Preimplantation Genetic Testing (PGT)
What is PGT, or preimplantation genetic testing?
Preimplantation genetic testing (PGT) is the process of removing a cell from an in vitro fertilization embryo for genetic testing before transferring the embryo to the uterus.
The term PGT is often loosely used to refer to any testing performed on an embryo prior to it being transferred to the uterus. There are actually three distinct types of PGT:
- Preimplantation genetic testing for aneuploidy (PGT-A), formerly known as preimplantation genetic screening (PGS)
- Preimplantation genetic testing for monogenic disorders (PGT-M), formerly referred to as PGD
- Preimplantation genetic testing for structural rearrangements (PGT-SR)
PGT-M/PGD involves removing a cell from an IVF embryo to test it for a specific genetic condition (cystic fibrosis, for example) before transferring the embryo to the uterus. PGT-A/PGS is the proper term for testing for overall chromosomal normalcy in embryos. PGT-A/PGS is not looking for a specific disease diagnosis – it is screening the embryo for normal chromosome copy number. PGT-SR looks for structural abnormalities in the embryo’s chromosomes.
History of PGT / PGD / PGS
IVF, in vitro fertilization, was first successfully used in 1978. It was not until years later that scientists began tinkering with the possibility for extracting one or more cells from the embryo to get information about the potential health of the child that might result following implantation of that embryo.
The first report of pre-implantation genetic testing in humans with a pregnancy resulting was published in 1990. Major improvements in these technologies have been developed since then. Both the embryo biopsy techniques as well as the genetics technologies used on cells removed from embryos have improved dramatically.
Much has been learned in the PGT/PGD/PGS field over the past 20 years – and much remains to be learned.
How are embryos biopsied?
There are 3 basic stages at which eggs or embryos are biopsied at the present time.
- Polar body biopsy is performed at an early stage when the polar body of the egg is removed to have its genetic material tested. A mature egg has one polar body and a fertilized egg has 2 polar bodies. Either the first polar body of the egg or the first and second polar bodies can be removed for testing.
This technique does not involve taking anything from the embryo from the cells that would become part of the fetus or the placenta. However there is still potential for damaging the developmental potential of the resulting embryo with polar body biopsy. There is some evidence that the polar bodies may help direct differentiation of cells in the very early embryo.
- Biopsy at the cleavage stage is done on day 3 of embryo development. At this point the embryo usually has 6 to 10 cells. Removal of a portion of the zona pellucida (outer shell of embryo) is performed. Then 1 or 2 cells are pulled out of the embryo for subsequent testing. The day 3 cleavage stage biopsy technique has been shown in several studies to be detrimental to embryo development. Therefore, many IVF programs have stopped doing it.
- The third type of PGTD biopsy is called trophectoderm biopsy. It is performed at the expanded blastocyst stage after the embryo has differentiated into an inner cell mass, a trophectoderm component and a fluid filled cavity.
With trophectoderm biopsy at the blastocyst stage, a small hole is made in the shell of the embryo and several cells that are precursors to the placenta (trophectoderm) are removed for testing. This technique has shown excellent results in many US IVF programs, including ours. It is now considered by many experts to be the biopsy method of choice for PGT/PGD/PGS.
Cost of PGT/PGD/PGS
Another issue is cost. PGT/PGD/PGS costs in the US vary from about $3000-$9000 plus all other associated IVF costs. There are PGT/PGD/PGS related costs associated with the embryo biopsy procedure itself and there are also costs involved for the genetics laboratory performing the chromosomal analysis on the cells.
- In order to do PGT/PGD/PGS one must first do IVF (and pay for) standard IVF costs
- Then there is the biopsy cost and the genetics analysis of the cells
- There may be additional fees for the frozen embryo transfer cycle that is done after the genetics results come back.
Which are the best PGT clinics?
In general, the best clinics for PGT/PGD/PGS will be the clinics with the best IVF success rates. It is critical to have an excellent culture system in order to get a maximal number of quality blastocysts for biopsy. Also, the skills that lead to successful IVF are the same skills that facilitate blastocyst culture, embryo biopsy and blastocyst vitrification (freezing) and successful frozen-thawed embryo transfers. Check the CDC and SART sites to find an IVF clinic in your area with a good volume of cases and high success rates. Then make sure that they have been doing trophectoderm biopsies. Come to our clinic for IVF and PGT/PGD/PGS.
