Cancer Research UK funded scientists have for the first time identified a common genetic variant that can increase a person’s risk of developing bowel cancer. Their findings appear in two papers published online in Nature Genetics1,2 today (Sunday).
Several genes are already known to contribute to bowel cancer risk. However, these are extremely rare among the population – only around one person in every 2,500 carries any of the known bowel cancer genes and they account for less than five per cent of bowel cancer cases arising annually. Around 35,000 new cases of bowel cancer are diagnosed in the UK each year and it is estimated that genetic risk contributes to around a third of cases of the disease.
Reporting their findings in two separate papers, research teams based in Edinburgh and London studied the genetic make-up in a total of over 30,000 people in a hunt for the genes that make up the rest of this risk. Around half of the participants were bowel cancer patients and half were healthy people.
Each team carried out a ‘whole genome search’ and pinpointed a gene that is faulty more often amongst bowel cancer patients than in people without the disease. They narrowed down the gene’s location within the genome to a region called 8q24. Scientists recently found that men who have the same genetic variant are at an increased risk of developing prostate cancer.
Around half of the general population carry the genetic variant, which results in a 20 per cent increased risk of developing bowel cancer. Lifetime risk of the disease rises from around one in 20 for people who do not carry a faulty copy of the gene to one in 16 for people who do. The evidence in these papers indicates that around one in 10 bowel cancers diagnosed in the UK are linked to this genetic fault – equating to around 3,500 cases each year.
Because the increased risk incurred by this genetic fault is relatively small, it would not be suitable for genetic testing at this stage. But it may be possible to design a test for a combination of genes as more ‘low risk’ variants are found. Identifying people who have an increased risk of developing bowel cancer will improve prevention, diagnosis and treatment of the disease in the future.
In the first study, Professor Malcolm Dunlop, from the University of Edinburgh and the Medical Research Council’s Human Genetics Unit, and an international team of researchers compared the DNA of around 8,000 bowel cancer patients from North America, France and Scotland, to that of around 8,000 healthy people.
Professor Dunlop said: “Scanning the entire genome of large numbers of people has enabled us to identify the first common genetic variant that increases bowel cancer risk. We are now using an even more refined “genome-wide scan” to discover yet more genes linked to bowel cancer risk.
“Understanding all the genes involved is a bit like putting together a jigsaw puzzle in the dark. First we have to feel around for the genes involved and only then will we be able to find out how they all fit together to contribute to increased risk. By identifying these genetic variants, we will be in a better position to understand how such changes can lead to cancer.”
The second study was jointly led by Professor Ian Tomlinson from Cancer Research UK’s London Research Institute and Professor Richard Houlston from The Institute of Cancer Research. They examined the DNA of a similar number of patients and healthy people from England.
Professor Tomlinson said: “This is an important first step but we still have a long way to go before we have a complete picture of all the genes that are involved in inherited bowel cancer risk. Eventually it may be possible for scientists to design treatments to prevent people at increased risk of the disease from developing bowel cancer altogether.”
Both teams used a multi-stepped approach to find the region of the genome that was linked to bowel cancer risk. They studied thousands of ‘tags’ – distinct blocks of DNA that act as signposts for genes – in hundreds of people. The tags which were more common among the bowel cancer patients than the healthy people were then reassessed in new, larger groups of patients and healthy people.
After repeating this process many times the researchers whittled down the tags to just one – 8q24. This painstaking work led researchers to a still to be identified gene within this part of the genome that is responsible for the increased risk of bowel cancer among the patients they studied.
Cancer Research UK is launching similar genome wide studies for lung and ovarian cancer and scientists hope to find out more about the genes linked to these cancers as a result.
Harpal Kumar, chief executive of Cancer Research UK, which has instigated this series of genome-wide studies for common cancers, said: “This is an extremely important discovery which will significantly improve our understanding of the biology of bowel cancer and what causes it. In the future we hope studies like this across a range of cancers will help people at increased risk of the developing the disease through the development of tailored screening and treatment programmes.”
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- “A colorectal cancer susceptibility locus on chromosome 8q24 identified by a genome wide association scan.” Brent W. Zanke, et al. (2007) Nature Genetics. Working with Canadian collaborators on a “whole genome scan” to identify genes that might contribute to bowel cancer risk, the research group in Edinburgh narrowed the search down and pinpointed one genetic region on the long arm of chromosome 8 (8q24) where a common variation is associated with bowel cancer risk. This study was supported by Cancer Research UK, the Medical Research Council, the Scottish Executive Chief Scientist’s Office and CORE.
- “A genome-wide association scan of tag SNPs identifies a novel susceptibility variant for colorectal cancer at 8q24.21.” Ian Tomlinson et al. (2007) Nature Genetics. The London team and collaborators also undertook a genome scan of their own. Among several potential locations of new bowel cancer genes, the strongest evidence independently pointed to 8q24. This study was supported by Cancer Research UK, the Bobby Moore Fund for Cancer Research UK, CORE, the European Union, the Thomas Falknor Fund and Leukaemia Research.
The technology used in these studies was provided by Illumina.
Every year in the UK, around 35,000 people are diagnosed with bowel cancer. It is the third most common cancer after breast and lung and the disease causes almost 16,100 deaths in the UK every year. Around two-thirds of cases are in the large bowel (colon) and the remaining third are in the back passage (rectum). The occurrence of bowel cancer is strongly related to age, with over 80 per cent of cases arising in people who are 60 years of older.
One factor that can increase risk of developing bowel cancer is an inherited faulty gene (genetic mutation). An inherited genetic mutation may mean that several people on the same side of a family develop bowel cancer. This is called a ‘strong family history’. There are a number of known genetic conditions linked to bowel cancer, including familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC).
A strong family history usually means several relatives have been diagnosed with bowel cancer on the same side of the family. It can also mean there are one or more relatives diagnosed at a particularly young age. People who are concerned they may have a strong family risk should talk to their GP. The doctor may refer them to a genetics clinic for specialist advice about their risk, screening and the availability of genetic testing.
Bowel cancer screening
Screening means looking for early signs of a particular disease in ‘healthy’ people who don’t have symptoms. Results from large scale Government pilot studies published in 2003 suggested that a national bowel screening of the normal population would help save lives. The Government is now phasing in a national screening programme across the country.
Men and women aged between 60-69 years old are being offered a test every two years. The screening method used is called faecal occult blood testing (FOB). Faecal occult blood testing means looking for hidden (occult) blood in your stool (faeces). People can do the test themselves at home.
For more information about bowel cancer or screening visit our patient information website CancerHelp UK.
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