VIEWS: 13 PAGES: 113 POSTED ON: 3/5/2011
NEOPLASIA All tumors, benign and malignant, have two basic components: (1) proliferating neoplastic cells that constitute their parenchyma proliferating "cutting edge" determine their nature, (2) supportive stroma made up of connective tissue and blood vessels. growth and evolution of neoplasms are critically dependent on their stroma. adequate stromal blood supply is requisite, and the stromal connective tissue provides the framework for the parenchyma. stromal support is scant - neoplasm is soft and fleshy. parenchymal cells stimulate the formation of an abundant collagenous stroma--referred to as desmoplasia. The nomenclature of tumors based on the parenchymal component. Benign Tumors. In general, benign tumors are designated by attaching the suffix -oma to the cell of origin. However nomenclature of benign epithelial tumors is more complex. They are variously classified, some based on their cells of origin, others on microscopic architecture, and still others on their macroscopic patterns. Adenoma benign epithelial neoplasm that forms glandular patterns as well as to the tumors derived from glands but not necessarily reproducing glandular patterns. Benign epithelial neoplasms producing microscopically or macroscopically visible finger-like or warty projections from epithelial surfaces are referred to as papillomas Those that form large cystic masses - cystadenomas. Some tumors produce papillary patterns that protrude into cystic spaces and are called papillary cystadenomas. When a neoplasm, benign or malignant, produces a macroscopically visible projection above a mucosal surface and projects - a polyp. Malignant Tumors. The nomenclature essentially follows the same schema used for benign neoplasms, with certain additions. Malignant tumors arising in mesenchymal tissue - sarcomas Malignant neoplasms of epithelial cell origin, derived from any of the three germ layers - carcinomas. a glandular growth pattern microscopically is termed an adenocarcinoma. the organ of origin Not infrequently, a cancer is composed of undifferentiated cells and must be designated merely as a poorly differentiated or undifferentiated malignant tumor. In most neoplasms, benign and malignant, the parenchymal cells bear a close resemblance to each other divergent differentiation of a single line of parenchymal cells creates what are called mixed tumors. . The great majority of neoplasms, even mixed tumors, are composed of cells representative of a single germ layer. The teratoma, in contrast, is made up of a variety of parenchymal cell types representative of more than one germ layer, usually all three. They arise from totipotential cells and so are principally encountered in the gonads, although, rarely, they occur in sequestered primitive cell rests elsewhere. An ectopic rest of normal tissue is sometimes called a choristoma- Analogously, aberrant differentiation may produce a mass of disorganized but mature specialized cells or tissue indigenous to the particular site, referred to as a hamartoma. CHARACTERISTICS OF BENIGN AND MALIGNANT NEOPLASMS in general, there are criteria by which benign and malignant tumors can be differentiated, and they behave accordingly: (1) differentiation and anaplasia, (2) rate of growth, (3) local invasion, (4) metastasis. Differentiation and Anaplasia These terms apply to the parenchymal cells of neoplasms. Differentiation - extent to which parenchymal cells resemble comparable normal cells, both morphologically and functionally. Well-differentiated tumors are thus composed of cells resembling the mature normal cells of the tissue of origin of the neoplasm Poorly differentiated or undifferentiated tumors have primitive-appearing, unspecialized cells. In general, benign tumors are well differentiated Malignant neoplasms range from well differentiated to undifferentiated. . Lack of differentiation, or anaplasia - hallmark of malignant transformation. means "to form backward," implying a reversion from a high level of differentiation to a lower level. arise from stem cells present in all specialized tissues. Marked by a number of morphologic and functional changes. Both the cells and the nuclei characteristically display pleomorphism--variation in size and shape Cells may be found that are many times larger than their neighbors, and other cells may be extremely small and primitive appearing. Characteristically the nuclei contain an abundance of DNA and are extremely dark staining ( hyperchromatic). The nuclei are disproportionately large for the cell, and the nuclear-to-cytoplasmic ratio may approach 1:1 instead of the normal 1:4 or 1:6. The nuclear shape is usually extremely variable, and the chromatin is often coarsely clumped and distributed along the nuclear membrane. Large nucleoli are usually present in these nuclei. undifferentiated tumors usually possess large numbers of mitoses, reflecting the higher proliferative activity of the parenchymal cells. The presence of mitoses, however, does not necessarily indicate that a tumor is malignant or that the tissue is neoplastic. . More important as a morphologic feature of malignant neoplasia are atypical, bizarre mitotic figures sometimes producing tripolar, quadripolar, or multipolar spindles . Formation of tumor giant cells, some possessing only a single huge polymorphic nucleus and others having two or more nuclei. . In addition to the cytologic abnormalities described here, the orientation of anaplastic cells is markedly disturbed (i.e., they lose normal polarity). Sheets or large masses of tumor cells grow in an anarchic, disorganized fashion. Although these growing cells obviously require a blood supply, often the vascular stroma is scant, and in many anaplastic tumors, large central areas undergo ischemic necrosis. Dysplasia - means disordered growth. is encountered principally in the epithelia, and it is characterized by a constellation of changes that include a loss in the uniformity of the individual cells as well as a loss in their architectural orientation. also exhibit considerable pleomorphism (variation in size and shape) and often possess deeply stained (hyperchromatic) nuclei, which are abnormally large for the size of the cell. Mitotic figures are more abundant than usual, although