After birth, some scientists have hypothesized that XIST could play a protective role against immune diseases, such as SLE, by silencing or dampening expression of X-encoded, pro-inflammatory genes. Dysregulation of XIST, they reasoned, may reactivate some of those genes and aid the development of autoimmunity.
XIST, in fact, is necessary to limit the expression of SLE-linked TLR7, although scientists have also shown that XIST can permit the expression of both alleles of TLR7 and other X-linked genes in some scenarios. The TLR7 gene, a 2018 study found, can escape silencing by X chromosome inactivation in immune cells from women and from men with Klinefelter syndrome who have additional X chromosomes.4 XIST can escape X inactivation as well, as has been documented in several cell types and disease states, although the exact mechanism responsible for the loss of XIST-mediated X inactivation remains unclear.5
In the new study, the team at Johns Hopkins conducted multiple searches for female sex-biased, self-RNA transcripts that can bind TLR7, based in part on recent studies showing that TLR7 has a high affinity for UU-containing RNA sequences. The assays converged on the 19 kb-long XIST transcript as the strongest candidate in the human genome. The researchers found it has the most pronounced female expression bias of any RNA transcript in adults: It was 472-fold more abundant in blood samples from female donors than from their male counterparts. The long noncoding RNA, the study found, is also a uniquely rich source of putative TLR7 binding sites, with an especially high binding capacity in its so-called “A-repeat region.”
A synthesized version of the XIST A-repeat region stimulated interferon production by human plasmacytoid dendritic cells expressing the TLR7 receptor. That interferon production, in turn, was wholly dependent on the presence of TLR7.
“A big unknown question when we sought to do this study was, ‘What are the specific self-RNAs that are actually driving—stimulating—toll-like receptor 7 in the setting of lupus, or is it any old cellular RNA?’” Dr. Darrah says. In female-derived epithelial cells, the researchers found, knocking out XIST expression significantly diminished the ability of the full pool of cellular RNAs to activate TLR7.
The experiment, in effect, allowed the researchers to ask how much XIST was contributing to the pool of potential TLR7 agonists. “The answer was a lot, because by completely deleting XIST, we lost a huge proportion of that signal downstream of TLR7,” Dr. Antiochos says. Out of the thousands of RNA transcripts, in fact, the loss of XIST reduced the signaling by 67%. “It really shows that there is some sequence specificity to this effect,” Dr. Darrah adds.
