter class of genes act to inhibit cellular growth and are believed to contribute to the tumorigenic phenotype only when their activities are absent. This new class of cancer genes is referred to by a number of different names including; anti-oncogenes, recessive oncogenes, growth suppressor genes, tumor suppressor genes and emerogenes. Although only a few of these cancer genes have been identified, to date, it is likely that many additional genes of this class await identification. A third class of genes, necessary for the development of the cancer phenotype, is comprised of the transformation effector genes. These are normal cellular genes that encode proteins that cooperate with or activate oncogene functions and thereby induce the development of the neoplastic phenotype. The inactivation of transformation effector functions would therefore inhibit the ability of certain dominantly acting oncogenes to transform cells. The approaches outlined here describe functional assays for the isolation and molecular characterization of transformation effector and suppressor genes.     

Somatic selection for and against cancer

In multicellular organisms, cells cooperate within a well-defined developmental program. Cancer is a breakdown of such cooperation: cells mutate to phenotypes of uncoordinated proliferation. We study basic principles of the architecture of solid tissues that influence the rate of cancer initiation. In particular, we explore how somatic selection acts to prevent or to promote cancer. Cells with mutations in oncogenes or tumor suppressor genes often have increased proliferation rates. Somatic selection increases their abundance and thus enhances the risk of cancer. Many potentially harmful mutations, however, increase the probability of triggering apoptosis and, hence, initially lead to cells with reduced net proliferation rates. Such cells are eliminated by somatic selection, which therefore also works to reduce the risk of cancer. We show that a tissue organization into small compartments avoids the rapid spread of mutations in oncogenes and tumor suppressor genes, but promotes genetic instability. In small compartments, genetic instability, which confers a selective disadvantage for the cell, can spread by random drift. If both deleterious and advantageous mutations participate in tumor initiation, then we find an intermediate optimum for the compartment size.     

The molecular biology of cancer

The process by which normal cells become progressively transformed to malignancy is now known to require the sequential acquisition of mutations which arise as a consequence of damage to the genome. This damage can be the result of endogenous processes such as errors in replication of DNA, the intrinsic chemical instability of certain DNA bases or from attack by free radicals generated during metabolism. DNA damage can also result from interactions with exogenous agents such as ionizing radiation, UV radiation and chemical carcinogens. Cells have evolved means to repair such damage, but for various reasons errors occur and permanent changes in the genome, mutations, are introduced. Some inactivating mutations occur in genes responsible for maintaining genomic integrity facilitating the acquisition of additional mutations. This review seeks first to identify sources of mutational damage so as to identify the basic causes of human cancer. Through an understanding of cause, prevention may be possible. The evolution of the normal cell to a malignant one involves processes by which genes involved in normal homeostatic mechanisms that control proliferation and cell death suffer mutational damage which results in the activation of genes stimulating proliferation or protection against cell death, the oncogenes, and the inactivation of genes which would normally inhibit proliferation, the tumor suppressor genes. Finally, having overcome normal controls on cell birth and cell death, an aspiring cancer cell faces two new challenges: it must overcome replicative senescence and become immortal and it must obtain adequate supplies of nutrients and oxygen to maintain this high rate of proliferation. This review examines the process of the sequential acquisition of mutations from the prospective of Darwinian evolution. Here, the fittest cell is one that survives to form a new population of genetically distinct cells, the tumor. This review does not attempt to be comprehensive but identifies key genes directly involved in carcinogenesis and demonstrates how mutations in these genes allow cells to circumvent cellular controls. This detailed understanding of the process of carcinogenesis at the molecular level has only been possible because of the advent of modern molecular biology. This new discipline, by precisely identifying the molecular basis of the differences between normal and malignant cells, has created novel opportunities and provided the means to specifically target these modified genes. Whenever possible this review highlights these opportunities and the attempts being made to generate novel, molecular based therapies against cancer. Successful use of these new therapies will rely upon a detailed knowledge of the genetic defects in individual tumors. The review concludes with a discussion of how the use of high throughput molecular arrays will allow the molecular pathologist/therapist to identify these defects and direct specific therapies to specific mutations.     

