Pregnant women who are at high risk of having a baby with a serious genetic condition are most likely to terminate their pregnancy. Nowadays, there are prenatal diagnosis that can help parents to make an educated decision with regard to their baby’s risk of inheriting a genetic disease. These tests are called amniocentesis and chorionic villus biopsy. However, not all couples decide to have these tests done and they prefer a different kind of testing called pre-implantation genetic diagnosis (PGD). It is a procedure used prior to implantation to identify genetic defects within embryos created through IVF (In Vitro fertilization) to prevent certain diseases or disorders from being passed on to the child. the The process helps potential parents prevent the birth of a child with a serious genetic condition.

Pre-implantation genetic diagnosis can be offered for three major categories of disease:

  1. Firstly, it can be used to determine the sex of the embryo for sex linked disorders such asDuchenne muscular dystrophy.
  2. Secondly, it can be used to identify single gene defects such as cystic fibrosis, where the molecular abnormality can be tested with molecular techniques likepolymerase chain reaction (PCR).
  3. Thirdly, it can be used in chromosomal disorders through fluorescence in situ hybridization which detects a variety of chromosomal rearrangements including translocations, inversions, and chromosome deletions.

         General process for pre-implantation genetic diagnosis &pre-implantation genetic screening.

 

The procedure for PGD is usually as follows:

  • Female partner takes medications to stimulate the ovaries to make multiple eggs grow.
  • Eggs are retrieved with a trans-vaginal needle by a physician.
  • Eggs are fertilized by sperm using a process called ICSI (Intra-cytoplasmic sperm injection) meaning sperm is injected into eggs.
  • The embryo is grown in the embryology laboratory for two to three days until the cells have divided and the embryo consists of around eight cells.
  • A trained embryologist removes one or two of the cells (blastomeres) from the embryo.
  •  The cells are tested to see if the embryo from which they were removed contains the gene that causes the genetic condition in the family.
  • The embryo unaffected by the genetic disorder or chromosomal abnormalityis transferred into the womb to allow for development.
  • Any suitable remaining unaffected embryos can be frozen for later use. Those embryos that are affected by the condition are allowed to perish or, with your consent, used for research.

 

If successful, the procedure will result in pregnancy and the child should not be affected by the condition for which it was tested.

 

Biopsy Techniques

In order to obtain genetic material and be able to carry out a genetic diagnosisa procedure named embryo biopsy is required. In the last two decades, embryo biopsy at the cleavage stage has been the most performed procedure. However, recently there have been alternative methods which have obtained consensus such as blastocyst stage biopsy or polar body biopsy. Let’s delve into the specifics of each method:

Polar body biopsyPolar body biopsy works only for female chromosomal disorders. The mature metaphase II egg extrudes a single polar body. This polar body can be removed and tested, providing information on only the chromosomal content of the egg. Because only information about the mother can be obtained by analyzing polar bodies, chromosomal abnormalities occurring after fertilization (when the sperm meets the egg) are not detected.

Cleavage-stage embryo biopsyThe most common approach for PGD is to biopsy a single blastomere from day 3 embryos. This allows extraction of a single blastomere from a developing embryo. The removal of the blastomere is a technically challenging procedure. The embryologist's goal, is to remove an intact cell with minimal trauma to the remaining embryo .

 

Before extracting the single cell from a 6-10 cell embryo, the embryo is incubated in calcium-and magnesium-free medium for approximately 20 minutes in order to reduce blastomere-to-blastomere adherence.

The embryo is then anchored on one side with a holding pipette; simultaneously, a small opening within the zona pellucidais made in order to readily access the blastomeres. The zona pellucida (ZP) is a specialized extracellular matrix surrounding the developing oocyte (egg, ovum) within each follicle within the ovary.This opening procedure is called assisted hatching.After the small opening is made, a pipette is placed through the opening and focused on the blastomere of choice. The blastomere is subsequently gently aspirated into the pipette and expelled into the surrounding medium.The embryo, now containing one less blastomere, is returned to the incubator into the appropriate culture medium. The blastomere is then processed for either FISH or PCR, depending on the genetic condition to be studied.

Blastocyst biopsy

Blastocyst formation begins on day 5 post-egg retrieval and is defined by the presence of an inner cell mass and the outer cell mass or trophectoderm. At this stage of development, the embryo is formed of more than 100 cells. A hole is breached in the zonapellucida in a similar manner as described for a cleavage-stage embryo biopsy, and cells are removed from the trophectoderm using a fine biopsy pipette. The inner cell mass is left undisturbed. Genetic analysis is performed via FISH or PCR analysis as described below.

Genetic evaluation is performed using PCR, FISH, or comparative genomic hybridization (CGH). Nonaffected or normal embryos are then transferred into the uterus for subsequent implantation pregnancy.

Polymerase chain reaction

PCR is used for the diagnosis of single gene defects, including dominant and recessive disorders. PCR, sometimes called DNA amplification, is a technique in which a particular DNA sequence is copied many times in order to facilitate its analysis. PCR rapidly multiplies a single DNA molecule into billions of molecules

Fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH is the technique of choice to detect chromosome imbalance associated with chromosome rearrangements, and to select female embryos in families with X-linked disease for which there is no mutation-specific test. A human cell contains 23 pairs of chromosomes; however, FISH analysis allows accurate assessment of only 7-9 chromosomes in each biopsied cell. Consequently, many abnormal embryos, incapable of forming a successful pregnancy, remain undetected and may be transferred.However, due to the unacceptably low predictive accuracy of this test using FISH, it is not recommended for routine clinical use.

Comparative genomic hybridization

Using CGH, the embryo nucleus is labeled with a fluorescent dye and a control cell is labeled using another color (i.e., red or green). The two cells are then co-hybridized onto a control metaphase spread.The ratio between the 2 colors is compared. If the chromosomal analysis shows an excess of red, the embryo nucleus contains an extra chromosome. If an excess of green is apparent, then the embryo nucleus is missing one of these chromosomes. CGH enables not only enumeration of all 23 chromosomes but provides a more detailed picture of the entire length of the chromosome which may detect imbalance of chromosomal segments. Currently, this technique takes 72 hours.

In the past, people with a genetic disease or people who are carriers frequently choose not to have children in order to avoid the risk of passing on the disease to future generations. Now, PGD allows these couples the opportunity to have a child free of their particular disease.PGD has been performed in about 7,000 cases. 1,000 healthy children have been born, most since 1999.

 

COPYRIGHT: This article is property of We Speak Science, a non profit institution co-fonded by Dr. Detina Zalli (Harvard University) and Dr. Argita Zalli (Imperial College London). The article is written by Detina Zalli and Brisilda Pashaj (Plovdivski Universitet Paisii Hilendarski, Bulgaria ( Msc. Medical Biology).

 

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