Chapter 064. The Practice of Genetics in Clinical Medicine (Part 8)

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Preventive measures and therapeutic interventions are not restricted to metabolic disorders. Identification of familial forms of long QT syndrome, associated with ventricular arrhythmias, allows early electrocardiographic testing and the use of prophylactic antiarrhythmic therapy, overdrive pacemakers, or defibrillators (Chap. 226). Individuals with familial hypertrophic cardiomyopathy can be screened by ultrasound, treated with beta blockers or other drugs, and counseled about the importance of avoiding strenuous exercise and dehydration (Chap. 231). Likewise, individuals with Marfan syndrome can be treated with beta blockers and monitored for the development of aortic aneurysms (Chap. 242). ...

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Chapter 064. The Practice of Genetics in Clinical Medicine (Part 8) Preventive measures and therapeutic interventions are not restricted to metabolic disorders. Identification of familial forms of long QT syndrome, associated with ventricular arrhythmias, allows early electrocardiographic testing and the use of prophylactic antiarrhythmic therapy, overdrive pacemakers, or defibrillators (Chap. 226). Individuals with familial hypertrophic cardiomyopathy can be screened by ultrasound, treated with beta blockers or other drugs, and counseled about the importance of avoiding strenuous exercise and dehydration (Chap. 231). Likewise, individuals with Marfan syndrome can be treated with beta blockers and monitored for the development of aortic aneurysms (Chap. 242). Individuals with α1 antitrypsin deficiency can be strongly counseled to avoid cigarette smoking and exposure to environmental pulmonary and hepatotoxins. Various host genes influence the pathogenesis of certain infectious diseases in
humans, including HIV (Chap. 182). The factor V Leiden allele increases risk of thrombosis (Chap. 59). Approximately 3% of the worldwide population is heterozygous for this mutation. Moreover, it is found in up to 25% of patients with recurrent deep-vein thrombosis or pulmonary embolism. Women who are heterozygous or homozygous for this allele should therefore avoid the use of oral contraceptives and receive heparin prophylaxis after surgery or trauma. The field of pharmacogenomics seeks to identify genes that alter drug metabolism or confer susceptibility to toxic drug reactions. Pharmacogenomics permits individualized drug therapy, resulting in improved treatment outcomes, reduced toxicities, and more cost-effective pharmaceutical care. Examples include succinylcholine sensitivity, thiopurine methyltransferase (TPMT) deficiency, malignant hyperthermia, dihydropyrimidine dehydrogenase deficiency, the porphyrias, and glucose-6-phosphase dehydrogenase (G6PD) deficiency. As noted above, the identification of genes that increase the risk of specific types of neoplasia is rapidly changing the management of many cancers. Identifying family members with mutations that predispose to FAP or hereditary nonpolyposis colon cancer (HNPCC) can lead to recommendations of early cancer screening or prophylactic surgery (Chap. 87). Similar principles apply to familial forms of melanoma, basal cell carcinoma, and cancers of the breast, ovary, and thyroid gland. It should be recognized, however, that most cancers harbor several distinct genetic abnormalities by the time they acquire invasive or metastatic
potential (Chaps. 79 and 80). Consequently, the major impact of genetic testing in these cases is to allow more intensive clinical screening, as it remains very challenging to predict disease penetrance, expression, or clinical course. Although genetic diagnosis of these and other disorders is only beginning to be used in the clinical setting, predictive testing holds the promise of allowing earlier and more targeted interventions that can reduce morbidity and mortality. We can expect the availability of genetic tests to expand. A critical challenge for physicians and other health care providers is to keep pace with these advances in genetic medicine and to implement testing judiciously. Further Readings Clayton EW: Ethical, legal, and social implications of genomic medicine. N Engl J Med 349:562, 2003 [PMID: 12904522] Collins FS, Watson JD: Genetic discrimination: Time to act. Science 302:745, 2003 [PMID: 14593134] Ensenauer RE: Genetic testing: Practical, ethical, and counseling considerations. Mayo Clin Proc 80:63, 2005 [PMID: 15667031]
Guttmacher AE, Collins FS: Genomic medicine—a primer. N Engl J Med 347:1512, 2002 [PMID: 12421895] Harper PS: Practical Genetic Counselling, 5th ed. Oxford, Butterworth Heinmann, 1998 McCandless SE et al: The burden of genetic disease on inpatient care in a children's hospital. Am J Hum Genet 74:121, 2004 [PMID: 14681831] Wolfberg AJ: Genes on the web—Direct-to-consumer marketing of genetic testing. N Engl J Med 355:543, 2006 [PMID: 16899772]