Preimplantation Genetic Screening
Varied genetic prenatal conditions impact pregnancy and fetal development. Accordingly, preimplantation genetic screening (PGS) has emerged as an important clinical tool for identifying chromosomal aberrations.
Traditional prenatal screens have a number of limitations. Some screens, such as amniocentesis or chorionic villi sampling, are invasive; others, such as ultrasound or biochemical screening, are less invasive but limited in their sensitivity and specificity.
“In contrast to these techniques, noninvasive PGS-based on NGS has both superior detection sensitivity and specificity for chromosomal abnormalities,” said Keith Jones, Ph.D., vp of development at Illumina. “The Illumina verifi® test detects greater than 99% of all true-positive cases and has a cumulative false-positive rate of <0.2%.”
The verifi test uses sequence information from across the genome. This approach, Dr. Jones suggested, allows for the rapid adoption of additional tests that may find abnormalities not readily detected using traditional screening approaches. Such abnormalities include sex chromosome aneuploidy, microdeletions, trisomy 9, and trisomy 16.
Approximately 1.3 million in vitro fertilization (IVF) procedures are performed globally each year; however, only 25% of the procedures meet with success. The low success rate is usually attributed to complicating factors associated with advanced maternal age and chromosomal aneuploidy in the embryo.
“The aim of PGS in the IVF setting is to select chromosomally balanced embryos during the IVF process and ensure that only euploid embryos—those with a normal number of chromosomes—are implanted during IVF procedures,” explained Dr. Jones. He added that PGS has been shown to improve implantation success rates and reduce the number of high-risk pregnancies associated with multiple egg transfers.
In Illumina’s VeriSeq™ PGS platform, genomic DNA from a single cell is amplified and sequenced to provide a genome-wide view of the copy number state of the embryo. The protocol takes less than a day and allows multiplexing of up to 24 samples per sequencing run, translating to an increased likelihood of identifying a viable embryo and decreasing the time between biopsy and an answer. “The broad dynamic range derived from the sequencing data makes interpretation clear with a high degree of confidence,” Dr. Jones asserted.