VIRUS AND CANCERS...?

                 Cancer is a class of diseases in which a group of cells display uncontrolled growth, invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood).Cancer may affect people at all ages, even fetuses. Cancer causes nearly 13% of all death around the world.

                                          There are many causes for cancer for cancer including chemical carcinogens, ionizing radiations, viral or bacterial infections, hormonal imbalances, immune system dysfunction and heredity.

Some cancers can be caused by infection with pathogens. There are many cancers that originate from a viral infection, and these viruses are responsible for 15% of human cancers worldwide. Many viral oncogenes have been discovered and identified to cause cancer. (An oncogene is an agent associated with cancer).Oncoviruses come in tow different forms: viruses with a DNA genome, and viruses with a RNA genome.

                                           There are many examples of DNA viruses in the world. Homan Papilloma Virus (HPV), is a DNA virus, which causes transformation in cells through interfering with tumor suppressor proteins such as p53. It cancers in cervix, skin, anus mouth, and may be lung cancer. Human herpes virus 8 is associated with Kaposi’s sarcoma, a type of skin cancer. Epstein Barr Virus (EBV) is associated with four types of cancers and they are Burkitt’s lymphoma, Hodgkin’s lymphoma, B lymph proliferative disease and nasopharyngeal carcinoma.

                                            There are RNA viruses too which can cause cancer. Hepatitis Virus causes Hepatocellular carcinoma (liver cancer).Human T cell leukaemia virus1 (HTLV1) cause adult T cell leukemia.
                             Mainly the DNA viruses insert their information directly into the cells of their hosts, although the Epstein Barr virus instead appears to exist in multiple copies as nucleic material known as a plasmid in the host cell’s nucleus, separate from the host DNA. The RNA viruses such as the HTLV 1 virus require first that their genetic information be transcribed into DNA by using the enzyme reverse transcriptase, supplied by the virus.
                              The oncogenic mechanism is ether to insert additional oncogenic genes in the host DNA, or to enhance already existing oncognic genes in the genome. These oncogenic  are best characterized in the genomes of oncogenic RNA viruses.
            Some oncogenic retroviruses (RNA viruses causing cancers) insert their genomic material into the host cell and use reverse transcriptase enzyme to make new DNA. This DNA is then incorporated into the cell DNA along with Powerful Promoter Sequences (LTRs) that Promote transcription Of the Viral DNA to reproduce more Viruses. Sometimes the Viral DNA incorporates a section of the host DNA which contains genes for growth promotion. These types of genes are called proto oncogenes in their normal state, which become oncogenic once incorporated into the viral DNA because of the increased transcription caused by the viral LTRs. This is the causes for the increased growth of the infected cell, leading to cellular proliferation and the tumor formation. A large number of oncogenes have been discovered in the genomes of transforming retroviruses. 

                  Other oncogenic retroviruses transform cells by integrating into the host gene near a proto oncogene. If the viral LTRs are close enough to that oncogene, they will up regulate transcription not only of the viral DNA but of the proto oncogene nearby, causing growth, cell proliferation and by consequence tumor formation.