Guest Author - Nina Shavel

Cancer is a disease, in which the cells proliferate uncontrollably, forming a malignant tumour. It is caused by mutations (alterations) in different genes that can arise due to a variety of causes:
� Spontaneous
� Radiation
� Chemical damage
� Infection

Spontaneous mutations generally occur when DNA in our cells is replicated with a mistake as the molecular machinery involved in copying our DNA is not perfect. Some damage to DNA also occurs spontaneously, for example, each cell in your body loses thousands of DNA building blocks a day. Most of the time, both types mistakes are repaired by specific enzymes but sometimes they are not, giving rise to a mutated gene. If spontaneous mutations that arise in important genes are passed on to children, predisposition to cancer can be inherited, e.g. BRCA1 and BRCA2 gene mutations confer increased risk of breast and ovarian cancer.

Radiation that causes mutations in cancer can be divided into two groups � ionising, e.g. alpha and beta rays, and UV. Ionising radiation damages the DNA by causing breaks in one or even both strands of the DNA molecule. When these breaks are joined together, mistakes can be made, giving rise to mutations. UV acts in a different way by causing the thymidine bases (one of the four DNA bases) to join together. This causes problems during copying of the DNA, as the replication enzyme cannot read through this obstacle and so makes a mistake. This is why UV exposure can give rise to skin cancer.

Many chemicals have carcinogenic (cancer causing) properties, e.g. cigarette tar components. This was demonstrated by painting tar onto the skin of mice, which resulted in tumour formation. Benzpyrene, a component of cigarette smoke, when metabolised by the body, binds to DNA and forms a bulky adduct, which hinders correct replication. This is one of the ways, in which smoking causes lung cancer.

Some infections can be involved in causing cancer, e.g. human papilloma virus (HPV) in cervical cancer. HPV can cause the cervical cells to replicate fast, pushing them a few steps along the way to cancer, which develops if, in addition to the viral infection, certain mutations occur. Recent developments in the vaccine for HPV are very exciting because they could help to bring down the number of cases of cervical cancer, especially in developing countries.