10 - Viral Oncogenesis

Question 1

Current estimate is the ~12% of human cancers are caused by __ of __ viruses?

Answer 1

1 of 7

Question 2

What are common traits associated with viral oncogenesis?

Answer 2

Oncoviruses are necessary byt not sufficient for cancer development, and many who have the virus do not get the cancer.

Viral cancers appear in contexrt of persistent infections and occur many years to decades after acute infection.

Question 3

What is the role of the immune system in viral oncogenesis?

Answer 3

Can play a deleterious role or a protective role.

Question 4

How can viruses cause cancer indirectly? What infections is this seen with?

Answer 4

Hep B and C

Chronic infection can cause high levels of cell division.

This increases the probability that hepatocytes will develop mutations and chromosomal aberrations that derail their growth control.

Question 5

How can viruses cause cancer directly? What infections is this seen with?

Answer 5

Papillomavirus

Some DNA viruses stimulate cells to enter S phase and ready themselves for DNA synthesis.

If the virus doesn’t complete its lifecycle and kill the cell, the same viral proteins can continue to override the normal controls on cell growth and divide inappropriately.

Question 6

What are the structural characteristics of papillomavirus (HPV)? What human disease is it associated with?

Answer 6

Icosahedral capsid, naked virus.

Small 8 kbp dsDNA circular genome.

Associated with cervical cancer, penile cancer, head and neck cancer (~5.2%)

Question 7

Describe the strong association of merkel cell polyoma virus and merkel cell carcinoma? What are characteristics of this virus

Answer 7

1. common deletion in the viral genome renders it unable to replicate
2. persistent infection
3. Immunosuppressed

dsDNA, icosahedral capsid, naked virus

Question 8

What does papillomavirus infect and what does replication depend on?

Answer 8

Infects cutaneous and mucosal epithelia hands, feet, and anogenital tract.

Replication depends on life cycle of keratinocyles and epithelial cells of the skin and mucosa.

Question 9

What are the functions of E1 and E2 genes encoded by the papillomavirus?

Answer 9

Early 1 and 2 proteins mediate the replication and transcription of viral DNA

Question 10

What are the functions of E3 and E5 genes encoded by the papillomavirus?

Answer 10

Early genes.

E3: disrupts keratin to facilitate virus egress (leaving)

E5: stimulates constitutive cellular growth factor receptor signaling

Question 11

What are the functions of E6 and E7 genes encoded by the papillomavirus?

Answer 11

Early genes.

E6 and E7 neutralice the major “breaks” that regulate the cell cycle - p53 and Rb-uncoupling cell division from key regulatory controls.

Question 12

What are the functions of L1 and L2 genes encoded by the papillomavirus?

Answer 12

Late genes.

Compose the self-assembling capsid and bind cell surface integrins.

Question 13

E5, E6, and E7 intersect with key elements of ______ _______ _______ and are associated with __________.

Answer 13

E5, E6, and E7 intersect with key elements of cellular growth control and are associated with oncogenesis.

Question 14

How many subtypes of papillomavirus are there? How do they differ?

Answer 14

Over 200, based on DNA sequence homology, tissue tropism, and disease penetration.

There’s cutaneous HPB and mucosal HPV.

There’s high rick and low risk HPV.

Question 15

Where does HPV infect and replicate? What is a common location of these infections?

Answer 15

In squamous epithelium, and viral infection remains local in skin (warts) and mucous membranes (genital, oral, and conjunctival papillomas).

1/3 infect the genital tract and are sexually transmitted.

Question 16

Which subtypes of HPV cause anogenital warts and are associated with condyloma acuminatum (genital warts)? What is the risk associated with these subtypes?

Answer 16

Types 6 and 11

Low risk

Question 17

Which subtypes of HPV cause anogenital warts and are associated with cervical intraepithelial neoplasia and cancer? What is the risk associated with these subtypes?

Answer 17

Types 16 and 18

High risk

Question 18

What is the life cycle of papillomavirus? What arethe potential outcomes of papilloma replication?

Answer 18

Normally only basal cells of the skin divide but HPV stimulates the cell cycle and causes cells in the stratum spinosum to divide instead of differentiate.

Altered keratinocyte division and differentiation are required for HPV replication.

Potential outcomes: maintain papilloma (wart-like growth) or regress

Question 19

Describe the regulation of the cell cycle?

Answer 19

1. Cylin-dependent kinases (CDKs) are the engine of the cycle that move it ahead via phosphorylating key substrates
2. Enzymes also regulated to make sure the cell is ready

Question 20

What are the major checkpoints of the cell cycle?

Answer 20

G1 (before entry to S): Is the environment favorable?

