- Also called:
- genetic embryo screening
preimplantation genetic diagnosis
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What is preimplantation genetic diagnosis (PGD)?
Preimplantation genetic diagnosis (PGD) is a procedure used with in vitro fertilization (IVF) to test embryos for genetic defects before implantation in the uterus. PGD is an alternative to later prenatal genetic diagnosis which carries the option of terminating the pregnancy.
PGD utilizes genetic analyses like polymerase chain reaction (PCR) to identify single-gene disorders and to determine the sex of the embryo, as well as fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) to find chromosomal defects. PGD can help couples with a family history of single-gene disorders, such as autosomal dominant, autosomal recessive, and X-linked abnormalities, to avoid passing these conditions on to their children. It is also used to improve IVF success in women over 38 or those with repeated IVF failures by screening for embryonic genetic abnormalities that could cause miscarriage or implantation failure.
PGD has been the source of ethical debate since its creation. This includes concerns about discarding normal embryos due to false positives, the desire to avoid knowing parental carrier status, sex selection, screening for late-onset disorders, and the possibility of eliminating individuals with disabilities, such as deafness.
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Why do some people use PGD during in vitro fertilization (IVF)?
During in vitro fertilization (IVF), some people use preimplantation genetic diagnosis (PGD) to test embryos for genetic defects before implanting the fertilized egg in the uterus. PGD helps avoid later pregnancy termination due to inherited genetic diseases. It can also improve IVF success in women over 38 or who have had repeated IVF failures.
PGD is applied when couples have a family history of single-gene disorders, including autosomal dominant, autosomal recessive, and X-linked abnormalities. For women of advanced maternal age (35 or older), PGD can screen for chromosomal disorders. Genetic analyses used in PGD include polymerase chain reaction (PCR), fluorescence in situ hybridization (FISH), and comparative genomic hybridization (CGH).
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How does PGD help identify genetic disorders?
Preimplantation genetic diagnosis (PGD) is a technique used with in vitro fertilization (IVF) to test embryos for genetic defects before implantation. This is done prior to the implantation of the fertilized egg within the uterus. PGD can also be performed on eggs before fertilization.
PGD utilizes genetic analyses such as polymerase chain reaction (PCR) to detect single-gene disorders and to determine the sex of the embryo, as well as fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) to detect chromosomal defects. It is commonly used to screen for autosomal dominant, autosomal recessive, and X-linked abnormalities. PGD can be used by couples with a family history of a single-gene disorder that places the fetus at high risk of inheriting that condition.
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What steps are involved in the PGD process?
Preimplantation genetic diagnosis (PGD) involves testing embryos, produced via in vitro fertilization (IVF), for genetic defects before implanting the fertilized egg in the uterus. PGD can also be performed on eggs before fertilization.
The process includes extracting a single cell from an embryo that has divided into more than five cells. This cell is then analyzed using methods such as fluorescence in situ hybridization (FISH) to detect chromosomal structural abnormalities, polymerase chain reaction (PCR) to detect gene mutations, or comparative genomic hybridization (CGH) to identify chromosomal abnormalities.
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What are some benefits and risks of using PGD?
Preimplantation genetic diagnosis (PGD) is a procedure used with in vitro fertilization (IVF) to test embryos for genetic defects before implantation. PGD can eliminate the risk of later pregnancy termination due to inherited genetic diseases. It may also improve IVF success in women over 38 or with repeated IVF failures by detecting embryonic genetic abnormalities that could cause miscarriage or implantation failure.
However, PGD has ethical concerns. It can produce false positives, leading to discarding normal embryos, or false negatives, where an embryo with a genetic disorder is transferred. There are debates around sex selection and screening for late-onset disorders. The procedure's high cost raises concerns about equal access, and some worry about the potential to eliminate individuals with disabilities. Extracting a cell for testing also carries an inherent risk of damaging the embryo.
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How does PGD raise ethical questions in medicine and society?
Preimplantation genetic diagnosis (PGD) brings forth several ethical questions for medicine and society. Though PGD can prevent later pregnancy termination, it can produce false positives, where a normal embryo is discarded, and false negatives, where an affected embryo is transferred.
