MPSS is a random, sequencing technique that analyzes millions of cfDNA fragments. MPSS analyzes the level of gene expression in a specimen by totaling the number of individual messenger-RNA (mRNA) molecules that are produced by each gene. The method sequences short segments of cfDNA from the mother and the fetus and then assigns them to specific chromosomes. After comparing the number of chromosome counts to a control value of other chromosomes, any excess of a particular chromosome (eg, 21) would suggest a trisomy.
In the MPSS procedure, tagged polymerase chain reaction (PCR) products produced from complementary DNA (cDNA), which is the DNA synthesized from an mRNA, are amplified so that each corresponding mRNA molecule will yield about 100,000 PCR products with unique tags. The tags are used to attach the PCR products to microbeads. Several rounds of sequence determinations are performed and a sequence pattern or signature is identified from each microbead. The process is performed in parallel with about 1 million sequence signatures produced per overall assay. Each signature sequence is then analyzed, compared with all other signatures, and all identical signatures are counted. The level of expression of any single gene is then calculated. MPSS does require analysis of very large numbers of DNA fragments per sample (about 25 million), which could potentially limit its clinical utility.