2026-07-03 | Single-Cell dissection of fibrodysplasia ossificans progressiva identifies SPP1 as a mediator of macrophage-fibroadipogenic progenitors crosstalk.

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder caused by gain-of-function mutations in ACVR1/ALK2, leading to progressive heterotopic ossification (HO) through endochondral bone formation. Inflammatory flare-ups often precede new ossification events, but the cellular and molecular mechanisms linking immune responses to progenitor cell fate remain incompletely understood. Here, using a tamoxifen-inducible Acvr1R206H mouse model of FOP and a reproducible muscle injury protocol, we combined single-cell RNA sequencing with in vitro assays to dissect early events during lesion formation. We identified an expansion of macrophages (MPs) and fibro-adipogenic progenitors (FAPs) in FOP mice, with both populations exhibiting inflammatory and osteochondrogenic transcriptional signatures. Cell-cell interaction analysis revealed a self-reinforcing network of cytokine signaling among MPs and a prominent MP-FAP communication axis centred on SPP1. Functional studies confirmed that SPP1 enhanced FAP osteogenic differentiation and that its inhibition partially reversed this phenotype in vitro and attenuated HO in vivo. Our findings highlight the critical role of inflammatory MPs in shaping the fate of resident stromal mesenchymal progenitors (e.g. FAP) and suggest that early immune-stromal interactions set the stage for HO. Targeting this immune-mesenchymal crosstalk may represent a potential complementary strategy for preventing or mitigating disease progression in FOP.

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2026-06-29 | Precision RNAi for Fibrodysplasia Ossificans Progressiva: a combinatorial, unimolecular, allele selective approach.

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder caused by a dominant mutation in the ACVR1 gene (R206H, 97% of cases), leading to debilitating heterotopic ossification (HO) characterized by abnormal bone growth triggered by inflammatory flare-ups. Here, we report the development of disease-modifying, allele-selective small interfering RNA (siRNA) targeting ACVR1 R206H . Allele selectivity is essential as wildtype ACVR1 is crucial for many functions including skeletal homeostasis and development. When conjugated to docosanoic acid (DCA), administration of the fully modified ACVR1 siRNA, either alone or in combination with an siRNA targeting IL1B (a key regulator of inflammation), results in profound reduction of HO using both responsive (post-trauma) and preventative (pre-trauma) intervention strategies in a murine FOP model. Notably, the combination therapy outperforms modulation of either target alone. We also describe the chemical engineering of a new class of lipophilic divalent siRNAs that target both pathways with a single compound, demonstrating superior muscle accumulation and therapeutic efficacy. siRNA treatment inhibits key signaling pathways (e.g. inflammatory, WNT, Notch, Hedgehog, and TGF-β), within muscle-resident fibroadipogenic progenitors (FAPs), leading to a significant reduction in cartilage, bone, and connective tissue formation. This work establishes a foundation for the development of disease-modifying treatments for FOP and offers a platform for targeting other musculoskeletal disorders involving multi-pathway dysregulation.

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2026-06-22 | Single-base 2'OMe-modified LNA and MOE gapmers selectively silence ACVR1 R206H in fibrodysplasia ossificans progressiva.

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder caused by gain-of-function mutations in ACVR1, most commonly c.617G>A (R206H), leading to progressive heterotopic ossification. In this study, we developed novel antisense gapmers selectively targeting the mutant ACVR1 R206H transcript while sparing the wild-type allele. We engineered locked nucleic acid (LNA) and 2'-O-methoxyethyl (MOE) gapmers incorporating a single 2'-O-methyl (2'OMe) modification at gap position 2. This is hypothesized to synergize with the wild-type sequence mismatch to restrict RNase H1 cleavage, limiting wild-type degradation while preserving mutant target engagement. In FOP patient-derived fibroblasts carrying the endogenous ACVR1 R206H mutation and in murine-derived C2C12 cells ectopically expressing ACVR1 R206H constructs, 2'OMe-modified gapmers demonstrated robust and preferential suppression of ACVR1 R206H at both RNA and protein levels. Gapmer treatment also reduced osteogenic differentiation, as shown by decreased alkaline phosphatase and Alizarin Red S staining, and lower expression of osteogenic markers. In wild-type mice, 2'OMe modification was associated with higher apparent gapmer levels in skeletal muscle and tendon and lower hepatic and renal stress marker readouts. These findings provide preliminary proof-of-concept that a single-base chemical modification can modulate allele selectivity and biodistribution of gapmers targeting ACVR1 R206H . Further studies in disease-relevant FOP models will be needed to establish therapeutic efficacy and long-term safety.

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