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.

The best scientists always want to know more

Sir Isaac Newton

Sir Isaac Newton

Some years ago I was in the Wren Library at Trinity College, Cambridge in England when I noticed a display case with a cloth over it. Being a naturally curious person, downright nosy in fact, I lifted the cloth. In the display case was a first edition of Sir Isaac Newton’s Principia Mathematica and in the margins were notes, corrections put there by Newton for the second edition.

It highlighted for me how the best scientists never stop working, never stop learning, never stop trying to improve what they do.

That came back to me when I saw a news release from ViaCyte, a company we are funding in a Phase 1 clinical trial to treat type 1 diabetes.  The news release announced results of a study showing that insulin-producing cells, created in the lab from embryonic stem cells, can not only mature but also function properly after being implanted in a capsule-like device and placed under the skin of an animal model.

VC-01-cross-section-5

Now the clinical trial we are funding with ViaCyte uses a similar, but slightly different set of cells in people. The device in the trial contains what ViaCyte calls PEC-01™ pancreatic progenitor cells. These are essentially an earlier stage of the mature pancreatic cells that our body uses to produce insulin. The hope is that when implanted in the body, the cells will mature and then behave like adult pancreatic cells, secreting insulin and other hormones to keep blood glucose levels stable and healthy.

Those cells and that device are being tested in people with type 1 diabetes right now.

Learning more

But in this study ViaCyte wanted to know if beta cells, a more mature version of the cells they are using in our trial, would also work or have any advantages over their current approach.

The good news, published in the journal Stem Cells Translational Medicine,  is that these cells did work. As they say in their news release:

“The animal study also demonstrated for the first time that when encapsulated in a device and implanted into mice, these more mature cells are capable of producing functional pancreatic beta cells. ViaCyte is also the first to show that these further differentiated cells can function in vivo following cryopreservation, a valuable process step when contemplating clinical and commercial application.”

This does not mean ViaCyte wants to change the cells it uses in the clinical trial. As President and CEO Paul Laikind, PhD, makes clear:

“For a number of reasons we believe that the pancreatic progenitor cells that are the active component of the VC01 product candidate are better suited for cell replacement therapy. However, the current work has expanded our fundamental knowledge of beta cell maturation and could lead to further advances for the field.”

And that’s what I mean about the best scientists are the ones who keeping searching, keeping looking for answers. It may not help them today, but who knows how important that work will prove in the future.