Viruses are to reproduce only inside living cells .Some DNA and RNA viruses affects host cell’s genome in a way that may cause it to become cancerous.
The virus that may cause cancer is called oncogenic virus. This name is originated from the studies of transforming retroviruses in the 1950–1960s, usually termed oncogenic viruses to indicate their RNA virus origin. It is now referred to any virus with a DNA or RNA genome that cause cancer and it is also called “tumor virus” . Most of human and animal viruses do not cause cancer, probably due to the longstanding common evolution between the virus and the affected host. Oncogenic viruses have been important in both epidemiology, and investigations of cell cycle control mechanisms such as Retinoblastoma protein.
It is essential to note that the process from the primary viral infection to tumor generation is slow and inefficient; only few number of viral infections complicate to cancer after the initial infection. Other factors may enhance the chance of cancer occurrence including immune system complications, cell mutations, cancer agents exposure and hereditary susceptibility.
Tumor viruses have many forms: Viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the Hepatitis C virus (HCV), as can retroviruses having both DNA and RNA genomes (Human T- lmphotropic virus and hepatitis B virus, which replicates normally as a mixed single and double-stranded DNA virus but also has a retroviral replication component). In many cases, tumor viruses do not cause cancer in their native hosts but only in dead-end species
Information about virus life cycle:
For most virus: (replication -; lysis -; progeny Virions) lytic life Cycle: genome -; all viral proteinslatent life Cycle: viral cell -; cell -; integration (usually) -; transformation.
– Some virus specific protein expressed (early function) – Sometimes latency may terminate cell must be infected by complete virus.
Transformation:1/The loss of growth control 2/reduced adhesion3/motility4/invasion5/ability to form tumorsThe interfere of viral genes with cell replication control and other sides of the cell phenotype
The transformed cells frequently exhibit chromosomal aberrations
Both RNA and DNA tumor virus can transform cells .
Integration of viral genes into the host chromosomes occurs usually.
The DNA Tumor viruses
The human papilloma virus
The Kaposi’s sarcoma associated Herpes Virus
-Primary effusion lymphoma
-The metacentric castleman’s disease (MCD)- a rare lymphoproliferative disorder
Epstein-Barr virus (EBV)
Merkel cell polyoma virus (polyoma virus )
Human cytomegalo virus (associated with mucoepidermoid carcinoma )
Human papilloma virus
There are over 100 different species of human papilloma viruses that can cause warts on the skin, mouth, genital organs and larynx. They are spread by human contact and are a common sexually transmitted disease. Currently there are no effective treatments for the HPV other than the removal of infected cells. Later on ,the immune system of the body usually kills the virus.
Human papillomavirus is a widespread sexually transmitted infection. Most sexually active men and women are exposed to the virus at some point during their lifetime. Specific human papilloma viruses are known to cause cervical cancer, which is the 2nd most common cancer among women globally.
HPV is estimated to cause 5% of cancers worldwide according to the National Cancer Institute. An oral HPV infection may also cause oropharyngeal cancers .
papilloma Viruses -; epidermodysplasia verruciformiswart -; malignant sq.c.cmost cervical vulvar and penile cancers are associated with types 16 and 18 (70% of penile cancer ).
effective vaccine :quadrivalent recombinant HPV126.96.36.199 proteins made in yeast – Gardasil – Cervarix Kaposi’s sarcoma associated Herpes Virus
Kaposi’s sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV-8) is present in all clinical variants of Kaposi’s sarcoma (which is a cancer commonly occurring in AIDS patients) and two lymphoproliferative diseases: primary effusion lymphoma and multicentric Castleman’s disease (MCD). Studies have proved the link between KSHV/HHV-8 infection and development of Kaposi’s sarcoma. The virus encodes a number of genes homologous to human genes involved in cell proliferation, anti-apoptosis, angiogenesis and cytokine action. In Kaposi’s sarcoma and primary effusion lymphoma most tumour cells only show evidence of latent infection, with a restricted gene expression pattern.
