Genes control the way cells work, and in particular how they grow and divide. When something goes wrong with one or more of the genes in a cell, changes occur that can lead to cancer.
Such changes are commonly referred to as faults or mutations. Although cancer is mainly the result of life choices and not related to family gene pools, it can also be caused by these faults or mutations.
As a result, cancer can be inherited from parents and grandparents and passed on to future generations.
A cell must typically contain six or more faults to become cancerous. Such faults can cause the cell to stop functioning normally, become cancerous, and grow and divide uncontrollably.
This may happen as people get older as a result of random mistakes when a cell is dividing. Changes can also occur due to exposure to substances that can cause cancer called carcinogens. These include cigarette smoke or sunlight.
Though potentially cancerous, these gene changes don't affect all body cells, are not inherited, and cannot be passed on to children.
Cancer is such a common disease that many families have several members who have had cancer. Certain types of cancer seem to run in some families, but only a small portion of all cancers are inherited.
How is cancer risk passed on?
Faulty genes that increase the risk of cancer can be passed on from parent to child. Such genes are called inherited cancer genes. They occur when there is a mistake or a fault in the genes contained in an egg or sperm cell.
Genes that increase the risk of cancer are known as cancer susceptibility genes. Their normal job is to correct DNA damage that naturally occurs when cells divide, protecting people against cancer.
Inheriting a faulty copy of one these genes means that it is unable to repair damaged DNA in cells. As a consequence, the cells may become cancerous.
People inherit genes from both parents. If a gene fault occurs in each parent, a child has a 1 in 2 chance of inheriting it. While some children will have the faulty gene and an increased risk of developing cancer, others won't.
Cancers caused by inherited faulty genes are much less common than cancers caused by gene changes due to aging or other factors.
Most cancers develop through a combination of chance and the environment rather than inheriting a specific cancer gene. However, about 5 to 10 percent of all cancer cases occur in people who have inherited genetic mutations that raise the risk of cancer.
Mutations and syndromes that can increase the risk of cancer include:
- BRCA1 and BRCA2 genetic mutations - raised risk of breast, ovarian, and prostate cancer
- Cowden syndrome - raised risk of developing breast, uterine, and thyroid cancer
- Familial adenomatous polyposis - raised risk of colorectal cancer, and soft tissue and brain tumors
- Li-Fraumeni syndrome - raised risk of developing many different types of cancer
- Lynch syndrome - raised risk of colorectal cancer, some skin cancers, and brain tumors
- Multiple endocrine neoplasia - raised risk of endocrine cancers
- Von Hippel-Lindau disease - raised risk of kidney and other cancers
Tests are currently available for gene faults that increase the risk of breast, bowel, ovarian, womb, and prostate cancer. Tests are also available for rare gene faults that can increase the risk of kidney, skin, and thyroid cancer, and even a type of eye cancer called retinoblastoma.
Tests are not presently available for other types of cancer genes. Research is always being conducted, however, with tests being developed for more and more gene faults.
Predictive genetic testing refers to the type of testing done on genes that increase the risk of cancer. They do not usually give precise answers about inherited diseases. The tests will only show if there is a specific gene mutation, not whether cancer is present.
A positive result means there is a raised risk of developing cancer but does not mean that cancer is present or will develop. For example, a particular gene mutation may be found through testing, but it might not be clear if this raises the risk of cancer.
In short, the test can show what might happen, but it cannot show what will happen. A negative result does not mean there is no risk of getting cancer. The risk can change over time due to non-genetic reasons, such lifestyle choices or simply getting older.
Genetic tests may be flawed, and test results can be interpreted wrongly. Although this is not common, different labs may have different ways of interpreting changes because genetic testing is not tightly regulated.
The makers of tests often advertise and promote their tests to doctors and the public. In doing so, they can make the test sound more helpful and decisive than it actually is. This can be misleading as decisions about testing may be made on incomplete or wrong information.
Reputable genetics counselors should, however, be able to help people know what to expect from test results.
Who should get genetic testing?
If a person has a strong family history of cancer and the genetic specialist thinks they could have inherited a faulty gene, they will offer that person a genetic test.
According to Cancer Research UK, the following scenarios signal what is considered to be a strong family history of cancer:
- If more than two close relatives on the same side of the family have had cancer
- The relatives have had the same cancer or different cancers that can be caused by the same gene fault
- The cancers appeared when the relatives were below the age of 50
- One relative has had a gene fault found by genetic tests
What to do if you think you may have a hereditary risk of cancer
People are advised to talk to their family doctor if they think that they may have a strong family history of cancer. Specialists will ask about their family in order to determine how many members have been diagnosed with cancer. If they think that a person has an increased risk, they will refer them to a genetics clinic for testing.
According to the American Cancer Society, the most effective ways of reducing the risk of cancer are simple things, such as:
- Eating a healthy well-balanced diet
- Not smoking
- Exercising regularly
- Keeping to a healthy weight
- Staying safe in the sun