Introduction
Preimplantation genetic screening (PGS) involves the assessment of the chromosomal composition of embryos and evaluating for numerical chromosomal abnormalities with a view to selecting normal embryos for transfer. PGS should be differentiated from preimplantation genetic diagnosis (PGD). PGD allows couples where one or both partners carry a known genetic condition to have their embryos tested to avoid the transfer of a genetically affected embryo.
The application of PGD has expanded, with over 200 single gene and chromosomal disorders currently amenable to PGD. It has enabled couples affected by such disorders to avoid the need to undergo prenatal diagnosis.
What is Preimplantation Genetic Diagnosis (PGD)?
Preimplantation genetic diagnosis is a ray of hope for the couples with metabolic disorders, single gene disorder, X-linked disorder, translocation and severe male factor infertility. In this procedure, patients are given injections for ovarian stimulation like normal IVF (in vitro fertilization). Eggs are removed and ICSI (intracytoplasmic sperm injection) is done. Embryo biopsy can be done on day 3 or trophectoderm biopsy on day 5. The single cell is removed from the embryo and tested for various disorders.
It is preimplantation genetic testing to identify genetic defects in embryos through IVF (in vitro fertilization). Only unaffected embryos are transferred into the uterus for implantation. PGD is alternative to post conception testing like amniocentesis and chorionic villus sampling, which are difficult. It is an attractive way of preventing heritable genetic diseases
Who Should Go for Preimplantation Genetic Diagnosis (PGD)?
- It is recommended when couple are at high risk of transmitting a known genetic abnormality to their children. In common conditions like family history of X-linked disorders, they have risk of transmission to 50 percent male babies if the mother is a carrier of conditions like DMD (Duchenne muscular dystrophy), hemophilia, fragile X syndrome (FXS), neuromuscular dystrophies and Rett syndrome.
- Couples with chromosomal translocation, which is a common cause of recurrent abortions.
- Recurrent pregnancy loss. About 50 to 80 percent abortions shows chromosomal abnormalities in recurrent pregnancy loss, high rate of aneuploidies (abnormal number of chromosomes in a human cell).
- Recurrent IVF failures, that is three or more IVF failures. It has been proven that embryos of good quality and free of chromosomal defects increase implantation of the embryo and pregnancy rate.
- Single gene disorders like cystic fibrosis, sickle cell anemia, Huntington disease, Tay-Sachs disease require molecular diagnosis with PCR (polymerase chain reaction).
- In women with advanced maternal age, the risk of aneuploidy increases. Chromosomes in the egg are less likely to divide properly in women who have crossed 35 years of age.
- Male factor infertility - normal sperm count has 3 to 8 percent of abnormal sperm. Male with low sperm count and motility are associated with a high rate of abnormal sperm like 27 to 74 percent. Commonly male factor infertility is seen in Klinefelter syndrome and Robertsonian translocation. In such cases, the chromosome microdeletion can be transmitted genetically and lead to abnormal embryos and failure of implantation.
- HLA matching that is required in cases like thalassemia, leukemia for stem cell or bone marrow transplantation.
What Is the Need for Preimplantation Genetic Diagnosis (PGD)?
The need for Preimplantation Genetic Diagnosis (PGD) arises from the increased risks associated with advanced maternal age, particularly after the age of 35. As women age, the likelihood of implantation failure, multiple IVF failures, and pregnancy loss due to aneuploidy (abnormal number of chromosomes in the embryo) becomes higher. This can be a major challenge for couples seeking to conceive through in vitro fertilization (IVF) and can lead to emotional distress and financial strain.
PGD offers a valuable solution to this problem by allowing for the screening of embryos for genetic abnormalities before they are implanted in the uterus. With PGD, embryologists can identify embryos with the correct number of chromosomes, thereby selecting normal embryos with higher chances of successful implantation and development. This process could potentially improve the overall implantation rates and success rates of IVF procedures, providing hope for couples facing the complexities of fertility treatments.
By ensuring that only genetically healthy embryos are transferred during IVF, PGD minimizes the risk of miscarriage and enhances the chances of a successful pregnancy for women of advanced maternal age and those with known genetic conditions. It not only offers hope for a higher rate of successful pregnancies but also helps reduce the emotional burden and financial costs associated with repeated IVF failures.
However, it is important to acknowledge that PGD raises ethical considerations, including the potential for embryo selection based on non-medical traits or sex preferences. It is crucial for healthcare providers to exercise responsible and ethical use of this technology, adhering to patient autonomy and informed consent while upholding the principles of equitable access to reproductive options.
The need for PGD is driven by the desire to enhance the success rates of IVF for couples facing genetic risks and those with advanced maternal age. By providing a means to select genetically healthy embryos, PGD presents a promising tool in the field of assisted reproductive technologies, giving prospective parents a chance to have healthy pregnancies and build healthy families. However, responsible implementation and ongoing research are essential to ensure that PGD is utilized ethically and responsibly, respecting the diverse needs and values of patients while promoting the well-being of future generations.
What Are The Other Techniques of Preimplantation Genetic Diagnosis?
The conventional method of diagnosis of genetic defects is by a technique called PCR that is polymerase chain reaction. This is used to detect target specific whole chromosomal anomalies like Trisomy 13, 18 and 21 and sex chromosomal anomalies X and Y. It is relatively cheap and rapid to provide results within 48 hours to 72hours. Recent advances in the field of preimplantation genetic testing are-
- FISH (Fluorescence In Situ Hybridization) is a useful adjunct to full conventional karyotyping when a high degree of suspicion of specific chromosome abnormality is present.
- CGH (Comparative Genomic Hybridization) provides a higher detection rate of chromosomal abnormalities and more definitive diagnosis. The only drawback is the high cost.
- Other newer technologies include NGS (Next Generation Sequencing). This technology allows sequencing DNA as well as RNA more quickly and can give complete genetic mapping.
Conclusion
In conclusion, Preimplantation Genetic Diagnosis (PGD) has emerged as a groundbreaking reproductive technology that allows prospective parents to make informed decisions regarding genetic disorders before embryo implantation. This powerful screening technique offers hope to couples with known genetic conditions, increasing the chances of a successful pregnancy while reducing the risk of passing on inherited disorders to their offspring. As PGD continues to advance, it holds the promise of reshaping the landscape of assisted reproductive technologies and contributing to a future where families can make well-informed choices for the health and well-being of their children. However, ethical considerations and ongoing research will be crucial in ensuring responsible and ethical use of this technology to uphold the principles of patient autonomy, informed consent, and equitable access to reproductive options.
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