Oncogenic Viruses

Front 1

What organism causes a large percentages of cancer?

Back 1

Viruses

Front 2

What are some examples of viruses that can cause cancer?

Back 2

Human papillomavirus, Epstein Barr Virus, Kaposi's Sarcoma Herpes Virus, Hepatitis B virus, Hepatitis C virus

Front 3

To cause cancer what does the viral life cycle look like?

Back 3

1) Attachment (binding and then entry into the host cell
2) Gene expression and genome replication (might or might not occur) (genes are expressed that are important for genome replication)
3) Assembly and exit from the host cell is BLOCKED

Front 4

What is the early and late phase in the viral life cycle?

Back 4

Early phase= Attachment, entry, gene expression, genome replication

Late phase= gene expression, genome replication, assembly, exit

Front 5

What are the genes we are most interested in for oncoviruses?

Back 5

The genes the virus expressed upon first entry into the cell that lead to oncogenesis.

Front 6

What do oncogenic viruses hijack the cell for?

Back 6

The viruses hijack the cell to replicate themselves and promote their own survival

Front 7

What need of the cell results in cancer as a byproduct?

Back 7

Cancer is the byproduct of the need of the virus to have its host cell be in S phase. The virus needs to use the DNA polymerase and DNA machinery that is expressed during S phase to replicate and survive. Cancer results when the cell is not kill following infection with the virus (as would occur in a lytic life cycle of the virus) but rather lytic replication is blocked resulting in latency of the virus in the cell with uncontrolled growth (the cell remains in S phase).

Front 8

Where do oncogenic DNA viruses replicate and what do they require/ what do they do to the host cell?

Back 8

Oncogenic viruses replicate in the nucleus. They require some or all of the host DNA synthetic machinery which means that the cell must be in S phase. They also result in the disruption of the pRb and p53 pathways (growth control pathways) in the host cell.

Front 9

How do cells usually respond to an HPV viral infection and how do the viruses counteract this response?

Back 9

Cells usually respond to a viral infection by activating an apoptotic program. They can sense that the right stimuli have not induced it to enter S phase. The viruses respond by activating functions that stop apoptosis from occurring. This allows the virus to keep the cell alive as long as possible to maximize the number of progeny it can generate. It is this unregulated growth of the cells being held in S phase that results in oncogenesis.

Front 10

How many HPV viruses are there and what is the difference between low risk and high risk HPV viruses?

Back 10

There are over 100 types of HPV viruses (types refers to genetic differences not serotypes). Low risk HPV viruses cause benign warts. High risk HPV viruses cause cancer including cervical, anal, penile and head and neck

Front 11

What is the HPV life cycle?

Back 11

1) The virus enters the skin through a microabrasion
2) The virus gets down into the basal layers of the epithelium by damaging the cells along its path (it cannot transcytose across cells)
3) The virus begins to produce its early HPV genes which encode for the proteins: E1, E2, E4, E5, E6, and E7
4) E7 induces the cell to enter into S phase (the basal cells are already dividing at a low level so not much E7 has to be expressed to keep the cells dividing); E1 and E2 drive replication of the genome (also expressed at low levels).
5) The viral genome only replicates enough that each new daughter cell being produced has a viral genome that can go along with it (viral genomes as episomes aka plasmids)
6) The cells move up the epithelium towards the top and as they do this they differentiate.
7) The differentiated cells stop making early HPV proteins and begin to express late HPV genes (L1 and L2) that encode for structural proteins, etc... It also results in the amplification of viral genome replication.
8) New viral particles are make
9) The viral particles are shed into the environment with the sloughing off of the dead epithelial cells at the surface of the skin

Front 12

Is HPV a lytic virus?

Back 12

NO

Front 13

Where in the body does HPV grow?

Back 13

HPV grows in keratinized epithelium

Front 14

How does a wart form compared to the normal HPV life cycle?

Back 14

The HPV virus continues to produce early HPV protein products (via E1, E2, E4, E5, E6, and E7) instead of switching to the late HPV gene expression. This results in the cells continuing to grow at a low level near the surface of the skin instead of differentiating and a resulting hyperproliferation of cells that are not sloughed off (wart).

Front 15

What occurs during the episomal replications in the basal layers of the skin with a normal HPV infection?

Back 15

You are getting maintenance level replication that is instigated by the E2 protein. The E2 protein, along with activating and inducing replication of the viral genome, acts as a repressor of the E7 and E6 proteins (oncogenes of HPV). This allows the cells to maintain their low level of replication and prevents the virus from causing cancer.

Front 16

What change in the HPV infection turns it from a latent infection into cancer?

Back 16

The HPV viral genome integrates into the host chromosome randomly. This blocks the viral genome replication (it can no longer replicate as a virus but instead replicates as part of the host chromosome). Generally this integration event selects for the viral genome inserting into the host chromosome in a way that disrupts the E2 gene so it no longer can be expressed (splits the gene in half). This results in the expression of the E6 and E7 oncogenes (causing the cell to enter into S phase) leading to deregulated cell growth.

Front 17

Are HPV viruses strong or weak oncogenic viruses?

Back 17

STRONG

Front 18

What does HPV require from the host to replicate and what is the battle that ensues between host and virus?

Back 18

The HPV virus requires host cell DNA synthetic machinery to replicate (the virus does not encode for any enzymatic functions except E1 and E2 uncoating of DNA). The E7 protein induces the cell to enter S phase by binding to and inactivating the pRb tumor suppressor (stops Rb from binding to E2F and repressing it resulting in the constitutive activation of E2F; E2F increases the entry of the cell into S phase and increases DNA synthesis). The host cell responds by activating a p53 dependent apoptotic pathway. However, the expression of E6 by the virus allows it to bind to p53 and cause its degradation. This results in cells being continually induced to divide and a block of apoptosis of these cells resulting in oncogenesis.

Front 19

What is the difference in the actions of E6 in a high vs. low risk HPV virus?

Back 19

High risk HPV viruses have E6s that cause the degradation of p53. Low risk HPV viruses have E6s that DO NOT cause the degradation of p53.

Front 20

What is the HPV vaccine composed of?

Back 20

The HPV vaccine is composed of virus-like particles. These are capsids that contain L1 and are structural identical to the capsids of the infectious HPV. L1 can be expressed in bacteria to form the capsids. L1s are divergent across HPV viruses. These capsids lack L2 protein on the inside and lack viral DNA.

Front 21

What is the function of the current HPV vaccines?

Back 21

These vaccines serve as neutralizing antibodies. They do not contain live viruses and thus are used as prophylaxis (people MUST be vaccinated PRIOR to exposure. This vaccine ONLY works against the 2 most common forms of HPV (HPV 16 and HPV 18) but 20-30% of those HPVs that cause cancer are not covered by this vaccine.

Front 22

What are some future HPV vaccines people are working on?

Back 22

Pan-HPV VLP- this vaccine would contain the L2 protein which is conserved across all HPV strains; this L2 protein is exposed to the environment right when the capsid binds to the host cell allowing it to mediate an antibody response.

Cell-Mediated Immunity inducing vaccine- this vaccine would need to involve viral replication either using an attenuated virus or a recombinant virus. It would be used to treat people who already have HPV cancer.

Front 23

Do HPV vaccines cause autism or mental handicaps?

Back 23

NO

Front 24

What is a type of cancer both HBV and HCV can cause and what are HBV and HCV?

Back 24

HBV is the Hepatitis B virus and HCV is the Hepatitis C virus. Both of these viruses can cause hepatocellular carcinoma. They both cause chronic infections with ongoing low levels of replication of these viruses in the cells.

Front 25

How can HBV and HCV cause hepatocellular carcinoma?

Back 25

The HBV and HCV viruses are constantly damaging liver cells requiring them to regenerate. This constant liver regenerating increases the chances of multiple mutations appearing and thus the chance of multiple genetic lesions. Also the viral proteins themselves stimulate cell division and interfere with DNA repair (they increase the mutation rate which can lead to cancer).

Front 26

What type of cancer is EBV linked with?

Back 26

EBV (Epstein Barr Virus) is linked with Burkitt's lymphoma, a B cell lymphoma. It can also cause other lymphoproliferative disorders in people with compromised immunity.

Front 27

How does EBV cause Burkitt's lymphoma?

Back 27

EBV translocates the viral oncogene (c-myc) to the immunoglobulin locus of the B cell. This results in a constitutive high level of expression of the myc gene which promotes cancer. Along with this translocation, the EBV virus induces hyperproliferation of the cell via expression of EBNAs and LMPs.

Front 28

What state is the EBV virus in while it induces hyperproliferation?

Back 28

EBV is in a latent state when it induces hyperproliferation. It is ONLY expressing the EBNA and LMP genes.

Front 29

What is latency?

Back 29

Latency is a state of viral infection where the viral genome is present and few, if any, viral genes are being expressed and NO viral replication is occurring.

Front 30

What are the EBNAs and LMPs expressed by EBV and what do they do?

Back 30

EBNAs are Epstein Barr Nuclear Antigens. They activate signal transduction pathways and inhibit tumor suppressor pathways (such as p53) resulting in inducing cell growth and inhibiting apoptosis. LMPs are Latent Membrane proteins (found in the plasma membrane of the cell). They stimulate proliferation and survival pathways of the cell.

Front 31

What does Kaposi's Sarcoma Herpes Virus cause?

Back 31

This is a virus that causes Kaposi's Sarcoma within the AIDs population and immunosuppressed individuals. it encodes a number of cellular mimics. These include:
v-cyclin- activates the cyclin-dependent kinases promoting cell proliferation
c-IL6- growth stimulatory cytokine
v-Bcl2- antiapoptotic protein

Front 32

What is one problem that might arise from a pan-HPV virus?

Back 32

HPV viruses are found all over the skin. A pan-HPV vaccine might destroy the good viruses and have unforeseen consequences.

Front 33

What was the previous method used in viral discovery and why was it not very good?

Back 33

Viral culture was the previous method used to identify viruses but the problem is that not many viruses can be cultured.

Front 34

What are some new techniques used to identify and discover new viruses?

Back 34

Virus chips- you have the DNA of all known viruses on a microarray; you take your sample and hybridize it to the microarray plate to see if you can find a new virus
Deep sequencing
Subtractive genomics- you take the tumor, isolate its DNA, and use bioinformatics protocols to subtract out all of the cellular genes and see what is left (might be a virus). This was how Merkel cell Polyomavirus was discovered.

Front 35

Is there such a thing as beneficial viruses?

Back 35

Yes, they are the equivalent to the microbiome and play a positive role in health. For example, it is possible that some latent viruses can prime the immune system and make you less susceptible to bacterial infections.