1 tumor viruses for most viruses: genome viral proteins replication lysis progeny virions lytic life...

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Tumor VirusesTumor Viruses

For most viruses:

Genome viral proteins

Replication Lysis Progeny virions

Lytic Life Cycle

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Tumor VirusesTumor VirusesLatent Life Cycle

Virus

Cell

Integration (usually)

Transformation

Virus-specific proteins expressed - No mature virus

Changes in the properties of host cell - TRANSFORMATION

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Tumor VirusesTumor VirusesTransformation:

Loss of growth control

Ability to form tumors - viral genes interfere with control of cell replication

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TRANSFORMATIONBoth 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

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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

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Two Major Classes of Tumor Viruses

DNA Tumor Viruses

DNA viral genome

Host RNA polymerase

Viral mRNA

Viral protein

DNA-dependentDNA polymerase

(Host or viral)

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RNA Tumor VirusesViral RNA genome

Reverse transcriptase (Virus-encoded)

Viral DNA genome (integrated)

DNA-dependent RNA polymerase (Host Host RNA pol II)

Viral genomic RNA

Splicing (Host splicing enzymes)

messenger RNA

viral protein

Virus

Important: Use HOSTRNA polymerase

to make its genome

An enzyme that normally

makes mRNA

IMPORTANT

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DNA Tumor VirusesDNA Tumor VirusesDNA genome

mRNA

protein

virus

Host RNA polymerase II

Host enzymes

OR TRANSFORMATIONIn transformation usually only EARLY functions are expressed

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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

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Papilloma Viruses

• epidermodysplasia verruciformis

wart malignant squamous cell carcinoma

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Epidermodysplasia verruciformis

Papilloma virus

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Papilloma Viruses

urogenital cancer

wart malignant squamous cell carcinoma

Papilloma viruses are found in 91% of women with cervical cancer

Squamous cell carcinoma:LarynxEsophagus All histologically similarLung

10% of human cancers may be HPV-linked

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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

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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

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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

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ONCOGENEA 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

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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

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Hepatitis B Virus

DNA genome

RNA polymerase II

RNA Provirus

Reverse transcriptase

DNA genome

Host enzyme

Viral enzyme

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Hepatitis B continued

• Vast public health problem

• 10% of population in underdeveloped countries are chronic carriers

•Long latency

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Hepatitis B continued

Epidemiology:

Strong correlation between HBV and hepatocellular carcinoma

China: 500,000 - 1 million new cases of hepatocellular carcinoma per year

Taiwan: Relative risk of getting HCC is 217 x risk of non-carriers

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Summary

• Can transform cells or have lytic life cycle

• Often integrate into host genome

• In transformation ONLY early genes are transcribed

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RNA Tumor VirusesRNA Tumor VirusesRNA Genome - Retroviruses

RNA-dependent DNA Polymerase encoded by virus

REVERSE TRANSCRIPTASE

RNA genome


DNA genome

Integrase

Integrates

Host RNA polymerase II

RNA genome

virus

virus

host

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RNA Tumor Viruses

RNA Tumor Viruses

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RNA Tumor VirusesRNA Tumor Viruses

POL: Enzymes


Integrase

Protease

A normal retrovirus has:

3 genes

GAG : internal proteins

ENV: Envelope glycoproteins

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RNA Tumor VirusesRNA Tumor VirusesRNA 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

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RNA Tumor VirusesRNA Tumor VirusesGroups of Retroviruses

• Oncovirinae

Tumor viruses and similar

• Lentiviruses

Long latent period

Progressive chronic disease

Visna HIV

• Spumavirinae

important

important

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• Human T cell lymphotropic virus -2 (HTLV-2)

Hairy cell leukemia

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

• HIV?

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RNA Tumor VirusesRetrovirus Life Cycle

Endocytosis

Fusion of membranes

Release of nucleocapsid to cytoplasm

Nucleus

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RNA Tumor VirusesRNA Tumor VirusesParental RNA

RNA/DNA Hybrid

Linear DNA/DNA duplex

Circular Duplex DNA

Integration Replication (DNA genome in cell)

Transcription Viral RNA genome mRNA protein



Integrase

Host RNA pol II

Host DNA polymerase

Host splicing enzymes

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RNA Tumor VirusesDrawback to this lifestyle

Genomic RNA

DNA

Genomic RNA

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

Host RNA pol II


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primer

Viral genomicRNA


dsDNA

promotor

RNA synthesis initiation site

RNA pol IIRNA synthesis termination

site

Result: New copy of viral RNA is shorter - lacks control sequences

Problem of using RNA pol II to copy a gene

RT

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?