almost invariably they conform to normal patterns. Frequently the mitoses appear in abnormal locations within the epithelium. There is considerable architectural anarchy. almost invariably antedates the appearance of cancer, dysplasia does not necessarily progress to cancer. the better the differentiation of the cell, the more completely it retains the functional capabilities found in its normal counterparts. Highly anaplastic undifferentiated cells, whatever their tissue of origin, come to resemble each other more than the normal cells from which they have arisen. The more rapidly growing and the more anaplastic a tumor, the less likely it is that there will be specialized functional activity. The cells in benign tumors are almost always well differentiated and resemble their normal cells of origin; the cells in cancer are more or less differentiated, but some loss of differentiation is always present. Rate of Growth The generalization can be made that most benign tumors grow slowly over a period of years, whereas most cancers grow rapidly, sometimes at an erratic pace, and eventually spread and kill their hosts. Such an oversimplification, however, must be extensively qualified. Some benign tumors have a higher growth rate than malignant tumors. Moreover, the rate of growth of benign as well as malignant neoplasms may not be constant over time. In general, the growth rate of tumors correlates with their level of differentiation, and thus most malignant tumors grow more rapidly than do benign lesions. There is, however, a wide range of behavior. Some malignant tumors grow slowly for years then suddenly increase in size virtually under observation, explosively disseminating to cause death within a few months of discovery. It is believed that such behavior results from the emergence of an aggressive subclone of transformed cells. At the other extreme are those that grow more slowly than benign tumors and may even enter periods of dormancy lasting for years. On occasion, cancers have been observed to decrease in size and even spontaneously disappear Local Invasion Nearly all benign tumors grow as cohesive expansile masses that remain localized to their site of origin and do not have the capacity to infiltrate, invade, or metastasize to distant sites, as do malignant tumors. Because they grow and expand slowly, they usually develop a rim of compressed connective tissue, sometimes called a fibrous capsule, that separates them from the host tissue. This capsule is derived largely from the stroma of the native tissue as the parenchymal cells atrophy under the pressure of expanding tumor. Such encapsulation tends to contain the benign neoplasm as a discrete, readily palpable, and easily movable mass that can be surgically enucleated . Although a well-defined cleavage plane exists around most benign tumors, in some it is lacking. Most malignant tumors are obviously invasive and can be expected to penetrate the wall of the colon or uterus, for example, or fungate through the surface of the skin. They recognize no normal anatomic boundaries. Such invasiveness makes their surgical resection difficult, and even if the tumor appears well circumscribed, it is necessary to remove a considerable margin of apparently normal tissues about the infiltrative neoplasm. Next to the development of metastases, invasiveness is the most reliable feature that differentiates malignant from benign tumors. We noted earlier that some cancers seem to evolve from a preinvasive stage referred to as carcinoma in situ. This is best illustrated by carcinoma of the uterine cervix (Chapter 24) . In situ cancers display the cytologic features of malignancy without invasion of the basement membrane. They may be considered one step removed from invasive cancer, and with time, most penetrate the basement membrane and invade the subepithelial stroma. Metastasis tumor implants discontinuous with the primary tumor. unequivocally marks a tumor as malignant because benign neoplasms do not metastasize. The invasiveness of cancers permits them to penetrate into blood vessels, lymphatics, and body cavities, providing the opportunity for spread. With few exceptions, all cancers can metastasize. The major exceptions are most malignant neoplasms of the glial cells in the central nervous system, called gliomas, and basal cell carcinomas of the skin. Both are highly invasive forms of neoplasia (the latter being known in the older literature as rodent ulcers because of their invasive destructiveness), but they rarely metastasize. It is evident then that the properties of invasion and metastasis are separable. In general the more aggressive, the more rapidly growing, and the larger the primary neoplasm, the greater the likelihood that it will metastasize or already has metastasized. There are innumerable exceptions, PATHWAYS OF SPREAD Dissemination of cancers may occur through one of three pathways: (1) direct seeding of body cavities or surfaces, (2) lymphatic spread, and (3) hematogenous spread. Seeding of Body Cavities and Surfaces. may occur whenever a malignant neoplasm penetrates into a natural "open field." Most often involved is the peritoneal cavity, but any other cavity--pleural, pericardial, subarachnoid, and joint space--may be affected. Lymphatic Spread. Transport through lymphatics is the most common pathway for the initial dissemination of carcinomas, but sarcomas may also use this route. The pattern of lymph node involvement follows the natural routes of drainage. .Hematogenous Spread. is typical of sarcomas but is also used by carcinomas. Arteries, with their thicker walls, are less readily penetrated than are veins, however, may occur when tumor cells pass through the pulmonary capillary beds or pulmonary arteriovenous shunts or when pulmonary metastases themselves give rise to additional tumor emboli Venous invasion, the blood-borne cells follow the venous EPIDEMIOLOGY Study of cancer patterns in populations - origins of cancer. Cause of cancer can be obtained by epidemiologic studies that relate particular environmental, racial (possibly hereditary), and cultural influences certain diseases associated with an increased risk of developing cancer Cancer Incidence Individual's likelihood of developing a cancer is expressed by