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ZOO 405, Week 5ZOO405 by Rania Baleela is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License

This week

• Cancer can be caused by viruses• Oncogenesis• Anti-oncogenesis

Cell cycle• M- mitosis• G1 - cells grow• S - DNA synthesis• G2 - growth and preparation for mitosis

• G1/S decision point for going to dividing state

• Problem for DNA viruses that need S phase machinery.

Cell cycle control proteins

• Activation of cell cycle progression => cyclins, cyclin dependent protein kinases (Cdks), Cdk inhibitors

• Inhibitors of cell cycle progression => tumor suppressors

Cancer

Is the uncontrolled growth of

abnormal cells in the body.

Viruses can give rise to a tumor by

• Bring an oncogene into the cell = V-onc

Or • Can take control of a cellular proto-

oncogene = proto/C-onc

Viral transformation of cells

What is transformation?

"the introduction of inheritable changes in a cell that causes changes in the growth phenotype

and immortalisation"

Viral transformationThe 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

Viruses causes transformation via:• Loss of growth control

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

Schematic comparison of 2 types of interaction between a papovavirus & a host cell

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most viruses

Changes in the properties of host cell = TRANSFORMATION

May lead a latent Life Cycle

Transformation

• Both DNA & RNA tumor viruses can transform cells

• Integration occurs (usually)

• Similar mechanisms

• Means of oncogenesis:1. Loss of anchorage dependence2. Loss of contact inhibition (foci)3. Decreased requirements for growth factors

Some properties of the TRANSFORMED CELL

• Loss of growth control

• Tumor formation

•  Mobility

• Reduced adhesion

• Transformed cells frequently exhibit chromosomal aberrations

Receptors of viruses

Oncogenes

“A gene that is capable of transforming a normal cell into a cancerous cell”

• Oncogenes result from:1.  The mutation of normal genes (proto-

oncogenes).2. As an oncogenic viruses. 

OncogenesViruses are a contributing factor in

~20% of all human cancers.

• Most oncogenes (onc) code for proteins with growth promoting properties.

• Expression can lead to uncontrolled proliferation of the infected cells

Proto-oncogenes Proto-oncogenes

• A cellular oncogene can only induce transformation after

1.  mutation

2.  some other change in the cell’s genome

A cellular (host/ or C-onc/ or proto-onc) gene is a gene that is homologous with a similar gene that is found in a transforming virus

HISTORYHuman Cancer Viruses

Oncogenic viruses may be RNA or DNA

• 20% of human cancers believed to be of viral origin

• These include:–Cervical cancer–Burkitt’s lymphoma–Hepatocarcinoma–Kaposi’s sarcoma

• Virus is not only factor

• 1964: Epstein-Barr virus=> 95% of the world population,  some carriers may develop Burkitt’s lymphoma and nasopharyngeal carcinoma.

• Mid 1960s: isolation of HBV=> hepatocellular carcinomas.• 1980: HTLV1=> which cancer?• 1984–86: viruses HPV16 & HPV18 => ~ 70% of cervical 

cancers.•  1987: HCV => a major contributor to hepatocellular 

carcinoma worldwide.• 1994: Kaposi sarcoma-associated virus HHV8 associated 

with AIDS=> skin cancer.

Age-standardized death from lymphomas and multiple myeloma per 100,000 inhabitants in 2004 (WHO, 2009)

www.wikepida.org

Examples of retroviral oncogenes

Virus OncogeneRous sarcoma virus  srcSimian sarcoma virus sis

Avian erythroblastosis virus erbA or erbBKirsten murine sarcoma virus krasMoloney murine sarcoma virus mosMC29 avian myelocytoma virus myc

Retroviruses

May be divided into 2 groups according to their

ability to induce tumors:

1. Acutely transforming retroviruses;

2. Slowly transforming retroviruses.

1. Acutely transforming retroviruses

(e.g. Avian, murine & feline sarcoma)

• cause tumors in 1 to a few weeks,• highly carcinogenic,• carry oncogenes,• transform cells in vitro,• nearly all are defective.

2. Slowly transforming retroviruses

(e.g. Avian & murine leukemia, HTLV1)

• cause tumors in 6-12 months

•less carcinogenic• do not carry oncogenes• no visible effects on cells in culture• most are replication competent• some are endogenous

Structure: normal vs. oncogenic

Oncogenic genes

• Rous sarcoma virus (RSV) has managed to retain its whole genome of gag/pol/env + an oncogene (src).

• In sharp contrast to RSV, many retroviruses have lost part of their genome to accommodate an oncogene .

