Download presentation
Presentation is loading. Please wait.
1
Replication Lysis Progeny virions
Tumor Viruses For most viruses: Genome viral proteins Replication Lysis Progeny virions In previous lectures we have looked at the replication of viruses. How the genome gives rise to viral proteins Lytic Life Cycle
2
Tumor Viruses Latent Life Cycle
Cell Integration (usually) Transformation Virus-specific proteins expressed - No mature virus Changes in the properties of host cell - TRANSFORMATION Some viruses may lay latent in the cell for a very long time (and then may possibly lyse the cell). In this case only early functions of these viruses may be expressed. This results in virus proteins being expressed but no mature virus particles. The expression of these early virus proteins may nevertheless alter the properties of the infected cell. THE CELL IS SAID TO BE VIRALLY TRANSFORMED.
3
Tumor Viruses Transformation:
Loss of growth control Ability to form tumors - viral genes interfere with control of cell replication Viral genes interfere with cell replication control mechanism. NOTE: The early functions of DNA viruses do not make structural proteins but control cellular and viral genome replication.
4
TRANSFORMATION VIRAL TRANSFORMATION
Both DNA and RNA tumor viruses can transform cells Integration occurs (usually) Similar mechanisms VIRAL TRANSFORMATION The changes in the biological functions of a cell that result from REGULATION of the cell’s metabolism by viral genes and that confer on the infected cell certain properties characteristic of NEOPLASIA These changes often result from the integration of the viral genome into the host cell DNA
5
Among the many altered properties of the TRANSFORMED CELL are:
TRANSFORMATION Among the many altered properties of the TRANSFORMED CELL are: Loss of growth control (loss of contact inhibition in cultured cells) Tumor formation Mobility Reduced adhesion Transformed cells frequently exhibit chromosomal aberrations
6
Two Major Classes of Tumor Viruses
DNA Tumor Viruses DNA viral genome Host RNA polymerase Viral mRNA Viral protein DNA-dependent DNA polymerase (Host or viral)
7
RNA Tumor Viruses Reverse transcriptase (Virus-encoded)
Viral RNA genome Reverse transcriptase (Virus-encoded) Viral DNA genome (integrated) DNA-dependent RNA polymerase (Host RNA pol II) Viral genomic RNA Splicing (Host splicing enzymes) messenger RNA viral protein Virus IMPORTANT Important: Use HOST RNA polymerase to make its genome An enzyme that normally makes mRNA
8
DNA Tumor Viruses DNA genome mRNA protein virus
Host RNA polymerase II Host enzymes Permissive cells: Replication, lysis and death Non-permissive cells: transformation. Usually DNA is integrated. Early functions only are expressed. Control information, rather than structural proteins OR TRANSFORMATION In transformation usually only EARLY functions are expressed
9
DNA Tumor Viruses In Human Cancer
Papilloma Viruses cause natural cancers in animals cause benign warts ubiquitous epitheliotropic - most human tumors are malignancies of epithelial cells
10
DNA Tumor Viruses In Human Cancer
Papilloma Viruses epidermodysplasia verruciformis wart malignant squamous cell carcinoma Squamous cell carcinomas of larynx, espophagus and lung are all histologically similar to cervical carcinomas. HPV also linked to penile and vulval cancers 10% of all human cancers and 16% of all female cancers may be HPV-linked
11
DNA Tumor Viruses In Human Cancer
Epidermodysplasia verruciformis Papilloma virus
12
DNA Tumor Viruses In Human Cancer
Papilloma Viruses urogenital cancer wart malignant squamous cell carcinoma Papilloma viruses are found in 91% of women with cervical cancer Squamous cell carcinomas of larynx, espophagus and lung are all histologically similar to cervical carcinomas. HPV also linked to penile and vulval cancers 10% of all human cancers and 16% of all female cancers may be HPV-linked Squamous cell carcinoma: Larynx Esophagus All histologically similar Lung 10% of human cancers may be HPV-linked
13
DNA Tumor Viruses In Human Cancer
Papilloma Viruses 51 types identified - most common are types 6 and 11 most cervical, vulvar and penile cancers are ASSOCIATED with types 16 and 18 (70% of penile cancers) EPIDEMIOLOGIAL STUDIES BUT: HPV 16 and HPV 18 do transform human keratinocytes No final proof that these viruses cause cancer as Koch’s postulates cannot be fulfilled. Papilloma viruses cannot be grown in culture It appears that there is free plasmid in the cell rather than integration
