Objectives: The incidence of Down syndrome increases with maternal age and a rapid and accurate method for prenatal diagnosis is a necessity. This study was devised to evaluate and compare the methods for detecting trisomy 21 by polymerase chain reaction (PCR)-associated analysis of small tandem repeats (STR) of D21S11 and semiquantitative analysis of S100B of chromosome 21.
Methods: PCR was performed with DNA template obtained from 20 normal samples (10 blood, 10 amniotic fluid) and 12 Down syndrome samples (10 blood, 2 amniotic fluid). 32P-labelled primers for D21S11 and S100B were used. PCR products for D21S11 were subjected to polyacrylamide urea gel (6%) electrophoresis, followed by exposure to X-ray film, and then the densities of signals were recorded by densitometer. PCR products for S100B and insulin-like growth factor-I (IGF-I) as an internal control were subjected to agarose gel (2%) electrophoresis and the relative amounts of radioactivity in their products were measured to assess the quantitation of template DNA.
Results: Analysis of D21S11 STR showed equivalent triplets in 4 cases and unequivalent doublets (1:2) in 8 Down syndrome samples. The normal control group showed singlets in 5 cases and equivalent doublets in 15 cases. In the analysis of S100B, the ratios of S100B to IGF-I was 1.4-1.6 in 7 of 12 Down syndrome samples, while the ratios of S100B in all normal samples were close to 1.0. All the results were obtained within 24 h. The D21S11 STR analysis managed to distinguish more clearly between normal and trisomy 21, while semiquantitative PCR analysis of S100B was less able to assess trisomy 21.
Conclusion: Prenatal diagnosis of trisomy 21 by PCR-associated STR analysis of D21S11 and semiquantitative analysis of S100B are useful, innovative, accurate and rapid diagnostic methods, while D21S11 STR analysis is more discriminating in detecting trisomy 21 and also may be employed in preimplantation diagnosis of Down syndrome.