For the first time, scientists in Japan have cloned a mouse using just a drop of blood taken from its tail. The result is important because it gives scientists a new way to preserve strains of lab mice for the study of human diseases.
The female clone proved to be fertile by natural mating and lived for 23 months, which is about normal for a lab mouse, researchers from RIKEN BioResource Center in Tsukuba, Japan, report in a paper published online in the journal Biology of Reproduction on 26 June.
Since the world’s first successful reproductive animal cloning that resulted in Dolly the Sheep in 1996, nearly 20 different mammal species have been cloned.
Genetically modified mice are used extensively in the study of diseases, from anxiety to cancer, obesity to substance abuse, and researchers often spend years developing strains with the exact genetic features of the disease being studied.
But in some cases, engineering mice with unique gene mutations produces animals that are infertile.
There are currently two ways to preserve a valuable strain of lab mouse. If there is at least one male left and he has healthy sperm, then researchers use an IVF method called intracytoplasmic sperm injection (ICSI) where a single sperm is injected directly into an egg. This is the preferred method.
But if the only surviving mice are female, or the male’s sperm is not viable, then researchers turn to a cloning method called somatic-cell nuclear transfer (SCNT).
In SCNT, the nucleus of a host’s egg cell is removed and replaced with the nucleus of a donor somatic cell (any cell of the body that is not a sex cell). This means that when the egg cell multiplies it will carry the genetic code of the donor.
The egg cell carrying the donor’s nucleus is inserted into the host egg and is given an electric shock that triggers it to start dividing.
An early version of this method was used to produce Dolly the Sheep. Since then, the technique has been refined by labs all over the world. However the process is time-consuming and not cheap, so one of the challenges is how to increase the success rate of producing viable offspring.
The RIKEN team in Japan have succeeded in increasing the success rates from the standard 3% in first-generation clones to 10% in first-generation and 14% in higher-generation clones.
The somatic cell that is used in SCNT cloning is also important. At present the preferred type is cumulus cells, the cells that surround the oocytes in the ovarian follicle and after ovulation. However, these are not easy to retrieve and risks damage to the donor mouse.
So the team at RIKEN decided to explore alternatives. They had an idea that white blood cells in peripheral blood that can be easily accessed (such as from the mouse’s tail) might be worth exploring.
For this latest study, the team experimented with several types of white blood cells to see which would produce the most successful yield.
When they used lymphocyte nuclei from the donor mouse, only 1.7% of embryos developed into offspring. This was more or less expected because lymphocytes are small and not easy to filter from the blood sample.
They had the best success with the largest white blood cells, granulocytes and monocytes. 2.1% embryos of their embryos produced offspring, nearly as good as the 2.7% of the preferred cell type, the cumulus cells.
Although these results are not as high as the success rate of cumulus cells, the study proves, for the first time, that it is possible to produce genetic copies of a mouse using nuclei of cells taken from peripheral blood of a live donor.
This opens the door to a way of producing clones almost as soon as the cells are retrieved, with minimal risk to the donor, helping to continue important genetic strains that cannot be preserved by other means, as the researchers note in their conclusions:
“This strategy will be applied to the rescue of infertile founder animals or a “last-of-line” animal possessing invaluable genetic resources.”
The researchers also hope to improve the success rate with this new source of somatic cells. They noticed that embryos from granulocytes fragmented at a much higher rate than embryos from lymphocytes and cumulus cells and are planning to investigate this further.
The idea of somatic cloning is not a human invention: as with many things in science, nature got their first. Studying somatic cloning in nature yields some interesting results. For instance, in 2011, scientists reported how marine organisms that reproduce asexually by somatic cloning have special mechanisms that delay aging and keep them in exceptionally good health.
Written by Catharine Paddock PhD