Embryos with abnormal chromosomes can repair themselves

CVS

In a chorionic villus sampling (CVS) test, cells from the fetal side of the placenta are collected and tests for genetic defects.
Image credit: ADAM Health Solutions

Like an increasing number of women, Magdalena Zernicka-Goetz waited later in life to have kids and was pregnant at 44 with her second child. Because older moms have an increased risk of giving birth to children with genetic disorders, Zernicka-Goetz opted to have an early genetic screening test about 12 weeks into her pregnancy. The test, which looks for irregular amounts of chromosomes in the cells taken from the placenta, showed that a quarter of the cells in the developing fetus had genetic abnormalities.

Expectant mothers and tough choices

If she carried the child to term, would the baby have a birth defect? Zernicka-Goetz learned from geneticists that this question was difficult to answer due to a lack of data about what happens to abnormal cells in the developing fetus. Fortunately, her baby was born happy and healthy. But the experience motivated her to seek out a better understanding for the sake of other women who would be faced with similar difficult decisions based on screening tests.

As a professor of developmental biology at Cambridge University, Zernicka-Geotz had the expertise to follow through on this challenge. And in a Nature Communications journal article published yesterday, she and her team report a fascinating result: the very early embryo has the ability to essentially repair itself by getting rid of abnormal cells.

Aneuploidy: You Have the Wrong Number

aneuploidy

Aneuploidy in the developing fetus can lead to genetic disorders. Image credit: Deluca Lab Colorado State University

To reach this finding, the team first had to recreate chromosomal abnormalities in mouse embryos. If you remember your high school or college biology, you’ll recall that before a cell divides, it duplicates each chromosome and then each resulting “daughter” cell grabs one chromosome copy using a retracting spindle fiber structure. The scientists took advantage of the fact that treating dividing cells with the drug reversine destabilizes the spindle fibers and in turn causes an unequal divvying up of the chromosomes between the daughter cells. In scientific jargon the condition is called aneuploidy.

Rescuing the embryo by cellular suicide

Blog embryo repair fig 3

Generating early mouse embryos with an equal mix of normal cells and cells with abnormal chromosome numbers (induced via reversine treatment). Image credit: Bolton et al. Nat Commun. 2016 Mar 29;7:11165

The researchers created mosaic embryos at the eight cell stage in which half the cells had a normal set of chromosomes while the other half we’re the reversine-treated cells with abnormal numbers of chromosomes. With these genetically mosaic embryos, the team tagged the cells with fluorescent dye and used time-lapsed imaging to track the fate of each cell for 48 hours. They found a decrease specifically in the portion of cells that stemmed from the abnormal cells.

A follow up experiment examined cell death as a way to help explain the reduced number of abnormal cells. The researchers found that compared to the normal set of cells in the embryo, the abnormal cells had a significantly higher evidence of apoptosis, or programmed cell death, a natural process that occurs to eliminate harmful or damaged cells. According to Zernicka-Geota and the team, this is the first study to directly show the elimination of abnormal cells in the growing embryo.

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Time lapse images showing an abnormal cell (green cell indicated by arrow) being eliminated by apoptosis (programmed cell death) and then engulfed by normal (red) cells (engulfment indicated by star).
Image credit: Bolton et al. Nat Commun. 2016 Mar 29;7:11165

To look at their fate beyond the very early stages of development, the mosaic mouse embryos were implanted into foster mothers and allowed to develop to full term. Thirteen of the twenty-six embryos transferred to foster mothers gave rise to live pups which were all healthy after four months of age.

As Zermicka-Geota stated in a university press release picked up by Medical Express, if these findings reflect what goes on in human development, then decisions based on genetic screening results may not be clear cut:

“We found that even when half of the cells in the early stage embryo are abnormal, the embryo can fully repair itself. It will mean that even when early indications suggest a child might have a birth defect because there are some, but importantly not all abnormal cells in its embryonic body, this isn’t necessarily the case.”

Implications for genetic testing on days-old IVF embryos

These new results don’t suggest that current genetic testing is obsolete. For instance, the amniocentesis test, which collects fetal tissue from the mother’s amniotic fluid between 14 and 20 weeks of pregnancy, can detect genetic disorders with 98-99% accuracy. But this study may have important implications for testing done much earlier. When couples conceive via in vitro fertilization, a so-called pre-implantation genetic diagnosis (PGD) test can be performed on embryos that are only a few days old. In the test, a single cell is removed – without damaging the embryo – and the cell is tested for chromosomal defects. Based on this study, a positive PGD test may be misleading if that abnormal cell was destined to be eliminated from the embryo.