national incidence and mortality rates. Geographic and Environmental Factors Remarkable differences can be found in the incidence and death rates of specific forms of cancer around the world. There is no paucity of environmental factors: ambient environment, workplace, in food, personal practices. Age important influence on the likelihood of being afflicted with cancer. Most carcinomas occur in the later years of life (55 years). Each age group has its own predilection to certain forms of cancer striking increase in mortality from cancer in the age group 55 to 74 years. The decline in deaths in the 75-year-and-over group merely reflect the dwindling population reaching this age children under the age of 15 are not spared acute leukemias and CNS - 60% account for all death other common neoplasm of infancy and childhood neuroblastoma Wilms tumor retinoblastoma rhabdomyosarcoma Heredity large number of types of cancer, including the most common forms, there exist not only environmental influences, but also hereditary predispositions. NO MORE THAN 5 % TO 10% of all human cancers fall into the following categories Hereditary forms of cancers contribution of heredity to the fatal burden of human cancer - no more than 5 to 10% of all human cancers. 1. Inherited Cancer Syndromes. Several well-defined cancers in which inheritance of a single mutant gene greatly increases the risk of developing a tumor. Predisposition to these tumors shows an autosomal dominant pattern of inheritance. Childhood retinoblastoma Familial adenomatous polyposis (FAP) Often associated with a specific marker phenotype. multiple adenoma >100 in Familial adenomatous polyp there are abnormalities in tissue that are not the target of transformation Lisch nodules and café-au-lait spot – neurofibromatosis specific sites and tissues - MEN syndrome In each syndrome, tumors involve 2. Familial Cancers. Virtually all the common types of cancers that occur sporadically have also been reported to occur in familial forms. brain, colon, breast, ovary Characteristic features a. Early age at onset b. tumors arising in two or more close relatives of the index case, and c. sometimes multiple or bilateral tumors. d. Not associated with specific marker phenotypes. e. Transmission pattern is not clear. In general, sibs have a relative risk between 2 and 3. predisposition to the tumors is dominant, but multifactorial inheritance cannot be easily ruled Certain familial cancers can be linked to the inheritance of mutant genes - linkage of BRCA-1 and BRCA-2 genes to familial breast and ovarian cancers. 3. Autosomal Recessive Syndromes of Defective DNA Repair. Small group of autosomal recessive disorders is collectively characterized by chromosomal or DNA instability. Xeroderma pigmentosum - in which DNA repair is defective. There is an emerging evidence that the influence of hereditary factors is subtle and indirect The genotype may influence the likelihood of one’s developing environmentally induced cancers Acquired Preneoplastic Disorders Predispositions - cell replication is involved in cancerous transformation - Great majority of instances they are not complicated by neoplasia. regenerative, hyperplastic, and dysplastic proliferations are fertile soil for the origin of a malignant neoplasm Precancerous conditions well-defined association with cancer great majority of instances no malignant neoplasm emerges, but it calls attention to the increased risk. 1. non-neoplastic disorders chronic atrophic gastritis of pernicious anemia; solar keratosis of the skin; chronic ulcerative colitis; and leukoplakia of the oral cavity, vulva, and penis- 2. benign neoplasia - most do not become cancerous villous adenoma of the colon, tubular adenoma Are benign tumor cancerous? In general, the answer is NOT, but invariably there are exceptions, and perhaps it is better to say that each type of benign tumor is associated with a particular level of risk, ranging from HIGH to virtually NONEXISTENT MOLECULAR BASIS OF CANCER Nonlethal genetic damage lies at the heart of carcinogenesis. genetic damage (or mutation) may be acquired by the action of environmental agents inherited in the germ line. Mutations permanent changes in the DNA Germs cells transmitted to the progeny and may give rise to inherited diseases Somatic cells not transmitted to the progeny but are important in the causation of cancers and some congenital malformations Mutation is corrected carcinogens by a mechanism of DNA repair 1. recognition and incision of the affected DNA strands by an ENDONUCLEASE 2. excision and broadening of the gap by an EXONUCLEASE - DNA polymerase 3.filling of the gap by repair replication – DNA polymerase 4. covalent joining of the polynucleotides by LIGASE Genetic hypothesis of cancer tumor mass results from the clonal expansion of a single progenitor cell that has incurred the genetic damage Tumors are monoclonal DNA repair genes affect cell proliferation ( regulate repair of damaged DNA ) affect cell proliferation or survival indirectly by influencing the ability of the organism to repair nonlethal damage in other genes, including protooncogenes, tumor-suppressor genes, and genes that regulate apoptosis. A disability in the DNA repair genes can predispose to mutations in the genome - neoplastic transformation. Both alleles of DNA repair genes must be inactivated to induce such genomic instability; When the genes that normally sense and repair DNA damage are impaired or lost, the resultant genomic instability favors mutation in genes that regulate six acquired capabilities of cancer cells MOLECULAR BASIS OF CANCER Three classes of normal regulatory genes principal targets of genetic damage. 