Project Summary
Heterotopic ossification (HO), the formation of bone in skeletal muscle and associated soft tissues, can result
from traumatic injury or disease. The most extreme form of HO is manifested in the rare, autosomal-dominant
genetic disorder, fibrodysplasia ossificans progressiva (FOP), in which HO continues progressively throughout
life, resulting in devastating effects on health, life expectancy and quality of life. We developed a new genetic
mouse model of FOP based on conditional expression of the disease-causing BMP receptor, ACVR1(R206H).
Using this model, we identified fibro/adipogenic progenitors (FAPs), a multipotent mesenchymal progenitor of
muscle tissue, as the offending cell population that gives rise to the heterotopic skeleton. Our genetic studies
indicate that the wild type (WT) ACVR1 receptor functions as a direct or indirect competitive inhibitor of
ACVR1(R206H) in heterozygous cells. Based on these findings, the overarching hypothesis that provides the
conceptual framework and justification for this exploratory grant posits that WT and mutant ACVR1 compete for
limiting osteogenic signaling components and that disease severity is dictated by the stoichiometric balance of
these receptors. The primary experimental objective of the current proposal is to determine whether ACVR1
over-expression mitigates the deleterious effects of ACVR1 (R206H) in FOP mice, a result that would provide
proof-of-concept for a novel and non-obvious therapeutic approach for FOP. Using a newly developed mouse
knockin line that conditionally over-expresses WT human ACVR1 (R26ACVR1), Aim 1 proposes functional studies
that utilize quantitative µCT imaging and histological analyses to determine whether ACVR1 over-expression in
FAPs effectively inhibits HO. Cell transplantation studies will determine whether ACVR1 over-expression can
function cell-non-autonomously, perhaps by binding key osteogenic ligands that drive ACVR1(R206H) signaling.
As a sensitive test of the neutralizing effects of ACVR1 receptor over-expression on ACVR1(R206H) function,
Aim 1 will also determine whether global embryonic over-expression of ACVR1 can rescue the neonatal-lethal
phenotype of mice that broadly express Acvr1R206H. Since severe muscle loss can be a significant contributing
factor to patient morbidity, Aim 1 will determine whether ACVR1 over-expression restores muscle regenerative
capacity. Aim 2 will use RNA sequencing to define the FAP mRNA transcriptome at early times after injury to
identify new direct or indirect transcriptional targets that are associated with entry of multipotent FAPs into the
endochondral pathway, and to define the extent to which over-expression of ACVR1 “normalizes” the FAP
transcriptome. Finally, transcriptome analysis of both normal and mutant FAPs derived from pathogenic FOP
muscle will address the relative extent to which environmental and intrinsic genetic factors dictate FAP cell fates
and transcriptional outcomes. The proposed research will contribute significantly to an understanding of
molecular mechanisms of pathological FAP reprogramming and may lead to the development of novel
therapeutic strategies for FOP based on ACVR1 over-expression.