Preimplantation genetic diagnosis: state of the art.

Preimplantation genetic diagnosis (PGD) is used to analyze embryos genetically before their transfer into the uterus. It was developed first in England in 1990, as part of progress in reproductive medicine, genetic and molecular biology. PGD offers couples at risk the chance to have an unaffected child, without facing termination of pregnancy. Embryos are obtained by in vitro fertilization with intracytoplasmic sperm injection (ICSI), and are biopsied mostly on day 3; blastocyst biopsy is mentioned as a possible alternative. The genetic analysis is performed on one or two blastomeres, by fluorescent in situ hybridization (FISH) for cytogenetic diagnosis, or polymerase chain reaction (PCR) for molecular diagnosis. Genetic analysis of the first or second polar body can be used to study maternal genetic contribution. Only unaffected embryos are transferred into the uterus. To improve the accuracy of the diagnosis, new technologies are emerging, with comparative genomic hybridization (CGH) and microarrays. In Europe, depending on national regulations, PGD is either prohibited, or allowed, or practiced in the absence of recommendations. The indications are chromosomal abnormalities, X-linked diseases or single gene disorders. The number of disorders being tested increases. In Europe, data collection from the year 2004 reports that globally 69.6% of cycles lead to embryo transfer and implantation rate is 17%. European results from the year 2004 show a clinical pregnancy rate of 18% per oocyte retrieval and 25% per embryo transfer, leading to 528 babies born. The cohort studies concerning the paediatric follow-up of PGD babies show developmental outcomes similar to children conceived after IVF-ICSI. Recent advances include human leucocyte antigen (HLA) typing for PGD embryos, when an elder sibling is affected with a genetic disorder and needs stem cell transplantation. The HLA-matched offspring resulting can give cord blood at birth. Preimplantation genetic screening (PGS) consists in euploid embryo selection; it could be used for advanced maternal age, repeated implantation failure, single embryo transfer or idiopathic recurrent pregnancy loss. These applications are controversial. PGD for inherited cancer predispositions is discussed and social sexing remains prohibited in Europe. PGD requires a close collaboration between obstetricians, fertility specialists, IVF laboratory and human geneticists. It needs intensive effort, expensive techniques and is demanding for the patients, but it offers tremendous opportunity for couples whose previous child has exhibited genetic abnormalities. The debate on certain indications is ongoing.