PGS – aneuploidy screening
Preimplantation genetic screening (PGS) or Preimplantation Genetic Testing of Aneuploidy (PGT-A) is a method which enables to find out whether the embryos from in vitro fertilization (IVF) show randomly developed changes in the number of chromosomes which could be caused by external environment or age of parents. PGS basically analyses the genetic make-up of an egg or embryo to determine whether the genetic material is normal or abnormal.
In theory, by excluding abnormal eggs and embryos, the procedure should increase the chances of a successful pregnancy. It also increases the chances of having no embryos to transfer in a treatment cycle if all the embryos are chromosomally abnormal.
Historically, evidence supporting PGS as beneficial was poor and earlier forms of PGS may have shown little benefit. The newer, more comprehensive approaches, are seen to benefit patients undergoing IVF and the latest evidence from several clinics has been very encouraging.
The assumption is that almost half of the embryos have a chromosome aberration which leads to their early miscarriage. Early miscarriage can be explained as due to a „hormonal disorder“. The frequency of chromosome aberrations in recognized pregnancies is 7,5% at the beginning and only 0,6% at birth. This means that the majority of chromosome aberrations are miscarried in the course of pregnancy. The spectrum of chromosomal defects detected in new-borns is due to the fact whether the defect is compatible with intrauterine development.
The most common chromosomal defect diagnosed during pregnancy and at childbirth is Down syndrome – trisomy of chromosome 21 (there are three chromosomes 21 instead of two in the cells). Trisomy of chromosome 18 and 13 is less common. The majority of embryos with those chromosome aberrations are miscarried at an early stage of pregnancy. Other chromosomes which display alterations in their number more often are chromosomes 15, 16, 22 and sex chromosomes X and Y (deviations in number of chromosomes 15, 16 and 22 always lead to a miscarriage).
PGS is offered to couples with a higher risk of numerical chromosome aberrations:
- Couples with recurrent miscarriages up to week 12 of the pregnancy, older women (over 35 years)
- Couples with recurrent failures of IVF cycles
- It should be noted here that the efficiency of PGS which is expressed by the increase in number of children born compared to attempts at in vitro fertilization without PGS has not been reliably proved yet.
PGS is performed during a cycle of in vitro fertilisation (IVF). The woman will complete hormonal treatment for controlled ovarian stimulation to produce a number of mature oocytes (eggs). The oocytes are removed during a simple operative procedure and fertilised using the partner's or donor sperm in the laboratory. Using micromanipulation a portion of the egg, embryo or blastocyst can be removed to be tested. Each of these parts can be examined for the presence or absence of chromosomes that may create embryos that do not have the potential to implant, create a baby with a disease or a pregnancy that is destined to end in miscarriage. Embryos that contain a chromosome imbalance cannot be transferred. Usually, aneuploidy of 8 most frequently endangered chromosomes is investigated: 13, 15, 16, 18, 21, 22, X and Y.
Success rate and risks of PGS
Success rate of IVF with PGS (number of pregnancies after embryo transfer in the uterus) is 36% (GENNET, 2007-2012).
Like most interventions, PGS is not always 100% accurate, which is indicated around 10% for diagnosis of chromosome aberrations. The risk of an error is given by the fact that the probes are not 100% reliable and that there is so-called mosaicism occurring in embryos – the number of chromosomes in the investigated blastomere does not always have to correspond to the number of chromosomes in the remaining cells of the embryo. Therefore, PGS does not replace prenatal diagnosis in the pregnancy.
It should be noted that only a limited number of chromosomes can be observed in PGD. A child born after a PGD can therefore have a different chromosomal defect or genetic disease which has not been observed by us.
The risk of congenital developmental defects is according to the studies which were conducted so far the same as in the population, namely 3-5%.
Sex selection with X-linked diseases
Preimplantation genetic diagnosis with sex selection is offered to couples with genetic disease which is linked to the sex chromosome X when the PGD cannot be performed using the PCR method.
- Sex selection is uniquely performed for genetic reasons.
- Examples of X-linked diseases for which PGD-FISH can be performed are Duchenne/Becker muscular dystrophy, hemophilia, or SCID.
PGD for X-linked diseases is performed in a similar way as PGS.
Success rate and risks
Success rate of IVF with PGS (number of pregnancies after embryo transfer in the uterus) is around 42% (GENNET, 2007-2012). If the genetic disease is linked to maternal X chromosome (mother is an unaffected carrier), then 50% of the boys are affected by the disease and 50% of the boys carry the „healthy“ X chromosome. Therefore, the biggest hindrance of the FISH method when performing PGD of X-linked diseases is the fact that by sex selection only we do not recommend 50% of healthy boys for the transfer as well. Therefore, this method is only used in cases when it is not possible to perform a targeted PGD using PCR.
New method Array Comparative Genomic Hybridisation (Array CGH)
We have introduced the screening of aneuploidy using the method Array CGH, which enables us to control 24 chromosomes. The aim of the test is to choose embryos without genetic abnormalities for the transfer in the uterus. Those genetic abnormalities could lead to serious developmental defects and they would also increase the risk of a miscarriage and reduce the chance of a successful pregnancy. Microchips are now used for the screening of the most appropriate embryos – the array CGH method replaced the now obsolete FISH method.
The method can be immediately used in a fresh cycle for a three- or five-day-old embryo. However, we mainly try not to use one cell of a three-day-old embryo, but a higher number of cells of a five- or six-days-old blastocyst and perform the analysis only as soon as we have several samples, which is economically more advantageous for the patient.
In addition, cell extraction on day 5 does not practically present any additional risk for the embryo and it has a higher diagnostic value. The embryos are frozen using the modern vitrification method and the transfer of the embryo follows in the next cycle after preimplantation analysis. Besides, we achieve the optimal preparation of the endometrium in this subsequent cycle, which results in a high success rate. This procedure is called PGD dislocation. In this case, vitrification is included in the price, as well as the storage of the material until the transfer.