1. Growth-promoting protooncogenes, 2. Growth-inhibiting cancer-suppressor genes (antioncogenes), 3. Genes that regulate programmed cell death, or apoptosis 4. DNA repair genes Mutant alleles of protooncogenes - oncogenes are considered dominant because they transform cells despite the presence of their normal counterpart. Tumor-suppressor genes both normal alleles of the must be damaged for transformation to occur - recessive oncogenes. Genes that regulate apoptosis may be dominant, as are protooncogenes, or may behave as cancer-suppressor genes. Genetic changes that fuel tumor progression involved Growth-regulatory genes Genes regulate angiogenesis, invasion, and metastases Cancer related genes in the context of six fundamental changes in cell physiology that together dictate malignant phenotype 1. self-sufficiency 2. insensitivity to growth-inhibitory signals 3. Evasion of apoptosis 4. limitless replicative potential – overcome cellular senescence 5. sustained angiogenesis 6. ability to invade and metastasize Carcinogenesis molecular basis of cancer Multistep process at both the phenotypic and the genetic levels A malignant neoplasm has several phenotypic attributes these characteristics are acquired in a stepwise fashion Tumor Progression. as excessive growth, local invasiveness, ability to form distant metastases. At the molecular level, progression results from accumulation of genetic lesions that in some instances are favored by defects in DNA repair. A. Self-sufficiency in growth signal Oncogenes – mutant allele of protooncogenes genes that promote autonomous cell growth in cancer cells promote cell growth in the absence of normal growth-promoting signals products – oncoproteins resemble the product of normal protooncogenes except it is devoid of important regulatory elements and their production in transformed cells does not depend on growth factors or other external signals Steps of cell proliferation 1. Binding of a GF to its specific receptor on CM 2. Transient and limited activation of the growth factor receptor, which in turn activates several signal- transducing proteins on the inner leaflets of the CM 3. Transmission of the transduced signal across the cytosol to the nucleus via a second messengers 4. Induction and activation of nuclear regulatory factors that initiate DNA transcription 5. Entry and progression of the cell into the cell cycle, resulting ultimately in cell division Signal transduction cascade and Cell Cycle Regulation I. GROWTH FACTORS normal cell require stimulation by growth factors to undergo proliferation – most soluble GF are made by one cell type and act on a neighboring cell to stimulate proliferation ( paracrine action) Tumor cells 1. ability to synthesize the growth factor to which they responsive – autocrine 1.1 altered/mutated growth factor genes – excessive production Growth Factor 1.2 Most common Growth factor genes itself is not altered or mutated, but the products of other oncogene cause overexpression of growth factor genes cells are forced to secret large amount of Growth Factors II. GROWTH FACTOR RECEPTORS 1. Mutation oncogenes encode growth factor receptors – overexpression of mutant receptor proteins deliver continuous mitogenic signals to cells even in the absence of the growth factors in the environment 2. Most common growth factor receptor genes itself is not altered or mutated but there are overexpression of GF from other sources – can render cancer cells hyperresponsive to normal levels of growth factors, a level that would not normally trigger proliferation HER2 (EGF receptor family ERBB1, ERBB2)in breast CA III. SIGNAL TRANSDUCING PROTEINS mutation in genes that encode various components of the signaling pathways ( overexpression of signaling proteins ) – couple with GF receptors to their nuclear targets RAS and ABL 30% of all human tumor contain mutated version of the RAS genes – undergo point mutation Mutated RAS Genes produced Mutant RAS Proteins can bind GAPs but their GTPase activity fails to be augmented – mutant RAS is trapped in its activated GTP- bound form and the cell is led to believed that it must continue to proliferate Neurofibromin1 Nonreceptor-associated tyrosine kinase - signal transduction pathways ABL – protooncogene is dampened by negative regulatory domains chromosome 9 and is translocated to chromosome 22, where it fuses with part of the break point cluster region ( BCR) gene – BCR-ABL hybrid gene has a potent tyrosine kinase activity, and it activates several pathways including RAS-RAF cascade chronic myeloid leukemia certain acute leukemia IV. NUCLEAR TRANSCRIPTION FACTORS responder genes in the nucleus that orchestrate the cells orderly advance through the mitotic cycle – genes that regulate transcription of DNA oncogenes - MYC, MYB, JUN, FOS productions of oncoproteins MYC oncogene – associated with persistent expression or overexpression – overexpression of MYC oncoproteins MYC proteins – bind to the DNA – transcriptional activation of several growth-related genes, including CYCLIN-DEPENDENT KINASE – drives cells into cell division translocation t(8:14) Burkitt’s lymphoma amplification breast, colon, lung and others Cyclin and Cyclin-dependent kinase growth promoting stimuli – entry of quiescent cells into cell cycle cyclin are synthesized during specific phase of the cell cycle and their function is to activate the CDK which is in inactive form and ones activated by cyclins phosphorylate crucial target proteins on completion of their task cyclin degenerate CDK inhibitors silence the CDK and exert negative control over the cell cycle B.INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS Normal cell antigrowth signal may cause dividing cell to go to G0 ( queiscence) or enter postmitotic, differentiated pool and lose replicative potential exert their effects on G1 to S phase check point of the cell cycle – this transition is controlled by RB protein 1.RB gene suppressor gene – product is a DNA-binding proteins RB protein serves as a brake in the advancement of cells from G1 to S phase when the cells are activated by growth factor – RB protein is inactivated by phosphorylation, ( phosphorylated RB – allow cell proliferation ) the break is released and the cells traverse the G1 to S phase when the cells enter S phase they are committed to divide without additional GF stimulation. During M phase the phosphate groups are removed from RB by cellular phosphate generating the dephosphorylated form of RB – prevent cell proliferation Mutated RB genes – absence or RB protein or its ability to sequester transcription factors is derailed by mutation 2. TGF B pathways TGF B in normal cells potent inhibitor of proliferation it regulate cellular processes by binding to three receptors Type I, II,III antiproliferative effects are mediated in large part by regulating RB pathways arrests cells in the G1 