Preimplantation Genetic Diagnosis (PGD)
Preimplantation genetic diagnosis, which is performed on embryos, was established about a quarter-century ago as a substitute for prenatal diagnosis. Preimplantation genetic diagnosis (PGD) was first used in assisted reproduction to detect chromosome aneuploidy caused by advanced maternal age or structural chromosomal rearrangements in couples at risk for single-gene disorders such as cystic fibrosis, spinal muscular atrophy, and Huntington’s disease. Moving away from earlier, less effective technologies like fluorescence in situ hybridization (FISH) and toward newer molecular tools like DNA microarrays and next-generation sequencing has resulted in significant advancements in PGD analysis. Reduced utilization of Day 3 blastomere biopsy in favor of polar body or Day 5 trophectoderm biopsy has started to yield better outcomes.
Preimplantation Genetic Diagnosis Definition
The preimplantation genetic test is a method for detecting genetic problems in in vitro fertilization (IVF) embryos prior to pregnancy. Preimplantation genetic diagnosis (PGD) arises when one or both genetic parents have a known genetic disorder, and testing is done on the embryo to see if it too has a genetic disorder. Preimplantation genetic screening (PGS), on the other hand, refers to approaches for screening embryos from supposed chromosomally normal genetic parents for aneuploidy.
Preimplantation genetic testing offers an alternative to conventional postconceptional diagnostic techniques (such as amniocentesis or chorionic villus sampling), which are often followed by the difficult option of terminating a pregnancy if the findings are unfavorable. PGD and PGS are now the sole alternatives for avoiding a high chance of a child being born with a genetic illness before implantation. It's an appealing way to prevent inherited genetic disorders and avoid the problem of having to terminate a baby due to a negative prenatal diagnostic.
Preimplantation Genetic Diagnosis Indications
When a couple is at risk of passing on a known genetic defect to their children, preimplantation genetic diagnostic (PGD) is recommended. Only healthy and normal embryos are implanted into the mother's uterus, lowering the danger of passing down a genetic defect and preventing late pregnancy termination (after a positive prenatal diagnosis). Aneuploidy is the cause of the majority of early pregnancy losses. The likelihood of first and second-trimester loss is significantly lowered since only chromosomally normal embryos are placed into the uterus. There is currently no comprehensive list of indications for preimplantation genetic diagnosis (PGD).
The following are some of PGD’s primary candidates:
- Couples having a history of X-linked illnesses in their family (Couples with an X-linked disease in their family have a 25% chance of conceiving an afflicted embryo.)
- Couples with chromosomal translocations (which might result in implantation failure, frequent pregnancy loss, or mental or physical disorders in their children)
- Autosomal recessive disease carriers (Carriers of autosomal recessive illnesses have a 25% chance of having an embryo afflicted.)
- Autosomal dominant disease carriers (Carriers of autosomal dominant illness have a 50% chance of having an embryo afflicted.)
- Advanced maternal age
- Couples who have had previous miscarriages
- Couples who have had many failed IVF
- Severe male factor infertility
Conditions Diagnosed Using PGD
Sex-Linked (X-Linked) Disorders
X-linked disorders are passed down to a child through a carrier mother. They are transmitted through an aberrant X chromosome and manifest in sons who do not inherit the father's normal X chromosome. Because the X chromosome is passed down through the mother to offspring/embryos, affected men produce unaffected sons, but their daughters have a 50% chance of being carriers if the mother is healthy. Hemophilia, fragile X syndrome, most neuromuscular dystrophies (about 900 neuromuscular dystrophies are documented), and hundreds of other diseases are all sex-linked recessive diseases. Rett syndrome, incontinentia pigmenti, pseudohypoparathyroidism, and vitamin D-resistant rickets are all sex-linked dominant illnesses.
Single gene abnormalities such as cystic fibrosis, Tay-Sachs disease, sickle cell anemia, and Huntington’s disease are identified via PGD. Molecular techniques such as polymerase chain reaction (PCR) amplification of DNA from a single cell can discover abnormalities in these disorders. Despite progress, some single gene abnormalities, such as cystic fibrosis, are known to have numerous mutations. Only 25 variants are presently frequently examined in cystic fibrosis. Because most of these rare mutations aren't regularly screened, a parent with no cystic fibrosis symptoms could nevertheless be a carrier. This permits a parent who carries a rare mutation gene to be tested negative yet still pass on the defective cystic fibrosis to their children.
The last group includes chromosomal diseases in which fluorescent in situ hybridization (FISH) can identify a variety of chromosomal rearrangements, such as translocations, inversions, and deletions. FISH uses telomeric probes that are specific to the loci that are being studied. Because prior conceptions resulted in chromosomally imbalanced embryos that spontaneously miscarried, some parents may never have been able to produce a successful pregnancy without PGD.