14
DNA Tumor Viruses In Human Cancer
Polyoma Viruses Simian virus 40 - juvenile hamster sarcomas, transformation Polyoma - mouse leukemia, in vitro transformation Human polyomas (JC and BK) - monkey sarcoma, transformation PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY Polyoma virus transforms cells when the genome is incomplete Early functions are necessary SV40 was in early batches of polio vaccine Normally these viruses cause lytic infection and transform when they are incomplete
15
DNA Tumor Viruses In Human Cancer
Adenoviruses Highly oncogenic in animals Only part of virus integrated Always the same part Early functions E1A region: 2 T antigens E1B region: 1 T antigen E1A and E1B = Oncogenes Adenovirus can be involved in RETINOBLASTOMA and maybe in some other rare cancers
16
DNA Tumor Viruses In Human Cancer
ONCOGENE A gene that codes for a protein that potentially can transform a normal cell into a malignant cell An oncogene may be transmitted by a virus in which case it is known as a VIRAL ONCOGENE v-onc
17
DNA Tumor Viruses In Human Cancer
Herpes Viruses Considerable evidence for role in human cancer Some very tumorigenic in animals Viral DNA found in small proportion of tumor cells: “hit and run” Epstein-Barr Virus Burkitt’s Lymphoma Nasopharyngeal cancer Infectious mononucleosis Transforms human B-lymphocytes in vitro Most people have antibodies against EBV Why some populations get mononucleosis while others get tumors in not known Causes lymphoma in marmosets
18
DNA Tumor Viruses In Human Cancer
Hepatitis B Virus DNA genome RNA polymerase II RNA Provirus Reverse transcriptase Host enzyme Viral enzyme
19
DNA Tumor Viruses In Human Cancer Hepatitis B continued
Vast public health problem 10% of population in underdeveloped countries are chronic carriers Latency up to 30 years Long latency
20
DNA Tumor Viruses In Human Cancer Hepatitis B continued
Epidemiology: Strong correlation between HBV and hepatocellular carcinoma China: 500, million new cases of hepatocellular carcinoma per year Taiwan: Relative risk of getting HCC is 217 x risk of non-carriers
21
DNA Tumor Viruses In Human Cancer
Summary Can transform cells or have lytic life cycle Often integrate into host genome In transformation ONLY early genes are transcribed
22
RNA Tumor Viruses RNA Genome - Retroviruses
RNA-dependent DNA Polymerase encoded by virus REVERSE TRANSCRIPTASE RNA genome Reverse transcriptase DNA genome Integrase Integrates Host RNA polymerase II virus Reverse transcriptase is not a capacity possessed by normal eucaryotic cells Many features are unique to retroviruses Very unusual mode of replication which gives them the potential to transform the cell But when it comes to how these viruses cause neoplasia, they may be very similar to DNA viruses Rous sarcoma virus in chickens was the first retrovirus to be discovered. It causes an aggressive acute cancer in chickens host
23
RNA Tumor Viruses
24
RNA Tumor Viruses A normal retrovirus has: 3 genes
GAG : internal proteins ENV: Envelope glycoproteins POL: Enzymes Reverse transcriptase Integrase Protease
25
RNA Tumor Viruses RNA is: Diploid Capped and polyadenylated
Positive sense (same as mRNA) Viral RNA cannot be read as mRNA New mRNA must be made Virus must make negative sense DNA before proteins are made Therefore virus must carry REVERSE TRANSCRIPTASE into the cell Even though the virus RNA is same sense (positive) as mRNA, it cannot be translated directly as it is encapsulated by proteins. Thus it must be copied via a negative sense nucleic cid (in this case via DNA). Virus contains about 10 copies of reverse transcriptase. NOTE: Both copied of single strand RNA are identical
26
RNA Tumor Viruses
27
Groups of Retroviruses
RNA Tumor Viruses Groups of Retroviruses Oncovirinae Tumor viruses and similar Lentiviruses Long latent period Progressive chronic disease Visna HIV Spumavirinae important important
28
RNA Tumor Viruses Retroviruses known to cause human cancer
Human T cell lymphotropic virus -1 (HTLV-1) Adult T cell leukemia, Sezary T-cell leukemia Africa, Caribbean, Some Japanese Islands Human T cell lymphotropic virus -2 (HTLV-2) Hairy cell leukemia HIV?