Perhaps virus could integrate downstream of a promotor etc so that the cell provides sequences

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

OR

Virus provides its own promotors etc

BUT not copied!

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RNA Tumor VirusesRNA Tumor VirusesClue: Difference in the two forms

RNA

R U5 GAG POL ENV U3 R

LTR

Repeatregion

Repeatregion

DNA

U3 R U5 GAG POL ENV U3 R U5

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promotor

Viral RNA


R U5 U3 R

U3 R U5 U3 R U5

Long terminal repeats are formed

RNA initiation site RNA termination site

POLIIPOLII

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Retroviruses can have only one promotor

Retroviruses can have only one promotor

LTR LTR

U5

RNA initiation site RNA termination site

Therefore only one long RNA can be made

Therefore mRNA requires processing

Explains why RNA has to be positive sense

POLIIPOLII

Contained in U3

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Rous Sarcoma Virus


Some retroviruses have an extra gene

Some retroviruses have an extra gene

“typical retrovirus”

SRC

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Feline Sarcoma Virus (FSV)

R U5 dGAG FMS dENV U3 R

Avian Myelocytoma Virus (MC29)

R U5 dGAG MYC dENV U3 R

Avian Myeloblastosis Virus

R U5 GAG POL MYB U3 R

Some retroviruses have an oncogene instead of their regular genes

Some retroviruses have an oncogene instead of their regular genes

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Viral Oncogene

V-onc

Cellular Proto-oncogene

C-onc

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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

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RNA Tumor VirusesRNA Tumor VirusesThe 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

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RNA Tumor VirusesRNA Tumor VirusesIn contrast:

Chronically transforming retroviruses

cause tumors inefficiently after prolonged period of time

No oncogene! – How does it cause a tumor?


Avian Leukosis Virus (causes lymphomas)

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Suggest tumor arose from one cell

• Something must be important about this site for transformation

• Crucial event must be rare

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)

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RNA Tumor VirusesRNA Tumor VirusesWhat 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

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Could C-oncs be involved in NON-VIRAL cancers?

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Genes can be assigned to

sites on specific

chromosomes

mos and myc : chromosome 8

fes: chromosome 15fes

mosmyc

myb

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Cancers often result from gene translocations

Cancers often result from gene translocations

Burkitt’s Lymphoma

8:14 translocation

Break in chromosome 14 at q32

Acute myelocytic leukemia7:159:18

11:15:17

myc

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Oncogenesis by rearrangementOncogenesis 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)

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What do oncogenes encode?Proteins that are involved in growth control and

differentiation

Growth factorsGrowth factor receptors

Signal transduction proteinsTranscription factors

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OncogenesOncogenesMutations 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

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Proto-oncogenesProto-oncogenes

Heterozygote Homozygote

Allele 1 Allele 2 Allele 1 Allele 2

Normal Mutant Mutant Mutant

Function gained Function gained

Dominant mutations

Binds under special

circumstances

Mutant always binds

Mutant always binds

Mutant always binds

Always binds Always binds

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Anti-OncogenesAnti-Oncogenes

Rb Gene Mutant Rb Mutant Rb

Rb

Rb

Rb protein

Binds and controls cell cycle No binding - Growth continues

Mutant Rb

Recessive mutations

Function lost

Mutation growth

Heterozygote Homozygote

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Anti-OncogenesAnti-OncogenesRetinoblastoma gene has normal regulatory function in many cells

Involved in

Retinoblastoma

Lung carcinomas

Breast carcinomas

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Anti-OncogenesAnti-OncogenesP53

Inactivated by

• deletion

• point mutation

In a series of colorectal cancers all showed:

• Allele 1: partial or complete deletion

• Allele 2: Point mutation

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DNA Tumor VirusesOncogenes

DNA Tumor VirusesOncogenes

• Adenovirus E1A region 2

• SV 40 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

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Anti-OncogenesAnti-Oncogenes

Rb Gene

RbRb protein

Rb

Stops replication

Rb

Adenovirus E1A

Cell cycle continues

Retinoblastoma

105kD

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Anti-OncogenesAnti-Oncogenesp53

P53 gene P53 gene P53 gene

P53

P53 DNA

Stops replication

Hepatitis C

P53

replication replication

Papilloma proteolysis

P53

Papilloma

1 tumor viruses for most viruses: genome viral proteins replication lysis progeny virions lytic life...

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