phase by stimulating CDK1 p15 ( inhibitors of CDK1), and by inhibiting the transcription of CDK2, CDK4and cyclin A and E Mutation of TGF B signalling pathways affect type II receptor colon stomach and endometrium SMAD molecules pancreatic tumor serve to transduce antiproliferative signal from the receptor to the nucleus 3. Adenomatous polyposis coli B-catenin pathway APC gene – suppressor gene - APC protein it is a cytoplasmic protein whose dominant function is to regulate the intracellular level of B catenin protein – bind to the to the cytoplasmic portion E- cadherin – a cell surface protein that maintain intercellular adhesiveness translocate to nucleus and activate cell proliferation – important component of WTN signaling pathways allows the transcription of growth promoting genes – D1 and MYC APC genes one mutant gene – develop hundreds to thousands of adenomatous polyps in the colon during their teens or 20s, one or more polyps undergo malignant transformation – both copies of APC genes must be lost for tumor development – ADENOMA 70% to 80% of sporadic colon cancers TP53 gene - guardian of the genome suppressor gene one of the most commonly mutated genes in human cancers antiproliferative – cell cycle regulate apoptosis DNA repair gene senses DNA damage and assists in DNA repair by causing G1 arrest and inducing DNA repair genes A cell with damaged DNA that cannot be repaired is directed by TP53 to undergo apoptosis Mutated TP53 DNA damaged is goes unrepaired, mutations becomes fixed in dividing cells, and the cell turnsonto a one-way street leading to maligant transformation 70% of human cancers have defect in this gene and the remaining have defects in genes up-stream or down-stream TP53 Homozygous loss – found virtually every type of cancer including carcinomas of the lung, colon, and breast. Heterozygous allele – inherited, predisposes to develop malignant tumor – Li Fraumeni Normal TP53 can be rendered nonfunctional by certain viruses proteins encoded by oncogenic HPVs, HBV, EBV – bind to normal TP53 proteins and nullify their protective functions C. EVASION OF APOPTOSIS CD95 is bound to its Ligand – CD95L – trimerizes and its cytoplasmic death domain attract the intracellular adaptor protein –FADD recruit – Procaspase 8 to form death-inducing signaling complex activated to caspase 8 –activates caspase 3 executioner caspase that cleaves DNA and other substrate mitochondria – release cytochrome c form complex with apoptosis-inducing factor 1( APAF 1 ) activates Procaspase 9 to caspase 9 – which activates caspase 3 release of cytochrome c regulated by genes of the BCL2 family BCL2 and BCL-X - inhibit apoptosis BAD, BAX, BID - promote apoptosis by favoring cytochrome release TP53- regulate BAX synthesis caspase 8 – activates BID D. LIMITLESS REPLICATIVE POTENTIAL Normal cells Have a capacity of 60 to 70 doubling. After this, the cells lose the capacity to divide and enter a nonreplicative senescence. - due to progressive shortening of the TELOMERES At the end of chromosomes with each cell division – massive chromosomal abnormalities and death To avoid senescence – acquire telomerase is active in normal stem cell but is absent from most somatic cells Tumor cells telomerase maintenance present in virtually all types of cancers E. DEVELOPMENT OF SUSTAINED ANGIOGENESIS tumor cannot enlarge beyond 1 to 2 mm in diameter unless they are vascularized required for tumor growth and metastasis Neovascularization 1. supplies nutrients and oxygen 2. newly formed endothelial cells stimulate the growth of adjacent tumor cells by secreting polypeptides – IGF, PDGF, GM-CSF IL-1 Tumor-associated angiogenic factor VEGF – vascular endothelial growth factor Basic fibroblast growth factor early tumor growth do not induce angiogenesis they remain small or in situ for years until the angiogenic switch terminates the stage of vascular quiscence molecular basis increase production of angiogenic factors or loss of angiogenesis inhibitors wild type TP53 – inhibit angiogenesis by inducing synthesis of thrombospondin-1, with mutation of both allele of TP53 Decrease in thrombospondin-1 tilt the balance in favor of angiogenic factors hypoxia release hypoxia-inducible factor – HIF-1 – control transcription of VEGF - also under the control RAS Proteases can release FGF stored in ECM cleavage of plasmin – give rise to angiostatin – potent F. ABILITY TO INVADE AND METASTASIZE A carcinoma first must breach the underlying BM, then traverse the interstitial CT, and ultimately gain access to the circulation by penetrating the vascular BM Detachment of tumor cells from each other E-cadherins intercellular glue and transmit antigrowth signals via B-catenin Mutation of E-cahedrin gene or activation B-catenin gene Free B-catenin – activate transcription of growth promoting genes Attachment of tumor cells to matrix components receptors for BM laminin are distributed around the cell membrane loss of integrins Degradation of extracellular matrix tumor cells secrete proteolytic enzymes or induce the host cell to elaborate proteases metalloproteinases gelatinases type IV collagenases stromelysins Migration of tumor cells mediated by tumor cell-derived cytokines cleavage products of matrix components some growth factor stromal cell paracrine effectors of cell motility Genomic Instability - Enabler of malignancy are encountered in inherited disorders in which genes that encode proteins involved in DNA repair are defective HNPCC syndrome hereditary nonpolyposis colon carcinoma characterized by familial carcinoma of the colon affecting predominantly the cecum and proximal colon Xeroderma pigmentosum increased risk for the development of cancers of the skin exposed to the UV light contained in sun rays UV causes cross-linking of pyrimidine residues preventing normal DNA replication Autosomal recessive disorders characterized by hypersensitivity to other DNA – damaging agents Ionizing radiation Bloom syndrome ataxia-telangiectasia DNA cross-linking agents Fanconi anemia Hereditary breast cancer mutation in two genes account for 80% of cases BRCA 1 and also higher risk of epithelial ovarian cancers BRCA 2 also increased risk of breast CA in male and women and cancers of the ovary, prostate pancreas bile ducts stomach and melanocytes Tumor