Preimplantation Genetic Testing
Preimplantation genetic diagnosis (PGD) & preimplantation genetic screening (PGS)
Preimplantation genetic testing at a glance
- An embryo formed through in vitro fertilization (IVF) can be tested for genetic abnormalities before it is transferred into a woman’s uterus and ultimately lead to a live birth.
- Preimplantation genetic testing can dramatically increase the odds that only an embryo free of genetic defects is implanted in the woman’s uterus, and thus improve the likelihood of a healthy pregnancy and baby.
- Preimplantation genetic diagnosis (PGD) tests an embryo for a single, specific genetic disorder the parent(s) may be carrying, such as a cystic fibrosis or a balanced translocation or inversion.
- Preimplantation genetic screening (PGS) tests an embryo for a range of genetic disorders, such as Trisomy 21 (Down syndrome), caused by chromosomal aneuploidy, meaning there is either an extra or a missing chromosome.
- Aneuploidy accounts for approximately 71 percent of all spontaneous miscarriages and also plays a significant role in failure to achieve a pregnancy.
What is preimplantation genetic testing?
The PGD and PGS forms of preimplantation genetic testing were developed to prevent transfer during in vitro fertilization (IVF) of genetically abnormal embryos into the uterus. Depending on maternal age and reproductive history (recurrent miscarriages) in approximately 35 to 85 percent of IVF cases, the embryos have flawed chromosomal composition and result in failed pregnancies.
By identifying in the embryo a condition known as aneuploidy (too many or too few chromosomes) or a specific genetic disorder the parent(s) are at risk of carrying, preimplantation genetic testing has dramatically improved the success rates of IVF, increasing successful pregnancies and births of healthy infants.
Preimplantation genetic testing began in 1990 to evaluate embryos created through IVF for genetic traits known to exist in parental DNA. This early form of preimplantation testing expanded to include preimplantation genetic screening (PGS) for aneuploidy (also called comprehensive chromosomal screening), a significant reason for pregnancy failure in IVF. PGS evaluates for the same genetic problems sometimes searched for in normal pregnancies using any of three prenatal tests: chronic villus sampling, amniocentesis and cell-free DNA testing.
Both PGS and PGD are often recommended for people who have experienced infertility, have had recurrent loss of pregnancy through miscarriage, or have had unsuccessful IVF treatment.
Benefits of recent technological advances
New techniques for preimplantation genetic testing have resulted in more accurate diagnoses, easier processes and more detailed genetic information. A very new technology, known as next-generation genetic sequencing (NGS) promises to further reduce the risks of an unsuccessful IVF pregnancy or parents passing along a genetic disease. NGS can identify aneuploidy, mitochondrion copy number (indicators of embryos under stress or potentially identifying aging-related health problems), and single-gene mutations (which can cause birth defects and genetic disease).
NGS relies on computers to sequence an entire genome, the map of one person’s code of approximately 25,000 genes. This exciting advancement means that doctors and scientists can learn more than ever about one person’s genetic make-up behind and his or her likelihood of passing on one or more of more than 6,000 debilitating genetic diseases such as cystic fibrosis, certain muscular dystrophies and Tay-Sachs disease. These diseases occur in about 1 out of every 200 births.
What is PGD used for?
Preimplantation genetic diagnosis (PGD) evaluates an IVF embryo for a single, specific gene mutation that one of the parents is known to have or that is evident in a parent’s extended family. Single-gene disorders are variances in a DNA sequence that cause a disease, such as cystic fibrosis, or cause a specific characteristic, such as the inherited genetic BRCA1 and BRAC2 mutations that predispose a woman to breast and ovarian cancer.
The presence of certain gene disorders can affect the chances of successful IVF and the health of a resulting child. Disorders can be inherited at birth from a parent or develop as a first event in an early embryo.
Many single-gene disorders are known as Mendelian disorders, meaning they are passed from one generation to the next in the pattern of inheritance. Single-gene inheritance patterns may be autosomal dominant (AD), autosomal recessive (AR) and X-linked dominant or X-linked recessive.