Other ethical issues arise when parents seek testing without wanting to know their own carrier status, or for sex selection. Screening for late-onset disorders and using PGD to eliminate individuals with disabilities also spark debate. The limited access to PGD for those who can afford it raises concerns about equitable access.
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preimplantation genetic diagnosis (PGD), the testing of embryos produced through in vitro fertilization (IVF) for genetic defects, in which testing is carried out prior to the implantation of the fertilized egg within the uterus. Preimplantation genetic diagnosis (PGD) also may be performed on eggs prior to fertilization. PGD differs from prenatal genetic diagnosis, in which testing is carried out in utero. PGD is an important accessory to assisted reproductive technologies, since it can eliminate the risk of later elective pregnancy termination for reasons of debilitating inherited genetic disease. There are multiple different genetic analyses used in PGD; examples include polymerase chain reaction (PCR), which can be used to determine the sex of the embryo and to detect single-gene disorders, and fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH), which are used to detect chromosomal defects.
Historical background
PGD was pioneered by American embryologist and geneticist Alan Handyside, who first used the approach in 1989 to test for the presence of the gene defects that cause cystic fibrosis, an X-linked disease (caused by a genetic mutation carried on the X chromosome). The following year, Handyside carried out biopsies on IVF embryos, removing one or two cells from each embryo. The biopsy procedure was performed three days after fertilization. Of three couples who volunteered for the procedure, one woman had two embryos that were fertilized normally and were negative for the cystic fibrosis gene. The woman underwent an embryo transfer with the normal embryo and gave birth to a healthy female who was free of cystic fibrosis and was not a carrier of the gene defects. The results of the trial proved that single-gene diseases could be identified prior to implantation, marking an important development for families who are affected by inherited genetic mutations.
Applications of PGD
PGD can be used for couples who have a family history of a single-gene disorder that places the fetus at high risk of inheriting that condition. Such single-gene disorders include those that are autosomal dominant (a mutation inherited from one parent causes disease), autosomal recessive (the same mutation inherited from both parents causes disease), X-linked dominant (a mutation on an X chromosome from only one parent causes disease), X-linked recessive (the same mutation on the X chromosome from both parents causes disease), Y-linked (the mutation is inherited on the Y chromosome from the father), and mitochondrial disorders (the mutation occurs in mitochondrial DNA and is inherited from the mother). PGD commonly is used to screen for autosomal dominant, autosomal recessive, and X-linked abnormalities. It is less commonly used to screen for mitochondrial disorders, owing in large part to the unpredictable nature of mitochondrial heteroplasmy, in which the cell has some mitochondria that carry the mutation and other mitochondria that do not. The proportion of mutant mitochondria determines both disease expression (whether it will affect offspring) and disease severity.
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Women aged 35 or older are considered to be of advanced maternal age, which raises the risk of transmitting a chromosomal disorder to the offspring. Both aneuploidy (abnormal chromosome number) and chromosomal translocation (part of one chromosome is transferred to another) can be detected by methods such as FISH and CGH.
Ethical considerations
PGD has been at the centre of ethical debate since its inception. Although the procedure can prevent later pregnancy termination, it has been known to produce false positives (in which a normal embryo is discarded) and false negatives (in which the embryo is transferred to the uterus and later develops signs of a genetic disorder). Other ethical concerns have been raised when parents want testing but do not want to know their own carrier status. Other issues have been raised with sex selection; in some countries PGD has been used specifically to obtain the “more desired” male child. In the late 1990s PGD was developed to screen for certain late-onset disorders with hereditary predisposition. Testing for potentially life-threatening diseases that may or may not appear in adulthood has created considerable controversy. There is also debate about using PGD to eliminate individuals with certain disabilities, such as deafness. Other ethical debate concerned the exclusive use of the technique by only those who could afford it.
In some countries cases for PGD are first reviewed by experts, who take into consideration the specific genetic disease and the seriousness of the condition and then make recommendations as to whether the procedure should be approved. The Preimplantation Genetic Diagnosis International Society was formed in 2002 to provide multidisciplinary research and education in the PGD arena and to advance the science of PGD.