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The Fertility Show


 

Reassurance for mothers over mosaic aneuploidy in CVS

03 May 2016

By Professor Magdalena Zernicka-Goetz

Department of Physiology, Development and Neuroscience, University of Cambridge

Appeared in BioNews 849

Professor Caroline Ogilvie's recent article ('Abnormal fetuses are highly unlikely to heal themselves', BioNews 847) is very helpful in reflecting the current state of understanding of mosaic aneuploidy in human embryo development.

Our recent findings about the fate of aneuploid cells in mosaic embryos (see BioNews 845) have indeed been inaccurately interpreted in the press in some cases, and those reports have occasionally included some very misleading statements.

I am in total agreement with Professor Ogilvie that our results do not in any way question the legitimacy of prenatal testing as it is currently employed. Early tests of fetal DNA in the mother’s blood or by chorionic villus sampling (CVS) are particularly important in assessing aneuploidy arising in meiosis where all of the embryonic cells are affected. These tests therefore represent an excellent means of spotting aneuploidy of the small chromosomes, such as chromosome 21, that can be tolerated but not without affecting the developing person.

Professor Ogilvie is also correct that we know that a normal baby can arise from a conceptus showing mosaic aneuploidy as detected by CVS. In our study we sought to find: first, why this should be so – what is the fate of those early anueploid cells; and second, what degree of mosaicism can be tolerated in the whole embryo (not just in the placenta). We showed for the first time that aneuploid cells are eliminated by programmed cell death (apoptosis) in the pluripotent cells that give rise to the fetus whereas they are tolerated in the extra-embryonic cells of the placenta where they divide more slowly.

We also gained an indication of the proportion of aneuploid cells that can be tolerated in the conceptus by varying the proportions of normal and aneuploid cells in experimental mouse embryos. This gives some insight into the numbers of normal pluripotent cells that are needed to make a viable fetus. The mouse embryo is similar to the human embryo in that at the blastocyst stage it usually has about 12 pluripotent epiblast cells out of a total of around 100 cells altogether.

One of our previous studies indicated that a minimum of four such pluripotent cells was required to make viable offspring in the mouse. Our new results are in broad agreement with this earlier finding.

Thus we believe our findings may constitute a strand of hope for mothers who have had early test results showing mosaic aneuploidy. Of course they should then have amniocentesis that will most likely show a normal karyotype. But they can also have some reassurance that although the amniocentesis cannot detect cells from all tissues of the fetus, experiments in mice show that cells with abnormal karyotypes are generally eliminated during early development.

Hopefully future studies will cast more light upon the underlying mechanism and at which stage of development these abnormal cells can be eliminated.

SOURCES & REFERENCES

RELATED ARTICLES FROM THE BIONEWS ARCHIVE

15 August 2016 - by Dr Katie Howe 
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A central tenet of prenatal testing is to promote reproductive autonomy by providing women with information that can assist in pregnancy management. Bringing NIPT into the NHS will improve an established screening programme...
09 May 2016 - by Julianna Photopoulos 
Scientists from the UK and US have grown human embryos in the lab for 13 days after fertilisation – the longest ever recorded. This is beyond the stage when embryos would normally implant in the womb, but just before the 14-day legal limit in the UK...

18 April 2016 - by Professor Caroline Ogilvie 
Press coverage of a recent study on mouse embryos speculates that termination of human pregnancies diagnosed with aneuploidy (a family of conditions including Down's syndrome) at prenatal diagnosis could be unnecessary, due to the ability of the fetus to 'heal itself'. In fact, the paper's findings are not relevant to aneuploidy detected at prenatal diagnosis...
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15 August 2011 - by Dr Tamara Hirsch 
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Published by the Progress Educational Trust

CROSSING FRONTIERS

Moving the Boundaries of Human Reproduction

Public Conference
London
8 December 2017

Speakers include

Professor Azim Surani

Professor Magdalena Zernicka-Goetz

Professor Robin Lovell-Badge

Sally Cheshire

Professor Guido Pennings

Katherine Littler

Professor Allan Pacey

Dr Sue Avery

Professor Richard Anderson

Dr Elizabeth Garner

Dr Jacques Cohen

Dr Anna Smajdor

Dr Andy Greenfield

Vivienne Parry

Dr Helen O'Neill

Dr César Palacios-González

Philippa Taylor

Fiona Fox

Sarah Norcross


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