Parents who carry an AD disorder have a 50-percent risk of having a child with that disorder. (AdvaGenix recommends that PGD be performed on embryos if one of the parents has an AD gene disorder.)
If both the man and woman have an AR disorder, then the couple has a 25-percent chance of having a child affected by that disorder, and PGD is recommended. PGD is also recommended for women with a disease such as Hemophilia A that is caused by an X-linked recessive mutation, as they have a 25-percent chance of passing that on to their children.
PGD is also used for Human Leukocyte Antigen (HLA) typing to identify an embryo that carries the same HLA as a couple’s existing child with leukemia. This child may need a bone marrow transplant and the new embryo with compatible HLA can result in the birth of a sibling who may serve as a tissue donor.
For all couples interested in single gene PGD, AdvaGenix strongly recommends that aneuploidy testing is also included in the analysis.
Preimplantation genetic diagnosis can also detect structural chromosome aberrations in embryos due to parental chromosome rearrangements that involve chromosome translocations (reciprocal and Robertsonian), in which pieces of chromosomes attach to or interchange with other chromosomes, or inversions (pericentric and paracentric), meaning a chromosome breaks in two places and the broken piece reverses and reinserts itself into the larger chromosome.
Common disorders PGD tests for:
- Cystic fibrosis
- Huntington’s disease
- Sickle cell disease
- BRCA1 mutations (a cancer causing gene)
- Tay-Sachs disease
- X-linked disorders
- Myotonic dystrophy
Some parental rearrangements can result in a greater risk of having a miscarriage or delivering an abnormal child.
How is PGD performed?
To determine if mutations are present, geneticists usually perform PGD through a process called genotyping or next generation sequencing. Cells to be used in these processes can be obtained through biopsy of a three-day-old embryo at the cleavage stage of development or at the blastocyst stage at approximately 5-6 days post fertilization. At this stage, the cells of an embryo have separated into two types, an inner-cell mass, which will form the fetus, and cells that will form the placenta, also called the trophectoderm. AdvaGenix strongly recommends biopsy at the blastocyst stage of development.
To help identify problem areas within chromosomes, geneticists use a technique called DNA amplification. In this technology, scientists are able to make multiple copies of the chromosome DNA where the suspected gene resides; in turn, this approach permits them to perform a thorough analysis of the embryonic cells.
DNA amplification is normally done by a process called whole genome amplification (WGA) which amplifies multiple copies of the cellular DNA.
In order to minimize risks to an embryo, geneticists will often perform a trophectoderm biopsy when the embryo is five to six days old and is at what’s called the blastocyst stage of development. At this stage the embryo has 100 to 150 cells.
Once PGD identifies an embryo as free of the genetic issues being searched for, the embryo can be transferred in the woman’s uterus in order to achieve a pregnancy. Any remaining healthy embryos can be frozen for later use.
Who can benefit from PGD?
PGD can alert couples in IVF treatment to the presence of a specific genetic mutation in their DNA that can jeopardize their pregnancy and the health of their child.
These genetic mutations can be a certain gene known to cause a disease, or a mitochondrial disease that can lead to cellular death and ultimately severe physical debilitation, or a variety of abnormalities in chromosome structure, such as rearrangements of genes or fusion of genes.
These disorders are known by their pattern of transmission in families. A parent does not have to have the disease or characteristic in order to pass it on to his or her child.
The first step for a couple suspected of having such a mutation is for their physician or geneticist to do a complete family history of both partners, identifying family members who have had a particular genetic disease.
That information is summarized in a pedigree, which is a family-tree-like diagram indicating presence of the disease from one generation to the next. This will help the couple and their physician assess the couple’s risk for passing along the disease to their children.
