HISTORICAL PERSPECTIVE AND BACKGROUND
Cystic fibrosis (CF) has been an obvious target for population carrier screening ever since the gene responsible, cystic fibrosis transmembrane conductance regulator (CFTR), was discovered and characterized in 1989. The reasons for this include its high carrier frequency in the general US population (approximately 1 in 25-30 non Hispanic Caucasians; progressively less in individuals of Hispanic (1 in 46), African (1 in 65) or Asian (1 in 90).
CF has recessive mode of inheritance, with the result that most carriers have a negative family history and do not know they are at risk of having an affected child.
CF is usually thought of as a chronic pulmonary disease that may cause progressive, life-threatening respiratory infections However, it is actually a multisystem disease, with defects in many organs associated with defects in many organs associated with abnormally thick and viscous secretions from epithelial cells obstructing airways and ducts. It is true that chronic obstructive pulmonary disease is the major source of morbidity and early death; however, CF patients may also suffer from pancreatic insufficiency, intestinal obstruction, diabetes, biliary cirrhosis, growth retardation, dehydration due to excessive salt loss in sweat, and sinusitis.
In addition, virtually all adult males with CF are infertile due to congenital bilateral absence of the vas deferens (CBAVD).
With newer antibiotic and other therapies, the median life ecpectancy has extend to over 30 years, though some patients still die in early childhood from meconium ileus, hiponatremic dehydration, severe protein energy malnutrition or respiratory failure.
MOLECULAR BASIS OF CISTIC FIBROSIS
Initial molecular genetic chararcterization of the CFTR gene in affected patients revealed a recurring mutation, a three-nucleotide of codon 508, which specifies phenylalanine in the protein product and is designated F508.
Within a few months of the gene’s discovery, reports from many laboratories began to document a growing number of less common mutations, mostly missense or nonsense but also encompassing delections and splice-side defects.
The catalog has continued to increase ever since, with over 1000 mutations now listed.
Most of these are extremely rare or even private (found only in a single family) and thus would not be cost effective to screen for in the general population. However, there are a number of other mutations that recur at an appreciable but far lower frequency than F508.
CTFR MUTATION TESTING
CFTR mutational analysis utilizes a single nucleotide polymorphism-based technology of which there are many choices.
There are no US FDA (Food and Drug Administration) approved kits for CFTR testing. Laboratories may choose to develop home-brew CFTR tests or use analyte-specific reagents (ASRs) that have been developed by manufacturers for use on several different technical platforms.
Reporting and residual risk -
Clinical laboratory reports for CF carrier screening will in most cases be either positive
(heterozygous for a CF mutation) or the mutations tested and a residual risk will be given.
A positive test result in which an individual is found to carry a CF causing mutation will be considered first.These reports are relatively strainghtforward : identify the mutation and the patient as a heterozygote carrier and recommend testing the partner.
Negative results require an estimate of the individual’s residual risk for being a carrier and this risk on the family history of CF, the patient’s ethnicity and the mutation detection frequency.
While most laboratories report the individual’s test result, physicians screening reproductive couples to identify a CF fetus and some laboratories using this programmatic approach will focus on the couple’s result.
Depending upon the model used in testing, a couples report may vary. For example, if a sequential screening model is used and the first partner tests negative, then the second partner would not be tested.
The couples residual risk to have a child affected with CF (assuming no family history in this and following examples) would be calculated as follows :
1/200 (first partner,negative) x 1/25 (second partner, untested) x 1/4 = 1/20000
In contrast, using a concurrent approach, both are tested and if both are negative their residual risk is lowered as follows:
1/200 (first partner, negative) x 1/200 (second partner negative) x ¼ = 1/160,000
In both cases, no further testing or genetic counseling is recommended.
On the other hand, if the first individual tested is found to be positive, then the partner is tested. In this case, there are two possible outcomes (actually more if one considers the CBAVD issue, however, this discussion will be limited to the obvious ones).
- The partner could be found to be negative,
resulting in what has come to be known as the positive-negative couple. The residual calculation for these couples is as follows:
1 (she is a carrier) x 1/200 (his test is negative) x ¼ = 1/800 to have a child with CF
While these couples are it increased risk over the general untested population, they are not suitable candidates for prenatal diagnosis.
- Another outcome for testing the partner of a CF carrier is that he too could be found to be a carrier.
In this case, the couple has a
one in four risk for having a child affected with CF and genetic counselig is recommended and prenatal diagnosis is offered.
PRENATAL DIAGNOSIS OF CF
Prenatal diagnosis is indicated for couples in which both partners are carriers of CFTR mutations and thus the fetus is at one in four risk of being affected. Thus, it is important to perform carrier testing early in pregnancy in order to identify such at risk couples in sufficient time (before 20 weeks of gestation) to allow the option of prenatal diagnosis and possible termination of a fetus. An additional indication for prenatal testing is echogenic bowel of the fetus found on ultrasound in the second trimester. Echogenic bowel is found in approximately 0,1% of pregnancies in the second trimester and can be due to CF, normal variation, chromosomal abnormality or congenital viral infection.
Prenatal specimen types include both direct and cultured amniocytes and chorionic villi. Chorionic villus sampling (CVS) is performed during the first trimester at approximately 10 weeks, while Amniocentes is performed in the second trimester at around week 16.