Hedgehog signalling linked to heterotopic ossification in POH

“We set out to ask how bone formation is spatially regulated by studying the molecular mechanisms underlying a genetic disease of ectopic bone formation, progressive osseous heteroplasia (POH)”, explains Yingzi Yang, from the National Human Genome Research Institute at the NIH. This research, now published in Nature Medicine, found that loss of GNAS expression in POH results in activation of Hedgehog signalling, which leads to the formation of extraskeletal bone (heterotopic ossification).

Frederick Kaplan and Eileen Shore (University of Pennsylvania), also coauthors on this paper, found that POH is caused by a defect in GNAS, the gene encoding Gα_s, which is a downstream mediator of signalling via G protein-coupled receptors; however, the molecular mechanisms responsible for the pathogenesis of this disease remain poorly understood. Here, the researchers generated mice that lacked expression of both alleles of Gnas in limb mesenchymal progenitor cells (Prrx1-cre;Gnas^fl/− and Prrx1-cre; Gnas^fl/fl mice). Homozygous loss of Gnas expression resulted in progressive heterotopic ossification and POH-like phenotypes in these mice. The authors established that this phenotype resulted from ectopic osteoblast differentiation resulting in progressive noncartilaginous intramembranous bone formation and that these mice, therefore, represent a model of POH.

Bone-forming cells (brown nuclear staining, arrows) in the ossicles (light blue) between two ossifying digits (arrowheads) in Prrx1-cre; Gnas^fl/fl mice. Image courtesy of Y. Yang.

Previous work by Yang and colleagues demonstrated a link between Gα_s and the Wnt-β-catenin pathway, which led them to investigate whether Gα_s regulates bone formation in POH by influencing the Wnt-related Hedgehog signalling pathways. First, they demonstrated that Hedgehog target genes were expressed at a higher level in Prrx1-cre;Gnas^fl/− limbs at embryonic day 14.5 in comparison with control embryos. Next they showed that Gα_s modulates the strength of Hedgehog signalling pathways by regulating the activation of the Gli family of zinc-finger transcription factors in embryonic limbs, through cAMP and PKA. Finally, the authors showed that activation of Hedgehog signalling is necessary and sufficient to result in heterotopic ossification.

“The significance of our work is that we have figured out a fundamental mechanism that restricts bone formation to its normal location, which has to be strictly controlled in skeletal development and regeneration”, says Yang. “We plan to test whether inhibition of Hedgehog signallingcould lead to suppression of heterotopic ossification caused by nongenetic causes, such as occurs after trauma or surgery”.