DNA typing to avoid need for prenatal diagnosis of cystic fibrosis

First-trimester prenatal diagnosis of cystic fibrosis using fibroblasts from a deceased index child to establish haplotypes

First-trimester prenatal diagnosis of cystic fibrosis (CF) using linked DNA markers is usually only possible if there is an index affected child to establish the haplotype of the parental chromosomes. We describe a prenatal diagnosis where fibroblasts, cultured from the skin of a deceased affected child and then held in frozen storage for 3 years, were used as the starting point for tracking the CF gene. The fetus was diagnosed as a homozygous normal and the diagnosis confirmed by immunoreactive trypsin testing after birth. It was also possible to establish heterozygosity in the aunt of the affected child.

Prenatal diagnosis of cystic fibrosis by microvillar enzyme assay on a sequence of 258 pregnancies

Prenatal diagnosis of cystic fibrosis by microvillar enzyme assay on amniotic fluid supernatant has been carried out on 258 sequential pregnancies with a 1 in 4 recurrence risk, all with known outcome. In general the three enzymes evaluated, gamma-glutamyltranspeptidase, aminopeptidase M and the intestinal isoenzyme of alkaline phosphatase, showed a high degree of concordance. However, there were two unusual patterns of microvillar enzyme activity; in seven cases a low gamma-glutamyltranspeptidase activity was associated with elevated values of intestinal alkaline phosphatase, and in ten cases there were isolated low values of intestinal alkaline phosphatase. The former pattern was found to be associated with cystic fibrosis in five cases, while the latter was associated with a normal outcome in all ten cases. A retrospective analysis of enzyme values suggested that the optimal system for minimizing false positives and false negatives was to define foetal cystic fibrosis as a sample where two of the three microvillar enzymes were below a cut-off of half the median value for the gestational week. If such scoring were applied to the cases where conventional microvillar enzyme patterns were observed, the false positive rate was 2.3% and the false negative rate 4.4% between 17 and 20 weeks of gestation.

Fetal diagnosis of cystic fibrosis

Two different methods of prenatal diagnosis of CF are currently available. Both are usually only suitable for couples with a 1 in 4 risk of bearing an affected child. The method of choice is first trimester diagnosis on a chorionic villus sample using linked DNA probes. The advantages are: 1. The very high degree of accuracy of diagnosis, 2. The opportunity of first trimester termination of pregnancy when the fetus is affected. The disadvantages are: 1. The need for a blood sample from an index affected child to establish phase, 2. The requirement for early workup of the nuclear family to ascertain informativeness, 3. The chance that some families will be uninformative or only partially informative for available DNA probes. An alternative form of diagnosis is that using microvillar enzyme testing on second trimester amniocentesis samples. The advantages are: 1. It is suitable for all pregnancies with a 1 in 4 risk of CF, 2. No prior workup of the family is required, 3. Assays can be completed within one hour of receipt of the sample. The disadvantages are: 1. The accuracy of diagnosis leaves some scope for error, 2. If termination of pregnancy is indicated this can only be carried out at about 18 or 19 weeks of gestation.