Gene therapy for lung cancer

Lung cancer continues to be the largest killer of Americans due to cancer. Although progress has been made, with advances in chemotherapy, the majority of patients diagnosed with lung cancer ultimately succumb to the disease. A better understanding of the molecular pathogenesis of lung cancer is demonstrating how alterations in oncogenes and tumor suppressor genes control lung cancer initiation, growth, and survival. In this article, attempts to target molecular alterations in lung cancer using gene therapy techniques are reviewed. These include introducing suicide genes into tumor cells, replacement of defective tumor suppressor genes, inactivating oncogenes, and immunotherapy-based approaches using gene therapy technology. The major barrier for these techniques continues to be the inability to specifically target tumor cells while sparing normal cells. Nonetheless, these approaches are likely to yield important biologic and clinical data which will further the progress of lung cancer treatment.     

Genetics of colorectal cancer

Colorectal cancer (CRC) is the second leading cause of cancer-related mortality in the United States. As such, it assumes a significant role in both health policy decision-making and scientific research. CRC has been a model for investigating the molecular genetics of cancer development and progression; this is in part due to the easily detectable, sequential transition of cells from normal colonic epithelium to adenoma and then to adenocarcinoma. In addition, familial syndromes that predispose to CRC, such as familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC), have significantly contributed to our understanding of the genetic mechanisms underlying CRC formation. It is now well recognized that hereditary CRC syndromes are due to germline mutations of genes that function as tumor suppressors or, less frequently, oncogenes. Accumulation of subsequent mutations in other genes with related functions results in the stepwise progression to carcinoma. It is important to note that somatic changes in similar genes are involved in the formation of sporadic CRC. The identification of these important CRC-related genes may help facilitate the early diagnosis, prevention, and treatment of CRC. This article reviews the various familial CRC syndromes along with their genetic etiology, as well as discusses the principle of genetic testing for these conditions.     

Linear model of colon cancer initiation

Cancer results if regulatory mechanisms of cell birth and death are disrupted. Colorectal tumorigenesis is initiated by somatic or inherited mutations in the APC tumor suppressor gene pathway. Several additional genetic hits in other tumor suppressor genes and oncogenes drive the progression from polyps to malignant, invasive cancer. The majority of colorectal cancers present chromosomal instability, CIN, which is caused by mutations in genes that are required to maintain chromosomal stability. A major question in cancer genetics is whether CIN is an early event and thus a driving force of tumor progression. We present a new mathematical model of colon cancer initiation assuming a linear flow from stem cells to differentiated cells to apoptosis. We study the consequences of mutations in different cell types and calculate the conditions for CIN to precede APC inactivation. We find that early emergence of CIN is very likely in colorectal tumorigenesis.     

Linear Model of Colon Cancer Initiation

Cancer results if regulatory mechanisms of cell birth and death are disrupted. Colorectal tumorigenesis is initiated by somatic or inherited mutations in the APC tumor suppressor gene pathway. Several additional genetic hits in other tumor suppressor genes and oncogenes drive the progression from polyps to malignant, invasive cancer. The majority of colorectal cancers present chromosomal instability, CIN, which is caused by mutations in genes that are required to maintain chromosomal stability. A major question in cancer genetics is whether CIN is an early event and thus a driving force of tumor progression. We present a new mathematical model of colon cancer initiation assuming a linear flow from stem cells to differentiated cells to apoptosis. We study the consequences of mutations in different cell types and calculate the conditions for CIN to precede APC inactivation. We find that early emergence of CIN is very likely in colorectal tumorigenesis.     

Thrombospondin-1 as an endogenous inhibitor of angiogenesis and tumor growth

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein that influences cellular phenotype and the structure of the extracellular matrix. These effects are important components of the tissue remodeling that is associated with angiogenesis and neoplasia. The genetic mutations in oncogenes and tumor suppressor genes that occur within tumor cells are frequently associated with decreased expression of TSP-1. However, the TSP-1 that is produced by stromal fibroblasts, endothelial cells and immune cells suppresses tumor progression. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival and through indirect effects on growth factor mobilization. TSP-1 that is present in the tumor microenvironment also acts to suppress tumor cell growth through activation of transforming growth factor β in those tumor cells that are responsive to TGFβ. In this review, the molecular basis for the role of TSP-1 in the inhibition of tumor growth and angiogenesis is summarized.     

