ly defined. Since the genetic causes of hypertension are likely to differ among individuals, definition of the genetic-biochemical defect(s) in a given person may lead to more rational therapy directed to the underlying cause. Thus, salt restriction might be advisable for salt-sensitive persons but not for all individuals, and a specific class of drug to lower blood pressure could be selected when drug therapy becomes indicated. Medications that lower blood pressure are currently used empirically, without defining the specific cause of high blood pressure. There is now some evidence that hypertension among African-Americans is less likely to respond to certain classes of drugs (beta-blockers) than hypertension among Caucasians, suggesting genetic heterogeneity in the mechanism of hypertension in these populations. Much more work will be needed before genetic tests can be useful as practical tools for predicting high blood pressure and directing therapy.
Unlike those cancers with inherited susceptibility, discussed earlier, most cancers have complex etiology; they are caused by alteration of the genetic material (DNA) in somatic cells of various organs (such as breast and colon) that occur after birth. Most cancers, therefore, may be classified as somatic genetic disease. Unlike classic heritable diseases where every body cell carries the altered DNA, the characteristic molecular and cellular alterations in cancer affect only the descendants of the original cancer cell in a given organ, which grow in an unregulated fashion and may spread to other organs.
Detection of the somatic mutations of cancer is becoming increasingly possible by genetic techniques, and many of these techniques are already coming into clinical use. In hematologic cancers, different specific alterations characteristic of a given disorder were initially observed in the chromosomes of the involved tissue (IOM, 1992). The genes affected by these cytogenetic anomalies are being identified and can sometimes be utilized for molecular diagnosis. Occasionally, detection of a specific chromosomal or molecular abnormality is helpful in predicting the clinical severity of the cancer and in a few cases may aid in selecting the most appropriate treatment (IOM, 1992). In colon cancer and probably in many other cancers, a sequential series of different mutations of the involved somatic tissue is required for the ultimate development of the malignant tumor (Fearon and Vogelstein, 1990). Once characteristic genetic alterations have been recognized, increased monitoring may make possible early diagnosis of a tumor in the affected organ and provide the appropriate treatment.
Better still, a tumor may be recognized in its premalignant stages, allowing prevention of its growth by life-style changes, chemoprevention, or a variety of other interventions. Mutations of common tumor suppressor genes such as the p53 gene are often involved (Frebourg and Friend, 1992). Screening of the general population for common somatic cancers for which everyone is at risk may become feasible using such tumor biomarkers by examining DNA from cells in