PGD PGS PGT

PGD/PGS is a  boon in Assisted Reproductive Techniques /IVF

What is PGD/PGS?

Preimplantation genetic diagnosis (PGD) is a technology widely used in IVF and involves genetic testing of embryos. It allows couples who are at risk of transmitting a genetic disease or chromosomal disorder to have children not affected by that condition.

The desiring couple needs to undergo IVF, the eggs and the sperms are fertilized in the IVF  laboratory, the embryos which are created are tested on day3 and more so later on day 5/6 .5-8 cells are removed from the outer layer (Trophoectoderm) of each embryo which will eventually become the placenta, without harming the inner cells (inner cell mass)  that will eventually become the baby. The cells removed are sent to the genetic lab for the presence or absence of the relevant mutation or chromosome abnormality. After the biopsy is taken, the embryos are frozen and the embryo transfer is planned next cycle where only healthy and disease-free embryos are transferred. PGD refers to the whole trajectory, from biopsy to embryo selection and transfer.

PGD can be used to identify approximately 2,000 inherited single-gene disorders and is 98 percent accurate in identifying affected and unaffected embryos.

How is PGD different from PGS?

PGD is done for specific genetic disorders like thalassemia and is different from PGS which selects euploid embryos (embryos containing the normal number of chromosomes, which is 46 )  for transfer, in order to increase the efficacy of the IVF cycle.

What is PGT?

The terms PGD and PGS are now being replaced by Preimplantation Genetic Testing (PGT). When done for aneuploidies, it is PGT-A; for monogenic/single gene defects PGT-M; and for chromosomal structural rearrangements, it is PGT-SR. So, what was called PGD is now PGT-M, and what was called PGS is now PGT-A or PGT-SR.

What are the Indications of  PGD/PGS?

1. The PGD /PGT –M has proved to be very beneficial in single gene disorders which can either be Autosomal recessive, Autosomal Dominant, or X-linked disorders. The various indications are mentioned in the mentioned table.

2. Indications for PGS/ PGT-S/- Structural Chromosomal Abnormalities  –. The Chromosomal Structural abnormalities may be the cause of Recurrent Implantation Failure or Recurrent Pregnancy Loss. They are usually Robertsonian, Reciprocal (Male and Female )  sex chromosome abnormalities, etc.

3. Indications for PGS/ PGT-A are – Preimplantation genetic screening is being proposed to improve the effectiveness of in vitro fertilization by screening for embryonic aneuploidy in the following conditions

• Advanced maternal age ( > 35 years)
• Recurrent miscarriage
• Repeated IVF/ICSI –ET failure cycles
• Unexplained infertility
• Severe Male factor

WHAT IS THE ROLE OF  PGS IN THE CURRENT SCENARIO?

 The application of PGS has become controversial in recent years as eleven randomized control trials (RCTs) (2004-2010) showed that preimplantation genetic screening (PGS) with FISH did not increase delivery rates with some demonstrating adverse outcomes.  The possible reason was that only 1-2 cells were biopsied on Day 3  which had a higher rate of mosaicism and were not representative of the rest of the embryo. Moreover  FISH detects abnormalities in only 5  chromosomes, embryos diagnosed as euploid by FISH may harbor aneuploidies of other chromosomes that were not tested for.  With a shift towards blastocyst biopsy where 5-8  cells can be biopsied instead of 1-2, mosaicism being less and FISH being replaced by newer technologies like array CGH (comparative genomic hybridization )  SNP arrays (single nucleotide polymorphism ), NGS(next generation sequencing ) all chromosomes can be analyzed in a single cell, this approach will result in improved outcomes for PGS.

WHAT ARE THE CONCERNS /DRAWBACKS OF PGD?

The following are considered concerns or disadvantages associated with the use of PGD.

