PGD is the process of removing one or more cells from an embryo, originating from in vitro fertilization, with the purpose of carrying out genetic tests – and ensuring its normality – before being transferred to the uterus.
The term PGD is often used loosely to refer to any test performed on an embryo before it is transferred to the uterus. However, it is necessary to make a distinction between the terms PGD and PGS.
PGD (Pre Gestational Diagnosis) involves removing one or more cells from an IVF embryo to test it for a specific genetic condition (cystic fibrosis, for example) before transferring it to the uterus.
PGS (Pre Gestational Screening) is the appropriate term to test for the normality of all chromosomes in embryos. PGS does not seek for a diagnosis of a specific disease, it is tracking embryos for any changes that may be found.
HISTORY OF PGD – PGS
In vitro fertilization was successfully obtained for the first time in 1978. It did not take a few years for researchers to begin to check the possibility of extracting one or more cells from the embryo to obtain information on the health of the potential child that could result from this treatment.
The first human pregnancy report obtained after the PGD was published in 1990. Numerous improvements in these technologies have been developed since then. The techniques of embryonic biopsy, as well as the genetic technologies used to analyze the cells taken from the embryos have improved dramatically. Much has been learned in this field over the past 25 years and much remains to be learned.
Biopsy can be done at different stages of eggs and embryos
Today there are 3 fundamental stages in which eggs or embryos are biopsied.
- Polar body biopsies can be made on eggs (a structure derived from the cell division of the egg). It is performed at an early stage, when the polar corpuscle is removed and can have its genetic material tested. A mature egg (in the Metaphase II phase) has only one polar corpuscle, while a fertilized egg has two. Both the first polar corpuscle of the egg or both of the fertilized embryo can be removed for testing.This technique does not involve the removal of cells that would become part of the fetus or placenta. However, there is still a risk of damaging the embryo after the biopsy.There is evidence in the medical literature that polar corpuscles can help differentiate cells in the embryo, and may be involved in determining which cells will become precursors of the fetus (internal cell mass) or precursors of the placenta (trophectoderm).
- The biopsy in the cleavage phase is performed on day 3 of the embryo’s development. At this point, the embryo usually has 6 to 10 cells. The removal of the portion of the pellucid zone (outer layer of the embryo) is carried out, and then one or two cells are removed from the embryo for subsequent analysis.This biopsy technique in the cleavage phase has been questioned a lot, as it has been shown by several studies to be harmful to the development of the embryo. In addition, at this stage of embryonic development, the embryo has a natural characteristic, called “mosaicism”.Mosaic embryos are embryos that have some genetically altered cells and other normal cells. As the days pass and as they progress in their development, the altered cells do not proliferate and the embryo becomes totally normal or with a small percentage of altered cells, with no repercussions on the health of the future child to be generated. The problem consists precisely in the fact that, at random, the biopsied cell is the altered cell that “would be normal” afterwards. That is, the test gives a false positive result for the disease (on that day there was the change, but it is not possible to predict whether it will remain later).
- The third type of biopsy is called a biopsy of the tropectoderm (embryonic portion of the blastocyst that will differentiate into the placenta). It is performed in the blastocyst phase, after the embryo differentiates into an internal cell mass (it will originate the embryo itself), in tropectoderm (it will originate the placenta) and in a fluid-filled cavity (called a blastocele). In the biopsy of tropectoderm, a small hole is made in the external embryonic cells (hatching in the pellucid zone) and some cells of the tropectoderm are removed for analysis. This technique is now considered the procedure of choice for PGD and PGS.
WHAT IS IMPORTANT IN THE PGD LAB?
In general, the best laboratories that perform PGD or PGS are the same with the best success rates in IVF. It is critical to have an excellent culture system, with modern and well-controlled incubators, which can take the embryos to the blastocyst stage and with biopsy quality.
In addition, the technical skills of embryologists that lead to successful in vitro fertilization are the same as those required for blastocyst culture, embryo biopsy and vitrification (freezing) and thawed embryo transfers.
WHEN TO DO PGD?
In general, there are some indications for doing PGD:
- Advanced maternal age (over 40 or 42 years old)
- History of genetic diseases in the family of one of the partners
- History of recurrent abortion
- Background of child born with genetic disease
- Successive failures in IVF treatments
There is a point that needs to be highlighted: human eggs and embryos are mostly chromosomally abnormal. In general, about 30-60% of human embryos have some type of chromosomal abnormality. This is a characteristic of our species: we are poor breeders compared to other mammals.
