2. AIM OF REVIEW
This study aims at highlighting the distribution and Epidemiology of Viruses
Associated with Cancer.
OBJECTIVE OF THE REVIEW
1. To highlight cancer causing viruses in human.
2. To highlight the mechanisms they use to cause cancer.
3. To highlight the epidemiology and distribution of these cancer causing viruses.
3. INTRODUCTION
Viruses are only able to reproduce by entering a living cell and manipulating the
cell’s machinery to create more viruses. During the viral replication process, certain
virus’s DNA or RNA affect the host cell’s genes in ways that may cause it to become
cancerous. These viruses are known as oncogenic viruses, meaning viruses that
cause or give rise to tumors. Oncogenic viruses belong to a number of viral
families, including the RNA virus families Retroviridae and Flaviviridae and the DNA
virus families Hepadnaviridae, Herpesviridae, Poliomaviridae and Papillomaviridae
(Klein, 2002).
4. Some of the viruses now known or suspected of being linked to cancer in humans
include; human papilloma viruses, the Epstein-Barr virus, the hepatitis B and C
viruses, the human herpes virus 8, human immunodeficiency virus (HIV) and the
human T-lymphotrophic virus-1 (Liao, 2006).
5. BODY OF REVIEW
Table 1: Carcinogenic mechanisms of oncogenic viruses
Source: (International Agency for Research on Cancer, 2009)
When an oncogene is activated by mutation, the structure of the encoded protein is changed in a way that enhances its transforming activity. Many types of mutation occur in oncogenes (Rodenhuis,
6. GROUPS OF ONCOGENIC VIRUSES
Oncogenic viruses can be divided into 2 groups, based on their genetic
material, as DNA and RNA tumor viruses (Zheng, 2010).
Table 2: Human oncogenic RNA viruses.
Source: (Zheng, 2010) HTLV: Human T-cell leukemia virus.
7. Table 3: Human oncogenic DNA viruses.
Source: (Butel, 2000; Zheng, 2010; McLaughlin-Drubin and Munger, 2008; Zheng and Ou, 2009) HBV:
Hepatitis B virus, EBV: Epstein-Barr virus, KSHV: Kaposi’s sarcoma-associated herpesvirus, HHV: Human herpes virus,
HPV: Human papillomavirus, MCV: Molluscum contagiosum virus
8. Epidemiology of viruses associated with human cancer
The International Agency for Research on Cancer has comprehensively assessed
the carcinogenicity of the biological agents to humans based on epidemiological
and mechanistic evidence (IARC 2009). Seven viruses including Epstein–Barr
Virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), Kaposi’s sarcoma
herpes virus (KSHV), human immune deficiency virus, type-1 (HIV-1), human T
Cell lymphotrophic virus, type-1 (HTLV-1),and several types of human papilloma
virus (HPV) have been classified as Group1 human carcinogen.
13. CONCLUSION
Globally, viral infections contribute to about 15% to 20% of all human cancers (Zur, 2001).
Several viruses with oncogenic potential stimulate cell proliferation and cause tumors and
cancer in animals and humans. They act with different mechanisms depending on different host
factors. The tumor viruses with small genomes integrate into host cell chromosomal DNA and
cause mutations and chromosomal rearrangements that predispose to cancer (Murat, 2012).
Viral infection is now considered as the second major preventable cancer risk factor after
tobacco use. The universal immunization program against HBV in Taiwan and Gambia has
successfully lowered the incidence of Hepatocellular carcinoma (HCC) in the vaccinated
population. The first HPV vaccine approved in 2006 are expected to reduce deaths from cervical
cancer by 75%, a number that should be a great inspiration for similar research on other human
oncogenic viruses as well (Zheng and Ou, 2009).
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