Progression and Heterogeneity acquisition of of more aggressive behaviour and greater malignant potential Karyotypic Changes in Tumors The genetic damage that activates oncogenes or inactivates tumor-suppressor genes may be subtle (e.g., point mutations) or be large enough to be detected in a karyotype. In certain neoplasms, karyotypic abnormalities are nonrandom and common. Specific abnormalities have been identified in most leukemias and lymphomas and in an increasing number of nonhematopoietic tumors. The common types of nonrandom structural abnormalities in tumor cells are (1) balanced translocations, - chromosome 22 and chromosome 9 (2) deletions - chromosome 13q14 – retinoblastoma 17p, 5q, 18q – colorectal CA 3p small cell CA (3) cytogenetic manifestations of gene amplification. homogeneously staining regions on single chromosomes double minutes In addition, whole chromosomes may be gained or lost. The study of chromosomal changes in tumor cells is important on two accounts. First, molecular cloning of genes in the vicinity of chromosomal breakpoints or deletions has been extremely useful in identification of oncogenes (e.g., bcl-2, c- abl) and tumor-suppressor genes (e.g., APC, Rb). Second, certain karyotypic abnormalities are specific enough to be of diagnostic value, and in some cases they are predictive of clinical course. ETIOLOGY OF CANCERS Carcinogenic agents several may act in concert or sequentially to produce the multiple genetic abnormalities characteristic of neoplastic cells 1. chemicals 2. radiant energy 3. microbial agents Chemical carcinogen indirect-acting procarcinogen require metabolic conversion before they become active Direct-acting ultimate carcinogen require nometabolic conversion to become carcinogen Mechanism of action RAS and TP53 Initiator – mutational activation of an oncogene Promoter nontumorigenic cause clonal expansion of mutated cells Radiation carcinogenesis 1. chromosomal breakage 2. translocation 3. point mutation Nonlethal doses of radiation may induce genomic instability Fanconi anemia Bloom syndrome ataxia-telangiectasia Double stranded DNA breaks – most important UV light causes mutation in the TP53 gene Microbial Carcinogenesis RNA oncogenic viruses retroviruses transform cell by two mechanism acute transforming viruses – contains transforming viral oncogene ( v-onc ) slow transforming viruses – do not contain a v-onc, but the proviral DNA is always found inserted near a cellular oncogene – insertional mutagenesis Human T-cell leukemia virus type no v-onc no consistent integration next to a cellular oncogene retroviral gene with unique region pX – encodes TAX protein activate transcription of host cell gene including gene encoding – IL2, GM-CSF repress tumor supressor genes – CDK1, CDKN2A/p16, TP53 DNA oncogenic viruses HPV, EBV, HHV-8, HBV Human papillomavirus infection with high-risk HPV type simulate the loss of tumor suppressor genes activate cyclins inhibit apoptosis and combat cellular senecence infection with is not sufficient for carcinogenesis cotransfection with a mutated RAS genes – full malignant transformation Helicobacter pylori chronic gastritis – development of reactive T-cell which cause – polyclonal B-cell proliferation - additional accumulated mutation – monoclonal B-cell proliferation Tumor immunity - Host defense against tumor Tumor antigens cancer-testis antigens – MAGE family of gene Tissue-Specific antigens differentiation-specific antigen expressed on tumor cells and their untransformed counterpart Antigens resulting from mutational change in proteins B-catenin, TP53, RAS, CDK4 genes Overexpressed antigens product of normal genes that overexpressed –amplification HER-2 Viral antigens Other tumor antigens MUC – 1 antigen Oncofetal antigens CEA, alpha-feto proteins Differentiation-specific antigens CD10, PSA Antitumor effector mechanism Cytotoxic T lymphocytes NK cells Macrophages Humoral mechanism activation of complements induction of antibody-dependent cellular cytotoxicity by NK cells Immunosurveillance increased frequency of cancers in immunodeficient host in about 5% of patient with congenital immunodeficiencies immunosuppressed transplant patient with AIDS most are lymphopmas Cancer cells evades immune system by the following mechanism selective overgrowth of antigen negative variant loss or reduced expression of histocompatibility antigens HLA class I lack costimulation – B7-1 – prevent sensitization render T cell anergic or cause them to undergo apoptosis Immunosuppression oncogenic agents – suppress host immune response tumor or tumor products also may be immunosuppressive TGF-B CLINICAL FEATURES OF NEOPLASIA protooncogenes are named after their viral homologs. (viral oncogenes [v- oncs]) sequences were almost identical to sequences found in the normal Molecular dissection of their genomes revealed the presence of unique transforming sequences not found in the genomes of nontransforming retroviruses.. From this evolved the concept that during evolution, retroviral oncogenes were transduced (captured) by the virus through a chance recombination with the DNA of a (normal) host cell that had been infected by the virus. Insertional mutagenesis - mode of protooncogene activation proviral DNA is always found to be integrated (inserted) near a protooncogene. One consequence of proviral insertion near a protooncogene is to induce a structural change in the cellular gene, thus converting it into a cellular oncogene ( c-onc) . Alternatively, strong retroviral promoters inserted in the vicinity of the protooncogenes lead to dysregulated expression of the cellular gene. Human tumors, which (with rare exceptions) are not caused by infection with retroviruses. Do nonviral tumors contain oncogenic DNA sequences? DNA of spontaneously arising cancers contains oncogenic sequences, or oncogenes. Protooncogenes may become oncogenic by retroviral transduction (v-oncs) or by influences that alter their behavior in situ converting them into cellular oncogenes (c-oncs) Two questions follow 1. what are the functions ofoncogene product? 