A new study suggests a simple letter change can reverse sex development, causing a mouse with female chromosomes (XX) to develop male genitalia.
The mutation responsible for the change was not in protein-coding genes, which comprise a relatively tiny fraction of the genome, but in a section of the vast (and until recently, thought to be “junk”) non-coding portion of the genome, which plays a key role in gene expression.
“This is a remarkable finding because such a tiny change – just one DNA letter out of ~2.8 billion – was enough to produce a dramatic developmental outcome,” Nitzan Gonen from the Goodman Faculty of Life Sciences and Institute of Nanotechnology and Advanced Materials at Bar-Ilan University said in a press release. “It shows that non-coding DNA can have a profound effect on development and disease.”
The research, published in Nature Communications, highlights the significance of a specific regulator (Enhancer 13, or Enh13) when it comes to sex development.
Protein-Coding Genes and Genital Development
Only a small fraction of the human genome is made up of protein-coding genes. Approximately 98 percent is made up of non-coding sequences. While this section of the genome is not fully understood, parts of it are involved in gene expression and influence when, where, and how genes are turned on and off.
Explore All OptionsThis includes the subject of the study: a regulator called Enhancer 13 (Enh13), which controls the activity of a gene that plays a key role in genital development (Sox9).
The researchers describe Enh13 as a “binary switch.” Male genitals are formed when certain factors bind to Enh13 and activate Sox9, whereas female genitals develop when a different set of factors binds to Enh13 and represses Sox9.
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Gene Editing with CRISPR
To explore the influence of Enh13 on sex development, Gonen and colleagues used CRISPR, a gene-editing technique, to make two mutations to the regulator: one involved adding an extra nucleotide (or letter), the other involved deleting three existing letters. In both cases, the Sox9 gene was activated, and the XX mice developed male genitalia.
“Adult XX mice appear externally and internally as males, but with small testes devoid of sperm due to the lack of the Y chromosome,” the study’s authors wrote in the recent study.
This, the researchers explain, is because the mutations prevent factors associated with female genitalia (NR5A1, GATA4, and RUNX) from muting Sox9. As a result, the Sox9 gene can be activated (or “turned on”), which in turn triggers the development of testes.
Building on Existing Research
The study builds on existing research showing tweaks to Enh13 caused mice with male chromosomes (XY) to develop female genitalia. A 2018 study published in Science found removing Enh13 entirely triggered sex reversal in XY mice. Another study, published in Nucleic Acids Research in 2024, found that subtle mutations in Enh13 can cause mice with male chromosomes to appear female.
The researchers say this adds to what we know about Enh13, showing it not only plays a key role in the development of testes but in the development of ovaries too.
“These findings implicate Enh13 as a regulatory element essential for both testicular and ovarian development, placing it at the critical junction of sexual differentiation pathways,” they wrote in the study. “This also highlights that even small changes within regulatory elements can induce dramatic phenotypic outcomes.”
From a clinical perspective, the researchers believe it could improve diagnosis rates for those born with Differences of Sex Development (DSD), an umbrella term describing a range of conditions that involve atypical sexual or reproductive development, many of whom do not have a genetic diagnosis. It also highlights the importance of non-coding regions of the genome.
“Our findings show that it is not enough to look only at genes,” lead author Elisheva Abberbock, a Ph.D. student at Bar-Ilan University, said in a statement. “Important disease-causing mutations may also lie in the non-coding genome, in DNA regions that control gene activity rather than encode proteins.”
This article is not offering medical advice and should be used for informational purposes only.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Science: Sex reversal following deletion of a single distal enhancer of Sox9
- This article references information from a study published in Nucleic Acids Research: Two redundant transcription factor binding sites in a single enhancer are essential for mammalian sex determination
- This article references information from a study published in Nature Communications: A single-nucleotide enhancer mutation overrides chromosomal sex to drive XX male development
