Embryo Enhancement Strategies: Preimplantation Genetic Diagnosis
What is PGD?
PGD, or preimplantation genetic diagnosis, is a procedure performed on embryos prior to implantation in the womb. During in vitro fertilization (IVF), embryos are created through fertilization in a laboratory dish and are allowed to develop for a few days. At this early stage, one or two cells are gently removed from the embryo and tested for genetic abnormalities and conditions. Only healthy embryos, as determined by PGD, are selected for implantation. The History of PGD
The first baby born through PGD was in 1990 after testing for cystic fibrosis. In the years since, the technique has improved and can now test for hundreds of genetic disorders. Initially, PGD was used for serious hereditary conditions like cystic fibrosis, Tay-Sachs disease, sickle cell anemia, and muscular dystrophy. Today, it is also applied to detect chromosomal abnormalities like Down syndrome and gender selection for X-linked disorders. PGD has helped many couples avoid passing on serious genetic diseases to their children. The PGD Process
The general steps involved in PGD are:
1) IVF treatment is performed as normal to collect multiple eggs from the female patient. 2) Eggs are fertilized with sperm in the lab to create embryos. 3) Typically on day 3 of development, when the embryo contains 6-10 cells, one or two cells are gently removed for biopsy without harming embryonic development. 4) The biopsied cells are analyzed using techniques like fluorescence in situ hybridization (FISH) or polymerase chain reaction (PCR) to detect abnormalities. 5) Only healthy embryos, as confirmed by PGD results, are selected and implanted back into the woman's womb. Diseases Screened by PGD
Some of the more common genetic disorders for which PGD is often used include: - Cystic fibrosis: A lung disease caused by mutations in the CFTR gene. PGD can detect affected embryos. - Huntington's disease: A neurodegenerative condition caused by CAG repeat expansions. PGD finds embryos free of expansions. - Fragile X syndrome: The most common form of inherited intellectual disability caused by mutations in the FMR1 gene. Embryos can be tested. - Myotonic dystrophy: A muscular dystrophy caused by CTG repeat expansions that PGD can identify in embryos. - Beta thalassemia: An anemia caused by mutations in the HBB gene. PGD selects unaffected embryos. - Sickle cell anemia: Another anemia caused by a mutation in the HBB gene. PGD accurately diagnoses embryos. How Accurate is PGD?
As the techniques used for PGD continue to improve, the tests have become highly accurate. FISH analysis has an accuracy rate of about 95-99% while PCR-based methods detect abnormalities with over 99% accuracy. However, there is still a small chance of diagnostic errors. PGD is not currently able to test for all known genetic disorders and conditions either. While it has helped many couples, PGD is not perfect and misdiagnosis is still possible in a small number of cases. Continued clinical testing and experience will help further enhance the accuracy and reliability of PGD. Overall, when performed by experienced centers, PGD provides an effective method for screening embryos. Limitations and Ethical Considerations
While PGD addresses the desire to have healthy, genetically related children, it is not without limitations and ethical concerns. Aside from occasional misdiagnosis risks, PGD is not accessible or affordable for all couples and families affected by genetic disorders. The procedure is also limited to testing only those disorders with a known genetic cause and diagnostic test. There are ongoing debates around the widening scope of what conditions PGD is used for, including gender selection for social reasons alone. Some argue this could eventually lead to selecting embryos for traits like intelligence or athletic ability. Most experts agree PGD should only be offered for preventing medical disease or conditions. Overall, PGD is an advancing technology that provides benefit but also responsibility in its application. In conclusion, through the advances of preimplantation genetic diagnosis, modern science now offers new hope for couples at high risk of passing on life-altering genetic disorders. When performed carefully by trained professionals, PGD has proven an effective tool for pre-implantation diagnosis of embryos. Continued progress is still needed to further increase accuracy, expand testing options, and ensure judicious application in line with ethical standards. Looking ahead, PGD promises more couples the ability to have healthy children free of devastating genetic diseases.
