Preimplantation genetic screening (PGS) involves testing pre-implantation embryos for chromosomal numerical abnormalities (aneuploidy). Preimplantation genetic diagnosis (PGD) involves genetic testing of embryos for specific genetic conditions. The objective of both PGS and PGD is to identify embryos that are aneuploid or genetically defective so as to selectively transfer the most “competent” so as to improve embryo implantation potential, reduce the risk of miscarriages and minimize the chance of birth defects
PREIMPLANTION GENETIC SCREENING (PGS)
PGS is the process whereby the chromosomes in the cells of an embryo (or the polar body of an egg) are examined (karyotyped). Embryo cells that have all 46 chromosomes intact are termed euploid. Those with additional chromosomal material and those with deficient chromosomal material are aneuploid.
Should PGS be done routinely in IVF? When Levent Keskintepe and I first introduced PGS testing into the clinical IVF arena (2005) initial results were most-encouraging. Embryo implantation rates of >50% and birth rates of 50-60% when up to two euploid blastocysts were transferred, were being reported. In addition, the reported incidence of miscarriages and chromosomal birth defects was likewise greatly reduced. Initially we believed that a time would come where full embryo karyotyping through PGS would become a routine part of IVF. Alas, we were soon to be disappointed when following the widespread introduction of PGS testing success rates started dropping. This was especially the case when PGS was performed on embryos derived from the eggs of older women and women with severely diminished ovarian reserve (DOR). We soon began to recognize that other factors are also operative:
- The chromosomal integrity of the egg is certainly the most important factor that influences the subsequent ability of an embryo to propagate a viable pregnancy. However, aside from the woman’s age and her ovarian reserve, the type of protocol used for ovarian stimulation and its implementation can also significantly impact on egg/embryo competency.
- Aside from the embryo’s karyotype, there are likely also epigenetic and metabolic factors (perhaps also influenced by advancing age and DOR) that likely impact egg/embryo competency.
- Technical skill in performing embryo transfer (ET) varies and, needless to say, the “competency” of the embryo, will not correct for this.
- There are anatomical and immunologic implantation issues (addressed elsewhere) which, unless addressed, will thwart the ability of even the most competent embryo to implant and propagate a viable pregnancy.
- Not all PGS-aneuploid embryos are “incompetent”. Some are capable of “autocorrecting” upon being transferred to the uterus (i.e. are “mosaic”. While most embryo aneuploidy (>70%) originates from aneuploidy that occurs during reproductive division (meiosis) of either the egg or the sperm, the vast majority of cases are egg related. Such meiotic aneuploidy is irreversible and is responsible for >80% of IVF failures and early miscarriages. In contrast, in some cases where both the egg and sperm are euploid and upon fertilization propagate a euploid fertilized egg (zygote), during subsequent mitosis where the embryos cells multiply, some undergo “mutation” and become aneuploid while the majority maintain euploid division. This is referred to as embryo “mosaicism”.
- Upon reaching the uterine environment, mosaic embryos have the potential to leach out their aneuploid blastomeres, while allowing the euploid cells to multiply in an orderly fashion. This results in autocorrection and in most cases, in the subsequent development of a normal, euploid conceptus/baby. Since some mitotically aneuploid (“mosaic”) embryos can, and indeed do “autocorrect’ while meiotically aneuploid embryos cannot, it follows that an ability to reliably differentiate between these two varieties of aneuploidy would potentially be of considerable clinical value. The introduction of a variety of preimplantation genetic screening (PGS) known as next generation gene sequencing (NGS) has vastly improved the ability to reliably and accurately karyotype embryos and thus to diagnose embryo “mosaicism”.
- The ability of “mosaic embryos” to autocorrect is influenced by the stage at which the condition is diagnosed as well as the percentage of mosaic cells. Many embryos diagnosed as being mosaic while in the earlier cleaved state of development, subsequently undergo autocorrection to the euploid state (normal numerical chromosomal configuration) during the process of undergoing subsequent mitotic cell to the blastocyst stage.
- Similarly, mosaic blastocysts can also undergo autocorrection after being transferred to the uterus. The lower the percentage of mosaic cells in the blastocyst the greater the propensity to autocorrect and propagate chromosomally normal (euploid) offspring. By comparison, a blastocyst with 10% mosaicism could yield a 30% healthy baby rate with 10-15% miscarriage rate, while with >50% mosaicism the baby rate is roughly halved, and the miscarriage rate doubled.
