Over the years, it’s become apparent that certain viruses, bacteria and parasites can cause cancer, as exemplified by the Human papillomavirus, or HPV, which is implicated in cervical cancer and now subject of the new vaccination programme for teenage girls in the UK.
What many people don’t realise is that nearly 20 per cent of all cancers worldwide (and around 8 per cent in the developed world) are thought to be linked to infections (although it’s important to stress that cancer itself cannot be passed on between people except under vanishingly rare circumstances)
But while it’s relatively easy to find out which viruses, bacteria or parasites cause a particular infection, it’s very difficult to pin down an infection as being linked to cancer because the usual rules don’t apply.
For starters, many thousands of people are often infected, but only a few will ever go on to develop cancer. And finding the infectious agent in a sample of the tumour isn’t enough either – how can scientists know whether it’s causing the cancer, or simply along for the ride?
There’s also the issue that it can take many years for cancer to develop – by which time the original infection that caused it may well have cleared up.
So clearly, pinning down exactly which infections are responsible for which cancers, and how they affect a person’s chances of developing that cancer, requires well-planned, thoughtful, long-term research.
In a breakout session on the first full day of the NCRI Cancer Conference, we heard about some great case-studies of cancer-causing infections, and about the scientists’ work to find out exactly how these infections cause cancer, and who’s most at risk.
Kaposi sarcoma virus – a mean machine
First up was Dr. Thomas Schulz, from Hannover Medical School in Germany, who works on Kaposi sarcoma virus, also known as Kaposi sarcoma-associated herpesvirus, or KSHV. Discovered 15 years ago, this virus is linked to a cancer called (unsurprisingly) Kaposi’s sarcoma.
Dr. Schulz pointed out what a remarkable virus KSHV was. “If you were designing a perfect oncogenic [cancer-causing] virus, this would be it”, he told the session attendees.
He then went on to detail KSHV’s full armoury, which allows it to transform normal cells into a cancerous ones. Scientists have discovered that the virus can produce many proteins that can make cells grow and divide, and more that can stop cells killing themselves if they become too damaged.
But even though it’s a mean machine, people with a fully-functioning immune system appear to completely control this cancer-causing virus, and seldom develop Kaposi’s Sarcoma. It’s only when the immune system stops working, for example because of HIV infection or because of being treated with immunosuppressive drugs, that KSHV can take a hold and cause cancer. Indeed, people with an immune deficiency are about 1000 times more likely to develop the disease.
Helicobacter pylori and stomach cancer
Next, we heard from Dr. Jean Crabtree at Leeds University, whose particular interest is in the bacterium Helicobacter pylori, which is behind the vast majority of stomach cancers. She’s trying to find out exactly how the bacteria cause stomach cancer, and to find out what affects people’s risk of developing the disease.
It turns out that things about both the host and the bacteria affect how likely people are to develop chronic infections, and stomach cancer. From the host side, variations in the chemicals which the immune system produces in response to the infection can mean that the bacteria are more likely to avoid being killed and go on to cause stomach cancer.
But these bacteria have advanced weaponry too, including an ‘island’ of genes called ‘cag’ that they can pass on between one another. The genes act as a portable weapons depot that provides the bacteria with many tools for infecting their host. For example, they provide them with a ‘syringe’ that allows them to inject their own proteins directly into the cells that line the stomach. They break up the junctions between cells in the stomach lining. They promote inflammation. They mutate the p53 gene, the powerful ‘guardian of the genome’ that helps to protect us from cancer. They even hijack the cell’s normal communication lines to encourage them to become cancerous.
Understanding all of this can help to prevent disease too – a recent review of all the evidence has found that treating H. pylori infection in people who haven’t yet developed stomach cancer can cut the risk of the disease by 35 per cent.
New links between infections and cancer are being discovered all the time, like the new evidence presented in this session that Merkel Cell carcinoma, a rare but aggressive form of skin cancer, might be triggered by a virus. There’s also recent evidence that prostate cancer might be linked to a virus and a parasite. And understanding these associations, and the reasons why these infectious agents are causing cancer, can give us new ideas and approaches for both prevention and treatment.
(image from Wikipedia)
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