A group of scientists from the University of Pennsylvania may have found part of the reason lupus is more prevalent in women than in men: a defect that prevents the full inactivation of women’s second X chromosome in immune T cells.
The findings of the study, “Altered X-chromosome inactivation in T cells may promote sex-biased autoimmune diseases,” were published in JCI Insight.
Systemic lupus erythematosus (SLE), the most prevalent form of lupus, is a chronic autoimmune disease characterized by tissue inflammation, skin rash, pain, fatigue, depression, and impaired cognition. According to previous studies, up to 85% of all patients diagnosed with lupus are women. The reason for that may lie on their second X chromosome.
All females have two X chromosomes, while males only have one, along with an unmatched Y chromosome. However, despite having two X chromosomes, female cells only have one active chromosome at a time. The other chromosome is permanently shut down early on during embryonic development, to ensure that all genes in the X chromosome are expressed equally in men and women. This is broadly known as X chromosome inactivation (XCI).
However, according to this new study, women with lupus may be unable to fully shut down, or silence, their second X chromosome in T cells, resulting in overactivation of certain genes in the X chromosome that may be involved in the onset of the disease.
“In normal circumstances, the inactive X should be silenced, and what we show is, in lupus, it’s not,” Montserrat Anguera, a biologist at the University of Pennsylvania School of Veterinary Medicine (PennVet) and senior author of the study, said in a press release. “And it’s ultimately affecting gene expression.”
Previous studies by the same team of researchers have shown that in females, T cells and B cells (the immune cells responsible for the production of antibodies) may be unable to completely inactivate the second X chromosome due to defects in Xist, an RNA molecule necessary for XCI that is encoded by the XIST gene. (RNA is the template used for the production of a protein.)
In this study, the researchers focused on investigating how this process could be affected in T cells from SLE patients and animal models of disease.
First they studied the process of XCI in T cells from healthy female mice and found that as T cells develop, Xist is temporarily removed from the inactive X chromosome. However, when these cells become active — as they would in the presence of pathogens — Xist returns to the X chromosome, efficiently silencing it.
Conversely, in T cells from female mice with late-stage lupus, this pattern was dramatically altered, preventing normal XCI.
“The only differences we detected happened at late stages of disease,” Anguera said. “What this means is that abnormal X inactivation is a consequence of the disease; it’s not predisposing the animal to develop the disease.”
Interestingly, they found the same alterations in Xist localization in T cells isolated from children with SLE who were in remission.
“Even though they don’t have active disease, there’s something missing that’s preventing the RNA from staying targeted at that inactive X chromosome,” Anguera said.
After zeroing in on the genes whose expression might be altered because of the abnormal Xist localization in female SLE patients, researchers found that some of them were involved in controlling the organization and structure of the cells’ nuclei.
“What we think is happening is that in lupus, this Xist RNA is diffusing all over the place, these chromosomal proteins are changing their expression, and nuclear organization in the territory of the inactive X is changing,” Anguera said. “And that may also be contributing to the relaxed silencing of the inactive X and the changes in gene expression that we’re seeing.”
“The list of X-linked genes overexpressed in SLE patient T cells contains genes that are known to escape XCI (such as KDM5C and JPX) and also genes that should be silenced (FOXP3 and IL2RG). It is intriguing to speculate that particular regions are poised for reactivation or increased expression of escape genes from the [inactive X chromosome] and that loss of XIST RNA [may] facilitate gene reactivation,” the researchers stated.
The team is hoping to use single-cell sequencing technology to investigate how XCI is achieved and perturbed in the context of autoimmune disorders.
“I think it’s really promising,” she said. “If you can get Xist RNA to look like it should, then perhaps you can fix the aberrant X-linked gene expression.”