For some adults who have a genetic disease or suspect they carry genes that might cause a genetic disorder, the risk of passing that disease on to their children is simply too great. A woman with Huntington disease, for example, has a 50 percent chance of passing her debilitating and ultimately lethal neurological disorder on to her child. A man who carries the gene for cystic fibrosis has a 25 percent chance of fathering a child with the disease if his female partner is also a carrier. Many such people sacrifice their desire to bear children in order to stop a family disease in its tracks. But thanks to a technique called pre-implantation genetic diagnosis, some disease sufferers and carriers are finding it is possible to have children, with far less worry about genetic consequences.
Despite its short history -- the technique has been in use for only a few years -- pre-implantation genetic diagnosis (PGD) holds once-unimaginable potential for thousands of people with a strong family history of genetic disease. The technique is similar to other forms of genetic testing, but instead of focusing on adults, it allows doctors to look for gene and chromosome abnormalities in developing embryos so tiny they can only be seen with a microscope.
PGD capitalizes on the techniques of in vitro fertilization (IVF) developed in the late-1970s as a solution to infertility. In an IVF procedure, a woman's ovaries are stimulated with fertility drugs to produce many more eggs than they would normally in one cycle. These eggs are harvested and placed in a petri dish, where they are then fertilized with sperm. Doctors select one or more of the embryos that subsequently develop and, usually on the third day, place them back in the woman's uterus where one will hopefully "implant" and continue to develop.
With PGD, the standard IVF process is interrupted. On the third day, before implanting the embryo (a mass of just eight to ten cells at this point), geneticists extract one of the cells and analyze its DNA. Depending on the disease or diseases the geneticists are screening for, they may look for chromosome abnormalities, gene abnormalities, or both. If, for instance, their main concern is Down's syndrome, a disease caused by the presence of an extra chromosome 21, they may look only for that chromosome. If they are concerned about Huntington disease, caused by a single gene on chromosome 4, they look specifically for that gene.
There is tremendous potential for PGD, thanks to our newfound knowledge of the human genome. For the moment, however, the technique is limited in its usefulness. It remains prohibitively expensive and/or unavailable to most people. There may be only a few thousand performed worldwide this year. More importantly, experts say, PGD cannot yet provide a reliable diagnosis of an embryo's health, because doctors perform the analysis on just one or two of each embryo's cells. Chromosomal abnormalities can remain hidden in such a small sample. And, while a more developed embryo would provide doctors with more cells to analyze, embryos that are more than four or five days old do not easily implant in the uterus. Researchers are working now to address this problem.
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