Most of the thousands of gene mutations are rare and affect only about 1 percent of pregnancies with birth defects or debilitating or life-threatening illnesses. The most common for which couples consider PGD are:
- Cystic fibrosis
- Down syndrome
- BRCA1 and BRAC2 mutations (breast and ovarian cancer)
- Muscular dystrophy
- Fragile-X syndrome
- Huntington’s disease
- Tay-Sachs disease
- Sickle cell anemia
What are the risks and concerns of PGD?
Some embryos may be damaged during cell removal in the biopsy process. Further, results are not 100-percent accurate. For this reason, doctors often recommend further testing for single-gene disorders, such as amniocentesis, during pregnancy.
The health risks for children born from embryos that have received PGD have been shown in studies to be no different from the health risks for any child born through IVF. Those risks for IVF babies (low birth weight) are only slightly higher than the same risks for babies born through conventional pregnancies.
Misdiagnosis of the presence of a single genetic abnormality is a risk with PGD. This is due to faulty amplification of genetic material (known as an allele dropout), which can result in failure to identify a genetic mutation, incomplete amplification or contaminated amplification. The chance of an allele dropout – the main cause of single gene misdiagnosis – can be greatly reduced by performing a linkage analysis assay, a technique in which geneticists can establish and identify whether the DNA from the sperm and egg amplified from the cells of the embryo.
Everyone considering PGD should receive genetic counseling beforehand, because PGD involves ethical issues that affect people differently.
An embryo may inherit a genetic mutation discovered through PGD that might never develop into a lethal disease, yet the parents may elect to have the embryo discarded before transfer in order to avoid that risk. In cases of offspring carrying mutations considered mult-factoral, environmental and personal development factors can determine if that child ever gets the disease and to what extent.
For a woman with the BRAC mutations that predispose one to breast and ovarian cancer, counseling may help both partners make the decision of whether to discard any IVF embryos with that mutation. Detailed family history and the knowledge of the BRAC mutation typically creates considerable stress as to whether the child will indeed develop cancer ultimately.
The issue of “savior siblings” is also ethically controversial. PGD can identify a specific gene in an embryo that would represent a perfect match for an existing child of the parents who needs a life-saving bone marrow transplant. Producing this savior sibling, who may become a bone marrow donor for its older sibling, is precisely some the parents select IVF as a means of achieving pregnancy, because PGD can eliminate embryos that do not have the genetic match desired.
Is it ethical to have a child and force it to be a tissue donor? Is it ethical to put that child at risk by going through the procedure? These are the kinds of issues couples must make value-based decisions on. Well trained genetic counselors can often provide excellent assistance.
What is PGS used for?
Preimplantation genetic screening was originally used to improve IVF success for couples at risk for miscarriage due to advanced maternal age or recurrent pregnancy loss.
PGS tests an embryo for aneuploidy (an extra or missing copy of a chromosome), which if present can cause the implantation in the uterus to fail (no pregnancy), cause a miscarriage, or result in a child born with a serious genetic disorder. Discovering the existence of such genetic abnormalities in embryos, and not using them in IVF, is the primary reason couples turn to PGS.
PGS identifies aneuploidy, which is considered to be the greatest cause of pregnancy failure. Aneuploidy, which also causes birth defects, is the condition in which the number of chromosomes in a cell’s nucleus are mismatched, with either extra or missing chromosomes rather than a normal pair.
Healthy diploid cells have 23 pairs of chromosomes, chromosomes 1 through 22 and the two sex chromosomes X and Y. The two parents contribute a chromosome to each pair.
Monosomy, which means a missing chromosome, is generally more deadly than trisomy, an extra chromosome. The only chromosomal abnormalities that can generally result in a live birth are trisomies of chromosomes 13, 18 and 21 and of numerical abnormalities of the sex chromosomes (X, XXX, Y, YY, XXY, etc.). All other instances of chromosomal aneuploidy result in miscarriages or unsuccessful implantation of the embryo.
Trisomy 21 is Down syndrome, characterized by intellectual disabilities and delayed growth. Trisomy 21 affects 1 in 800 births and is the aneuploidy an infant is most likely to survive.
