Preimplantation Genetic Diagnosis

September 6, 2010Carole 2 Comments »

Preimplantation Genetic Diagnosis (PGD) is a technical procedure that was invented to determine if an embryo is likely to carry a genetic disease or other gene of interest. Based on this information, embryos without the genetic mutation are selected for transfer.  PGD can only be done a part of an in vitro fertilization cycle because the test is done on a single cell (sometimes two) removed from the early embryo, typically at the eight-cell stage.

How does PGD work? Fertilized eggs are cultured through early embryo development to day 3 of culture. On day 3, a typical embryo has approximately 8 cells. At this stage of development, the embryo’s cells are totipotent, meaning each cell has exactly the same genetic potential and theoretically, each cell could  give rise to one of eight identical octopulets. Therefore, each cell is likely to be* a genetic mirror image of every other cell in the embryo and so any of the cells could be analyzed to get the whole genetic picture.

*Mosaicism as a source of error in PGD. Sometimes there is an error in the genetic division of genes and the eight daughter cells are imperfect clones of one another. This imperfect replication is called genetic mosaicism. In that case, the cell sampled might not be representative of the entire embryo, resulting in erroneous information about the genetic makeup of the rest of the embryo and you might wrongly assume the embryo is normal (or abnormal). Some programs have elected to biopsy two cells from the embryo -to conform the diagnosis- but sometimes removing two cells slows down the embryo’s development and reduces the pregnancy rate.

Another approach to try to improve accuracy is to take more cells for the test. By waiting until day 5 of development when the embryo has between 50 and 100 cells, it is possible to biopsy relatively more cells from the trophectoderm, a portion of the embryo that gives rise to the placental tissue and not the fetus. Using trophectoderm tissue also has some drawbacks because there is some evidence that genetically mosaic cells may preferentially end up in the trophectoderm layer, again leading to genetic results that may not provide a true picture of the embryos genetic status.

Because of these test limitations, most programs suggest that amniocentesis be used as a back-up test once the pregnancy is established.

Embryo biopsy technique to sample a cell. On day 3 of culture, a highly trained embryologist uses either a laser or a acid solution to gently open a cell-sized window in the shell or zona pellucida surrounding the eight-cell embryo. Using a tiny glass holding straw (called a pipette) to hold the embryo in the ideal orientation and using a second acid-filled pipette or laser to create the hole, the embryologist opens a window and gently coaxes out an intact cell for analysis. It is important that the cell removal or biopsy of the embryo is performed before the embryo moves on to the next stage of development, the morula stage. At the morula stage, the poor embryo looks disorganized and disheveled, clearly on its way to better things. The morula is like the embryo equivalent of the gangly pimply-faced teenager.  But like a typical teenager, it is busy making lots and lots of (social) connections with adjacent cells and is more tightly attached to its neighboring cells than ever before. At this sticky stage, it is much more difficult to detach a cell from it’s neighbors without causing damage. If you are interested you can watch videos of day 3 embryo and trophectoderm biopsy  here.

Will removing one cell hurt the embryo? If the biopsy is done poorly, it is possible to damage neighboring cells. However, if done correctly, there should be no damage to the cells and the embryo should continue to divide without a problem. It is not uncommon for cells in the embryo to spontaneously die and fragment into pieces and the rest of the embryo motors on at this early stage. However, if fragmentation involves too many cells, it can cause problems with continued development. Patients have asked me if removing a cell from the eight cell embryo will result in the child missing an essential part, like an arm or leg. Because of the totipotent potential of each of the eight cells, this won’t happen. In nature, some cells might die at this early stage, so nature has built in some redundancy. Later in development, loss of cells that have started to mature into a tissue can have profound developmental consequences.

