Application of genetic testing in the clinical setting

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In the absence of systematic, family based screening programs, the presenting features are usually those of malignancy, such as weight loss and inanition, bowel obstruction, or bloody diarrhoea. In such cases, patient evaluation will frequently find a colorectal carcinoma. Occasionally, the extracolonic features of the condition may lead to presentation and diagnosis. Cases of new mutation still present in these ways, but in areas with well organized registers, gene carriers among relatives of affected patients are identified prior to symptoms either by DNA-based genetic tests or by bowel examination.

The most commonly used commercially available genetic testing for FAP involves identification of the mutant APC allele by in vitro detection of truncated APC protein {414}. This approach is referred to as in vitro protein synthesis (IVPS) testing. IVPS testing is able to detect mutation carriers in about 80% of families. Once evidence of a disease-causing mutation is found in an index case by this method, testing is near 100% predictive in other family members. It is imperative that genetic counselling be undertaken throughout the process of genetic testing. Without this, genetic testing and the use of the results are poorly applied in the clinical setting {1703}.

Screening in gene carriers is similar to that in families where genetic testing is not applied or does not work and usually involves sigmoidoscopy every 1 to 2 years, beginning between age 10 and 12 years. If a genetic diagnosis is made after that age, full colonoscopy should probably be done in view of the risk of lesions higher in the colon. Preventive total colectomy is proposed to gene carriers when polyposis becomes conspicuous. Genotype/phenotype correlations may be used to adapt clinical management to individual FAP patients. A family member who has a negative DNA based genetic test can forgo screening if (1) the mutation found in other affected family members is obviously deleterious and (2) if the individual with a negative test has been unambiguously shown to be a non-gene carrier by DNA testing. Such individuals need no further screening as their risk to develop colon cancer is similar to that of the general population.

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