Small molecules in DNA enable scientists to change the sex of fetuses from male to female

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Mammalian chromosomes have a major influence on an individual's development as male or female, but a new study reveals that the withdrawal of these sex chromosomes can be overridden by small molecules called microRNA.

The study, published May 7 in the journal Nature Communications, showed that deleting genes behind specific microRNA molecules can turn male mice into females in the womb, leading to a complete sex reversal.

“We did not expect the results to be as surprising as they are,” study co-author Rafael Jiménez, a professor of genetics at the University of Granada, told Live Science.

Sex determination in mammals depends on a delicate balance between “opposite” gene sets – one that drives the development of female characteristics, such as ovaries, and another that produces male characteristics, such as testicles.

Early in animal development, the scales flip one way or another, leading to an irreversible series of steps that culminates in the development of either set of sexual organs.

“At a very early stage in our evolution, all mammals have the ability to be either male or female,” explains Francisco Barrionuevo, co-author of the study and professor of genetics at the University of Granada. A gene called testis-determining factor or sex-determining region Y (SRY) protein, which is found only on the Y chromosome, triggers a cascade of events that form the testicles. The absence of the gene in individuals with only X chromosomes leads to the formation of ovaries.

Scientists know a lot about the genes involved in making the proteins needed for these processes. But a large portion of mammalian DNA, including about 98% of the human genome, does not code for any proteins, so scientists were not sure what, if any, role these other genes played in sex determination.

These stretches of genetic material, long considered "junk DNA," are converted into molecules called non-coding RNA, rather than proteins. RNA can influence many biological processes. About a quarter of these molecules are microRNAs, which can bind to many genes and regulate their activity levels.

Among the thousands of known microRNA molecules, the team focused on a group of six molecules known to interact with genes involved in sex determination. They deleted these molecules from developing mouse embryos that contained either XY or XX chromosomes.

The XX mice developed ovaries, as expected, but the XY mice showed early signs of uterine development and had ovaries that were indistinguishable from those of XX mice.

"We saw the gonads under the microscope and they were full of signal for this female marker," Alicia Hurtado, first author of the study and a postdoctoral researcher at the Andalusian Center for Developmental Biology in Seville, told Live Science.

To confirm the results, they repeated the experiments several times, using different strategies to delete microRNAs, which are RNA molecules responsible for controlling gene expression.

In order for the testicles to develop properly in XY animals, the protein made by the SRY gene must be made in the right quantities and at the right times. Scientists found that the absence of 6 microRNA molecules in XY mice caused this protein to be produced about 12 hours later than normal. This, in turn, affects the production of a different protein necessary for the growth of male sexual organs. Eventually, this series of events led to the mice being reversed in sex.

While the study was only conducted in mice, the six major microRNAs are present in all vertebrates and date back to the first vertebrates, about 500 million years ago. Therefore, it is very likely that this group of microRNAs functions similarly in other mammals as well, including humans.