Epstein-Barr virus (EBV)7
It is one of eight known human herpesvirus types in the herpes family, and is one of the most common viruses in humans. It is called human herpesvirus 4 (HHV-4). It is best known as the cause of infectious mononucleosis (glandular fever). It is also associated with particular forms of cancer, such as Hodgkin’s lymphoma, Burkitt’s lymphoma, gastric cancer, nasopharyngeal carcinoma. There is evidence that infection with EBV is associated with a higher risk of certain autoimmune diseases, especially dermatomyositis, systemic lupus erythematosus, rheumatoid arthritis, Sjögren’s syndrome, and multiple sclerosis. Some 200,000 cancer cases per year are thought to be attributable to EBV. Infection with EBV occurs by the oral transfer of saliva and genital secretions. The virus remains with the person throughout life by infecting epithelial cells and a type of white blood cells known as B lymphocytes or B cells. However, after a few weeks of initial infection, most people show no symptoms.
Merkel cell polyomavirus (polyomavirus )
Polyomaviruses are small, non-enveloped DNA viruses, which are widespread in nature. In immunocompetent hosts, the viruses remain latent after primary infection. With few exceptions, illnesses associated with these viruses occur in times of immune compromise, especially in conditions that bring about T cell deficiency . The family of this virus is called Polyomaviridae viruses whose natural hosts are primarily mammals and birds. the human polyomaviruses are known to cause, respectively, hemorrhagic cystitis in recipients of bone marrow transplantation and progressive multifocal leukoencephalopathy in immunocompromised patients, for example, by HIV infection. Recently, transplant nephropathy due to BKV infection has been increasingly recognized as the cause for renal allograft failure
polyoma virus transforms cells when the genome is incomplete and early functions are necessary.
Human cytomegalo virus
(associated with mucoepidermoid carcinoma )
Human cytomegalovirus (CMV) infection has tropism for salivary gland ductal epithelium and establishes a persistent and lifelong infection. Mucoepidermoid carcinoma (MEC) is the most common salivary gland tumor. Recent studies have established that mouse CMV-induced tumorigenesis that shows histologic and molecular characteristics similar to human MEC. It is a member of the viral family known as Herpesviridae or herpesviruses. It is also known as human herpesvirus-5 (HHV-5). It can be life-threatening for the immunocompromised, such as HIV-infected persons, organ transplant recipients, or newborn infants. Congenital cytomegalovirus infection can lead to significant morbidity and even death. After infection, HCMV remains latent within the body throughout life and can be reactivated at any time. Eventually, it may cause mucoepidermoid carcinoma and possibly other malignancies such as prostate cancer.
They are members of the family Adenoviridae. They are non enveloped viruses with an icosahedral nucleocapsid having double stranded DNA genome . They were first isolated in the 1950s in adenoid tissue–derived cell cultures, hence the name. These primary cell cultures were often noted to spontaneously degenerate over time. They have a broad range of vertebrate hosts; in humans, more than 50 distinct adenoviral serotypes have been found to cause a wide range of illnesses, from mild respiratory infections in young children (known as the common cold) to life-threatening multi-organ disease in people with a weakened immune system. Adenoviruses are now known to be a common cause of asymptomatic respiratory tract infection that produces in vitro cytolysis in these tissues.
Highly oncogenic and in animals only part of virus integrated.
RNA Tumor viruses
Not all oncoviruses are DNA viruses. Some RNA viruses have also been associated such as:
1) Hepatitis C virus
As well as certain retroviruses such as:
2) Human T-lymphotropic virus (HTLV-1) and 3) Rous sarcoma virus (RSV).
