Pre-implantation genetic diagnosis
PGD: who is it for?
Pre-implantation genetic diagnosis (PGD) is offered when one of the two members of a couple, or both, presents either a chromosome abnormality or a gene DNA mutation identified previously. In both cases, the issue must be acknowledged as particularly harmful to the unborn child and untreatable at the time of diagnosis. The requesting couple is a man and a woman of child-bearing age. Diagnosis is used to determine whether an embryo obtained from in vitro fertilisation has the family gene mutation sought before it is transferred to the uterus.
In France, PDG is regulated by the Agence de la Biomédecine (ABM) and is defined by two Acts: the Act dated 29th July 1994 “on donation and use of human body parts and products, medically assisted procreation and pre-natal diagnosis”1 and the Act dated 6th August 2004 “on bioethics”2. Nationally, the technique has been authorised since 1999 and in 2016, about 200 babies were born following transfer of an embryo subject to PDG.
When should PDG be requested?
Requests for PDG usually emanate from geneticists and gynaecologists who specialise in pre-natal diagnosis or infertility. More rarely, the couple requests it itself from their selected PDG centre. The request is then examined by a multi-discipline team which approves its eligibility, and genetic and gynaecological feasibility. For this, several examinations will be prescribed for the woman (doses of hormones, ultrasound-guided measuring of antral follicles), and the man (spermogram, spermocytogram, migration survival test) and both members of the couple, and if necessary, their close family (genetic feasibility tests).
Five centres in France are authorised to conduct PDG: Paris-Clamart, Montpellier, Strasbourg, Nantes and Grenoble. Before anything is done, couples must grant their free, enlightened consent in a dedicated consultation during which the whole procedure is explained, including its limits and constraints. As for Medically Assisted Procreation (MAP) provided for infertility, PDG requires ovarian stimulation for about 10 to 12 days. The purpose being to obtain several oocytes which are placed in contact with the spermatozoids of the spouse of the patient receiving the stimulation (fertilisation stage). Several embryos are thus obtained in vitro and those which develop as they should are removed for a gene test. Embryo cells (blastomers) are usually removed on the 3rd day of development. Depending on practices, this stage may also take place on the 4th or 5th day of embryo development. The technique entails removing one or more embryo cells for a gene test. The act requires a small hole to be made in the envelope surrounding the embryo. Then, using a micro-pipette, one or more cells is/are aspirated (Figure 1). Once the cell(s) has/have been removed, it is/they are sent for a gene test and the embryo is returned to culture to continue to develop. Developing embryos free of the mutation sought are transferred to the uterus or indeed frozen for transfer later. Once transfer has taken place, the chances of achieving pregnancy are about 30%.
Figure 1 Embryo biopsy for PDG, conducted on the 3rd (A), 4th (B) or 5th (C) day of development. Each insert represents the embryo before the biopsy.
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