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PGT for monogenic disorders (PGT-M)
We use PGT-M for genetic abnormalities responsible for autosomal dominant traits, autosomal recessive traits, or X-linked diseases. A specific PGT must be developed beforehand in that case. PGT development can take several months on average. In certain individual situations, this may even take longer. The lab will develop the test that is most suitable for your situation.
There are two options: a Polymerase Chain Reaction (PCR) test or a Single Nucleotide Polymorphism (SNP) test.
- A PCR test detects the known genetic abnormality using one cell that is removed on day 3 after fertilisation. The embryo then develops further in the incubator until day 5 or 6, after which it can potentially be used for ‘fresh’ transfer.
- In addition to the known genetic abnormality, an SNP test can also detect any additional chromosomal abnormalities in the embryo (PGT-A), such as trisomy 21 (which causes Down syndrome). This is done routinely and at no extra cost. On day 5 or 6, multiple cells are removed from all embryos of sufficient quality. Since genetic screening takes about four weeks, all embryos are frozen immediately after the biopsy. This means ‘fresh’ transfer is not possible.
PGT for structural chromosomal rearrangements (PGT-SR)
We use PGT-SR if one of the partners within a couple is the carrier of an (un)balanced chromosomal aberration. Prior PGT development is usually not necessary in that case. The lab needs about 3-4 weeks for preparations (again, exceptions are possible) and uses NGS (Next Generation Sequencing) for genetic analysis. NGS cannot distinguish between normal and balanced embryos, only between normal/balanced and unbalanced embryos.
Depending on the chromosomal abnormality, we can also use an SNP test for the genetic screening. In that case, specific PGT has to be developed beforehand, which can take several months. SNP can, however, distinguish between normal and balanced embryos. Like SNP, NGS, can detect any additional chromosomal abnormalities in the embryo (PGT-A). This is done routinely and at no extra cost. In both NGS and SNP, multiple cells are removed from all embryos of sufficient quality on day 5 or 6. Since genetic screening takes about four weeks, all embryos are frozen immediately after the biopsy. This means ‘fresh’ transfer is not possible.
PGT-A or pre-implantation genetic testing for aneuploidy is a form of PGT that counts whether there are an equal number of copies of all chromosomes in the embryo. Prior development is not required, we only need the karyotypes (blood test). Genetic analysis is performed on some cells taken from the embryo’s outer shell on day 5 or 6 after fertilisation. The embryos are then frozen. The results of the genetic screening are available about four weeks after biopsy.
Embryos with an abnormal number of chromosomes (aneuploid, e.g., trisomy 21 or Down syndrome) will not be transferred. These embryos are not viable or may give rise to the birth of a child with a genetic disorder. Embryos with a normal number of chromosomes (euploid) are eligible for transfer.
The lab can use PGT-A to select embryos based on their morphology (the shape and number of cells), but also on the chromosomes they contain.
Special indication for PGT
A special indication for PGT is HLA typing. The aim is to investigate which embryos have the same tissue type as an existing, seriously ill child of the same parents. Children with certain (fatal) blood or immune diseases can sometimes be cured thanks to the transplantation of stem cells from umbilical cord blood or bone marrow. This is only possible if the transplanted cells are HLA-compatible with the cells of the sick child. If they are not compatible, they will be rejected. In other words, a baby born from an HLA treatment can save a sick sibling. Such a baby is also called a ‘saviour baby’ or ‘donor baby’ in the media.
Special indication for PGT
Sometimes a condition occurs in the family that does not manifest itself until late adulthood and for which there is currently no prevention or treatment. Examples of such conditions include Huntington’s disease, hereditary forms of Alzheimer’s, and hereditary forms of amyotrophic lateral sclerosis (ALS). A prospective parent may not wish to be tested presymptomatically for this familial condition on the one hand, but he or she may still want to avoid passing on the condition on his/her child. In that case, PGT exclusion may be an option. Embryos are selected based on the presence or absence of the gene copy of the ‘grandparent who is a risk’ without knowing whether the prospective parent is a carrier of this familial disorder.
An embryo that is not eligible for transfer due to PGT-exclusion because there is a risk that it carries this disease will no longer be kept. It is considered ‘affected’.
If the prospective parent were to undergo presymptomatic testing at some stage in the future and it is established that he or she is not a carrier of this condition, then PGT no longer needs to be performed for this condition.
Special indication for PGT
Often, the genetic abnormality occurs in several family members (familial variant). If this is not the case and the genetic abnormality only occurs in the prospective parent, this is called a ‘de novo’ variant. In the case of a familial variant, the lab determines during test development how the causal variant is inherited between family members. The information is used for an accurate diagnosis of the embryos. In the case of a ‘de novo’ variant, we cannot fall back on family members and whether or not the condition is inherited will only become clear on examination of the cells obtained after embryo biopsy. We therefore need at least 4 embryos for examination during the first treatment (including cells from embryos not eligible for biopsy on day 5 or 6). If the lab has access to cells from 4 embryos, there is a good chance that this is sufficient for the accurate diagnosis of the embryos in the first cycle. Exceptionally, there is no diagnosis and the embryos remain frozen until inheritance can be determined during a subsequent treatment.
Where does the treatment take place?
Your embryo testing treatment will take place entirely in Brussels IVF; all consultations, examinations and procedures will take place on site. Feel free to take a look via the virtual tour!
5 common questions about having children when you have an inherited condition answered:
How long does test development take?
Can PGT that was developed by another centre also be used?
Can I conceive spontaneously?
Why do we need blood or DNA samples from (first-degree) relatives?
Can I choose the sex of my baby?