Achondroplasia is the most common form of dwarfism. It results from an activating mutation in the gene encoding fibroblast growth factor receptor 3 (FGFR3), which is expressed in growth-plate cartilage and bone. This receptor acts as a negative regulator of bone growth: its hyperactivation restrains chondrocyte proliferation and differentiation, disrupts endochondral ossification, and clinically manifests as short limbs, macrocephaly, midface hypoplasia, and foramen magnum stenosis. Longitudinal bone growth also depends on C-type natriuretic peptide and its receptor NPR2, a guanylate cyclase that produces cyclic GMP (cGMP). FGFR3 signaling, however, reduces NPR2 activity by dephosphorylating it. Achondroplasia and acromesomelic dysplasia, Maroteaux type—the latter linked to inactivating mutations in NPR2—thus converge on a common defect: decreased cGMP production in chondrocytes.
Building on the observation that the NPR2 agonist BMN-111 (vosoritide) stimulates bone growth in mice modeling achondroplasia (Fgfr3Y367C/+), the authors sought to potentiate this effect by blocking the FGFR3-induced dephosphorylation of NPR2. To this end, they combined BMN-111 with a phosphatase inhibitor, LB-100. cGMP production was measured directly in chondrocytes of live tibiae using a FRET sensor, and the phosphorylation state of NPR2 was assessed in primary rib chondrocytes by Phos-tag electrophoresis. The efficacy of the combination was then tested on femur and skull base explants from mutant embryos cultured ex vivo, with measurement of growth, histological analysis, and immunostaining for collagen X and the phosphorylated form of ERK1/2.
Experiments on tibiae and primary chondrocytes show that LB-100 counteracts the FGF-induced dephosphorylation and inactivation of NPR2. In Fgfr3Y367C/+ mouse explants, the combination of BMN-111 and LB-100 increases bone length and cartilage area, restores terminal chondrocyte differentiation, and expands the proliferative zone of the growth plate to a greater extent than BMN-111 alone. The combined treatment also reduces the abnormal elevation of MAP kinase activity in the growth plate and ameliorates skull base abnormalities.
This work provides proof of concept that a phosphatase inhibitor could be used in combination with an NPR2 agonist to increase cGMP production, with a view to therapeutic applications in achondroplasia.