PGD Technology Offers More Than Designer Babies
Pacific Fertility Center is a leading
PGD is a technique used in conjunction with IVF where a cell is removed from an embryo prior to transfer back to the mother. The DNA in the cell can be analyzed for novel or inherited genetic errors such as Down syndrome or cystic fibrosis. Normal embryos, based on the genetic analysis, are chosen for transfer back to the mother’s uterus to achieve pregnancy.
PGD allows diagnosis prior to implantation–or transfer of the embryo. Prior to discovery of the PGD technique, patients at risk for certain genetic illnesses faced diagnosis during pregnancy after undergoing amniocentesis or chorionic villous sampling. An abnormal result would result in genetic counseling, and termination of the pregnancy was a common choice if an abnormality was found. PGD allows diagnosis before implantation, while the embryo is still in the laboratory. Healthy embryos can be selected for transfer, well before the pregnancy is established.
Embryos that are tested using PGD are produced via conventional IVF techniques. For IVF, a woman is placed on fertility drugs to produce multiple eggs. When mature, the eggs are removed from the ovary with a needle, under anesthesia. The eggs are inseminated with sperm and the resulting embryos are cultured for several days in the IVF laboratory. For PGD, on the third day of embryo development, at the 6-8 cell stage, one or two cells are removed from the embryo. The cells are analyzed for chromosome count, specific DNA sequences, or markers linked to a specific inherited defect. After analysis, normal embryos are transferred directly into the uterus–the normal site for implantation–using a small catheter placed through the cervix. Embryo transfer is a simple procedure that requires a few minutes of office time.
For illnesses such as cystic fibrosis, sickle cell anemia, and other single gene defects where the DNA mutation is limited to a specific location on a gene, PGD can diagnose affected embryos. After the biopsy of one or two cells, polymerase chain reaction (PCR) is used to amplify the DNA sequence of interest. The normal and abnormal genetic sequences in the DNA can be amplified and tagged with specific probes, revealing which embryos are affected by the mutation. In circumstances where the specific mutation cannot be identified, genetic markers located near the mutation of interest can be used to identify affected embryos.
Using a technique known as Fluorescent In Situ Hybridization (FISH), PGD can also be used to count the chromosomes in the cell removed from an embryo. A normal cell has a complement of 23 pairs of chromosomes (22 pairs plus a pair of sex chromosomes: 2 X chromosomes for a female or an X and a Y for a male). Uniquely colored fluorescent probes attach, or hybridize, to specific chromosomes. A spot of color for each of the probes appears under fluorescent light, each color spot represents a chromosome under study. This analysis can show abnormal chromosome pairings such as three copies of chromosome 21, which indicates Down syndrome, or a single copy of the X chromosome which indicates Turner’s syndrome.
Similar techniques have been used for chromosomal translocations, common in miscarriages, X-linked genetic illness; gender identification for family balancing; and for HLA typing for use in cancer therapy.
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Pacific Fertility Center is a fertility medical practice and egg donor agency located in
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