Testing the Cell. The biopsy procedure is done in the fertility lab but the genetic testing is usually done at a special genetic testing lab that specializes in PGD. Your local hospital lab that does chorionic villus sampling (CVS) or amniocentesis is probably not going to do PGD. Depending on the type of test, the single test cell is either dissolved  in a test tube for single gene tests or placed on a glass slide to look for the presence or absence of chromosomes (aneuploidy testing). The sample is literally shipped off to the testing lab by Fedex or a medical courier. The lab completes their tests in a day or two and the fertility lab gets a written lab report faxed back with the genetic test result for each sampled embryo. The lab report is discussed with the patients and the patients make a decision how to proceed. A transfer of the embryo or embryos selected is typically scheduled for day 5 when they have reached the blastocyst stage. Sometimes, some of the tested embryos fail to progress and you may have progressive embryos that carry the  mutation and stalled embryos that tested negative for the mutation. Hopefully, there is at least one normal or unaffected progressive embryo available for transfer.

What should we test embryos for? Here’s where it gets problematic and controversial.  PGD was developed in the 1990’s to provide a test to identify embryos that carried mutations for very specific genetic diseases that would cause death or life-long misery.  Genesis Genetics is a pioneer in the detection of genes that cause disease in cells from the early embryo. You can see a list of the diseases that can be detected in embryonic cells using probes manufactured by Genesis Genetics. In my previous work as a lab director, I saw how this  amazing group of scientists and genetic counselors worked with our PGD cases and was always impressed with how well they handled the concerns and needs of our patients. (And, in case you were wondering-  I do not now and have never had a financial interest in Genesis Genetics but I think they are a great example of doing the right thing the right way. They never lost sight of the families involved).

We all roll the genetic dice when we have kids. Sometimes, if we are unlucky, we roll the genetic dice and our genes collide and cause illness in our children. Sometimes the illnesses are relatively minor or treatable. Other times, our children are killed by a genetic disease. If there was a way to avoid passing these misery-generating or deadly genes to our children, wouldn’t we choose to do so and spare them suffering or death? Wouldn’t any parent want the healthiest possible start for their children? Unlike the selfish villains commonly portrayed in the media who are looking for perfect children, parents looking to avoid passing genetic disease onto their children have in many cases already lost a child to a horrific disease and simply want to avoid burying more of their children.

PGD for other reasons. PGD is performed for other reasons in addition to looking for single gene mutations that cause disease. There is usually a variety or opinions as to if and when PGD should be used for these indications.

Sex selection. PGD is increasingly used for sex selection which has more people raising ethical concerns. Sex selection or identifying the gender of the embryo and transferring embryos of the desired  gender is commonly done for two reasons: either to prevent sex-linked diseases or for “family balancing”. When used to identify (and not transfer) male (or female) embryos  that carry a X or Y linked disease, it seems to be an extension of the disease prevention indications and less problematic. PGD done for family balancing , on the other hand is not performed for a medical indication. Couples who want PGD for family balancing typically have at least one child of one gender and now want to have children of the opposite gender. Although it is often depicted as a nefarious method to prevent the birth of female children, I haven’t seen it used that way.  At least in the practices I have been associated with, patients requested it just as often to try to have that much desired girl after having one or more boys.

I did see one case where a couple wanted to determine the birth order of their children, wanting the first child to be male and the second child to be female, which was problematic for some of us in the clinic because it seemed a comparatively trivial reason to use PGD. But people have resorted to extreme measures to control the composition of their family. If a family is determined to have a child of a certain gender, it seems to me that PGD is more ethical than other alternatives that have been used such as ultrasound and abortion of an existing pregnancy or even worse, infanticide.

Sex selection by sperm sorting. Sorting sperm is also touted as a method of gender selection but it is much less than 100% certain because the sperm sample is enriched to perhaps 80-90% for one gender or another, leaving 10-20% of a sperm of the non-desired gender to fertilize the egg. Sometimes families have only girls or only boys because the father is mostly producing sperm for one gender, not the other. Below are three articles describing small but interesting studies that found that fathers with high-stress jobs were more likely to have female children. From the review Sex Selection published by the Human Fertilization and Embryology Authority of the United Kingdom, three references to studies suggesting that fathers working in some occupations, particularly if they are stressful occupations may father more girls than boys.