Preimplantation Genetic Diagnosis Advantages
- An alternative to prenatal diagnostic testing. Couples avoid the anxiety of a diagnosis, especially if the pregnancy is already confirmed.
- An alternative to termination of pregnancy. If prenatal testing (through amniocentesis or chorionic villus sampling) finds a genetic anomaly, parents have the choice of having a child with the problem or terminating the pregnancy. This is a difficult and frequently traumatic choice. PGD, on the other hand, is done before the pregnancy begins, removing this difficult decision.
- The ability to conceive a baby that is biologically their own and unaffected by a genetic disorder in the family. Couples who do not want to have a child having a genetic condition may not want to take the danger of a child having a genetic problem. In the past, parents who had a genetic disease or who knew they were carriers typically chose adoption, embryo donation, or surrogacy over having children to reduce the danger of passing on the illness. PGD now gives these couples the chance to conceive a kid without the disease.
Before Preimplantation Genetic Diagnosis
Candidates should see a geneticist or a genetic counselor before requesting a preimplantation genetic diagnosis (PGD) to assess the risk of passing their genetic defect on to their children. Tests should be undertaken to establish the affected parent's diagnosis, identify the genetic alteration that causes the disorder, and ensure that the currently available technology can detect that genetic change in a polar body, cleavage state, or blastocyst embryo biopsy.
Patients must undergo in vitro fertilization (IVF) in order to obtain embryos for biopsy for PGD/PGS. Embryos are permitted to develop into cleavage-stage embryos after fertilization with sperm. A single blastomere is taken from the developing embryo on day 3 after egg retrieval for genetic testing. PCR, FISH, or comparative genomic hybridization (CGH) are used to assess genetics. After that, non-affected or normal embryos are put into the uterus for implantation and conception.
Preimplantation Genetic Diagnosis Process
Ovarian stimulation, egg retrieval, egg fertilization, embryo development, and embryo transfer are all part of the IVF technique.
The following are the steps of IVF in order:
- To release numerous eggs, ovarian stimulation is required. Frequent ultrasonographic scans and laboratory testing are undertaken to evaluate the development and maturation of follicles (egg-containing ovarian cysts) during the 8- to 14-day hormonal stimulation period.
- The process for retrieving oocytes is usually done under anesthetic. Follicles are pierced and their follicular fluid is collected under sonographic guidance using a transvaginal technique. Embryologists then identify the oocytes in the embryology laboratory. The treatment takes about 15 minutes on average.
- Following their retrieval, the eggs are cultured for a few hours to allow for ultimate maturation. A polar body can then be detached for PGD/PGS if required. Intracytoplasmic sperm injection (ICSI), in which a single sperm is infused into a single egg, is preferable for the PGD/PGS process at a later stage of embryonic development. ICSI eliminates the risk of polyspermy and the unintentional acquisition of "additional" chromosomal material from the sperm, which might affect the PGD/PGS readings (i.e., give false-positive results).
- Masturbation on the day of egg extraction is often used to acquire sperm from the male partner for egg fertilization.
- The eggs are tested for evidence of fertilization the morning following ICSI, which is determined by the existence of two pronuclei, which reflect the male and female contributions to the embryo.
- The division of embryos into multicellular entities continues. The embryos can be prepared for a cleavage-stage biopsy three days following egg retrieval which is normally at the 6-10 cell stage. Two days following egg extraction, normal development includes progression to the 2-4 cell stage, followed by 6-10 cells after three days.
A cleavage-stage embryo biopsy is performed in most clinics. However, one of the three procedures mentioned below can be utilized for PGD.
- Polar body biopsy
Only female chromosomal abnormalities can be addressed by polar body biopsy. A solitary polar body emerges from the mature metaphase II egg. This polar body can be removed and evaluated, yielding solely information about the egg's chromosomal makeup. Importantly, this does not reveal anything about the chromosomal makeup of the succeeding embryo.
Because polar bodies can only provide information about the mother, chromosomal abnormalities that develop after fertilization (when the sperm hits the egg) are not discovered.
- Cleavage-stage embryo biopsy
The most frequent method for PGD/PGS is to biopsy a single blastomere from a day 3 embryo, which permits a single blastomere to be extracted from a developing embryo. Blastomere removal is a technically difficult process. The embryologist’s aim is to extract an intact cell with minimal stress to the remaining embryo using an advanced microscope and micromanipulators.
To minimize blastomere-to-blastomere adhesion, a 6-10 cell embryo is cultured in calcium- and magnesium-free media for around 20 minutes before being isolated as a single cell.
A holding pipette is used to anchor the embryo on one side, while a tiny opening is produced within the zona pellucida to allow easy access to the blastomeres. This method of opening is known as assisted hatching. A dilute acidic Tyrode solution, a laser, or a sharp curette can all be used to help with assisted hatching. A pipette is inserted through the tiny opening and focused on the blastomere of choice, which has a visible nucleus. After that, the blastomere is softly aspirated into the pipette and released into the medium.