29
Release of nucleocapsid to cytoplasm
RNA Tumor Viruses Retrovirus Life Cycle Endocytosis Fusion of membranes Release of nucleocapsid to cytoplasm Nucleus
30
Reverse transcriptase Reverse transcriptase
RNA Tumor Viruses Parental RNA RNA/DNA Hybrid Linear DNA/DNA duplex Circular Duplex DNA Integration Replication (DNA genome in cell) Transcription Viral RNA genome mRNA protein Reverse transcriptase Reverse transcriptase When integrated viral genes may or may not be expressed If expressed, all are expressed at least in the simpler retroviruses (why we shall see later) Whether host polymerase makes mRNA or genomic RNA depends on the processing by the host cell splicing enzymes Nucleocapsid assembly is in the cytoplasm and reverse transcriptase is packed into the virus Virus buds through the plasma membrane where it picks up the viral glycoprotein Maturation occurs in the budded virus Integrase Host DNA polymerase Host splicing enzymes Host RNA pol II
31
RNA Tumor Viruses Host RNA pol II Drawback to this lifestyle
Genomic RNA DNA Pol II is a host enzyme that, in the uninfected cell, makes mRNA When making mRNA, pol II does not copy entire gene to RNA Reverse transcriptase Host RNA pol II
32
Problem of using RNA pol II to copy a gene
Viral genomicRNA RT primer Reverse transcriptase dsDNA promotor RNA synthesis initiation site To make an RNA using RNA pol II, sequences other than those to be transcribed are required. Thus the DNA “gene” is bigger than the mRNA that is transcribed from it. These extra sequences can be upstream or downstream from the transcribed portion. They include: promotors, enhancers, termination sequences. Thus these “control” sequences will be lost on reconversion of the viral genome to RNA. These sequences are necessary for transcription of mRNA but not the translation of mRNA RNA pol II RNA synthesis termination site Result: New copy of viral RNA is shorter - lacks control sequences
33
? RNA Tumor Viruses RNA polymerase II will not copy
Upstream sequences from transcription initiation site Promotors / Enhancers Down stream sequences from transcription termination site Enhancers / Poly A site / termination site ? Perhaps virus could integrate downstream of a promotor etc so that the cell provides sequences In fact, virus does provide its own promotors and enhancers. The clue to how it does this is in the differences in structure between the RNA and DNA forms OR Virus provides its own promotors etc BUT not copied!
34
Clue: Difference in the two forms
RNA Tumor Viruses Repeat region Repeat region Clue: Difference in the two forms RNA R U5 GAG POL ENV U R DNA U R U GAG POL ENV U3 R U5 LTR
35
Reverse transcriptase Long terminal repeats are formed
R U5 Viral RNA U3 R Reverse transcriptase U R U5 U R U5 Long terminal repeats are formed Enhancers may upstream or downstream. LTRs have promotors and enhancers promotor POLII POLII RNA initiation site RNA termination site
36
Retroviruses can have only one promotor
Contained in U3 LTR LTR POLII POLII RNA termination site RNA initiation site Note: Reverse transcriptase needs a PRIMER as does any other DNA polymerase. Prolyl tRNA is packaged in virus. This is necessary because reverse transcription starts in nucleocapsid in cytoplasm Therefore only one long RNA can be made Therefore mRNA requires processing Explains why RNA has to be positive sense U5
37
Some retroviruses have an extra gene
“typical retrovirus” R U GAG POL ENV U R A normal retrovirus has three genes, GAG, OPL, ENV – only these are necessary for the virus to replicate to more virus. Many, however, have another gene that allows them to transform the cell. NOTE: This extra gene is NOT necessary for a productive infection. C.f. DNA tumor viruses in which the oncogene is necessary for BOTH replication and transformation First oncogene to be discovered was the src gene of RSV. It is an extra gene at the 3’ end of the viral RNA Rous Sarcoma Virus R U GAG POL ENV U3 R SRC
38
Some retroviruses have an oncogene instead of their regular genes
Avian Myeloblastosis Virus R U GAG POL MYB U R Feline Sarcoma Virus (FSV) These viruses cannot make all of their proteins and so need a co-infecting “helper” virus Viral oncogenes have three letter names e.g. src, myb R U5 dGAG FMS dENV U R Avian Myelocytoma Virus (MC29) R U5 dGAG MYC dENV U R
