Full-term mouse offspring were derived from single unfertilised eggs using targeted CRISPR/Cas9 epigenome editing.

Parthenogenesis – generating offspring from a single unfertilised egg – is usually not possible in mammals due to a phenomenon known as genomic imprinting. Genomic imprinting describes how methylation in certain regions of the genome silences one allele inherited from either the mother or father, but not the other. These regions are often important for genetic control of embryonic development, which cannot go ahead without correct imprinting.

Now, scientists from Shanghai Jiao Tong University in China have described how they achieved parthenogenesis in a mouse who survived to adulthood and later gave birth to viable offspring by targeting these regions. 

In the study published in the journal Developmental Biology the authors wrote: 'Together, these data demonstrate that parthenogenesis can be achieved in mammals by appropriate epigenetic regulation of multiple imprinting control regions. This is consistent with the famous parental conflict hypothesis (also known as the Haig hypothesis), which proposes that the imprinting-mediated balance between paternal and maternal genomes is critical for mammalian development.'

Seven imprinting control regions of DNA were targeted for induced methylation or demethylation in the study. These targeted imprinting control regions were shown in the literature to play key roles in the regulation of fetal and postnatal growth as well as support the development of bimaternal and bipaternal embryos.

Eggs were removed from a donor mouse and injected with multiple single guide RNAs attached to either Cas9 or messenger RNA that induced methylation or demethylation in the imprinting control region, respectively, in one allele of each gene and not the other. These edited regions maintained methylation during the early stages of development. Quantitative real-time PCR was used to assess the expression of the modified gene in the embryos and suggested that the techniques significantly improved parthenogenetic development. The modified embryos were then transferred into the uteruses of different mice. 

Of 192 embryos transferred at the blastocyst stage, 14 developed into pregnancy, three gave birth, and only one survived to adulthood. Of the two pups who died within 24 hours of birth, further testing demonstrated that at least one of the seven imprinting control regions exhibited loss of methylation confirming imprinting at all seven regions was crucial for development and viability. 

Identifying and editing additional imprinting control regions could improve the efficiency of the parthenogenetic process, the authors suggested. Epigenetic imprinting may also result in unknown off-target effects. Here, off-target analysis of likely sites showed no significant changes, suggesting high-specificity of the technology. Although, future studies are needed to fully assess any effects.

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Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.