Above: X chromosomes. Image courtesy of Johns Hopkins Medicine.

‍

The Immune System and Autoimmune Diseases

The immune system is a network of cells, tissues, and organs that defend the body against harmful invaders like bacteria, viruses, and parasites. There are two main parts of the immune system: innate and acquired. The innate immune system is the first line of defense against a pathogen and consists of barrier tissues like the skin, eye cornea, and mucous membranes of the respiratory and gastrointestinal tracts. The acquired immune system triggers a more targeted response, as the body makes specific antibodies to combat pathogens to which it has been exposed. These antibodies then bind to the foreign pathogen and flag it to be destroyed by T-cells.

Above: COVID virus and antibodies. Image courtesy of Yale School of Medicine.

‍

Autoimmune diseases occur when the body’s immune system can’t distinguish between its own cells and foreign pathogens and begins to mistakenly attack normal cells and tissues. It is estimated that up to 50 million Americans have an autoimmune disease, and as many as 80% of them are women. Of the more than 80 known autoimmune disorders, lupus and rheumatoid arthritis are examples of common immune disorders that more often affect women. The uneven gender distribution of autoimmune diseases has long puzzled scientists. Stanford researchers suspect that the culprit might be the most fundamental sex difference: the X chromosome. 

The X Chromosome and Silencing by Xist

Above: Process of X chromosome inactivation via Xist coating. Image courtesy of Katopodis et al.

‍

Chromosomes are thread-like structures made of histone proteins and single molecules of DNA. They are located in the nucleus of every somatic cell and serve to store genetic information within the body. Humans have 22 pairs of numbered chromosomes and one pair of sex chromosomes for a total of 46 chromosomes or 23 chromosome pairs. The X chromosome in particular contains hundreds of protein-specifying genes that are necessary for survival. Because females have two X chromosomes, many expect that they would produce twice the amount of X chromosome-encoded proteins as males. However, in XX individuals, one of the X chromosomes in each cell is silenced via a process known as X-inactivation.

Above: Image highlighting the location of the Xist gene on chromosome and Xist molecules coating the chromosome. Image courtesy of The Scientist.

‍

X chromosome inactivation begins early in embryonic development and can proceed via two paths: random and imprinted. Random inactivation is the pathway relevant to ensuring normal doses of X chromosome-encoded proteins in women. In this process, each bodily cell randomly chooses either the paternal or maternal X chromosome to inactivate. This ensures that females have only one functional copy of the X chromosome, while the other lies dormant. 

But how does this process actually happen? The answer lies in a long non-coding RNA molecule known as Xist. A soon-to-be inactive X chromosome generates a lncRNA called X-inactive specific transcript, or Xist, from the Xist gene. Xist coats the chromosome from which it’s transcribed, marking it for inactivation. Various chromatin modifying factors are recruited to alter the structure of chromatin. These modifications can include direct methylation of DNA to silence nearby genes or methylation of histone proteins, which wraps DNA more tightly around the histone. Such a compacted structure makes it difficult for transcription factors to access DNA, thus reducing transcription and effectively silencing the entire chromosome.

Stanford scientists have recently found that several of Xist’s collaborator proteins are also linked to a plethora of autoimmune disorders.

The Stanford School of Medicine Study

Above: Summary of hypothesized connections between X-chromosome regulation and female-biased autoimmunity. Image courtesy of Dou et al.

‍

A new study led by Stanford School of Medicine researchers explored the connection between  Xist-mediated X-inactivation and the higher prevalence of autoimmune disease in women. Previous studies have investigated the roles of hormones or abnormal protein production as potential causes of autoimmune disease in women. To eliminate these causes as variables, researchers decided to use both a male and female mouse model for their study. They bred one mouse line to be autoimmune-resistant and another to be autoimmune-prone. They then incorporated the Xist gene into the genome of male mice, as Xist is not naturally occurring in males. From here, scientists asked two key questions: is a male mouse with an active Xist gene more likely to develop autoimmunity than a male with an inactive Xist gene? What about a male mouse with no Xist gene at all?

By injecting an irritant that causes lupus-like symptoms, scientists could compare the prevalence of autoimmune symptoms in each mouse model. They found that merely inserting the gene didn’t have immediate effects on the mice, but once the gene was activated, male mice developed autoimmunity at a rate comparable to females. However, this finding was only true for autoimmune-prone mice. In mice engineered to be resistant to autoimmunity, activating Xist had no effect. Additionally, autoimmune-resistant female mice seldom developed autoimmunity. This demonstrates that not only Xist activation but also a genetic predisposition and a tissue-damaging stressor are required to trigger autoimmunity.

Above: Antibodies bind to antigens on the surface of a cell flagged to be destroyed. Image courtesy of News-Medical.

‍

Dr. Howard Chang, the geneticist and dermatologist at Stanford who led the study, suspects that autoimmune diseases may arise during the normal process of cell death, wherein cellular contents are released into the bloodstream. Examining blood samples of 100 patients with autoimmune disorders revealed autoantibodies against several Xist protein complexes. Though the exact mechanism by which these antibodies form is unclear, it’s hypothesized that immune cells may become confused by these large complexes and mistakenly produce antibodies against them, causing the body to attack itself. Some of these autoantibodies were specific to particular immune disorders and thus may serve as early diagnostic markers of autoimmune disease far before symptoms develop. 

Looking to the Future

The present study highlights the critical role of Xist in the increased prevalence of autoimmunity among women. Every cell in a woman’s body produces Xist. Despite this, Dr. Chang explains, male cell lines have been used as the standard of reference for decades—cells that do not produce Xist or its associated complexes. As a result, the impact of Xist on autoimmunity has been long overlooked. The study has established that Xist plays a role in female autoimmunity although the precise mechanism is still undetermined. Several important steps between the production of this RNA and the immune system malfunction that lead to autoimmune disease remain unclear. In an interview with journalist Lisa Kim, Dr. Chang explains that he is confident that a deeper understanding of these processes could pave the way for new therapies to treat autoimmune diseases in the future.