Pre-Implantation Genetic Screening and Diagnosis: Current Technology
The field of Assisted Reproduction has always been one of rapidly evolving technologies, but nowhere more so than in the area of screening embryos. Screening is possible for not only genetic disease (PGD) and but also abnormal numbers of chromosomes (PGS). Along with the revolution in human DNA biotechnology, new companies such as Gene Security Network of Redwood City, California have emerged. They are able to apply information from the Human Genome Project to the analysis of DNA from single human embryonic cells.
Until recently, a single embryonic cell could only be analyzed for either a mutated gene sequence using a limited number of markers (usually about 10) or for chromosome copy number (karyotype), but not both at the same time. In the last year, Gene Security Network has offered testing of DNA for BOTH genetic mutations (when parents are at risk for having affected offspring) AND for chromosome copy number to rule out Down Syndrome, Trisomy 18, or any other aneuploidies that can cause implantation failure or miscarriage. Both tests can be done on the amplified DNA from a single cell. Pacific Fertility Center has been participating in their pilot studies on this project and, due to the success of the initial group of patients PFC is now offering this type of testing routinely to couples that need this service. We welcome this change because it means we can now not only select unaffected embryos, but also have a fairly high confidence that the embryos we select for embryo transfer have normal chromosome copy numbers and will have a good chance of establishing a normal pregnancy.
Another area in which this DNA micro-array technology has found application is in the area of testing miscarriages for chromosome copy number. In the past, if we wanted to know if the reason a miscarriage had occurred was due to abnormal chromosomes, we had to do a D&C procedure, obtain placental DNA and send it to a cytogenetics lab. At the cytogenetics lab, the placental tissue had to be put into cell culture to try to capture dividing cells, which is the only way a karyotyping analysis could be performed. If the placental tissue contained no viable, living cells, the culture would fail and there would be no results. If the analysis revealed a 46 XX karyotype, we could not be sure that this was a normal female miscarried or if the cell culture was contaminated with maternal DNA. Now we can send the placental tissue with a sample of the mothers blood, and the lab can tell if the DNA is maternal or not and the tissue does not need to be viable to get a result. This then allows us to determine if a pregnancy loss was due to abnormal fetal chromosomes, one of the most common causes of miscarriage.
There is no doubt that all of these new genetic technologies will continue to evolve over time, becoming even more rapid and accurate than they are today. It is exciting to be involved with applying the latest science and biotech has to offer to help solve clinical problems for our fertility patients.
-Carolyn Givens, M.D.
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