The percentage of eggs with chromosomal abnormalities increases with increasing age of the female. However, the vast majority of these altered embryos cannot even develop and implant in the uterus. Another large part, due to its genetic alterations, implant and abort. Only a small part can implant and generate a child with some syndrome, as shown in the table below (percentage of embryos with aneuploidies according to the woman’s age).
Thus, when performing chromosomal tests of embryos, we can those who have abnormal chromosomal structures and choose the healthy embryo (s) for transfer to the woman’s uterus.
We currently have three technologies that can be used to assess chromosomal normality in biopsied embryos:
- Comparative Genomic Hybridization by Arrays (aCGH)
- Microarrays for single nucleotide polymorphism (SNP)
- Real-time quantitative polymerase chain reaction (qPCR)
PGD FOR GENERIC HEREDITARY DISEASES:
This is a situation where the couple knows that they carry a gene that would put their child at risk for serious genetic disease. In such a scenario, PGD can solve the problem by selecting normal embryos that do not carry the altered gene or chromosome.
For example, if in the couple both the male and the female partner have a recessive disease (such as cystic fibrosis, for example), their children (naturally designed) would have a 25% chance of having this terrible disease.
When performing IVF with PGD, they can have healthy embryos transferred to the uterus, ensuring that the child generated will not have cystic fibrosis.
PGD FOR PATIENTS WITH CHROMOSOME TRANSLOCATIONS
In this scenario, one of the partners is known to have a chromosomal defect called balanced translocation. It is when there is an exchange of pieces between chromosomes, but that in the individual does not cause clinical manifestations or medical problems.
However, someone who has a balanced chromosomal translocation, even though it is normal, when the chromosomes in his sperm or egg join with those of his partner in the fertilized embryo, he has a high percentage chance of inheriting chromosomal abnormalities.
These embryo (s) have a very high risk of miscarriage or result in the birth of a child with genetic defects.
Can embryos be traumatized by the biopsy procedure? Theoretically yes, especially for embryos biopsied in the cleavage phase (day 3 of life). However, experimental embryologists perform biopsies with skill and we do not observe any risk of damage. As with any new technique, there is a “learning curve” and some embryologists will be more skilled in the biopsy procedure.
IF SO MANY EMBRYOS HAVE ABNORMAL CHROMOSOMES, WHAT IS THE RISK OF HAVING A CHROMOSOMICALLY ABNORMAL BABY WHEN WE DON’T MAKE PGD
In human beings there is a natural selection process that prevents the implantation of abnormal embryos. The vast majority of chromosomally abnormal embryos cannot maintain themselves from the beginning of development and do not survive for long enough for the implant in the uterus. Some will implant and result in early abortions.
Only an extremely small percentage can continue and result in pregnancy and can progress to the birth of a chromosomally abnormal baby – if not detected during pregnancy.
GENETIC TESTS DURING PREGNANCY:
There are non-invasive screening tests, such as blood tests (NIPT, performed after 8 weeks of pregnancy) or evaluation of the baby’s ultrasound (first trimester morphological ultrasound, performed from 11 to 13 weeks of gestation), which can be done in very early stages and allow the detection of genetic diseases.
There are also invasive tests in early pregnancy, such as chorionic villus biopsy or amniocentesis. The chorionic villus biopsy is done around 11-12 weeks of pregnancy. Amniocentesis is performed around 16-18 weeks. In the general population, the risk of a live birth with a chromosomal abnormality is:
The overall risk of an abnormal chromosomal live birth does not appear to increase when we do IVF or ICSI (intracytoplasmic sperm injection).
WHAT ELSE CAN BE DONE TO HELP COUPLES HAVE A SUCCESSFUL RESULT FROM IVF WITH A NORMAL AND HEALTHY BABY?
Classifying the quality of embryos in the IVF laboratory can help to choose chromosomally normal embryos for transfer. Embryos that are “classified ” at the top end of the scale have lower rates of chromosomal abnormalities compared to embryos that have lower grades.
Embryos that reach blastocysts on day 5 of development have lower rates of chromosomal abnormalities compared to embryos that do not make blastocysts or in those that take more than 5 days to reach this stage. Therefore, we always prefer – when indicated and possible – to use embryo culture until the blastocyst stage, in order to be able to select those with the greatest potential for implantation and the lowest rates of chromosomal anomalies (when compared to embryo transfer in the cleavage, on day 2 or day 3).
Thus, we have embryonic selection techniques as a great safe option to avoid genetic diseases that would be transmitted to children. It should be noted that there are ethical and religious issues involved, in addition to legal standards (Biosafety Law) and professionals (CFM Resolution) to be complied.
References in Medical Literature:
- Maternal Fetal Medicine: Practice and Principles. Creasy and Resnick, eds.
- W.B. Saunders, Philadelphia, PA, 1994.