Preimplantation genetic diagnosis (PGD) is offered to couples that are carriers or suffer from a genetic disorder and want to avoid an affected pregnancy. PGD is based in IVF. Following egg retrieval and fertilisation and before embryo transfer the embryos are biopsied. The biopsied embryonic cells are analysed by specialised techniques for the detection of the disorder and only those embryos that are found to be free of the disorder are chosen for transfer.
Traditionally the biopsy is performed on the third day post fertilisation at the cleavage stage. The experienced embryologist drills a hole on the zona pellucida that surrounds the embryo with the aid of a laser under the microscope. One or two cells are removed carefullly with a fine pipette and the analysis is performed.
Recently, with the aid of improved culture media the embryo transfer can be performed on day five or six post ferilisation at the blastocyst stage. The advantage of blastocyst culture is that mainly embryos of good quality become blastocysts and therefore have increased chances of successful implantation and further development. Blastocyst biopsy is possible with the use of modern equipment and has the additional advantage that six to ten cells are biopsied and not one or two that are biopsied on the cleavage stage, thus producing more clear and robust results.
The majority of genetic disorders that are caused by DNA mutations can be detected by PGD. This can be autosomal recessive disorders like thalassaemia and cystic fibrosis, where both parents are carriers for the mutation, autosomal dominant, like inherited cancer (neurofibromatosis, breast and ovarian cancer), myotonic dystrophies and sex linked disorders, in which the mutations are located mainly on the X chronosome, like Duchenne mustrular dystrophy and fragile X syndrome. PGD is also used for the detection of structural chromosomal abnormalities like reciprocal and Robertsonian translocations.
According to the most recent ESHRE PGD Consortium data in 2008, 60 PGD centers that co-operate with ESHE, performed 1710 egg collections for single gene disorders and 870 egg collections for structural chromosomal abnormalities (Moutou et al, 2014). These numbers are just indicators as the exact number of PGD cycles performed around the world is greater.
The techniques used in PGD depend on the type of the disorder. For those that are caused by mutations, the points on the DNA chain that are responsible for the disorder are amplified by PCR and the mutations are detected by specialised techniques (eg haplotyping, sequencing). The protocols that are used in PGD differ from thosed used in routine genetic analysis on blood or other samples (that contain millions of cells). The PGD protocols are specially designed and optimised to work efficiently in minute DNA quantities (often as low as one cell DNA). Many centres prefer to use protocols that are not specific for one mutation only but the same for each disorder. For example, a PGD centre in Europe presented recently the optimisation and application of general protocols in couples with breast and ovarian cancer (Drusedau, 2013).
The traditional technique for the detection of chromosomal abnormalities is FISH (fluorescence in situ hybridization). With this technique only a limited number of chromosomes can be checked. In recent years the technique aCGH (array comparative genomic hybridization) has been applied for the detection of chromosomal abnormalities. It can be used successfully for all Robertsonian translocation cases and for most of reciprocal translocations. If aCGH is used to detect structural chromosomal abnormalities it can also detect abnormalities in the number of the chromosomes. The aCGH technique is the same for every couple/disorder and does not need optimization.
aCGH example of an embryo with normal chromosomes
aCGH may also be used in PGS (preimplantation genetic screening). PGS is the screening of embryos to avoid a pregnancy with a numerical chromosomal abnormality (eg trisomy for chromosomes 13, 18 and 21). It is known that embryos with chromosomal abnormalities cannot be successfully implanted, or will lead to a miscarriage or to an affected pregnancy. For this reason PGS is offered to couple that have increased chances of having embryos with chromosomal abnormalities, like those of advanced maternal age (over 38 years old), recurrent miscarriage, recurrent IVF failure, severe male factor infertility and unexplained infertility. According to the ESHRE PGD Consortium committee, in 2009, 3551 egg collections were performed for PGS .
aCGH example of an embryo with trisomy for chromosomes 7 and 21 and monosomy for chromosome 20.
- Moutou C, Goossens V, Coonen E, De Rycke M, Kokkali G, Renwick P, SenGupta SB, Vasela K, Traeger-Synodinos J. 2014. ESHRE PGD Consortium data collection XII: cycles from January to December 2009 with pregnancy follow-up to October 2010. Hum Reprod. 29(5):880-903.
- Drusedau M, Dreesen JC, Derks-Smeets I, Coonen E, van Golde R, van Echten-Arends J, Kastrop PM, Blok MJ, Gomez-Garcia E, Geraedts JP, de Die-Smulders CE, Paulussen AD. 2013. PGD for hereditary breast and ovarian cancer: the route to universal tests for BRCA1 and BRCA2 mutation carriers. Eur J Hum Genet. 21(12):1361-8.