Variable phenotype related to a novel PAX 6 mutation (IVS4+5G>C) in a family presenting congenital nystagmus and foveal hypoplasia

Inherited Eye Diseases with Retinal Manifestations through the Eyes of Homeobox Genes

Retinal development is under the coordinated control of overlapping networks of signaling pathways and transcription factors. The paper was conceived as a review of the data and ideas that have been formed to date on homeobox genes mutations that lead to the disruption of eye organogenesis and result in inherited eye/retinal diseases. Many of these diseases are part of the same clinical spectrum and have high genetic heterogeneity with already identified associated genes. We summarize the known key regulators of eye development, with a focus on the homeobox genes associated with monogenic eye diseases showing retinal manifestations. Recent advances in the field of genetics and high-throughput next-generation sequencing technologies, including single-cell transcriptome analysis have allowed for deepening of knowledge of the genetic basis of inherited retinal diseases (IRDs), as well as improve their diagnostics. We highlight some promising avenues of research involving molecular-genetic and cell-technology approaches that can be effective for IRDs therapy. The most promising neuroprotective strategies are aimed at mobilizing the endogenous cellular reserve of the retina.

A family with a mild form of congenital nystagmus and optic disc coloboma caused by a novel PAX6 mutation

Congenital nystagmus (CN) is a heterogeneous disease that shows variable clinical features. There are a few mutations that are known to cause CN. Among them, a PAX6 mutation is known to cause CN with an extremely high frequency of aniridia. Here, we report on a family with an autosomal dominant PAX6 mutation, c.214G > A (p.Gly72Ser.), who presented with CN in the absence of aniridia. This study describes detailed clinical findings, including videonystagmography and fundus photography findings and emphasizes the importance of screening for the PAX6 gene in patients who present with CN in the absence of aniridia, as this will further elucidate the known phenotypes of PAX6-related diseases.

A novel PAX6 nonsense mutation identified in an Iranian family with various eye anomalies

Purpose

The aim of this study was to detect the genetic defects in a large pedigree of affected individuals with various phenotypes of ocular anomalies including partial aniridia, congenital cataract, and nystagmus.

Methods

The entire coding region of paired box gene 6 (PAX6) was amplified by polymerase chain reaction (PCR), sequenced, and compared with a GenBank database.

Results

A novel mutation (c.1170 C > T; p.Gln297X) was found in the proband and all affected members. This nonsense mutation leads to PAX6 protein truncation.

Conclusions

Our findings suggest that this novel mutation is most likely responsible for the pathogenesis of the congenital aniridia, cataract, and nystagmus in this pedigree. To the best of our knowledge, this is the first report of this mutation of PAX6 gene in a kindred pedigree with various ocular abnormalities.

Pax6 is essential for the generation of late-born retinal neurons and for inhibition of photoreceptor-fate during late stages of retinogenesis

In the developing retina, as in other regions of the CNS, neural progenitors give rise to individual cell types during discrete temporal windows. Pax6 is expressed in retinal progenitor cells (RPCs) throughout the course of retinogenesis, and has been shown to be required during early retinogenesis for generation of most early-born cell types. In this study, we examined the function of Pax6 in postnatal mouse retinal development. We found that Pax6 is essential for the generation of late-born interneurons, while inhibiting photoreceptor differentiation. Generation of bipolar interneurons requires Pax6 expression in RPCs, while Pax6 is required for the generation of glycinergic, but not for GABAergic or non-GABAergic-non-glycinergic (nGnG) amacrine cell subtypes. In contrast, overexpression of either full-length Pax6 or its 5a isoform in RPCs induces formation of cells with nGnG amacrine features, and suppresses generation of other inner retinal cell types. Moreover, overexpression of both Pax6 variants prevents photoreceptor differentiation, most likely by inhibiting Crx expression. Taken together, these data show that Pax6 acts in RPCs to control differentiation of multiple late-born neuronal cell types.

Panel-based whole exome sequencing identifies novel mutations in microphthalmia and anophthalmia patients showing complex Mendelian inheritance patterns

Background

Microphthalmia and anophthalmia (MA) are congenital eye abnormalities that show an extremely high clinical and genetic complexity. In this study, we evaluated the implementation of whole exome sequencing (WES) for the genetic analysis of MA patients. This approach was used to investigate three unrelated families in which previous single‐gene analyses failed to identify the molecular cause.

Methods

A total of 47 genes previously associated with nonsyndromic MA were included in our panel. WES was performed in one affected patient from each family using the AmpliSeq^TM Exome technology and the Ion Proton^TM platform.

Results

A novel heterozygous OTX2 missense mutation was identified in a patient showing bilateral anophthalmia who inherited the variant from a parent who was a carrier, but showed no sign of the condition. We also describe a new PAX6 missense variant in an autosomal‐dominant pedigree affected by mild bilateral microphthalmia showing high intrafamiliar variability, with germline mosaicism determined to be the most plausible molecular cause of the disease. Finally, a heterozygous missense mutation in RBP4 was found to be responsible in an isolated case of bilateral complex microphthalmia.

Conclusion

This study highlights that panel‐based WES is a reliable and effective strategy for the genetic diagnosis of MA. Furthermore, using this technique, the mutational spectrum of these diseases was broadened, with novel variants identified in each of the OTX2,PAX6, and RBP4 genes. Moreover, we report new cases of reduced penetrance, mosaicism, and variable phenotypic expressivity associated with MA, further demonstrating the heterogeneity of such disorders.

