Hmx1- A Regulator Of Lateral Facial Development

Objectives: The oculoauricular syndrome (OAS) in humans, the “dumbo” and “misplaced ears” strains in mice, the ”dumbo” strain in rats, and the “crop ear” phenotype in cattle all converge on a single gene, Hmx1. It encodes a homeodomain transcription factor expressed in the developing eyes, peripheral nervous system, and posterior aspects of the first (BA1) and second (BA2) branchial arches. Posterior BA1 and BA2 give rise to lateral craniofacial structures including the external ear. The mouse and human phenotypes are due to coding region mutations, whereas we have shown that a 5.7kb deletion, ~80kb distal to the Hmx1 locus is responsible for the rat dumbo phenotype. This deleted non-coding sequence contains a ~600bp evolutionarily conserved region (ECR) that is predicted to serve as a regulatory element.
Methods: 3D morphological studies were performed on staged mouse embryos and also on postnatal day 28 (adult) mice to characterize in detail the phenotype of the mutants. An air-blowing test was used to explore craniofacial neuro-muscular function. A regulatory function for the ECR was investigated using reporter transgenesis.
Results: In addition to the rotated low-set position of the dumbo ear, an extra fold of the pinna was also noted that could be traced back to embryonic day (E)12. Various subtle bony defects and a likely neuromuscular defect were also uncovered in adult dumbo mice. Transient transgenesis revealed the ECR to be sufficient to direct Hmx1 expression within specific lateral and frontonasal mesenchyme.
Conclusions: We have identified previously unappreciated bony anomalies in dumbo mice that supports a role for Hmx1 in specification of lateral neuromuscular tissue and the ear. In addition, we show that the distal ECR functions as a lateral facial mesenchyme-specific enhancer of Hmx1 expression. Future fate-mapping studies will ultimately determine the specific craniofacial tissues derived from the embryonic Hmx1 expressing mesenchyme.