Editorial to the special issue on “Molecular Genetics of Developmental Eye Disorders”

Phenotype–genotype correlations and emerging pathways in ocular anterior segment dysgenesis Abstract Disorders of the anterior segment of the eye encompass a variety of clinical presentations including aniridia, Axenfeld and Rieger anomalies, primary congenital glaucoma, Peters anomaly, as well as syndromal associations. These conditions have a significant impact on vision due to disruption of the visual axis, and also secondary glaucoma which occurs in over 50% of patients. Ocular anterior segment disorders occur due to a complex interplay of developmental, embryological and genetic factors, and often have phenotypic overlaps and genetic heterogeneity. Here we present a review of the clinical features and genes associated with aniridia, Axenfeld and Rieger anomalies, primary congenital glaucoma, Peters anomaly, and syndromic forms of these conditions. We also highlight phenotype–genotype correlations, recent discoveries with next-generation sequencing which broaden known phenotypes, and new anterior segment genes and pathways. We provide a guide towards genetic diagnosis for clinicians investigating patients with anterior segment dysgenesis.

Through the looking glass: eye anomalies in the age of molecular science

Prospects and modalities for the treatment of genetic ocular anomalies Abstract Over the last three decades, genetic studies have made great strides toward the identification of genes and genetic mechanisms underlying congenital disorders of the eye. However, despite the vast knowledge available this has not translated into treatments to prevent or repair the damage in the clinical setting. Recently, new research in technologies, such as tissue regeneration, next generation designer drugs, and genome editing, have become available for some genetic disorders that might be applicable to congenital ocular diseases in the near future. Here, we provide an overview of the emerging therapeutic modalities and the future prospects they hold for debilitating ocular defects.

An update on the genetics of ocular coloboma Abstract Ocular coloboma is an uncommon, but often severe, sight-threatening condition that can be identified from birth. This congenital anomaly is thought to be caused by maldevelopment of optic fissure closure during early eye morphogenesis. It has been causally linked to both inherited (genetic) and environmental influences. In particular, as a consequence of work to identify genetic causes of coloboma, new molecular pathways that control optic fissure closure have now been identified. Many more regulatory mechanisms still await better understanding to inform on the development of potential therapies for patients with this malformation. This review provides an update of known coloboma genes, the pathways they influence and how best to manage the condition. In the age of precision medicine, determining the underlying genetic cause in any given patient is of high importance.

Genetic landscape of isolated pediatric cataracts: extreme heterogeneity and variable inheritance patterns within genes Abstract Pediatric cataract represents an important cause of pediatric visual impairment. While both genetic and environmental causes for pediatric cataract are known, a large proportion remains idiopathic. The purpose of this review is to discuss genes involved in isolated pediatric cataract, with a focus on variable inheritance patterns within genes. Mutations in over 52 genes are known to cause isolated pediatric cataract, with a major contribution from genes encoding for crystallins, transcription factors, membrane proteins, and cytoskeletal proteins. Interestingly, both dominant and recessive inheritance patterns have been reported for mutations in 13 different cataract genes. For some genes, dominant and recessive alleles represent distinct types of mutations, but for many, especially missense variants, there are no clear patterns to distinguish between dominant and recessive alleles. Further research into the functional effects of these mutations, as well as additional data on the frequency of the identified variants, is needed to clarify variant pathogenicity. Exome sequencing continues to be successful in identifying novel genes associated with congenital cataract but is hindered by the extreme genetic heterogeneity of this condition. The large number of idiopathic cases suggests that more genes and potentially novel mechanisms of gene disruption remain to be identified.

Human eye conditions: insights from the fly eye Abstract The fruit fly Drosophila melanogaster has served as an excellent model to study and understand the genetics of many human diseases from cancer to neurodegeneration. Studying the regulation of growth, determination and differentiation of the compound eyes of this fly, in particular, have provided key insights into a wide range of diseases. Here we review the regulation of the development of fly eyes in light of shared aspects with human eye development. We also show how understanding conserved regulatory pathways in eye development together with the application of tools for genetic screening and functional analyses makes Drosophila a powerful model to diagnose and characterize the genetics underlying many human eye conditions, such as aniridia and retinitis pigmentosa. This further emphasizes the importance and vast potential of basic research to underpin applied research including identifying and treating the genetic basis of human diseases.

