Preimplantation Genetic Diagnosis/Screening – Update July 2016
The idea of genetically testing embryos to select the best embryo for transfer is not a new concept. We have been applying one technology or another for over a decade and yet we remain unsettled in terms of answering some of the basic questions surrounding proper clinical application of this technology. In this blog post we address the current controversies surrounding this technology and attempt to answer the questions of who should undergo testing, and which specific testing should be offering to our patients.
Why consider preimplantation genetic screening?
The reason to consider preimplantation genetic testing for aneuploidy (abnormal chromosomes) is to identify the embryo most likely to result in the birth of a healthy child. It is well recognized at this time that most IVF failure is caused by the transfer of embryos that are genetically abnormal (aneuploid). Any test that allows differentiation with perfect accuracy whether any embryo is genetically normal or abnormal, while not harming that embryo in any way, would be the perfect test. Unfortunately no existing technology achieves this perfect differentiation. While our current technology for genetic analysis of cells from the embryo has advanced over the past decade from FISH (fluorescent in-situ hybridization) to CGH (comparative genomic hybridization) to qPCR (quantitative polymerase chain reaction), and most recently HR-NGS (high resolution next generation sequencing). As we have developed these technologies, the error rate has dropped from 7% with PCR to 1-2% with CGH. The errors that do persist are most likely due to mosaicism (a mixture of genetically normal and genetically abnormal cells in one biopsy specimen).
What is the best available technology for obtaining genetic information about the embryo?
The most recently developed technology (HR-NGS) is quite sensitive in terms of picking up mosaicism. However, it is important to note that many mosaic embryos will result in the birth of a normal child if given the opportunity. This has been proven in studies in which only the only embryos available for transfer were mosaic embryos, and the couple accepted this risk and decided to transfer one or more of these embryos. The number of embryos classified as mosaic is significant as indicated in the table below.
AGE Genetically Genetically Mosaic Mosaic
Normal Abnormal (Abnormal) (Normal)
Egg Donors 61.2% 14.8% 6.5% 17.4%
Less than 35 48.2% 18.6% 6.6% 26.6%
35-37 43.9% 26.4% 9.3% 20.5%
38-40 33.1% 35.6% 13.3% 17.9%
41-42 17.0% 51.6% 17.5% 13.9%
Greater that 42 10.6% 57.6% 21.2% 10.5%
It is important to recognize that 30% of embryos at the blastocyst stage are mosaic. Therefore, using high resolution NGS, the most sensitive testing currently available, 30% of any group of blastocysts from patients of all ages will be found to be mosaics. Importantly, depending on maternal age, a percentage of embryos classified as mosaic, will in fact prove to be genetically normal.
As a specific example, for women less than 35 years of age, 26.6% of embryos found to be mosaic, will ultimately be found genetically normal. For women older than 42, 10.5% of mosaic embryos are actually genetically normal.
This information is problematic. In essence, we are subjecting these embryos to biopsy at a cost of $4000-$5000. When the testing is complete the couple will be given information suggesting that approximately 30% of their embryos are mosaics and may be genetically abnormal. In reality, many of these embryos are genetically normal but the current technology cannot precisely identify which ones these are. For couples who have some genetically normal embryos to choose from, the choice is simple, they will transfer the genetically normal embryo. For patients who have only genetically abnormal embryos to choose from, the answer is straightforward; none of these embryos should be transferred. However, for those patients with only mosaic embryos to choose from, the dilemma is that some of these embryos are genetically normal and some are not. We do know that the percentage of these embryos that is genetically normal decreases with increasing maternal age.
Of interest is that some clinics have transferred embryos thought to be genetically abnormal (when no genetically normally embryos were available for transfer), and normal babies have resulted. This phenomenon may be attributed to one of the following three possibilities:
1) The embryo transferred was actually a “mosaic” embryo that was misidentified as abnormal.
2) Natural conception occurred from an egg released around the time of the embryo transfer.
3) The embryo “self corrected”.
There is little question at this point in time that the transfer of a genetically normal embryo is the best possible outcome following pre-implantation genetic screening, leading to a 65-70% live birth rate and miscarriage rates of 7-10%. However, even patients who achieve a genetically normal embryo must deal with the stress of dealing with mosaic embryos. They must choose to either cryopreserve or discard them, in each case with the awareness that they may be discarding normal embryos or cryopreserving genetically abnormal embryos.
PGS For Younger Patients?
Reviewing the data presented earlier, until age 37 roughly half of all embryos are genetically normal (44-61%). Using the microscopic appearance of the embryo prior to transfer as a method of selecting the best embryo will result in a 50% chance of resulting in an ongoing pregnancy. If pregnancy does not occur, transfer of a 2nd embryo (frozen/warmed) in the next cycle will result in another 50% chance of an ongoing pregnancy. It is true that if the first embryo transferred was genetically abnormal, a miscarriage might result. In addition to the emotional stress of a miscarriage, the pregnancy loss would lead to a delay in the continuation of treatment. These disadvantages must be weighed against the expense and ambiguity that may be introduced by pre-implantation genetic screening. Patients deserve careful discussion and counseling before deciding for or against preimplantation screening. Some of the potential advantages of not doing pre-implantation genetic screening are:
1) Success rates when transferring high quality embryos are quite good with young patients (37 or younger).
2) The costs of pre-implantation genetic screening are eliminated
3) The psychological effects of deciding what to do about mosaic embryos is eliminated
4) Embryos are spared the trauma of biopsy
PGS For Older Patients?
Considering patients older than 37, other considerations must be addressed. Based on available data, only 11-33% of embryos will be found through PGS to be chromosomally normal. Roughly 36-58% of these embryos will be found to be genetically abnormal. About 30% of these embryos will be found to be mosaic when HR-NGS is the technology used. It can be argued that in this group of patients (older than 37) finding the genetically normal embryo serves the important purpose of reducing the number of failed cycles that the couple must endure before achieving success. If transfer of the genetically “normal” embryo(s) fails to result in successful pregnancy, mosaic embryos may be considered for transfer with careful and proper patient counseling. Alternatively, these patients may choose not to transfer mosaic embryos, preferring to go through additional egg retrievals in the hopes of identifying genetically normal (euploid) embryos.
The advantages of PGS in older patients include the following:
1) This approach decreases the “time to successful pregnancy” by reducing the number of failed cycles as well as the number of miscarriages, both of which cause not only delays, but also emotional hardship on couples.
2) Since most of these patients have some degree of diminished ovarian reserve, using PGS for these women reduces the likelihood of further loss of ovarian function related to the passage of time.
Looking to the future, the hope is that technological advances will reduce the percentage of embryos identified as mosaic, lifting this emotional burden that couples must deal with today. Reducing the financial burden on couples that choose genetic screening could be achieved by reducing the cost of the technology or by greater insurance coverage for this type of testing, either of which would be a welcome relief for couples.
Munne, S, Grifo, J, Well, D Mosaicism: “survival of the fittest” versus “no embryo left behind”. Fertility and Sterility 2016; 105:1146-1148.
Melrum, David R. Lightening the burden of care in assisted reproductive technology. Fertility and Sterility 2016; 105:1144-1145.