ONCOGENIC VIRUSES

INTRODUCTION

Oncogenic viruses cause cancer.
The oncogenic viral DNA is always present in the tumor cells caused by the oncogenic virus.
In some tumor cells, the virus DNA gets fused into the cell chromosome.
In some cases, the virus DNA is also present as multiple copies of covalently closed DNA (cccDNA).
The protein of oncogenic viruses can be detected in tumor cells.
Some of the human adenoviruses are seen to be oncogenic.
They are can transform cells in culture and cause tumors after inoculating into animals.
Adenoviruses do not cause cancer in humans to date.
About 20 percent of cancer cases in human females and about eight percent in males are seen to be caused by viruses

TYPES OF CANCER

Papillomavirus-linked cancers

Most of the infections caused by papillomavirus can result in cervical carcinoma in women.
It is a small DNA virus that infects mammals and birds.
There is approximately 100 human papillomavirus (HPV) types.
They are distinguished based on their DNA sequencing.
This virus enters the body through small cuts.
It infects keratinocytes present in the skin or a mucous membrane.
Each type of this virus either infects the hands or the genitals.
It results in a wart or carcinoma.
Most of these infections do not become persistent.
The one who gets a persistent infection has a small risk of cancer
HPV 16 and 18 are the high-risk types of human papillomavirus(HPV).
The host cells of this virus i.e. keratinocytes do not divide after differentiation.
But when infected the virus induces keratinocyte into the S phase of the cell cycle.
When a cell is infected with a high-risk HPV, its division continues unchecked.
This eventually results in cancer.
HPV can also cause a rare type of skin cancer.

Polyomavirus-linked cancers

Epstein-Barr virus-linked cancers

Anthony Epstein grew cell lines from the B cell tumor of a patient
He observed that the cells were continuously infected with a herpesvirus.
He named it Epstein Barr virus (EBV)
Nasopharyngeal carcinoma (NPC) is another tumor that is associated with EBV.
In both the tumors, the EBV genome is present as cccDNA.
Other cancers that are associated with EBV include – some cases of Hodgkin’s lymphoma, non-Hodgkin’s lymphoma in AIDS, and post-transplant lymphoproliferative disorder.

HOW DO VIRUSES CAUSE CANCER

Interference in cell cycle

A virus manipulates the internal environment of its host cell.
Through this, it gets all the requirements for its replication.
The control in the cell cycle is regulated by many proteins.
p53 and retinoblastoma protein (pRb) are the two proteins in humans that mediate the cell cycle control.
Oncogenic viruses produce several proteins that interact with these host proteins which results in repeated cycles of division.
HPV early proteins E6 and E7 can interfere with control in the cell cycle.
E6 binds to p53, which leads to its degradation and E7 binds to pRb–E2F complexes, resulting in their dissociation.
The cell is then pushed to the S phase.

Activation of untargeted cell genes

Some virus proteins interact with the cell proteins that may not be the intended targets.
As a result, events that may be harmful to the host are triggered.
A virus protein pushes a cell towards a cancerous state by activating a gene that is switched off.

Damaging the immune defense

Interaction between host protein and protein produced by the oncogenic virus can damage the immune defenses.
This can lead to the development of cancer.