Some neurological diseases and developmental disorders affect women less frequently and less severely than men. Experiments with brain organoids now show a possible reason for this. Using this brain tissue grown in the laboratory, researchers examined the role that women’s second X chromosome plays as a possible genetic reserve. This excess X chromosome is normally silenced in female cells and is not read. However, the experiment showed that selected genes on this chromosome can be reactivated during embryonic development. In the case of the genetic developmental disorder Opitz-BBB/G syndrome, which was studied in the laboratory, this ensured that the female brain organoids developed fewer malformations than the male ones. Because the “reserve gene” on the second X chromosome was still intact, it was able to step in at least in part. As a result, the signs of illness were weaker.
In all mammals, males and females differ in terms of their sex chromosomes – this also applies to us humans. Men have one X and one Y chromosome, while women have two X chromosomes. The genes on this chromosome are therefore present in duplicate. So that the body can still develop normally, it switches off one of the two X chromosomes very early in embryonic development in women. This chromosome remains in the cell, but its genes are largely deactivated and are not read. “However, it was already known in research that this mechanism does not affect the entire chromosome,” explains co-author Sven Falk from the University of Erlangen-Nuremberg. Studies show that some of the genes on the actually inactive second X chromosome can escape this “silencing”. This means that certain proteins encoded by these genes are produced in higher concentrations in female cells than in men.
Brain organoids as a test case
Researchers have long suspected that this partial activation of some X chromosome genes could explain some health differences between men and women, for example why women live longer and are often less severely affected by neurodegenerative diseases and neurodevelopmental disorders. However, this partial reactivation has so far been difficult to understand. Although this can still be partially detected in the tissue of deceased people, it remains unclear when and for how long these genes were switched on. The study on animals can only partially be transferred to humans. That’s why Falk, first author Marco Bertin from the University of Mainz and their colleagues have chosen a different approach. They used induced human stem cells to grow brain organoids. These tissue structures, a few millimeters in size, recreate early steps in human brain development in the laboratory. “In this way, we were able to investigate processes that occur differently in humans than in animal models,” explains Falk.
For their study, the research team examined brain organoids that replicate the rare genetic developmental disorder Opitz-BBB/G syndrome. This manifests itself in an abnormally wide distance between the eyes, often combined with a cleft lip and palate and malformations of the larynx. Mental performance is also reduced. The trigger is often a genetic defect in a gene on the X chromosome. Similar to other developmental disorders based on genetic causes, women are affected less frequently and less severely by this disease. Bertin and his colleagues have now used their brain organoids to investigate what differences there are between brain organoids made from female and male cells and what role reactivation of the genes on the second X chromosome plays. It turned out that the signs of these developmental disorders were significantly less severe in female brain tissue. “Brain organoids from the stem cells of a male donor showed a dramatic decrease in differentiated neurons, accompanied by a delayed cell cycle of neuronal stem cells and progenitor cells,” report the researchers. “The female brain organoids with the same mutation, on the other hand, showed a significantly milder course.”
Genetic reserve reactivated
The reason for this was revealed by closer analyzes of the gene activity. “We were able to show that the switched off X chromosome is used again later in embryonic development,” says Falk. “We do not see this reactivation uniformly in all cells, but rather differently in different groups of cells. It occurs increasingly in cells that form additional brain cells.” An experiment using the CRISPR/Cas9 gene scissors demonstrated that this temporary reactivation actually represents a kind of buffer or reservoir for the female brain. The researchers used it to remove both alleles of the defective gene in the female cells. As a result, brain organoids grown from these homozygous stem cells showed similarly severe malformations as in the male organoids. “This proves that this reactivation makes a measurable difference,” says Falk. “It helps ensure that signs of illness are less pronounced in the female brain.” The study shows for the first time that the second X chromosome in the female brain actively helps to mitigate the consequences of illness.
The new results provide a possible biological reason why female sufferers often show milder symptoms of many genetic brain development disorders. The switched off X chromosome can partially switch back in during brain development. The researchers speak of a genetic reserve. “This second X chromosome is not only available, it is actually used during brain development,” says Falk. In the long term, this knowledge could help develop new approaches to dealing with certain genetically determined developmental disorders.
Source: Marco Bertin (University of Mainz) et al., Nature Communications, doi: 10.1038/s41467-026-68428-x