Advances in the late twentieth century in the fields of in vitro fertilization (IVF), human embryo culture and manipulation, molecular genetics, and fluorescence in situ hybridization (FISH), set the stage for the...
Advances in the late twentieth century in the fields of in vitro fertilization (IVF), human embryo culture and manipulation, molecular genetics, and fluorescence in situ hybridization (FISH), set the stage for the development of preimplantation genetic diagnosis (PGD). From an essentially research-based exercise in a very few laboratories in the early 1990s, it has progressed to being, in the 2010s, a diagnostic tool available through a number of larger IVF clinics. PGD is applied in two main genetic settings: for the diagnosis of chromosome disorders, and for the detection of a mendelian condition. Initially, the two categories were distinguished by the methodology applied: FISH in the former, DNA testing in the latter; a distinction that is now blurring, as molecular methodologies advance. There has arisen a praiseworthy tradition of excellent communication between the major centers that do this work, with the majority contributing their data to an international clearing house (under the aegis of ESHRE, the European Society for Human Reproduction and Embryology), and detailed analyses of the accumulated experience of the participating clinics are documented in the annual reports of the ESHRE PGD consortium, which appear in the journal Human Reproduction (Harper et al., 2010b); and the International Society for Prenatal Diagnosis sponsors a regular conference devoted to PGD. Thus, new knowledge from the leading centers can translate readily into improved services to patients worldwide.
Chromosomal PGD is typically done on a blastomere (single cell) from a 6–10 cell embryo1 at day 3. Alternative approaches are blastocyst biopsy at day 5 and polar body biopsy. The selected embryo is then transferred that same day (day 3 transfer) or maintained in culture a little longer (day 5 or 6 transfer). In principle, and barring the presence of mosaicism (a very important issue; see later), the pregnancy can proceed in the knowledge that the baby will be unaffected. There are two main categories of chromosomal PGD: focused PGD with respect to a particular parental translocation, or other rearrangement; and PGD being done as a general aneuploidy screen (PGD-AS).
Chapter. 7551 words. Illustrated.
Subjects: Clinical Genetics
Full text: subscription required