39
Cellular Proto-oncogene
RNA Tumor Viruses Viral Oncogene V-onc Retroviruses that cause acute transformation (rapid onset of neoplasia) such as RSV have an extra gene. These neoplasms are usually only seen in laboratory situations. The viral oncogene causes cells to be released from their normal growth controls. These extensively characterized genes are NOT UNIQUE to retroviruses. cDNA probes against viral oncogenes show similar homologues in NORMAL eucaryotic cells. These cellular homologues are not identical to the viral oncogene and it seems that the virus has picked up a cellular gene during its evolution. The discovery of normal homologues of viral oncogenes was a great step forward in the cell biology of cancer. The cellular proto-oncogenes are a family of genes that may underlie much of carcinogenesis. BUT a cellular oncogene does not cause cancer normally. Why does it do so in a virus? Cellular Proto-oncogene C-onc
40
RNA Tumor Viruses Proto-oncogene
A cellular (host) gene that is homologous with a similar gene that is found in a transforming virus A cellular oncogene can only induce transformation after mutation some other change in the cell’s genome Remember: A virus has only one end: to reproduce. This means that the genome and proteins have to be made in large numbers. So many more copies of the v-onc mRNA are made than the c-onc RNA. This over expression may be the basis of the transformation that is seen
41
These viruses cause rapid cancer in animals in the laboratory
RNA Tumor Viruses The discovery of the acutely transforming retroviruses that contain v-oncs explains how cancers may arise as a result of infection These viruses cause rapid cancer in animals in the laboratory
42
RNA Tumor Viruses No oncogene! – How does it cause a tumor?
In contrast: Chronically transforming retroviruses cause tumors inefficiently after prolonged period of time R U GAG POL ENV U R Avian Leukosis Virus (causes lymphomas) No oncogene! – How does it cause a tumor?
43
RNA Tumor Viruses ALV can integrate into the host cell genome at MANY locations but in tumor it is always at the SAME site (or restricted number of sites) Suggest tumor arose from one cell Something must be important about this site for transformation Crucial event must be rare
44
What is special about this site?
RNA Tumor Viruses What is special about this site? Myelocytoma tumors from several birds all have the oncogene close to this site It is close to C-myc! Oncogenesis by promotor insertion The virus is always at the same site in ALV-induced tumors. VERY SUGGESTIVE! This insertion next to a cell oncogene is similar to taking in the virus’s own oncogene. But why should c-onc be switched on as indeed it is in the tumor?
45
Could C-oncs be involved in NON-VIRAL cancers?
RNA Tumor Viruses Could C-oncs be involved in NON-VIRAL cancers?
46
RNA Tumor Viruses Genes can be assigned to sites on specific chromosomes fes mos myc myb We can map genes to precise locations on chromosomes mos and myc : chromosome 8 fes: chromosome 15
47
Cancers often result from gene translocations
Burkitt’s Lymphoma 8:14 translocation Break in chromosome 14 at q32 myc Could the break and exchange of parts of chromosomes bring the c-onc under the control of a very active cell promotor? In Burkitt’s lymphoma there is a 8:14 translocation. Myc is at the break site on chromosome 8. What does it come next to on chromosome 14? Acute myelocytic leukemia 7:15 9:18 11:15:17
48
Oncogenesis by rearrangement
Tumor c-onc new promotor Burkitt’s lymphoma myc (8) Ig heavy (8 to 14) Ig light (8 to 2) T cell chronic lymphocytic myc T cell receptor (8 to 14) leukemia B-cell chronic lymphocytic bcl-1 Ig heavy (11 to 14) leukemia bcl-2 Ig heavy (18 to 14) T cell chronic lymphocytic tcl-1 T cell receptor leukemia (14 inversion) Thus in Burkitt’s lymphoma, the myc gene is brought under the control of a Ig promotor which is very active in this lymphocyte. Thus this is similar to putting the cellular oncogene under the control of the very active promotor in the viral LTR Proof that deregulated c-myc may be involved in tumor formation comes from transgenic mice. If a gene consisting of c-myc linked to the Ig promotor is integrated into the chromosomes of these mice there is a high frequency of lymphomas.
