18.05.11 PGD Flashcards
What is preimplantation genetic testing?
Preimplantation genetic (PG) testing is the practice of obtaining a cellular biopsy sample from a developing human oocyte (prior to fertilisation) or embryo (prior to implantation), obtained via a cycle of in vitro fertilisation (IVF); evaluating the sample’s genetic composition; and using this information to determine which embryos will be optimal for subsequent uterine transfer.
What are the two broad categories of preimplantation genetic testing?
PGD (diagnosis)PGS (screening)
What is the purpose of PGD?
PGD is a technique that involves testing cells from embryos created outside the body by IVF for a genetic disorder. This technique aims to prevent the birth of affected children from parents (usually fertile) who are affected by, or who are carriers of a known genetic abnormality.
What is the purpose of PGS?
The purpose of PGS is to identify optimal embryos for uterine transfer in an IVF cycle. Parents are assumed to be normal and the screen is primarily used to look for abnormalities in chromosome numbers with the overall aim of improving implantation efficiency and therefore live birth rates.
What types of disease may PGD be suitable in?
Structural chromosome rearrangements (to minimise the risk of spontaneous abortion or unbalanced offspring)Mendelian disorders (very high accuracy (99%); >40% rate of embryo transfer in leading PGD centres)Susceptibility to certain cancers
What is current opinion on the use of PGS in the NHS?
The routine use of PGS remains controversial, with at least 11 randomised trials since 2004, showing no clinical benefit of the procedure (Harper et al 2010) and one study showing that PGS significantly lowers the birth rate when applied to women of advanced maternal age (Mastenbroek et al 2011). Based on these findings, the NHS has no intention of supporting the introduction of PGS into NHS clinical practise.
Under what circumstances will the NHS fund PGD? Give some of the criteria required.
- The couple should be at risk of having a child with a serious genetic condition.2. The couple should have been referred to the PGD provider by an NHS Clinical Genetics Service.3. The risk of conceiving a pregnancy affected by a serious genetic condition should be 10% or more.4. The couple should have received genetic counselling from a clinical geneticist or a registered genetic counsellor.5. The female partner should be under 40 years of age at the time of treatment (In France it is 43y).6. The female partner should have a BMI of more than 19 and less than 30.7. Both partners should be non smokers.8. There should be no living unaffected child from the current relationship.9. The HFEA must have licensed the indication for PGD.10. The test must be included in the list of UKGTN approved tests, or suitable for inclusion.11. The couple should not be seeking PGD primarily because they are infertile.
Give some of the aims of the NHS England Clinical Commissioning Policy for PGD.
- Reduce variation in access to PGD2. Ensure PGD is commissioned for those conditions for which there is acceptable evidence of clinical benefit and cost effectiveness3. Reduce unacceptable variation in clinical practise in conditions referred for PGD and4. Promote the cost effective use of healthcare resources.
Give some examples for uses of PGD which are not permitted in the UK.
- Non medical gender selection e.g. for the purpose of family balancing. This is illegal in the United Kingdom (UK).1 Makes up 1.4% of global PGD cycles.2. Using PGD to address infertility or to prevent miscarriages of unknown aetiology.3. Pre-implantation Genetic Screening (PGS). Here, genetic testing is used to screen embryos for various abnormalities in chromosomes typically the number of chromosomes. This makes up 60.6% of PGD cycles although there is no evidence that this improves the rate of pregnancy or live IVF births.
At which stage of embryo development is cell biopby for PGD most commonly performed?
BlastomereMore recently, blastocentesis, a procedure where fluid is removed from the blastocoelic cavity of the day 5 blastocyst has been investigated as an alternative source of embryonic DNA (Gianaroli et al 2014).
How many cells can be biopsied at blastomere stage? How is this performed?
Here 1-2 cells are removed from the 3-day cleavage-stage embryos. These are obtained by aspiration or extrusion via opening of the zona pellucida. Up to 60% of embryos at cleavage stage of development exhibit mosaicism, where at least one cell has a different ploidy from other cells in the embryo but many “self-correct” by blastocyst stage.
Other than the blastomere stage, give examples of other stages of embryonic development that can be used for PGD and how many cells can be obtained safely.
1st and 2nd polar bodies: Testing of the first (pre- fertilisation) and second (subsequent to fertilisation only) polar bodies has the advantage of testing material that does not comprise a functional part of the embryo. This technique is not in routine use apart from in countries where legislation prohibits the genetic testing of cells derived from cleavage pre-implantation embryo Blastocyst: 10-30 cells can be removed at this stage providing more material for testing. However, <40% of embryos survive to the day 5-6 stage in vitro and they need to be transferred by day 6 (may be cryopreserved and transferred at a later date). Despite fewer embryos surviving to this stage it does mean that those being tested are more robust and more likely to be successfully implanted.
What is the disadvantage of biopsy at 1st or 2nd polar body stage?
Does not allow analysis of the paternal contribution; lacks information for aneuploidies of paternal and mitotic origin; needs analysis for each polar body therefore generating unnecessary work (some of the oocytes will not be fertilized and some of the zygotes will not reach the blastocyst stage); being highly expensive; chance for aneuploidy compensation (2–4%); and high risk of aneuploidy.
What genetic testing methods are used for PGD
Traditionally: FISH - cytogenetic abnormalities, PCR - molecular analysisaCGH and SNP arrays can improve diagnostic accuracyThe single-cell whole genome amplification (WGA) method allows subsequent mutation study, directly by minisequencing and/or indirectly by linkage analysis alongside the mutation test. It allows PGD for more than one indication.
Which sample type could be used for a non-invasive preimplantation genetic screening approach ?
Embryonic DNA isolated from blastocyst culture conditioned medium (BCCM) combined with blastocoel fluid (BF) could be used for blastocyst stage non-invasive preimplantation genetic testing for chromosomal aneuploidy (non-invasive preimplantation genetic screening, NIPGS).
