Comprehensive chromosome screening is the screening of all chromosomes (23 pairs of 24 different chromosomes) in the embryo cell with microarray technology; it is used for couples planning to undergo a Pre-implantation Genetic Diagnosis (PGD).
The purpose of CCS is to analyse, select and transfer only embryos that do not have abnormalities in their number of chromosomes. Screening embryos in advance can help achieve higher implantation rates and fewer pregnancy losses, particularly for women who are 35 or older, couples with multiple-failed IVF cycles or implantation failure, and couples with repeated miscarriages.
Compared to the FISH method used frequently on embryos for routine aneuploidy screening in which only a limited number of chromosomes are screened, with CCS it is possible to study all chromosomes for digital and structural abnormalities. Embryos found to have chromosome abnormalities can be excluded from the treatment, only leaving embryos that have completely healthy chromosomes. If a normal embryo is found during genetic analysis in the PGD procedure, it is possible to offer older women a higher chance of conception and maintaining a pregnancy.
What are the advantages of comprehensive chromosome screening compared to aneuploidy screening with conventional PGD?
There are an increased number of analysed chromosomes, and diagnostic accuracy rate is very high.
Conventional PGD which uses the FISH technique can only study 5-9 chromosomes at one time. These chromosomes (e.g. chromosomes 21, 13 and 18) contain anomalies leading to abnormal pregnancies or early stage miscarriages. Because the other chromosomes are not analysed it is impossible to understand whether the remaining chromosomes contribute to failed attempts of implantation. The CCS method is capable of screening all 23 of the chromosome pairs in an embryo. That is why it is possible to determine in detail the chromosomal condition of the embryo or the effects of the chromosomal anomalies have on the development of the embryo with almost 98% accuracy.
It is possible to minimise mosaicism (which is the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilised egg) by postponing embryo biopsy from day 3 to the blastocyst stage.
Early stage cells in a developing embryo might appear similar under the microscope however they might not have the same chromosomal content. That is why a cell collected from an 8-celled embryo during an embryo biopsy on day 3 of embryo development in scope of conventional PGD might have a different chromosomal structure compared to the rest of the embryo cells. This difference of the potential of difference is termed as “chromosomal mosaicism” and is common amongst embryos in early development stage.
This is also the biological source of diagnostic “errors”. The only way to detect chromosomal mosaicism is to collect multiple cells from the embryo. However; this is not preferred because collecting more than one cell during the development phase of an embryo can seriously hinder the development of the embryo itself.
By allowing the embryo to grow until the blastocyst stage (day 5 or 6 of embryo development) and collecting multiple cell samples it is possible to:
- Understand which embryo has high implantation potential;
- See the level of mosaicism in the developing blastocyst. Because these cells are from the trophectoderm layer of the day 5 or day 6 embryo (which will create the placenta) the biopsy does not in any way affect the foetus cells (baby) or reduce volume.
Chances of conception are higher with the transfer of embryos frozen in CCS cycles.
Analysing all the chromosomes of an embryo is not an easy procedure; it requires a certain amount of time. If a biopsy is performed on the embryo on the morning of day 5, the results of the analysis could be ready on day 6 and a fresh embryo transfer may take place. However postponing this cycle to a day 6 transfer does present certain setbacks for the treatment:
- All embryos might not reach the blastocyst stage on the morning of day 5. Therefore a limited number of embryos would be ready for CCS;
- It is noticed that endometrium receptivity (the strength of embryo holding on to the uterus) decreases on day 6. An embryo that has normal chromosomes may not yield conception due to endometrial reasons.
In order to avoid such problems it is recommended to freeze day 5 or 6 embryos that have undergone biopsy for transfer to a uterus of natural composition at a later date. This stands out as the most ideal strategy for CCS. With this approach it is possible to maximise the number of embryo analysed with CCS while also increasing implantation potential of selected blastocysts because the transfer will be made to a near natural uterus environment.
Potential risks related to the PGD procedure
Other than the mentioned advantages of PGD, there are certain risks involved with the technique:
- Although extremely rare there is the chance that the biopsy will damage the embryo (<0.1%). This is why PGD is not recommended for patients other than candidate couples;
- Embryo cryopreservation will be necessary if PGD is performed together with frozen embryo transfer (as in CCS). In such cases there will be a need to implement embryo-freezing programmes that yield high vitality. Clinics with a record of limited post-thawing vitality rates should be avoided;
- Transferring an embryo, which is chromosomally normal increases chances of conception, however it does not guarantee it. Successful conception also requires a healthy (receptive) endometrium to which the embryo can attach to. Presence of an unreceptive endometrial environment or anatomic disorders of the uterus is in some cases, an important cause of failure to conceive;
- Conception with a “chromosomally normal embryo” does not guarantee that the child to be born will be “free of genetic diseases”. Chromosomal level tests like FISH and CCS aim to determine chromosome level abnormalities and related genetic diseases. These tests cannot identify gene level diseases like thalassemia or cystic fibrosis. If the families of the couple have a background of a single gene disease then the option of “PGD for single gene disease” should be considered;
- PGD tests performed on day 3 and days 5/6 only give insight into the current chromosomal condition of the embryo and do not guarantee they will remain unchanged until birth. However, potential embryonic or spontaneous chromosomal changes after the test date will only cause regional differences in the foetus and in most cases will not cause any serious life threatening problems in the child.
- There is a 2-10% chance of wrong diagnosis based on the day of the embryo biopsy or the technique used. Having said that, chances of error during blastocyst stage is less than 3% in experienced hands.