There are lots of different types of cancer which affect different tissues in the body, and two cancers of the same type may not have the same causes. This makes cancer interesting, but also potentially confusing. So it helps to break the issues down to basics.
Firstly, cancer is defined as pathological cellular hyperplasia. ‘Hyperplasia’ means that cells are multiplying, and ‘pathological’ means that this process is causing problems.
Some cell types are constantly multiplying throughout life (like those in skin, hair, bone marrow and the gut wall) but others do not multiply at all (like those in muscle, or nerve cells). This rate of multiplication is carefully controlled by a large number of genes which work together to make sure that a cell only divides when the body needs it to.
Cancers can develop when mutations arise these genes. These may be inherited, happen at random during normal cell division, or have environmental causes like cigarette smoke (‘mutagens’) which make alterations to the DNA molecules of a cell. The effect of these mutations is that cell division is no longer controlled according to the needs of the body as it has evolved, but according to the new, mutated cell.
There are two basic types of genes which are involved in the development of cancers:
- ‘Oncogenes’ encourage cell multiplication. These contribute to cancerous growth when they are overexpressed, or when they act too efficiently.
- ‘Tumour suppressor genes’ slow down cell multiplication. These contribute to cancerous growth when they are underexpressed, or do not work properly.
A mutation which makes a cell multiply faster than its neighbours eventually leads to a large collection of similar, rapidly multiplying cells with the same mutation. This is the basis of a tumour.
Because these cells are no longer controlled by the normal system of genes which regulate cell turnover, further mutations in these cells then occur at an increasing rate which help the cancer by improving its blood supply, helping to defend against the immune system, disabling the proteins which correct genetic mistakes and so on. Typically, five or more of these mutations occur before a cancer grows large enough to cause problems.
Mutations also occur which weaken bonds between cells so that they can drift away and start new tumours elsewhere in the body. This is called metastasis and often marks the point at which a cancer becomes incurable.
So really, there is no single cause of cancer but a process involving changes to genes, changes in the behaviour of cells, and changes in the structure of body tissues. Beyond this general picture, understanding the causes of cancer relies on looking at the ways that individual cancers tend to develop.