Research from JAMA Ophthalmology — Intravitreal Triamcinolone for the Treatment of Macular Edema AssociatedWith Central Retinal Vein Occlusion

Purpose.: The homeobox transcription factor PITX2 is a known regulator of mammalian ocular development, and human PITX2 mutations are associated with Axenfeld-Rieger syndrome (ARS). However, the treatment of patients with ARS remains mostly supportive and palliative. Methods.: The authors used molecular genetic, pharmacologic, and embryologic techniques to study the biology of ARS in a zebrafish model that uses transgenes to mark neural crest and muscle cells in the head. Results.: The authors demonstrated in vivo that pitx2 is a key downstream target of retinoic acid (RA) in craniofacial development, and this pathway is required for coordinating neural crest, mesoderm, and ocular development. pitx2a knockdown using morpholino oligonucleotides disrupts jaw and pharyngeal arch formation and recapitulates ocular characteristics of ARS, including corneal and iris stroma maldevelopment. These phenotypes could be rescued with human PITX2A mRNA, demonstrating the specificity of the knockdown and evolutionary conservation of pitx2a function. Expression of the ARS dominant negative human PITX2A K50E allele also caused ARS-like phenotypes. Similarly, inhibition of RA synthesis in the developing eye (genetic or pharmacologic) disrupted craniofacial and ocular development, and human PITX2A mRNA partially rescued these defects. Conclusions.: RA regulation of pitx2 is essential for coordinating interactions among neural crest, mesoderm, and developing eye. The marked evolutionary conservation of Pitx2 function in eye and craniofacial development makes zebrafish a potentially powerful model of ARS, amenable to in vivo experimentation and development of potential therapies.

It has been previously shown that genes transcribed by RNA polymerase II (RNAP II) are subject to position effect variegation when located near yeast telomeres. This telomere position effect requires a number of gene products that are also required for silencing at the HML and HMR loci. Here, we show that a null mutation of the DNA repair gene RAD6 reduces silencing of the HM loci and lowers the mating efficiency of MATa strains. Likewise, rad6-delta reduces silencing of the telomere-located RNAP II-transcribed genes URA3 and ADE2. We also show that the RNAP III-transcribed tyrosyl tRNA gene, SUP4-o, is subject to position effect variegation when located near a telomere and that this silencing requires the RAD6 and SIR genes. Neither of the two known Rad6 binding factors, Rad18 and Ubr1, is required for telomeric silencing. Since Ubrl is the recognition component of the N-end rule-dependent protein degradation pathway, this suggests that N-end rule-dependent protein degradation is not involved in telomeric silencing. Telomeric silencing requires the amino terminus of Rad6. Two rad6 point mutations, rad6(C88A) and rad6(C88S), which are defective in ubiquitin-conjugating activity fail to complement the silencing defect, indicating that the ubiquitin-conjugating activity of RAD6 is essential for full telomeric silencing.

In the anterior vertebrate head, a population of neural crest cells (NCCs) migrates to the periocular mesenchyme and makes critical contributions to the developing eye and orbit. Improper migration and differentiation of these NCCs have been implicated in human diseases such as congenital glaucoma and anterior segment dysgenesis syndromes. The mechanisms by which these cells migrate to their target tissues within and around the eye are not well understood. We present a fate map of zebrafish diencephalic and mesencephalic NCC contributions to the eye and orbit. The fate map closely resembles that in chick and mice, demonstrating evolutionary conservation. To gain insight into the mechanisms of anterior NCC guidance, we used the eyeless mutant chokh/rx3. We show that, in chokh mutants, dorsal anterior NCC migration is severely disorganized. Time‐lapse analysis shows that NCCs have significantly reduced migration rates and directionality in chokh mutants. Developmental Dynamics 237:1645–1652, 2008.

The volume of the adult human orbit (eye socket) is 30 mL and contains the ocular globe and a surrounding complex set of neuronal, vascular, muscular, and glandular structures. These include 6 extraocular muscles (EOMs) that differentiate contemporaneously with the eye vesicle, form precise anatomic attachments to sclera and bone, and are innervated by cranial nerves 3, 4, and 6. Vertebrate EOMs are required for the precise control of eye movements to achieve directionality of gaze. In binocular animals with overlapping visual fields (eg, primates, birds), eye movement control is required for stereopsis and fusion. Extraocular muscles are skeletal muscles with unique properties, including patterns of gene expression different from skeletal muscle and altered susceptibility to systemic myopathies.^1-5