If Star Wars is anything to go by, cloning techniques of the future will be able to create a whole army of clones in the blink of an eye. Yet, back on planet Earth, the cloning of today is still relatively inefficient. A new study published in Cell identifies an important epigenetic road block in cloning and proposes a way to overcome this barrier.
Since Dolly the sheep, around 20 mammalian species have been cloned by somatic cell nuclear transfer (SCNT). This technique involves taking a nucleus from a differentiated cell and injecting it into an empty oocyte. If all goes well, the nucleus will be reprogrammed to totipotency, add a surrogate mother into the mix, et voila, you have a recipe for making a clone.
However, the efficiency of reproductive cloning is low (around 1-5%) for most species, and this has been attributed to uncharacterized epigenetic barriers that prevent the activation of the zygotic genome in SCNT embryos.
A talented team of researchers from the Howard Hughes Medical Institute used RNA-seq to compare gene expression profiles between mouse embryos derived from SNCT and those derived from IVF at the stage when the zygotic genome starts to become expressed. Here’s what they uncovered:
- Using a sliding window approach, they identified 222 regions of 100-800kb that failed to be properly activated in SCNT embryos, which they called ‘reprogramming resistant regions’ (RRRs).
- These RRRs are gene poor, but are enriched in particular DNA repeats.
- RRRs are marked by H3K9me3 in many somatic cell types and show characteristics of silent chromatin.
With a steady hand, the team injected mRNA encoding a H3K9me3-specific demethylase into SCNT embryos. This greatly reduced H3K9me3 levels and led to the activation of 184/222 of the RRRs. Remarkably, these embryos developed into blastocysts and implanted with substantially higher efficiency than control SCNT embryos to produce healthy, fertile cloned mice.
Thus, with a little molecular trickery, epigenetic barricades can be broken, bringing us one step closer to the realm of science fiction.