Though it is difficult to pinpoint who can be more susceptible to a particular type of cancer, there are certain biological, gender and ethnic factors which make the occurrence of a particular type of cancer more probable in some individuals than others. Speculations about the causes of cancer and the mechanisms that lead to its occurrence have been constrained by some of its biological characteristics. These include the relationships between incidence and genetic susceptibility, age, sex, and the delay (which is sometimes misleadingly called the ‘latent period’) that occurs between exposure to a causative agent and the appearance of clinical disease.
Genetic susceptibility to cancer: It has been demonstrated by various studies that if one member of a family develops a specific type of cancer, other members are somewhat more likely to develop that same type than would be expected in the population as a whole. Several genes have such a great effect on susceptibility that bearers of one such gene (if it is dominant) or two (if they are recessive) almost invariably develop a particular type of cancer. Examples include the dominant genes for polyposis coli and Gardner’s syndrome that lead to cancer of the large bowel, and the recessive genes for retinoblastoma and xeroderma pigmentosum that lead (in the latter case) to squamous carcinoma and (less commonly) melanoma of the skin.
The genetics of breast cancer is well established. Breast Cancers occur when a buildup of genetic mutations in critical genes-those that control cell growth and division or the repair of damaged DNA-allow cells to grow and divide uncontrollably to form a tumor. These genes are BRCA1, BRCA2, CDH1, PTEN, STK11, and TP53. In most cases, the genetic changes that trigger breast cancer are acquired during a person’s lifetime and are present only in certain cells. A history of breast cancer in closely related family members is also an important risk factor, particularly if the cancer occurred at an early age. Some breast cancers that cluster in families are associated with inherited mutations in particular genes, such as BRCA1 or BRCA2. BRCA1 and BRCA2 are major genes related to hereditary breast cancer. Women who have inherited certain mutations in these genes have a high risk of developing breast cancer. The proportion of all cancers that occur in people who are highly susceptible to cancer in this way is, however, very small. It means that though cancer can run through generations it is not necessarily so in most of the cases. Those with no close relatives who have this disease do develop it in increasing numbers.
Race too plays an important role in the susceptibility to a particular type of cancer. Caucasians (people who are from or trace their origin to Europe, West Asia or North Africa) are at a greater risk of developing basal-cell and squamous carcinomas of the sun-exposed skin. This risk is greatly diminished in Chinese, Japanese, and Asian Indians. Caucasians have a 30% chance of developing basal-cell cancer in their lifetime and maximum number of such basal cell cancer cases occur in neck and head. There is also a relationship between type of blood group and cancer.For example a particular gene that is present in those with blood group A, increases the risk of gastric cancer by about 20 per cent over that of people belonging to blood groups O or B.
Such genetic, race and other relationship to cancer should help doctors, researchers, scientists and other health care specialists to identify high risk groups. This can help in early diagnosis and prevention of aggravating factors.
by Andy Kahn