Scientists use genetic engineering to create mice with two male parents

By Will Dunham

(Reuters) - Scientists in China have manipulated embryonic stem cells to create laboratory mice with two male parents that managed to live to adulthood - though with significant developmental abnormalities - in what they are calling a noteworthy achievement in mammalian genetic engineering.

If further refined, the research could prove valuable, according to the scientists, in potential future efforts to prevent the extinction of critically endangered species and could inform the field of regenerative medicine, which involves replacing tissue or organs damaged by disease or other factors.

The study, published in the journal Cell Stem Cell, involved targeting a certain set of genes used in reproduction in order to create what are called bipaternal offspring, with two biologically male parents rather than a female and a male parent. This is called unisexual reproduction.

While mammals are known to reproduce only through sexual reproduction, certain other vertebrates including some reptiles, amphibians, birds and fish have produced offspring using unisexual reproduction. These have involved a phenomenon called parthenogenesis in which an embryo forms from an egg without fertilization.

In the new study, mice were created through genetic editing of mouse embryonic stem cells targeting a class of mammal-specific genes known as imprinted genes, of which there are about 200, the researchers said.

Stem cells are cells that can develop into various bodily tissues. Embryonic stem cells, present in early-stage embryos, have the capability of developing into all the cell types of the developing fetus. Most kinds of stem cells are less versatile and may help to sustain and restore only the tissues and organs in which they are situated.

The researchers said they modified 20 of the imprinting genes and used cloning technology in order to create the bipaternal mice.

"Generally speaking, imprinted genes have been thought to serve as a 'lock' preventing mammals from undergoing unisexual reproduction. However, this widely accepted hypothesis had not been conclusively proven before," said developmental biologist Wei Li of the Chinese Academy of Sciences, who helped lead the study.

"This is a fundamental science study focused on animal models. There are no plans to extend this type of experimentation to humans," he added.

The researchers created 1,081 embryos. Of those, about 12% survived to birth, much lower than normally would be the case.

A total of 84 male and 50 female pups were born. More than half died before reaching adulthood. Those reaching adulthood all had developmental defects, experienced a shortened lifespan and were sterile.

"The bipaternal mice exhibited developmental disorders, including craniofacial deformities, where their facial width-to-length ratio was broader compared to normal mice. They also had difficulty suckling," said developmental biologist and study co-author Guan-Zheng Luo of Sun Yat-sen University.

The researchers devised a way to significantly alleviate those two defects by inactivating a specific developmental gene.

"However, the mice still exhibited behavioral abnormalities, such as a tendency to enter the center of an open-field test, which is contrary to the instinctive behavior of rodents, who typically prefer to stay near the edges. The sterility of the bipaternal mice may be due to excessive modifications of imprinted genes, as these regions play various roles in development," Luo added.

This is the latest research involving bipaternal laboratory mice.

A team of scientists led by developmental biologist Katsuhiko Hayashi of Osaka University in Japan used a different technique in research published in 2023 to create bipaternal mice by turning skin cells into what are called induced pluripotent stem cells, which can transform into any cell type.

"The main difference between their work and ours," Hayashi said of the new study, "lies in the use of genome editing for generating offspring."

Hayashi said his team's method does not involve genome editing, except in a minor way, and all the pups derived from this technique have grown into adults without premature death.

"That said, this report makes a significant contribution to a deeper understanding of the function of genomic imprinting in mammalian development and reproductive technologies," Hayashi said.

Developmental biologist Zhi-kun Li of the Chinese Academy of Sciences, first author of the new study, said it represents "a notable accomplishment in mammalian genetic engineering." He said the technique used in this study enhanced the pluripotency - the ability to develop into numerous cell types - of embryonic stem cells, which he said play a critical role in regenerative medicine.

"For endangered species, this technique provides a potential method for producing offspring through unisexual reproduction, without relying on complex stem cell differentiation processes," he added.

(Reporting by Will Dunham in Washington, Editing by Rosalba O'Brien)