What is Preimplantation genetic diagnosis (PGD)?

Preimplantation genetic diagnosis (PGD) is a test which is done to detect whether genetic abnormalities, which are present in one of the parents, are passed over to the embryos obtained by In-Vitro-Fertilization (IVF) or Intra-cytoplasmaic-sperm-injection (ICSI) prior to the transfer of the embryos into the uterus of the woman.

The aim is to select healthy embryo(s) for embryo transfer to avoid the passing on of a hereditary disease to the offspring. The goal of PGD testing is to help couples build a healthy family. The PGD test is done before conceiving a pregnancy and results of the test help avoid difficult decisions and situations.

What are the requirements to do PGD?

PGD helps couples who have an increased risk of a genetic disorder conceive a healthy pregnancy. The DNA of the human being is organized into small segments called genes. There are about 25,000 genes in humans, all of which influence our growth and development. Just like chromosomes come in pairs, most genes also come in pairs, one copy inherited from the egg and the other from the sperm.

When the function of a gene is altered by a change (called a mutation) in the DNA sequence, it results in a genetic disease. These mutations can be transmitted in families from generation to generation or can be a new change in an individual (de novo).

To perform PGD testing, a set-up test is required. A unique probe is customized for each couple and used to perform PGD testing on embryos. To build the probe, blood samples from the couple and often from other family members are required.

How and when is Preimplantation genetic diagnosis (PGD) done?

The test has to be done on the cells from the embryo, therefore the biopsy procedure always involves two steps: the opening of the zona pellucida,the outer shell of the embryo, and the removal of the cell(s). The biopsy will be done either day 3 or day 5.

A 3 day old embryo has approximately 6 to 8 cells, and so only a single cell is removed for the biopsy done at day 3. A blastocyst, a 5 day old embryo, has over a hundred cells, and so several cells can be safely removed during the biopsy.

The cells of the embryo will be tested with different methods, depending on the previous results

  • FISH-technique: Fluorescence in situ hybridization (FISH) provides researchers with a way to locate and visualize a specific gene or DNA(Deoxyribonucleic acid is a molecule that encodes the genetic information of a human being) sequence using fluorescentprobes. The fluorescent probes bind to specific areas on the chromosomesand detect any alterations that may cause a genetic condition.
  • PCR-technique: polymerase chain reaction is a laboratory procedure in which millions of copies of a specific piece of DNA (Deoxyribonucleic acid is a molecule that encodes the genetic information of a human being) is made. PCR can be used in the analysis of mutations that occur in many genetic diseases (e.g. cystic fibrosis, sickle cell anaemia, phenylketonuria, muscular dystrophy).
  • aCGH-technique: Array Comparative Genomic Hybridization (aCGH) is a method of genetic testing that allows the whole genome (all the genetic material present)to be analysed. It identifiesminute alterations in chromosomes that would not be seen by a microscope.It can be used to quickly scan an entire genome for imbalances that may cause a genetic condition.
  • NGS: Next-generation sequencing applies to genome sequencing, and resequencing. It seems to be slightly more accurate that the aCGH.

In case you want to have more information on the different tests, please go to: http://www.igenomix.com/

Is it harmful for the embryo to do PGD?

If the embryo is handled correctly by a skillful embryologist, the embryo develops normally after embryo biopsy. Published studies demonstrate that there is no increased rate of birth defects in IVF babies that are born after biopsy compared to IVF babies that are born without embryo biopsy.

What are the reasons to do Preimplantation genetic diagnosis (PGD) and who should use it?

PGD is indicated for couples who have increased risks of having a child with a chromosomal abnormality or with a single gene disorder.

PGD is recommended in:

  • Couples with a personal or family history of high risk for severe monogenic disorders
  • Fertile couples who have a known history of a genetic condition in their families

Common examples of monogenic disorders are cystic fibrosis, fragile X syndrome, spinal muscular dystrophies (SMA), Duchenne muscular dystrophy (DMD), Huntington disease.

Does PGD include all genetic diseases?

No genetic test can detect all potential genetic abnormalities. PGD will test only for a specific already known abnormality.

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