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You will usually only be offered genetic testing if your family history makes it likely that there is a susceptibility gene for breast|, ovarian | or bowel cancer| in your family.
So far, only a few of the genes involved in hereditary cancer (that can be tested for) have been found. These include some of the genes that affect bowel|/ womb| cancer, breast/ovarian cancer| and a few other rare conditions.
If you have a number of 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 in which scientists are trying to identify more cancer genes or are testing ways of preventing cancer. Your blood sample may be kept, so that 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.
Genetics tests can only be done for people who may have one of the cancer susceptibility genes that have been identified. You will usually only be offered genetic testing if your family history makes it likely that there is a susceptibility gene for breast, ovarian or bowel cancer in your family.
You can usually only be offered predictive genetic testing 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, not everyone in the family will have inherited the gene. If a parent has a faulty gene there is a 1 in 2 (50:50) chance that this is passed on to their child. So someone in your family who has been diagnosed with cancer is usually tested first to find out whether they have genetic cancer susceptibility, and if so, what the particular genetic change is. This makes it easy for their relatives to be tested for that gene change, when 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) ancestry, or belong to some other small 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 involves checking a blood sample for gene changes (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.
A mutation search 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 in which the chemical ‘letters’ A T C G of the genetic code are found within a gene.
A member of your family who has been diagnosed with cancer has to be tested first. The test involves taking a blood sample. This can be arranged in your own clinic or by your relative’s GP.
If a gene change that is known to increase the risk of cancer is identified in your family, you and other members of your family can be tested. Predictive testing is much faster than mutation searching, as the researchers already know what they are looking for (the exact gene mutation) and where to find it.
If a mutation is found this is called a positive test result. A positive test result means that the person being tested has inherited a mutation in a cancer susceptibility gene that increases their risk of getting a specific cancer. They can also pass on the gene to their children.
Having a cancer susceptibility gene doesn’t usually mean that you will definitely get cancer but it significantly increases the likelihood you will develop cancer during your lifetime. The increase in risk varies according to the particular genetic 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.
If you have a cancer susceptibility gene mutation, other members of your family may have inherited the same genetic change that you have. Each of your children will have a 1 in 2 chance (50%) of inheriting the change 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. You have to decide whether and who to tell about your test. Some other members of your family may decide to see a genetic counsellor to help them make decisions about their cancer risk.
As a general rule, children aren’t tested to see whether they may have inherited a genetic mutation that increases their cancer risk. 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 family bowel and breast cancers don’t affect children. The main exception to this is family adenomatous polyposis (FAP)|. This is a particular kind of bowel cancer susceptibility that can start very early in life.
It’s important to remember that having a cancer gene mutation isn’t your fault. No one is to blame for the genes they have 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 have inherited a cancer gene mutation.
If you have a positive test, genetic counselling and predictive testing can be offered to other family members who may be carriers of the gene. This allows identification of other family members who have an increased risk of getting cancer, and those who don’t. For this reason it’s advisable to tell your relatives you’ve had a test and that it is positive.
This is your decision to make and can be very difficult. Your genetic counsellor can talk through all the issues with you. Other people can help you to deal with the emotional issues that may arise. You can also call the Macmillan Support Line for support.
Once a mutation has been identified in a family, other family members can choose to have predictive testing to see if they have inherited the mutation.
Family members who don’t have the mutation won’t have a significantly increased risk of developing the cancer, although they may still have a slightly increased risk due to other genetic factors. 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 for people who have one of the known cancer susceptibility genes. So in this case a negative test is good news.
If no mutation is found in an affected person in the family, the test is said to be inconclusive. 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. Figures vary, but overall, mutations are identified in only about 10 to 20 out of 100 samples (10–20%) taken in the genetic clinic.
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 members of this family are at increased risk of that cancer type, even if the laboratory can’t find a specific genetic mutation.
There are several possible reasons for a test not showing a mutation (an inconclusive result):
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 what the test result means, and which of the reasons above applies.
Sometimes an alteration may be found in a susceptibility gene which may or may not alter the function of the gene. It isn’t always possible to be certain whether such an alteration matters, and it is called a variant of unknown significance. Research is ongoing to determine the importance of such variants.
As not everyone who inherits a cancer susceptibility gene actually gets cancer, it can sometimes look as if cancer skips a generation in families where many members have cancer. But you either inherit the gene from one of your parents or you don’t. There is a 50/50 chance.
If you don’t inherit the gene, you can’t pass it on to your children. So the genetic mutation can’t skip a generation. 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 women’s 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 50/50 chance of passing the genetic mutation on to their children.
So, it can seem as though the genetic mutation has skipped a generation, if a daughter inherits the breast cancer susceptibility gene from her father.
For answers, support or just a chat, call the Macmillan Support Line free (Monday to Friday, 9am-8pm)
If you have any questions about cancer, need support or just want someone to talk to, ask Macmillan.