Cancer occurs when the mechanisms that control cell reputation get damaged. This could happen due to four main genes being damaged:
* Oncogenes that instruct the cell to divide and so if these are damaged then the cell keeps dividing;
* Tumour suppressor genes, these do the opposite and stop the cells from dividing but if these are damaged then the genes will never stop dividing;
* Suicide genes, these cause the cell to break itself down if something goes wrong, this process is called apoptosis, but without this function, if the cell was to become damaged it would replicate in its damaged form could eventually cause cancer;
* DNA-Repair genes, which are parts of the DNA that code of proteins that repair the DNA as it is constantly under attack, but if this gene is damaged then the repair proteins can no longer be made and that DNA can degrade.
The damage to the DNA as stated above can be caused by exposure to x-rays, UV-light, cigarette smoke, an unhealthy diet and even some viruses as well as many other things. So it is extremely difficult to prevent cancer from occurring in the first place. Instead we have to be able to kill the cancer after it has invaded the body. There are many techniques available to do this and they are constantly getting better, examples of a few are radiotherapy, chemotherapy and surgery. (Biological Science review, volume 5, number 1, September 2002) But this still isn’t good enough when, according to a article by the BBC published in 2003, only 50.8% of women and only 37.1% of men were still alive 5 years after being diagnosed. (http://news.bbc.co.uk/1/hi/health/3136874.stm)
This means that new ways of fighting cancer are desperately needed and one such way is the use of viruses or virotheropy. Viruses can be harnessed in many ways that make them unique cancer fighting mechanism. This isn’t a new idea; ‘In 1912 an Italian gynaecology journal reported the case of a woman with advanced cervical cancer who, after being bitten by a dog, was vaccinated with a live but weakened strain of the rabies virus. To the doctors surprise, her tumour shrank.’ But due to a lack of understanding of DNA and how viruses worked it wasn’t till 1991 that the next breakthrough was made when Robert Martuza and his team deleted the gene for the enzyme, called thymidine kinase, from the virus. The virus uses this enzyme to replicate. This renders the virus harmless to the human body cell but the enzyme is present in replicating cells, meaning that it thrives in the cancers.
Another method used is to mutate the virus so that its protein coat will only bind with the antigens on the cancer cells, this means that the virus can only attack the cancerous cells only and so can kill over a thousand cancer cells for every healthy cell where as chemotherapy drugs have a ration more like 6 cancerous cells to every health cell! (New Scientist magazine, 19 November 2005)
The reason the viruses are so good at destroying the cancer cells is that they are organisms that are designed to kill cells, they enter a cell, use the cell to replicate themselves then burst the cell, this process is called lysis or in the case of cancer cells oncolysis. And from the burst cell thousands more of the viruses emerge to infect the other cells. Another reason the viruses are particularly suited to attack the cancer cells is that when a healthy cell is infected by a virus it under goes apoptosis but as mentioned in the first paragraph cancer cells often have the “suicide gene” knocked out of them, meaning that cancer cells are much less resistant to viruses. (Advanced Biology by J Simpkins and JI Williams)
But putting a live virus into the human body is a risky idea and so the viruses must be weakened so that they cannot spread and pose a risk to healthy cells. But in weakening these viruses it has meant that success in the field so far has been limited. The weakened viruses are a easy target for the immune system. (Advanced Biology by Michael Kent) This leaves two options, immunosuppressant drugs can be used to stop the immune system from fighting back but many scientists believe that we should instead leave the immune system intact and use viruses to harness the immune system, getting it to attack the tumour. This is done by adding an immune-stimulating protein to the virus, this means that when the virus enters the cell the protein is replicated with the rest of the virus and so when the cell undergoes oncolysis the protein is released around the tumour causing the immune system to attack the tumour.
None of these methods are proven to work with an extremely high success rate but are under clinical trials at the moment, it is most likely that they will be used in conjunction with radiotherapy and chemotherapy to treat cancer. One of the main problems with cancer at the moment is that it is very difficult to stop it spreading, then the main cancer is treated it can spread round the body just to pick up in another organ. Viruses may play a vital role in stopping this as a “stealth” virus can be released into the blood stream to mop up any cancer cells that have escaped the other treatments.
These viruses can’t be detected by the immune system because they are covered in an inert polymer that stops they from being recognised as it hides any antigens. Then you can add specific cancer binding proteins to the polymer coat and you have a virus that will only bind with specific cancer cells. This does mean that the daughter cells won’t have the stealth coat and so the second generation will be destroyed by the immune system.
So although the different methods of virotheropy are still in a stage of testing they seem a promising tool in the fight against cancer as they are so versatile may soon offer new hope.