Preimplantation Genetic Diagnosis Comes of Age

On July 15, 2011, preimplantation genetic diagnosis (PGD) came of agewith the 21st birthdayof Natalie and Danielle, the first twin girls to be born in theworld to a couple at risk of the X-linked condition adrenoleukodystrophy.1 Since then, many thousands of couples have undergone in vitro fertilization (IVF) and genetic analysis of single or small numbers of cells biopsied from preimplantation embryos, and thousands of infants were born following transfer of embryos diagnosed as unaffected by genetic defects identified in one or both parents.2 The goal of helping at-risk couples avoid the trauma of prenatal diagnosis and the possibility of terminating affected pregnancies has thus been achieved, and PGD is now clinically well established worldwide. In this issue, we have assembled a series of expert comprehensive and detailed reviews on the achievements and controversies of the last 2 decades as well as more recent developments in PGD, focusing mainly on the current clinical state of the art. PGD is a complex clinical procedure. To help a couple have a healthy child free of severe genetic disorders requires the close collaboration of several disciplines including clinical genetics, reproductive medicine, clinical embryology, andmolecular genetics to identify the genetic defect in the family, inform the couples of the risks as well as the potential benefits of PGD, manage the ovarian stimulation safely to maximize the chance of identifying unaffected embryos, biopsy the embryos minimizing any damage, and finally perform highly accurate single-cell genetic testing. With hindsight, PGD was born prematurely. In the late 1980s, pregnancy rates following IVF were much lower than those routinely achieved today. The genes involved in common inherited diseases, such as cystic fibrosis and Duchenne's muscular dystrophy, had only recently been identified. The discovery of the polymerase chain reaction (PCR) for amplifying short fragments of DNAwas promising, but the amplification of single-copy sequences from single cells was an enormous challenge. The oligonucleotide primers required to define the fragment to be amplified were difficult to design, with often limited access to sequence information, and they were unreliable. Heroic numbers of PCR cycles were required to produce enough DNA to visualize by basic methods, and consequently theywere exquisitely sensitive to contamination with previously amplified PCR products. Single-cell PCR was also error prone because one parental copy (allele) sometimes failed to amplify (i.e., allele dropout [ADO]), a problem that remains unsolved today. PGD therefore gained a reputation as a difficult and inefficient technology with a high risk of misdiagnosis, which it has never completely shaken off. Furthermore, fueled by the intense debate over the ethics of human embryo research, particularly in the United Kingdom, and the publicity surrounding the first cases that involved identifying the gender of the embryo to avoid sex-linked diseases typically only affecting males, but with obvious potential for social sex selection, the epithet “designer baby” was coined and to this day hasmired PGD in unnecessary regulation not applied to other methods of prenatal diagnosis.3 Today, with improvements to embryo culture media that enable extended culture of human embryos to the blastocyst stage, implantation rates per embryo transferred have increased significantly. Thus prospects for at-risk couples to achieve a pregnancy following PGD have increased correspondingly. This is particularly so if they are young and fertile and have one or more good quality unaffected blastocysts for transfer. Until recently, PGD has been performed either on the two by-products of female meiosis, the first and second polar bodies, as pioneered by Yury Verlinsky and colleagues,4 or on single cells biopsied from embryos at cleavage stages when the embryo has divided to the 6to 10-cell stage early on the Alan H. Handyside, Ph.D. Kangpu Xu, Ph.D., H.C. L.D.