Mysteries of Aneuploidy

Nicole Crown

I’ve often wondered why humans are so bad at reproduction.  It’s been estimated that 10-30% of all fertilized eggs are aneuploid, and approximately one third of all miscarriages are due to aneuploidy (Hassold et al.).  In striking contrast, only 1-2% of fertilized eggs are aneuploid in mice.  As a researcher that studies meiosis, this baffles me.  The meiotic program is astoundingly complex, coordinating the repair of programmed DNA damage, finding and pairing homologous chromosomes, along with attaching the spindle in the correct orientation.  This complexity requires multiple checkpoints throughout the process to ensure everything is going as planned; if something goes wrong, the checkpoint stops the cell. Given multiple opportunities to ensure chromosomes are properly segregated, how is it that, in humans, aneuploid cells not only make it through a complete meiosis, but also go on to complete oogenesis?

Recent work from Dokshin et al. suggests that one potential way this can happen is to decouple oogenesis from meiosis.  In mice, early germ line cells initiate meiosis after Stra8 expression is turned on by a retinoic acid signal.  In the absence of Stra8, cells never initiate meiosis, and by 6-8 weeks of age, the ovaries contain no germ cells. However, a very small percentage of cells are able to escape the Stra8 phenotype, and while they still don’t initiate meiosis, they do proceed through oogenesis.  These “oocyte-like cells”, as the authors call them, have the same morphological and physiological characteristics as normal oocytes: they are able to make a zona pellucida, generate follicles, can be ovulated and fertilized, and the embryo can undergo the first division.  When the authors looked at the chromosome complement of the oocyte-like cells, they found that the chromosomes were randomly distributed between the polar body and the oocyte-like cell.

Certainly, these data show that in the absence of meiosis, the vast majority of female germ cells will not differentiate; therefore, there must be some way of monitoring the meiotic state of a cell before oocyte differentiation begins.  However, this monitoring clearly can fail, as the Stra8 deficient mice do produce oocyte-like cells capable of being fertilized.  The authors suggest some cases of human infertility may be explained by a disconnect between oocyte differentiation and meiosis.  It will be fascinating to compare the mechanism of communication between meiotic state and oocyte differentiation in humans to other organisms, and determine if the apparently higher rate of aneuploidy in humans is sometimes due to miscommunication.


Stick a PIN in it

Nicole Crown

Post-translational modification (PTM) of proteins is an essential cellular process used to regulate protein function and stability.  PTMs are assumed to have an impact on protein structure and conformation, but the effects of specific protein conformations on biological processes are difficult to understand and test in vivo.  The importance of protein conformation is of particular interest in DNA repair as many of the same proteins are involved in context specific repair processes.  For example, the Mre11-Rad50-Nbs1 complex is an essential DNA double-strand break sensor that is found at every type of double-strand break.  It has been proposed that this complex can, in theory, adopt up to 216 conformational states and acts as a “molecular computer” to detect damage and regulate repair pathway choice (i.e. should the break be repaired via nonhomologous end joining or homologous recombination)1In vivo studies of the roles that particular protein conformations play are rare, yet critical for understanding the regulation of DSB repair.

New work from Steger and colleagues2 shows that Pin1, a prolyl isomerase that catalyzes cis/trans isomerization, binds multiple DNA repair proteins, including CtIP, a key player in double-strand break end resection.  The authors show that the interaction between Pin1 and CtIP is induced by phosphorylation of CtIP at two S/T-Proline sites, and that Pin1 binding does indeed cause a conformational change in CtIP.  This conformational change leads to polyubiquitylation of CtIP and subsequent degradation. Overexpression of Pin1 causes hyporesection and a decrease in homologous recombination; similarly, depleting Pin1 causes hyperresection and decreased NHEJ.  These data and others presented in the paper lead to a model in which CtIP is phosphorylated, causing Pin1 to bind and isomerize CtIP.  This isomerization leads to ubiquitin-mediated CtIP degradation and appropriate end resection.  Pin1 overexpression leads to reduced CtIP activity and therefore hyporesection and decreased homologous recombination, whereas Pin1 depletion leads to increased CtIP activity, hyperresection and decreased NHEJ.