Who might benefit from PGT?
In general, there are 5 main groups of patients that might utilize PGT/PGD/PGS (list is not exhaustive):
- Patients that are having IVF with advanced female age – 38 or older (common)
- Patients of any age with repeated IVF failure – usually defined as 3 or more failed attempts
- To screen for inherited genetic diseases
- Patients that are carriers of chromosomal translocations
- Patients that have had recurrent miscarriages
Preimplantation genetic testing for monogenic disorders (PGT-M) / Preimplantation Genetic Diagnosis (PGD)
PGT-M/PGD for single gene defects to prevent transmission of genetic disease
Preimplantation genetic testing for monogenetic disorders (PGT-M), sometimes referred to as preimplantation genetic diagnosis (PGD) is a technique that is used along with in vitro fertilization (IVF) and allows testing of embryos for certain characteristics such as their chromosomal makeup and also testing for genetic diseases that are passed on through families.
When one or both partners in a couple are carriers of the genetic mutation that could lead to a serious medical condition in the child, in vitro fertilization and preimplantation genetic diagnosis testing can be performed on their embryos.
In past years couples that carried a genetic mutation could choose between not having children or having prenatal testing done with either amniocentesis at about 16 to 18 weeks of pregnancy or with chorionic villus sampling at about 11 to 12 weeks to see if the baby was affected with the genetic condition. The couple then has the option of terminating the pregnancy if the baby is affected with the disorder.
Now couples that are carriers for genetic diseases have the option of having IVF and PGT-M/PGD to screen their embryos prior to transferring them to the uterus.
Not all diseases can be tested for in this manner. Single gene disorders are caused through the inheritance of a defective gene. These disorders are classified as either recessive or dominant. A recessive disorder requires 2 bad copies of the gene to pass the disease on to the baby. With a dominant single gene disorder only one copy of the defective gene is needed to cause the disease.
There are over 1000 single gene disorders that have been identified at the present time. Many of these disorders are very rare. However, some are so common in certain ethnic groups that routine screening to check whether someone is a carrier is recommended prior to getting pregnant. This is often referred to as carrier genetic testing (or screening).
- Cystic fibrosis
- Tay-Sachs disease
- Spinal muscular atrophy (SMA)
- Sickle cell disease
- Duchennes muscular dystrophy
However, there are hundreds more genetic diseases that can have single gene testing done using IVF and PGT-M/PGD. A partial list of relatively common single gene diseases is below.
Autosomal recessive disorders
Sanhoff disease, Gaucher disease, adenosine Deaminase deficiency, glycogen storage disease, Fanconi anemia, adrenal hyperplasia, phenylketonuria (PKU).
Autosomal dominant disorders
Neurofibromatosis, Von-Hippel Lindau, myotonic dystrophy, Huntington’s Disease, Marfan syndrome, osteogenesis imperfecta, Charcot-Marie-Tooth, APP early onset Alzheimer’s, polycystic kidney disease, retinitis pigmentosa, familial adenomatous polyposis, achondroplasia.
Ornithine carbamyl transferase deficiency, Fragile X, X-linked hydrocephalus.
Preimplantation genetic testing for aneuploidy (PGT-A) / Preimplantation Genetic Screening (PGS)
What is PGT-A, or preimplantation genetic testing for aneuploidy?
- Preimplantation genetic testing for aneuploidy (PGT-A), sometimes referred to as preimplantation genetic screening (PGS), refers to removing one or more cells from an in vitro fertilization embryo to test for chromosomal normalcy.
- PGT-A/PGS screens the embryo for normal chromosome number.
- Humans have 23 pairs of chromosomes – for a total of 46.
- Having an extra or a missing chromosome causes problems.
- One example is Down syndrome which has an extra chromosome number 21. This should be detected by PGT-A/PGS.
- PGT-A/PGS does not test for a specific disease such as cystic fibrosis. That process is referred to as PGT-M/PGD (please see above).
Many human embryos are chromosomally abnormal
Several studies have shown that overall about 50% of human preimplantation embryos from IVF are chromosomally abnormal. The rate of abnormalities is affected greatly by female age, as shown in the graph below. Chromosomes in eggs from older women have a significantly increased rate of abnormalities.
