RNA VIRUSES OF VIRAL CARCINOGENESIS IN CANCER
6.2.1. RNA viruses
RNA virus infect competent cells. Their RNA is converted into DNA that incorporated into the host genome. Hence, that is classification as retroviruses. Example of retrovirus: human T-lymphotropic virus(HTLV-1) and Rous sarcoma virus(RSV). RNA viruses show two general ways: provision of an oncogene or insertional mutagenesis in which regulatory sequences alter host gene activity.
Provision of an oncogene: Many oncogenic RNA viruses contain an oncogene additional to the sequences for viral replication. The Rous sarcoma virus, in which the oncogene v-src codes for a 60 kDa phoshoprotein(pp60src) that has tyrosine kinase activity.
Insertional mutagenesis: Mouse mammary tumour virue(MMTV) has regulatory sequences that can stimulate host genes. The regulatory sequences are repeated at each end of viral genme(long terminal repeats, LTRs). The LTRs contain enhancer sequences capable of increasing transcription from nearby genes(int) that codes for fibroblast growth factor. HTLV code for transcription factors that regulate host genes such as fos.
6.2.2. DNA viruses
Several DNA viruses are specific in human cancers: human papilloma virus(HPV) , hepatitis B virus(HBV), Epstein-barr virus(EBV), Kaposi sarcoma virus(KSV). The cellular responses of these viruses derive from the proteins E6 and E7. The E6 and E7 mRNAs are generated by cleavage of larger RNA. Carcinogenesis is time-dependent, multistage process; inactivation of Rb and p53 by viral E6 and E7 proteins. The progress of the disease is favoured by various cofactors such as tobacco, smoke and oral contraceptives.
If a protooncogene under goes a somatic mutation, control of cell growth is lost in the cell in which the mutation occurs and cancer can occur. Those protooncogenes which have been shown to mutate in any individual are called cellular oncogenes and are designated by the prefix "c" (i.e. c-myc, c-abl) to distinguish them from the viral oncogenes. Those protooncogenes that have not been found to mutate are called normal oncogenes and are designated "n" (i.e. n-ras).
6.2.3. Reterovirus Associated Oncogenes
The oncogene of those viruses transform a cell to unproliferated growth carry an oncogene in addition to the three primary genes required of all retroviruses. The figure below is the generalized structure of a retrovirus.
Example of Reterovirus
Rous Sarcoma Virus.
The oncogene found in this retrovirus is src. Src is a receptor Tyrosine kinase and involve in cell division and cell proliferation. After the infection of Rous Sarcoma Virus the cell start expressing more src which leads to tumor formation.
Avian erythroblastosis Virus
The oncogene found in this virus is V-erbB. V-erbB is epidermal growth factor receptor. V-erbB is a receptor Tyrosine kinase and involve in cell division and cell proliferation. After the infection of Avian erythroblastosis Virus the cell start expressing more V-erbB receptor on cell surface which leads to tumor formation.
Harvey Murine Sarcoma
The oncogene found in this virus is V-H-Ras. V-H-Ras binds to the GTP. V-H-Ras involve in cell division and cell proliferation. After the infection of Harvey Murine Virus the cell start expressing more V-H-Ras which leads to tumour formation.
6.3. Tumour suppressor genes
Tumor suppressor genes are genes that slow down cell division, involve in DNA Repair and apoptosis. When tumor suppressor genes don't work properly, cells can grow out of control, which can lead to cancer that is created by loss of function mutation. In contrast activate mutation inactivated oncogene from protooncogenes, tumor suppressor genes and proteins that are encode. Tumor suppressor gene suppress the tumor. Tumor suppressor gene control the processes of genetic of integration, the progression of cell cycle, differentiation , cell-cell interactions and apoptosis.