49
RNA Tumor Viruses What do oncogenes encode?
Proteins that are involved in growth control and differentiation Growth factors Growth factor receptors Signal transduction proteins Transcription factors Only a few genes in the cell behave as c-oncs. They are expressed in normal cells at some time in the cell’s life. They perform a vital function. They are involved, as might be expected, in growth control. There are now about 40 of these c-oncs. A surprisingly small number that fall into several groups such as growth factors, signal transduction proteins or transcription factors
50
Oncogenes Anti-Oncogenes
Mutations in a proto-oncogene are dominant “gain of function” mutations However other oncogenic genes show recessive mutations Anti-Oncogenes Loss of function mutations Retinoblastoma p53 So far we have seen that viruses can cause cancer but it must be admitted that most cancers do not arise as a result of the mechanisms that have so far been described.
51
Binds under special circumstances
Proto-oncogenes Dominant mutations Heterozygote Homozygote Allele Allele Allele Allele 2 Normal Mutant Mutant Mutant Normally, the unmutated protein binds only under special circumstances such as when a growth signal is received at the cell plasma membrane. In the heterozygote, the mutant oncogene protein (e.g. myc protein) can bind to the DNA (for example) in the absence of a signal and cause growth. Thus the mutation is dominant over the wild type as there will be mutant protein in the heterozygote that can always signal growth Binds under special circumstances Mutant always binds Mutant always binds Mutant always binds Always binds Function gained Function gained
52
Binds and controls cell cycle No binding - Growth continues
Anti-Oncogenes Recessive mutations Mutation growth Rb Gene Mutant Rb Mutant Rb Mutant Rb Rb Rb protein Instead of being like myc which, when expressed, promotes cell division, Rb protein controls cell growth so that it is shut down. Thus when one gene is mutated so that it cannot function, there is still the other gene in the hereozygote that can produce Rb protein to control growth. Only in the homozygous mutant is there no functional Rb protein and so growth is no longer controlled. Heterozygote Homozygote Function lost Rb Binds and controls cell cycle No binding - Growth continues
53
Retinoblastoma gene has normal regulatory function in many cells
Anti-Oncogenes Retinoblastoma gene has normal regulatory function in many cells Involved in Retinoblastoma Lung carcinomas Breast carcinomas
54
Anti-Oncogenes P53 Inactivated by deletion point mutation
In a series of colorectal cancers all showed: Allele 1: partial or complete deletion Allele 2: Point mutation What has this got to do with tumor viruses? The discovery of anti-oncogenes led to the elucidation of how DNA tumor viruses cause tumors.
55
DNA Tumor Viruses Oncogenes
Adenovirus E1A region 2 SV Large T Polyoma Large T BK virus Large T Lymphotropic virus Large T Human papilloma Virus-16 E7 All have a sequence in common Mutations in this region abolish transformation capacity The oncogenes of DNA viruses such as polyoma or adenovirus have been identified to early function genes and all of these seem to have characteristics in common, indeed, they have some sequence similarities and mutations in this common region abolish tumorigenicity
56
Anti-Oncogenes Retinoblastoma Adenovirus E1A Rb Gene Rb protein Rb
105kD It is found that in adenovirus cells, the virus E1A gene makes a protein that complexes with a 105kD cellular protein. This turned out to be Rb protein. Binding Rb protein so that it cannot control growth is the same as mutating both copies of the gene for the Rb protein and so the cell continues to replicate Rb Rb Cell cycle continues Stops replication
57
Papilloma proteolysis
Anti-Oncogenes p53 P53 gene P53 gene P53 gene Hepatitis C Papilloma P53 P53 P53 A similar thing happens to p53 protein in hepatitis C-infected cells and p53 is bound so that it becomes inactive. Again, lack of p53 results in loss of growth control. In papilloma-infected cells, things happen a little differently. In this case the virus makes a protease that destroys p53 protein. Thus, our knowledge of how RNA tumor viruses cause tumors led to the discovery of viral oncogenes. This led to the discovery of cellular oncogenes which in turn led to anti-oncogenes. The discovery of anti-oncogenes showed how DNA tumor viruses cause tumors. Papilloma proteolysis P53 DNA Stops replication replication replication
Similar presentations
