Human chromosomes in prenatal diagnosis: A one step high resolution technique

A simple high resolution technique for human chromosomes is described for fibroblasts obtained from amniotic fluid cell cultures. The application and clinical significance of this technique in prenatal diagnosis is discussed.     

Prenatal determination of dihydropteridine reductase in a normal fetus at risk for malignant hyperphenylalaninemia

Assay of dihydropteridine reductase in amniotic cells from a fetus at risk for malignant hyperphenylalaninemia is described. A normal result was obtained, this was confirmed after delivery. On two-dimensional polyacrylamide gel electrophoresis the enzyme from amniotic cells showed the same mobility as the mature liver reductase.     

Disappearing lung echogenicity in fetal bronchopulmonary malformations: A reassuring sign?

Congenital bronchopulmonary malformations detectable on prenatal ultrasound include cystic adenomatoid malformation (CAM), lobar sequestration, and upper airway atresia. We describe three fetuses with prenatally detected intrathoracic lesions in which the associated pulmonary hyperechogenicity disappeared before delivery. In the first case of pulmonary sequestration, the infant was asymptomatic after birth. However, in a case of CAM and another with laryngeal atresia, respiratory distress developed after delivery, despite recent scans showing apparently normal lung fields. This experience suggests that ultrasonic resolution of hyperechogenic lung lesions in utero does not necessarily indicate resolution of the underlying pathology.     

False non-paternity in a family for prenatal diagnosis of β-thalassaemia

Initial screening for the common β-thalassaemia mutations with allele-specific oligonucleo-tide probes in an at-risk family suggested non-paternity. Subsequent DNA fingerprinting of the members proved otherwise. The mother had a codon 41/42 frameshift mutation and the father's defect, determined by direct sequencing of PCR-amplified β gene, was a codon 43 nonsense mutation. In the affected children, the close proximity of these two defects resulted in the absence of a hybridization signal to the normal probe in that region and a wrong assumption of homozygosity for the codon 41/42 mutation. The non-reactivity of the father's amplified DNA to the codon 41/42 thalassaemic probe accounted for the initial wrong conclusion of non-paternity. Since prior screening for β-thalassaemia mutations is done in all prenatal diagnosis programmes and concomitant inheritance of these two defects is relatively common in the Chinese, this ‘artefact’ of false non-paternity is worth noting.     

Human chorionic gonadotropin levels in pregnancies with aneuploid fetuses

Maternal serum human chorionic gonadotropin (hCG) and the free alpha-hCG subunit were evaluated in 249 women from 9 to 11 weeks gestation who subsequently underwent chorionic villus sampling for determination of fetal karyotype and in 20 women of 18 or more weeks gestation who were ascertained to have an aneuploid fetus by genetic amniocentesis. Seven of the first-trimester pregnancies were determined to be aneuploid and six had hCG levels in the normal range (one triploid pregnancy had elevated hCG levels) whereas 12 of the 20 secondtrimester cases had abnormal hCG levels and an additional three had elevated levels of alpha-hCG. This study confirms the previous report of abnormal maternal serum hCG levels in women with an aneuploid fetus at ≥ 18 weeks gestation and demonstrates that hCG evaluation is not useful at 9–11 weeks gestation for selecting pregnancies at risk for fetal aneuploidy.     

Prenatal diagnosis of the pterygium syndrome

We report two second trimester pregnancy terminations in the same woman following intrauterine ultrasonic findings of hydrops fetalis, polyhydramnios, lack of fetal movements, and short, fixed malformed limbs. One fetus also showed a cystic mass at the back of the head. Radiographic and anatomic studies of the fetuses demonstrated multiple pterygia, flexion contracture of multiple joints, abnormal facial appearance, cleft palate, pulmonary hypoplasia, and gracile bones. The cystic mass of the back of the head was found to be a cystic hygroma. These findings are consistent with the lethal variant of multiple pterygium syndrome. Early prenatal diagnosis of this condition is possible using ultrasonography.     

Evaluation of maternal serum dimeric inhibin a as a first-trimester marker of down's syndrome

While second-trimester prenatal screening programmes for Down's syndrome have become established in prenatal care, it would be advantageous to be able to offer screening in earlier preganancy. To this end, we have evaluated a new potential maternal serum marker, dimeric inhibin A, as a possible first-trimester marker. Dimeric inhibin A was measured in propsectively collected maternal serum from 23 cases of Down's syndrome and matched chromosomally normal controls, at 11–13 weeks' gestation. Levels of this protein were significantly elevated in the Down's pregnancies compared with the control pregnancies. The median multiple of the normal median (MOM) for the Down's samples was 2.46 (95 per cent confidence interval: 2.11–3.26, P<0.0001 vs. controls). These results suggest that dimeric inhibin A is a useful discriminator of Down's-affected pregnancies from normal pregnancies in the first trimester and that sensitive screening in combination with maternal age and other possible markers may be practicable in the first trimester.     

Infantile hypophosphatasia: Successful prenatal assessment by testing for tissue-non-specific alkaline phosphatase isoenzyme gene mutations

We successfully assessed a fetus at risk for lethal infantile hypophosphatasia using amniocyte DNA and allele-specific oligonucleotide (ASO) probes for two missense mutations in the tissue-non-specific alkaline phosphatase isoenzyme (TNSALP) gene. The nucleotide changes had been discovered in a sister who died at 8 months of age from this inborn error of metabolism. The mother was known to carry the 747 (cDNA) G→A transition, whereas her husband and 5-year-old daughter, who were also healthy, carried the 1309 A→T transversion. Amniocytes, obtained at 16 weeks' gestation, provided genomic DNA for polymerase chain reaction (PCR) amplification of the appropriate TNSALP gene exons. ASO hybridization revealed absence of the 747A mutation and presence of the 1309T base change in the fetus, indicating a carrier for hypophosphatasia. At 8 months of age, the offspring was in excellent health and without any radiological evidence of skeletal disease. His serum ALP activity and plasma pyridoxal 5′-phosphate level were decreased and increased, respectively, at levels consistent with the prenatal assessment. The ASO studies were confirmed postnatally using peripheral blood leukocyte DNA. This is the first application of direct mutational analysis to assess a fetus at risk for hypophosphatasia.     

Prenatal diagnosis of sanfilippo disease type C using a simple fluorometric enzyme assay

A new fluorogenic substrate, 4-methylumbelliferyl β-D-glucosaminide, was used for the assay of acetyl CoA:glucosaminide N-acetyltransferase in chorionic villi, cultured villus cells, and amniocytes. Optimal conditions for the assay and the ranges of enzyme activity were established for the various types of fetal cells. This simple fluorometric assay provides a reliable method for early prenatal diagnosis of Sanfilippo disease type C which is more convenient than current methods using radiolabelled substrates. The method was applied to amniotic fluid cells and fetal fibroblasts from an at-risk pregnancy in which an affected fetus was diagnosed by two-dimensional electrophoresis of glycosaminoglycans in the amniotic fluid.