Preimplantation genetic diagnosis: Its time is now
Dr Simpson, a member of the Contemporary OB/GYN editorial board, is Executive Associate Dean for Academic Affairs and Professor of Obstetrics and Gynecology and Human and Molecular Genetics at Florida International University College of Medicine, Miami.
Preimplantation genetic diagnosis (PGD) is 25 years old. Its technology and novel indications have long dazzled, but some continued to feel that the field was just boutique medicine. In 2016 we can now confidently state that PGD is an integral part of not only medical genetics but also, increasingly, reproductive medicine and infertility care. Evolving technology has welded pragmatism with vision. Here I review developments that have made this transition feasible.
Obtaining DNA through trophectoderm biopsy
There have always been 3 sources for obtaining embryonic DNA for PGD: (1) polar body biopsy, prior to or at the time of fertilization, (2) blastomere biopsy from the 3-day 6- to 8-cell cleaving embryo, and (3) trophectoderm biopsy from the 5- to 6-day blastocyst. Initial work in PGD involved removal of a single blastomere, the zona pellucida traversed by mechanical or laser means.
Today, the preferred approach for PGD involves biopsy of the trophectoderm in the 5- to 6-day blastocyst, since more than a single cell can be safely removed, and thus more DNA is available. A further advantage is that the trophectoderm forms the placenta; thus, the 5â10 cells removed were never destined to be part of the embryo itself (inner cell mass).
PGD for single-gene disorders
When first performed, the intention of PGD was to diagnose single-gene disorders. Approximately 20% of PGD cases are now performed on couples at risk for one or more single-gene disorders. Despite the miniscule amount of DNA, PGD can be performed whenever the chromosomal location of the gene causing the disorder is known. The causative mutation need not even be known so long as affected and unaffected family members are available to determine markers linked to the mutation site. This allows one to deduce whether a given embryo has or has not inherited the mutation.