RUNX2-modifying enzymes: therapeutic targets for bone diseases

Abstract

RUNX2 is a master transcription factor of osteoblast differentiation. RUNX2 expression in the bone and osteogenic front of a suture is crucial for cranial suture closure and membranous bone morphogenesis. In this manner, the regulation of RUNX2 is precisely controlled by multiple posttranslational modifications (PTMs) mediated by the stepwise recruitment of multiple enzymes. Genetic defects in RUNX2 itself or in its PTM regulatory pathways result in craniofacial malformations. Haploinsufficiency in RUNX2 causes cleidocranial dysplasia (CCD), which is characterized by open fontanelle and hypoplastic clavicles. In contrast, gain-of-function mutations in FGFRs, which are known upstream stimulating signals of RUNX2 activity, cause craniosynostosis (CS) characterized by premature suture obliteration. The identification of these PTM cascades could suggest suitable drug targets for RUNX2 regulation. In this review, we will focus on the mechanism of RUNX2 regulation mediated by PTMs, such as phosphorylation, prolyl isomerization, acetylation, and ubiquitination, and we will summarize the therapeutics associated with each PTM enzyme for the treatment of congenital cranial suture anomalies.

Fig. 1. Hypothesis of the molecular mechanism involving RUNX2 in cranial suture anomalies.

RUNX2 transcriptional, translational and PTM level regulations are closely associated with cranial suture anomalies. Based on the amount of RUNX2 expressed in the normal skull, hypoostotic disease, typically CCD, results from a reduced RUNX2 level in the cranial sutures due to haplodeficiency. In contrast, osteopenic disease, typically CS, results from an increased RUNX2 level in the cranial sutures due to constitutive FGFR2 signaling. (asterisk means hyper-active mutation of gene; $ means hypo-active mutation of gene).

Fig. 2. Therapeutic intervention points targeting RUNX2 activity regulation for the treatment of craniosynostosis.

The soluble FGFR2 decoy receptor sFGFR2S252W may sequester and inhibit FGF ligands from binding to cell surface receptors. The receptor tyrosine kinase inhibitors PLX052, PD173074 and ARQ087 may block ligand binding-induced signaling propagation to cytosolic mediators. The MEK1/2 inhibitor U0126 or p38 inhibitor SB203580 blocks cytosolic mediator or nuclear transcription factor phosphorylation. The PIN1 inhibitors juglone and DTM block cis-trans structural modification of phosphorylated proteins. The HAT inhibitor anacardic acid blocks protein stabilization and activation by acetylation.