- I advise all patients who subsequently conceive after undergoing ET using such “potentially mosaic” embryos to undergo prenatal genetic testing to rule out the development of an aneuploid fetus so that they can terminate affected pregnancies if they so choose.
Gender identification: Potential applications include:
- Gender selection for non–medical reasons (see elsewhere). This indication has become the commonest application of PGD with >50% of all such testing being done for this reason. Gender selection done simply for family balancing remains controversial, challenging concern that if it became widely accessible and freely available, such practice could distort the natural sex ratio leading to a population gender imbalance.
- However, for this to happen, there would have to be a significant population preference for sex selection. However, the contrary seems to apply since studies conducted in western societies discount both of these concerns. In fact, the relatively high cost of IVF with the added cost of gender selection in the United States makes it unlikely that the demand would ever become large enough to impact on population gender bias.
- In addition, several studies done in Western countries have shown that the majority does not seem to be concerned about the gender of their offspring and that with a few notable exceptions, gender preference does not appear to be slanted in the direction of either male or female. Thus, from a practical standpoint such concerns are overstated.
- So, given that in the United States most do not care about the sex of their offspring and only a minority are interested in selecting the sex of their children, it is my opinion that freedom of choice should prevail and as such, a service for sex selection for non-medical reasons should be freely available.
- Gender selection to avoid X-linked diseases such as : Duchenne muscular dystrophy (DMD), and hemophilia A and B,
- To prevent non-Mendelian disorders that are significantly more prevalent in one sex.
PREIMPLANTATION GENETIC DIAGNOSIS (PGD):
Approximately 1 in 500 babies born in the united states are afflicted by a sex-linked disorder (when a genetically defective Y (male) chromosome is transmitted to offspring). Another 1 in 300 newborns has an autosomal genetic disorder, an abnormality of 1 or more genes involving the 44 remaining autosomes (non-sex chromosomes). This means that approximately 1 in 20,000 babies born annually in the U.S. will have one or other genetic or chromosomal disorder. In addition, about 1: 50 babies are born with an identifiable major genetic abnormality.
Many couples who parent a child with a severe birth defect will subsequently elect not to have another child or to adopt. These facts and figures offer a glimpse at the magnitude of the challenge confronting the medical profession, government, and society in general. The advent of IVF/ET provides a unique opportunity to diagnose and/or exclude genetic chromosomal (structural or numerical) disorders that have the potential to impact adversely on pregnancy outcome and the very quality of life after birth, using preimplantation genetic diagnosis (PGD or preimplantation genetic screening (PGS) for numerical chromosomal defects (aneuploidy).
The procedures both start with biopsying an IVF-generated embryo, 3-6 days post-fertilization. The biopsied material is then subjected to genetic testing whereupon 1 or more, advanced embryos, presumably free of the chromosomal/genetic defect are selectively transferred to the uterus (almost always) during a subsequent frozen embryo transfer (FET) cycle. As such PGD/PGS, has provided the ability to prevent some diagnosable chromosome/genetic disorders prior to the initiation of pregnancy and thereby, provide many desperate couples who might transmit a potential genetic catastrophe to their offspring, with real hope.
PGD allows studying the DNA of eggs or embryos to select those that carry certain mutations for genetic diseases. It is used to identify:
- Monogenic conditions: Disorders due to a single gene (Monogenic disorders), (autosomal or X-linked)—or structural of chromosomal defects (e.g. balanced translocation). Relatively common autosomal recessive disorders that are so detectable include: sickle-cell anemia, Tay Sachs disease. Relatively common autosomal Dominant disorders include: Huntington’s disease and Charcot–Marie–Tooth disease; myotonic dystrophy etc. X-linked diseases include: Hemophilia A; fragile X syndrome, and Duchenne muscular dystrophy.
- Cancer predisposition: Examples include BRCA1 & II mutations and multiple polyposis of the colon
- Minor disabilities such as deafness, in order that the child would share that characteristic with the parents.
Both PGD and PGS, are additional steps in the IVF treatment process. Performance of PGS is less complex than PGD. The cost is roughly $3,00-=$3500, while PGD which examines individual genes costs $4,000-$6000, depending on the nature and extent of gene assessment. For a variety of reasons, the results reported following PGS/PGD testing can sometimes be incorrect (very rarely) or inconclusive (about 5% of the time) and neither PGS or PGD are devoid of risk to the embryos. However, the reliability of the test as well as the likelihood of causing damage to embryos is very much dependent on the skill and experience of the embryologist. This having been said, the introduction of PGS/PGD technology has literally changed the entire field of IVF.