be caused by mistakes in copying DNA, or a genetic predisposition as a result of inheritance. Many tumours are caused by activated oncogenes, which are special genes with the potential to cause cancer. Oncogenes may either be normal genes that have become altered, or they may be genes that are expressed at abnormally high levels. Activated oncogenes can cause cells that should die during apoptosis to survive and divide instead. Most oncogenes become active as a result of some additional process such as mutation in another gene (often those which regulate cell growth or differentiation), direct exposure to a mutagen or another environmental factor such as a viral infection. Because of their importance in human cancers, oncogenes are specifically targeted in many new cancer treatments that are being developed in laboratories all over the world.
Smoking and cancer Smoking is a major cause of several types of cancer. There is strong evidence to show that it increases the risk of cancer of the bladder, cervix, kidney, larynx and stomach, and smokers are seven times more likely to die of these cancers than non-smokers. In the UK, approximately 85% of lung cancers in both men and women are related to smoking. The risk of contracting lung cancer increases with the number of cigarettes that a person smokes and the number of years that they continue to smoke. If a person gives up smoking, their risk of developing cancer decreases (Figure 1.34). Cumulative risk of lung cancer for men at age 75, according to age at which they stopped smoking
Relative risk of lung cancer, according to duration and intensity of smoking, for men 45
45 40
Relative risk
35
Cumulative risk of lung cancer / %
<10 cigarettes per day 10–19 20+
30 25 20 15 10
40 35 30 25 20 15 10 5
5 0 <20
20–29
30–39 Years of smoking
40–49
50+
0 nonsmoker
<30 years
40 years
50 years
60 years
current smoker
Age at which smoking stopped
Figure 1.34 Graphs to show the relationship between smoking and lung cancer, and the cumulative risk of lung cancer among men in the UK at age 75 according to the age at which they stopped smoking (data from Cancer Research UK).
1 CELL BIOLOGY
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