The screening of early embryos in IVF treatment has long been a contentious issue in the media. A study published in the journal Nature has reported that abnormalities in the early stages of mouse embryonic development may not ultimately become birth defects, potentially due to the positive effects of the process of cell death.
Dr Neil Vargesson, Senior Lecturer in Biomedical Sciences, University of Aberdeen, said:
“In this elegant study the mouse ’embryos’ used are essentially a ball of cells, which is around the first week of development in human terms – most women wouldn’t know they were pregnant. Human pregnancies are scanned for problems around weeks 10 and 20. By week 10 the human embryo is fully formed and if any damage is apparent (or even not apparent) will likely be irreversible. In terms of Down’s syndrome I believe several other tests are performed around week 10-12 and if these suggest risk then a Chorionic Villus Sampling or Amniocentesis is performed soon after to confirm or disprove the condition.
“From this study the suggestion is that early ’embryos’ that have mutant cells with more healthy cells in early development have the ability to recover (somehow) so by the time of scans at week 10 and 20 the baby is normal. This is very interesting. In contrast those ’embryos’ with many cell defects in early development will either spontaneously terminate or continue to develop with abnormality that will then be identified in the scans at week 10 and 20 (the time when the pregnancy is normally assessed to check things are okay). So I don’t think terminations are being carried out unnecessarily. A big question now is what time does the embryo lose this ability to recover from unhealthy or mutant cell numbers?”
Professor Caroline Ogilvie, Consultant Scientist & Honorary Chair in Cytogenetics, Kings College London, said:
“This is a beautifully executed and presented study, which, by using a mouse model, will help us to a better understanding of early events in human development. With regard to its relevance to prenatal diagnosis by CVS sampling in the first trimester, it’s important to point out that following a finding of mosaic aneuploidy, diagnostic laboratories will generally recommend a follow-up amniocentesis to rule out confined placental mosaicism (CPM).
“There are numerous studies and publications on CPM, the frequency, the chromosomes involved, and the outcome of the pregnancies. There can be no generalisations as to the phenotypic outcome; at amniocentesis, low-grade mosaicism can be benign, depending on the chromosome involved, and similarly, the absence of abnormal cells in amniotic fluid does not exclude the possibility that low grade mosaicism is present in some other fetal tissues.
“Selection against abnormal clones is to be expected, especially where the genetic imbalance is large, so the results from this paper are not surprising. Detailed and expert ultrasound assessment and genetic counselling should be offered to any woman with a mosaic finding at CVS, before any decisions are made on the management of the pregnancy.
“’Unnecessary’ terminations are therefore likely to be infrequent, although for some women, any risk of fetal abnormality may be unacceptable.”
Prof. Jan Brosens, Chair of Obstetrics and Gynaecology, University of Warwick, said:
“It is well known that most human embryos are mosaic and that these embryos can develop into healthy babies. This study elegantly investigates how embryos cope with abnormal cells. Importantly, the findings cast further doubt on the use of pre-implantation genetic screening (PGS) of human embryos in IVF.”
Prof. Daniel Brison, Honorary Professor of Clinical Embryology and Stem Cell Biology and Scientific Director of the Department of Reproductive Medicine, University of Manchester, said:
“This outstanding paper provides direct experimental evidence that cell death (apoptosis) in early embryos has a positive role in eliminating abnormal cells from development. Apoptosis is often considered a bad thing in early embryos, and some culture media used in assisted conception contain growth promoting ingredients which may block cells from dying. This new work however suggests that early embryonic cell death should be allowed to proceed normally to protect the future health of the fetus and baby.”
‘Mouse model of chromosome mosaicism reveals lineage-speciﬁc depletion of aneuploid cells and normal developmental potential’ by Bolton et al. published in Nature Communications on Tuesday 29th March.
Prof. Jan Brosens: I have no conflicts of interests to declare.
Dr Vargesson: I have no conflicts of interest with this study. I am or have been funded by the Wellcome Trust, Royal Society, Newlife. I have previously (several years ago now) given advice to lawyers representing alleged thalidomide survivors – namely to discuss the mechanisms of action of the drug.
All others: None declared