Genetic architecture of retinoic-acid signaling-associated ocular developmental defects Abstract Ocular developmental anomalies are among the most common causes of severe visual impairment in newborns (combined incidence 1–2:10,000). They comprise a wide range of inborn errors of eye development with a spectrum of overlapping phenotypes and they are frequently associated with extraocular malformations, neuropsychomotor developmental delay and/or intellectual disabilities. Many studies from model organisms have demonstrated the role of retinoic acid (RA) during organogenesis, including eye development, and have revealed the wide spectrum of malformations that can arise from defective RA signaling. However, genes coding for homeobox proteins and morphogenetic factors were implicated in anomalies of ocular development long before genes coding for RA-signaling proteins. The purpose of this review is to discuss current knowledge about the highly complex genetic architecture of RA-signaling-associated ocular developmental anomalies in humans. Despite less than a dozen genes identified thus far, all steps of RA-signaling, from vitamin A transport to target cells to transcriptional activation of RA targets, have been implicated. Furthermore, the majority of these genetic disorders are associated with both dominant and recessive inheritance patterns and a wide spectrum of ocular malformations, which can dominate the phenotype or represent one of many features. Although some genotype–phenotype correlations are described, in many cases, the variability of clinical expression cannot be accounted for by the genotype alone. This observation and the large number of unsolved cases suggest that the relationship between RA signaling and eye development deserves further investigation.

Mouse models for microphthalmia, anophthalmia and cataracts Abstract Mouse mutants are a long-lasting, valuable tool to identify genes underlying eye diseases, because the absence of eyes, very small eyes and severely affected, cataractous eyes are easily to detect without major technical equipment. In mice, actually 145 genes or loci are known for anophthalmia, 269 for microphthalmia, and 180 for cataracts. Approximately, 25% of the loci are not yet characterized; however, some of the ancient lines are extinct and not available for future research. The phenotypes of the mutants represent a continuous spectrum either in anophthalmia and microphthalmia, or in microphthalmia and cataracts. On the other side, mouse models are still missing for some genes, which have been identified in human families to be causative for anophthalmia, microphthalmia, or cataracts. Finally, the mouse offers the possibility to genetically test the roles of modifiers and the role of SNPs; these aspects open new avenues for ophthalmogenetics in the mouse.

Expanding the phenotype of the X-linked BCOR microphthalmia syndromes Abstract Two distinct syndromes arise from pathogenic variants in the X-linked gene BCOR (BCL-6 corepressor): oculofaciocardiodental (OFCD) syndrome, which affects females, and a severe microphthalmia (‘Lenz’-type) syndrome affecting males. OFCD is an X-linked dominant syndrome caused by a variety of BCOR null mutations. As it manifests only in females, it is presumed to be lethal in males. The severe male X-linked recessive microphthalmia syndrome (‘Lenz’) usually includes developmental delay in addition to the eye findings and is caused by hypomorphic BCOR variants, mainly by a specific missense variant c.254C > T, p.(Pro85Leu). Here, we detail 16 new cases (11 females with 4 additional, genetically confirmed, affected female relatives; 5 male cases each with unaffected carrier mothers). We describe new variants and broaden the phenotypic description for OFCD to include neuropathy, muscle hypotonia, pituitary underdevelopment, brain atrophy, lipoma and the first description of childhood lymphoma in an OFCD case. Our male X-linked recessive cases show significant new phenotypes: developmental delay (without eye anomalies) in two affected half-brothers with a novel BCOR variant, and one male with high myopia, megalophthalmos, posterior embryotoxon, developmental delay, and heart and bony anomalies with a previously undescribed BCOR splice site variant. Our female OFCD cases and their affected female relatives showed variable features, but consistently had early onset cataracts. We show that a mosaic carrier mother manifested early cataract and dental anomalies. All female carriers of the male X-linked recessive cases for whom genetic confirmation was available showed skewed X-inactivation and were unaffected. In view of the extended phenotype, we suggest a new term of X-linked BCOR-related syndrome.