The expression of Pax6 variants is subject to posttranscriptional regulation in the developing mouse eyelid

Gene dosage manipulation alleviates manifestations of hereditary PAX6 haploinsufficiency in mice

In autosomal dominant conditions with haploinsufficiency, a single functional allele cannot maintain sufficient dosage for normal function. We hypothesized that pharmacologic induction of the wild-type allele could lead to gene dosage compensation and mitigation of the disease manifestations. The paired box 6 (PAX6) gene is crucial in tissue development and maintenance particularly in eye, brain, and pancreas. Aniridia is a panocular condition with impaired eye development and limited vision due to PAX6 haploinsufficiency. To test our hypothesis, we performed a chemical screen and found mitogen-activated protein kinase kinase (MEK) inhibitors to induce PAX6 expression in normal and mutant corneal cells. Treatment of newborn Pax6-deficient mice (Pax6Sey-Neu/+ ) with topical or systemic MEK inhibitor PD0325901 led to increased corneal PAX6 expression, improved corneal morphology, reduced corneal opacity, and enhanced ocular function. These results suggest that induction of the wild-type allele by drug repurposing is a potential therapeutic strategy for haploinsufficiencies, which is not limited to specific mutations.

Role of EGF receptor signaling on morphogenesis of eyelid and meibomian glands

The epidermal growth factor receptor (EGFR) signaling has a pivotal role in the regulation of morphogenesis during development and maintenance of homeostasis in adult eyelid and its adnexa. Studies have demonstrated that during eyelid morphogenesis the EGFR signaling pathway is responsible for keratinocyte and mesenchymal cell proliferation and migration at the eyelid tip. For meibomian gland morphogenesis, EGFR signaling activation stimulates meibomian gland epithelial cell proliferation. EGFR signaling pathway functions through multiple downstream signals such as ERK, Rho/ROCK and integrin and is regulated by a variety of upstream signals including Adam17, GPR48 and FGFR signaling. Herein we review the literature that describe the role of EGFR and its related signaling pathways in eyelid and meibomian gland morphogenesis.

Genetic background-dependent role of Egr1 for eyelid development

EGR1 is an early growth response zinc finger transcription factor with broad actions, including in differentiation, mitogenesis, tumor suppression, and neuronal plasticity. Here we demonstrate that Egr1^-/- mice on the C57BL/6 background have normal eyelid development, but back-crossing to BALB/c background for four or five generations resulted in defective eyelid development by day E15.5, at which time EGR1 was expressed in eyelids of WT mice. Defective eyelid formation correlated with profound ocular anomalies evident by postnatal days 1-4, including severe cryptophthalmos, microphthalmia or anophthalmia, retinal dysplasia, keratitis, corneal neovascularization, cataracts, and calcification. The BALB/c albino phenotype-associated Tyr^c tyrosinase mutation appeared to contribute to the phenotype, because crossing the independent Tyr^c-2J allele to Egr1^-/- C57BL/6 mice also produced ocular abnormalities, albeit less severe than those in Egr1^-/- BALB/c mice. Thus EGR1, in a genetic background-dependent manner, plays a critical role in mammalian eyelid development and closure, with subsequent impact on ocular integrity.