Knockout of miR-184 in zebrafish leads to ocular abnormalities by elevating p21 levels

miR-184 is one of the most abundant miRNAs expressed in the lens and corneal tissue. Mutations in the seed region of miR-184 are responsible for inherited anterior segment dysgenesis. Animal models recapitulating miR-184-related anterior segment dysgenesis are still lacking, and the molecular basis of ocular abnormalities caused by miR-184 dysfunction has not been well elucidated in vivo. In the present study, we constructed a miR-184^-/- zebrafish line by destroying both two dre-mir-184 paralogs with CRISPR-Cas9 technology. Although there were no gross developmental defects, the miR-184^-/- zebrafish displayed microphthalmia and cataract phenotypes. Cytoskeletal abnormalities, aggregation of γ-crystallin, and lens fibrosis were induced in miR-184^-/- lenses. However, no obvious corneal abnormalities were observed in miR-184^-/- zebrafish. Instead of apoptosis, deficiency of miR-184 led to aberrant cell proliferation and a robust increase in p21 levels in zebrafish eyes. Inhibition of p21 by UC2288 compromised the elevation of lens fibrosis markers in miR-184^-/- lenses. RNA-seq demonstrated that levels of four transcriptional factors HSF4, Sox9a, CTCF, and Smad6a, all of which could suppress p21 expression, were reduced in miR-184^-/- eyes. The predicted zebrafish miR-184 direct target genes (e.g., atp1a3a and nck2a) were identified and verified in miR-184^-/- eye tissues. The miR-184^-/- zebrafish is the first animal model mimicking miR-184-related anterior segment dysgenesis and could broaden our understanding of the roles of miR-184 in eye development.