Preimplantation Genetic Diagnosis

There are an increasing number of genetic disorders that can be diagnosed by direct DNA analysis, such as cystic fibrosis. For couples at high risk for having affected offspring, the options for prenatal diagnosis were previously limited to analyzing fetal DNA samples obtained by chorionic villus sampling or amniocentesis. However, couples then face the dilemma of whether or not to terminate the pregnancy if the genetic abnormality is present. In some cases this may not be a viable option for religious or personal reasons. 

An alternative approach, referred to as preimplantation genetic diagnosis, is to sample DNA from embryos which are derived from the in vitro fertilization of the mother's eggs by the father's sperm. Following several embryonic cell divisions, one or more cells are removed from the developing embryos and the DNA amplified and analyzed for the genetic abnormality. The results can often be obtained within 12-24 hours. Embryos without the genetic defect are then transferred into the mother's uterine cavity to develop into a normal pregnancy. 

Research towards developing techniques for early genetic diagnosis in humans was initiated in the UK in the late 1980s. The first report of the successful application of this technique came from the Hammersmith Hospital, London, UK, which currently is the center with the highest number of births following preimplantation diagnosis. Over 30 pregnancies have now been reported globally including the US. The conditions diagnosed include cystic fibrosis, Tay-Sachs disease, hemophilia A, and fragile-X syndrome. 

Only a few centers worldwide are offering preimplantation diagnosis. The number of successful cases is small enough such that this method of diagnosis is still considered to be experimental. Efforts continue to be focused on improving methods to obtain an accurate diagnosis from only one or two cells. Techniques are now available to screen for more than one condition simultaneously, however the accuracy of these modifications needs to be tested further. Although there is certainly a demand for this approach, it will continue to be available only in select specialized institutions with advanced in vitro fertilization and molecular biology laboratories.