To a great extent, chromosomal abnormalities are responsible for failure of implantation of IVF embryos. Chromosomal abnormalities are also responsible for about 70% of miscarriages in early pregnancy.
Problems in the past with aneuploidy screening of IVF embryos
IVF and PGT-A/PGS for aneuploidy (an abnormal number of chromosomes) has been used at some clinics in the United States since the mid 1990s. However, studies showed that performing embryo biopsy on day 3 and performing the genetic analysis using FISH technology (fluorescent in situ hybridization) did not result in an increase in the chance for the patient to have a successful IVF cycle.
There were 2 main problems with that approach.
- FISH technology was usually looking at only 5 chromosomes out of 23. Therefore, the FISH test would miss many chromosomal abnormalities. This resulted in abnormal embryos being transferred after the screening test came back “normal”.
- The biopsies on day 3 were removing a cell (or 2 cells) from a 6 to 10 cell embryo. This required a relatively large hole being made in the shell of the embryo and then removal of a significant percentage of the “biomass” of that embryo (one sixth to one tenth of it removed, or more).
Recent advances allow for better IVF success rates after aneuploidy screening
Improvements in genetics technologies
Advances in the field have led to utilization of improved genetics technologies that allow assessment of all 23 pairs of chromosomes.
There are currently 4 technologies that can be utilized for assessment of normality of all 23 chromosomes:
- Next Generation Sequencing (NGS)
- Array Comparative Genomic Hybridization (aCGH)
- Single nucleotide polymorphism microarrays (SNP)
- Quantitative real time polymerase chain reaction (qPCR)
Comparative Genomic Hybridization (often referred to as CGH, or aCGH) is a microarray technology that is often used now instead of the older and far less comprehensive FISH. With microarray CGH, the actual DNA in the embryo is compared to a known normal DNA specimen utilizing thousands of specific genetic markers. This gives a more accurate result, with far fewer false normal or false abnormal results.
Some studies have determined that the error rate using array CGH technology is about 2%. FISH has an error rate of about 5-10%. Additionally, many other abnormal embryos would be reported by FISH as normal because the abnormality was in a chromosome that was not part of the FISH panel being used.
Next Generation Sequencing (NGS) is a newer technology that has been increasingly utilized for testing IVF embryos since about 2015. NGS appears to be better at detecting smaller segmental changes compared to aCGH. It is also thought to be better at detecting partial aneuploidy and small unbalanced translocations. Mosaicism is probably more likely to be discovered using Next Generation Sequencing.
Improvements in embryo biopsy techniques
Trophectoderm biopsy is done at the blastocyst stage on day 5 and 6. At this stage there are many more cells present in the embryo. This allows multiple cells to be removed from the trophectoderm (precursors to the placenta). The inner mass cells (precursors to the fetus) can be left undisturbed during the biopsy.
With trophectoderm biopsy, about 5 cells are snipped off for the genetic testing. This does not significantly weaken the embryo because it has about 70-150 cells at this stage.
The combination of these two modifications (advanced genetics and trophectoderm biopsy) has led to significantly improved pregnancy success rates for patients that want to utilize PGT-A/PGS for their IVF treatment.
Some clinics in the US have been using trophectoderm biopsy and the newer genetics technologies to screen embryos in some IVF patients. The results seen in some IVF programs (including ours) have been very promising.
We are seeing substantially improved ongoing pregnancy rates in patients that are having trophectoderm biopsy performed at the blastocyst stage with subsequent freezing of their embryos.
A frozen thawed transfer cycle is done after the chromosomal analysis results come back.
Uterine receptivity issues
There is some interesting speculation that the uterine lining could be less receptive during a stimulated cycle as compared to the controlled or “artificial” embryo replacement cycle.
- Some fertility doctors believe that transferring embryos in a controlled cycle (using frozen embryos) gives a higher pregnancy rate than in a “fresh” cycle
- This has not yet been carefully studied with multiple controlled clinical trials
- The improved success rate seen following blastocyst biopsy and comprehensive chromosomal analysis is mainly due to the benefit of transferring chromosomally normal embryos
- In some women there is some additional benefit derived from transferring the embryo(s) in a frozen thawed cycle rather than in a stimulated cycle
Which couples should we be offering aneuploidy screening to?
