Prenatal diagnosis of trisomy 21, 18 and 13 by quantitative pyrosequencing of segmental duplications

Chromosomal aberration mostly occurs in chromosomes 21, 18 and 13, with an incidence approximately 1 out of 160 live births in humans, therefore making prenatal diagnosis necessary in clinics. Current methods have drawbacks such as time consuming, high cost, complicated operations and low sensitivity. In this paper, a novel method for rapid and accurate prenatal diagnosis of aneuploidy is proposed based on pyrosequencing, which quantitatively detects the peak height ratio (PHR) of different bases of segmental duplication. A direct polymerase chain reaction (PCR) approach was undertaken, where a small volume of amniotic fluid was used as the starting material without DNA extraction. Single-stranded DNA was prepared from PCR products and subsequently analyzed using pyrosequencing. The PHR between target and reference chromosome of 2.2 for euploid and 3:2 for a trisomy fetus were used as reference. The reference intervals and z scores were calculated for discrimination of aneuploidy. A total of 132 samples were collected, within trisomy 21 (n = 11), trisomy 18 (n = 3), trisomy 13 (n = 2), and unaffected controls (n = 116). A set of six segmental duplications were chosen for analysis. This method had consistent results with karyotyping analysis, a correct diagnosis with 100% sensitivity and 99.9% specificity.

Prenatal Diagnosis of Chromosomal Aneuploidies by Quantitative Pyrosequencing®

One of the major reasons for pregnant women to ask for prenatal diagnosis is to detect fetal chromosomal aneuploidies. Analysis of allele ratios of SNPs has been used for prenatal detection of fetal aneuploidies using MALDI-TOF mass spectrometry (MS). However, quantitative SNP genotyping by MALDI-TOF MS is challenging. To obtain a better quantification of allelic ratios, a Pyrosequencing(®) protocol for SNP genotyping has been developed to perform prenatal diagnosis of aneuploidies.To avoid the laborious process and risk of cross-contamination brought in by DNA extraction procedures, a PCR assay, which can amplify DNA directly from cells in amniotic fluid, has been developed. Pre-amplification steps such as cell enrichment and heating are required to obtain sufficient amounts of amplification products.In this chapter, SNPs on chromosome 21 are used to detect trisomy 21 as an example of aneuploidy by quantifying the allele ratio using Pyrosequencing. Primer selection for PCRs and Pyrosequencing reactions, optimization of nucleotide dispensation orders, establishment of cutoff values for trisomy 21, and interpretation of data are all factors essential for a successful diagnosis and are discussed in detail herein.