Genetic counseling: DNA testing for the patient

Posted: February 14, 2015 at 4:40 pm

Proc (Bayl Univ Med Cent). 2005 Apr; 18(2): 134137.

1From the Department of Pathology, The University of Texas Southwestern Medical Center, Dallas, Texas.

Presented at the Department of Pathology Fall Symposium, Baylor University Medical Center, November 23, 2004.

Genetic counseling deals with the human problems associated with the occurrence, or the risk of occurrence, of a genetic disorder in the family (1). Genetic counseling is essentially a communication processabout medical facts, the contribution of heredity to certain conditions, the interpretation of test results, and the options available. It also involves supportive counseling to enable patients to make decisions and to make the best possible adjustment to the presence or risk of genetic disease. Genetic counselors have master's degrees from certified programs and are certified by either the American Board of Medical Genetics or the American Board of Genetic Counseling.

This article reviews several case scenarios in order to highlight some themes and lessons from genetic counseling.

In recent years, obstetricians have begun offering cystic fibrosis carrier screening to all couples either planning a pregnancy or in the early stages of pregnancy. The incidence of cystic fibrosis in Caucasians is about 1 in 2500. Caucasian men and women with no family history of cystic fibrosis would each have a 1 in 25 chance of being a carrier, i.e., of having one of over 1300 different mutations in the CFTR gene. The screening test usually includes analysis for only 23 to 25 of the most common mutations. At this time, sequencing the gene from beginning to end to search for a mutation would be too expensive and time consuming as a screening test.

The decision to undergo carrier screening is a personal one. Some people consider cystic fibrosis a serious disorder appropriate for screening, and others do not. Those who do not may focus on the fact that half of those with the disease survive until the age of 30 or 31, and cystic fibrosis does not involve mental retardation or birth defects. A couple may opt for screening for one or several reasons: because the chance of being a carrier seems high to them, because they would consider prenatal diagnosis if they were shown to be carriers, or because results are usually reassuring. Similarly, those who don't feel the odds of being a carrier are high enough may not be interested in the screening test. Others may not be interested because the cost is not covered by their insurance, the test is imperfect and will not identify all carriers, or they would rather not have the information. Indeed, getting information can provoke anxiety, and some patients prefer not to go down that path.

Some practitioners discourage screening unless the patient plans to pursue prenatal diagnosis. However, my experience has shown that people often change their minds. Some come in with very set ideas about what they would and would not do; until they're faced with a particular circumstance, they may not re-ally know. The point is, options need to be made available.

We will discuss a case in which the woman was screened and found to have mutation G542X (Figure ). This means that in the 542 amino acid position, glycine has been converted to a stop codon. She is a cystic fibrosis carrier. Since cystic fibrosis is an autosomal recessive disorder, this should not affect her health. However, her husband was subsequently screened, and he was shown to have the most common mutation, F508, a deletion of phenylalanine at position 508. Even though they have different mutations, the mutations are in the same gene. The child now has a 1 in 4 risk of inheriting both mutations and therefore having cystic fibrosis.

Pedigree of a couple presenting for carrier screening for cystic fibrosis. (See discussion in the text.) Squares represent males, circles represent females, diamonds are used when the gender is unknown or unspecified, and a P within ...

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Genetic counseling: DNA testing for the patient

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