Molecular analysis of patients with aniridia in Russian Federation broadens the spectrum of PAX6 mutations

Congenital aniridia is a severe autosomal dominant congenital panocular disorder, mainly associated with pathogenic variants in the PAX6 gene. The objective of the study was to investigate the mutational and clinical spectra of congenital aniridia in a cohort of 117 patients from Russia. Each patient underwent detailed ophthalmological examination. From 91 unrelated families, 110 patients were diagnosed with congenital aniridia and 7 with WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, and mental Retardation syndrome). The clinical presentation in aniridia patients varied from the complete bilateral absence of the iris (75.5%) to partial aniridia or iris hypoplasia (24.5%). Additional ocular abnormalities were consistent with previous reports. In our cohort, we saw a previously not described high percentage of patients (45%) who showed non-ocular phenotypes. Prevalence of deletions coherent with WAGR syndrome appeared to be 19.4% out of sporadic patients. Among the other aniridia cases, PAX6 deletions were identified in 18 probands, and small intragenic changes were detected in 58 probands with 27 of these mutations being novel and 21 previously reported. In 3 families mosaic mutation was transmitted from a subtly affected parent. Therefore, PAX6 mutations explained 96.7% of aniridia phenotypes in this study with only 3 of 91 probands lacking pathogenic variants in the gene.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Alternative splicing in development and function of chordate endocrine systems: a focus on Pax genes

Genome sequencing has facilitated an understanding of gene networks but has also shown that they are only a small part of the answer to the question of how genes translate into a functional organism. Much of the answer lies in epigenetics-heritable traits not directly encoded by the genome. One such phenomenon is alternative splicing, which affects over 75% of protein coding genes and greatly amplifies the number of proteins. Although it was postulated that alternative splicing and gene duplication are inversely proportional and, therefore, have similar effects on the size of the proteome, for ancient duplications such as occurred in the Pax family of transcription factors, that is not necessarily so. The importance of alternative splicing in development and physiology is only just coming to light. However, several techniques for studying isoform functions both in vitro and in vivo have been recently developed. As examples of what is known and what is yet to be discovered, this review focuses on the evolution and roles of the Pax family of transcription factors in development and on alternative splicing of endocrine genes and the factors that regulate them.

Pituitary stalk duplication in association with moya moya disease and bilateral morning glory disc anomaly - broadening the clinical spectrum of midline defects

BACKGROUND

Duplication of the pituitary stalk, morning glory disc anomaly and moya moya are rare malformations. The combination of these findings may be syndromic and may have an underlying genetic etiology.

METHODS

Case report and review of the literature of neurological, ophthalmological, and neuroradiological findings including ophthalmic examination, MRI and MRA.

CASE REPORT

A 2 year-old girl presented with reduced visual acuity and roving eye movements since birth. Ophthalmological workup revealed bilateral morning glory disc anomaly. MRI showed duplication of the pituitary stalk and caudal displacement of the floor of the third ventricle. MRA showed narrowing of the supraclinoid internal carotid arteries with focal narrowing of the proximal middle cerebral arteries consistent with early moya moya disease.

CONCLUSIONS

Review of the literature of pituitary gland duplication and of the combination of morning glory disc anomaly and moya moya disease revealed only one previously reported case. However, the spectrum of this possibly syndromic presentation may be much broader and include various types of anterior midline defects and may have a common underlying genetic cause.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Ocular findings in Gillespie-like syndrome: association with a new PAX6 mutation

BACKGROUND

Gillespie syndrome is a rare variant form of aniridia, characterized by mental retardation, nonprogressive cerebellar ataxia, and iris hypoplasia. Unlike the more common dominant and sporadic forms of aniridia, there have been no associated PAX6 mutations or Wilms' tumor reported in Gillespie syndrome patients. Ocular findings in 21 cases published since Gillespie's initial description in 1965 include iris and foveal hypoplasia, nystagmus, and small optic discs with pigmentary retinopathy.

CASE REPORT

We herein report a case of atypical Gillespie syndrome associated with bilateral ptosis, exotropia, corectopia, iris hypoplasia, anterior capsular lens opacities, foveal hypoplasia, retinal vascular tortuosity, and retinal hypopigmentation. Neurologic evaluation revealed a mild hand tremor and learning disability, but no ataxia or cerebellar abnormalities on neuroimaging. Sequencing studies revealed a substitution in intron 2 of the PAX6 gene (IVS2 + 2T > A). To our knowledge, this is the first mutation of PAX6 gene reported in association with a Gillespie-like syndrome.

Developmental eye disorders

In developed countries, malformations of the eye are among the most common causes of serious visual impairment in newborns. The identification of pathogenic mutations in autosomal and X-linked transcription factors has advanced our understanding of the critical stages in human eye development and has begun to explain some unusual inheritance characteristics of these disorders. The functional characterisation of these genes in model organisms has prompted reinvestigation of affected individuals to identify previously unrecognized but consistent extra-ocular malformations. This dialogue between clinical genetics and basic developmental biology provides a paradigm to enhance our understanding of many critical developmental processes in human embryogenesis.

Continuous expression of the homeobox gene Pax6 in the ageing human retina

PURPOSE

In the past few years, the essential role of the homeobox gene Pax6 for eye development has been demonstrated unambiguously in a variety of species including humans. In humans, Pax6 mutations lead to a variety of ocular malformations of the anterior and posterior segment. However, little is known about PAX6 expression in the adult human retina. We have therefore investigated PAX6 levels and localization in the human retina at various ages.

METHODS

Adult human eyes of various ages (17-79 years) were obtained from the Zurich Eye Bank. PAX6 expression levels and patterns were analysed by Western blot analysis of total retinal protein and by immunohistochemistry on paraffin sections, respectively.

RESULTS

PAX6 expression in the retina was detected up to 79 years of donor age and was predominantly localized to the ganglion cell layer and the inner part of the inner nuclear layer.

CONCLUSIONS

PAX6 remains distinctly expressed throughout the lifespan of the human retina suggesting a role for PAX6 in the retina after completion of eye morphogenesis.