Chapter 16: Viruses and Cancer

Most common cancers today

Meneses says lung cancer is #1 In US - for men, prostate is #1, then lung and colon In US - for women, breast is #1, then lung and colon

Viruses and Cancer: causal or correlative?

Often correlative - viruses are thought to be cocarcinogens in the development of human tumors

oncogenes

cancer-causing genes

driver vs. passenger genes

drivers - directly involved in driving cancer progression passengers - just tag along

pRb and p53 involved in?

pRb = cell cycle p53 = apoptosis (2 hits)

What are cellular oncogenes involved in?

production of cytokines, cytokine receptors, protein kinases, G-proteins, transcription factors, and other nuclear proteins that regulate cell growth

Cancer

unregulated/uncontrolled cell growth/division

Metastasis

when a cell or clump of cells separates from a tumor and spreads to another location

Challenges to Virotherapy

Immunity: -viruses are vulnerable to antiviral host defenses in that they are usually eliminated from the body before they have had the chance to cause substantial damage to the tumor -sometimes, there is no negative effect, and the immune response may actually help activate antitumor immune surveillance systems Delivery: -viruses have to transverse a lot before they can reach a tumor - research into best methods of delivery Biomarkers to Track Progress: -biomarker genes incorporated into the genome of oncolytic viruses facilitate noninvasive monitoring of viral replication and production of viral progeny in cancers

Epstein-Barr Virus (EBV)

-the first human virus to be directly associated as a cause of cancer in humans -HHV-4 - lytic and latent cycles -EBV persistently infects B cells -Burkitt's lymphoma - solid tumor of aberrant B cells -mostly occurs in immunosuppressed people, especially African children -affects the jaw and spreads to soft tissues

transformation

- the conversion of a normal cell into a cancer cell - transformed cells no longer grow as cell monolayers and have lost contact inhibition

Exactitudes of HPV

-E6/E6AP causes the ubiquitylation of p53, targeting it for destruction in proteosomes -E7 does the same for pRb

Cervical Cancer Oncogenesis: Benign vs. Malignant

-HPV-16, 18, 45, and 31 DNA is found in nearly all cervical cancer cells 2 methods: 1. Benign: HPV can be maintained as an independently replicating circular extrachromosomal episome in benign tumors -HPV-16 mRNAs transcribed from the episome contain a destabilizing sequence (5' AUUUA 3') - results in unstable mRNA 2. Malignant: HPV-16 genome in cervical cancer cells can be integrated into the host DNA -the HPV-16 genome is interrupted upstream of the E6/E7 ORFs -no destabilizing sequence - results in stable mRNA -the integrated E6/E7 early genes are overexpressed - their proteins inactivate the products of p53 and pRb -disruption of the normal cell cycle - unregulated growth and cell division, along with chromosomal abnormalities

More on proto-oncogenes

-If the gene itself is mutated, its product is abnormal and might be active at all times, constantly signaling growth and division -If the promoter is mutated, the mRNA may be overexpressed, leading to more cell signaling or cell signaling at inappropriate times

Insertional Activation

-Insertional Activation of the expression of a normal proto-oncogene occurs when viral promoters or enhancer elements are inserted near a proto-oncogene -this causes elevated or abnormal expression of the proto-oncogene -since cellular proto-oncogenes are involved in signal transduction and cell cycle regulation, this results in uncontrolled growth and division of cells due to hyperactive or inappropriate timing expression

Src history

-It was found that injecting filterable agents (viruses) into chickens could produce tumors in soft tissues, called sarcomas -The agent was a retrovirus called the Rous sarcoma virus

2 other polyomaviruses

-JCV and BKV - can cause cancer in about 5% of AIDS patients

Special proteins on SV-40

-Large T antigen - involved in transcriptional activation and repression, differentiation, and the stimulation of the cell cycle -helicase activity; recruits DNA replication machinery -interacts with both pRb and p53 -Small T antigen - interacts with a cellular phosphatase (PP2A) keeping phosphorylated proteins always on!

