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Preimplantation Genetic Diagnosis (PGD)

Posted in Q&As on June 18th, 2008.

What is Preimplantation Genetic Diagnosis (PGD)?

Preimplantation Genetic Diagnosis (PGD) is a procedure for genetically testing early-stage human embryos.  Its main use is to test for inherited genetic conditions and chromosomal abnormalities, such as Down Syndrome. It is also used to find embryos that are a tissue match for siblings who are affected with certain genetic conditions.

How is PGD carried out?

To carry out PGD, an embryo is created via IVF (in vitro fertilisation). One or two cells are removed when the embryo is at the 6-10 cell stage (usually on the third day after fertilisation). These cells are tested for particular genetic conditions or chromosomal abnormalities, or tissue typing is carried out. Unaffected embryos or those with the desired tissue profile are then transferred to a woman’s uterus, and if all goes well, a pregnancy results.

What genetic disorders can PGD test for?

There are many genetic disorders that PGD can test for. The first category is single gene disorders, such as cystic fibrosis and Huntington’s disease. PGD for these conditions is usually carried out only when there has been a history of the condition in a particular family.  The next category that PGD can test for is sex-linked genetic disorders, such as haemophilia or Fragile-X syndrome. Sex-linked genetic disorders occur when the gene involved in the condition is located on the X or Y chromosome. Females have two X chromosomes, while males have one X and one Y. Because of this, sex-linked genetic disorders can affect males and females differently. For example, if a faulty gene is on the X chromosome, males can be affected, while females may not be. This is because males have only one copy of the gene, while females have another copy that can compensate for the faulty one.

In this case, PGD is used to determine the sex of the embryo, meaning that only embryos of the non-affected sex are implanted. This technique is an advantage where no specific test for the condition itself is available. PGD for sex-linked conditions is also usually tested for only when there has been a history of the condition in a particular family.

The third category that PGD can test for is chromosomal disorders. These include translocations and incorrect numbers of chromosomes, such as an extra chromosome 21 (which causes Down syndrome). Sometimes, PGD is carried out because translocations run in the family, and other times it is carried out because a woman is older (and therefore at greater risk of having a baby with chromosomal abnormalities) or has had repeated miscarriages or fertility problems.

Why is PGD used for tissue typing?

As well as testing for genetic disorders, PGD can also be used for tissue typing to select embryos with a specific Human Leukocyte Antigen (HLA) profile. This is carried out when an existing sibling has a genetic condition which means they are in need of donor tissue, which can be provided by a sibling who is a tissue match. An example of such a condition is Faconi’s anaemia. Once selected, the embryo is implanted into the mother’s uterus with a view to creating a child whose tissue can then be used (within strict boundaries) to help their sibling.

What regulations are there for the use of PGD?

There are government guidelines for the use of PGD in New Zealand. Clinics that offer PGD have to be approved by the government’s Ethics Committee on Assisted Reproductive Technology, and some uses of PGD require submissions to the committee for approval on a case by case basis before they can be carried out. For example, ethics approval is not needed for serious single gene, sex-linked and chromosomal disorders, but it is required for tissue typing, and there are several conditions on this use of it.

Some potential uses of PGD are banned altogether. For example, PGD cannot be used for sex selection for social reasons or to genetically alter embryos. It also is not allowed to be used to select for embryos with a particular genetic impairment (for example, if a parent wants a child with the same genetic condition as they have).

What ethical issues are associated with PGD?

A number of ethical questions have been raised regarding PGD, which have been the subject of much discussion. Some of these are:

  • Is PGD safe, or could it contribute to congenital disorders? No serious short term risks have been found, but long-term risks are not yet known as the technology is still relatively new.
  • Does PGD discriminate against those with a disability? PGD involves discarding affected early-stage embryos.
  • Is this ethically acceptable? PGD is expensive, so without funding is not accessible to all.
  • Is this fair? Should it be publicly funded? Using PGD for HLA tissue typing involves selecting an embryo that is a tissue match for an existing sibling, thus potentially being the source of a life saving therapy.
  • Is it acceptable to select embryos on this basis?

What is the advantage of PGD over other screening methods?

PGD has advantages over other methods of screening for genetic conditions. An alternative to PGD (which was developed earlier) is prenatal diagnosis, which involves genetic testing at the fetal stage, rather than the embryo stage. With this option, pregnancy has already occurred and termination is usually the only option for avoiding an affected baby if testing reveals the fetus has a genetic defect. In PGD, the genetic testing occurs much earlier, with embryos that have not yet been implanted in the mother’s uterus, therefore avoiding the need for a termination.

Could PGD be used for non-medical reasons?

Potentially, PGD could be used for much more than is currently permitted, and this is one of the concerns raised about the technology. Currently, it can only be used for medical reasons, but it could in principle be used to select for or against any characteristic with a known genetic origin. Concern has been raised that it could be used to select for or against traits such as sexual orientation, appearance, athletic ability or intelligence. Currently, though, not only is it forbidden to use it for these purposes, it is not scientifically possible, as there is no known, simple genetic basis for these characteristics. It also cannot be used to create new traits, only to select for or against existing ones.

Is PGD available overseas?

PGD is also available overseas, in countries such as Australia, the United Kingdom, USA, Canada, Denmark, France, Belgium, Hungary, Sweden, Greece and Spain.

Sources:

http://www.moh.govt.nz/moh.nsf/0/E21D842F – CF50CC256FC4
http://fertilityassociates.co.nz/services11.asp
Guidelines on Preimplantation Genetic Diagnosis (Prepared by the National Ethics Committee on Assisted Human Reproduction, March 2005)

This Science Byte was reviewed by Professor Kenneth McNatty (School of Biological Sciences, Victoria University)

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