X Chromosome Dosage Tunes Interferon Sensitivity via Epigenetic Feedback Buffering

Hypothesis

Progressive loosening of the inactive X chromosome (Xi) in aging females increases the dosage of X-linked immune genes (e.g., TLR7, TLR8, IRAK1), driving chronic JAK-STAT activation. Rather than simply exhausting the pathway, this sustained stimulus triggers a compensatory epigenetic program that reinforces negative feedback regulators (SOCS1/3, PIAS1, PTPN2) at the same X-linked loci, creating a bistable switch: low interferon sensitivity in youth, high sensitivity during mid‑life Xi escape, and refractory insensitivity in late age. The switch is mediated by XIST‑dependent recruitment of HDAC3 and DNMT3A to promoter regions of feedback genes, which becomes destabilized as Xi escape spreads.

Mechanistic Reasoning

1. Dosage‑Dependent Feedback Induction – X‑linked immune genes and their corresponding feedback inhibitors are co‑located in topologically associating domains (TADs) that retain Xi‑specific chromatin contacts. When immune gene transcription rises due to Xi escape, enhancer‑promoter looping also brings feedback gene promoters into proximity with active RNA polymerase II, facilitating their transcription.
2. Epigenetic Buffering – In young females, XIST RNA coats Xi and recruits HDAC3/DNMT3A, keeping feedback genes poised but silent. Chronic IFN signaling activates STAT1, which phosphorylates HDAC3, reducing its affinity for chromatin and allowing transient feedback expression. With age, Xi erosion diminishes XIST spread, weakening HDAC3/DNMT3A recruitment and causing feedback genes to become constitutively expressed, thereby dampening JAK‑STAT signaling despite high ligand levels.
3. Bistable Outcome – The system exhibits hysteresis: early‑life IFN pulses produce transient STAT1 activation (protective); mid‑life Xi escape yields sustained STAT1 driving inflammation and longevity benefits; late‑age loss of buffering locks the pathway in a refractory state, contributing to immunosenescence and increased infection susceptibility.

Testable Predictions

• Prediction 1: In aged female mice, immune gene loci on Xi will show increased H3K27ac and decreased HDAC3 occupancy, whereas feedback gene loci (Socs1, Socs3, Pias1, Ptpn2) will display the opposite trend.
• Prediction 2: Restoring XIST coating specifically at immune gene TADs using dCas9‑XIST fusions will reduce feedback gene expression, increase IFN‑stimulated STAT1 phosphorylation after ligand challenge, and improve viral clearance without exacerbating autoimmunity.
• Prediction 3: Pharmacologic inhibition of HDAC3 in young females will mimic the aged Xi escape phenotype, causing premature feedback gene upregulation and reduced JAK‑STAT responsiveness.

Experimental Approach

1. Chromatin Profiling – Perform CUT&RUN for H3K27ac, HDAC3, and RNA Pol II on sorted splenic B cells from young (3 mo), mid‑aged (12 mo), and old (24 mo) XX and XY mice; focus on TADs containing TLR7/TLR8/IRAK1 and nearby feedback genes.
2. CRISPR‑Epigenetic Editing – Deploy dCas9‑XIST or dCas9‑HDAC3 to immune gene TADs in aged XX mice; validate Xi spread by RNA‑FISH for Xist and measure feedback gene mRNA by qRT‑PCR.
3. Functional Readouts – Stimulate ex vivo splenocytes with IFN‑α; assess phospho‑STAT1 (flow cytometry), ISG expression (Nanostring), and survival after sub‑lethal vesicular stomatitis virus challenge.
4. Longevity & Autoimmunity Cohort – Track edited mice for lifespan, autoantibody titers (ANA), and kidney pathology over 18 months.

Falsification

If maintaining Xi silencing at immune loci does not alter feedback gene expression, JAK‑STAT sensitivity, or healthspan outcomes, the hypothesis that Xi escape drives a compensatory epigenetic buffering mechanism would be refuted, suggesting that observed longevity differences arise from alternative X‑linked processes.