Preimplatation Genetic Diagnosis for single gene disordersPreimplatation Genetic Diagnosis (PGD) is a reproductive option for couples at risk of passing a genetic disease on to their children.
Introduction
Preimplantation Genetic Diagnosis (PGD) is a reproductive option for couples at risk of passing a genetic disease on to their children. PGD involves screening IVF generated embryos for genetic disease prior to transfer, with unaffected embryos transferred to the uterus. PGD provides the opportunity to commence a pregnancy knowing that the baby is unaffected by the disease in question. Thus, PGD avoids the difficult decision of whether or not to terminate an affected pregnancy. Couples choose PGD over prenatal diagnosis for many reasons including:
- Objection to termination of pregnancy
- Previous pregnancy terminations
- Loss of a child from the disease.
Genetic counselling is an important step to ensure that PGD is the right option for you.
PGD at Monash IVF
Monash IVF has offered a clinical PGD service for single gene disorders since January 2000. Monash IVF has been responsible for developing new PGD technologies with the aim of offering testing for any genetic condition where the cause is known. PGD has now been performed for many disorders, including Cystic Fibrosis, Beta-thalassemia, Huntington’s Disease, Spinal Muscular Atrophy and Fragile X.
The steps involved in PGD:
STEP 1: Genetic testing
One or both partners must have had previous genetic testing to determine the gene change/s causing the genetic condition in their family. If the causative gene change/s has not yet been identified, the couple should be referred to Genetic Health Services Victoria (phone +61 03 9594 2026), who will be able to organise this genetic testing. The PGD team need to know the causative gene change/s in order to try to develop a PGD test.
STEP 2: Genetic Counselling in PGD Clinic
Once the specific gene change has been identified, the couple should make an appointment to attend the Monash IVF PGD clinic. The PGD clinic is a free service which is run on a fortnightly basis. The appointment will take approximately one hour and will involve a one-on-one consultation with a Clinical Geneticist and Genetic Counsellor. During this appointment, the couple will be provided with information on the PGD program at Monash IVF and will have an opportunity to have any questions answered. A PGD clinic appointment can be made by calling the Monash IVF genetic counsellor on +61 03 9590 8336. This appointment can be made either before or after you have met with your IVF doctor.
STEP 3: Feasibility Testing
Prior to commencement of a PGD cycle, it is necessary for the couple to undergo a feasibility test in order to determine if PGD will be possible for their particular genetic condition. Feasibility testing will require a blood sample from both partners. In most cases, a blood or DNA sample will also be required from one of the following:
- Both partner's parents
- The couple's child
- A DNA sample from prenatal testing in a previous pregnancy
These additional samples are used to track the inheritance of the particular gene change/s within the family. The genetic counsellor can organise for the collection of these samples following the PGD clinic appointment.
The feasibility test is required to ensure that the Monash IVF genetics laboratory is able to develop a test that can detect the specific gene change/s and that this test is sensitive enough to provide a result from a single cell (which contains the same amount of DNA that we would be able to biopsy from an embryo).
In most cases, specific "linked markers" are included in the final test in order to increase the accuracy of the results. Linked markers are segments of DNA that are located close to the specific gene of interest. Ideally, each partner will have different marker sizes which will enable the laboratory to distinguish between each gene copy. These markers are used to confirm the result of the direct gene testing.
In some instances, due to the particular genetic combination of the couple, an accurate test may not be possible to develop and PGD may not be available. There is a non-refundable fee for the feasibility test.
A feasibility report outlining the results of the feasibility testing is sent to the IVF doctor and genetic counsellor. The genetic counsellor will contact the couple to go through the results of the feasibility testing process.
STEP 4: IVF Cycle
Once feasibility testing has been completed and PGD is feasible, the couple will need to make an appointment with their IVF doctor to discuss their IVF treatment. An IVF counselling session will also be required (this is a legal requirement in Victoria). More information about the IVF side of the procedure can be obtained from Monash IVF on request.
