Preimplantation Genetic Screening & Diagnosis (PGS & PGD)
Preimplantation genetic screening refers to the removal of one or more cells from an in vitro fertilization embryo to test for chromosomal normalcy. PGS screens the embryo for a normal chromosome number. PGS is a method of chromosome disorder screening on IVF embryos.
Preimplantation genetic diagnosis involves the following steps:
First, one or two cells are removed from the embryo.
The cells are then evaluated to determine if the inheritance of a problematic gene is present in the embryo.
Once the PGD procedure has been performed and embryos free of genetic problems have been identified, the embryo will be placed back in the uterus, and implantation will be attempted.
Any additional embryos that are free of genetic problems may be frozen for later use, while embryos with the problematic gene are destroyed.
There are two kinds of PGD / PGS biopsy:
1. Trophectoderm Biopsy
Trophectoderm biopsy involves removing some cells from the trophectoderm component of an IVF blastocyst embryo. The removed cells can be tested for overall chromosome normality (PGS) , or for a specific gene defect (PGD).
The embryo should be at the expanded blastocyst stage (or beyond) at the time of cell removal
This stage is reached on day 5 to 6 after fertilization
Trophectoderm cell removal is much less traumatic compared to blastomere removal
2. Blastomere Biopsy
Blastomere biopsy is removal of a cell on Day 3 at the “cleavage” stage - before a blastocyst is formed. Day 3 embryo biopsy is traumatic and lowers the embryo’s
potential for implantation.
We generally choose to do trophectoderm biopsy.
Advantages of trophectoderm biopsy and PGD on success rates
The advantage of trophectoderm removal is that the embryo is much less traumatized by the procedure compared to blastomere removal done on day 3
Since the embryo has many more cells (about 100) at the blastocyst stage than on day 3 (about 6-10), we can remove about 5 cells with trophectoderm biopsy with little or no impact on its potential
Trophectoderm biopsy allows the potential to increase IVF live birth success rates by screening for chromosomally normal embryos prior to transfer back to the uterus
Disadvantages of trophectoderm biopsy
At this point performing the genetics techniques for chromosomal or genetic analysis on the cells requires 24-48 hours
So a day 3 biopsy can be done with the information about the chromosomes or genetics coming back in time to do transfer on day 5 with fresh embryos
However, when the cell removal is done on a day 5 embryo the genetic or chromosomal results will not be back in time to do a fresh transfer procedure
So we must do the trophectoderm removal, then freeze the embryos, and wait to get the genetics results back before we thaw and transfer back blastocysts to the uterus
Day 5 embryo -blastocyst- just hatching
The disadvantages of this approach are:
Increased cost from adding a frozen embryo transfer cycle
Delay of about a month before the transfer can be done
The need for development to the blastocyst stage to do a biopsy procedure. Not all couples will have embryos develop to blastocyst stage.
Traditionally, morphology-based grading ( looking embryos under microscope ) had been the primary technique used in in vitro fertilization (IVF) to evaluate and select the most competent embryos for transfer. Technologies have been developed try to assist in the selection of the best embryos. However, a focus has been on analysis of 24-chromosome copy number for evaluation and transfer of only diagnosed genetically normal embryos, also known as preimplantation genetic testing for aneuploidy (PGT-A). Several molecular techniques have been utilized during IVF cycles to determine genetically normal embryos including fluorescence in situ hybridization (FISH), comparative genomic hybridization (CGH), array CGH (aCGH), digital polymerase chain reaction (dPCR), single-nucleotide polymorphism (SNP) array, real-time quantitative PCR (qPCR), and next-generation sequencing (NGS). These technologies vary in terms of cost and time to completion, and few of these methods allow for fresh embryo transfer.Mostly we nedd to freeze embryos to wait for result. Because the embryo biopsies are done at day 3 or day 5 . Embryos need to implant in uterus at day 5 otherwise day die. So if we do biops at day 3 we have 48 hours to transfer them , day 3 biopsy genetic analysis are simple , can't check all the chromosomes but gives us the chance of fresh transfer, day 5 biopsies genetically can be searched for whole chromosomes but it takes more than a week so after biopsy completed embryo must be freezed. Day 5 biops also is less destructive to the embryos theoretically, the cells are taken from around the embryo but not directly itself.
