Family history and inherited cancer genes

This information looks at how inherited faulty genes can cause cancer and what you can do if you have a strong family history of cancer.

How can genes cause cancer?

Inside almost every single cell in your body is a structure called the nucleus. The nucleus is the control centre of the cell. Inside the nucleus are 23 pairs of chromosomes made up of genes. Genes are coded messages that tell cells how to behave. They control how our bodies grow and develop. We each have about 25,000 genes.

chromosome-diagram.jpg

All cancers develop because something has gone wrong with one or more of the genes in a cell. A change in a gene is called a ‘fault’ or ‘mutation’.

These faults can make a cell stop working properly. It may then become cancerous and divide and grow uncontrollably. Most gene changes happen during our lifetime but some can be inherited from a parent.

Most cancers are caused by gene faults that develop during our lifetime. They usually happen as we get older due to random mistakes when a cell is dividing. Or they may happen because of something we are exposed to, such as cigarette smoke or sunlight. Doctors call these things carcinogens.

These gene changes don’t affect all body cells. They are not inherited and can't be passed on to our children. They are called acquired mutations. Cancers caused by acquired mutations are called sporadic cancers. They are the most common type of cancer.

Inherited cancer gene faults

Some faulty genes that increase the risk of cancer can be passed on from parent to child. These are inherited cancer gene faults. They occur when there is a fault in the genes in an egg or sperm cell at the time of conception. These faults in the initial sperm or egg cell are copied into every single cell in the body. The faulty genes can then pass from generation to generation. They are called germline mutations.

We inherit genes from both our parents. If a parent has a gene fault, then each child has a 1 in 2 chance (50%) of inheriting it. So, some children will have the faulty gene and an increased risk of developing cancer and some children won’t.

Being born with inherited faulty genes doesn’t mean that a person will definitely get cancer. But they have a higher risk of developing particular types of cancer than other people. They are also more likely to develop cancer at a younger age. Doctors call this having a genetic predisposition to cancer. For a cancer to develop, further gene changes (mutations) need to happen. This usually happens over many years.

How common are cancers caused by inherited faulty genes?

Cancers due to inherited faulty genes are much less common than cancers due to gene changes caused by ageing or other factors. Most cancers develop because of a combination of chance and our environment, not because we have inherited a specific cancer gene fault.

Genetic specialists estimate that between 5 and 10 in every 100 cancers (5 to 10%) diagnosed are linked to an inherited faulty gene. 

Different gene faults increase the risk of different types of cancer. Some faults increase the risk by a small amount and some increase the risk much more.

Read about inherited genes and cancer types.

A family history of cancer

Most people who have relatives with cancer will not have inherited a faulty gene. Cancer mostly occurs in older people. It is a common disease. 1 in 2 people in the UK (50%) born after 1960 will be diagnosed with some form of cancer during their lifetime. So, most families will have at least one person who has or has had cancer. Having a couple of relatives diagnosed with cancer doesn’t mean there is a cancer gene fault running in the family.

In families with an inherited faulty gene, there is usually a pattern of specific types of cancer running in the family. The strength of your family history depends on:

• who in your family has had cancer
• the types of cancer they have had
• how old they were at diagnosis
• how closely related the relatives with cancer are to each other

The more relatives who have had the same or related types of cancer, and the younger they were at diagnosis, the stronger someone’s family history is. This means that it is more likely that the cancers are being caused by an inherited faulty gene.

This family tree shows a pattern of cancer that could be caused by an inherited faulty gene.

family-tree.jpg

• the cancers developed when the family members were young – for example below the age of 50 for breast, bowel and womb cancer
• multiple close relatives on the same side of your family have had cancer – the same side of your family means either your father’s relatives or your mother’s relatives
• the relatives have had the same type of cancer, or different cancers that can be caused by the same gene fault
• one of your relatives has had a gene fault found by genetic tests

It is important to remember that cancer is most common in older people. Each year more than a third (36%) of all cancer cases in the UK are diagnosed in people aged 75 and over. Cancer in older people is less likely to be due to an inherited cancer gene.

What if I have a strong family history?

Talk to your GP if you think that you may have a strong family history of cancer. Your GP will ask you about your family and how many members have had cancer, so it is important to have as much information about your relatives’ cancer diagnoses as possible. If they think that you might be at increased risk, they can refer you to a genetics clinic.

Read about genetic testing for cancer risk.

What if I have a known gene fault?

Your doctor or a genetic counsellor will be able to give you an idea of how much your cancer risk is increased compared to the general population. They might suggest that you have regular monitoring for particular cancers. Or treatment such as surgery to try to reduce the risk of developing cancer.

You can find out more about the options if you have a known gene fault on our page about getting your genetic test results.

Faulty genes and other factors

Some inherited cancer genes are more likely to cause cancer than others. As well as a gene fault, many other factors need to be in place for a cancer to develop. Because of this, the effect of the cancer gene may appear to skip a generation. For example, a parent may have the gene fault and not develop cancer but their child who inherits the same gene fault does develop cancer.

Researchers need to find out more about what these other factors are for each type of cancer. They are also trying to find out more about how genes work together to cause cancer so that we can reduce the risk of developing it.