Confirmation of FZD5 implication in a cohort of 50 patients with ocular coloboma

Age and light damage influence Fzd5 regulation of ocular growth-related genes

Genetic and environmental factors can independently or coordinatively drive ocular axis growth. Mutations in FRIZZLED5 (FZD5) have been associated with microphthalmia, coloboma, and, more recently, high myopia. The molecular mechanism of how Fzd5 participates in ocular growth remains unknown. In this study, we compiled a list of human genes associated with ocular growth abnormalities based on public databases and a literature search. We identified a set of ocular growth-related genes from the list that was altered in the Fzd5 mutant mice by RNAseq analysis at different time points. The Fzd5 regulation of this set of genes appeared to be impacted by age and light damage. Further bioinformatical analysis indicated that these genes are extracellular matrix (ECM)-related; and meanwhile an altered Wnt signaling was detected. Altogether, the data suggest that Fzd5 may regulate ocular growth through regulating ECM remodeling, hinting at a genetic-environmental interaction in gene regulation of ocular axis control.

Clinical, genetic and biochemical signatures of RBP4-related ocular malformations

BACKGROUND

The retinoic acid (RA) pathway plays a crucial role in both eye morphogenesis and the visual cycle. Individuals with monoallelic and biallelic pathogenic variants in retinol-binding protein 4 (RBP4), encoding a serum retinol-specific transporter, display variable ocular phenotypes. Although few families have been reported worldwide, recessive inherited variants appear to be associated with retinal degeneration, while individuals with dominantly inherited variants manifest ocular development anomalies, mainly microphthalmia, anophthalmia and coloboma (MAC).

METHODS

We report here seven new families (13 patients) with isolated and syndromic MAC harbouring heterozygous RBP4 variants, of whom we performed biochemical analyses.

RESULTS

For the first time, malformations that overlap the clinical spectrum of vitamin A deficiency are reported, providing a link with other RA disorders. Our data support two distinct phenotypes, depending on the nature and mode of inheritance of the variants: dominantly inherited, almost exclusively missense, associated with ocular malformations, in contrast to recessive, mainly truncating, associated with retinal degeneration. Moreover, we also confirm the skewed inheritance and impact of maternal RBP4 genotypes on phenotypical expression in dominant forms, suggesting that maternal RBP4 genetic status and content of diet during pregnancy may modify MAC occurrence and severity. Furthermore, we demonstrate that retinol-binding protein blood dosage in patients could provide a biological signature crucial for classifying RBP4 variants. Finally, we propose a novel hypothesis to explain the mechanisms underlying the observed genotype-phenotype correlations in RBP4 mutational spectrum.

CONCLUSION

Dominant missense variants in RBP4 are associated with MAC of incomplete penetrance with maternal inheritance through a likely dominant-negative mechanism.

Clinical and functional heterogeneity associated with the disruption of retinoic acid receptor beta

PURPOSE

Dominant variants in the retinoic acid receptor beta (RARB) gene underlie a syndromic form of microphthalmia, known as MCOPS12, which is associated with other birth anomalies and global developmental delay with spasticity and/or dystonia. Here, we report 25 affected individuals with 17 novel pathogenic or likely pathogenic variants in RARB. This study aims to characterize the functional impact of these variants and describe the clinical spectrum of MCOPS12.

METHODS

We used in vitro transcriptional assays and in silico structural analysis to assess the functional relevance of RARB variants in affecting the normal response to retinoids.

RESULTS

We found that all RARB variants tested in our assays exhibited either a gain-of-function or a loss-of-function activity. Loss-of-function variants disrupted RARB function through a dominant-negative effect, possibly by disrupting ligand binding and/or coactivators' recruitment. By reviewing clinical data from 52 affected individuals, we found that disruption of RARB is associated with a more variable phenotype than initially suspected, with the absence in some individuals of cardinal features of MCOPS12, such as developmental eye anomaly or motor impairment.

CONCLUSION

Our study indicates that pathogenic variants in RARB are functionally heterogeneous and associated with extensive clinical heterogeneity.

