Pre-implantation Genetic Diagnosis
- Preimplantation genetic diagnosis (PGD) is a type of embryo screening performed prior to implantation.
- Requires in vitro fertilisation to obtain the embryos (or oocytes) for evaluation.
- Aim of PGD is to try to ensure that the baby will be free from that particular disease: we are searching for a specific inherited genetic defect that will give rise to a specific serious disease.
- PGD is somewhat of a misnomer: diagnosis means to identify an illness. However, at the time at which PGD occurs, the embryo has no symptoms.
- Can be used to screen for a specific disorder in couples with a high risk of transmitting an inherited condition (determined by previous pregnancies with serious genetic conditions or family history). Different categories of diseases screened for are:
- Autosomal dominant: Huntington's disease, Charcot-Marie-Tooth
- Autosomal recessive: Cystic fibrosis
- X linked: Fragile X, Haemophilia A, Duchenne's
- Chromosomal structural abberation
- In addition, there is also a process called preimplantation genetic screening (PGS). This is offered to patients who are undergoing IVF with advanced maternal age/history of recurrent miscarriage/ family history of chromosomal problems/ several unsuccessful IVF cycles. This is to screen for aneuplodies which can cause failure of implantation, miscarriages or diseases (e.g. trisomy 21: Down's syndrome).
- These examples are just a small fraction of the potential diseases that can be screened for in licensed clinics.
- The Human Fertilisation & Embryology Authority (HFEA) is the UK body that is in charge of determining whether a condition is serious enough to be included. Most of the conditions on the list can be carried out for any patient who requests it, and the list is regularly updated with other diseases, including Ehlers-Danlos, currently awaiting consideration for inclusion (at the time of writing)1
- For certain life limiting conditions the best available treatments is a transfusion of stem cells from cord blood provided by a tissue-matched donor.
- In these cases Preimplantation tissue typing (PTT) gives the best chance of having a tissue matched child. This is beneficial because using a tissue matched donor who is a close relative is more likely to be successful than treatment using a tissue matched unrelated donor.
- PTT is similar to PGD and ensures that the child is free from the disease and is a tissue match for it's older affected sibling. There are 10 diseases that are licensed for PTT including Diamond Blackfan Anaemia, Beta and Alpha Thalassaemia, Fanconi's Anaemia, and Aplastic Anaemia. However, each individual case still needs to considered by HEFA on a case by case basis.2
- HEFA determines which serious conditions get included as permissable for PGD.
- However, some worry that this is just the start of a slippery slope that will enable embryos to be selected on a "trivial" basis. For example, sex selection is currently used to exclude embryos with sex-linked diseases.
- There are some that argue that couples should be free to choose the sex of their child. By following the trajectory of this argument ("Designer babies") it is clear to see that this could have serious implications for the future of society.3
Inaccuracy of techniques
- Screening relies on the theory that all of the embryo cells are identical. However, some embryos are mosaic: the cells are non-chromosomally identical. As the embryologist only takes 1-2 cells (blastomeres) from the developing embryo it is possible that something may be identified in that cell which is not present in the rest of the embryo (so causing a viable embryo to be discarded)- a false positive.
- Alternatively, a serious genetic condition may be missed causing a false negative.
- Additionally, it is important to note that PGD tests for a specific condition (or conditions) based on what the child is deemed to be at high risk for. It is possible that the child is affected by another disease other than the one that is tested for.
- Furthermore, the child is still at risk of other complications that are present with every normal pregnancy (e.g. Cerebral palsy)4.
The rights of an embryo
- Ongoing discussion, particularly from religious figures, over whether an embryo should be afforded the same rights and respect as a child /adult.5
- For those that hold this view, destruction of an unsuitable embryo is morally objectionable. Potentially, PGD can be seen as less acceptable than a termination because (potentially) more embryos can be discarded.
- This issue is further compounded by the relatively low success rate: as it currently stands, only about 20% of treatment cycles result in live births.
Impact on those with disabilities
- There are two facets to this issue. The first is that the practice of PGD can be seen to reinforce negative stereotypes of those with disabilities, possibly suggesting that lives of those affected by such disorders are worth less than those of healthy people.
- The second issue, although somewhat rarer, is the selection for certain disabilities. For example, PGD can be used to select for deafness or dwarfism. Both can be genetic conditions and can be considered as having their own unique culture and identity. For this reason, a parent may want to select for such an embryo despite a potential negative impact on the child's future life6
Given below are some of the most important events deteriming the legal aspects of PGD:
- 1989: PGD was first used to select a female embryo to be free from Duchenne's musclar dystrophy (a severe sex linked condition)
- 1990: Human Fertilisation and Embryology Act to set up the HFEA
- 1999: HFEA carries out public consultation on the use of PGD for couples at risk of genetic diseases.
- 2000: Reccomendation that PGD should be limited to specific and serious conditions. HFEA grants licence to perform PGS
- 2001: HFEA grants licence to Mr & Mrs Hashmi (both carriers of thalassaemia). Their first son is affected with thalassaemia and they want to find an embryo free from disease that could serve as a blood donor. They hoped to harvest umbilical stem cells to cure their first son
- 2002: License issued in 2001 was deemed unlawful, and High Court Judgement that PGD must only be used in interests of child to be concieved
- 2003: Court of Appeal allows tissue typing under strict conditions
- Human Fertilisation and Embryology Act 2008
- The Parental Orders (Human Fertilisation and Embryology) Regulations 2010
- Surrogacy Arrangements Act 1985 (c.49)
- Bromham DR, Journal of Assisted Reproduction and Genetics 12 (8) 1995 "Surrogacy: Ethical, Legal, and Social Aspects"
- Royal College of Obstetricians and Gynaecologists: Male and Female sterilisation Guidelines 2004
- Recommendations on Ethical Issues in Obstetrics and Gynaecology (FIGO 2000)