Viruses are very small organisms. Most of them cannot be seen with an ordinary microscope. A virus has to enter a living cell and take over the machinery of the cell in order to reproduce more viruses.
Some viruses perform this by inserting their own DNA or RNA into the DNA of the host cell. When the DNA or RNA affects the genes of the host cell then it can drive the cell on the way to becoming cancer.
Each type of virus is liable to infect only a particular type of cell in the body. Various viruses are associated with cancer, which has led to the formulation of vaccines to assist cancer prevention. However, these vaccines can only provide protection against infections if they are administered before the person is exposed to the virus promoting the cancer.
Like the chemicals and radiation, viruses are another source of mutation. They can disturb the cell behavior in many different ways:
- They can directly cause mutations by inserting their genomes into the DNA of the host cell. The combination can interrupt vital regulatory genes.
- The viruses may have their own genes, which interrupt the cell regulation. This can be seriously harmful to the host.
- Several viruses actually hold modified altered versions of genes, which have been picked up from the previous host cells.
These modified genes do not perform well. When inserted into a new host cell, they lead to dysregulation and can promote cancerous growth.
Viruses can play a considerable role in the development of specific cancer through their mutagenic activity or effects on cell behavior.
In this article we will discuss
1. Viruses known to cause human cancers
- Epstein-Barr Virus (EBV)
- Hepatitis B Virus (HBV)
- Human T-lymphotrophic virus-1 (HTLV-1)
- Merkel cell polyomavirus (MCV)
- Human herpes virus 8 (HHV-8)
- Human papillomaviruses (HPVs)
Epstein-Barr Virus (EBV)
It is linked to lymphoproliferative disorder, which is most commonly known as Burkitt’s lymphoma. According to data EBV is also linked with Hodgkin lymphoma.
It is predictable that more than 90% of the people are infected with EBV in the world. It is probably transmitted through saliva.
EBV infection generally starts in the epithelial cells of the oropharynx, parathyroid gland and posterior nasopharynx, where it infects the B cells and continual infection is recognized.
B cell infection is needed for EBV mediated carcinogenesis. Very less percentage of infections result in cancer. Most of the cases arise in transplanted or immunocompromised persons because they lack adequate immune function to block the development of infected B cells.
EBV-mediated carcinogenesis is usually caused due to the actions of viral gene products. EBV nuclear antigen (EBNA’s) and latent membrane proteins (LMP’s) are the two proteins that usually are thought to perform the primary role in B cell immortalization.
Hepatitis B Virus (HBV)
This virus is linked with Human Hepatocellular Carcinoma (HCC). The HBV is transmitted through contact with contaminated tears, blood or sweat. It can also be transmitted from mother to baby and via sexual contact.
HBV infection arises mostly in the liver, but viral antigens can be identified in the blood. The chronic infection is seen by the occurrence of viral antigens in the blood for more than 6 months. It can cause cirrhosis of the liver and progress of HCC.
Viral integration into the host genome is frequently observed in chronic liver infection and cancer, which reveals a considerable role of virus in carcinogenesis.
HBV encodes a protein, which may stimulate the cell production and hinder the DNA repair.
Human T-lymphotrophic virus-1 (HTLV-1)
HTLV-1 has been associated with adult T-cell lymphoma/ leukemia (ATL). This virus can cause other medical conditions also.
These viruses use RNA for their genetic code. They change their RNA genes into DNA to reproduce. Various new DNA genes can become part of the human cell chromosomes infected by the virus. This can cause modification in the cell growth and division that can lead to cancer.
HTLV-1 is transmitted similarly as HIV, such as through sexual contact and using injection needles of an infected person. It can pass through the breastfeed of infected mothers.
Merkel cell polyomavirus (MCV)
The majority of people are infected with MCV at certain points usually in childhood and it generally causes no symptoms.
However, in some people this infection can affect the DNA within cells that can lead to Merkel cell cancer. Almost all Merkel cell cancers are now believed to be associated with this infection.
Human herpes virus 8 (HHV-8)
It is also called Kaposi sarcoma–associated herpes virus (KSHV). Kaposi sarcoma is a slow-growing cancer, which usually appears as blue-brown or reddish-purple tumors underneath the skin.
In this sarcoma, the cells, which line the lymph and blood vessels, are infected with HHV-8. This infection causes the cells to divide too much and live longer. Such changes may ultimately change them into cancer cells.
HHV-8 is transmitted through sexual contact and may be through other means such as saliva and blood.
Human papillomaviruses (HPVs)
Human papillomaviruses (HPVs) are a group of above 150 related viruses. Certain types of HPV only grow in skin, though the others grow in mucous membranes like vagina, mouth or throat.
All HPVs spread through contact and more than 40 types of HPV transmit by sexual contact. Minimum, a dozen of HPV types are known to cause cancer.
Some types of HPV are the major cause of cervical cancer. HPV can also cause cancers of the penis, throat, anus, vagina, vulva and mouth.
Now, vaccines are available to assist in protecting the young adults and children against infection of major cancer-causing types of HPV.
The vaccines can help only to prevent HPV infection; they cannot stop or assist the treatment of existing infection.
There are various vaccinations available that can prevent viral infections. You can also follow certain good lifestyle habits to prevent viral disease and infection, such as:
- washing your hands frequently
- not sharing personal items such as razors and toothbrushes
- using barrier protection during sexual activity
- getting regularly screened for HPV, HIV and HCV
- not sharing needles
It is important to know that having an infection by an oncogenic virus does not mean that you will develop cancer. It just means you may have a higher risk of developing cancer than those people who never had the infection.