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Many DNA and some RNA viruses, especially the retroviruses, can transform normal cells into abnormal cells called tumors (benign or malignant). This process is called viral transformation, and these viruses are oncogenic viruses. Viruses that can either cause tumors in their natural hosts or other species or can transform cells in vitro are considered to have oncogenic potential. Specifically, a tumor is an abnormal growth of cells and is classified as benign or malignant—depending on whether it remains localized or has a tendency to invade or spread by metastasis. Therefore, malignant cells have at least two defects. They fail to respond to controlling signals that normally limit the growth of nonmalignant cells, and they fail to recognize their neighbors and remain in their proper location.
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Several DNA, and some RNA, viruses can transform normal cells into tumors
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When grown in tissue culture in the laboratory, these tumor cells exhibit a series of properties that correlate with the uncontrolled growth potential associated with the tumor in the organism. They have altered cell morphology and fail to grow in the organized patterns found for normal cells. In addition, they grow to a much higher cell density than do normal cells under conditions of unlimited nutrients and can lose contact inhibition and the requirement for growth on a solid substrate; therefore, they appear unable to enter the resting G0 state. Furthermore, they have lower nutritional and serum requirements than normal cells and are able to grow indefinitely in cell culture. These transformed or tumor cells often are used as cell lines for the culture or propagation of viruses in the laboratory.
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Malignant cells fail to respond to signals controlling the growth and location of normal cells
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In addition to the listed properties, viral transformation usually, but not always, endows the cells with the capacity to form a tumor when introduced into the appropriate animal. Although the original use of the term transformation referred to the changes occurring in cells grown in the laboratory, current usage often includes the initial events in the animal that lead to the development of a tumor. In recent years, it has become increasingly clear that some, but not all, of these viruses cause cancers in the host species from which they were isolated.
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Transformation by DNA Human Viruses
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The oncogenic potential of human DNA viruses is summarized in Table 7–4. With the exception of parvoviruses, most DNA virus families have some members capable of causing aberrant cell proliferation under some conditions. For some viruses, transformation or tumor formation has been observed only in species other than their natural host. Apparently, infections of cells from the natural host are so cytocidal that no survivor cells remain to be transformed. In addition, some viruses have been implicated in human tumors without any indication that they can transform cells in culture.
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Some oncogenic viruses cause tumors in species other than their natural hosts
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In nearly all cases that have been characterized, viral transformation is the result of the continual expression of one or more viral genes that are directly responsible for the loss of growth control. Two targets have been identified that appear to be critical for the transforming potential of these viruses. Adenoviruses, papillomaviruses, and simian virus 40 all code for either one or two proteins that interact with the tumor suppressor proteins known as p53 and Rb (for retinoblastoma protein) to block their normal function, which is to exert a tight control over cell-cycle progression. The end result is endless cell cycling and uncontrolled growth.
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Several DNA viruses encode proteins that interfere with cell cycle causing uncontrolled growth and transformation
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In many respects, transformation is analogous to lysogenic conversion and requires that the viral genes be incorporated into the cell as inheritable elements. Incorporation usually involves integration into the chromosome (with a high efficiency for retroviruses and a low efficiency for adeno-, polyo-, papilloma- viruses), although the DNAs of some papillomaviruses and some herpesviruses are found in transformed cells as extrachromosomal plasmids. Unlike some of the temperate bacteriophages that code for the enzymes necessary for integration, papillomaviruses, polyomaviruses, and adenoviruses integrate by nonhomologous recombination using enzymes present in the host cell. The recombination event is, therefore, nonspecific—both with respect to the viral DNA and with respect to the chromosomal locus at which insertion occurs. It follows that for transformation to be successful, the insertional recombination must not disrupt a viral gene required for transformation. In summation, two events appear to be necessary for viral transformation: a persistent association of viral genes with the cell, and the expression of certain viral “transforming” proteins.
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Transformation by DNA viruses is analogous to lysogenic conversion
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Transformation by Retroviruses
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Two features of the replicative cycle of retroviruses are related to the oncogenic potential of this class of viruses known as oncoretroviruses. First, most retroviruses (exception human immunodeficiency virus, HIV) do not kill the host cell, but rather set up a permanent infection with continual virus production. Second, a DNA copy of the RNA genome is integrated into the host cell DNA by a virally encoded integrase (IN); however, unlike bacteriophage λ integration, a linear form of the viral DNA, rather than a circular form, is the substrate for integration. Furthermore, unlike λ, there does not appear to be a specific site in the cell DNA where integration occurs.
