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Monkey see, monkey clone

31/10/24, 11:37

A leap forward in primate research

Chinese scientists have recently unlocked the secrets of cloning Rhesus monkeys offering new hope for medical breakthroughs.


Introduction


When we think of cloning, perhaps the first thing that comes to mind is Dolly the sheep, the first mammal ever cloned from an adult cell back in 1996. This groundbreaking achievement inspired a revolution leading to the successful cloning of other mammals such as cattles and pigs. However, cloning primates, especially Rhesus monkeys, has proven to be a significant challenge due to the low success rates and high embryonic losses during development.


What is cloning?


Cloning is the process of creating an identical genetic copy of an organism. In mammals, this is typically done through a technique called somatic cell nuclear transfer (SCNT). In SCNT, the nucleus (the compartment storing genetic material) from a cell of the animal to be cloned is transferred into an egg cell that has had its own nucleus removed. This hybrid egg cell then develops into an embryo which is implanted into a surrogate mother to grow into a new individual. Despite the success in cloning other mammals, cloning primates has proven to be a significant challenge. However, the potential benefits of cloning primates for medical research make it a worthwhile endeavour.


The importance of cloning primates


You might be wondering why being able to clone primates is so important. Well, primates like the Rhesus monkey are invaluable models for studying human diseases and create new therapies! The reason we can use them as disease models is because they share about 93% genetic identity and have very similar physiological characteristics with humans. For instance, Rhesus monkeys also  experience a decline in their cognitive abilities as they age, and they lose important connections between brain cells in the part of the brain responsible for complex thinking, even when there's no severe brain damage.


Moreover, Rhesus monkeys also develop the same kinds of brain changes that we see in people with Alzheimer's disease, such as the buildup of sticky proteins called amyloid-beta and tangled fibres of another protein called tau.These similarities make them excellent models for understanding how human diseases progress and for developing new treatments.


So, by cloning these animals, researchers might be able to create monkeys with specific genetic changes that mimic human diseases even more closely. This could allow scientists to study these diseases in greater detail and develop more effective therapies. Cloning primates could give us a powerful tool to fight against some of the most challenging disorders that affect the human brain!


A breakthrough in primate cloning


Now, a group of scientists in China have made a breakthrough in primate cloning. They successfully cloned a Rhesus monkey using a novel technique called trophoblast replacement (TR).This innovative approach not only helps us better understand the complex process of cloning but also offers a promising way to improve the efficiency of primate cloning, bringing us one step closer to unlocking the full potential of this technology for medical research and beyond.


The awry DNA methylation of cloned conkey embryos


To understand why cloning monkeys is so challenging, Liao and colleagues (2024) took a closer look at the genetic material of embryos created in two different ways. They compared embryos made through a standard fertility treatment called intracytoplasmic sperm injection (ICSI) with those created via the cloning technique, SCNT. What they found was quite surprising!


To make matters worse, the scientists also noticed that certain genes, known as imprinted genes, were not functioning properly in the SCNT embryos. Imprinted genes are a special group of genes that play a crucial role in embryo development. In a healthy embryo, only one copy of an imprinted gene (either from the mother or the father) is active, while the other copy is silenced. But in the cloned embryos, both copies were often incorrectly switched on or off.


Here's the really concerning part: these genetic abnormalities were not just present in the early embryos but also in the placentas of the surrogate monkey mothers carrying the cloned offspring. This suggests that the issues arising from the cloning process start very early in development and continue to affect the pregnancy.


Liao and colleagues suspect that the abnormal DNA methylation patterns might be responsible for the imprinted gene malfunction. It's like a game of genetic dominos – when one piece falls out of place, it can cause a whole cascade of problems down the line.

Piecing together this complex genetic puzzle is crucial for understanding why primate cloning is so difficult and how we can improve its success in the future. By shedding light on the mysterious world of DNA methylation and imprinted genes, Liao and colleagues have brought us one step closer to unravelling the secrets behind monkey cloning.


Digging deeper: what does the data reveal?