Most retroviruses have an oncogen instead of their

regular genes

V-onc genes - transducing 

• Virus LTR is a strong promotor• V-onc is altered form of c-onc• rapid onset, high efficiency 

tumorigenesis (acute transforming)

time

% transformed

Consequences of losing genome to accommodate oncogenes

1) The protein encoded by the oncogene is often part of a fusion protein with other virally-encoded amino acids attached

2) Virus is in trouble as it cannot make all of itself. To replicate and bud from the host cell needs products of another virus, that is a helper virus.

“Hit and run” mechanism

• Virus thought to cause mutation in cell genes and then virus is no longer needed

• Tumors may start with virus but leave no evidence of infection

How are cells transformed?

• Cell cycle control changes due to viral genes that – Interact directly with the proteins in the cycle• Bind to and inhibit or degrade

– Interfere with expression of host cell cycle control genes

HUMAN RETROVIRUSES & CANCER

4 human retroviruses identified

• Discovered in the 1980's after it became possible to culture T-cells in vitro. 

• All infect CD4 bearing cells. 1. HTLV 12. 2. HTLV 2 3. HIV-1/AIDS (Europe & America)  4. HIV-2/AIDS (Africa)

RNA Tumor Viruses

RNA Tumor Viruses

A Slow transforming retrovirus: How to cause cancer

A Slow transforming retrovirus: How to cause cancer

It cause tumors inefficiently after prolonged period of time

Avian Leukosis Virus (ALV) => causes lymphomas

      R       U5      GAG        POL        ENV    U3       R

ALV carcinogenesisALV carcinogenesis

• 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

What is special about this site?What is special about this site?

• All myelocytoma tumours from several birds have the oncogene close to the site

C-myc!Oncogenesis by promotor insertion

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

Genes can be assigned to sites on specific chromosomes

mos and myc : chromosome 8

fes: chromosome 15

fes

mosmyc

myb

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 leukemia7:159:1811:15:17

myc

DNA TUMOR VIRUSES

DNA tumor virusesDNA tumor viruses

• Can transform cells or have lytic life cycle

• Often integrate into host genome

• In transformation ONLY early genes are transcribed

IntegrationIntegration• Virus binds to the cellular receptor. • Internalization occurs via receptor-mediated 

endocytosis followed by release from the endosome. 

• After endosomal release, the viral capsid disassembles then journeys to the nuclear pore.

• The viral DNA does not integrate into the host genome but remains in an episomal state

DNA oncogenic viruses • Human papilloma virus (HPV) interferes with tumor 

suppressor proteins (e.g. p53), forcing the cell into the S phase

• HHV8.• Epstein-Barr virus (EBV or HHV-4) => associated with 

4 types of cancers.

1. Papilloma viruses (HPV)1. Papilloma viruses (HPV)

•Cause natural cancers in animals

• Cause benign warts

•Seeming  to be in all places (i.e. ubiquitous),

• Epitheliotropic - most human tumors are malignancies of epithelial cells 

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

Papilloma viruses51 types identified (types 6 & 11 are common)

Papilloma viruses51 types identified (types 6 & 11 are common)

Disease HPV type

Common warts (lump) 2, 7

Plantar warts 1, 2, 4, 63

Flat warts 3, 10

Anogenital warts 6, 11, 42, 44 and others

Genital cancers•Highest risk: 16, 18, 31, 45•Other high-risk: 33, 35, 39, 51, 52, 56, 58, 59•Probably high-risk:26, 53, 66, 68, 73, 82

Epidermodysplasia verruciformis more than 15 types

Focal epithelial hyperplasia (oral) 13, 32

Oral papillomas 6, 7, 11, 16, 32

Oropharyngeal cancer 16

Laryngeal papillomatosis 6,11

Genome organization of HPV type 16 (cause cervical cancer): E1-E7 early genes, L1-L2 late genes: capsid

How does it cause CA?How does it cause CA?• The viral produced E6/E7 (onc) proteins inactivate the 

tumour suppressor proteins p53 (E6) & pRb (E7).

•  The viral oncogenes E6 & E7 are thought to modify the cell cycle to retain a state favourable to the amplification of viral genome replication and consequent late gene expression.

•  E6 associates with host E6 associated protein leading to the degradation of p53.

• E7 acts as the primary transforming protein. E7 competes for retinoblastoma protein (pRb) binding thus pushes the cell cycle forward.

