Discovering miR6891-5p: guardian of XX allelic balance and barrier to Sjgren's syndrome pathogenesis

This study, investigates a molecular mechanism that may underlie the extreme female predominance of primary Sjogren's syndrome (pSS). Focusing on minor salivary gland-derived mesenchymal stromal cells (MSCs), the authors performed genome-wide, allele-specific expression profiling of primary, non-immortalized cells from female pSS patients and matched controls. In control female MSCs, X-linked genes displayed balanced expression from both parental X chromosomes, with a median paternal expression ratio near 0.5. In striking contrast, female pSS MSCs exhibited widespread preferential expression from a single X chromosome - allelic skewing -accompanied by decreased expression of the long noncoding RNA XIST and reorganization of H3K27me3-positive nuclear foci that mark the inactive X. Mechanistic investigation tied these alterations to decreased levels of the HLA-locus-encoded microRNA miR6891-5p in pSS MSCs. Experimental inhibition of miR6891-5p in control MSCs recapitulated key pSS-associated features: dysregulation of XIST, ectopic silencing of X-linked loci, and allelic skewing.

Allelic skewing was not a benign transcriptional observation: protein products encoded by skewed X-linked genes were mislocalized in cells with skewed expression, and disruption of XIST or miR6891-5p in control MSCs produced an inflammatory phenotype during MSC differentiation similar to that observed in pSS MSCs. The study links a noncoding RNA, miR6891-5p, with maintenance of X-chromosomal allelic balance in salivary gland stromal cells and positions loss of this miRNA as a contributor to X skewing and downstream cellular dysfunction in pSS. The authors contextualize these findings within broader observations that X chromosomal abnormalities (for example, trisomy X and rare mosaicism) are overrepresented in women with pSS and other autoimmune diseases, and they note that escape from X inactivation contributes to sex differences in expression of X-linked genes. By using primary MSCs derived from salivary gland biopsies - cells relevant to the hallmark secretory dysfunction of pSS - the work emphasizes tissue-specific mechanisms rather than peripheral blood surrogates.

The discovery that miR6891-5p reduction causes XIST disruption and allelic imbalance, with associated protein mislocalization and inflammatory differentiation responses, suggests that restoring X-chromosomal allelic balance could be a therapeutic consideration. Specifically, the findings raise caution for autologous MSC therapies unless X skewing is corrected, and they highlight miR6891-5p and XIST-related pathways as potential targets for interventions aimed at re-establishing allelic balance in the salivary gland microenvironment. Overall, the study identifies X skewing as a molecular hallmark of pSS MSCs and advances understanding of how noncoding RNAs can protect X-chromosomal allelic balance and thereby constrain Sjogren's syndrome pathogenesis.