Tuesday, February 15, 2011

Preimplantation genetic diagnosis (PGD)

The subject of PGD has come up several times in the last couple of years regarding the potential parent’s concerns with Kennedy’s Disease. At the KDA conference this year, attendees heard of the success of one family using PGD to terminate the defective gene in their lineage.  

Since I know very little about the subject, I went to Wikipedia to learn more.  From there I read more about it at Suite 101 and the Advanced Fertility Center of Chicago. Below are excerpts from these three websites. I can understand why potential parents might want to know if their child has the defect, but I also can see the possible risks of the process since it is a relatively new technology and appears to be not always accurate.

Because I know so little about this procedure and am somewhat biased, I will just provide information that I found on the internet and let you, if interested, investigate the process further.


In medicine and (clinical) genetics pre-implantation genetic diagnosis (PGD or PIGD) (also known as embryo screening) refers to procedures that are performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization. PGD is considered another way to prenatal diagnosis. Its main advantage is that it avoids selective pregnancy termination as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation.

Suite 101
PGD Testing for Familial Disease
It’s understandable that a family that carries a devastating disease would want to eliminate this from future children. But the problems of using PGD in these cases are similar to those for gender selection; the results may not be accurate, and there’s a chance of damaging a normal embryo. Genetic testing laboratories can only test for some genetic abnormalities, not all, so it’s possible an embryo that doesn’t have a test for disease could have another disease. Parents should also consider that the deadly diseases of today may have cures by the time their child needs them.
The Unknowns of PGD Testing
Some studies have shown that embryos with abnormal chromosomes on day three, when the cell is removed for PGD testing, may "self correct” and have normal chromosomes by day five. Obviously, this isn’t always the case, but it could mean that normal embryos are destroyed. 

Advanced Fertility Center of Chicago
What is PGD, or preimplantation genetic testing?
Preimplantation genetic diagnosis involves testing done on in vitro fertilization IVF embryos prior to transferring them to the mother's uterus. The testing is done either to check for a specific genetic abnormality (such as a disease like cystic fibrosis), or it can be done to determine if the embryos are chromosomally normal (also called aneuploidy screening). 
PGD is done by removing 1 or 2 cells usually at about the 8-cell stage (day 3 after fertilization). A hole is made in the shell of the embryo, and then a pipette is used to suck the cell(s) away from their neighbors - removing them from the embryo so that testing can be done. This process is called embryo biopsy or blastomere biopsy. Some programs biopsy the polar bodies of the egg, rather than the cells from the early embryo. 
Specialized techniques are then used to either check for the genetic disease in question, or to investigate for overall chromosomal normality. The testing can generally be completed in 1 day. Therefore, embryos can be tested on day 3 after fertilization and transferred back to the wife on day 4 or 5 after the results are back. 

What are some of the additional problems and concerns with PGD at this time?
The embryos are traumatized with PGD. The question remains unanswered as to how often they can "recover enough from the beating" to retain viability. It seems that many embryos do not recover completely. Said another way, too many normal, strong embryos are weakened to the point of being non-viable by the embryo biopsy procedure.
As with any new technique and technology, there is a "learning curve". Some technicians will be more proficient at the biopsy procedure. Some labs will also be more proficient at the diagnostic component after the cells are removed - giving a higher percentage of accurate results. Therefore, there will potentially be large differences between centers performing these techniques, and possibly even between different technicians within the same center.
PGD test results are not always correct
  • Sometimes the embryo has abnormal chromosomes, but PGD testing shows a normal result
  • Sometimes the embryo has normal chromosomes, but PGD testing shows an abnormal result
  • Therefore, some chromosomally normal embryos will be discarded, and some chromosomally abnormal embryos will be transferred after PGD
IVF and PGD Costs - How much is PGD?
  • PGD is expensive and costs about $3000 to $5000 in the US, in addition to all of the other IVF costs

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