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Preimplantation genetic diagnosis(PGD)

Written by dr- zhila abedi asl. Posted in Infertility

Preimplantation genetic diagnosis
Pre-implantation genetic diagnosis (PGD or PIGD) refers to genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal diagnosis. When used to screen for a specific genetic disease, its main advantage is that it avoids selective pregnancy termination as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation. The term preimplantation genetic screening (PGS) is used to denote procedures that do not look for a specific disease but use PGD techniques to identify embryos at risk.
The procedures may also be called preimplantation genetic profiling to adapt to the fact that they are sometimes used on oocytes or embryos prior to implantation for other reasons than diagnosis or screening.
 Procedures performed on sex cells before fertilization may instead be referred to as methods of oocyte selection or sperm selection, although the methods and aims partly overlap with PGD.
Indications and applications
PGD can potentially be used to select embryos to be without a genetic disorder, to have increased chances of successful pregnancy, to match a sibling in HLA type in order to be a donor, to have less cancer predisposition, and for sex selection.
Monogenic disorders
PGD is available for a large number of monogenic disorders — that is, disorders due to a single gene only (autosomal recessive, autosomal dominant or X-linked)— or of chromosomal structural aberrations (such as a balanced translocation). PGD helps these couples identify embryos carrying a genetic disease or a chromosome abnormality, thus avoiding diseased offspring. The most frequently diagnosed autosomal recessive disorders are cystic fibrosis, Beta-thalassemia, sickle cell disease and spinal muscular atrophy type 1. The most common dominant diseases are myotonic dystrophy, Huntington's disease and Charcot-Marie-Tooth disease; and in the case of the X-linked diseases, most of the cycles are performed for fragile X syndrome, haemophilia A and Duchenne muscular dystrophy. Though it is quite infrequent, some centers report PGD for mitochondrial disorders or two indications simultaneously.
PGD is also now being performed in a disease called Hereditary multiple exostoses (MHE/MO/HME).
In addition, there are infertile couples who carry an inherited condition and who opt for PGD as it can be easily combined with their IVF treatment.
Pregnancy chances
Preimplantation genetic profiling (PGP) has been suggested as a method to determine embryo quality in in vitro fertilization, in order to select an embryo that appears to have the greatest chances for successful pregnancy. However, as the results of PGP rely on the assessment of a single cell, PGP has inherent limitations as the tested cell may not be representative of the embryo because of mosaicism.
A systematic review and meta-analysis of existing randomized controlled trials came to the result that there is no evidence of a beneficial effect of PGP as measured by live birth rate. On the contrary, for women of advanced maternal age, PGP significantly lowers the live birth rate. Technical drawbacks, such as the invasiveness of the biopsy, and chromosomal mosaicism are the major underlying factors for inefficacy of PGP.
Alternative methods to determine embryo quality for prediction of pregnancy rates include microscopy as well as profiling of RNA and protein expression.
Sex discernment
Preimplantation genetic diagnosis provides a method of prenatal sex discernment even before implantation, and may therefore be termed preimplantation sex discernment. Potential applications of preimplantation sex discernment include:
•    A complement to specific gene testing for monogenic disorders, which can be very useful for genetic diseases whose presentation is linked to the sex, such as, for example, X-linked diseases.
•    Ability to prepare for any sex-dependent aspects of parenting.
•    Sex selection. A 2006 survey [1] found that 42 per cent of clinics that offer PGD have provided it for sex selection for non-medical reasons. Nearly half of these clinics perform it only for “family balancing”, which is where a couple with two or more children of one sex desire a child of the other, but half do not restrict sex selection to family balancing. In India, this practice has been used to select only male embryos although this practice is illegal {{PNDT ACT NO. 57 OF 1994}}. Opinions on whether sex selection for non-medical reasons is ethically acceptable differ widely, as exemplified by the fact that the ESHRE Task Force could not formulate a uniform recommendation.
In the case of families at risk of X-linked diseases, patients are provided with a single PGD assay of gender identification. Gender selection offers a solution to individuals with X-linked diseases who are in the process of getting pregnant. The selection of a female embryo offspring is used in order to prevent the transmission of X-linked Mendelian recessive diseases. Such X-linked Mendelian diseases include Duchenne muscular dystrophy (DMD), and hemophilia A and B, which are rarely seen in females because the offspring is unlikely to inherit two copies of the recessive allele. Since two copies of the mutant X allele are required for the disease to be passed on to the female offspring, females will at worst be carriers for the disease but may not necessarily have a dominant gene for the disease. Males on the other hand only require one copy of the mutant X allele for the disease to occur in one's phenotype and therefore, the male offspring of a carrier mother has a 50% chance of having the disease. Reasons may include the rarity of the condition or because affected males are reproductively disadvantaged. Therefore, medical uses of PGD for selection of a female offspring to prevent the transmission of X-linked Mendelian recessive disorders are often applied. Preimplantation genetic diagnosis applied for gender selection can be used for non-Mendelian disorders that are significantly more prevalent in one sex. Three assessments are made prior to the initiation of the PGD process for the prevention of these inherited disorders. In order to validate the use of PGD, gender selection is based on the seriousness of the inherited condition, the risk ratio in either sex, or the options for disease treatment.
