SUN-725 Sex-Specific Effects of the Gut Microbiome onHeterotopic Ossification inFibrodysplasia Ossificans Progressiva

Abstract Disclosure: H. Herzog: None. C. Fang: None. L. Lam: None. A. Zamarioli: None. A. Sharma: None. T. Moody: None. C. Lal Gupta: None. D. Dumlao: None. K.L. Wentworth: None. K. Yu: None. V. Lu: None. K. Pollard: None. C. Hernandez: None. D. Perrien: None. E.C. Hsiao: None. Heterotopic ossification (HO) is the formation of bone in soft tissues, caused by trauma, surgeries, or genetic mutations. HO leads to the loss of mobility due to joint fusion, resulting in pain, physical disabilities, and deformities. Treatments for HO are limited and focus on symptom and pain management; current available therapies are moderately effective at reducing new HO, no therapies exist to reverse HO formation. HO is difficult to study due to varied triggers and the poor ability to predict timing and location of lesions. Genetic forms of HO provide a consistent driver of the condition, creating an opportunity to study factors that influence lesion formation. Fibrodysplasia Ossificans Progressiva (FOP) is a severe genetic form of HO caused by the ACVR1R206H mutation, which enhances BMP signaling and creates an inappropriate signaling response to Activin A. FOP pathology manifests as flare-ups of ectopic bone formation driven by local and systemic inflammatory responses including elevated inflammatory cellular activity and cytokines that precede HO. Despite the highly conserved ACVR1R206H mutation, FOP patients exhibit disease variability not explained by genotype, suggesting a contribution of environmental factors to disease progression. The gut microbiome influences systemic inflammation through the release of microbial metabolites into the blood and modulation of the gut microbiome has shown promise for treatment of RA. Thus, we examined if the gut microbiome may be a regulator of HO formation using a murine model of FOP (PDGFRα-cre/ACVR1R206H).PDGFRα-Cre mice were crossed with ACVR1flR206H mice to induce the ACVR1R206H mutation in PDGFRα cells, generating a murine FOP model. FOP mice exhibit a significantly shortened lifespan, averaging ∼40 days, primarily due to HO formation. To test the role of the gut microbiome in HO formation, we ablated the gut microbiome (GMA) in FOP mice starting at weaning using a broad-spectrum antibiotic cocktail (1 g/L ampicillin, 0.5 g/L metronidazole, 0.5 g/L neomycin, and 0.5 g/L vancomycin). GMA significantly increased early survival in female FOP mice to an average of 80 days (p=0.001) but had little effect in males, which remained at 40 days (p=0.6). MicroCT scans after 4 weeks of GMA showed a 37.8% reduction in whole-body HO volume compared to Non-GMA mice (p = 0.20). Metagenomic and metabolomic analyses indicated gender-specific inflammatory responses and bile acid metabolism changes in GMA mice. Five candidate anti-inflammatory metabolites were differentially abundant in GMA vs. non-GMA mice, with higher levels in females and lower levels in males. These results suggest that microbially-derived metabolites may drive HO severity in FOP mice by a sex-specific mechanism, highlighting the potential role the gut microbiome or specific microbially derived products as therapeutic targets for FOP and potentially other HO conditions. Presentation: Sunday, July 13, 2025