Question

Scenario
Doug was a newly single 42-year-old man, whose reentry into the dating world led to a few casual sexual encounters with women he met while out in bars.

After 8 months apart, Doug and his wife reconciled. He decided it would be best not to mention the other women he had intercourse with because he was pretty sure his wife would get upset even though they were not together at the time.

Signs and symptoms
Several months later, during a dentist’s appointment, the hygienist noted what looked like a series of small bumps on the back and side of Doug’s tongue, which had not been noted in his chart before.

At his next visit, the bumps had expanded into lesions and Doug mentioned them as a concern. The dentist noticed that his tonsil on the side of the tongue lesions was swollen, as were the lymph nodes. He prescribed an antibiotic, which Doug took.

The lesions didn’t get better but they also got no worse. A few weeks later Doug developed a stubborn sore throat, so he went to see his family physician.

Testing
When Doug’s doctor saw the lesion at the back of his tongue, he sent Doug for an oral brush biopsy procedure.

When the biopsied tissue was stained with hematoxylin and eosin stain, the pathologist noted a well-differentiated squamous cell carcinoma.

A second sample was obtained by scalpel biopsy and tested for human papillomavirus (HPV) DNA, specifically HPV-16 and HPV-18. The results were returned as positive for viral DNA.

Question 1: Referring to the image below obtained by transmission electron microscopy (TEM), describe the following characteristics of HPV: Enveloped or nonenveloped? Capsid symmetry?

Question 2: HPV has a double-stranded DNA genome. What is the Baltimore group for HPV?

Question 3: Other than genome, what criteria are used in the Baltimore model to create distinct groups of viruses?

Diagnosis
Doug’s diagnosis was stage IV squamous cell carcinoma.



Question 4: How is the detection of HPV DNA in the cancer tissue pertinent to this diagnosis?

Question 5: How does HPV facilitate the replication of its genome and production of mRNA transcripts from the viral genes?

Treatment
Doug had surgery to remove the cancerous tissue, followed by six months of radiation therapy. After two years, there was no recurrence of the cancer.

Question 6: A vaccine, Gardisil, is available to prevent HPV infection. How does a vaccine that protects against a viral infection prevent cancer?

Additional Considerations
Question 7: In a human host cell, where does the process of viral DNA replication occur?

Question 8: Where does translation of HPV mRNA occur in an infected host cell?

Question 9: How are viral latency and lysogeny related?

Answer 1

1. Human Papillomavirus Characteristics: It is a small, non-enveloped, DNA virus that infects skin or mucosal cells. The circular, double-stranded viral genome id about 8-kb in its length. This genome encodes for 6 early proteins responsible for virus replication and two late proteins, L1 and L2 which are for viral structural proteins. The virus consists of icosahedral symmetry with 72 capsomeres that surround a genome containing double-stranded circular DNA with approximately 8000 base pairs.

2. Baltimore classification, developed by David Baltimore is a virus classification system, where virus are classified into two families depending on their type of genome and their method of replication. HPV has double stranded DNA and belongs to the higher taxa - Papillomaviridae.

3. These types of viruses must enter the host nucleus before they are able to replicate. They host cell polymerases to replicate the viral genome and are highly dependent on the cell cycle. Proper infection and production of progeny requires that the cell be in replication, as it is during replication that the cell's polymerases are active. The virus may induce the cell to forcefully undergo cell division, which may lead to transformation of the cell and ultimately into a cancer. Example:  Herpesviridae, Adenoviridae and Papovaviridae.

4. Some papillomavirus types can cause cancer in the epithelial tissues they inhabit, cancer is not a typical outcome of infection. An association with vulval cancer and urothelial carcinoma with squamous differentiation in patients with neurogenic bladder are also been noticed. There are cancer causing papillomavirus genome that encodes two small proteins called E6 and E7 that mimic cancer causing oncogenes. The way they work is that they stimulate unnatural growth of cells and block their natural defenses and also they act on many signaling proteins which control the proliferation and apoptosis.

5. After the initial infection, the viral genome replicates at low levels in the dividing cells of the epithelium but the receptors are unknown and these cells form a reservoir of infection that can last for decades. The individual isolates for papillomavirus are very species-specific. However, the later gene expression, expression of structural proteins and vegetative DNA synthesis is restricted to terminally differentiated cells of the epidermis which implies a link between cellular differentiation and viral gene expression. The functions of each Gene:

E1-- Replication and replication repression
E2-- Activates transcription in HPV types 6, 11 and 16, represses transcription and binds to long control region.
E3-- No known product or function
E4-- Cytoplasmic protein in HPV-1-induced warts.
E5-- Transformation in HPV-6.
E6-- Transformation in cooperation with E7 in HPV-16 and HPV-18.
E7-- Transformation in cooperation with E6.
E8-- No known product or function.
L1-- Major capsid protein.
L2-- Minor capsid protein.

6. HPV vaccines are vaccines that protect against infection with human papillomaviruses. HPV is a group of more than 200 related viruses, of which more than 40 are spread through direct sexual contact. Among these, two HPV types cause genital warts, and about a dozen HPV types can cause certain types of cancer like cervical, anal, oropharyngeal, penile, vulvar and vaginal. Three vaccines that prevent infection with disease-causing HPV types are Gardasil, Gardasil 9, and Cervarix. All the three vaccines prevent infection with HPV types 16 and 18, the two high-risk HPVs that causes about 70% of cervical cancers and an even higher percentage of some of the other HPV-caused cancers. Gardasil also prevents infection with HPV types 6 and 11, which cause 90% of genital warts. Gardasil 9 prevents infection with the same four HPV types plus five additional cancer-causing types (31, 33, 45, 52, and 58) that together account for 10 to 20% of cervical cancers.

7. In the human host cells, Human papillomavirus (HPV) begins its life cycle by infecting the basal cells of the epithelium, within these proliferating cells, the viral genomes are replicated, maintained and passed on to the daughter cells. There are two mechanisms involved in genome replication:
Plasmid Replication: involves the E1 protein and occurs in cells in the lower levels of the dermis. The virus DNA is amplified to 50-400 diploid genomes and then it replicates once per cell division, with the copy number/cell remaining constant.
Vegetative Replication: occurs in terminally differentiated cells in the epidermis. Here, the control over the number of copies seems to be lost and the DNA is amplified to hundreds of copies/cell. The virus is then shed from epidermal cells when these are sloughed off and is transmitted by both direct and indirect contact.

8. After the host cell is infected, HPV16 early promoter is activated and a polycistronic primary RNA containing all six early ORFs is transcribed. This polycistronic RNA contains three exons and two introns and undergoes active RNA splicing to generate multiple isoforms of mRNAs. The viral life cycle is linked to the differentiation of the infected epithelial cell and greatly from those of the DNA genomes of the infected host organism.

9. Virus latency or viral latency is the ability of a pathogenic virus to lie dormant (latent) within a cell, called as the lysogenic part of the viral life cycle. A latent viral infection is a type of persistent viral infection which is distinguished from a chronic viral infection. As a result of this condition, it results that the virus can be reactivated and can produce large amounts of viral progeny without the host becoming reinfected by new outside virus and stays within the host indefinitely.