Minor disabilities
A 2006 survey reveals that PGD has occasionally been used to select an embryo for the presence of a particular disease or disability, such as deafness, in order that the child would share that characteristic with the parents
Technical aspects[edit]
PGD is a form of genetic diagnosis performed prior to implantation. This implies that the patient’s oocytes should be fertilized in vitro and the embryos kept in culture until the diagnosis is established. It is also necessary to perform a biopsy on these embryos in order to obtain material on which to perform the diagnosis. The diagnosis itself can be carried out using several techniques, depending on the nature of the studied condition. Generally, PCR-based methods are used for monogenic disorders and FISH for chromosomal abnormalities and for sexing those cases in which no PCR protocol is available for an X-linked disease. These techniques need to be adapted to be performed on blastomeres and need to be thoroughly tested on single-cell models prior to clinical use. Finally, after embryo replacement, surplus good quality unaffected embryos can be cryopreserved, to be thawed and transferred back in a next cycle.
Obtaining embryos for preimplantation genetic diagnosis
Currently, all PGD embryos are obtained by assisted reproductive technology, although the use of natural cycles and in vivo fertilization followed by uterine lavage was attempted in the past and is now largely abandoned. In order to obtain a large group of oocytes, the patients undergo controlled ovarian stimulation (COH). COH is carried out either in an agonist protocol, using gonadotrophin-releasing hormone (GnRH) analogues for pituitary desensitisation, combined with human menopausal gonadotrophins (hMG) or recombinant follicle stimulating hormone (FSH), or an antagonist protocol using recombinant FSH combined with a GnRH antagonist according to clinical assessment of the patient’s profile (age, body mass index (BMI), endocrine parameters). hCG is administered when at least three follicles of more than 17 mm mean diameter are seen at transvaginal ultrasound scan. Transvaginal ultrasound-guided oocyte retrieval is scheduled 36 hours after hCG administration. Luteal phase supplementation consists of daily intravaginal administration of 600 µg of natural micronized progesterone.
Oocytes are carefully denudated from the cumulus cells, as these cells can be a source of contamination during the PGD if PCR-based technology is used. In the majority of the reported cycles, intracytoplasmic sperm injection (ICSI) is used instead of IVF. The main reasons are to prevent contamination with residual sperm adhered to the zona pellucida and to avoid unexpected fertilization failure. The ICSI procedure is carried out on mature metaphase-II oocytes and fertilization is assessed 16–18 hours after. The embryo development is further evaluated every day prior to biopsy and until transfer to the woman’s uterus. During the cleavage stage, embryo evaluation is performed daily on the basis of the number, size, cell-shape and fragmentation rate of the blastomeres. On day 4, embryos were scored in function of their degree of compaction and blastocysts were evaluated according to the quality of the throphectoderm and inner cell mass, and their degree of expansion.
Biopsy procedures
As PGD can be performed on cells from different developmental stages, the biopsy procedures vary accordingly. Theoretically, the biopsy can be performed at all preimplantation stages, but only three have been suggested: on unfertilised and fertilised oocytes (for polar bodies, PBs), on day three cleavage-stage embryos (for blastomeres) and on blastocysts (for trophectoderm cells).
The biopsy procedure always involves two steps: the opening of the zona pellucida and the removal of the cell(s). There are different approaches to both steps, including mechanical, chemical, and physical (Tyrode’s acidic solution) and laser technology for the breaching of the zona pellucida, extrusion or aspiration for the removal of PBs and blastomeres, and herniation of the trophectoderm cells.
Embryo transfer and cryopreservation of surplus embryos
Embryo transfer is usually performed on day three or day five post-fertilization, the timing depending on the techniques used for PGD and the standard procedures of the IVF centre where it is performed.
With the introduction in Europe of the single-embryo transfer policy, which aims at the reduction of the incidence of multiple pregnancies after ART, usually one embryo or early blastocyst is replaced in the uterus. Serum hCG is determined at day 12. If a pregnancy is established, an ultrasound examination at 7 weeks is performed to confirm the presence of a fetal heartbeat. Couples are generally advised to undergo PND because of the, albeit low, risk of misdiagnosis.
It is not unusual that after the PGD, there are more embryos suitable for transferring back to the woman than necessary. For the couples undergoing PGD, those embryos are very valuable, as the couple's current cycle may not lead to an ongoing pregnancy. Embryo cryopreservation and later thawing and replacement can give them a second chance to pregnancy without having to redo the cumbersome and expensive ART and PGD procedures.