Genetic analysis of cancer in families.

Cancer is genetic, in the sense that it is caused by DNA alterations at the cellular level. On the other hand, the most important risk factors for the common cancers are environmental: cigarette smoking, environmental pollution, occupational exposures, poor diet, and so on. These two observations are not in conflict: the DNA alterations that lead to cancer are very likely to be caused by environmental mutagens. It would be valuable to know exactly what genes are altered to cause a specific cancer, because the effects of these alterations might then be reversible before cancer has a chance to develop. A key to identifying these cancer genes may lie with rare families at extremely high risk of a specific cancer. Unlike most cancer patients, members of these families may inherit an alteration that confers increased susceptibility to cancer. In these rare instances, cancer is a genetic disease at the level of the family, as well as at the level of the cell. Therefore, in these families, genes predisposing to cancer can be mapped in the same way as genes for purely genetic diseases like sickle cell anaemia, cystic fibrosis, and Huntington's disease. The hypothesis that underlies the mapping of cancer genes in families is that the genes inherited in altered form in these rare families are the same genes that are altered in somatic cells of individuals without a remarkable family history of cancer. This hypothesis has proved correct for retinoblastoma. Genes responsible for other rare cancers have been mapped in families as well: neurofibromatosis, multiple endocrine neoplasia, Wilms' tumour, and colon cancer following familial adenomatous polyps, among others. Genes responsible for common cancers are also being defined by genetic analysis, most notably breast cancer and colon cancer. This review summarizes why, how, and what genetic analysis of families can reveal about human cancers.