Introduction to Next Generation Sequencing for PGD
Next Generation Sequencing or NGS is a new technology for comprehensive chromosome testing of IVF embryos. Next Generation Sequencing for PGD is likely going to replace other technologies as the predominant method for preimplantation genetic screening (PGS) due to lower cost and reduced errors.
Background – Why do we need PGD / PGS?
Successful in vitro fertilization (IVF) is based in part on successful selection and transfer of healthy, viable embryos. Initially, embryo selection was performed by examination of the embryo appearance under a microscope. This is known as morphology assessment. Unfortunately, it became clear that many embryos with a normal appearance under the microscope were not able to produce a live born baby.
We now know that many normal appearing embryos have abnormalities in the number of chromosomes (aneuploidy). These abnormalities are the leading cause of IVF implantation failure and spontaneous miscarriage.
Based on these facts, studies were performed to try to identify and exclude from transfer those embryos with chromosome abnormalities. This type of testing is known as preimplantation genetic screening or PGS. In the initial studies on PGS, a single cell was removed from an eight cell embryo and tested using a technology known as fluorescence in situ hybridization (FISH). These early studies showed promising results and resulted in the widespread use of PGS in IVF. Subsequently, however, better large scale studies failed to find an improvement in the chance for live birth using FISH from biopsies at the eight cell stage. As a consequence, this type of testing has become largely obsolete.
One possible reason for the failure of this technique is that the results obtained FISH were limited to only a few of the 23 total chromosomes in human beings. We now know that abnormalities may occur in any of the 23 chromosomes. This led to the use of more robust, automated technologies which were capable of looking at all 23 pairs of chromosomes.
At IVF1, we have commonly used comparative genomic hybridization (CGH microarray) for comprehensive chromosome screening. This technology has been extensively validated and is now used extensively around the world. Clinical studies have demonstrated improved IVF outcomes resulting in a reduction in the time to achieve a healthy live birth and a reduction in the rate of miscarriage or a profoundly disabled child due to an abnormal number of chromosomes.
Next Generation Sequencing
Recently, advances in another technology have sparked interest in its use for screening embryos for chromosome abnormalities. This new technology is called Next Generation Sequencing or NGS. There are some potential advantages with the use of NGS:
- Reduced cost
- Enhanced detection of structural abnormalities such as chromosomes with missing or duplicate segments
- Better ability to detect when an embryo may have cells with differing results (mosaicism)
- Reduction of human errors by increasing use of automation
Studies have now been performed to validate the use of Next Generation Sequencing for use in preimplantation genetic screening. The results from NGS correlated well with CGH microarray and researchers were able to realize many of the advantages listed above.