Individuals with heterozygous variants in the Wnt-signalling pathway gene FZD5 delineate a phenotype characterized by isolated coloboma and variable expressivity

BACKGROUND

Anophthalmia, microphthalmia and coloboma are a genetically heterogenous spectrum of developmental eye disorders. Recently, variants in the Wnt-pathway gene Frizzled Class Receptor 5 (FZD5) have been identified in individuals with coloboma and rarely microphthalmia, sometimes with additional phenotypes and variable penetrance.

MATERIALS AND METHODS

We identified variants in FZD5 in individuals with developmental eye disorders from the UK (including the DDD Study [www.ddduk.org/access.html]), France and Spain using whole genome/exome sequencing or customized NGS panels of ocular development genes.

RESULTS

We report eight new families with FZD5 variants and ocular coloboma. Three individuals presented with additional syndromic features, two explicable by additional variants in other genes (SLC12A2 and DDX3X). In two families initially showing incomplete penetrance, re-examination of apparently unaffected carrier individuals revealed subtle ocular colobomatous phenotypes. Finally, we report two families with microphthalmia in addition to coloboma, representing the second and third reported cases of this phenotype in conjunction with FZD5 variants.

CONCLUSIONS

Our findings indicate FZD5 variants are typically associated with isolated ocular coloboma, occasionally microphthalmia, and that extraocular phenotypes are likely to be explained by other gene alterations.

First evidence of SOX2 mutations in Peters' anomaly: lessons from molecular screening of 95 patients

Peters' anomaly (PA) is a rare anterior segment dysgenesis characterized by central corneal opacity and irido-lenticulo-corneal adhesions. Several genes are involved in syndromic or isolated PA (B3GLCT, PAX6, PITX3, FOXE3, CYP1B1). Some Copy Number Variations (CNVs) have also been occasionally reported. Despite this genetic heterogeneity, most of patients remain without genetic diagnosis. We retrieved a cohort of 95 individuals with PA and performed genotyping using a combination of Comparative genomic hybridization, whole genome, exome and targeted sequencing of 119 genes associated with ocular development anomalies. Causative genetic defects involving 12 genes and CNVs were identified for 1/3 of patients. Unsurprisingly, B3GLCT and PAX6 were the most frequently implicated genes, respectively in syndromic and isolated PA. Unexpectedly, the third gene involved in our cohort was SOX2, the major gene of micro-anophthalmia. Four unrelated patients with PA (isolated or with microphthalmia) were carrying pathogenic variants in this gene that was never associated with PA before. Here we described the largest cohort of PA patients ever reported. The genetic bases of PA are still to be explored as genetic diagnosis was unavailable for 2/3 of patients. Nevertheless, we showed here for the first time the involvement of SOX2 in PA, offering new evidence for its role in corneal transparency and anterior segment development. This article is protected by copyright. All rights reserved.

Identification of missense MAB21L1 variants in microphthalmia and aniridia

Microphthalmia, coloboma, and aniridia are congenital ocular phenotypes with a strong genetic component but often unknown cause. We present a likely causative novel variant in MAB21L1, c.152G>T p.(Arg51Leu), in two family members with microphthalmia and aniridia, as well as novel or rare compound heterozygous variants of uncertain significance, c.184C>T p.(Arg62Cys)/c.-68T>C, and c.658G>C p.(Gly220Arg)/c.*529A>G, in two additional probands with microphthalmia, coloboma and/or cataracts. All variants were predicted as damaging by in silico programs. In vitro studies of coding variants revealed normal subcellular localization but variable stability for the corresponding mutant proteins. In vivo complementation assays using the zebrafish mab21l2 Q48Sfs*5 loss-of-function line demonstrated that though overexpression of wild-type MAB21L1 messenger RNA (mRNA) compensated for the loss of mab21l2, none of the coding variant mRNAs produced a statistically significant rescue, with p.(Arg51Leu) showing the highest degree of functional deficiency. Dominant variants in a close homolog of MAB21L1, MAB21L2, have been associated with microphthalmia and/or coloboma and repeatedly involved the same Arg51 residue, further supporting its pathogenicity. The possible role of p.(Arg62Cys) and p.(Gly220Arg) in microphthalmia is similarly supported by the observed functional defects, with or without an additional impact from noncoding MAB21L1 variants identified in each patient. This study suggests a broader spectrum of MAB21L1-associated disease.