Liao et al. (2024) discovered that nearly half of the cloned monkey foetuses died before day 60 of the gestation period, indicating developmental defects in the SCNT embryos during implantation. They also found that the DNA methylation level in SCNT blastocysts was 25% lower compared to those created through ICSI (30.0% vs. 39.6%). Furthermore, out of the 115 human imprinting genes they examined in both the embryos and placentas, four genes - THAP3, DNMT1, SIAH1, and RHOBTB3 - showed abnormal expression and loss of DNA methylation in SCNT embryos. These findings highlight the complex nature of the reprogramming process in SCNT and the importance of imprinted genes in embryonic development. By understanding these intricacies, scientists can develop targeted strategies to improve the efficiency of primate cloning.


The power of trophoblast replacement


To avoid the anomalies in SCNT placentas, the researchers developed a new method called TR. In this method, they transferred  the inner cell mass (the part of the early embryo that develops into the baby) from an SCNT embryo into the hollow cavity of a normal embryo created through fertilisation, after removing its own inner cell mass.


The idea behind this technique is to replace the abnormal placental cells in the SCNT embryo with healthy ones from the normal embryo. And it worked! Using this method, along with some additional treatments, Liao et al. (2024) successfully cloned a healthy male Rhesus monkey that has survived for over two years (FYI his name is Retro!).


The ethics of cloning


While the scientific advances in primate cloning are exciting, they also raise important ethical questions. Some people worry about the potential misuse of this technology, for instance to clone humans, which is widely considered unethical. Others are concerned about the well-being of cloned animals, as the cloning process can sometimes lead to health problems.


As scientists continue to make progress in cloning technology, it is essential to have open discussions about the ethical implications of their work. Rules and guidelines must be put in place to ensure that this technology is developed and used responsibly, with the utmost care for animal welfare and the concerns of society.


Looking to the future


The successful cloning of a rhesus monkey using TR opens up new avenues for primate research. This technology can help scientists create genetically identical monkeys to study a wide range of human diseases, from neurodegenerative disorders like Alzheimer's and Parkinson's to infectious diseases like HIV and COVID-19. The trophoblast replacement technique developed by Liao et al. (2024) increases the likelihood of successful cloning by replacing the abnormal placental cells in the SCNT embryo with healthy ones from a normal embryo. However, it is important to note that this technique does not affect the genetic similarity between the clone and the original monkey, as the inner cell mass, which gives rise to the foetus, is still derived from the SCNT embryo.


Moreover, this research provides valuable insights into the mechanisms of embryonic development and the role of imprinted genes in this process. By understanding these fundamental biological processes, scientists can not only improve the efficiency of cloning but also develop new strategies for regenerative medicine and tissue engineering.


As we look to the future, cloning monkeys could help us make groundbreaking discoveries in medical research and develop new treatments for human diseases. However, we must also carefully consider the ethical implications of cloning primates and ensure that this powerful tool is used responsibly and for the benefit of society.


Written by Irha Khalid



REFERENCES


Beckman, D. and Morrison, J.H. (2021). Towards developing a rhesus monkey model of early Alzheimer’s disease focusing on women’s health. American Journal of Primatology, [online] 83(11). doi:https://doi.org/10.1002/ajp.23289.


Liao, Z., Zhang, J., Sun, S., Li, Y., Xu, Y., Li, C., Cao, J., Nie, Y., Niu, Z., Liu, J., Lu, F., Liu, Z. and Sun, Q. (2024). Reprogramming mechanism dissection and trophoblast replacement application in monkey somatic cell nuclear transfer. Nature Communications, [online] 15(1), p.5. doi:https://doi.org/10.1038/s41467-023-43985-7.


Morrison, J.H. and Baxter, M.G. (2012). The ageing cortical synapse: hallmarks and implications for cognitive decline. Nature Reviews Neuroscience, [online] 13(4), pp.240–250. doi:https://doi.org/10.1038/nrn3200.


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Shi, L., Luo, X., Jiang, J., Chen, Y., Liu, C., Hu, T., Li, M., Lin, Q., Li, Y., Huang, J., Wang, H., Niu, Y., Shi, Y., Styner, M., Wang, J., Lu, Y., Sun, X., Yu, H., Ji, W. and Su, B. (2019). Transgenic rhesus monkeys carrying the human MCPH1 gene copies show human-like neoteny of brain development. National Science Review, [online] 6(3), pp.480–493. doi:https://doi.org/10.1093/nsr/nwz043.

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