PGT is a diagnostic method intended for couples who are concerned about the risk of passing on a genetic condition to their offspring. PGT is defined as the analysis of DNA from oocytes or embryos carried out for the purpose of HLA typing or identifying genetic abnormalities. The most common aim of pre-implantation genetic testing is to detect abnormal embryos before they are transferred to the uterus.
Pre-implantation genetic diagnosis – is a method that enables the genetic material of oocytes to be assessed prior to fertilisation, or that of embryos prior to transfer to the uterus. R.G. Edwards and R.L. Gardner are regarded as the pioneers of PGT; in 1968, they performed a biopsy on a rabbit blastocyst to determine its sex. The use of pre-implantation genetic testing on human germ cells in 1989 marked the beginning of a new phase in the development of this field in terms of its diagnostic capabilities. In 1990, Handyside et al. described the first case of pregnancy following PGT in patients at risk of passing on an X-linked genetic disorder (adrenoleukodystrophy) to their offspring.
Following testing using the polymerase chain reaction (PCR) method, female embryos free from the risk of this condition were transferred to the uterus. In the same year, a team led by Verlinsky successfully carried out pre-implantation diagnosis for another condition – cystic fibrosis. The simultaneous development of diagnostic techniques allowed for the expansion of PGT’s diagnostic capabilities. The use of fluorescent in situ hybridisation (FISH) enabled the rapid analysis of aneuploidy and chromosomal aberrations in oocytes and embryos. Trophoectoderm (TE) biopsy and next-generation sequencing (NGS) are currently the preferred techniques for pre-implantation genetic testing.
The indications for PGT can be divided into two groups:
1. The first group, characterised by a high risk of passing on genetic defects to offspring:
2. Group two, with a low risk of passing on genetic defects to offspring, in which PGT is performed to increase the chances of a successful IVF (In Vitro Fertilisation) procedure. This type of diagnostic test is also known as PGS (Pre-implantation Genetic Screening):
This procedure is an alternative to the prenatal diagnosis previously recommended for couples with a family history of genetic conditions who wish to have a healthy child. PGT helps to reduce the risk of having to make difficult decisions about a possible termination of pregnancy in the event that genetic abnormalities are diagnosed in the developing foetus.
High risk of passing on genetic disorders to offspring, i.e.:
A lower risk of passing on genetic conditions to offspring, in order to increase the chances of a successful IVF procedure:
There is a risk of damage to the embryo during the collection of cells for PGT testing. This risk is less than 1 per cent in the case of polar body sampling. Furthermore, the result of pre-implantation genetic testing may be inconclusive. This is because not every cell collected for analysis contains an equivalent amount of genetic material compared with the other cells.
In 2005, the INVICTA Clinic was the first in Poland to introduce pre-implantation genetic diagnosis into the in vitro fertilisation procedure. Today, we can boast some of the most significant successes in this field. We have helped many couples at high risk of genetic disorders to have healthy babies. These include serious conditions such as cystic fibrosis, SMA, COX syndrome, Huntington’s disease and many others. We are the only clinic in Poland to be a full member of the ESHRE PGT Consortium. According to data from this international organisation, we rank sixth in the world in terms of the number of diagnostic tests performed. Over 1,000 PGT diagnostic tests had been carried out by the end of 2011. We are the only clinic in Poland to perform so-called ‘full PGT’, testing both embryos and oocytes.
At INVICTA, the selection of embryos for transfer based on their sex is carried out solely on medical grounds. The use of pre-implantation genetic testing requires familiarisation with the procedure, specialist medical consultations – including genetic counselling – and the signing of the relevant documents.
In summary, PGT is a method of genetic testing of embryos prior to implantation, which enables the detection of genetic and chromosomal abnormalities that may lead to genetic disorders in offspring. This method is one of the most advanced solutions in the field of reproductive medicine and genetics.
The NGS method allows all 23 pairs of chromosomes to be analysed with a high degree of accuracy. Based on the results, it is possible to determine whether any chromosomes are missing or present in excess. The risk associated with an abnormal number of chromosomes in the embryo increases with the mother’s age; we therefore particularly recommend this test for women aged 35 and over. The test detects, for example, trisomy 21, which causes Down’s syndrome.
By carrying out this type of diagnostic test, it is possible to check whether the embryo has the correct number of chromosomes. In this case, the cause of the abnormal number of chromosomes is the fusion of two acrocentric chromosomes, which consequently means that a chromosome is missing from the embryo’s karyotype
PGT testing for monogenic disorders involves analysing material taken from an embryo to check for the presence of a mutation carried by one or both parents. There are two types of this testing:
This type of pre-implantation diagnosis makes it possible to rule out sex-linked hereditary disorders. It involves determining which sex chromosome the embryo has inherited, and on this basis, the geneticist assesses whether the embryo in question carries the mutation or not.
There are situations in which it is worth considering carrying out dual PGT testing. From a single embryo biopsy, we can perform the following combined tests:
