Preimplantation genetic diagnosis (PGD or PIGD) is the 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 abortion, 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. Embryos are generally obtained through blastomere or blastocyst biopsy. The latter technique has proved to be less deleterious for the embryo, therefore it is advisable to perform the biopsy around day 5 or 6 of development.
The world's first PGD was performed by Handyside, Kontogianni and Winston at the Hammersmith Hospital in London. "Female embryos were selectively transferred in five couples at risk of X-linked disease, resulting in two twin and one singleton pregnancy."
The term preimplantation genetic screening (PGS) refers to the set of techniques for testing whether embryos (obtained through IVF/ ICSI have an abnormal number of chromosomes (aneuploidy). PGS is also called aneuploidy screening. PGS was renamed preimplantation genetic diagnosis for aneuploidy (PGD-A) by the Preimplantation Genetic Diagnosis International Society (PGDIS) in 2016.
The PGD allows studying the DNA of eggs or embryos to select those that carry certain mutations for genetic diseases. It is useful when there are previous chromosomal or genetic disorders in the family and within the context of in vitro fertilization programs.How does DGP work? Infographic Retrieved 28. June 2015
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.Page 205 in:
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.
History In 1968, Robert Edwards and Richard Gardner reported the successful identification of the sex of rabbit blastocysts. It was not until the 1980s that human IVF was fully developed, which coincided with the breakthrough of the highly sensitive polymerase chain reaction (PCR) technology. Handyside, Kontogianni, and Winston's first successful preimplantation diagnostic tests took place in October 1989, with the first births in 1990, though the preliminary experiments had been published some years earlier. In these first cases, PCR was used for sex determination of patients carrying X-linked diseases. A 2001 report on worldwide use of PGD since 1990 noted that embryo or polar body biopsies had occurred in more than 3000 clinical cycles, with a 24% pregnancy rate — comparable to assisted reproductive practices not involving biopsy.
First clinical cases
Elena Kontogianni was studying for her PhD at the Hammersmith Hospital, on single-cell PCR for sexing, which she did by amplifying a repeated region of the Y chromosome. It was this approach that she used for the world's first PGD cases.
Female embryos were selectively transferred in five couples at risk of X-linked disease, resulting in two twins and one singleton pregnancy. Because the Y chromosome region Kontogianni was amplifying contained many repeats, it was more efficient than trying to amplify a unique region. A band on the PCR gel indicated that the embryo was male and the absence of a band indicated that the embryo was female. However, amplification failure or an anucleate blastomere also resulted in absence of a band on the PCR gel. To reduce the risk of misdiagnosis, Kontogianni went on to co-amplify sequences on the X and Y (Kontogianni et al., 1991).Kontogianni, E. H., Hardy, K. and Handyside, A. H. (1991). "Co-amplification of X- and Y-specific sequences for sexing human preimplantation embryos". In «Proceedings of the First Symposium on Preimplantation Genetics», pp 139–142. Plenum, New York At that time nothing was known about allele dropout, cumulus cell contamination, or amplification failure from single cells. During the 1980s, human IVF embryos were exclusively transferred on day two of development as the culture medium used was incapable of reliably growing embryos past this stage. Since the biopsy was to be performed on day three, the first diagnoses were all performed in one day, with transfer of the embryos late on day three. A comparison of day two and day three transfers indicated that this would not adversely affect pregnancy rates. The worry of embryos arresting was so high that some transfers took place in the early hours of day four so that the embryos were removed from culture as soon as possible. There were many evenings at the Hammersmith when a transfer was performed at 1 a.m. on day four and researchers returned to the laboratory at 7 a.m. to start the next case. Winston helped deliver most of the first PGD babies.
PGD became increasingly popular during the 1990s when it was used to determine a handful of severe genetic disorders, such as sickle-cell anemia, Tay–Sachs disease, Duchenne's muscular dystrophy, and beta-thalassemia.
Indications and applications PGD is used primarily for genetic disease prevention, by selecting only those embryos that do not have a known genetic disorder. PGD may also be used to increase 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.
PGD is frequently employed for the detection of autosomal dominant, autosomal recessive, and X-linked abnormalities. However, its utilization in screening for mitochondrial disorders is less common, primarily due to the unpredictable characteristics of mitochondrial heteroplasmy. In cases of mitochondrial heteroplasmy, some mitochondria within a cell bear the mutation, while others do not. The ratio of mutant mitochondria plays a crucial role in determining both the expression of the disease (its impact on offspring) and the severity of the disease.
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. Furthermore, a study found that diagnoses of the biopsies from the same embryos at two separate laboratories matched up only 50% of the time.
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. Normal live births of healthy offspring after transfers of embryos deemed aneuploid by PGP have been reported worldwide.
Alternative methods to determine embryo quality for prediction of pregnancy rates include microscopy as well as profiling of RNA and protein expression.
HLA matching
Human leukocyte antigen (HLA) typing of embryos, so that the child's HLA matches a sick sibling, availing for cord-blood stem cell donation.Pattinson 2003 The child is in this sense a "savior sibling" for the recipient child. HLA typing has meanwhile become an important PGD indication in those countries where the law permits it. The HLA matching can be combined with the diagnosis for monogenic diseases such as Fanconi anaemia or beta thalassemia in those cases where the ailing sibling is affected with this disease, or it may be exceptionally performed on its own for cases such as children with leukaemia. The main ethical argument against is the possible exploitation of the child, although some authors maintain that the Kantian imperative is not breached since the future donor child will not only be a donor but also a loved individual within the family.
Cancer predisposition A more recent application of PGD is to diagnose late-onset diseases and (cancer) predisposition syndromes. Since affected individuals remain healthy until the onset of the disease, frequently in the fourth decade of life, there is debate on whether or not PGD is appropriate in these cases. Considerations include the high probability of developing the disorders and the potential for cures. For example, in predisposition syndromes, such as BRCA mutations which predispose the individual to breast cancer, the outcomes are unclear. Although PGD is often regarded as an early form of prenatal diagnosis, the nature of the requests for PGD often differs from those of prenatal diagnosis requests made when the mother is already pregnant. Some of the widely accepted indications for PGD would not be acceptable for prenatal diagnosis.
For late-onset conditions A woman carrying the gene for early-onset Alzheimer's disease used preimplantation genetic diagnosis (PGD) to ensure that her child would not inherit this condition. Ethical questions arise regarding the decision to allow an individual aware of their own susceptibility to a late-onset disease to have a child free of the gene, despite the risk that the child may lose a parent prematurely. Some argue that this decision is ethical, asserting that the desire for reproduction is as legitimate for these individuals as for others seeking infertility services. Comparisons are drawn with other medical situations, such as assisted reproduction for individuals with HIV or other serious illnesses. Although the child may face risks of early bereavement, the argument put forth is that the psychological trauma does not render the child's life devoid of clear benefits. Therefore, assisting parents in reproducing in these circumstances is not considered to cause undue or unnecessary suffering to the child.
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.