STEP 5: Embryo Biopsy
Couples consenting to PGD will have their embryos created using intracytoplasmic sperm injection (ICSI). This involves the injection of a single sperm into each egg in an attempt to achieve fertilisation. ICSI is used exclusively in PGD for single gene disorders to avoid misdiagnosis of the genetic condition due to other contaminating sperm that could be present after conventional IVF. Following fertilisation, the embryos are cultured in the laboratory for 3 days before embryo biopsy (Figure 1). The embryo biopsy process involves removal of 1 or 2 cells for genetic analysis.
Figure 1: Embryo Culture and Biopsy.
STEP 6: Genetic diagnosis
Biopsy samples are tested using a very powerful technique called fluorescent polymerase chain reaction (F-PCR). F-PCR makes millions of copies of the relevant region of DNA so that a reliable diagnosis can be made on each embryo. The product from the F-PCR reaction is tested for the presence or absence of the known parental gene change/s using a range of genetic techniques and a genetic analyser. Final results are usually obtained 12 to 24 hours after biopsy. A qualified PGD scientist will discuss your results with you prior to transfer.
STEP 7: Embryo transfer
Unaffected embryos are transferred to the uterus on Day 4 or Day 5. Only one or two embryos can be transferred. Surplus unaffected embryos will be grown in culture to Day 5 or Day 6. If they reach an appropriate stage of development (ie: form a blastocyst), they will be frozen. These embryos may be used in a subsequent IVF cycle if you do not achieve a pregnancy with the fresh embryos.
STEP 8: Pregnancy testing
A pregnancy test will be performed on Day 16. Your IVF nurse will organise this test for you.
Accuracy of diagnosis
The accuracy of the test is detailed in a feasibility report sent to your IVF doctor. Many factors influence the accuracy of the test, including the nature of the gene change/s, the number of cells biopsied from the embryo, and the clarity of the results obtained. PGD is based on the assumption that the biopsied cell is representative of the entire embryo which may not be the case. Every effort is made to ensure that the test offered for your treatment cycle has the highest possible accuracy using the currently available technology. Confirmatory prenatal diagnosis is highly recommended as PGD is NOT 100% accurate.
Other important information
Due to the complexity of the genetic tests, it is possible that a conclusive result is not obtained from all biopsied embryos. In this case, the embryos will be left to grow further and if they develop to blastocyst stage they will be frozen. It is possible that a second biopsy procedure, called blastocyst biopsy, can be performed on these embryos to try and obtain a conclusive result. If this is possible and unaffected embryos are identified, these can be transferred in a frozen embryo transfer cycle.
- Embryo biopsy does not appear to affect embryo development.
Embryo biopsy has been performed extensively at major IVF clinics throughout the world. Follow up studies have shown that the procedure is safe with no known adverse affects on the embryo’s potential to implant and develop normally.
- Only embryos that are good quality and adequately developed can be biopsied.
Embryos must have at least 5 cells by Day 3 to be suitable for biopsy (ideally they should have 6 to 8 cells). One or two cells will be biopsied from the embryo depending on the number of cells in the embryo.
- It may not be possible to obtain a result from every embryo.
It is expected that some analysed cells will only produce a partial result. This may not provide an informative result for the embryo. Therefore, the embryo may not be suitable for transfer. It is possible that no result will be obtained for some embryos due to technical difficulties and/or poor embryo quality.
- PGD only tests for the condition of interest
The PGD test is specifically designed to test for the gene change/s that have previously been identified in one or both partners. The test does not give any information relating to other single gene or chromosomal abnormalities.
What are the costs?
Information relating to the cost of feasibility testing, embryo biopsy and single gene testing is available from Monash IVF. Please note that additional costs will be incurred for IVF services.
Please contact Monash IVF for a list of current prices.
Quality Systems
Monash IVF employs a very high standard of quality assurance. Through the application of quality systems we monitor and provide standards of excellence in quality service, care and advice.
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