If a cancer occurs more frequently in a family than would be expected in the general population, this may be a sign that some people in the family have inherited a cancer susceptibility gene. In many cancers, it’s thought that about 5-10% of cases (between 5-10 out of every 100 cases) may be linked to inherited cancers.
Scientists have discovered cancer susceptibility genes for some of the more common cancers that can run in families, such as cancers of the breast, bowel, ovary and womb. There are other cancers, such as prostate, pancreatic and testicular cancer, which may run in families but specific cancer susceptibility genes for these cancers haven’t been identified yet.
Sometimes there are a number of different types of cancers in a family. Usually these cancers are unrelated and are due to the effects of a combination of risk factors such as age, lifestyle and the environment. But in some cases, when certain pairs of cancers run together in a family, such as bowel and womb cancer, or breast and ovarian cancer, it can be a sign of an inherited susceptibility gene.
There are also some very rare inherited gene mutations that increase a person’s risk of getting several different types of cancer at a young age. In families that have these cancer susceptibility genes several people may be diagnosed with cancer in childhood, adolescence or in their twenties or thirties.
If someone has an inherited cancer susceptibility gene, they have a significantly increased risk of developing certain cancers compared to other people in the population. In most cases, people who inherit a cancer susceptibility gene won’t definitely get cancer. You can’t inherit cancer from someone in your family, but you might inherit an increased risk of developing a certain type of cancer.
In many families with higher than expected numbers of cancers, the cancers aren’t caused by a cancer susceptibility gene. It’s thought that for some families, the increased risk may be caused by several genes which, in combination with lifestyle and environmental risk factors, affect the risk of developing certain cancers. These genes have a weaker effect on the risk of cancer than cancer susceptibility genes so they are sometimes called 'low risk' genes or ‘low penetrance’ genes.
Only a few low penetrance genes have been identified so far, but there aren’t tests available for checking if someone has them. Researchers are trying to find out more about what these genes are, and how, in combination with lifestyle and environmental factors, they might affect an individual’s cancer risk.