PGD (Pre-genetic Diagnosis) is used in tandem with in vitro fertilization (IVF) to test embryos for genetic disorders prior to transfer into the uterus.
What is PGD?
PGD is a general term that includes all methods of removing and testing the cells of an embryo before it is replaced in the uterus during an IVF cycle. We are able to look at whole chromosomes or individual genes.
What can we test for?
* The wrong number of chromosomes (Comprehensive Chromosome Screening (CCS))
* Translocations
* Single Gene defects
PGD (Pre-Implantation Genetic Diagnosis)
From 2004-2009 there have been over 30,000 cycles of IVF using PGD in the United States alone, resulting in the births of many healthy babies. This number still represents just 5% of the total number of IVF cycles carried out in the USA. As technology improves the number of patients undergoing testing on their embryos is set to rise dramatically.
When do we test the embryo?
Day 5 or day 6 after egg retrieval some of the embryos will develop into a blastocyst (consisting of about 70-100 cells) At this point the cells which are going to make the baby and the placenta have been decided. Choosing those cells to test gives us more accurate results. During the biopsy 3-10 cells are removed from outer part (trophectoderm) of the blastocyst.
Sperm injection (ICSI) is required for fertilization to prevent genetic contamination from extra sperm outside the embryo.
When do we replace embryos in the uterus?
If embryos are biopsied on day 5 and 6 the cells reach the testing lab on day 7.
We get results back on approximately day 8. Embryos need a uterus to survive by day 6 or 7 at the very latest. Biopsy at the blastocyst stage therefore requires embryos to be frozen (vitrified) and then transferred in a frozen embryo cycle (FET) at a later date, once we know which embryos are normal. The frozen cycle process takes about 8-12 weeks.
Comprehensive Chromosome Screening (CCS)
What is Comprehensive Chromosome Screening (CCS)?
CCS is the process of analyzing the cells of the embryo to determine if the embryo has a normal number of chromosomes PRIOR to replacing the embryo inside the uterus. This test of an embryo’s genetics can reduce the chance of transferring embryos with abnormal numbers of chromosomes such as 3 copies of chromosome 21 (which may result in a Downs’ Syndrome baby) or other permutations of chromosome numbers which may end in a miscarriage.
Who may benefit from this screening process?
* Patients who have had multiple miscarriages
* Patients who have had previous failed IVF cycles
* Patients who we need more information about their embryos to help us to diagnose the cause of infertility
* Patients who want to know if the embryos we freeze for future use are chromosomally normal
This test does not detect non-chromosomal birth defects. It also does not guarantee that the embryo will implant and not miscarry.
HOW is CCS performed?
CCS is done via CGH (comparative genomic hybridization).
All chromosomes including X & Y are analyzed and compared to a sample of “normal” DNA stuck to a slide. The embryo DNA is labeled green and the normal DNA is labeled red. If the embryo has a normal number of chromosomes, then there will be equal amount of embryo (green) DNA and “normal” (red) DNA, and the test result should appear yellow. If embryo is missing a chromosome, that area appears more red (i.e. extra “normal” DNA) If the embryo has an extra chromosome, that area appears more green
Options for testing:
* Option 1. “Embryo Banking” IVF cycle:
o Test all day 5 embryos (blastocysts) and freeze them all.
o This means NO embryo transfer for that cycle.
o Once genetic results are known (after 1 week), set up frozen embryo transfer cycle
+ This is most useful for patients who have had repeated miscarriages or several failed IVF cycles in the past
* Option 2. Fresh IVF cycle with testing of spare embryos:
o Transfer 2 fresh embryos from IVF cycle without testing them.
o Biopsy the remaining day 5 and 6 blastocysts that would otherwise have still been cryopreserved (vitrified)
o One week later, once we have genetic results, discard abnormal embryos and only keep normal embryos that could potentially produce a baby.
+ This is most useful for patients who have a high chance of pregnancy with their fresh embryo transfer (younger patients, patients using donor egg) and who want to know the status of their frozen embryos for future cycles.
Translocations
Patients who have had miscarriages or years of unexplained infertility may have a chromosome defect called a translocation. This is tested for by doing a karyotype on the blood of each partner. There are different types of translocation:
Reciprocal Translocations: Approximately one in 625 individuals has a reciprocal translocation. These translocations involve any of the chromosomes. This type of translocation is when chromosomes have swapped material. Breaks occur anywhere in the chromosomes allowing for pieces to be interchanged between them.
Robertsonian Translocations: Approximately one in 900 individuals has a Robertsonian translocation. These translocations involve chromosomes 13, 14, 15, 21 or 22. These chromosomes have a unique structure in that they are primarily made of a bottom half. This translocation results from fusion of two of these chromosomes such that the two bottoms are attached.
If a chromosome pair ends up with all the parts of the broken chromosome in equal amounts then the translocation is balanced and can result in a normal baby. If it does not have equal amounts of the broken chromosome it is unbalanced and will end in miscarriage or no pregnancy. We can do IVF with PGD and replace only balanced embryos which have a good chance of making a baby. Depending what kind of translocation a patient has will define what the ratio of unbalanced to balanced embryos should be.
The proportion of abnormal embryos found from patients who have a balanced translocation can range from 0 to 100%, with an average of 65% abnormal embryos. Fewer embryos will be available for transfer than for patients who do not have a balanced translocation and therefore the pregnancy rates may be lower for this patient group.
If the individual with the translocation is a male, we recommend FISH analyses of sperm prior to undergoing PGD. Determination of the percentage of unbalanced sperm will allow for estimates of the percentage of embryos that will be unbalanced, and therefore, determination of whether PGD is the best option.
Single gene defects
Patients who have a known inherited genetic disease can do IVF with PGD to weed out embryos affected with the particular disease. Examples of such diseases are: cystic fibrosis, Huntingtons disease, Marfan syndrome, Polycystic kidney disease, Duchennes muscular dystrophy plus hundreds more. There has to be a family history of a disease so that we can test for a particular gene.