  • Lyster, W.R. 1982. Altered sex ratio in children of divers (letter). Lancet. 8290: 152.
  • Goerres, H.P. and Gerbert, K. 1976. Sex ratio in offspring of pilots: a contribution to stress research. Aviation Space Environ. Med. 47: 889-892.
  • Little, B.B., Rigsby, C.H. and Little, L.R. 1987. Pilot and astronaut offspring: possible G-force effects on human sex ratio. Aviation Space Environ. Med. 58: 707-709.

Sometimes, it’s really tough to fight mother nature.

Aneuploidy Screening. Preimplantation genetic screening for abnormal chromosomes is another use of genetic testing of embryos. As women age, our eggs naturally have more chromosomal aneuploidies which simply means that eggs are more likely to have more (or fewer) chromosomes than they should have. For instance, Down’s syndrome is caused by an extra chromosome number 21 (which is why it is also called Trisomy 21) and is more common in children born to older mothers. For decades, amniocentesis, CVS sampling and ultrasound  have been used to detect Down’s syndrome in the developing fetus and some expectant parents have chosen elective abortion rather rather than give birth to a child with Down’s Syndrome. In my opinion, avoiding establishing a pregnancy with a Trisomy 21 embryo seems to be less ethically troubling than aborting an existing pregnancy.

Aneuploidy screening to prevent recurrent pregnancy loss. Because aneuploid embryos are more likely to result in early pregnancy losses, it seems to make a lot of sense to avoid transferring embryos that are doomed to fail. In practice, when preimplantation genetic screening is used for this reason, it hasn’t been a surefire way to avoid recurrent pregnancy rate and may actually reduce the pregnancy rate.

Older women are more likely to have aneuploid embryos so they are the obvious patient group to benefit from PGS, but they also tend to have the fewest embryos. PGS could show that none of the embryos are normal. Given the possibility of false positive results from genetic mosaicism, many older patients often decide to roll the dice and have a transfer, hoping the test was wrong.  A test that is not used to make clinical decisions is not a good test.

Varying medical practice opinions Re: Aneuploidy screening to prevent miscarriage. Some doctors have decided after trying PGS in their clinics, that it wasn’t really useful. Other clinics still offer it and stand behind it so the verdict is still out.  One doctor’s summary of various studies regarding the mixed results using PGD for aneuploidy screening and his thoughts on whether PGS is useful can be found here. Another physician’s explanation of testing and his endorsement of PGS for recurrent pregnancy loss. Be sure to ask your doctor about the pros and cons of using PGS for recurrent pregnancy loss.

Tissue matching for bone marrow transplant. Some diseases can be cured by a bone marrow transplant, if a suitable donor can be found that is a good genetic match. Some families have used PGD to have a baby whose cord blood (otherwise discarded at birth) could be used for a bone marrow transplant for an older ill sibling. Human leukocyte Antigen or HLA is the type of matching that is necessary to identify a compatible donor. Probes can be made to detect specific HLA antigens on a test cell from an embryo to determine if that embryo will be a tissue match for the existing sibling. This use of PGD is controversial because it has been portrayed as creating a child for parts as was depicted in the fictional film “My Sister’s Keeper”. While it may make thrilling and dramatic film, the truth is more benign and understandable in human terms. Parents with a sick child who want more children anyway can use PGD to have more kids and Oh, by the way, save the older child too.

The whole idea that normal loving parents who love their first child would create a second child for parts is just insane. Are we to assume that parents automatically love the second and third child less? I would think that the real danger might be that you love the child that kept your family whole even more, not the other way around. But that would make for a boring film, I guess. Dr. Marcus Hughes, president of Genesis Genetics seperates fact from fiction in this comment on his website. In actual PGD with HLA matching, the newborn’s umbilical cord blood is used for the treatment and no further interventions are expected of the second child. Doing to the second child what was depicted in “My Sister’s Keeper” is simple child abuse and there are laws against that.