- This is currently an evolving issue in the field of reproductive medicine
- Some potential candidates could be:
- Women over (about) 35 years old that are doing IVF
- Patients at any age that have failed multiple IVF cycles. They want answers about why they are failing. They also want to know what to do in order to improve their chances to have a baby. PGS sometimes provides answers.
- PGS can also provide a “weeding out” of the abnormal embryos. For example, if one embryo out of 6 is chromosomally normal and 5 out of 6 are abnormal – we transfer the one normal embryo and should have a very good chance for a baby.
- Couples with recurrent miscarriages
- Anyone that wants to use this technology to screen their embryos in order to transfer one that tests as chromosomally normal – and therefore has a high chance to implant and make a baby
PGT-A/PGS for patients that are carriers of chromosomal translocations
This is a rare situation in which a couple knows that one of them has a chromosomal arrangement called a balanced translocation. When someone has a balanced chromosomal translocation they are normal – until they try to have a child.
When the chromosomes in their sperm or eggs join with those of their partner in the fertilized embryo, they have a high percentage of chromosomal abnormalities.
These embryos are at very high risk for miscarriage or could result in the birth of a child with birth defects. This is another situation where PGS can help. By having IVF aneuploidy screening of the embryos, they can have chromosomally normal embryos transferred. This greatly reduces their risk for miscarriage and birth defects.
What are some concerns with PGT/PGD/PGS?
- The embryos could be traumatized by the biopsy procedure – particularly for day 3 embryo biopsies.
- There is some evidence that carefully performed trophectoderm biopsies done on day 5 and day 6 blastocysts might not weaken the embryo at all.
- As with any new technique and technology, there is a “learning curve.”
- Some technicians will be more proficient at the biopsy procedure.
- Some genetics labs will be more proficient at the diagnostic component after the cells are removed – giving a higher percentage of accurate results.
- Therefore, there could be large differences between centers performing these techniques, and even between technicians within the same IVF center.
- Mosaicism can complicate matters. An embryo is a mosaic if there are 2 (or more) different chromosomal patterns in the cells of that embryo.
- There is evidence that mosaic embryos sometimes “self-repair”, or possibly designate abnormal cells preferentially to the placenta instead of the fetus. More research on mosaicism is needed.
If so many embryos have abnormal chromosomes, what is the risk for having a chromosomally abnormal baby for couples that do not have PGT-A/PGS with their IVF?
- In humans, there is a natural selection process that prevents implantation of abnormal embryos.
- The large majority of chromosomally abnormal embryos will arrest in early development and not survive long enough to implant in the female partner’s uterus.
- Some will implant and result in early miscarriages.
- An extremely small percentage can continue further into pregnancy and could progress to a live birth of a chromosomally abnormal baby – if not detected during pregnancy.
- Testing for this can be done in early pregnancy:
- Non-invasive screening tests such as blood tests or ultrasound evaluation of the baby
- Invasive testing in early pregnancy such as CVS or amniocentesis.
- CVS, chorionic villus sampling is done at about 11-12 weeks of pregnancy.
- Amniocentesis can be done at about 16-18 weeks.
In the general population, the risk for a live birth with a chromosomal abnormality is:
|Female Age||Risk of a live birth with any chromosomal abnormality|
The overall risk for a chromosomally abnormal live birth does not appear to be increased by having IVF or IVF with ICSI (intracytoplasmic sperm injection).
What else can be done to help couples to have a successful IVF outcome with a normal, healthy baby?
Grading of embryo quality in the IVF lab can help pick the chromosomally normal embryos for transfer. Embryos that are “graded” on the higher end of the scale have lower rates of chromosomal abnormalities as compared to those embryos that have lower grades.
Embryos that make normal looking blastocysts on day 5 have lower rates of chromosomal abnormalities as compared to those embryos that do not make blastocysts. Therefore, some clinics are using blastocyst culture and transfer in order to be able to select embryos with higher implantation potential and lower rates of chromosomal abnormalities as compared to transferring embryos back on day 2 or day 3.
Women that are of advanced reproductive age (such as 38 and older) will sometimes have chromosomal abnormalities in a very high percentage of their remaining eggs.
Rather than having IVF with PGT/PGD/PGS using their eggs, many of these women will need to do IVF with egg donation in order to have a successful pregnancy.
Because egg donors are young (usually under 30) and chromosomally abnormal eggs are much less common in young women, PGT/PGD/PGS is generally not used with donor eggs.