2. Polyoma Viruses2. Polyoma Viruses

• Simian virus 40 - juvenile hamster sarcomas, transformation 

•  Polyoma - mouse leukemia, in vitro transformation

•  Human polyomas - infects the respiratory system, kidneys, or brain

dsDNA, ~5kb, circular genome, 40-50nm in diameter, naked icosahedral, normally cause lytic infection BUT transform when they are incomplete

3. Adenoviruses(dsDNA, linear genome, 36 to 38 kb, 90–100 nm in size, naked icosahedral)

3. Adenoviruses(dsDNA, linear genome, 36 to 38 kb, 90–100 nm in size, naked icosahedral)

Adenovirus can be involved in RETINOBLASTOMA and may be in some other rare cancers

• Highly oncogenic in animals• 47 distinct serotypes and 93 varieties• In human they infect the ocular, respiratory or GI epithelium

• Adenoviral infection is highly complex. 

OncogenesisOncogenesis

• Subgroup A (e.g. adenovirus 12 (Ad12)) induce tumours with high frequency and short latency,

• Subgroup B (such as Ad3 and Ad7) are weakly oncogenic. 

• Subgroup C (e.g. Ad2 & Ad5), D, E and F are non-oncogenic. 

• All human adenoviruses studied can transform rodent cells in culture, however, only cells transformed by viruses of subgroup A and B are oncogenic in newborn rodents.

Schematic diagram of the linear adenovirus genome (E=Early genes, L= Late genes).

Oncogenes

4. Herpes viruses (Epstein-Barr Virus)Linear dsDNA, encode 100-200 genes, icosahedral, enveloped

4. Herpes viruses (Epstein-Barr Virus)Linear dsDNA, encode 100-200 genes, icosahedral, enveloped

• Considerable evidence for role in human cancer.

• Some very tumorigenic in animals

• Viral DNA found in small proportion of tumor cells: “hit and run”

Herpes VirusesHerpes Viruses

•  Epstein-Barr Virus

1.  Burkitt’s Lymphoma

2.  Nasopharyngeal cancer

3.  Infectious mononucleosis

4.  Transforms human B-lymphocytes in vitro

Herpes papilloma

5. Hepatitis B Virus5. Hepatitis B Virus

• 8 known genotypes (A-H)• Causes Hepatitis B, • Can lead to liver cirrhosis• Can lead to hepatocellular carcinoma• Suspected to increase the risk of pancreatic cancer.

• Vast public health problem

•  In Sudan: exposure to the virus from 47%–78%,

•  Prevalence: in central Sudan= 6.8%, in southern Sudan= 26% (Mudawi et al., 2008) and in Nyala= 6.25% (Abou et al., 2009). 

ANTI-ONCOGENES

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

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

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

Retinoblastoma gene (pRb)Retinoblastoma gene (pRb)has normal regulatory function in many

cells

Involved in

Retinoblastoma (cancer of the retina)

Lung carcinomas

Breast carcinomas

Tumor suppressor Rb

• Rb binds to transcription factor E2F and prevents gene expression of proteins needed to go to S phase

RetinoblastomaRetinoblastoma

pRb Gene

RbpRb protein

Rb

Stops replication

Rb

Adenovirus E1A

Cell cycle continues

105kD

Tumor suppressor p53• P53 halts progression 

when DNA damaged – to give cell time to repair or

– triggers apoptosis of damaged cell by activating Bcl-2 causing mitochondria to release cytochrome C and activate caspase system

• If damaged (mutated) cell moves to S phase then it may replicate

p53p53• Inactivated by

1.  deletion 

2.  point mutation

• In a series of colorectal cancers all showed:

1.  Allele 1: partial or complete deletion

2.  Allele 2: Point mutation

p53p53

P53 gene P53 gene P53 gene

P53

P53 DNA

Stops replication

Hepatitis C

P53

replication replication

Papilloma proteolysis

P53

Papilloma

How should these proteins be similar?

AAs sequence similarities in Rb binding site

V P E V I D L T C H E A G F P P S D D

Q P E T T D L Y C Y E Q L N D S S E E

F N E E - N L F C S E E M - P S S D D

L X C X E

AdE1a

HPV E7

Sv40 Tag

HPV E7 sequences differ in low and high risk strains

6/11 P V G L H C Y E Q L N D

16 T T D L Y C Y E Q L N D

18 P V D L L C H E Q L S D

31 A T D L H C Y E Q L P S

33 P T D L Y C Y E Q L S D

Affects binding affinity to Rb

Viruses are not always the culprit!

Disease cellular proto-oncogenes (C-onc) translocation

Burkitt's lymphoma  myc  8 to 14Acute myeloblastic leukemia mos  8 to 21Chronic myelogenous leukemia abl 9 to 22Acute promyelocytic leukemia fes 15 to 17Acute lymphocytic leukemia myb 6 deletionOvarian cancer myb  6 to 14

HOWEVER