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Thursday, January 3, 2013

CANCER & VIRUSES

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The link between viruses & cancer was of the pivotal discoveries in cancer research.

  • In 2010, viruses were found to be linked to around 9,750 (3%) cancer cases in the United Kingdom, with human papillomavirus (HPV) responsible for around half of these.
  •  The Epstein-Barr virus has been linked to Burkitt's lymphoma. This virus infects B cells of the immune system and epithelial cells.
  • Worldwide, viruses are associated with the development of around 15% of cancers.
  •  Human herpes virus-8 has been linked to the development of Kaposi sarcoma. Kaposi sarcoma causes patches of abnormal tissue to create in various area of the body including under the skin, in the lining of the mouth, nose, and throat or in other organs.
  • Cancer develops in only a little proportion of individuals infected with cancer-linked viruses, usually.
  • The hepatitis B virus has been linked to liver cancer in people with chronic infections.
    Human papilloma viruses have been linked to cervical cancer. They also cause warts and benign papillomas. 

Viral infection is just one step in the process of cancer development. While this infection is necessary for certain cancers to develop, e.g. HPV in cervical cancer, the vast majority of these infected individuals will not develop cancer. Tumour viruses can therefore be described as risk factors for certain cancers.Viruses are parasites that require a host cell to replicate. Once inside a host cell, viruses hijack the cell’s replication machinery to make copies of themselves. The new virus particles can then spread to other cells in the same host or spread to a different host. Some viruses can persist in host cells without fully replicating for long periods of time, a process known as ‘latent’ infection. 

During latent infection, tumor viruses can cause genetic disruption to the host cell cycle machinery. Usually this disruption means activating genes that drive cell division forwards (oncogenes), or suppressing genes that restrict cell division (tumor suppressor genes). Together, these genetic disruptions act to drive host cell division forwards, predisposing the cell to further genetic mutations and increasing the likelihood of cancer development. These events occur by accident, as a result of the biological make-up of the virus.Some viruses are indirect tumor viruses. They do not genetically disrupt the host cell cycle machinery themselves. Instead, they set up an environment within the body that makes disruption from other sources more likely. For example, human immunodeficiency virus (HIV) depletes an individual’s immune system, making that person more susceptible to cancer caused by direct tumor viruses.


 
Cancer Treatment & Prevention:
The importance of the identification of an association between viruses & various types of cancer is that it opens up new possibilities for cancer prevention & treatment. Because virus-associated cancer cells express viral antigens, they can be recognized as 'foreign' by the immune technique. So vaccines can be developed which induce an effective immune response to the virus & can thereby prevent infection & consequent tumor production. Vaccines for HBV & HPV are at present being tested in clinical trials & are giving encouraging results. Also, where tumors create in the setting of immunosuppression, the key elements of the immune response controlling the virus infection, cytotoxic T cells, can be grown in the laboratory & given to patients to prevent or treat the tumor. With these various strategies, hopefully it won't be that long before the worldwide incidence of virus-associated cancers is dramatically reduced.


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