Premature Vertebral Mineralization in hmx1-Mutant Zebrafish

H6 family homeobox 1 (HMX1) regulates multiple aspects of craniofacial development, and mutations in HMX1 are linked to an ocular defect termed oculoauricular syndrome of Schorderet-Munier-Franceschetti (OAS) (MIM #612109). Recently, additional altered orofacial features have been reported, including short mandibular rami, asymmetry of the jaws, and altered premaxilla. We found that in two mutant zebrafish lines termed hmx1 and hmx1, precocious mineralization of the proximal vertebrae occurred. Zebrafish hmx1 and hmx1 report mutations in the SD1 and HD domains, which are essential for dimerization and activity of hmx1. In hmx1, the bone morphogenetic protein (BMP) antagonists chordin and noggin1 were downregulated, while bmp2b and bmp4 were highly expressed and specifically localized to the dorsal region prior to the initiation of the osteogenic process. The osteogenic promoters runx2b and spp1 were also upregulated. Supplementation with DMH1-an inhibitor of the BMP signaling pathway-at the specific stage in which bmp2b and bmp4 are highly expressed resulted in reduced vertebral mineralization, resembling the wildtype mineralization progress of the axial skeleton. These results point to a possible role of hmx1 as part of a complex gene network that inhibits bmp2b and bmp4 in the dorsal region, thus regulating early axial skeleton development.