Genetic testing

 

You might also find it helpful to watch our video about genetic counselling|. 

Who is offered genetic testing? Back to top

Genetics tests can only be done for people who may have one of the cancer susceptibility genes| that have been identified.

So far, only a few of the genes involved in hereditary cancer risk have been identified and can be tested for. These include some of the genes that affect the risk of bowel/ womb cancer, breast/ovarian cancer and a few other rare conditions. We have more information on specific cancer types and genetics|.

If you have several cases of other types of cancer in your family, there probably won’t be a genetic test available to you. But you may be offered the chance to take part in research that is trying to identify more cancer genes, or that is testing ways of preventing cancer. A blood sample may be taken and kept so it can be tested for genetic changes that may be identified in the future. You’ll be asked whether you agree to your blood being stored for this purpose.

You can usually only be offered a genetic test to look for a gene fault if you have a living relative who has been diagnosed with cancer and is willing to be tested first. This is because if there is a cancer susceptibility gene in a family, you won’t definitely have inherited it. If a person has a faulty gene, there is a 50% (1 in 2) chance that they will pass it on to their child. So someone in your family who has been diagnosed with cancer is tested first to find out whether they have a genetic cancer susceptibility.

The test will also look for what the particular gene fault is. Relatives can then be tested for that particular gene fault, once it is known to be the cause of the susceptibility in the family.

There is one exception to this. If you have Ashkenazi (Eastern European Jewish) or Polish ancestry, which are communities where a common gene mutation has been found, you may be offered testing without a relative who has developed cancer being tested first.

Genetic testing – a two-step process Back to top

Genetic testing involves checking a blood sample for gene faults (mutations) that are known to increase the risk of cancer. This is a complex process and it can take months for the results to be available.

Step 1 – mutation search

First, the laboratory looks for the particular gene fault (mutation) that may run in your family by testing a member of your family who has been diagnosed with cancer. This step is called the mutation search. It involves taking a blood sample, which can be arranged in your own clinic or by your relative’s GP.

Searching for a mutation in a gene is like trying to find a single spelling mistake in a big book. It involves reading the whole (or large parts) of the gene. The test looks at the order that the chemical ‘letters’ A, T, C and G (that make up the genetic code) are found within a gene.|

Step 2 – predictive testing

If a gene fault known to increase the risk of cancer is identified in your family, you and other family members who may be at risk can be offered a test. This is called genetic screening, or predictive or predispositional testing.

Predictive testing is much faster than mutation searching, as the laboratory already know what they are looking for (the exact gene fault) and where to find it.

Results of genetic testing may show that:

• the person affected by cancer who was tested wasn’t shown to have a mutation in a known cancer susceptibility gene - this is known as an inconclusive result
• there is a gene mutation in the family, but you haven’t inherited it
• there is a gene mutation in the family and you have inherited it.

If a mutation is found in the family Back to top

This is called a positive test result. It means that the person being tested has inherited a mutation in a cancer susceptibility gene, which increases their risk of getting a specific cancer. They can also pass the gene on to their children.

Positive results of predictive testing

Having a cancer susceptibility gene doesn’t mean you will definitely get cancer, but it does significantly increase the likelihood you will develop cancer during your lifetime. The increase in risk varies according to the particular gene mutation and the type of cancer it relates to.

Some people with a strong family history decide to have a genetic test when they have already been diagnosed with cancer. If you already have cancer and you are found to have a cancer susceptibility gene, you may have an increased risk of developing another new cancer.

Other members of the family

If you have a cancer susceptibility gene mutation, other members of your family may also have inherited the same genetic fault that you have. Each of your children will have a 50% (1 in 2) chance of inheriting the fault from you. Other relatives may also be affected because you share your genetic heritage with them. As medical test results are always confidential, no one but you can tell your relatives about your test result.

Your genetic counsellor will discuss with you who in your family needs to be made aware of the result. However, you will be able to decide when and how is the best way to tell them. Your genetic counsellor will often give you a letter that you can pass to relatives to help explain the result and the implications it has for them.

