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Reproductive(Fertility) Health

Preimplantation Genetic diagnosis

Written by Dr. Ashwini Nanasaheb Talpe.  

Preimplantation genetic screening (PGS) involves the assessment of the chromosomal composition of embryos and evaluating for numerical chromosomal abnormalities with a view to selecting normal embryos for transfer. PGS should be differentiated from preimplantation genetic diagnosis (PGD). PGD allows couples where one or both partners carry a known genetic condition to have their embryos tested to avoid the transfer of a genetically affected embryo.

The application of PGD has expanded, with over 200 single gene and chromosomal disorders currently amenable to PGD. It has enabled couples affected by such disorders to avoid the need to undergo prenatal diagnosis.

What is PGD?

Preimplantation genetic diagnosis is a ray of hope for the couples with metabolic disorders, single gene disorder, X-linked disorder, translocation and severe male factor infertility. In this procedure, patients are given injections for ovarian stimulation like normal IVF (in vitro fertilization). Eggs are removed and ICSI (intracytoplasmic sperm injection) is done. Embryo biopsy can be done on day 3 or trophectoderm biopsy on day 5. The single cell is removed from the embryo and tested for various disorders.

It is preimplantation genetic testing to identify genetic defects in embryos through IVF (in vitro fertilization). Only unaffected embryos are transferred into the uterus for implantation. PGD is alternative to post conception testing like amniocentesis and chorionic villus sampling, which are difficult. It is an attractive way of preventing heritable genetic diseases

Who Should Go for PGD?

  • It is recommended when couple are at high risk of transmitting a known genetic abnormality to their children. In common conditions like family history of X-linked disorders, they have risk of transmission to 50% male babies if the mother is a carrier of conditions like DMD (Duchenne muscular dystrophy), hemophilia, fragile X syndrome (FXS), neuromuscular dystrophies and Rett syndrome.
  • Couples with chromosomal translocation, which is a common cause of recurrent abortions.
  • Recurrent pregnancy loss. About 50% to 80% abortuses shows chromosomal abnormalities in recurrent pregnancy loss, high rate of aneuploidies (abnormal number of chromosomes in a human cell).
  • Recurrent IVF failures, that is three or more IVF failures. It has been proven that embryos of good quality and free of chromosomal defects increase implantation of the embryo and pregnancy rate.
  • Single gene disorders like cystic fibrosis, sickle cell anemia, Huntington disease, Tay-Sachs disease require molecular diagnosis with PCR (polymerase chain reaction).
  • In women with advanced maternal age, the risk of aneuploidy increases. Chromosomes in the egg are less likely to divide properly in women who have crossed 35 years of age.
  • Male factor infertility - normal sperm count has 3% to 8% of abnormal sperm. Male with low sperm count and motility are associated with a high rate of abnormal sperm like 27% to 74%. Commonly male factor infertility is seen in Klinefelter syndrome and Robertsonian translocation. In such cases, the chromosome microdeletion can be transmitted genetically and lead to abnormal embryos and failure of implantation.
  • HLA matching that is required in cases like thalassemia, leukemia for stem cell or bone marrow transplantation.

Need for PGD:

  1. The risk of implantation failure, multiple IVF failure and pregnancy loss secondary to aneuploidy increase with advanced maternal age, particularly after the age of 35 years.
  2. In theory, PGS would allow selection of normal embryos and could potentially improve implantation rates and successful outcomes with IVF.

Other Newer Techniques:

The conventional method of diagnosis of genetic defects is by a technique called PCR that is polymerase chain reaction. This is used to detect target specific whole chromosomal anomalies like Trisomy 13, 18 and 21 and sex chromosomal anomalies X and Y. It is relatively cheap and rapid to provide results within 48 hours to 72hours. Recent advances in the field of preimplantation genetic testing are,

FISH (Fluorescence In Situ Hybridization) is a useful adjunct to full conventional karyotyping when a high degree of suspicion of specific chromosome abnormality is present.

CGH (Comparative Genomic Hybridization) provides a higher detection rate of chromosomal abnormalities and more definitive diagnosis. The only drawback is the high cost.

Other newer technologies include NGS (Next Generation Sequencing). This technology allows sequencing DNA as well as RNA more quickly and can give complete genetic mapping.

To know more about preimplant genetic diagnosis, consult an infertility specialist online --> https://www.icliniq.com/ask-a-doctor-online/infertility-specialist

Last reviewed at: 07.Sep.2018

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