Genetic profiling of embryos prior to implantation
1♂︎—Sperm is collected from a male. 1♀︎—Eggs are collected from a female by a thin needle passed either through vagina or through abdomen under ultrasound guidance [transvaginal or transabdominal ultrasound aspiration]. 2a—The sperm and eggs are fertilized. 2b—The resulting embryos are kept safe and watched to see which will thrive. 3a—The embryos are allowed to develop; those that thrive are given identifiers. 3b—A genetic test is run on each embryo for a given trait and the results are matched with the embryos. 4—The embryos without the desired trait are identified and discarded. 5—The remaining embryos are allowed to grow to the point that they can be implanted. 6a—The embryos with the desired trait are implanted. 6b—The embryos result in a healthy pregnancy. 6c—Fraternal twins with the desired trait, not expressed in their mother, are born.
Preimplantation genetic diagnosis (PGD or PIGD) is the genetic profiling of embryos prior to implantation (as a form of embryo profiling),[1] 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.[2]
The world's first PGD was performed by Handyside,[3] 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."[3][4]
The term preimplantation genetic screening (PGS) refers to the set of techniques for testing whether embryos (obtained through IVF/ICSI) have abnormal chromosomes' number. In other words, it tests if an embryo is aneuploid or not. PGS is also called aneuploidy screening. PGS was renamed preimplantation genetic diagnosis for aneuploidy (PGD-A) by Preimplantation Genetic Diagnosis International Society (PGDIS) in 2016.[5]
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.[6]
This kind of tests are need because hereditary disorders are related with 20% of child deaths in developed countries. It is estimated that, as a whole, they are responsible for around 18% of pediatric hospital admissions.
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.[7]
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.
^"PGD / PGS – A boon for couples with genetic issues – Times of India". The Times of India. 31 May 2019.
^Sullivan-Pyke C, Dokras A (March 2018). "Preimplantation Genetic Screening and Preimplantation Genetic Diagnosis". Obstetrics and Gynecology Clinics of North America. 45 (1): 113–125. doi:10.1016/j.ogc.2017.10.009. PMID 29428279.
^ abHarper, Joyce C. (2009). "Introduction to preimplantation genetic diagnosis" (PDF). In Harper, Joyce (ed.). Preimplantation Genetic Diagnosis. pp. 1–10. doi:10.1017/CBO9780511581571. ISBN 978-0-511-58157-1.
^Handyside AH, Kontogianni EH, Hardy K, Winston RM (April 1990). "Pregnancies from biopsied human preimplantation embryos sexed by Y-specific DNA amplification". Nature. 344 (6268): 768–70. Bibcode:1990Natur.344..768H. doi:10.1038/344768a0. PMID 2330030. S2CID 4326607.
^"PGDIS Position Statement on Chromosome Mosaicism and Preimplantation Aneuploidy Testing at the Blastocyst Stage". Preimplantation Genetic Diagnosis International Society. July 19, 2016.
^How does DGP work? Infographic Retrieved 28. June 2015
^Page 205 in: Zoloth, Laurie; Holland, Suzanne; Lebacqz, Karen (2001). The human embryonic stem cell debate: science, ethics, and public policy. Cambridge, Mass: MIT Press. ISBN 978-0-262-58208-7.
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