Simian Virus 40

-SV-40 is a polyomavirus - small, simple, circular dsDNA genome -binds to MHC I molecules and enters cells via endocytosis mediated by caveolae -early genes encode gene products essential for genome replication -late genes encode structural proteins for viral assembly -also, noncoding regulatory sequences, like promoters, enhancers, and an ORI -assembly in nuclear viral factories -released by lysis -natural host is Asian macaque -can cause sarcomas in hamsters -SV-40 proteins interact with p53 and pRb, disrupting their cellular pathways and inactivating cell cycle control

Human Endogenous Retroviruses (HERVS)

-about 8% of our genome -have sequences similar to integrated proviral RVs -now defective

Low-Risk HPV

-benign in regard to cancer -cause genital warts - not that bad

HBV & HCV

-cause chronic hepatitis, which can progress to liver cirrhosis and hepatocellular cancer (HCC) -liver damage leads to need for more cellular division, more mutations, and cancer -HBV - dsDNA genome with RT of an RNA pregenome - usually integrated -HCV - +ssRNA virus that replicates in cytoplasm - no obvious v-onc or integration -recent research - HCV causes chromosomal instability i.e., higher mutation frequency -both are spread by blood/body fluids/needles/mother to baby -HBV is STD; not really HCV -HCV more quickly progresses to HCC than HBV -HCC is usually not detected until advanced stage - death in one year of diagnosis -HBV vaccine is the first vaccine to prevent a cancer

Kaposi's Sarcoma-Associated Herpesvirus (HHV-8)

-classic Kaposi's sarcoma - skin cancer endemic to older Mediterranean males -also, tell-tale sign of HIV/AIDS - the lifetime risk for KS on homosexual male AIDS patients is 50% -mostly latent -v-oncs are in the genome of HHV-8

Is cancer clonal or multi clonal?

-clonal - originates with one aberrant cell!

HPV

-common among sexually active people - transmission of HPV DNA can occur by semen -genital HPV infection is one of the most prevalent STDs in the world today -high-risk types of HPVs cause cervical cancer - a major cause of death among women in developing countries - the third most common cancer among women worldwide -Pap smear to detect early

Oncogenes are

-dominant -primarily TFs (like Myc) -also growth factor receptors

Intermediate-Risk HPV

-frequently found in precancerous lesions but less often in cancers

Adenoviruses

-in humans, causes respiratory tract, GI tract, and eye infections like pink eye -in animals, it can cause malignant tumors - has two genes that can cause oncogenic transformation in rodents - E1A inactivates pRb and E1B inactivates p53 -today, adenoviruses are used as gene therapy vectors b/c they cause only mild diseases and their genome replicates with high efficiency in the nucleus of host cells -researchers developed an adenovirus-transposon vector so that it would integrate into host DNA -used to treat cancers, cardiovascular disease, genetic disorders, and glaucoma -Jesse died from this vector, but now better

Comments on it

-most known oncolytic viruses can be engineered to eliminate their pathogenicity, making them safe to use without destroying their oncolytic potency -for viruses to be more widely approved and used as anticancer agents, they will have to meet stringent safety and efficacy standards and be amenable to study/monitoring in human tissues -some animal viruses lack pathogenicity in humans but are capable of destroying human tumor tissue -the problem is that there can be virus evolution giving rise to a new human pathogen -oncolytic viruses are designed to target tumor cells by targeting the transcription, attachment, defective interferon signaling, and nonfunctional apoptosis activities of the cells -adenovirus -we have viruses that can only infect mutant p53 cells

Example - PDGF (platelet-derived growth factor)

-mutated PDGF gene makes PDGF that gets stuck to its receptor and keeps kinase active indefinitely, leading to constant cell growth -mutated PDGF receptor doesn't even need PDGF and keeps the kinase constitutively active, driving unregulated growth and division