G2 (before entry into M): Is all DNA replicated? Is the environment favorable?

Metaphase checkpoint: Are all chromosomes attached to the spindle?

Question 21

What is the Rb protein and what is its role in the cell cycle?

Answer 21

It’s the brake that blocks progression into S phase.

When sufficient Cdk/cyclin has accumulated in the G1 phase, Rb is inactivated by phosphorylation and the cell can then more into S phase.

Question 22

What does E7 do to alter the cell cycle? What else does E7 make?

Answer 22

Binds Rb and targets it for proteasomal degradation, which prevents it from blocking progression into S phase. (Ie E7 removes a key regulator of cell cycle progression and allows S phase entry)

E7 also causes production of p16, which is a diagnostic marker of cervical cancer.

Question 23

What is the function of p53?

Answer 23

“gaurdian of the genome” that is mutated in >50% of all human cancers.

It induces inhibitors of CDKs to stop the cell cycle. Also a major inducer of apoptosis.

Question 24

What effect does HPV E6 protein have on the cell cycle?

Answer 24

It recruits ligase to target p53 for degradation to prevent it from blocking the cell cycle or inducing apoptosis.

It also induces expression of telomerase to maintain the chromosomal telomeres, leading to immortalization.

Question 25

What data supports the hypothesis that HPV causes cervical cancer?

Answer 25

Sexually active populations have a higher risk of cervical cancer.

> 99% of all cervical cancer specimens show evidence of high risk strains of HPV

High risk E6 and E7 can immortalize keratinocytes

Question 26

What occurs in the earlier stages of HPV infection?

Answer 26

Acute and persistent infection and virus is maintained as a circular episome.

Question 27

What occurs during HPC genome integrateion in the later stages?

Answer 27

Development of carcinoma

• genome integration into chrom
• Disrupts expression of E2 repressor when inserting, which causes constitutive action and expression of E6 and E7.

Integrated DNA triggers chromosomal abnormalities.

Question 28

What is the key step of an HPV infection that moves the infection towards oncogenesis?

Answer 28

Genome integration. This is what results in invasive cancer.

Question 29

How common is HPV?

Answer 29

~79 million US are infected with HPV, 14 million new/year

~17,600 women and 9,300 men are diagnosed with a cancer caused by HPV

• cervical (mostly HPV) usually have no symptoms until advanced
• Other HPV-related cancers may not have symptoms until advanced and may be hard to treat (vulva, vaginal, penile, anal, and oropharynx)

Question 30

What are the two cytological identifications associated with HPV-induced cervical lesions?

Answer 30

Low-grade squamous intraepithelial lesion (LISL)- very mild

High grade squamous intraepithelial lesion (HSIL) - range from moderate dysplasia to invasive carcinoma

Question 31

What is the appearance of HPV-induced cervical lesions on histology?

Answer 31

Cervical intraepithelial neoplasia (CIN) ranging from normal and CIN I - CIN III

CIN III will show an increased number of cells retaining their nucleus, even at the surface where cells are normally keratinized.

Question 32

What is one way to get cytological evidence of cervical dysplasia or neoplasia?

Answer 32

Pap smear: papanicolaou stain (multiple dyes) of exfoliated cervicovaginal squamous epithelial cells.

Can show cytological evidence of dysplasia or neoplasia.

Question 33

What are the different squamous cell abnormalities that can be seen with a pap smear?

Answer 33

Koilocytes: enlarged keratinocytes with vacuolated cytoplasm and clear halos around the nuclei

ASCUS: atypical squamous cells of undetermined significance

LSILs: mild abnormalities, early changes in cell size/shape caused by HPV

HSILs: severe abnormalities

Question 34

How does a hybrid capture assay work for the detection, strain analysis, and quantification of HPV DNA?

Answer 34

1. Treat specimen with base sln to release DNA
2. Hybridize RNA probe with target DNA
3. Capture hybrids on slide and label for detection with antibodies
4. Detect, read, and interpret results

Question 35

What factors can impact the outcome of an HPV infection>?

Answer 35

The host’s immune system

Smoking, oral contraception, viral load, and co-infections can all cause persistence of the infection as opposed to regression.

Question 36

How does the HPV vaccine work?

Answer 36

L1 and/or L2 proteins self assemble into virus-like particles (VLPs)

Capsid proteins induce protective humoral immunity and mucosal protection from multiple sub-types of HPV infections.

Question 37

Has the HPV vaccine helped reduce HPV? Is everyone currently getting vaccinated?

Answer 37

Since the vaccine in 2006, there has been a 56% reduction in vaccine type HPV infections among teen girls in the US.

4/10 girls remain unvaccinated
6/10 boys remain unvaccinated