Designer Babies. Here is where people really get upset but IMO, it’s a whole lot of media hype without a lot of science. I would be really surprised if we could ever make designer babies even if we wanted to. The “designer” traits people want in kids like beauty and brains and athleticism require a boat load of genes all acting together. Frankly, we don’t know which genes to pick, if we could pick them and how and when to turn them on etc etc.  And even if we could pick a bunch of genes we want from a Chinese Menu of traits, there’s this little thing called environment and self-determination that would foil that game plan. Even identical twins are actually NOT identical in every respect, especially if they are raised apart. I don’t lose sleep over this particular boogey man because claims that designer babies can be produced is just a scam sold to gullible people who don’t understand that you never get what you expect with your kids anyway. They are their own unique selves in utero even before they are born.

© 2010, Carole. All rights reserved.

2 Responses to this entry

  • Melissa Says:

    Hi Carole –
    First I’d like to thank you for this blog – your posts are incredibly informative without ever “talking down” to us patients.

    I’m curious about this post, and particularly if you have any new information or changed opinions in the almost two years since you wrote it. I am 38 with a history of 4 miscarriages in the last three years (we have no living children). Our losses have been at 12 weeks, 8 weeks (fetal testing revealed trisomy 13), and two at 5 weeks. My husband and I are currently undergoing IVF/ICSI (DH has 1% morphology) and, after lots of research and lengthy discussions with our RE, we have decided against using PGS for aneuploidy screening. However, there are a lot of women in my online support group in similar situations (either AMA, RPL, or both) who have chosen to use PGS in the hopes of reducing their risk of miscarriage. After several heated online discussions on the topic, it seems to me that there are a lot of REs out there recommending PGS to their patients as a magical panacea when there is not a lot of evidence, if any, supporting its use in these situations (and frankly, a lot of IVF patients who don’t do their own research and don’t question the recommendations of their RE). I am curious as to your thoughts on that. The most recent information I can find about it is at least two years old, and the current ASRM position statement on it dates to 2008. Are there any newer studies on PGS for aneuploidy screening?

    As a matter of interest, of the three women in my group who have made it through the testing already, two had no normal embryos to transfer. The third had only one, and she later miscarried at 10 weeks, due to a trisomy of a chromosome that was not tested in her PGS. Perhaps it’s just confirmation bias on my part, but I do have to wonder if these women are in any way benefiting from PGS….

    Thank you,
    Melissa

  • Carole Says:

    Hi Melissa,
    Your questions are good ones and your concerns are valid. You’ll want to read this paper,”Preimplantation genetic screening: a systematic review and meta-analysis of RCTs” free on-line at this link. The authors did a meta-analysis, meaning they compiled data from numerous random controlled trials (RCTs) which are considered the most rigorous type of research study for clinical applications. Their conclusions are “There is no evidence of a beneficial effect of PGS as currently applied on the live birth rate after IVF. On the contrary, for women of advanced maternal age PGS significantly lowers the live birth rate. Technical drawbacks and chromosomal mosaicism underlie this inefficacy of PGS. New approaches in the application of PGS should be evaluated carefully before their introduction into clinical practice. ” Now the trials reviewed employed mostly day 3 biopsy and FISH instead of the newer microarray methods using blastocyst biopsy material. Proponents of PGS hope that the new approaches of blastocysts biopsy coupled with micoarray technology will prove more effective in increasing the live birth rate. The authors of the first paper are skeptical and call for more trials using these new approaches before they are put into widespread us. This paper was published in 2011. Since then, I think there is little evidence that the enthusiasm for offering PGS has declined even though there is still no conclusive evidence that this technology is appropriate for most patients. Here is a newer 2012 article looking at PGS cycles using microarrays and the jury is still out. Honestly, the problem is as much in patient selection as in the technical limitations. The ideal patient for PGS testing has a large number of embryos available for testing so that after the losses due to biopsy problems, lab errors (no signal), and embryos that don’t progress, the patient still has some normal stage-appropriate embryos to transfer. Unfortunately, the patient who is older rarely has this abundance of embryos to choose from. In theory, a great option. in practice, not effective for everyone who might benefit. Hope this helps. Good Luck with your next cycle! Carole

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