What is PGT-SR? 

 

Preimplantation genetic testing for structural chromosomal rearrangements (PGT-SR) is a method for detecting chromosomal abnormalities in embryos in a case one or both partners are known carriers of a balanced translocation or another type of structural rearrangement. Translocations represent the most common structural abnormalities of chromosomes, with the frequency of 1:625. 

Individuals, who are carriers of balanced translocations, have no symptoms of congenital malformations nor other defects and diseases. Complications only occur during reproduction, where some eggs or sperm cells may contain a wrong amount of genetic material (some parts are missing or are duplicated). In case such cell participates in fertilization, it can cause the embryo to stop developing, decrease of the implantation potential or abortion of the fetus with a congenital defect. Some translocation types could even increase the risk of Down syndrome.

Who is PGT-SR suitable for?

PGT-SR is suitable for patients who are carriers of reciprocal or Robertson translocations, where two segments between different chromosomes are exchanged, as well as patients with inversions, in which there is a rotation of a segment within a chromosome. 
 

How does it work?

PGT-SR uses, similarly to PGT-A, the Massively Parallel Sequencing method (MPS). During the sample preparation, DNA of embryos is multiplied several times and subsequently, DNA of all 23 human chromosome pairs is read (sequenced). Information about the number of such sequence reads is then compared with the control sample, which has the right copy number of all chromosomes. This way we can reveal if some sequences are abundant or missing in embryo sample. Thus we detect numerical aberrations of all human chromosomes and choose euploid embryos with the highest implantation potential for transfer.

What is the benefit of PGT-SR using MPS? 

Thanks to the ability to examine all the chromosomes, even those that are not included in translocation, we can perform Preimplantation Genetic Testing for Aneuploidies in the same test. Oppose to Array Comparative Genome Hybridization (aCGH) method used in the past, MPS allows us to examine up to 96 embryos in one run. At the same time, thanks to much higher resolution, we can detect so-called segmental aneuploidies, where only part of the chromosome is abundant or missing in embryo cells (those are very often a consequence of reciprocal translocations). Moreover, a greater dynamic range of sequencing methods gives more precise information about chromosomal mosaicism - a situation when there are aneuploid cell lines along with the normal cells in a human embryo.