Tumor suppressor genes are inactivated by mutations contribute to the loss of tissue homeostasis –the hallmark of a developing neoplasm. The open reading frames of tumor suppressor genes are commonly truncated by nonsense mutations, frameshift mutations, small addition or deletions, or splice site mutations. Sometime exon removed by larger deletions. Tumor suppressor gene also inactivated by alteration of single nucleotide residues in tumor suppressor protein, there by encoded protein become non-functional.
TSG can be grouped into categories including caretaker genes, gatekeeper gene and landscaper genes Retinoblastoma.
It is caused by loss of both alleles of a gene. Rb is tumor suppressor protein. Mutation in Rb results in cancer. Normal cell express suppressor protein that inhibits the cancer.
6.3.1. Ratinoblastoins :
Rb is a large gene (300kb), although mostly mutations are in the 3kb coding region and mostly involve chromosomal changes. Several retinoblastoma can arise in one eye, each with a different Rb mutation. The Rb protein has about 10 phosphorylation sites. Rb has the ability to interact with other proteins. Over 25 Rb-binding protein have been identify with functions such as nucleosome structure(Brm), Tyrosine phosphorylation (abl), oncogenes (Mdm2) and transcription factors (E2F, DP) are responsible for proliferation. The Rb proiein has more than 10 phosphorylation (ser/thr) sites. Conversion from hypo- to hyperphosphorylated states changes the interaction of Rb with other proteins. Rb binds with transcription factor E2F. It is regulated by phosphorylation. In hypophosphorylated state of Rb the E2f is bounded this makes E2f inactive. The proliferation of cell cycle is regulated by cyclin dependent kinases (CDKs). These cyclin-dependent kinases phosphorylate and inactivate Rb, there by relieving the cycle block.
The released E2F stimulates the transcription of gene that regulate growth , such as cdc2, myc and DNA polymerase.
Rb also inhibits transcription from rRNA and tRNA genes by binding of UBF (upstream binding factor) and TF-IIIB(transcription factor IIIB) . Rb thus influences the mass of a cell(protein content). Rb suppression occurs normally by hyperphosphorylation and abnormally by Rb mutation or binding to other proteins.
The p53 tumor suppressor gene is activated in response to a wide variety of cellular stresses including DNA damage, ribonucleotide depletion, redox modulation, hypoxia, changes in cell adhesion, and the stresses created by activated oncogenes. The p53 protein work as transcription factors and activated the genes which is involved in DNA repair, apoptosis and growth arrest. These activities of P53 help in maintaining the genomic stability. Hence P53 is called as guardian of genome.
If normal p53 is mutated and non-functional by binding of other protens.p53 has four functional domains involved transcription activation domain(TAD), DNA binding domain(DBD), oligomerization domain(OD), autoinhibitory domain(AID). Each domains binds to several proteins that regulate p53 function. P53 binds to its response element present upstream to the gene to transcribe the gene. P53 binds in tetrameric form. p53 increase the expression of Bax, p21, insulin-like growth factor binding protein 3(IGFB3), GADD45 and thrombospondin. Expression of genes such as Bcl2, Fos and jun can be inhibited by p53. Thus p53 inhibits cell proliferation.
The gene p21 that code for cyclin-dependent kinase inhibitor. p21 inactivates CDK that is essential for DNA synthesis. GADD binds to a protein proliferating cell nuclear antigen(PCNA) that needed for both DNA synthesis and repair. Hence, p53 inhibit DNA synthesis while allowing repair to continue. DNA damage activates p53 function by post –transcriptional and cell-type specific mechanisms.
Normal p53 show inactivation by protein binding. For example- Adenovirus codes for E18 that binds to TAD of normal p53 and block its transcription. Some human sarcomas have mdm2 gene that have same end results. Human papilloma virus have protein E6 that binds to OD and prevents dimerisation, while the HBX protein of hepatitis B virus binds and inactivates p53.
P53 transcribe the thrombospondin, PAI, KAI, BAI and they block the angiogenesis. Mutation in p53 thrombospondin, PAI, KAI, BAI.