Female bias in autoimmune disease can be profound, with nine females developing lupus for every male affected, and nearly twice that ratio seen in Sjögren disease.
For years, researchers have worked to determine the reasons for sex-linked differences in immune response and autoimmunity, with environmental factors, sex hormones, and X-chromosome inactivation — the process by which a second X chromosome is silenced — all seen as having roles.
More recently, different groups of researchers have homed in on a long noncoding RNA fragment called X-inactive specific transcript, or Xist, as a potential driver of sex bias in autoimmune disease. Xist, which occurs in female mammals, has been known since the 1990s as the master regulator of X-chromosome inactivation, the process by which the second X chromosome is silenced, averting a fatal double dose of X-linked genes.
The inactivation process, which scientists liken to wrapping the extra X with a fluffy cloud of proteins, occurs early in embryonic development. After its initial work silencing the X, Xist is produced throughout the female’s life, allowing X inactivation to be maintained.
But is it possible that Xist, and the many dozens of proteins it recruits to keep that extra X chromosome silent, can also provoke autoimmunity? This is the question that several teams of researchers have been grappling with, resulting in provocative findings and opening exciting new avenues of discovery.
Xist Protein Complexes Make Male Mice Vulnerable to Lupus
In February, researchers Howard Chang, MD, PhD, and Diana Dou, PhD, of Stanford University in Stanford, California, made worldwide news when they published results from an experiment using male mice genetically engineered to carry a non-silencing form of Xist on one of their chromosomes.
Dr. Diana Dou
Xist acts like a scaffold, recruiting multiple protein complexes to help it do its job. Dr. Dou explained in an interview that her team has been eyeing suspiciously for years the dozens of proteins Xist recruits in the process of X-chromosome inactivation, many of which are known autoantigens.
When the mice were injected with pristane, a chemical that induces lupus-like autoimmunity in mice, the Xist-producing males developed symptoms at a rate similar to that of females, while wild-type male mice did not.
By using a male model, the scientists could determine whether Xist could cause an increased vulnerability for autoimmunity absent the influence of female hormones and development. “Everything else about the animal is male,” Dr. Dou commented. “You just add the formation of the Xist ribonucleoprotein particles — Xist RNA plus the associating proteins — to male cells that would not ordinarily have these particles. Is just having the particles present in these animals sufficient to increase their autoimmunity? This is what our paper showed: That just having expression of Xist, the presence of these Xist [ribonucleoproteins], is enough in permissive genetic backgrounds to invoke higher incidence and severity of autoimmune disease development in our pristane-induced lupus model.”
The Stanford group sees the Xist protein complex, which they have studied extensively, as a key to understanding how Xist might provoke autoimmunity. Nonetheless, Dr. Dou said, “It’s important to note that there are other contributing factors, which is why not all females develop autoimmunity, and we had very different results in our autoimmune-resistant mouse strain compared to the more autoimmune-prone strain. Xist is a factor, but many factors are required to subvert the checkpoints in immune balance and allow the progression to full-blown autoimmunity.”