Preimplantation genetic screening(PGS)
Preimplantation genetic screening(PGS)
Does PGS with chromosomal tests of embryos improve IVF success?
What is PGS, preimplantation genetic screening?
There is an equation that we can use to illustrate the implantation process:
- PGS, preimplantation genetic screening, refers to removing one or more cells from an in vitro fertilization embryo to test for chromosomal normalcy
- PGS screens the embryo for normal chromosome number
- We can think of PGS as chromosomal disorder screening on IVF embryos
- Humans have 23 pairs of chromosomes – for a total of 46
- Down syndrome has an extra chromosome 21, referred to as trisomy 21. This should be detected by PGS.
- PGS does not test for a specific disease – such as for cystic fibrosis – that process is referred to as PGD (preimplantation genetic diagnosis)
Many human embryos are chromosomally abnormal
Several studies have shown that overall about 50% of human preimplantation embryos from IVF are chromosomally abnormal. The rate of abnormalities is affected greatly by female age, as shown in the graph below. Chromosomes in eggs from older women have a significantly increased rate of abnormalities.
To a great extent, chromosomal abnormalities are responsible for failure of implantation of IVF embryos. Chromosomal abnormalities are also responsible for about 70% of miscarriages in early pregnancy.
Problems in the past with aneuploidy screening of IVF embryos
IVF and PGS for aneuploidy (an abnormal number of chromosomes) has been used at some clinics in the United States since the mid 1990s. However, studies showed that performing embryo biopsy on day 3 and performing the genetic analysis using FISH technology (fluorescent in situ hybridization) did not result in an increase in the chance for the patient to have a successful IVF cycle.
There were 2 main problems with that approach.
Sometimes, after repeated failure to implant any embryos with IVF, women will say;
- FISH technology was usually looking at only 5 chromosomes out of 23. Therefore, the FISH test would miss many chromosomal abnormalities. This resulted in abnormal embryos being transferred after the screening test came back “normal”.
- The biopsies on day 3 were removing a cell (or 2 cells) from a 6 to 10 cell embryo. This required a relatively large hole being made in the shell of the embryo and then removal of a significant percentage of the “biomass” of that embryo (one sixth to one tenth of it removed, or more).
Recent advances allow for better IVF success rates after aneuploidy screening
Improvements in genetics technologies
Advances in the field have led to utilization of improved genetics technologies that allow assessment of all 23 pairs of chromosomes.
There are currently 3 technologies that can be utilized for assessment of normality of all 23 chromosomes:
- Array Comparative Genomic Hybridization (aCGH)
- Single nucleotide polymorphism microarrays (SNP)
- Quantitative real time polymerase chain reaction(qPCR)
Comparative Genomic Hybridization (often referred to as CGH) is a microarray technology that is often used now instead of the older and less comprehensive FISH. With microarray CGH, the actual DNA in the embryo is compared to a known normal DNA specimen utilizing thousands of specific genetic markers. This gives a more accurate result, with far fewer false normal or false abnormal results.
Some studies have determined that the error rate using array CGH technology is about 2%. FISH has an error rate of about 5-10%. Additionally, many other abnormal embryos would be reported by FISH as normal because the abnormality was in a chromosome that was not part of the FISH panel being used.
Improvements in embryo biopsy techniques
Trophectoderm biopsy is done at the blastocyst stage on day 5 and 6. At this stage there are many more cells present in the embryo. This allows multiple cells to be removed from the trophectoderm (precursors to the placenta). The inner mass cells (precursors to the fetus) can be left undisturbed during the biopsy.
With trophectoderm biopsy, about 5 cells are snipped off for testing. This does not significantly weaken the embryo because it has about 100 cells at this stage.
The combination of these two modifications (advanced genetics and trophectoderm biopsy) has led to significantly improved pregnancy success rates for patients that want to utilize PGS for their IVF treatment.
There is some interesting speculation that the uterine lining could be less receptive during a stimulated cycle as compared to the controlled or “artificial” embryo replacement cycle.
- Some fertility doctors believe that transferring embryos in a controlled cycle (using frozen embryos) gives a higher pregnancy rate than in a “fresh” cycle
- This has not yet been carefully studied with multiple controlled clinical trials (as of 2014)
- The improved success rate seen following blastocyst biopsy and comprehensive chromosomal analysis is mainly due to the benefit of transferring chromosomally normal embryos
- There is probably some additional benefit derived from transferring the embryos in a frozen thawed cycle rather than in a stimulated cycle
Which couples should we be offering aneuploidy screening to?
- This is currently an evolving issue in the field of reproductive medicine
- Some potential candidates could be:
- Women over (about) 35 years old that want IVF and make enough embryos so that they will have multiple blastocysts available for biopsy.
- Patients at any age that have failed multiple IVF cycles. They want answers about why they are failing. They also want to know what to do in order to improve their chances to have a baby.
- PGS can sometimes provide answers in these cases.
- PGS can also can provide a “weeding out” of the abnormal embryos. For example, if one embryo out of 6 is chromosomally normal and 5 out of 6 are abnormal – we transfer the one normal embryo and should have a very good chance for a baby.
- Couples with recurrent unexplained miscarriage