Objective: Cartilage-hair hypoplasia (CHH) is an autosomal recessive metaphyseal chondrodysplasia characterized by bone dysplasia and many other highly variable features. The responsible gene for CHH has been identified to be the RNA component of the mitochondrial RNA processing endoribonuclease (RMRP) gene. Currently, the pathogenesis of osteochondrodysplasia and extraskeletal manifestations in CHH patients remains incompletely understood, furthermore, there are no viable animal models for CHH.
Methods: We used CRISPR/Cas9 system to generate a zebrafish model of CHH by knocking-out the RMRP ortholog in zebrafish, rmrp. Different zebrafish transgenic lines and vital dye were used to analyze phenotypes in mutant. Zebrafish live imaging were captured using a 20X water immersion objective mounted on an LSM880NLO confocal microscope.
Results: We found that rmrp is required for the patterning and shaping of pharyngeal arches. Rmrp mutation inhibits intramembranous ossification of skull bones and promotes vertebrae ossification. The abnormalities of endochondral bone ossification are variable depending on the degree of dysregulated chondrogenesis. Moreover, rmrp mutation inhibits cell proliferation and promotes apoptosis through dysregulating the expressions of cell cycle and apoptosis related genes. Furthermore, we demonstrate that rmrp mutation up-regulates canonical Wnt/β-catenin signaling and pharmacological inhibition of Wnt/β-catenin could partially alleviate the chondrodysplasia and increased vertebrae mineralization in rmrp mutants.
Conclusions: We have generated a zebrafish model for CHH that recapitulates chondrodysplasia and abnormal bone ossification of CHH patients, and studied the potential cellular and molecular mechanism for the phenotypes resulting from rmrp mutation. We provide evidence that inhibition of Wnt/β-catenin signaling could partially alleviate the phenotypes in rmrp mutants, suggesting that modulation of Wnt/β-catenin activity could be a potential therapy of CHH.