Papillomaviruses

  • Structure and Genome : Isocahedral virus with covalently closed circular double stranded DNA. Its virions are 50-55 mm large in diameter.
  • They are small DNA viruses of mammals and birds.
  • Most cervical carcinoma in women result from infection with papillomavirus.
  • Entry: They enter the body through small abrasions and infect keratin making cells keratinocytes in skin or mucous membrane.
  • Site of Infection: There are well over 100 human papillomavirus (HPV) types, differentiated by their DNA sequence. Each HPV type infects a preferred site, such as hands or the genitals, and infection may result in a benign wart or a cercinoma.
    The papilloma virus that infect the genitals are transmitted between individuals during sexual contact.
    Most papilloma virus infections do not become persistant, but in minority of the host the infection is not cleared by the host immune system and they have small risk of cancer developing. The risk is associated with about 15 of HPV types called ‘High-risk’ type such as HPV-16 and HPV-18. Other HPV types that infect genitals carries little or no risk of cancer, these ‘low-risk’ type include HPV-6 and HPV-2.
  • After infection: it has 3 possible results; immuned by body immune response, or remains longer period and cause genital warts or cause cervical cancer.
  • Detection: Changes can be detected by morphological observation of a cervical smear, If precancer cells are detected they can be killed or removed to prevent the development of tumor.
  • The presence of all or part of the genome of high risk HPV type can nearly always be demonstrated in precancer cells and in cancer cells; The virus DNA is integrated into a chromosome. The virus replication cycle is not complete in these cells and no progeny virions are formed.
  • HPVs are also involved in a very rare form of skin cancer; epidermodysplasia verruciforms. Patients are highly susceptible to infection with HPVs – mainly 5 and 8. Warts spread over entire body during childhood and 25-33% patients develop cancer (squamous cell carcinoma) in areas of the skin exposed to UV light. Contrast to cervical cancers, the DNA is rarely integrated into a cell chromosome.

How papilloma virus bring about the cancer?

Keratinocytes, the host cell of papilloma virus , stop dividing as they differenciate, but when infected presence of HPV induces an infected keratinocyte into the S phase of the cell cycle due to the fact that HPV need DNA replicating machinery of the host cell. The cell then undergoes cycles of cell division, which generally have a finite number but occasionally division of a cell infected with high risk HPV continues unchecked as a cancer.

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