Genetic tests for cancer?

In recent years more and more companies have started selling genetic tests to the public directly from the web. In some places genetic testing has even become a social activity.

But are these tests a good idea? And what does this mean for preventing and treating diseases such as cancer?

Cancer can be inherited

It’s true that certain cancers can run in families, and a higher risk of developing those cancers can be inherited. Let’s take breast cancer as an example.

We know that a woman’s risk of developing breast cancer is higher if one or more of her close relatives have had the disease. This is because in some cases people can inherit genes that make them more susceptible to the disease.

Some of these genes, such as faulty versions of the “high risk” BRCA1 or BRCA2 genes, greatly increase the risk of breast cancer, raising a woman’s chances of developing the disease from roughly one-in-ten to eight-in-ten.

But there’s another, more subtle way that genes can influence our inherited risk of cancer.

Each of us is different, thanks to tiny variations in the genes we carry. These variations, known as SNPs, or “snips”, have a much smaller effect than the “high risk” faults like those found in the BRCA genes – but they’re much more common – and their effects can add up.

Thanks to work by scientists around the world – including many here in the UK funded by Cancer Research UK – we now know a lot more about these gene variations that have a more subtle effect on cancer risk.

It’s worth pointing out here that most women who have relatives with breast cancer will not develop the disease themselves. And the majority – 85 percent – of women who do develop breast cancer do not have a family history of it. They develop it because of damage accumulated in their genes throughout their lives.

Other factors like hormones and body weight also have an influence on your risk of developing breast cancer. In fact it is a combination of genetic and environmental factors that determines a person’s overall risk of developing any type of cancer.

So what does a genetic test actually tell you?

Tests for a specific gene or genes (such as the BRCA genes) have been available for some time, and doctors generally offer them to families with a very strong history of cancer.

But the ‘whole genome’ tests that are now becoming available from companies via the internet are different – we blogged about the tests available from the company Decode Genetics back in October last year.

These tests are looking for SNPs – the common but weak-effect variations we mentioned above, which large population studies have linked to specific diseases. Typically a single test will look at many different SNPs in your DNA, rather than testing for a single fault like the BRCA tests.

But these SNPs usually only increase risk by a very small amount, and sometimes the link with disease might be weak. Also, the results of these tests can’t tell you about gene faults that might occur in the future as you age. Nor do they tell how your lifestyle will influence your risk. In short, they don’t paint anything like the complete picture.

As a result, these tests give results whose meaning and significance is not clear cut. Even scientists who specialise in the area can’t accurately predict who will get cancer by looking at test results alone. So it’s likely that ordinary members of the public will be unsure of how to act on their test results.

For example, say a test shows that a person has a 10 per cent increased risk of kidney cancer. This type of cancer is relatively rare and the chances of getting it are small – in the UK the risk of someone developing kidney cancer is less than 1 in 8000. So increasing this by 10 per cent is still a relatively small risk. Despite being told they are at ‘increased risk’, the person is very unlikely to develop kidney cancer. But knowing about the increased risk could make someone worry unnecessarily about harmless aches and pains.

Additionally, the risk of developing cancer depends on the combination of a variety of different SNPs (as well as environmental factors). But we just don’t know about all these SNPs, or about how their effects interact with each other.

We need more evidence that these tests accurately and reliably predict what they are advertising. Currently there is no standardised quality control for genetic tests.

Are there dangers to using tests which are available now?

Genetic information has the potential to be an extremely powerful and useful resource. Advances in technology mean that, over the next few years researchers will uncover many more genetic links to cancer, which may be of significant use in helping predict who might be susceptible to certain cancers. This could ultimately help doctors to advise people at high risk and offer them appropriate treatment and screening.

But are genetic test results received by post or email – with no ‘genetic counselling’ – an appropriate way to tell people about their risk of cancer?

We don’t think so – as well as all the grey areas mentioned above, there’s simply no way to ensure that the results are properly interpreted or acted upon.

We think that regulations or a mandatory code of practice should be in place to ensure that the delivery of genetic information or test results is appropriate and takes into consideration the potential sensitivity of such information.

One concern is that taking tests that claim to predict risk of cancer can significantly influence the health choices that you might make – choices which can have a huge impact on your wellbeing. The results could also cause unnecessary anxiety or even panic, as mentioned already.

Many people are concerned that the NHS could face additional burdens from people seeking help to understand the significance of the results of web-bought genetic tests.

Negative results from these tests could also lull you into a false sense of security, meaning you may not be as stringent about cancer screening programmes. It could also encourage complacency towards the effects of an unhealthy lifestyle – for example, whether or not you smoke is has much more influence over whether you will get lung cancer than the genes you carry.

This last point is especially important. Our knowledge of cancer genes is still in its infancy – as stated above, we haven’t yet found all the relevant gene variants, and we’re only starting to understand how the genes we carry interact with the choices we make.

For the moment, our understanding of cancer genetics can only give us a patchy and incomplete picture of our cancer risk.

What do the results mean?

How would you alter your behaviour after receiving the results of a genetic test? Is there something that can be done to lower your chance of getting cancer? Is there a screening programme to test for the cancer?

Sometimes there is. Where screening has proven to be accurate and useful, it has been implemented for the whole UK population – regardless of genetic make-up. Thousands of women are alive today thanks to breast and cervical screening, while the new bowel cancer screening programme for men and women has the potential to save thousands more lives.

Doctors will give woman with a family history of breast cancer a genetic test for the high risk genes – precisely because there are preventative options. As our understanding of the genes which influence cancer and the ways in which we can prevent it increase, more of these tests will become available from doctors, with the corresponding information, genetic counselling and appropriate screening or treatment.

But at the moment, the best cancer prevention advice is our general health messages: don’t smoke, keep a healthy bodyweight, don’t drink too much alcohol, be physically active, eat a healthy diet rich in fruit and veg, and go for screening when invited.

Half of all cancer could be prevented by changes to lifestyle, including the vast majority that are not related to inherited genes. These messages should be the core advice for anyone looking for effective ways to reduce their risk of cancer.

What does the future hold?

How our genes influence cancer is an exciting area and one which is an intensive subject of research. One day these tests may be an integral part of cancer treatment, helping to inform doctors about which treatments would be most useful for an individual patient or helping predict the outcome of a disease. Or perhaps well-researched combinations of genes may help pinpoint who is at higher risk of developing certain cancers, so they can be offered screening or prevention measures.

Taking tests which aren’t proven to work or are confusing may cause a loss of confidence and distrust amongst the public. This could damage the credibility of such tests in the future – when in fact they have the potential to be extremely useful when properly researched and used.

Laura Bell, Cancer Research UK Science Information Officer