Viruses that cause cancer

If you ever thought that cancer could not be transmitted from an organism to another organism through a pathogen, I must say you are wrong! In 1911, a scientist named Peyton Rous from the Rockefeller Institute for Medical Research published a paper on the transmission of sarcoma with cellfree filtrate in fowl (Rous, 1911). This was enough for scientists to look for the link between viruses and cancers. For the discovery of tumour-inducing viruses, Peyton Rous was awarded the Nobel Prize in Physiology in 1966 (Weiss & Vogt, 2011). Today the viruses that cause cancer are called onco viruses. Around 15 per cent of all cancer in human beings is caused by viruses (Liao, 2006). The incorporation of viral genes into the infected host cell was studied by Howard Temin, David Baltimore, and Renato Dulbecco; they were awarded the Nobel Prize in 1975. Both DNA and RNA viruse are known to cause cancer. Epstein Barr Virus (EBV), Human herpesvirus 8 (HHV-8), Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human Papillomavirus (HPV), Human T lymphotropic virus type 1 (HTLV-1), and Merkel Cell Polyomavirus (MCV) are the major viruses that have shown to induce cancer in human beings. HCV and HTLV-1 are the only two RNA viruses among them. J. Michael Bishop and Harold E. Varmus found that the retroviral oncogenes have a cellular origin: They were awarded the Nobel Prize in 1989. There are many viruses that cause cancer in other animals and this article talks about the ones that induce cancer in humans. Epstein Barr virus (EBV) is associated with different kinds of diseases including cancers and other developmental disorders in humans. It was first identified in Burkitt’s malignant lymphoma by M. A. Epstein, B. G. Achong, and Y. M. Barr (Epstein & Barr, 1964). EBV is also known as Human gammaherpesvirus 4, belonging to the family of herpes virus. EBV can cause a variety of cancers like Burkitt’s lymphoma, Hodgkin’s lymphoma, gastric cancer, nasopharynx cancers, and many more. Once the virus enters the body, B cells and epithelial cells of the oropharyngeal cavity are the destinations for EBV. In B cells the virus goes into a highly restricted environment where the gene expression is highly reduced and leads to a lifelong latent infection (Rezk, Zhao, & Weiss, 2018). While in epithelial cells the virus enters the lytic phase, multiplying in large numbers and is transmitted and propagated through saliva. Currently, there aren’t any vaccines for preventing EBV, while immunotherapy has shown some success but there still are many obstacles to be cleared (Liao, 2006). Human Herpesvirus 8 (HHV-8) is a virus that belongs to the herpes family. Kaposi’s sarcoma-associated herpesvirus is another name for HHV-8. As the name suggests it causes Kaposi’s sarcoma which affects people with compromised immunity like people with AIDS. It also causes primary effusion lymphoma and other lymphoid disorders. Even here the B cells are predominantly infected. The viral genome expresses transforming proteins and anti-apoptotic factors, which inhibit apoptosis (programmed cell death). Apoptosis is necessary for the normal functioning of the organism. HHV-8 encoded G protein-coupled receptor (vGPCR) is a viral oncogene found in the infected cells which helps in its cell signaling of transformation and angiogenesis. There are antiviral drugs that target DNA replication of the virus. Ganciclovir is an antiviral that helps in reducing Kaposi sarcoma in AIDS patients (Liao, 2006). Hepatitis B virus (HBV) belongs to the family hepadnaviridae, it's a DNA virus. It is a blood-borne pathogen that causes acute and chronic hepatitis. Chronic hepatitis can lead to liver cirrhosis. Prolonged infections can result in the development of hepatocellular carcinoma. In the 1980s Hepatitis B vaccine was introduced and which can be denoted as the first vaccine against cancer even though it was to prevent hepatitis. There is interferon-alpha therapy which prevents HBV replication. There are many antivirals against HBV including adefovir, entecavir, lamivudine, and telbivudine (Liao, 2006).

Hepatitis C virus (HCV) is one of the two RNA viruses that have been linked to cancer in humans. It belongs to the family Flaviviridae. Like HBV, it also causes acute and chronic hepatitis. 1 to 2 % with cirrhosis will develop primary hepatocellular carcinoma. For the discovery of HCV, the Nobel Prize in Physiology or Medicine was awarded to Charles M. Rice, Harvey J. Alter, and Michael Houghton in 2020. Currently, there is no vaccine for HCV infection because of the high mutation rates. A covalent conjugate of interferon-alpha and polyethylene glycol along with ribavirin, an antiviral has shown to reduce the infection in half of the patients (Liao, 2006).

Human Papillomavirus (HPV) are non-enveloped DNA viruses that are known to cause warts in humans. There are hundreds of HPVs. HPV16 and HPV18 infection will lead to cervical cancer in females. The link between HPV and cervical cancer was discovered by Harald Zur Hausen and he was awarded the Nobel Prize in 2008. HPV infects the epithelial cells and incorporates its DNA into the host cells. The production of oncoproteins will influence the natural tumoursuppressing mechanism of the host. Early detection using Pap smear can be game-changing for better treatments. A vaccine that targets the capsid protein L1 is available and provides 100% protection (Liao, 2006). Human T Lymphotropic virus type 1 (HTLV-1) is the second RNA virus that causes cancer (discovered yet). It is a single-stranded RNA retrovirus that causes adult T-cell leukaemia. 12-25 million people are affected worldwide. Less than 5% of infected people will get cancer. The latency period of the infection is very long, 20-30 years. The Tax gene in HTLV-1 has a major role in tumorigenesis. The tax gene is an important target for therapies (Liao, 2006). Merkel Cell Polyomavirus (MCV) is a nonenveloped, double-stranded DNA virus. It was discovered in 2008. It causes Merkel cell carcinoma which is rare but aggressive skin cancer. Survivin, an oncoprotein is activated with MCV, inhibiting survivin has shown to decrease tumour progression (Rotondo et al., 2017). Humans are able to identify only these seven viruses that cause cancer in humans. Would there be many more to uncover?

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