Abstract
Purpose :
The oculoauricular syndrome (OCACS) is an autosomal recessive disease manifested with symptoms including microphthalmia, cataract, and anomalies of the external ear. OCACS is caused by mutations in the HMX1 gene. Mutations of Hmx1 in other animals (cattle, rat, and mouse) cause similar defects. Our purpose is to understand the functions of Hmx1 in the ocular system by studying the defects of the Hmx1 knockout mouse.
Methods :
We first examined the expression of Hmx1 in the mouse eye at different developmental stages by RNAscope in situ hybridization. By gene targeting, we then created a mouse Hmx1 knockout (KO) allele. We then analyzed the eye defects of the homozygous Hmx1 KO mouse by histology and immunofluorescence staining. We also performed bulk RNA-seq to identify differentially expressed genes (DEGs) downstream of Hmx1.
Results :
RNAscope in situ hybridization revealed that Hmx1 is expressed in retinal progenitor cells and lens epithelial cells. The Hmx1 KO mice had microphthalmia and cataracts. Examination of the retina by histology and marker analysis did not find overt defects in the Hmx1 KO mice. However, these mice developed cataracts and their lenses are significantly smaller, as compared to wild-type controls. A significant reduction of proliferation as assayed by EdU labeling and proliferation marker analysis was observed in the lens epithelial cells, but not in retinal progenitor cells. Immunofluorescence staining also revealed aberrant expression of some lens epithelial cell markers such as E-cadherin and Cyclin D2. Consistent with the phenotypic defects, many genes involved in lens development were identified to be differentially expressed in the Hmx1 KO lens at different developmental stages, but very few DEGs were identified in the mutant retina.
Conclusions :
Hmx1 plays a significant role in lens development by regulating lens epithelial proliferation and differentiation. The role of Hmx1 in the retina remains to be further elucidated, as it has been reported to regulate retinal development in zebrafish. Our findings provide a molecular interpretation for the underlying cause of the ocular defects in OCACS.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.