Women are much more likely than men to have their immune systems turn against them, leading to a range of so-called autoimmune diseases, such as lupus and multiple sclerosis. HAS study published Thursday offers an explanation rooted in the X chromosome.

The research, published in the journal Cell, suggests that a special set of molecules acting on the extra X chromosome carried by women can sometimes disrupt the immune system.

Independent experts have said these molecules are unlikely to be the only reason autoimmune diseases affect women. But if the results are confirmed in other experiments, it could be possible to base new treatments on these molecules, rather than on current drugs that weaken the entire immune system.

“This may be a better strategy,” said Dr. Howard Chang, a geneticist and dermatologist at Stanford who led the new study.

Male and female embryos carry 22 identical pairs of chromosomes. The 23rd pair is different: females have two Xs, while males have one X and one Y, which lead to the development of male sexual organs.

Each chromosome contains genes that, when “turned on,” produce proteins that work inside cells. One might expect that women, having two copies of X, would produce twice as much X protein as men. Instead, they produce at about the same level. This is because one of the two X chromosomes is inactive.

A molecule called Xist clings to the second X chromosome “like Velcro,” Dr. Chang said. As hundreds of Xist molecules wrap around the X chromosome, they completely block it.

Keeping a silent crucial for women’s health. If a gene on the second

In 2015, Dr. Chang realized that silencing itself might also have a downside. His revelation came as he was preparing to take his medical exams to renew his dermatologist license.

As part of his studies, Dr. Chang had to brush up on autoimmune diseases, memorizing the names of human proteins that can be targeted by a misdirected immune system. When he looked at the list, he was surprised to see some familiar names.

When Dr. Chang is not working as a dermatologist, he conducts research on the X chromosome in his laboratory. I noticed that many proteins involved in autoimmune diseases also helped Xist silence the X chromosome.

Perhaps, Dr. Chang thought, this was no coincidence.

The new study is the result of years of research proving that Xist molecules could cause autoimmune diseases. He and his colleagues studied a strain of mice in which females are at high risk of contracting lupus, an autoimmune disease, while males never develop severe cases.

The researchers genetically modified male mice so that they, like females, produced Xist. “Once male mice express Xist, they have much worse levels of immune disease,” Dr. Chang said.

Researchers also found that people with lupus or two other autoimmune diseases had high levels of antibodies against Xist-related proteins in their blood.

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