Confirming and expanding the phenotypes of FZD5 variants: Coloboma, inferior chorioretinal hypoplasia, and high myopia

PURPOSE

Two frameshift and two indel variants in FZD5 have been reported to cause coloboma in two families with incomplete penetrance and in two isolated cases in previous studies, respectively. This study aims to confirm this association and expand related specific phenotypes based on the genotype-phenotype analysis of FZD5 variants.

METHODS

Variants in FZD5 were collected from our in-house exome sequencing data of 5,845 probands with different eye conditions. Multistep bioinformatics analysis was used to classify the variants. Potential pathogenic variants and phenotypic variations were further evaluated based on family segregation and genotype-phenotype analysis.

RESULTS

In total, 63 rare variants were detected in FZD5. Multistep bioinformatics and genotype-phenotype analyses suggested that eight rare heterozygous variants in nine families should be considered potential pathogenic variants: three novel frameshift variants (c.350_356delCGCCGCT/p.Ser117*, c.1403_1406dupACCT/p.Tyr470Profs*130, and c.1428delG/p.Ser477Alafs*130) and five novel missense variants (c.388C>A/p.Arg130Ser, c.794G>T/p.Arg265Leu, c.1162G>A/p.Gly388Ser, c.1232A>G/p.Tyr411Cys, and c.1510A>T/p.Met504Leu). Among the nine families, carriers of these variants showed overlapping phenotypes, including typical uveal coloboma (12 eyes of seven patients from four families), inferior chorioretinal hypoplasia (ICH) or optic disc hypoplasia (ODH; 12 eyes of eight patients from six families), and high myopia (10 eyes of five patients from five families) within individual families or among different families.

CONCLUSIONS

The data presented in this study confirmed that variants in FZD5, not only frameshift variants but also missense variants, are a common cause of uveal coloboma. In addition, ICH, ODH, and high myopia may be variant phenotypes that are frequently associated with FZD5 variants.

Animal and cellular models of microphthalmia

Microphthalmia is a rare developmental eye disorder affecting 1 in 7000 births. It is defined as a small (axial length ⩾2 standard deviations below the age-adjusted mean) underdeveloped eye, caused by disruption of ocular development through genetic or environmental factors in the first trimester of pregnancy. Clinical phenotypic heterogeneity exists amongst patients with varying levels of severity, and associated ocular and systemic features. Up to 11% of blind children are reported to have microphthalmia, yet currently no treatments are available. By identifying the aetiology of microphthalmia and understanding how the mechanisms of eye development are disrupted, we can gain a better understanding of the pathogenesis. Animal models, mainly mouse, zebrafish and Xenopus, have provided extensive information on the genetic regulation of oculogenesis, and how perturbation of these pathways leads to microphthalmia. However, differences exist between species, hence cellular models, such as patient-derived induced pluripotent stem cell (iPSC) optic vesicles, are now being used to provide greater insights into the human disease process. Progress in 3D cellular modelling techniques has enhanced the ability of researchers to study interactions of different cell types during eye development. Through improved molecular knowledge of microphthalmia, preventative or postnatal therapies may be developed, together with establishing genotype-phenotype correlations in order to provide patients with the appropriate prognosis, multidisciplinary care and informed genetic counselling. This review summarises some key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Plain language summary

Animal and Cellular Models of the Eye Disorder, Microphthalmia (Small Eye) Microphthalmia, meaning a small, underdeveloped eye, is a rare disorder that children are born with. Genetic changes or variations in the environment during the first 3 months of pregnancy can disrupt early development of the eye, resulting in microphthalmia. Up to 11% of blind children have microphthalmia, yet currently no treatments are available. By understanding the genes necessary for eye development, we can determine how disruption by genetic changes or environmental factors can cause this condition. This helps us understand why microphthalmia occurs, and ensure patients are provided with the appropriate clinical care and genetic counselling advice. Additionally, by understanding the causes of microphthalmia, researchers can develop treatments to prevent or reduce the severity of this condition. Animal models, particularly mice, zebrafish and frogs, which can also develop small eyes due to the same genetic/environmental changes, have helped us understand the genes which are important for eye development and can cause birth eye defects when disrupted. Studying a patient's own cells grown in the laboratory can further help researchers understand how changes in genes affect their function. Both animal and cellular models can be used to develop and test new drugs, which could provide treatment options for patients living with microphthalmia. This review summarises the key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.