Genetic Factors

Heredity and Cancer

Heredity is a relatively unimportant factor in carcinogenesis. But since growth is controlled by genes and as the two major criteria of cancer are increased rate of reproduction and uncontrolled growth, by implication, genetic factors must play a certain role in all cancers. However, compared with the influence of the environment, the contribution of genetic factors is rather slight for the more common forms of cancer. There is evidence that defects passed in the genes to infants can create a proneness to cancer in later life; in other words, a tendency to a certain type of cancer may sometimes be hereditary and such cancer inheritance is mostly site-specific i.e. the tumour arises in identical organs so that there appears to be a genuine family tendency for bowel, stomach and breast cancer, and to a lesser degree for lung cancer.

Bowel Cancer

This inherited condition usually develops in existing polyps which mostly line the whole of the large bowel. Without removal of the affected gut, 90 per cent of those affected would die of bowel cancer. Polyposis coli is inherited as a dominant trait and the primary cause is an abnormal gene.

Stomach Cancer

This is more common in the lower social classes, perhaps on account of cigarette smoking and an unhealthy diet. It has a genetic component which is only slight and is greatly influenced by environmental factors. The frequency in near relatives is small: about three per cent of close relatives are likely to develop the disease, as against one-and-a-half per cent in non-related patients. In a study of stomach cancers as many as 73% of identical twins were found to be discordant (presence of stomach cancer in only one of the twins). This emphasises the importance of environmental factors. If genetic factors were of greater importance the incidence of cancer in both twins would be much higher.

Breast Cancer

It is known that childbearing makes a woman less liable to this disease. But breast cancer is less common in childbearing women without a family history of breast cancer than it is in child-bearing women with relatives who have the disease. Therefore a family history of breast cancer in mother, sister, grandmother or aunt is a contributory factor, but it is a relatively unimportant one, when compared with such other predisposing factors as obesity, childlessness, a first menstruation at the age of 10 years, or excessive hard-fat consumption, particularly that of milk-fat (as in butter, cream, cheese and whole milk).

Skin Cancer

A defect in the production of enzymes that repair DNA damaged by ultraviolet light (the defect having probably been caused by a mutation) is responsible for a rare hereditary skin disease called Xeroderma pigmentosum. In this disease over sensitivity to ultraviolet sunlight also produces a tendency to multiple skin cancers on the exposed part of the skin. Pigmentation of the skin is also a hereditary trait that decreases susceptibility to skin cancer by protecting the skin against ultraviolet sunlight.

Retinal Cancer

Retinoblastoma, a growth affecting the retina, is inherited as a dominant trait; it develops in about one in 18,000 children. About 30 per cent have unilateral disease (in one eye only), whilst the remainder have bilateral disease (in both eyes).