PGSA: Pre-Implantation Genetic Screening Aneuploidy
(Note that genetic testing performed at different fertility clinics may vary, and that the information below refers to the genetic screening most commonly performed by fertility clinics in Australia. It is advisable to check with the fertility clinic or specialist on the exact nature of the genetic screening or testing which is to be performed.)
SUMMARY
The PGSA testing performed at most fertility centres tests the chromosomes only, not individual genes. PGSA Advanced embryo selection does NOT check for genetic defects for specific diseases. Patients should consult their specialist doctor in regards to having monogenetic testing for genetic disorders.
PGSA testing at is generally useful for:
- identifying chromosomally normal embryos thereby increasing the chance of implantation and ongoing pregnancy.
- detecting chromosomal abnormalities such as those associated with translocation and Down Syndrome.
- medically permitted sex selection.
This is a developing technology and the testing is only about 90-95% accurate.
Purpose of Pre-Implantation Genetic Screening Aneuploidy
PGSA is a laboratory procedure where a cell/cells are removed from a very early stage embryo in vitro and tested for some aspects of its chromosomal make-up. The purpose is to identify those embryos which are free of suspected chromosomal abnormalities so that they may be transferred to the uterus with the intention of achieving a normal pregnancy. The PGSA technique generally performed at fertility centres is advanced embryo selection which is discussed below.
It is now believed that disorders in the chromosomal make-up are responsible for most implantation and subsequent pregnancy failures (miscarriages) in women undergoing IVF treatment.
Definition of Terms
- Embryo/Blastocyst: After an egg is fertilized by a sperm it becomes an embryo which progressively divides prior to implanting in the uterus and becoming a pregnancy. On the third day (72hrs) post insemination, it should be at the 6-8cell stage. A blastocyst occurs on the fifth or sixth day (116-140 hrs) post insemination.
- ICSI Technique: A single sperm is injected into the oocyte using very fine microinjection equipment. ICSI is performed to reduce the chance of any extraneous DNA being tested.
- Trophectoderm: are cells forming the outer layer of a blastocyst, which provides nutrients to the embryo and develop into a large part of the placenta. Trophectoderm cells are the first stage of cells to differentiate from the fertilised egg.
- Biopsy: The removal of six to eight cells from the trophectoderm.
- Chromosomal Complement (CC): The normal number of chromosomes in a human cell is 46, comprising 23 pairs including the sex chromosomes. The most common chromosome disorders are termed ‘ploidies’, and refer to situations where one or more chromosomes are missing, or one or more chromosomes are added ie CC<46 or CC>46. For example, Down syndrome is characterised by having more than 46 chromosomes and the PGSA testing performed at most fertility centres does screen for this condition.
- Mosaicism: This term refers to a situation where the chromosome content of each cell in the embryo is different. It is estimated that there is a 7-10% risk of mosaicism in human embryos and this can be a source of misdiagnosis using PGSA. Trophectoderm biopsy is believed to reduce the risk of mosaicism. See ‘Incorrect embryo diagnoses’ in the section What Can Go Wrong? below.
- X & Y Chromosomes: Female embryos, and the female children that result from them, have two X chromosomes. Male embryos have one X and one Y chromosome. Thus, if we have a means of identifying the sex chromosomes in a cell from an embryo we can determine the sex of the child which would result from the embryo. Some genetic diseases are “carried” by the female but are only manifested in the male. In those cases it is preferable to transfer only female embryos to avoid the risk of having an affected male. Sex selection can be used for medical purposes such as the exclusion of sex linked disorders such as Duchenne muscular dystrophy. Sex selection for family balancing is not permitted in Australia and PGS performed at fertility centres can be used for medically permitted sex selection only.
- Advanced Embryo Selection (Array CGH): This uses array CGH technology to test for the correct numbers of all 22 pairs of numerical chromosomes and the two sex chromosomes.
- Translocation: Part of one chromosome is missing from its correct location and is attached to another chromosome. Chromosomal translocations are associated with a high incidence of pregnancy loss. Advanced embryo selection can be used to screen embryos for chromosomal translocations.
- Genetic Defects: These are changes (mutations) to the genes that make up the chromosomes. Advanced embryo selection does NOT check for these types of genetic defects. Patients should consult their specialist doctor in regards to having monogenetic testing for genetic disorders.
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Methods of Pre-Implantation Screening
The following provides an overview of the PGSA process, however each fertility clinic may have a slightly different protocol.
The female partner goes through conventional IVF to obtain a number of eggs. These are fertilised using the ICSI technique. The resulting embryos are grown in vitro until the morning of the fifth or sixth day when they should be a fully expanded blastocyst.
Using micromanipulation equipment and a laser, a hole is drilled in the zona (shell) of each embryo and cells are removed from the trophectoderm. The cells are placed in small vials identified with the details of the embryo from which they came.
For advanced embryo selection to be done the biopsied cells from the embryos are generally express shipped to a laboratory which performs this analysis and they are tested for the correct numbers of all chromosomes. Results are available within 10-14 working days.
Embryos which are chromosomally abnormal will generally not be used for embryo transfer. These embryos will generally be removed from storage.
What Can Go Wrong?
Pre-implantation genetic screening is still considered an evolving procedure worldwide and is continually being refined. As such there are a number of areas where things can go wrong:
- Destruction of an embryo during the biopsy procedure: The zona of the embryo shatters and the embryo loses its integrity.
- Failure of any embryos to develop to the blastocyst stage: A number of the embryos may have intrinsic defects which results in arrested development, irregular division or excess fragmentation occurring.
- Failure to amplify: To perform advanced embryo selection the DNA that comprises the chromosomes of the removed cells are amplified using a process called polymerase chain reaction (PCR). If this process fails then it is not possible to obtain an answer. This event is rare.
- All embryos are found to be abnormal or of the unwanted chromosomal type: Patients over the age of 40 years are more likely to have embryos with abnormal numbers of chromosomes 13, 18 & 21 as well as the sex chromosomes X & Y. Depending on the age of the woman and the number of embryos, it is possible that no normal embryos will be found and no embryo transfer can take place. In chromosome translocation cases, statistically only one in eight embryos will be normal. The embryos that are stored awaiting results that are returned abnormal will generally be removed from storage and will be unavailable for embryo transfer.
- Incorrect embryo diagnosis: The various cells within an embryo may not all have the same chromosomal constitution which is referred to as mosaicism. If the cells that are biopsied for testing are abnormal or of the unwanted type, it could lead to the discarding of an otherwise normal or desired embryo. If embryologists were to inadvertently biopsy the only normal cells from an embryo, it could lead to the transfer of an abnormal type of embryo. We estimate, on the basis of our experience and worldwide figures, that the risk of an incorrect diagnosis will be around 5%. For this reason, patients becoming pregnant from the transfer of embryos diagnosed by this procedure are still encouraged to have pre-natal diagnosis of the baby at approximately 12-14 weeks of pregnancy.