Trisomy 18 is known as Edwards syndrome, characterized by heart defects, kidney malformations, and a number physical deformities. Trisomy 13 is Patau syndrome, which has a low rate of fetal survival and results in infants affected by a variety of defects in the nervous system, musculoskeletal and skin disorders, genital abnormalities, kidney defects and others.
How is PGS performed?
In PGS, single cells from an IVF embryo are biopsied and tested for their chromosome compliment. AdvaGenix strongly recommends that all biopsies should be done at the blastocyst stage of development.
Different techniques can evaluate chromosomes. One older technology is called fluorescence in situ hybridization (FISH), evaluates chromosomes by way of luminescent probes designed to hybridize to specific DNA areas of target chromosomes. Microscopic evaluation detects the flourochromes that correspond to the hybridization site. If a flourochrome labeled for, say chromosome 21, is observed three times, the sample is identified as Trisomy 21.
This technique offers a couple of advantages. First, FISH does not require DNA amplification and thus eliminates errors from the amplification process. FISH evaluations can also be done in less than 10 hours from the time of PGS sampling, which allows for testing in fresh embryo IVF transfers so frozen embryos aren’t needed.
However, a major drawback of FISH is that it is impossible to accurately evaluate all 23 chromosomes using a single cell from an embryo. FISH also results in split signals in visualizing fluorescent dots, resulting in over-estimations of the presence of aneuploidy.
Though some geneticists still use FISH, generally for testing XY chromosomes or for a five-probe of chromosomes 13, 18, 21, X and Y.
Two often preferred methods are comparative genomic hybridization (CGH) microarray and the single nucleotide polymorphism (SNPs) microarray. Both PGS techniques evaluate for 23-chromosome aneuploidy. CGH and SNPs can identify large structural chromosome imbalances.
In CGH and SNP microarray, DNA from the embryo cell is amplified using a whole genome amplification protocol and mixed with chips that contain thousands of genetic markers. In this approach, a computer then gathers visual images of the embryonic cells and compares the embryonic DNA to a normal reference DNA to determine chromosome abnormalities.
There are significant differences in CGH and SNP. The high density of the SNP array makes it capable of detecting smaller clinically significant deletions and duplications in chromosomes harmful for developing embryos that would probably be missed by CGH microarrays.
Who can benefit from PGS?
Couples and women in IVF treatment who have a risk of passing aneuploidy chromosomal abnormalities to their child should consider PGS. Selecting embryos with no chromosome abnormalities also decreases the risk of miscarriage and increases IVF success.
AdvaGenix recommends PGS for couples with a previous pregnancy involving aneuploidy, for couples experiencing two or more miscarriages or the transfer of non-PGS embryos failing to achieve a pregnancy. Due to the high cost of donor egg IVF and / or surrogacy, the couple may also consider PGS in order to increase their chances of achieving a healthy pregnancy.
It is clear, that PGS on blastocysts for all age groups can potentially increase pregnancy rates and decrease miscarriage rates.
What are the risks and concerns of PGS?
In the process of PGS some embryos may be damaged during cell removal for testing. This risk is exceedingly low and could result in an embryo to stop developing. Otherwise, the medical risks from PGS are the same as for the IVF procedure.
A recent study showed that the health risks for children born from embryos that have received preimplantation genetic testing are no different from the health risks of any child born through IVF. Such risks (low birth weight) are only slightly higher for babies born through IVF than for babies born through conventional pregnancies.
All people considering PGS should first receive thorough genetic counseling. Such counseling should review the risk of damage to the embryo during biopsy, the labs mis-diagnosis rate and the couples expected pregnancy rate,
Counseling should address alternative testing, such as amniocentesis after pregnancy, as well as reviewing the genetics involved with the PGS procedure. Counseling after a PGS test discovers a genetic disease should address the fact that some disease symptoms don’t appear until the child matures and may not appear at all.
Other concerns about PGS revolve around the issue of genetic selection. Additionally, some are concerned that PGS may be used to reject an embryo with a certain gender.