Stochastic dynamics of metastasis formation

Tumor metastasis accounts for the majority of deaths in cancer patients. The metastatic behavior of cancer cells is promoted by mutations in many genes, including activation of oncogenes such as RAS and MYC. Here, we develop a mathematical framework to analyse the dynamics of mutations enabling cells to metastasize. We consider situations in which one mutation is necessary to confer metastatic ability to the cell. We study different population sizes of the main tumor and different somatic fitness values of metastatic cells. We compare mutations that are positively selected in the main tumor with those that are neutral or negatively selected, but faster at forming metastases. We study whether metastatic potential is the property of all (or the majority of) cells in the main tumor or only the property of a small subset. Our theory shows how to calculate the expected number of metastases that are formed by a tumor.     

Protein dynamics analysis reveals that missense mutations in cancer-related genes appear frequently on hinge-neighboring residues

Missense mutations have various effects on protein structures, also leading to distorted protein dynamics that plausibly affects the function. We hypothesized that missense mutations in cancer-related genes selectively target hinge-neighboring residues that orchestrate collective structural dynamics. To test our hypothesis, we selected 69 cancer-related genes from the Cancer Gene Census database and their representative protein structures from the Protein Data Bank. We first identified the hinge residues in two global modes of motion by applying the Gaussian Network Model. We then showed that missense mutations are significantly enriched on hinge-neighboring residues in oncogenes and tumor suppressor genes. We observed that several oncogenes (eg, MAP2K1, PTPN11, and KRAS) and tumor suppressor genes (eg, EZH2, CDKN2C, and RHOA) strongly exhibit this phenomenon. This study highlights and rationalizes the functional importance of missense mutations on hinge-neighboring residues in cancer.     

Absence of mutations in codon 61 of the Ha-ras oncogene in epithelial cells transformed in vitro by 7,12-dimethylbenz(a)anthracene

Epithelial cells of the respiratory tract of rats were transformed in vitro by 7,12-dimethylbenz(a)anthracene (DMBA) which has been reported to cause A----T transversion mutations of the second position of Ha-ras in codon 61 in several biological models. In this study Ha-ras exon 2 was amplified by the polymerase chain reaction (PCR) and then sequenced directly. In 10 transformed cell lines, of which 5 are known to be tumorigenic, no mutations in codon 61 were found. The results suggest that Ha-ras codon 61 mutations are not associated with cell transformation initiated with DMBA in this particular cell transformation system. These data imply that other genes (oncogenes) are responsible for transformation of these cells. The results are discussed in relation to observations in various transformation systems in vivo and in vitro.     

Activated leukemic oncogenes <Emphasis Type="Italic">AML1-ETO</Emphasis> and <Emphasis Type="Italic">c-kit</Emphasis>: Role in development of acute myeloid leukemia and current approaches for their inhibition

Acute myeloid leukemia (AML) is a malignant blood disease caused by different mutations that enhance the pro-liferative activity and survival of blood cells and affect their differentiation and apoptosis. The most frequent disorders in AML are translocations between chromosomes 21 and 8 leading to production of a chimeric oncogene, AML1-ETO, and hyperexpression of the receptor tyrosine kinase KIT. Mutations in these genes often occur jointly. The presence in cells of two activated oncogenes is likely to trigger their malignization. The current approaches for treatment of oncologic diseases (bone marrow transplantation, radiotherapy, and chemotherapy) have significant shortcomings, and thus many laboratories are intensively developing new approaches against leukemias. Inhibiting expression of activated leukemic oncogenes based on the principle of RNA interference seems to be a promising approach in this field.     