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Most animal retroviruses produce virions without causing host cell death
A DNA copy of the retroviral genome is integrated, but not at a specific site
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Retroviruses are known to transform cells by three different mechanisms. First, many animal retroviruses have acquired transforming genes called oncogenes. These retroviruses require a helper virus as the insertion of the oncogene replaces a viral gene. More than 30 such oncogenes have now been found since the original oncogene was identified in Rous sarcoma virus (called v-src, where the v stands for viral). Because normal cells possess homologs of these genes called protooncogenes (eg, c-src, where c stands for cellular), it is generally thought that viral oncogenes originated from host DNA. It is possible they were picked up by “copy choice” recombination involving packaged cellular mRNAs, as previously described. Because these transforming viruses carry cellular genes, they are sometimes referred to as transducing retroviruses. Most of the viral oncogenes have suffered one or more mutations that make them different from the cellular protooncogenes. These changes presumably alter the protein products such that they cause transformation. Although the mechanisms of oncogenesis are not completely understood, it appears that transformation results from inappropriate production of an abnormal protein that interferes with normal signaling processes within the cell. Uncontrolled cell proliferation is the result. Because tumor formation in vitro by retroviruses carrying an oncogene is efficient and rapid, these viruses are often referred to as acute transforming viruses. Although common in some animal species, this mechanism has not yet been recognized as a cause of any human cancers.
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Retroviruses may carry transforming oncogenes
Oncogenes encode a protein that interferes with cell signaling
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The second mechanism is called insertional mutagenesis and is not dependent on continued production of a viral gene product. Instead, the presence of the viral promoter or enhancer is sufficient to cause the inappropriate expression of a cellular gene residing in the immediate vicinity of the integrated provirus. This mechanism was first recognized in avian B-cell lymphomas caused by an avian leukosis virus, a disease characterized by a very long latent period. Tumor cells from different individuals were found to have a copy of the provirus integrated at the same place in the cellular DNA. The site of the provirus insertion was found to be next to a cellular protooncogene called c-myc. The myc gene had previously been identified as a viral oncogene called v-myc. In this case, transformation occurs not because the c-myc gene is altered by mutation, but because the viral promoter adjacent to the gene turns on its expression continuously and the gene product is overproduced. The disease has a long latent period because, although the birds are viremic from early life, the probability of an integration occurring next to the c-myc gene is very low. After such an integration event does occur, however, cell proliferation is rapid and a tumor develops. No human tumors are known for certain to result from insertional mutagenesis caused by a retrovirus; however, human cancers are known in which a chromosome translocation has placed an active cellular promoter next to a cellular protooncogene (Burkitt lymphoma and chronic myelogenous leukemia). In addition, a few retroviral gene therapy trials were stopped because of the induction of leukemia likely due to retroviral insertion near a protooncogene.
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Insertional mutagenesis causes inappropriate expression of a protooncogene adjacent to integrated viral genome
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The third mechanism was revealed by the discovery of the first human retrovirus. The virus, HTLV-I, is the causative agent of adult T-cell leukemia. HTLV-I sequences are found integrated in the DNA of the leukemic cells, and all tumor cells from a particular individual have the proviral DNA in the same location. This observation indicates that the tumor is a clone derived from a single cell; however, the sites of integration in tumors from different individuals are different. Thus, HTLV-I does not cause malignancy by promoter insertion near a particular cellular gene. Instead, the virus has a regulatory gene called tax that codes for Tax protein that acts in trans (ie, on other genes in the same cell) to not only promote maximal transcription of the proviral DNA, but also to transcriptionally activate an array of cellular genes. The resulting cellular proteins cooperate to cause uncontrolled cell proliferation. The tax gene is therefore different from the oncogenes of the acute transforming retroviruses in that it is a viral gene rather than a gene derived from a cellular proto-oncogene. HTLV-I is commonly described as a transactivating retrovirus.
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Human T-cell leukemia is caused by transactivating factor (Tax) encoded in integrated HTLV-I
Transactivating factor (Tax) turns on cellular genes, causing cell proliferation
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Transformation by Other RNA Viruses
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Hepatitis C virus (HCV) causes chronic infection in more than 80% of infected people. The chronicity in HCV infection increases the risk of cirrhosis of liver and hepatocellular carcinoma (HCC). HCC occurs on average approximately 20 to 30 years after chronic infection but alcohol and drug abuse can accelerate this process. It is thought that the constant inflammation and regeneration of hepatocytes leads to the eventual induction of the tumor and is, therefore, considered indirect oncogenesis. However, some studies suggest that HCV nonstructural proteins, NS3 and NS5A, and the HCV core protein may be involved in transformation.