First up, and probably most common, are the chemical carcinogens. Probably the first known connection between chemicals and cancer was made by Sir Percival Pott, who put two and two together and deduced that the extraordinarily high rate of scrotal cancer in chimney sweeps was most probably caused by the unfortunates' scrotal contact with the soot in the chimneys. Since then, a bewildering array of chemicals have been implicated in a huge variety of cancers. Common ones include elements in cigarettes (lung cancer) and some dyes (bladder cancer).
Then there is radiation. It's carcinogenic effects are well known. UV light is very well documented to be the cause of most skin cancers. Other types of more powerful radiation (e.g. X-rays, or sadly radiation from atomic explosions) can penetrate deeper, and are implicated in a wide range of tumours (e.g. leukaemias, breast cancers, etc.).
Remarkably, the final group contains actual organisms - the microbial carcinogens. This category is far less well known than either of the other two categories, and the majority of these organisms are viruses. Since viruses depend on getting into the cell and directing protein synthesis, and many of them even tamper with our own DNA, it is actually not that surprising that they can cause cancers in certain circumstances. Well-known examples include the hepatitis B virus (hepatic cancer) and human papilloma virus (cervical cancer). Vaccines for the latter are just being rolled out now, and have shown exceptional promise. Intruigingly, this therefore offers the potential of a vaccine for a cancer.
Not all microbial carcinogens are viruses, though. A notable exception is the extremely common bacterium Helicobacter pylori, which causes stomach ulcers. For some reason, in a small subset of people, it can lead to gastric (stomach) cancers.
OK, so we've covered the sorts of things that can cause DNA mutations. But not all DNA mutations cause cancer. In fact, very few of them do. Most either cause no change at all or else cause the death of the cell. Clearly, the DNA mutations have to be in certain specific areas coding for certain specific things. But what are those things within the cell that the mutations must alter to form a cancer?
I'll answer that in the next post.