In this particular case, the researchers were lucky to find a specific protein, Pin1, that induces a known conformational change, cis/trans isomerization, by acting directly on the substrate protein. However, most conformational changes may be caused indirectly by the effects of post-translational modifications.  While the identification of Pin1 as a critical regulator of DNA repair is a large step forward in understanding the role protein conformation plays in function, this same understanding for other proteins will require more cross-disciplinary studies that are able to modify protein conformation in vivo and determine the biological outcomes.

1. Williams, G. J., et al. (2010). "Mre11-Rad50-Nbs1 conformations and the control of sensing, signaling, and effector responses at DNA double-strand breaks." DNA Repair 9(12): 1299-1306.

2. Steger, M., et al. (2013). "Prolyl Isomerase PIN1 Regulates DNA Double-Strand Break Repair by Counteracting DNA End Resection." Molecular Cell 50(3): 333-343.

 


Turtle Power!

Nicole Crown

We often use the phrase “when pigs fly” to describe something that is extremely unlikely to happen.  But why is it so crazy that pigs could ever evolve wings?  In fact, why didn’t they evolve wings?

Maybe pigs never evolved wings because they couldn’t.  That is, there are developmental constrains on the basic body plan of pigs that prevent them from evolving wings.

This concept of developmental constraints occurs frequently in evolutionary and developmental biology.  Certain stages of development are non-negotiable; if development deviates too far from a given program, there are serious consequences for the organism.

On the other hand, there must be some breathing room in developmental programs so that organisms can evolve and adapt.

So, how has nature struck a balance between the need to stick closely to a developmental plan, but also allow for noise and fluctuation so that adaptation can occur?

Comparative studies of morphological data have led to an hourglass model of development in which the most constrained stages occur in mid-development when the basic body plan of an organism is established (called the phylotypic stage), whereas early and late stages are less constrained.  This theory has been most recently supported by molecular studies that show gene expression patterns are most conserved during mid-development.

In a huge collaborative effort, Wang and colleagues1 sequenced the genome and transciptome of two turtle species, the soft-shell turtle and the green sea turtle.  They were able to answer long standing questions about the evolutionary origins of turtles (they’re a sister group to crocodilians and birds), gain insight into the molecular mechanisms of unique turtle characteristics (they might live so long because of a gene with a role in antioxidative stress) and into how a turtle builds its shell (co-option of Wnt signaling normally used in limb bud formation).

But perhaps most pertinent here is their comparative analysis of turtle embryo development.  The authors’ previous studies made broad comparisons among vertebrates, sampling from different sub-taxa (for example, frogs vs. mouse) and they found that in this case, the most conserved stage was the vertebrate phylotypic stage.  In their present study, the authors asked what the most conserved stage of development is if the two organisms are both vertebrates and amniotes (a subtaxa of vertebrates).  Would it be the vertebrate or the amniote phylotypic period?

They compared gene expression in all developmental stages of the soft-shell turtle to all stages of the chicken embryo and found that the stage with the most shared gene expression corresponded to the vertebrate phylotypic stage, not the amniote.  They also found that turtle-specific expression of 223 genes begins after establishing the basic vertebrate body plan.

The authors’ findings suggest that, in the case of vertebrates, evolution is constrained by the developmental establishment of the vertebrate body plan, but that later developmental stages were fair game for natural selection to act on, ultimately ending up in morphological novelties like the turtle shell.  It would be interesting for the authors to expand their comparisons to other amniotes with unique morphological features to see if this pattern holds true.

  1. Want et al (2013). “The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan.” Nature Genetics 45:6. doi:10.1038/ng.2615

Further reading:

Irie, N. and S. Kuratani (2011). "Comparative transcriptome analysis reveals vertebrate phylotypic period during organogenesis." Nature Communications 2. doi: 10.1038/ncomms124