Pre -Implantation Genetic Screening PDF Print E-mail

Pre-implantation Genetic Screening/ Diagnosis (PGS/PGD)


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 Pre-implantation genetic testing (PGS) is the technique employed to investigate and identify genetic abnormalities in embryos which have resulted from in vitro fertilization undertaken by presumed normal genetic parents. Pre-implantation genetic diagnosis (PGD) is a laboratory procedure designed to identify a known genetic predisposition carried by either of the two or both parents. The genetic screening which is made possible at the pre-implantation stage using PGS/PGD substitutes the alternative post-conception diagnostic interventions such as amniocentesis and chorionic villus sampling. This powerful tool of laboratory science provides the only means towards preventing  heritable genetic disorders from being conveyed to the offspring.There are three different laboratory approaches to screening embryos before their replacement into the uterus of a recipient patient.


PCR (Polymerase Chain Reaction), FISH (In Situ Hybridisation) and CGH (Comparative Genome Hybridisation). Prior to enrolling in this treatment modality, potential candidates should seek advice from a qualified geneticist or a genetic counsellor in view of assessing the risk of conveying the genetic aberration to their children. A number of tests will be requested to confirm the presence of the abnormality to the parent and to identify accurately the genetic derangement(s) which cause the manifestation.


A pre-requisite to all three approaches is the incorporation of embryo biopsy during the IVF/ICSI cycle. Biopsies are normally performed on the 3rd day of embryonic development and by which time the embryo should have reached the 8 cell stage. Alternatively, embryos can be treated with biopsy at the blastocyst stage (64 cells). Embryos which have received a biopsy remain unaffected with regards to their developmental potential thereafter unless these are carrying major chromosomal derangements such as monosomies (presence of one rather than two chromosomes for a particular set).PCR is used for diagnosing single gene defects.


The basis of the technique resides on the million fold amplification of a particular DNA sequence related to the gene in question. Following this step the products of the reaction are ran onto a gel through electrophoresis and subsequently inspected using UV lighting. The technique which is relatively fast suffers from the pitfall that subtle contamination in the reaction will produce a misdiagnosis in the final result.FISH is a chromosome enumeration method where embryos can be screened in view of identifying whether they are currying a normal complement for a particular set of chromosomes. The basis of this technique is the hybridization of a particular chromosomal region with synthetic DNA which will fluoresce under a certain light band.


In this way the absence or presence of the chromosome in question can be identified. A disadvantage of the technique is the risk associated with mosaicism, a phenomenon often observed in pre-implantation embryos. The chromosomal constitution of the cell biopsy may be found to be normal whereas the rest of the embryo cells were abnormal and vice versa. In CGH, the nucleus of a cell recovered from the embryo is labeled with a fluorescent dye (green) whereas another cell used as a control is labeled using a different colour (red). The two cells are then co-hybridised and the ratio between the 2 colour intensities is compared. According to the higher or lower intensity exhibited the nucleus of the cell can be diagnosed to contain additional or less chromosomes.


The technique takes 72 hours suggesting that the embryos will have to be frozen since there is a limited duration of an embryo which can be maintained in vitro culture. Genesis has performed more than 25000 embryo biopsies and subsequent diagnoses to date rendering the centre one of the most experienced in the field of pre-implantation genetic diagnosis worldwide. 


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Last Updated ( Tuesday, 22 June 2010 )