Prenatal diagnosis of spinal muscular atrophy by direct molecular analysis: efficacy and potential pitfalls.

The efficacy of direct prenatal diagnosis for spinal muscular atrophy (SMA) is demonstrated, and the potential pitfalls with this type of analysis are highlighted in the largest prospective single-center prenatal series in the United States. The presence or absence of exons 7 and 8 of the SMN gene was determined from 66 fetuses from 51 families. Direct and cultured chorionic villus samples (CVS) and amniocytes were analyzed. DNA analysis to exclude maternal cell contamination was performed on all CVS. Follow-up was obtained for 48 cases; 13 pregnancies continue. One child predicted to be affected with SMA remains asymptomatic at 13 months. Thirty-three cases were confirmed to be clinically unaffected in agreement with the prenatal molecular results. Three of 24 CVS had maternal cell contamination. In conclusion, direct molecular analysis of either CVS or amniocytes is highly accurate in the prenatal diagnosis of SMA. However, maternal cell contamination of CVS samples can confound these analyses, and the possibility of contamination must be excluded routinely.

Calculating posterior cystic fibrosis risk with echogenic bowel and one characterized cystic fibrosis mutation: avoiding pitfalls in the risk calculations.

We describe a general approach to derive fetal risk following two separate test results that each raise the likelihood of the same fetal abnormality without clearly determining whether the abnormality exists. Echogenic bowel observed on fetal ultrasonography may have multiple causes, including an a priori risk of approximately 1% of cystic fibrosis (CF). On numerous occasions our laboratory tests have detected only normal cystic fibrosis transmembrane regulator (CFTR) alleles in fetuses with echogenic bowel. This result indicates that another cause most likely explains the abnormal ultrasound finding. One of our tested fetuses was heterozygous for the deltaF508 CFTR mutation and had a normal karyotype. Over 770 CFTR mutations have been described, and a significant proportion of parental mutant alleles could not be detected by our 25-mutation test. Further mutation analysis demonstrated that the fetus' mother carried the deltaF508 mutation but the father (of different ethnic background than the mother) did not carry a detectable mutation. Thus, this test result substantially increased the risk of the fetus having CF, while still not giving a definitive answer to whether the fetus was affected. A rigorous mathematical analysis determined that the 1% risk of CF following ultrasound study was increased to slightly under 12% following DNA analysis. The case is described, and the mathematical formulas are explained and illustrated with examples, along with a review of conditional probability (Appendix 2).