• The process of PGD/PGS requires the couple to undergo IVF even if the couple is fertile which adds to the financial burden on the couple.
• It's an invasive procedure as this requires the manipulation of gametes and embryos involved in IVF/ICSI.
• A condition called mosaicism, where some of the cells in a generally normal embryo are abnormal or some of the cells in a generally abnormal embryo are normal, can lead to test results that do not accurately reflect the chromosome status of the embryo.
• PGD helps in reducing the chances of conceiving a child with a genetic disorder, but it cannot completely eliminate this risk. Further testing is needed such as cell-free fetal DNA testing, chorionic villus sampling (CVS), or amniocentesis during pregnancy to ascertain if a genetic factor is still possible. Therefore, prenatal testing is still recommended and currently remains the standard of care.
• However cumulative data again confirm that pregnancies and babies born after PGD are similar to the pregnancies obtained and babies born after ICSI treatment.

Thus PGD/PGS are highly advanced techniques and especially benefit those couples with genetic disorders by not transferring the mutated genes to their offspring.

PGD /PGS/PGT

Cochrane Data Base Guidelines 2014, for PGD and PGS

Summary: 

Preimplantation genetic diagnosis is an alternative to prenatal diagnosis for the detection of genetic disorders in couples at risk of transmitting a genetic condition to their offspring. Preimplantation genetic screening is being proposed to improve the effectiveness of in vitro fertilization by screening for embryonic aneuploidy. Though FISH-based PGS showed adverse effects on IVF success, emerging evidence from new studies using comprehensive chromosome screening technology appears promising.

Recommendations

1. Before a preimplantation genetic diagnosis is performed, genetic counseling must be provided by a certified genetic counselor to ensure that patients fully understand the risk of having an affected child, the impact of the disease on an affected child, and the benefits and limitations of all available options for preimplantation and prenatal diagnosis. (III-A)

2. Couples should be informed that preimplantation genetic diagnosis can reduce the risk of conceiving a child with a genetic abnormality carried by one or both parents if that abnormality can be identified with tests performed on a single cell or on multiple trophectoderm cells. (II-2B) 3. Invasive prenatal or postnatal testing to confirm the results of preimplantation genetic diagnosis is encouraged because the methods used for preimplantation genetic diagnosis have technical limitations that include the possibility of a false result. (II-2B)

 4. Trophectoderm biopsy has no measurable impact on embryo development, as opposed to blastomere biopsy. Therefore, whenever possible, trophectoderm biopsy should be the method of choice in embryo biopsy and should be performed by experienced hands. (I-B)

 5. Preimplantation genetic diagnosis of single-gene disorders should ideally be performed with multiplex polymerase chain reaction coupled with trophectoderm biopsy whenever available. (II-2B)

6. The use of comprehensive chromosome screening technology coupled with trophectoderm biopsy in preimplantation genetic diagnosis in couples carrying chromosomal translocations is recommended because it is associated with favorable clinical outcomes. (II-2B)

7. Before preimplantation genetic screening is performed, thorough education and counseling must be provided by a certified genetic counselor to ensure that patients fully understand the limitations of the technique, the risk of error, and the ongoing debate on whether preimplantation genetic screening is necessary to improve live birth rates with in vitro fertilization. (III-A)

8. Preimplantation genetic screening using fluorescence in situ hybridization technology on day-3 embryo biopsy is associated with decreased live birth rates and therefore should not be performed with in vitro fertilization. (I-E)

9. Preimplantation genetic screening using comprehensive chromosome screening technology on blastocyst biopsy, increases implantation rates and improves embryo selection in IVF cycles in patients with a good prognosis. (I-B).

Canadian Task Force (CTF), level of evidence

• Level I: Evidence from at least one randomized controlled trial,
• Level II1: Evidence from at least one well-designed cohort study or case-control study, i.e. a controlled trial that is not randomized
• Level II2: Comparisons between times and places with or without the intervention
• Level III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.

The CTF graded their recommendations on a 5-point A–E scale: A: Good level of evidence for the recommendation to consider a condition, B: Fair level of evidence for the recommendation to consider a condition, C: Poor level of evidence for the recommendation to consider a condition, D: Fair level evidence for the recommendation to exclude the condition, and E: Good level of evidence for the recommendation to exclude the condition from consideration.