[Development of 3-enzyme pyrosequencing system and its application in rapid diagnosis of Down's syndrome]

To avoid sequencing error resulting from use of apyrase in conventional 4- enzyme pyrosequencing system, a non-apyrase 3-enzyme pyrosequencing system with a better performance of quantitative analysis was established. The method is to immobilize biotinylated DNA template, ATP sulfurylase and luciferase on streptavidin-coated magnetic beads for pyrosequencing. After pyrosequencing, ATP produced from the pyrosequencing reaction and excess dNTPs were removed by magnetic separation technique; another dNTP was then dispensed for sequencing reaction, and the components interfering with the next circle of pyrosequencing reaction were removed by the same way, achieving the circular sequencing. This new system can accurately measure base sequences of a target DNA template, and also can quantitatively determine the relative ratio of two alleles. The allele ratios in two SNPs (rs1042917 and rs4818219) having a higher heterozygote rate on chromosome 21 were successfully detected for 16 normal samples and 8 clinical samples from Down's syndrome patients. The results can accurately demonstrate whether or not the target sample has equal copies of chromosome 21 from mother and father. This paper established a non-apyrase 3-enzyme pyrosequencing method, which owns a good perform-ance of quantitative analysis. The method is especially suitable to allelic quantification of an SNP, enabling the rapid diagnosis of Down's syndrome by analyzing allele ratio of SNPs on chromosome 21.