Timing is everything: could CRISPR gene editing push CIRM to change its rules on funding stem cell research?

CRISPR

Talk about timely. When we decided, several months ago, to hold a Standards Working Group (SWG) meeting to talk about the impact of CRISPR, a tool that is transforming the field of human gene editing, we had no idea that our meeting would fall smack in the midst of a flurry of news stories about the potential, but also the controversy, surrounding this approach.

Within a few days of our meeting lawmakers in the UK had approved the use of CRISPR for gene editing in human embryos for fertility research —a controversial first step toward what some see as a future of designer babies. And a U.S. Food and Drug Advisory report said conducting mitochondrial therapy research on human embryos is “ethically permissible”, under very limited conditions.

So it was clear from the outset that the SWG meeting was going to be touching on some fascinating and fast moving science that was loaded with ethical, social and moral questions.

Reviewing the rules

The goal of the meeting was to see if, in the light of advances with tools like CRISPR, we at CIRM needed to make any changes to our rules and regulations regarding the funding of this kind of work. We already have some strong guidelines in place to help us determine if we should fund work that involves editing human embryos, but are they strong enough?

There were some terrific speakers – including Nobel Prize winner Dr. David Baltimore; Alta Charo, a professor of Law and Bioethics at the University of Wisconsin-Madison  ; and Charis Thompson, chair of the Center for the Science, Technology, and Medicine in Society at the University of California, Berkeley – who gave some thought-provoking presentations. And there was also a truly engaged audience who offered some equally thought provoking questions.

CIRM Board member Jeff Sheehy highlighted how complex and broad ranging the issues are when he posed this question:

“Do we need to think about the rights of the embryo donor? If they have a severe inheritable disease and the embryo they donated for research has been edited, with CRISPR or other tools, to remove that potential do they have a right to know about that or even access to that technology for their own use?”

Alta Charo said this is not just a question for scientists, but something that could potentially affect everyone and so there is a real need to engage as many groups as possible in discussing it:

“How and to what extent do you involve patient advocates, members of the disability rights community and social justice community – racial or economic or geographic.  This is why we need these broader conversations, so we include all perspectives as we attempt to draw up guidelines and rules and regulations.”

It quickly became clear that the discussion was going to be even more robust than we imagined, and the issues raised were too many and too complex for us to hope to reach any conclusions or produce any recommendations in one day.

As Bernie Lo, President of the Greenwall Foundation in New York, who chaired the meeting said:

“We are not going to resolve these issues today, in fact what we have done is uncover a lot more issues and complexity.”

Time to ask tough questions

In the end it was decided that the most productive use of the day was not to limit the discussion at the workshop but to get those present to highlight the issues and questions that were most important and leave it to the SWG to then work through those and develop a series of recommendations that would eventually be presented to the CIRM Board.

The questions to be answered included but were not limited to:

1) Do we need to reconsider the language used in getting informed consent from donors in light of the ability of CRISPR and other technologies to do things that we previously couldn’t easily do?

2) Can we use CRISPR on previously donated materials/samples where general consent was given without knowing that these technologies could be available or can we only use it on biomaterials to be collected going forward?

3) Clarify whether the language we use about genetic modification should also include mitochondrial DNA as well as nuclear DNA.

4) What is the possibility that somatic or adult cell gene editing may lead to inadvertent germ line editing (altering the genomes of eggs and sperm will pass on these genetic modifications to the next generation).

5) How do we engage with patient advocates and other community groups such as the social justice and equity movements to get their input on these topics? Do we need to do more outreach and education among the public or specific groups and try to get more input from them (after all we are a taxpayer created and funded organization so we clearly have some responsibility to the wider California community and not just to researchers and patients)?

6) As CIRM already funds human embryo research should we now consider funding the use of CRISPR and other technologies that can modify the human embryo provided those embryos are not going to be implanted in a human uterus, as is the case with the recently approved research in the UK.

Stay tuned, more to come!

This was a really detailed dive into a subject that is clearly getting a lot of scientific attention around the world, and is no longer an abstract idea but is rapidly becoming a scientific reality. The next step is for a subgroup of the SWG to put together the key issues at stake here and place them in a framework for another discussion with the full SWG at some future date.

Once the SWG has reached consensus their recommendations will then go to the CIRM Board for its consideration.

We will be sure to update you on this as things progress.