Cancer and genes
Cancer is the result of changes (mutations) in genes that control cell growth and death.
In normal adult tissues, the rate at which new cells are made (cell growth) and old cells die is in balance. Genes inside each cell maintain this balance by controlling how the cell behaves.
There are many different types of genes that control cell behaviour. Cancer is the result of faults (mutations) in genes that control cell growth and death, and that repair mistakes in our DNA. Cells with these mutations may begin to grow in an uncontrolled way, forming tumours. Usually, several gene mutations must happen in a group of cells before cancer develops. This can take many years, which is why cancer is more common in older people.
Oncogenes and tumour suppressor genes
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Changes in two types of genes are particularly important for cancer to develop: oncogenes and tumour suppressor genes.
Oncogenes encourage cells to grow and multiply. A change (mutation) in this type of gene can lead to a cell growing and multiplying out of control.
Tumour suppressor genes
Tumour suppressor genes help to protect against cancer by controlling cell growth and repairing damage to our genetic code (DNA).
Tumour suppressor genes that control cell growth tell the cell when to stop growing (multiplying). If a cell has a mutation in a tumour suppressor gene, the ‘brakes’ on its growth may be lost so it can multiply out of control.
Some tumour suppressor genes repair damage to DNA. They are called DNA repair genes or caretaker genes. When there is a fault (mutation) in a DNA repair gene, any new damage or mutations in the cell won’t be repaired. So cancer is more likely to develop.
Acquired and inherited mutations
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There are two types of mutations:
Acquired mutations - mutations that happen in a part of someone’s body (for example their lung or bowel) during their lifetime, which aren’t passed on to the next generation.
Inherited mutations - mutations that someone is born with and that are passed down from generation to generation.
Acquired mutations, also known as somatic mutations, happen during a person’s lifetime. An acquired mutation happens in one cell in a person’s body and only affects the cell it begins in, and any new cells made from the abnormal cell as it divides (multiplies). Acquired mutations can’t be passed on to your children.
Acquired mutations can happen by chance when a cell is multiplying or because a gene is damaged. Some substances, called carcinogens, are known to increase the chance of damage to genes. Examples of known carcinogens include radiation and chemicals in cigarette smoke.
Cancers caused by a build-up of acquired mutations (a number of mutations in a cell) during a person’s lifetime are called sporadic cancers. Most cancers are sporadic cancers.
Inherited mutations (also known as germline mutations) are gene faults inherited from your mother or father. All the cells of your body develop from one cell, formed when the sperm fertilises the egg. If you inherit a gene mutation from one of your parents, it will be in the original cell that goes on to form all the cells in your body. Therefore, all your cells will contain the genetic mutation. This includes reproductive cells (sperm cells in men and egg cells in women), which is why the mutation can be passed on from generation to generation. The diagram below shows how genes are inherited.
Inherited gene mutations that make a cancer more likely to develop are sometimes called cancer susceptibility genes. Inheriting a cancer susceptibility gene doesn’t mean a person has cancer, but that they are at an increased risk of developing certain types of cancer. Further gene changes (mutations) need to happen for a cancer to develop. This usually happens over many years.
Cancers that develop in a family because a gene fault causes a susceptibility to those cancers are sometimes referred to as inherited cancers.