Bone Mineralization Regulation: Using Zebrafish as a Model to Study ANKH-associated Mineralization Disorders

Craniometaphyseal Dysplasia (CMD) is a rare skeletal disorder that can result from mutations in the ANKH gene. This gene encodes progressive ankylosis (ANK), which is responsible for transporting inorganic pyrophosphate (PPi) and ATP from the intracellular to the extracellular environment, where PPi inhibits bone mineralization. When ANK is dysfunctional, as in patients with CMD, the passage of PPi to the extracellular environment is reduced, leading to excess mineralization, particularly in bones of the skull. Zebrafish may serve as a promising model to study the mechanistic basis of CMD. Here we provide a detailed analysis of the zebrafish ankh paralogs, ankha and ankhb, in terms of their phylogenic relationship with ANKH in other vertebrates as well as their spatiotemporal expression patterns during zebrafish development. We found a closer evolutionary relationship exists between the zebrafish ankhb protein and its human and other "higher" vertebrate counterparts. Furthermore, we noted distinct temporal expression patterns with ankha more prominently expressed in early development stages, and ankhb expression at larval growth stages. Whole mount in situ hybridization was used to compare spatial expression patterns of each paralog during bone development. Both paralogs showed strong expression in the craniofacial region as well as the notochord and somites, with only subtle patterning differences. Given the substantial overlap in spatiotemporal expression of ankha and ankhb, the exact roles of these genes remain speculative. However, this study lays the groundwork for functional analyses of each ankh paralog and the potential of using zebrafish to find possible targeted therapies for CMD.