An overview of genetic screening of egg
Many of the eggs that women produce are abnormal due to them containing an incorrect number of chromosomes. Chromosomes are minute rod shaped structures that exist in virtually every cell of the body. The chromosomes carry the genes; the chemical instructions that tell the embryo how to develop into a baby. The percentage of eggs that have abnormal chromosomes increases as women become older but even in younger women a significant minority of eggs have these abnormalities.
Abnormal eggs can fertilise with sperm but they will always produce abnormal embryos. Abnormal embryos may either not implant or they may implant and lead to miscarriage or, sometimes produce an ongoing pregnancy in which the baby has a chromosome abnormality, the commonest of which is known as Downs syndrome (also called trisomy 21).
It is now possible to identify eggs which contain an abnormal number of chromosomes using a very recent technique which measures all 23 chromosomes in the egg (array CGH).
How common are chromosome abnormalities in embryos?
Research has shown that chromosome abnormalities are very common in eggs and embryos. The risk of having an embryo with the wrong number of chromosomes is greatly affected by the age of the mother. For women in their early thirties about one quarter of the eggs and embryos have an abnormal number of chromosomes but for women over 40, it is usual for more than half of the embryos produced to be abnormal. It is true that some embryos have abnormal number of chromosomes due to the abnormality being introduced by the sperm but this is thought to account for less than 1 in 15 abnormal embryos. Unfortunately it is not possible to test whether the sperm is normal or abnormal without destroying it.
How do we assess the chromosomes in the egg?
At the time of ovulation the chromosomes in the egg split into 2 equal halves. One half is retained in the egg to mix with the chromosomes that will be introduced by the sperm when the egg is fertilised. The remaining half of the chromosomes are ejected in the form of the first polar body.
The first polar body has no function once it has been ejected.
It is this structure which is removed from the egg to assess for the chromosomes. This is known as polar body biopsy.
How is the biopsy performed?
Once the egg has been collected from the ovaries and the cells surrounding the egg have been removed, the embryologist is able to assess the egg to confirm that it contains a polar body. If the egg is not mature it will not contain a polar body and therefore can not be fertilised. These immature eggs can not be biopsied
Mature eggs do contain a polar body but to be able to do the biopsy, the polar body must be compact and clearly formed like the example in the picture above. Sometimes the polar body is fragmented into several small pieces and these can not be biopsied.
Once the embryologist has identified the eggs that are suitable to be biopsied, a small very precise opening is made with a laser in the zona pellucida (the coat around the egg, see picture above). This is to allow the passage of a tiny glass needle to remove the polar body from the egg. The polar body is gently aspirated by suction.
How is the egg fertilised by the sperm in the laboratory?
Normally in the absence of any problems with sperm, standard IVF treatment is performed to fertilise eggs. With IVF treatment, approximately 50,000 sperm are placed around each egg but only one is able to break through the zona pellucida to enter and then fertilise the egg. When a polar body biopsy is performed however, a hole is made in the zona pellucida so many sperm can gain access to the egg and cause it to be fertilised abnormally. To avoid this it is necessary to inject a single sperm into each egg (ICSI, sperm microinjection). This is done just before the biopsy takes place.
What happens after the egg has been biopsied?
The egg containing the sperm that has been injected is put into the incubator and left overnight as normal. The next day the eggs are examined to check those that have fertilised normally and these normally fertilised eggs are put back in the incubator and allowed to develop into embryos.
In the meantime the polar body that has been removed is sent for genetic analysis using a technique called “array CGH”. This test compares the total number of chromosomes in your egg with the number in a control sample which is known to contain the full set of 23 chromosomes. The results show if the egg has any extra chromosomes or conversely any missing chromosomes. It can also show which chromosomes are abnormal.
To reduce the cost to you of the testing procedure the genetics laboratory only perform the array CGH analysis on those eggs which have fertilised on the day after the biopsy avoiding having tested an egg which can not become an embryo. The results usually come back within 24 hours of the egg collection but sometimes up to 48 hours are required to get the results. By the time the results are back embryos are starting to develop so we know which embryos came from the normal eggs. With this knowledge it is possible to decide which embryos are the best one(s) to replace to give you the highest chance of pregnancy and the lowest risk of abnormality
What are the downsides?
Polar body biopsy and genetic analysis is a cutting edge technology and it is important to realise that there are several possible difficulties:
- When the polar body is fragmented it is not possible to biopsy that egg. In this case some of your eggs may not be biopsied but others (with normal shaped polar bodies) can still be.
- There is a small risk (1-5%) that the egg does not survive the biopsy or ICSI procedure. However the procedure can still carry on with the other eggs that do survive.
- Depending on your age it is possible/probable that most of the eggs may be abnormal. We can only replace embryos derived from normal eggs so the choice of embryos to replace will be limited. On rare occasions there will be no normal embryos to transfer.
- The genetic analysis of the eggs gives a result in about 95% of biopsies but in the remainder it may not be possible to find out if the chromosomes are normal or abnormal. If no other embryos are available it would be possible to replace embryos which have come from these eggs without a result and in this case it would be no different to having embryos transferred in cycles in which genetic screening has not been performed.
Have any studies produced data showing chromosome tests to be beneficial for IVF patients?
Some studies have suggested that the use of genetic screening increases IVF pregnancy rates, but other research has provided contradictory data. However, all previous studies used a technique that is now obsolete, which was only capable of testing about one third of the chromosomes in each embryo. We now use a screening method called array-CGH, which assesses all of the chromosomes in the egg and therefore we can find up to 40% more abnormalities. So far, there has only been one trial using this sort of technology, conducted in the United States. In that study, the chance of an embryo transferred to the womb making a baby was found to be increased by more than 50% in IVF cycles where embryos were chosen for transfer based upon the results of PGS analysis.
Who could benefit from this technique?
Pre-implantation genetic screening using array CGH is available for every patient who undergoes IVF treatment. The decision whether to have this or not will be taken after consultation. In particular this could be beneficial to couples with adequate number of eggs produced and:
- Repeated IVF failures (2 or more previous unsuccessful cycles)
- Repeated miscarriages
- Advanced maternal age ( over 38 years of age)
- Previous pregnancy with abnormal chromosomes.
Does genetic screening by array-CGH replace the need for routine prenatal testing?
Although array-CGH is likely to reduce the risk of having a pregnancy affected by a chromosome abnormality, it is not 100% accurate and therefore results cannot be guaranteed. For this reason, it is strongly recommend that you have a prenatal test (for example amniocentesis or chorionic villus sampling) if it is recommended to you by your doctor. This will provide a more definitive diagnosis and may detect some subtle abnormalities that array-CGH cannot.
Will it prevent me from miscarrying?
The embryos will be screened for all the chromosomes, including the ones that we believe are responsible for the majority of miscarriages. A miscarriage however can happen due to many reasons, not only chromosomal abnormalities. Therefore even though we believe that the miscarriage rate will decrease, it can still occur.
Retention of samples
After the polar body is removed it is destroyed in the genetics laboratory to measure the chromosome material (DNA). This DNA can not be used for any other purpose except the array CGH test. The DNA from these cells will be stored for a minimum of one year after the test. This is to allow the possibility that the sample can be re-analysed if necessary at a later date. After 1 year the sample is disposed of respectfully.