Some members of your family may decide to see a genetic counsellor to help them make decisions about their cancer risk. Their GP can refer them to their regional family cancer centre.

As a general rule, children aren’t tested to see whether they have inherited the genetic mutation. Even if a child has inherited a genetic mutation, testing and counselling can wait until the child is an adult and can make their own decisions. This is because most cancer susceptibility gene mutations don’t cause cancers in children.

The main exception to this is familial adenomatous polyposis (FAP).| This is a particular kind of bowel cancer susceptibility that can start very early in life. Testing and, if appropriate, screening, are usually recommend to start at around 10-12 years of age.

Your feelings

It’s important to remember that having a cancer gene mutation isn’t your fault. No one is to blame for the genes they’ve inherited. Families dealing with genetic risk sometimes have feelings of guilt, blame or responsibility, but genes are distributed by chance. We aren’t responsible for our eye colour or height, or whether we’ve inherited a cancer gene mutation. If you have a positive test result, genetic counselling and predictive testing can be offered to other family members who may be carriers of the gene. This can identify other family members who potentially have an increased risk of getting cancer, and those who don’t.

For this reason, you may wish to tell your relatives you’ve had a test and that it’s positive. This is your decision to make and it can be very difficult. Your genetic counsellor can talk through all the issues with you. Other people (such as your friends) can help you to deal with the emotional issues that may arise. You can also contact our cancer support specialists|.

Negative results of predictive testing

People who don’t have the mutation won’t have a definite increased risk of developing the cancer. Also, they can’t pass the mutation on to their children.

This doesn’t mean they definitely won’t get cancer. They still have at least the same risk of getting cancer as everybody else in the general population.

However, this is a much lower risk than it is for people who have one of the known cancer susceptibility genes. 

If no mutation is found in the family Back to top

If no mutation is found in the person affected by cancer who is tested, the test is said to be an inconclusive result. In most cases of genetic testing, no mutation is found in any of the genes known to be involved in hereditary breast/ovarian or bowel/womb cancer, even if there is a strong family history.

If a mutation isn’t found, it doesn’t necessarily mean that the family isn’t at increased risk of cancer. When there is a very strong family history of a certain cancer, doctors assume that the family members are at increased risk of that cancer type, even if a specific genetic fault can’t be found.

There are several possible reasons for a test not showing a mutation (an inconclusive result):

• There is an altered gene in the family but the fault is in a gene that hasn’t yet been identified.
• There are several ‘low risk’ genes in the family that, in combination with environmental factors, can still increase the risk of cancer.
• The cancer in the family or person isn’t due to an inherited gene, and the cancer has occurred by chance or for other reasons we don’t understand.

This uncertainty about inconclusive test results is the reason why the NHS doesn’t automatically offer genetic tests to everyone who wants to be tested. If the test is inconclusive, which is very likely, it will be impossible to know which of the above reasons applies.

Variants of unknown significance

Sometimes an alteration may be found in a susceptibility gene that may or may not alter the function of the gene. It isn’t always possible to be certain whether such an alteration matters. This is called a variant of unknown significance. Research is ongoing to determine the importance of such variants.

What you can do

Following an inconclusive test result, some people may decide to make changes to their lifestyle| and improve their general health by eating a more healthy diet or by getting fitter. You could talk to your doctor about whether there is any screening you may be suitable for.

Can cancer genes ‘skip’ a generation? Back to top

No - there is a 50% (1 in 2) chance of inheriting the gene from one of your parents, so you either inherit it or you don’t. If you don’t inherit the gene, you can’t pass it on to your children. However, not everyone with the mutation will necessarily develop cancer, so it can seem that the cancer skipped one generation.

For example, the gene mutations for female cancers such as ovarian or breast cancer can be passed on through the father’s side of the family. Men who have the genetic mutation responsible for breast and ovarian cancer often don’t develop cancer themselves. But they still have a 50% (1 in 2) chance of passing the genetic mutation on to their children.

So, if a daughter develops cancer after inheriting the breast cancer susceptibility gene from her father, it can seem as though the genetic mutation has skipped a generation. But this isn’t the case. The mutation can’t skip a generation.