Chapter 064. The Practice of Genetics in Clinical Medicine (Part 4) potx

In these cases, the choice of which genes to test is often determined by unique clinical and family history features, the relative prevalence of mutations in various genes, or test avail

Chapter 064 The Practice of Genetics

in Clinical Medicine

(Part 4) Figure 64-2

Many disorders exhibit the feature of locus heterogeneity, which refers to

the fact that mutations in different genes can cause phenotypically similar disorders For example, osteogenesis imperfecta (Chap 357), long QT syndrome (Chap 226), muscular dystrophy (Chap 382), homocystinuria (Chap 358), retinitis pigmentosa (Chap 29), and hereditary predisposition to colon cancer (Chap 87) or breast cancer (Chap 86) can each be caused by mutations in distinct genes The pattern of disease transmission, clinical course, and treatment may differ significantly, depending on the specific gene affected In these cases, the choice of which genes to test is often determined by unique clinical and family history features, the relative prevalence of mutations in various genes, or test availability

Methodologic Approaches to Genetic Testing

Genetic testing is performed in much the same way as other specialized laboratory tests In the United States, genetic testing laboratories are CLIA (Clinical Laboratory Improvement Act) approved to ensure that they meet quality and proficiency standards A useful source for various genetic tests is

www.genetests.org

DNA testing is most commonly performed by DNA sequence analysis for

mutations, although genotype can also be deduced through the study of RNA or protein (e.g., apoprotein E, hemoglobin, immunohistochemistry) The

determination of DNA sequence alterations relies heavily on the use of polymerase chain reaction (PCR), which allows rapid amplification and analysis

of the gene of interest In addition, PCR enables genetic testing on minimal amounts of DNA extracted from a wide range of tissue sources including leukocytes, fibroblasts, epithelial cells in saliva or hair, and archival tissues Amplified DNA can be analyzed directly by DNA sequencing or it can be hybridized to DNA chips or blots to detect the presence of normal and mutant DNA sequences Direct DNA sequencing is increasingly used for prenatal diagnosis as well as for determination of hereditary disease susceptibility Analyses of large alterations in the genome are possible using cytogenetics, fluorescent in situ hybridization (FISH), or Southern blotting (Chap 63)

Protein truncation tests (PTTs) are used to detect mutations that result in

the premature termination of a polypeptide occurring during protein synthesis In this assay, the isolated complementary DNA (cDNA) is transcribed and translated

in vitro, and the protein is analyzed by gel electrophoresis The truncated (mutant) gene product is readily identified as its electrophoretic mobility differs from that

of the normal protein This test is used most commonly for analyses of large genes

with significant genetic heterogeneity such as DMD, APC, and the BRCA genes

Like all laboratory tests, there are limitations to the accuracy and interpretation of genetic tests In addition to technical errors, genetic tests are sometimes designed to detect only the most common mutations In this case, a

negative result must be qualified by the possibility that the individual may have a mutation that is not included in the test In addition, a negative result does not mean that there is not a mutation in some other gene that causes a similar inherited disorder

In addition to molecular testing for established disease, genetic testing for susceptibility to chronic disease is being increasingly integrated into the practice

of medicine In most cases, however, the discovery of disease-associated genes has greatly outpaced studies that assess clinical outcomes and the impact of interventions Until such evidence-based studies are available, predictive molecular testing must be approached with caution and should be offered only to patients who have been adequately counseled and have provided informed consent In the majority of cases, genetic testing should be offered only to individuals with a suggestive personal or family medical history or in the context

of a clinical trial

Predictive genetic testing falls into two distinct categories Presymptomatic

testing applies to diseases where a specific genetic alteration is associated with a

near 100% likelihood of developing disease In contrast, predisposition testing

predicts a risk for disease that is less than 100% For example, presymptomatic testing is available for those at risk for Huntington's disease, whereas predisposition testing is considered for those at risk for hereditary breast cancer It

is important to note that, for the majority of adult-onset, multifactorial genetic

disorders, testing is purely predictive Test results cannot reveal with confidence whether, when, or how the disease will manifest itself For example, not everyone with the apolipoprotein E allele (ε4) will develop Alzheimer's disease, and individuals without this genetic marker can still develop the disorder (Chap 365)