Preimplantation Genetic Diagnosis (PGD) involves the removal of a single cell from an embryo (obtained from ICSI/IVF) for genetic testing prior to transfer into the uterus.
The testing of this cell will give us information regarding specific genetic abnormalities which could have a familial inheritance (cystic fibrosis, beta thallasmia, sickle cell disease, Duchenne muscular dystrophy,etc.).PGD can be used to screen a large number of monogenic disorders due to a single gene only.PGD is however an adjunct to assisted reproductive technology and requires IVF or ICSI to obtain oocytes.
The world’s first PGD was performed at Hammersmith Hospital in London by Dr. Handyside and Dr. Winston where female embryos were selectively transferred in five couples resulting in two twins and one singleton pregnancy.
Preimplantation Genetic Screening (PGS) involves the removal of a single cell from an embryo to test for the overall chromosomal normalcy in embryos. PGS is not looking for a specific disease, but rather to screen the embryo for normal chromosome copy number. Its most advantageous in screening women of higher age groups (over 37 years of age). The logic behind this is that higher age group women have an increased rate of chromosomally abnormal embryos. Most eggs might be chromosomally abnormal but thankfully only 30-60 % of human embryos have this abnormality transferred.
When are embryos biopsied?
- When one of the patents are carriers of some hereditary genetic disease and deicide to visit a genetic specialist. When one or both parents are having their karyotype altered due to presence of a chromosomal abnormality.
- Patients with repeated IVF / ICSI failures.
- Advanced mother’s age.
- Previous history of genetically abnormal fetus (aneuploidy)
Process of PGD?
After oocyte retrieval and ICSI. The embryos are cultured for 3 days at which point they are about 6 to 8 cells already.
One of the cells from the embryo are removed for a biopsy.
What types of cells can be removed for biopsy?
Polar body biopsy:
A mature egg has one polar body and a fertilized egg has two polar bodies. At both stages the polar bodies can be biopsied. This technique does not remove anything from the embryo (cells). The developing potential of the embryo, however could be damaged because there is evidence that such polar bodies may help direct differentiation of cells in the embryo.
On day 3 of embryo development, there are usually 6 – 10 cells within the embryo. At this stage, a portion of the zona pellucida (outer shell of the embryo) is removed either mechanically or with laser and, one or two cells are extracted for the biopsy.The day 3 clevage has been shown to be detrimental to the developing embryo, therefore, these embryos will be discarded.
This biopsy is performed at the blastocyst stage where the trophectoderm is biopsied.
This trophectoderm will later form the placenta. This biopsy shows excellent results and is the method of choice for a biopsy for PGD and PGS.
The sampled embryos will be frozen after the biopsy because results of the embryo transfer would come only after 2 – 3 weeks.
Fluorescence in situ hybridization (FISH) on the fixed nuclei of biopsied cells with target-specific DNA probes 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. FISH has also been used to screen embryos for sporadic chromosome aneuploidy (also known as PGS or PGD-AS) in order to try and improve the efficiency of assisted reproduction.
What happens if the biopsy is positive for genetic abnormality?
That particular embryo will be discarded
What happens if the biopsy is negative for genetic abnormality?
Since the embryo is still in an early stage of development, the embryo will compensate for the loss of that cell, therefore it should continue with the cleavage as it normally would.
What are the pros and cons of PGD / FISH?
Reduction in the risk of miscarriage and recurrent pregnancy loss.
Increase in the overall positive pregnancy rate.
The embryos have a chance to get damaged during the biopsy procedure.
Sex selection in PGD?
Although it is possible to check the sex of the embryo prior to transfer into the uterus, it is not done due to legal reasons. In certain countries when sex-linked diseases which affect female babies, then embryo selection is permitted.
How many embryos are necessary for PGD?
There is no minimum or maximum number of embryos that can be biopsied. Since the procedure in itself can incur additional charges, couples who have low number of embryos are advised to undergo more cycles in order to gather more than 5 embryos in total prior to PGD.
Spinal muscular atrophy, beta thalassemias (β thalassemias), Clouston syndrome (or hidrotic ectodermal dysplasia), Gaucher’s disease, cystic fibrosis, osteopetrosis, polycystic kidney disease, non-syndromic congenital sensorineural hearing loss, myotonic dystrophy, muscular dystrophy, tuberous sclerosis (type I and II), neurofibromatosis (type I), Huntington’s disease or chorea, retinitis pigmentosa, haemophilia A, Fragile X syndrome.