The earliest iterations of PGT-A evaluated a subset of the chromosomes primarily using FISH to examine 5-10 unique chromosomes. Despite the hypothesis that transfer of only euploid embryos should improve IVF outcomes, all but one serious scientific trial of this initial approach failed to demonstrate a benefit . Since 24-chromosome techniques have become available, there have been few well-designed studies providing Level-I evidence regarding IVF pregnancy outcomes in select populations with these techniques. Several opinion pieces have discussed advantages and disadvantages of PGT-A . The aim of this communication is to review the current evidence and to provide guidance for the use of PGT-A in IVF.
The detection of a genetically anomalous embryo in an IVF cycle can be an unwelcome occurrence for prospective parents. Anecdotal evidence suggests that most patients whose embryos contain a serious health-affecting genetic anomaly choose not to transfer those embryos, electing either discard or cryopreservation . Under certain circumstances, however, patients will request that such embryos be transferred, even when counseled about the near certainty their children will manifest symptoms of a serious genetic disorder. Three main reasons for such requests are: 1) the affected embryos are the only embryos the patient and/or her partner produced, thus providing the only opportunity for biologic parenthood; 2) the patient and/or her partner have religious or psychosocial beliefs that inform their decision to treat all their embryos with equal respect, thus permitting the transfer of genetically anomalous embryos in the face of some or no other healthy embryos; and 3) the intended parents themselves express the genetic anomaly and wish to rear children with the same characteristics. This latter scenario is sometimes referred to as “intentional diminishment” and primarily involves selection for sensory or mobility disorders such as deafness or achondroplasia (dwarfism)
While each of the above-mentioned rationales could motivate patient requests for transfer of genetically abnormal embryos , logic suggests the first scenario in which all embryos are affected is the most likely to present in clinical practice. Patients whose religious beliefs or other values would guide them to seek transfer of genetically anomalous embryos may be less likely to seek embryonic testing than patients for whom this information would impact decision making. While such patients may “want to know” in order to prepare for the birth of an affected child, other approaches to prenatal diagnostic testing are likely to be preferable. In cases in which a certain genetic anomaly is intentionally sought, patients are likely to have discussed this reproductive plan with their provider, giving the clinician an a priori opportunity to consider whether to assist or decline to assist in their reproductive efforts.
The high incidence of chromosome abnormalities in human gametes and embryos is a major cause of in vitro fertilization (IVF) failure and miscarriage . Most abnormalities arise in maternal side ( egg), and they increase exponentially in women over the age of 35 years, coinciding with rapidly declining IVF success and live birth rates in patients of advanced maternal age. For example, the Society for Assisted Reproductive Technology (SART) compilation of U.S. IVF cycle data for 2016 shows that final cumulative live birth rate per egg-retrieval cycle decreased from 54.5% in young patients to 13.4% in women aged 41—42 years . This is mirrored by the increased incidence of genetic abnormalities rom 30% to 50% in patients under 35 years of age to 80% in women 42 years of age or older . In contrast, the transfer of a genetically normal embryo results in similar implantation rates regardless of maternal age . Indeed, the 2016 SART data showed no age-related decrease in implantation rates after frozen-thawed normal embryo transfer following preimplantation genetic testing for chromosome aneuploidy (PGT-A) .
Morphologic assessment has always been the primary method of prioritizing IVF embryos for transfer, but the chromosome status of cultured embryos cannot be accurately ascertained through either ‘static or dynamic morphologic ( microscopic appearance) evaluation . PGT-A, formerly known as preimplantation genetic screening (PGS), was proposed as a method to select IVF embryos with the highest potential of ongoing implantation . Initial studies with the use of cleavage-stage biopsy on day 3 after insemination and fluorescence in situ hybridization (FISH) showed an improvement in pregnancy outcomes, but this was not confirmed in randomized control trials . With improvements in blastocyst culture ( day 5embryo) , embryo vitrification( fast freeze) , and molecular techniques that can test copy number of all chromosomes, several, mainly single-center, scientific researchs performed in the past decade have all shown significant improvement in ongoing pregnancy rates (OPRs) per embryo transfer procedure . Furthermore, the recent introduction of next-generation sequencing (NGS)—based methods have increased the sensitivity and resolution of copy number variation genome.