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Researchers from Kunming Institute of Zoology, Chinese Academy of Sciences, and the University of North Carolina have reported successfully editing a human gene into rhesus monkeys in a study published on March 27 in National Science Review. The gene is said to be important for human brain development and the treated monkeys subsequently showed human-like brain development, reports China Daily.

The human gene, microcephalin or MCPH1, is expressed during the fetal stage of brain development and is linked to brain size, according to the study. Researchers exposed monkey embryos to viruses containing the gene, which led to the differentiation of neural cells that resembled human development, they described in their study.

Human brains take much longer to develop in comparison to other primates in a process called neoteny, where the period of neural plasticity is extended through childhood. The researchers report that their study may be evidence that engineeredprimates can develop in a similar pattern.

The monkeys in the study also showed signs of better short-term memory, report the authors.

A number of scientists, even one involved with the study, have criticized the experiments. Unnamed scientists called the experiments “reckless” and “questioned the ethics of genetically modifying primates,” reports MIT Technology Review. “The use of transgenic monkeys to study human genes linked to brain evolution is a very risky road to take,” James Sikela of the University of Colorado who was not involved with the study tells MIT Technology Review. “It is a classic slippery slope issue and one that we can expect to recur as this type of research is pursued.”

Research using genetic modification of primates is active at Chinese institutions. A research group at the Institute of Neuroscience in Shanghai published results in January 2019 on disabling a gene necessary for the sleep-wake cycle, reports Nature.