Hmx homeobox gene function in inner ear and nervous system cell-type specification and development.

The Hmx homeobox gene family is comprised of three members in mammals, Hmx1, Hmx2, and Hmx3, which are conserved across the animal kingdom and are part of the larger NKL clustered family of homeobox genes. Expression domains of Hmx genes in distantly related species such as Drosophila and mouse suggest an ancestral function in rostral central nervous system development. During vertebrate evolution, the Hmx genes appear to have been recruited into additional roles in inner ear morphogenesis and specification of vestibular inner ear sensory and supporting cell types. Being derived from a common ancestor, the vertebrate Hmx gene family is thus a strong candidate to investigate functional overlap versus the unique roles played by multiple genes belonging to the same family. The functions of Hmx2 and Hmx3 were investigated via directed gene mutagenesis and the primary regions where Hmx2 and Hmx3 exert their individual functions are consistent with their expression domains, such as the vestibule and uterus. Meanwhile, it is notable that some tissues where both Hmx2 and Hmx3 are extensively expressed were not severely affected in either of the Hmx2 or Hmx3 single mutant mice, suggesting a possible functional overlap existing between these two genes. Compound Hmx2 and Hmx3 double mutant mice showed more severe defects in the inner ear than those displayed by either single knockout. Furthermore, novel abnormalities in the hypothalamic-neuroendocrine system, which were never observed in either of the single mutant mice, confirmed a hypothesis that Hmx2 and Hmx3 also function redundantly to control embryonic development of the central nervous system.