More on HPV - it's Meneses' favorite

-over 150 HPVs have been identified - 40 infect the genital area -most cause benign papillomas, or warts in the skin (esp. the hands, feet, and genitals) or mucus membranes due to a break in the skin -low, intermediate, and high risk types -HPV-2 caused the tree man

Exactitudes of Burkitt's Lymphoma

-overexpression of Myc -in B cells, the heavy chain/light chain promoter gets translocated next to the Myc gene, leading to over expression - drives cell cycle progression from G to S (due to cyclin D)

More on p53 and pRb in cancer cells

-p53 was thought to be an oncogene b/c it was found at high levels in cancer cells -really, it was just b/c the p53 DNA-binding domain was nonfunctional, inhibiting apoptosis in these cells - also, Ub was dysfunctional, causing the bad p53 to build up -pRb is normally bound to E2F, a TF, but when it is phosphorylated, it comes off and releases the E2F for cell cycle progression -this can be messed up

What do DNA tumor viruses target?

-pRb and p53 TS gene products - the gene products of DNA tumor viruses interact with proteins that have a negative regulatory role in cell proliferation - causes an alteration of cell cycle progression -low oncogenic potential - only transforms cells if there is an aborted replication cycle, when only early viral genes are expressed

Exactitudes of Kaposi's Sarcoma

-produces viral homologs of human stuff -v-cyclin D2- pushes cell cycle forward -v-IL 6- promotes proliferation -v-BCl2- inhibits apoptosis

TS genes are

-recessive - both must be knocked out -involved in DNA repair -cyclin/CDK regulators

High-Risk HPV

-responsible for cancers of the cervix, vulva, vagina, anus, anogenital area, and penis -about 70% of cervical cancer cases worldwide are caused by types 16 and 18 - 4 to 20 yr latency -HPV-16 also leads to cancer in the oral cavity, oropharynx, and throat

Insertional Inactivation of TS gene

-results in uncontrolled growth and cell division - all cancers involve the inactivation of a TS gene in addition to other changes in host cell DNA

HPV Structure and Genome

-small, nonenveloped, icosahedral, circular dsDNA -infects stratifying basal epithelial cells - must be metabolically active - through a break in the skin -genome replication, assembly, maturation occur in the nucleus

In-Depth on Retrovirus (RV) Genome

-the ssRNA is reverse transcribed into dsDNA prior to integration into the host chromosome -RVs contains 2 copies of the +ssRNA genome -normal RVs have three genes in between LTRs (long terminal repeats) containing unique and repetitive sequences -gag: matrix and core proteins that protect the viral genome from damage -pol: multifunctional protein with RT, RNase H, helicase, and integrase functions -env: a protein within the lipid bilayer that surrounds the nucleoprotein core - bind to host cell receptors -some RVs contain an extra v-onc gene that was hijacked from the genome of its host - encodes a protein that is capable of inducing cancer -when the RV DNA is integrated into host genome, it is called a provirus

Integration of v-onc gene

-the v-onc may become transcribed and functionally active, disrupting the cell cycle of the host cell and promoting unrestricted growth and division

DNA tumor viruses - how are they different from RNA tumor viruses?

-the v-oncs of DNA tumor viruses are essential genes used in replication -many code for nuclear proteins -some even encode cellular homologs that activate signal transduction pathways for enhanced cell growth and division

Rous Sarcoma genome

-three essential genes: gag-pol-env for viral replication and assembly -also has src gene - was required to transform cells - called oncogenic transformation - a kinase -using 32-P radioactively labeled single-stranded DNA probes, researchers found src sequences in the genomes of normal chicken and normal human cells - demonstrated that oncogenes were cellular genes that were hijacked by viruses from cells

Why Oncolytic Viruses Cause tumor cells to die

1. As a consequence of infecting and replicating in cancer cells 2. By inducing apoptosis 3. By causing the cancer cell to undergo lysis and the expulsion of progeny virions 4. Stimulates the host's immune system