The embryo is reintroduced to the incubator, now with one fewer blastomere, and placed in the proper culture media. Depending on the genetic disorder to be studied, the blastomere is next processed for FISH or PCR.
Because embryos might be mosaic, the blastomere obtained during the cleavage stage biopsy may not be totally representative of the entire embryo (i.e., the embryos may be composed of more than one type of cells).
- Blastocyst biopsy
The appearance of an inner cell mass and an outer cell mass, or trophectoderm, marks the start of blastocyst formation on day 5 after egg retrieval. The embryo consists of more than 100 cells at this stage of development. A hole in the zona pellucida is breached in the same way that a cleavage-stage embryo biopsy is breached, and cells are retrieved from the trophectoderm with a fine biopsy pipette. The inner cell mass remains intact. FISH or PCR analysis is used for genetic analysis.
Because of mosaicism, this method has the risk of obtaining cells from the trophectoderm that are not representative of the growing embryo (inner cell mass) (having multiple different types of cells). In addition, genetic/aneuploidy testing is required within 24-48 hours following the embryo biopsy; many embryos do not survive until embryo transfer due to the limited survival of embryos in the laboratory (6 days after egg retrieval). Biopsied blastocysts must therefore be frozen.
Preimplantation Genetic Diagnosis Risks
Risks of Treatment
The majority of the dangers associated with PGD treatment are comparable to those associated with traditional IVF:
- Fertility drugs reaction. Mild effects of fertility drugs include hot flashes, irritability, headaches, and restlessness; symptoms normally subside after a short period.
- Multiple pregnancy. the risk of difficulties is higher with a twin or multiple pregnancy (although some facilities minimize this risk by only transferring one embryo).
- Ovarian hyperstimulation syndrome (OHSS). Symptoms of Ovarian Hyperstimulation Syndrome (OHSS) include abdominal swelling and pain, breathlessness, nausea, vomiting, and a decrease in urine output; can cause mineral imbalances in the blood and coagulation problems.
- Pelvic inflammatory disease
- Miscarriage. The chance of miscarriage during IVF is the same as it is after a spontaneous conception.
- An ectopic pregnancy. An ectopic pregnancy occurs when a pregnancy develops outside of the uterus, in the fallopian tubes. This would not lead to a sustained pregnancy.
Risks of Baby Born after PGD
There is presently no convincing evidence to show that babies conceived by this method are at any higher risk of abnormalities than babies conceived through other methods of assisted reproduction.
PGD treatment does not appear to have any serious negative effects, according to several studies. There is evidence, however, that babies born through IVF are more prone to:
- A preterm birth
- At the same age, weigh less than spontaneously conceived babies.
Long-term consequences are unknown due to a lack of follow-up data. It's critical to keep track of the progress of infants born after PGD in order to learn more about the treatment's potential effects.
Preimplantation Genetic Diagnosis Future
New approaches for diagnosing monogenic disorders are rapidly emerging, some of which are particularly well suited to single-cell PCR, such as:
PCR product accumulation is assessed during cycling, while the PCR is in the exponential phase, rather than after the procedure is finished. This technique decreases allele dropout and delivers a more accurate picture of whether a single cell is homozygous or heterozygous.
This approach accurately detects single nucleotide polymorphisms (SNPs). SNPs are found throughout the genome as mutations or polymorphisms that don’t affect gene function. Small lengths of DNA amplification are considerably more efficient when starting from a single cell than larger fragments, hence SNP analysis is suitable for single-cell PCR.
Microarrays could be used to detect common genetic illnesses caused by many mutations, as well as in CGH to replace the metaphase spread, which combines a mixture of test and comparative DNA. The microarray sequence specificity would result in faster hybridization and computer analysis times than CGH, eliminating the requirement for polar body analysis or cryopreservation. Microarray CGH would also provide a higher resolution than metaphase spreads. More IVF clinics would be able to use microarray CGH for aneuploidy scram if the entire procedure could be computerized.
Prenatal diagnosis of genetic abnormalities has been accessible for decades, and it has significantly reduced the burden of these serious disorders. However, this necessitates the use of medical abortions, which is not widely accepted. The preimplantation genetic disease was developed with the goal of preventing the birth of a child with genetic problems without the need for abortion. PGD has gained widespread acceptability over the last decade, with thousands of PGD cycles completed and over 1000 normal, healthy children born as a result.
The PGD, like any powerful modern technology, has two sides and should be used according to strict ethical and legal guidelines. It is a labor-intensive and rather expensive method that requires a high degree of competence and can only work in tandem with a high-output successful IVF program and a cutting-edge genetic laboratory with single-cell diagnosis expertise. However, due to the huge possibilities in screening, diagnosis, and management of many types of genetic abnormalities, it has taken a significant place in preventive genetics programs.