胃癌细胞中Ras基因的表达、突变分析及新剪接型Kras△E2的发现

目的：传统Ras家族由Kras,Hras和Nras基因组成,这类基因的点突变经常在人类肿瘤中发现,突变热点位于12,13,61位密码子。ERas基因是2003年在鼠胚胎干（ES）细胞中发现的,其cDNA编码的蛋白与Kras,Hras和Nras分别有46％,43％和47％的相似性,故属于新的Ras家族成员,近几年发现ERas基因的表达与胃癌密切相关,而传统Ras基因在胃癌细胞中的表达及突变情况系统报道较少,本文旨在研究传统Ras基因Kras,Hras,Nras及其家族新成员ERas基因在胃癌细胞中的表达和突变情况。方法：选用7株不同来源不同分化程度的胃癌细胞系,利用RT—PCR及real-timePCR检测Ras基因在这些胃癌细胞系中的表达,并通过测序对传统Ras基因突变热点12,13,61位密码子及ERas基因全长进行突变分析。结果：QRas基因在这些胃癌细胞系中均有不同程度的表达,其中Hras和Nms基因在各株细胞中表达水平均一,而Kras和ERas基因则呈差异性表达;②在这些胃癌细胞中传统Ras基因突变热点12,13,61位密码子不存在突变,ERas基因全长亦未检测到突变．③发现Kras基因一新的剪接型,特点为第一、三外显子直接拼接,缺失第二外显子,命名为Kras△E2。结论：与在其他肿瘤中不同,传统Ras基因在胃癌细胞中不存在突变热点,家族新成员ERas基因全长亦无突变,在国际上首次报道新剪接型Kras△E2,从而得出创新性结论：Ras基因家族在胃癌细胞中并不是通过热点突变导致持续活化而致癌,而可能是通过ERas基因表达量的调节或形成新的剪接型KrasAE2而致癌。另外,Kras基因是一被受国际关注的肿瘤基因,新剪接型的发现可能会对Kras基因致癌机制产生新的认识,意义重大。     

Progressing toward a molecular description of colorectal cancer development.

During the past decade, the powerful techniques of molecular biology and genetics have been applied to the study of human cancer. Colorectal tumors have proved to be an excellent system in which to search for and study the mutations involved in the development and progression of a common human cancer because of their natural history and several inherited syndromes that strongly predispose to colorectal cancer. A current view is that colorectal cancer results, at least in part, from the accumulation of multiple mutations in oncogenes and tumor suppressor genes in an affected cell. Although the genetic alterations often occur in a preferred sequence, the total accumulation of changes, rather than their order with respect to one another, appears to be a critical determinant of the biological properties of the tumor cell. Study of the inherited and somatic mutations in colorectal tumor cells may provide insights not only into the pathogenesis of cancer, but may also lead to future understanding of the mechanisms of the origin of mutations and the role of environmental and dietary factors in colorectal tumor development.     

Intercellular communication and carcinogenesis

Two types of intercellular communication (humoral and cell contact-mediated) are involved in control of cellular function in multicellular organisms, both of them mediated by membrane-embedded proteins. Involvement of aberrant humoral communication in carcinogenesis has been well documented and genes coding for some growth factors and their receptors have been classified as oncogenes. More recently, cell contact-mediated communication has been found to have an important role in carcinogenesis, and some genes coding for proteins involved in this type of communication appear to form a family of tumor-suppressor genes. Both homologous (among normal or (pre-)cancerous cells) as well as heterologous (between normal and (pre)cancerous cells) communications appear to play important roles in cell growth control. Gap junctional intercellular communication (GJIC) is the only means by which multicellular organisms can exchange low molecular weight signals directly from within one cell to the interior of neighboring cells. GJIC is altered by many tumor-promoting agents and in many human and rodent tumors. We have recently shown that liver tumor-promoting agents inhibit GJIC in the rat liver in vivo. Molecular mechanisms which could lead to aberrant GJIC include: (1) mutation of connexin genes; (2) reduced and/or aberrant expression of connexin mRNA; (3) aberrant localization of connexin proteins, i.e., intracytoplasmic rather than in the cytoplasmic membrane; and (4) modulation of connexin functions by other proteins, such as those involved in extracellular matrix and cell adhesion. Whilst mutations of the cx 32 gene appear to be rare in tumors, cx 37 gene mutations have been reported in a mouse lung tumor cell line. Our results suggest that aberrant connexin localization is rather common in cancer cells and that possible molecular mechanisms include aberrant phosphorylation of connexin proteins and lack of cell adhesion molecules. Studies on transfection of connexin genes into tumor cells suggest that certain connexin genes (e.g., cx 26, cx 43 and cx 32) act as tumor-suppressor genes.