Genetics

PREIMPLANTATION GENETIC DIAGNOSIS BY NEXT GENERATION SEQUENCING (NGS)

Homapage / PREIMPLANTATION GENETIC DIAGNOSIS BY NEXT GENERATION SEQUENCING (NGS)
In the PGD technique, biopsy of cultured embryos, preferably blastocysts, is carried out until the fifth or sixth day, and the number and structure of chromosomes are examined by genetic analysis. If the embryo is diagnosed as being normal numerically and structurally...

Preimplantation Genetic Diagnosis (PGD) is used to test numerical and structural chromosomal abnormalities (aneuploidy) in the embryo. In the PGD technique, biopsy of cultured embryos, preferably blastocysts, is carried out until the fifth or sixth day, and the number and structure of chromosomes are examined by genetic analysis. If the embryo is diagnosed as being normal numerically and structurally, it will be defined as euploid and will therefore be recommended for transfer. As the clear majority of miscarriages occurring during early gestation are associated with numerical chromosomal abnormalities, the aim of PGD is to reduce aneuploid pregnancies, increase implantation rates and shorten the time to give birth to a healthy baby.

The microarray technology used for PGD, or array CGH (Comparative Genomic Hybridization), can examine all chromosomes in embryos with approximately 4000 genetic markers distributed all over the genome. However, the detection is made indirectly by comparing the tested DNA with two DNAs known to be normal. In the next generation sequencing (NGS) technique, 50 ng of DNA from each sample is enzymatically separated into millions of fragments after the amplification step. After several stages, the sequences of these DNA fragments are read and the total number of readings of each embryo is determined. Thus, when the number of readings pertaining to the embryo is evaluated by a special algorithm, the numerical chromosome examination is completed.

What are the advantages of NGS over aCGH?

  • NGS technology provides a potentially more accurate and sensitive diagnosis,
  • It is able to detect the amount of mitochondria that provide the energy of the cell,
  • NGS can detect mosaicism at a rate as low as 20% (vs. 50% in aCGH).

Embryonic mosaicism is a condition happening in embryos containing two or more cell populations with different chromosomal content. The picture below shows a 4-cell stage embryo with one blastomere having a different chromosomal structure and shown in blue, which further divides and is dispersed into different parts of the morula and the blastocyst. Therefore, schematically the mosaic rate varies depending on how much of the cells taken during the biopsy are red or blue. Embryonic mosaicism is caused by an abnormality occurring in cell division mechanisms. It is known that mosaic embryos are associated with decreased implantation and pregnancy rates, increased genetic anomalies and negative pregnancy outcomes.



It is possible to give more accurate and realistic information about the chance of a pregnancy to the couple who has undergone PGD with the mosaicism ratio reported by NGS. However, the transfer of euploid embryos should always be a priority over mosaic embryos. In the absence of normal embryos, the transfer of low-rate mosaic embryos affecting some of the chromosomes can be done with the patient's informed consent.

The transfer of mosaic monosomic embryos carries relatively little risk. However, the transfer of an embryo diagnosed as having a mosaic trisomy for chromosomes 2, 7, 13, 14, 15, 16, 18, or 21, which are considered risky, is not recommended because it can cause the birth of a child with a trisomy. In this case, genetic counseling is very important. Thus, in accordance with PGDIS (Preimplantation Genetic Diagnosis International Society), it is always recommended to begin another IVF-PGD cycle to find a normal (euploid) embryo. 

SAYFA BAŞINA DÖN