Assays to Check for Cancer

1. Focus-forming assays - normal cells form a monolayer of cells that do not grow on top of each other and undergo contact inhibition - when neighboring cells touch each other the cells stop dividing -cancer cells lose contact inhibition and form densely packed cells that pile on top of each other - called foci 2. Soft agarose assays - normal cells do not proliferate in soft agarose, but cancer cells will be able to divide and form free colonies in it 3. Reduced serum requirement - many cancer cells can grow in medium containing reduced serum or growth factors

6 viruses that contribute to cancer

1. Hepatitis B Virus (HBV) 2. Hepatitis C Virus (HCV) 3. Human Papillomavirus (HPV) 4. Epstein-Barr Virus (EBV) 5. Kaposi's Sarcoma-Associated Herpesvirus 6. Human T-lymphotropic Virus (HTLV-1 & 2) -80% of viral-associated cancers are cervical cancer (HPV) and liver cancer (HBV & HCV)

5 human retroviruses - not that important

1. Human foamy virus 2. HTLV-1 3. HTLV-2 4. HIV-1 5. HIV-2

Changes in Cancer Cells in Vitro

1. Increased number/size of nuclei and extra telomerase 2. Immortalized - divide indefinitely 3. Metabolic changes - dividing/growing rapidly 4. Lack of contact inhibition 5. Loss of need for adhesion to grow 6. Can grow independently without serum/growth factors 7. Loss of cell cycle control - no stopping at checkpoints 8. Changes in membrane structure/function - tumor-associated carbohydrate antigens on surface 9. Form tumors when injected into animals

Changes in Cancer Cells in the Body

1. Increased oncogene mRNA expression b/c the oncogene has undergone chromosomal translocations, amplification, or mutations 2. Cells lose tumor suppressor function due to a deletion or mutation in the TS gene 3. DNA methylation patterns are altered 4. Cells produce increased levels of growth factors 5. Cells divide uncontrollably 6. Telomerase activity is reactivated 7. Cells can avoid immune response

How Provirus Integration Leads to Cancer (3 Ways)

1. Insertional Activation of proto-oncogene 2. Integration of v-onc gene 3. Insertional Inactivation of TS gene

Meneses' Requirements for Cancer

1. Invasion into other tissues 2. Bypass of the cell cycle 3. Blocked apoptosis 4. Avoids immune response

How Retroviruses can cause cancer

1. May carry an oncogene (v-onc) into a cell 2. May activate a cellular proto-oncogene 3. May inactivate a TS gene

Cancer Treatments besides virotherapy

1. Surgery - anesthesia 1846 - ether 1st one 2. Radiation - X-rays and gamma rays from radium cause breaks in dsDNA, leading to greater death in more highly proliferating cells -can be targeted specifically to cancer cells 3. Chemotherapy - mustard gas - led to decreased lymphocyte counts - means that it attacks rapidly dividing cells - lymphoma and leukemia treatment, but toxic - also Farber - found that folic acid was higher in dead cancer patients - used a folic acid antagonist called aminopterin - 10/16 responded well -methotrexate 4. Immunotherapy - HER2 receptor breast cancer - created herceptin to bind and inactivate -mimicry - sarcoma disappeared when infected with Strep. - heat kill bacteria and see if you can get an immune response -Coley's toxins - LPS (lipopolysaccharide) on outside of bacteria induces a strong immune response, particularly tumor necrosis factor (TNF) - cytokine that causes necrosis of tumor cells

Book's Requirements for Cancer

The cancer cell: 1. bypasses apoptosis 2. circumvents the need for growth signals from neighboring cells 3. escapes immunosurveillance 4. commands its own blood supply 5. may metastasize to another location 6. Mutations in TS genes may be required for full malignancy to occur

HPV vaccine

Yeah, it exists. Get it. (Guardasil)

Is provirus integration random??

Yes, but it must be in a transcriptionally active cell!!

What percentage of cancers are associated with viruses?

about 20%

oncolytic virotherapy

an experimental form of cancer therapy in which viruses are used to target and destroy cancer cells without harming healthy cells