Susan H. Black, M.D. Genetics & IVF Institute
Preimplantation genetic diagnosis represents a new alternative to chorionic villus sampling or amniocentesis for parents at high risk of having offspring with certain genetic diseases. Assisted reproductive techniques interface with exciting advances in molecular genetic diagnosis to provide identification of an unaffected embryo prior to implantation.
Consider a woman known to be carrying an X-linked disease with a 50% risk of an affected male in each pregnancy. In addition, her daughters have a 50% risk of being carriers, but are unlikely to be clinically affected. She may not wish to become pregnant if she has to make decisions about an affected child in a viable pregnancy. However, she would become pregnant if she knew she had conceived a daughter, and with preimplantation diagnosis this possibility becomes a reality.
How is this done? Early embryos are obtained by IVF or ICSI: stimulated cycles, transvaginal ultrasound guided oocyte retrieval, and fertilization by sperm in vitro. The resulting embryos are then biopsied to obtain blastomeres (single cells) for molecular diagnosis. In one method to biopsy an embryo, the embryo is held against the tip of a pipette while a fine glass needle is used to make a small slit in the zona (outer layer) and a cell (or two) removed by gentle suction. The cell is then available for diagnosis, and the embryo, if healthy, is subsequently transferred to the uterus.
Analysis of genetic material (DNA) from a single cell has been made possible by a recent technique of amplifying minute quantities of DNA. This process, polymerase chain reaction (PCR), involves several steps that allow a specific DNA sequence to be replicated many times and then rapidly analyzed.
Success with preimplantation testing was first achieved in London, England by detecting X (female) and Y (male) embryos in families with X-linked diseases, such as Duchenne muscular dystrophy or hemophilia. Several unaffected girls have been born after embryo biopsy with sexing by Y-specific DNA amplification at GIVF and a few other leading centers. Scientists at GIVF can also test for other diseases such as cystic fibrosis, fragile X, Marfan syndrome, Huntington disease, and spinal muscular atrophy (SMA) in the preimplantation embryo. Once the appropriate molecular diagnosis is made, unaffected embryos can be transferred back into the uterus in the IVF cycle, or after freezing and thawing in a subsequent cycle. Prenatal confirmation of normality can be performed if desired by standard techniques: chorionic villus sampling, amniocentesis, ultrasound, or fetal blood analysis.
Preimplantation genetic diagnosis addresses the need of a special group of selected patients, and, because of the need for complex genetic and IVF services, it will probably continue to be limited to a few specialized centers such as GIVF. The GIVF preimplantation genetics program has several world "firsts" in its field. We were the first to perform specific preimplantation testing and achieve pregnancy in patients with Marfan syndrome and fragile X syndrome. We also obtained the world's first IVF pregnancy with a new method of X/Y sperm separation (MicroSort) that can enhance the proportion of female embryos for patients at risk for disease occurring only in males (sex-linked diseases).
For more information on preimplantation genetic testing, contact GIVF at 1-800-552-4363, by fax at 703-698-0418, or by email at GIVF@GIVF.com.
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rare genetic diseases.
Click here for review covering these and other topics: Recent Advances in Reproductive Genetic Technologies.
Click here for an essay covering recent advances in this and other fields: GIVF: A Decade of Achievement and Service.
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