PGD/PGS –A boon in Assisted Reproductive Techniques /IVF

Preimplantation genetic diagnosis (PGD) is a technology widely used in IVF and involves genetic testing of the 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 lately 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 send to 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.

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

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.

INDICATIONS OF PGD/PGS

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

Autosomal recessive Disorders
  • Cystic Fibrosis
  • Spinal Muscular Atrophy
  • Thalassemia
  • Sickle cell anemia
  • HLA Matching
Autosomal dominant Disorders
  • Myotonic dystrophy type
  • Huntington diseas (HD)
X-linked Disorders
  • Fragile X-syndrome
  • Duchenne muscular dystrophy (DMD)
  • >Haemophilia

2. Indications for PGS/ PGT-S/- Structural Chromosomal Abnormalities –. The Chromosomal Structural abnormalities may be the cause for Recuurent Implantation Failure or Recurrent Pregnancy losses . 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

ROLE OF PGS IN 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 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 harbour 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 analysed in a single cell, this approach will result in improved outcomes for PGS.

WHAT ARE THE CONCERNS OF PGD?

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

  • The process of PGD/PGS requires couple to undergo IVF even if the couple is fertile which adds to financial burden on the couple .
  • Its an invasive procedure as this requires 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, 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 specially benefit those couples with genetic disorders by not transferring the mutated genes to their offsprings .