[Practicality of rapid prenatal screening for Down syndrome with PCR-short tandem repeat method]

To assess the practicality of rapid prenatal screening for Down syndrome (DS) by polymerase chain reaction-short tandem repeat (PCR-STR) method, and to determine the genotypes of 7 STR loci in ethnic Chinese Han from Weifang region.

Seven STR markers (D21S11, D21S1411, D21S1412, D21S1413, D21S1414, D21S1432 and D21S2039) from chromosome 21q22.1-22.2 region were selected. Amniotic samples from 978 high-risk pregnancies and peripheral blood samples from 82 patients suspected with DS were tested with PCR-STR. And the results were verified with G-banding analysis.

(1) All of the 1060 samples were successfully tested by PCR-STR. For normal individuals, the patterns obtained by PCR-STR were two bands with 1:1 area ratio or a single band. For DS cases, by contrast, the patterns revealed either three bands with area ratio of 1:1:1 for two STR loci, or three bands with area ratio of 1:1:1 for one STR loci and two bands with 2:1 or 1:2 area ratio for two STR loci. Based on analysis of the 7 STR markers, 14 amniotic fluid samples and 26 peripheral blood samples were regarded as DS positive. (2) For the 978 amniotic fluid samples, cytogenetic analysis was successful in 961 (98.3%), among which 44 had an abnormal karyotype. These included 14 trisomy 21 (12 standard type, 1 mosaicism and 1 translocation). 17 cases which failed amniocytic culture were normal upon fetal blood karyotyping. (3) Cytogenetic analysis was successful in all of the 82 peripheral blood samples, and has identified 30 (36.6%) abnormal karyotypes, among which 26 were trisomy 21 (including 4 with translocation form). (4) For DS positive cases, STR 1-4 showed three bands with area ratio of 1:1:1, or there were 2-4 loci with two bands with an area ratio of 2:1 or 1:2. All of the DS cases detected by PCR-STR were confirmed by karyotyping. (5) All of the 7 selected loci were informative, with their degrees of heterozygosity ranging between 0.624 and 0.812. D21S2039 and D21S1412 had the highest heterozygosities (> 0.80), D21S1411 and D21S1432 had the lowest (< 0.70). D21S11 and D21S2039 showed the highest rate (≥ 40%) of three bands with area ratio 1:1:1. However, D21S1411, D21S1432 and D21S1413 markers had the highest rate of homozygosity (≥ 30%).

PCR-STR assay may provide an effective alternative for rapid prenatal DS screening. The 7 selected STR markers are highly informative. The result has featured good accuracy and reliability.