2. how do the normally “civilized” protooncogenes turn into “enemies” within PROTEIN PRODUCTS OF ONCOGENES Oncogenes encode proteins - oncoproteins, which resemble the normal products of protooncogenes, with the exception that (1) oncoproteins are devoid of important regulatory elements, and (2) their production in the transformed cells does not depend on growth factors or other external signals. Sequence of events that characterize normal cell proliferation Under physiologic conditions, cell proliferation can be readily resolved into the following steps: The binding of a growth factor to its specific receptor on the cell membrane Transient and limited activation of the growth factor receptor, which, in turn, activates several signal-transducing proteins on the inner leaflet of the plasma membrane Transmission of the transduced signal across the cytosol to the nucleus via second messengers Induction and activation of nuclear regulatory factors that initiate DNA transcription Entry and progression of the cell into the cell cycle, resulting ultimately in cell division SELECTED ONCOGENES, THEIR MODE OF ACTIVATION, AND ASSOCIATED HUMAN TUMORS Category Protooncogene Mechanism Associated Human Tumor Growth Factors PDGF-B chain sis Overexpression Astrocytoma Osteosarcoma Fibroblast growth factors hst-1 Overexpression Stomach cancer int-2 Amplification Bladder cancer Breast cancer Melanoma Growth Factor Receptors EGF-receptor family erb-B1 Overexpression SCS of lung erb-B2 Amplification Breast, ovarian, lung, stomach cancers erb-B3 Overexpression Breast cancers fm Point mutation Leukemia ret *Point mutation MEN 2A and B. Familial medullary thyroid CA Rearrangement Sporadic papillary CA of thyroid Proteins Involved in Signal Transduction GTP-binding ras Point mutations A variety of human cancers, including lung, colon, pancreas; many leukemias Nonreceptor tyrosine kinase abl Translocation Chronic myeloid leukemia Acute lymphoblastic leukemia Nuclear Regulatory Proteins Transcriptional activators myc Translocation Burkitt lymphoma N- myc Amplification Neuroblastoma Small cell CA of lung L- myc Amplification Small cell CA of lung Cell Cycle Regulators Cyclins cyclin D Translocation Mantle cell lymphoma Amplification Breast, liver, esophageal cancers Cyclin-dependent kinase CDK4 Amplification or Glioblastoma, point mutation melanoma, sarcoma ACTIVATION OF ONCOGENES Mechanisms by which protooncogenes are transformed into oncogenes. two broad categories of changes: 1. Changes in the structure of the gene, resulting in the synthesis of an abnormal gene product (oncoprotein) having an aberrant function 2. Changes in regulation of gene expression, resulting in enhanced or inappropriate production of the structurally normal growth-promoting protein. Point Mutations. The ras oncogene represents the best example of activation by point mutations – codon 12 reduce the GTPase activity of the ras protein Normal ras protein GTPase activity augmented by GAP mutant ras protein GTPase activity poorly stimulated by GAP Result – mutant ras remains in the active GTP-bound form Large number of human tumor carry ras mutations 90% - pancreatic adenocarcinoma and cholangiocarcinoma 50% - colon, endometrial and thyroid cancers 30% - lung adenocarcinoma and myeloid leukemia In general carcinoma has K-ras while hematopoeitic has N-ras ras mutations are infrequent or even nonexistent in certain other cancers ras mutation are extremely common, but their presence is not essential for carcinogenesis Chromosomal Rearrangements. Two types of chromosomal rearrangements can activate protooncogenes A. Inversions. B. Translocations - much more common – induced overexpression of a protooncogene activate protooncogenes in two ways: 1.In lymphoid tumors, specific translocations result in overexpression of protooncogenes by placing them under the regulatory elements of the immunoglobulin or T-cell receptor loci. Burkitt lymphoma, the most common form of translocation results in the movement of the c- myc- containing segment of chromosome 8 to chromosome 14q band 32 places c- myc close to the immunoglobulin heavy-chain (IgH) gene - coding sequences of the gene remain intact, and the c- myc gene is constitutively expressed at high levels. Mantle cell lymphoma, the cyclin D1 gene on chromosome 11q32 is overexpressed by juxtaposition to the IgH locus on 14q32. Follicular lymphoma, a t(14;18)(q32;q21) translocation causes activation of the bcl-2 gene 2.In many hematopoietic tumors, the translocations allow normally unrelated sequences from two different chromosomes to recombine and form hybrid genes that encode growth-promoting chimeric proteins Philadelphia chromosome oncogene formed by fusion of two separate genes. a reciprocal translocation between chromosomes 9 and 22 relocates a truncated portion of the protooncogene c- abl (from chromosome 9) to the bcr on chromosome 22. hybrid c- abl-bcr gene encodes a chimeric protein Chronic myeloid leukemia - chimeric protein 210 kD Acute lymphoblastic leukemias - chimeric protein 180-kD, abl- bcr fusion protein is formed. SELECTED EXAMPLES OF ONCOGENES ACTIVATED BY TRANSLOCATION Malignancy Translocation Affected Genes CML (9;22)(q34;q11) Ab1 9q34 bcr 22q11 AML and ALL (4;11)(q21;q23) AF4 4q21 MLL 11q23 (6;11)(q27;q23) AF6 6q27 MLL 11q23 Burkitt lymphoma (8;14)(q24;q32) c- myc 8q24 IgH 14q32 Mantle cell lymphoma (11;14)(q13;q32) Cyclin D 11q13 IgH 14q32 Follicular lymphoma (14;18)(q32;q21) IgH 14q32 bcl-2 18q21 T-cell acute lymphoblastic leukemia (8;14)(q24;q11) c- myc 8q24 TCR-alpha 14q11 (10;14)(q24;q11) Hox 11 10q24 TCR-alpha 14q11 Ewing sarcoma (11;22)(q24;q12) FL-1 11q24 EWS 22q12 Melanoma of soft parts (12;22)(q13;q12) ATF-1 12q13 EWS 22q12 . 