Gene-editing used to create single sex mice litters, 100% efficiency


Date:
December 3, 2021
Source:
The Francis Crick Institute
Summary:
Scientists have used gene editing technology to create female-only
and male-only mice litters with 100% efficiency.



FULL STORY ========================================================================== Scientists at the Francis Crick Institute, in collaboration with
University of Kent, have used gene editing technology to create
female-only and male-only mice litters with 100% efficiency.


==========================================================================
This proof of principle study, published in Nature Communications today
(Friday 3rd December), demonstrates how the technology could be used
to improve animal welfare in scientific research and perhaps also
agriculture.

In scientific research and also farming, there is often a need for either
male or female animals. For example, laboratory research into male or
female reproduction requires only animals of the sex being studied. And
in farming, only female animals are required for egg production and
in dairy herds. This means it is common practice for animals of the
unrequired sex to be culled after birth.

The researchers' new method uses a two-part genetic system to inactivate embryos shortly after fertilisation, allowing only the desired sex
to develop.

Such a genetically-based method to control the sex of offspring could drastically reduce culling in both industries.

The embryo selection is based on the fact that there are two elements of CRISPR-Cas9 -- the Cas9 enzyme that cuts the DNA, allowing scientists
to alter specific regions, and the guide RNA which carries the Cas9 to
the right location on the genome. The team placed one element of the
system on the father's X or Y chromosome, meaning that it will only be inherited by female or male embryos respectively. The other element is contributed by the mother, and is inherited by all embryos.

They targeted the Top1 gene, which is essential to DNA replication
and repair.

When an embryo formed from a sperm and egg, each containing one half of
CRISPR- Cas9, the gene-editing was triggered in the embryo and it was
not able to develop beyond a very early stage of around 16 to 32 cells.



========================================================================== Using this method, the researchers were able to control, with 100% effect,
the sex of a litter. To produce a male-only litter, the researchers edited
the father's X chromosome, meaning only females inherited the deleterious mutation, and for a female-only litter, they edited the Y chromosome.

Surprisingly, the method did not lead to a 50% decrease in the number
of offspring produced, rather the litter sizes were between 61% -- 72%
of the control litters. The researchers suggest this is because animals
such as mice produce more eggs than required during each ovarian cycle, allowing for a proportion of them to be lost during early development
without reducing litter size. This means that in situations where one sex
is needed, fewer breeding animals will be required in order to produce
the same number of the desired sex of offspring.

As the Top1 gene is well conserved across mammals, these results may
also be applicable to other animals.

Charlotte Douglas, first author and former PhD student and postdoctoral scientist at the Crick, says: "This method works as we split the genome
editing process in half, between a male and female, and it is only
when the two halves meet in an embryo through breeding, that it is
activated. Embryos with both halves cannot develop beyond very early
cell stages.

"We've also shown this process works successfully in different
combinations - - introducing either the Cas9 or the guide RNA elements
on to the mother's or father's chromosomes." As the offspring which
survive only contain half of the CRISPR-Cas9 elements within their genome,
this acts as a control preventing the sex-selection being passed down to further generations, unless they are selectively bred with an individual
of the opposite sex containing the other half. This is different to
genetic engineering through 'gene-drive' methods, which seek to spread
a genetic mutation widely amongst a population.



========================================================================== There are also no harmful effects of the gene edit in the surviving
offspring.

James Turner, author and group leader of the Sex Chromosome Biology
Laboratory at the Crick says: "This work could have immediate and
valuable impact in scientific laboratories, as we've shown how it
is safe and effective in mice, a common mammal used in medical and
scientific research. While a lot of research needs both sexes, there
are areas of study where only one is needed. For example, when studying
the reproductive system, sex-specific diseases, or certain hormones."
Peter Ellis, author and senior lecturer in molecular genetics and
reproduction at University of Kent, says: "The implications of this work
are potentially far-reaching when it comes to improving animal welfare,
but should be considered at ethical and regulatory levels.

"In particular, before any potential use in agriculture, there would
need to be extensive public conversation and debate, as well as changes
to legislation. On the scientific side, there is also much work to be
done over a number of years.

Further research is needed, first to develop
the particular gene editing toolkits for different
species, and then to check they are safe and effective." ========================================================================== Story Source: Materials provided by The_Francis_Crick_Institute. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Charlotte Douglas, Valdone Maciulyte, Jasmin Zohren, Daniel
M. Snell,
Shantha K. Mahadevaiah, Obah A. Ojarikre, Peter J. I. Ellis,
James M. A.

Turner. CRISPR-Cas9 effectors facilitate generation of single-sex
litters and sex-specific phenotypes. Nature Communications, 2021;
12 (1) DOI: 10.1038/s41467-021-27227-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211203081525.htm

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