They are classified into: 1) RNA Genome – Retroviruses
2) RNA-dependent DNA polymerase encoded by virus REVERSE TRANSCRIPTASE
3) RNA genome by reverse transcriptase DNA genome by integrase integrates by Host RNA polymerase RNA genome
A normal retrovirus has: 3 genes
GAG: internal proteins
ENV: Envelope glycoprotein
In the gag region of the virus, the synthesis of the internal virion proteins are maintained which make up the matrix, capsid and nucleocapsid proteins. In pol, the information for the reverse transcription and integration enzymes are stored. In env, it is derived from the surface and transmembrane for the viral envelope protein. There is a fourth coding domain which is smaller, but exists in all retroviruses. Pol is the domain that encodes the virion protease.
hepatitis C virus
Hepatitis B virus (HBV) and hepatitis C virus (HCV) HBV and HCV are two types of viruses that cause viral hepatitis, a type of liver infection. Hepatitis A can also cause viral hepatitis but HBV and HCV are known to cause long term infections that increase chances of liver cancer. Worldwide, most liver cancers are caused by HBV or HCV. Hepatitis C virus (HCV) is a small (55–65 nm in size), enveloped, positive-sense single-stranded RNA virus of the family Flaviviridae.
Both HBV and HCV are spread most often spread through unprotected sex, sharing needles or childbirth. Hepatitis B virus known to cause flu like symptoms and jaundice, or yellowing of the eyes and skin. Most people with HBV infection recover completely and are not chronic carriers of the virus. Hepatitis C infection may not cause any symptoms and is more likely to be chronic, which can lead to liver damage or even cancer.
Antiviral drugs are available to treat people with Hepatitis B and C. There is also a preventative but not therapeutic vaccine available for Hepatitis B, which is most commonly given to people who are at risk of contracting the virus, such as health care workers. There is no vaccine available for the Hepatitis C virus.
Hepatitis B virus
RNA polymerase II Host enzyme
Reverse transcriptase viral enzyme
human T-lymphotropic virus (HTLV-1)
It is a member of family of viruses are a group of human retroviruses that are known to cause a type of cancer called adult T-cell leukemia/lymphoma and a demyelinating disease called HTLV-I associated myelopathy/tropical spastic paralysis . During the replication process, the virus uses an enzyme called reverse transcriptase, which allows the virus to change its RNA genes into DNA. This allows the virus genes to become integrated into the genes of the host cell and can cause a mutation in the host cell genes that controls how the cell divides. This change can sometimes lead to cancer. HTLV-1 has been linked to a type of lymphocytic leukemia and non-Hodgkin lymphoma called adult T-cell leukemia/lymphoma (ATL). It is most common in populations of southern Japan, Caribbean, Central Africa, and some areas in the United States.
Human T-lymphotrophic virus-1 is spread by unprotected sex, needle sharing, pregnancy, childbirth or breastfeeding. Once infected, a person may live 20 years or more with no symptoms. There is about a 5% chance that the infection will lead to cancer.
Rous sarcoma virus (RSV)
It is the first oncovirus to have been described, it causes sarcoma (, a tumor of connective tissue) in chickens.As with all retroviruses, it reverse transcribes its RNA genome into cDNA before integration into the host DNA. Peyton Rous discovered that cancer could be induced in healthy chickens by injecting them with a cell-free extract of the tumor of a sick chicken. He ground up samples of the tumor and passed the material through a filter with pores so fine that not even bacteria could get through. However, the tumor filtrate was able to induce cancer when injected into chickens. This was the first demonstration of an oncogenic virus.
Virus Associated cancer types
Hepatitis B (HBV) HepatocarcinomaHepatitis C (HCV) HCV is a known carcinogen, causing hepatocarcinomaHuman T-lymphotropic virus (HTLV) Adult T-cell leukemiaHuman papillomaviruses (HPV) The types 16 and 18 are associated with cancers of cervix, anus, penis, vulva/vagina, and oropharyngeal cancer.
Kaposi’s sarcoma-associated herpesvirus (HHV-8) Kaposi’s sarcoma, multicentric Castleman’s disease and primary effusion lymphoma
Merkel cell polyomavirus (MCV) Merkel cell carcinoma
Epstein–Barr virus (EBV) Burkitt’s lymphoma, Hodgkin’s lymphoma, Post-transplant lymphoproliferative disease and Nasopharyngeal carcinoma.