Achondroplasia is the most common form of dwarfism. It results from an activating mutation of the gene encoding fibroblast growth factor receptor 3 (FGFR3). This constitutive activation of the receptor disrupts bone formation and elongation by dysregulating numerous intracellular signaling pathways, including the ERK1/2 and p38 kinases, the PI3K/AKT pathway, and the STATs, which govern chondrocyte proliferation and differentiation. FGFR3 also interacts with the Indian hedgehog (Ihh) signaling pathway, which is essential for chondrocyte differentiation and entirely dependent on the primary cilium. The team had previously demonstrated a defect in the elongation of this primary cilium in murine and human chondrocytes carrying the mutation: the regular alignment of primary cilia is responsible for the columnar stacking of chondrocytes within the growth plate, an organization indispensable to bone elongation. Identifying new molecules targeting FGFR3 signaling therefore represents a major therapeutic challenge, and natural plant-derived compounds are prime candidates.
The researchers focused on a phenolic compound, (-)-epicatechin, isolated from Theobroma cacao, the cocoa tree. They first showed in vitro, in human and murine chondrocytes as well as in HEK293 cells transfected with various mutated FGFR3 constructs, that this molecule efficiently inhibits the signaling pathways downstream of the receptor. Complementary molecular docking and molecular dynamics approaches were used to explore the interaction of (-)-epicatechin with the ERK1, ERK2, and p38 kinases. A transcriptomic analysis conducted in a murine Fgfr3 model revealed that the expression of ciliary messenger RNAs was altered and significantly influenced by the Indian hedgehog and PKA pathways. (-)-Epicatechin proved able to restore the altered expression of genes controlling both the structural organization of the primary cilium and ciliogenesis.
The effects on bone growth were then assessed in the Fgfr3^Y367C/+ murine model of achondroplasia. In ex vivo culture of femurs over six days, (-)-epicatechin abolished the bone growth defect. In vivo, daily subcutaneous injections administered to young mice from the first day of life, over two weeks, increased bone elongation and corrected the primary cilium abnormalities observed in chondrocytes. This correction promoted the columnar organization characteristic of flattened proliferative chondrocytes, thereby improving bone elongation.
This proof-of-concept study supports the potential of (-)-epicatechin as a candidate molecule for the treatment of achondroplasia.