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Nurse and patients talking about cancer How cancer starts

This page has information about how cancer starts. There is information about

* How cancer starts
* Genes and mutation
* Genes that are abnormal in cancer
* Oncogenes
* Tumour suppressor genes
* Genes that repair other damaged genes
* How mutations happen



How cancer starts

Doctors and scientists now know that each cancer starts with changes in one cell or a small group of cells. Usually, many years before you can feel a lump, or a doctor can see it on a scan, the cells have started to reproduce uncontrollably.

You can see from the differences between normal cells and cancer cells that the cancer cell seems to lose a number of vital control systems. This happens because some of the genes in the cell have been damaged or lost. Scientists call this 'mutation'.
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Genes and mutation

Genes are coded messages inside a cell that tell it how to behave. The genes are codes that tell the cell how to make many different proteins. One gene 'codes' for one protein.

Proteins are the building blocks that make up a cell. Some proteins act as 'on and off switches' that help to control how a cell behaves. For example, a hormone signal acts on a protein in or on the cell. The protein then sends a signal down a chain of switches. The final signal tells the cell to reproduce by dividing into two.

Mutation means that a gene has been damaged or lost. A mutation may mean that too much protein is made. Or that a protein is not made at all.

For example, a signalling protein may be permanently switched on. Other proteins, whose job is to control and limit cell division, may be permanently switched off.

Something that damages a cell and makes it more likely to be cancerous is called a 'carcinogen'. For example, there are carcinogens in cigarette smoke. There is more about this in the section about causes and cancer.
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Genes that are abnormal in cancer

There are three different types of genes that are important in making a cell cancerous

* Genes that encourage the cell to multiply (oncogenes)
* Genes that stop the cell multiplying (tumour suppressor genes)
* Genes that repair other damaged genes (DNA repair genes)


Oncogenes

Some genes encourage cells to multiply or 'double'. Normally, in adults, this would not happen very often. Cells would only multiply to repair damage, for example after a wound or operation. But if these genes become abnormal, they tell the cell to multiply all the time. Scientists call these genes oncogenes. This really means 'cancer genes'.
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Tumour suppressor genes

Some genes are in the cell specifically to stop the cell multiplying or doubling. If one of these 'tumour suppressor genes' becomes damaged and stops working, then the cell may carry on and on multiplying. In other words it becomes immortal, which is one of the properties of a cancer cell. The best known tumour suppressor gene is called p53. This gene normally stops cells with other damaged genes from reproducing and encourages them to destroy themselves (apoptosis). p53 is damaged or missing in most human cancers.
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Genes that repair other damaged genes

These genes normally repair any damage to the DNA that makes up the cell's genes. If these DNA repair genes are damaged, then other mutations are not repaired and the cell can copy the mutations into its daughter cells. These genes have been found to be damaged in some human cancers, including bowel cancer.
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How mutations happen

Mutations can happen by chance when a cell is reproducing. It is not easy for a normal cell to turn into a cancer cell. There have to be about half a dozen different mutations before this happens. Cells often destroy themselves if they carry a mutation. Or the cells might be recognised by the immune system as abnormal and killed. This means most precancerous cells die before they can cause cancer. Only a small number of the changes turn into a cancer. There is detailed information about the immune system in this section of CancerHelp UK.

It can take a long time before enough mutations happen for a cell to become cancerous. This is why most types of cancer are more common in older people. There has been more time to be exposed to carcinogens. And more time for accidents to occur when cells reproduce.

Some people are said to have a 'genetic predisposition' to a type of cancer. This means they are more likely to develop that type of cancer than most people. They are more at risk of cancer because they have been born with one of the mutations that starts to make a cell cancerous. They do not actually have the cancer because more than one mutation is needed for a cancer to develop. There are many steps in the process of a normal cell becoming a cancer cell. But people with particular mutations are naturally further along the road towards getting certain types of cancer than people who don't have the mutation.