3. Gene Amplification. Activation of protooncogenes associated with overexpression of their products may result from reduplication and manifold amplification of their DNA sequences may produce several hundred copies of the protooncogene in the tumor cell. detected by molecular hybridization with the appropriate DNA probe produce cytogenetic changes that can be identified microscopically. Two mutually exclusive patterns are seen: 1. double minutes (dms) Multiple small, chromosome-like structures or 2. Homogeneous staining regions (HSRs). assembly of amplified genes into new chromosomes lack a normal banding pattern – homogenous in a G - banded karyotype Cancer-Suppressor Genes apply brakes to cell proliferation. is to regulate cell growth - not to prevent tumor formation PROTEIN PRODUCTS OF TUMOR-SUPPRESSOR GENES . The tumor-suppressor genes seem to encode various components of this growth inhibitory pathway. SELECTED TUMOR-SUPPRESSOR GENES INVOLVED IN HUMAN NEOPLASMS Subcellular Location Gene Function Tumors Associated with Somatic Mutations Tumors Associated with Inherited Mutations Cell surface TGF-beta receptor Growth inhibition Carcinomas of colon Unknown E-cadherin Cell adhesion Carcinoma of stomach, breast Familial gastric cancer Under plasma membrane NF-1 Inhibition of ras signal transduction Schwannomas Neurofibromatosis type I and sarcomas Cytoskeleton NF-2 Unknown Schwannomas and meningiomas Neurofibromatosis type II; acoustic schwannomas and meningiomas Cytosol APC Inhibition of signal transduction Carcinomas of stomach, colon, pancreas; melanoma Familial adenomatous polyposis coli; colon cancer Nucleus Rb Regulation of cell cycle Retinoblastoma; osteosarcoma; carcinomas of breast, colon, lung Retinoblastomas, osteosarcoma p53 Regulation of cell cycle and apoptosis in response to DNA damage Most human cancers Li-Fraumeni syndrome; multiple carcinomas and sarcomas WT-1 Nuclear transcription Wilms tumor Wilms tumor p16(INK4a) Regulation of cell cycle by inhibiting cyclin-dependent kinases Pancreatic, esophageal cancers Malignant melanoma BRCA-1 DNA repair Carcinomas of female breast and ovary BRCA-2 DNA repair Carcinomas of male and female breast Genes That Regulate Apoptosis bcl-2 is a member of a large family of homodimerizing and heterodimerizing proteins, some of which inhibit apoptosis (such as bcl-2 itself and bcl-xL), whereas others (such as bax, bad, and bcl-xS) favor programmed cell death. Apoptosis is the end point of a cascade of molecular events that are initiated by several stimuli and lead ultimately to the activation of proteolytic enzymes responsible for cell death. The bcl-2 family of proteins regulates the activation of these proteolytic enzymes (caspases). The proapoptotic and antiapoptotic members of the bcl-2 family act as a rheostat in regulating programmed cell death. The ratio of death antagonists ( bcl-2, bcl-xL) to agonists ( bax, bcl-xS, bad, bid) determines whether a cell will respond to an apoptotic stimulus Two other cancer-associated genes are also intimately connected with apoptosis: the p53 gene and the protooncogene c- myc. The molecular mechanisms of cell death induced by these two intersect with the bcl-2 pathways Activation of p53, up-regulates bax synthesis and hence counteracts the antiapoptotic action of bcl-2. . c- myc induces apoptosis when cells are driven by c- myc activation, but growth is restricted by the limited availability of growth factors in the milieu. When confronted by such conflicting signals, the cells are programmed to die by up-regulation of p53 and other undefined signals. Overexpression of bcl-2 can rescue cells from c- myc-initiated apoptosis. Thus, it appears that myc and bcl-2 may collaborate in tumorigenesis: c- myc triggers proliferation, and bcl-2 prevents cell death, even if growth factors become limiting Genes That Regulate DNA Repair Humans literally swim in a sea of environmental carcinogens. Although exposure to naturally occurring DNA-damaging agents, such as ionizing radiation, sunlight, and dietary carcinogens, is common, cancer is a relatively rare outcome of such encounters. This happy state of affairs results from the ability of normal cells to repair DNA damage and thus prevent mutations in genes that regulate cell growth and apoptosis. In addition to possible DNA damage from environmental agents, the DNA of normal dividing cells is also susceptible to alterations resulting from errors that occur spontaneously during DNA replication. Such mistakes, if not repaired promptly, can also push the cells along the slippery slope of neoplastic transformation Hereditary non polyposis colon cancer ( HNPCC) syndrome - DNA mismatch repair. When a strand of DNA is replicating, mismatch repair genes act as "spell checkers." Thus, for example, if there is an erroneous pairing of G with T, rather than the normal A with T, the mismatch repair genes correct the defect Cells with such defects in DNA repair are said to have the replication error (RER) phenotype, which can be readily documented by examination of microsatellite sequences in the tumor cell DNA. Microsatellites are tandem repeats of one to six nucleotides scattered throughout the genome. Microsatellite sequences of an individual are fixed for life and are the same in every tissue. With errors in mismatch repair, there are expansions and contractions of these repeats in tumor cells, creating alleles not found in normal cells of the same patient. Such microsatellite instability is a hallmark of defective mismatch repair xeroderma pigmentosum, The basis of this disorder is also defective DNA repair. UV light causes cross-linking of pyrimidine residues, thus preventing normal DNA replication. Such DNA damage is repaired by the nucleotide excision repair system A group of autosomal recessive disorders comprising Bloom syndrome, ataxia telangiectasia, and Fanconi anemia are characterized by hypersensitivity to other DNA-damaging agents, such as ionizing radiation (Bloom syndrome and ataxia telangiectasia), or DNA cross-linking agents, such as nitrogen mustard (Fanconi anemia). Ataxia telangiectasia (AT) have a complex phenotype, characterized by gradual loss of Purkinje cells in the cerebellum, immunodeficiency, acute sensitivity to ionizing radiation, and profound susceptibility to lymphoid malignancies. Telomeres and Cancer with each cell division there is some shortening of specialized structures, called telomeres, at the ends of chromosomes Once the telomeres are shortened beyond a certain point, the loss of telomere function leads to end-to-end chromosome fusion and cell death. . Thus, telomere shortening is believed to be a clock that counts cell divisions. telomere shortening is a tumor-suppressive mechanism telomere shortening is prevented by the sustained function of the enzyme telomerase - telomerase hypothesis telomerase loss is causally associated with loss of replication ability. cancer cells must find a way to prevent telomere shortening. reactivate telomerase.
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