Today's lecture was a guest lecture concerning viruses and their role in cancer, particularly how they can cause cancer and can be used to treat cancer.
What is cancer?
Cancer, in short, is an uncontrolled proliferation of cells. It is the 2nd leading cause of death in developed nations - 6 million people worldwide die from cancer every year. Three main therapies are used to treat it: radiation, chemotherapy and surgery. The first two are specific in that they target cells that are rapidly dividing. This may be problematic in that tumor cells are not the only cells that divide rapidly; certain bone cells, for example, quickly divide and may be targeted by these treatments.
The process whereby a normal cell becomes cancerous is called transformation. A transformation may be a change in the morphological, biochemical or growth patterns of a cell. Cells which are transformed become immortal, that is, they can grow and divide indefinately. Normal cells are subject to what is called the Hayflick Limit, at which a cell will stop dividing (this occurs when the telomeres are shortened to a length below which becomes deleterious to the cell). Cells which are transformed may have one or more altered phenotypes, such as a loss of anchorage dependance (normal cells will not grow unless attached to the extracellular matrix), loss of contact inhibition (normal cells will stop growing when they fill the available space and contact one another), a decreased requirement for growth factors (i.e. they will continue to grow even when not supplied with the signal to grow), or gross morphological changes.
Both RNA viruses and DNA viruses may transform cells into cancer cells. There are three ways in which viruses can do this: by expressing a modified version of a cellular proto-oncogene, by cis-activation of proto-oncogenes or trans-activation of proto-oncogenes.
[Proto-oncogenes are genes that, when mutated, result in the formation of tumors]
Expression of Modified Proto-oncogenes
An example of this method of cancer induction comes from the Rous Sarcoma Virus, the first oncogenic virus studied. This virus contains a gene called v-src. It is very homologous to a cellular gene called c-src (v=viral, c=cellular). The c-src gene product normally encodes for a tyrosine kinase. It contains two domains at one end of the protein (in the kinase domain), the SH3 and SH2 domains. The SH2 domain is of particular importance; it binds to phosphotyrosine residues. At the opposite end of the c-src protein lies a regulatory domain. The 527th amino acid in the protein lies in this region, and is a tyrosine. This tyrosine gets phosphorylated, and the SH2 region of the kinase domain binds to it. This results in an inactive protein. The usual fuction of c-src is to work in a signaling cascade to turn on transcription factors that will express certain genes involved in the cell cycle; when it is on, cells grow and divide. When inactive, these genes do not get expressed, and no cell growth.
The v-src gene, however, encodes for a truncated protein. It is cut off at the 526th amino acid residue. That is, it does not contain the Tyrosine residue required to keep the enzyme inactive. When expressed in a host cell, it is always turned on, resulting in contant expression of the downstream genes, and uncontrolled cellular growth and division - in other words, tumor formation.
Cis-activation of proto-oncogenes
This method involves the integration of a viral genome in to the host genome. Eukaryotic cells are often tightly regulated, with promotor sequences, and regulatory or enhancer elements controlling the expression of the gene. Insertion of the viral genome is often a random process, and in the event that the genome were to insert into the repressor element of a gene, then the repressor would become nonfunctional. This would lead to an inability to turn the downstream gene off when it is not needed. The gene would be constitutively expressed. If this gene is a proto-oncogene, then the result is likely to be the formation of a tumor.
[This is called CIS-activation because the viral agent that is causing the activation is in cis, that is, is on the same chromosome, as the gene it is infecting. "Cis" comes from "cistron", the old name for genes.]
Trans-activation of proto-oncogenes
This method involves a viral protein directly acting on a transcription factor, which causes proto-oncogenes to be upregulated. The resulting upregulation can lead to increased or unconrtolable cellular growth.
An example of how viruses can cause cancer can be seen in the Papillomavirus family of DNA viruses. There are over 106 types of Papillomavirus, and over 40 of these are able to infect the mucosal surfaces of the the genetail and aero-digestive tracts. HPV (Human Papillomavirus ) has been detected in some 99.7% of cervical cancers.
HPV has two primary viral oncoproteins: E6 and E7. They are involved in disregulating the cell cycle. During the cell cycle, cells go through a phase of DNA replication, known as S-phase. If there is damage to the DNA, or errors with improper cell division, replicating the DNA during S-phase can result in dire problems for the cell. Tumor supressor genes will stop the cell from completing S-phase if this is the case. The HPV E6 and E7 proteins block the action of these tumor supressor genes, allowing cells to go through S-phase, and thus DNA replication, growth and division, unregulated.
One tumor supressor that is affected by HPV is the Retinoblastoma protein (Rb). Rb binds a transcription factor called E2F, and keeps it from fuctioning. When E2F is needed, Rb is phosphorylated and E2F dissociates and is allowed to activate the transcription of genes involved in DNA synthesis, and consequently, progression through the cell cycle. The HPV E7 protein interferes with the E2F:Rb complex - E2F is constantly active, leading to unscheduled cellular DNA synthesis as well as viral DNA synthesis. Also, there is a "High Risk" form of E7 where Rb is not only bound but is targeted for proteosome-mediated degredation.
The cell has a safeguard against this, however. The p53 protein is translated under cellular stress or DNA damage, and it halts the proliferation of cells, and plays a role in apoptosis. This ensures that defective cells do not run the risk of becoming cancerous. This is where the viral E6 protein comes into play - it binds p53 and blocks it from fuctioning. Much like E7, E6 has a 'high risk' variant. This form of E6 recruits an E2 ubiquitin ligase which polyubiquinates the p53, marking it for degradation via the proteosome.
From the above, it can be seen that DNA and RNA viruses have deifferent roles in cancer. RNA viruses encode for viral concogenes, while DNA viruses have mechanisms evolved to counter cellular tumor-supressor genes.
Another area of cancer that viruses play a role is in cancer therapy. Oncolytic viruses are viruses that are used as a form of cancer therapy. In this method, live viruses are used to selectively lyse cancerous cells, while normal cells are left unaffected. The viruses infect tumor cells, replicate, lyse the cells and then spread to other uninfected tumor cells. For some viruses, this selectivity is not perfect and normal cells can be infected; however, the viruses will grow better in tumor cells than in normal cells and (hopefully) will pick tumor cells over normal ones. The ideal features of such a virus are that they cause only mild human diseases that are well characterized, have secondary inactivation mechanisms, do not damage normal cells, have low mutation and recombination frequencies, do not spread from host to host and cannot integrate into the host genome.
Once such oncolytic virus is Onyx 015, a modified version of Adenovirus. This variant lacks the adenovirus E1b55K and E1b19K proteins which block p53 and apoptosis. It is hypothesized that its E1a protein binds Rb and E2F leading to increased DNA replication of the viral genome, and lysis of the tumor cells. What keeps it from destroying normal cells is that this also activates p53, and p53 keeps the infected normal cells from spreading the virus. The infected tumor cells do not have p53 and thus are unable to stop lysis and spread of the virus to other tumor cells.
Another such virus is myxoma virus. This virus usually infects European rabbits and not humans. However, it has the ability to infect human tumor cells instead. The benefit of this virus over the Onyx 015 is that it is natural rather than engineered.