Variants in gap junction protein alpha 8 (GJA8), the gene encoding connexin 50 (Cx50), are primarily associated with developmental cataract, although some are associated with severe structural eye anomalies, such as aphakia (absent lens), microphthalmia (small eyes), and sclerocornea. To further define the relationship of GJA8 variants to ocular developmental disorders, we screened four large international cohorts with structural eye anomalies, including anophthalmia, microphthalmia, and coloboma (AMC) or cataracts. We identified 15 new families carrying 14 different heterozygous GJA8 variants (12 missense variants and two 1q21 microdeletions). The missense variants comprised 10 previously reported alterations in cases with eye anomalies [p.(Gly22Ser), p.(Val44Met), p.(Asp67Gly), p.(Arg76Cys), p.(Pro88Leu), p.(Gly94Glu), p.(Gly94Arg), p.(His98Arg), p.(Pro189Ser), and p.(Arg198Trp)] and two not yet linked with disease [p.(Thr39Met) and p.(Tyr66Asp)]. Their associated phenotypes ranged from isolated cataracts to a combination of microphthalmia and cataract with/without sclerocornea. Our study confirms GJA8 variants as an important source of genetic diagnoses for families with structural eye anomalies in addition to cataract and highlights specific mutational hotspots. Furthermore, we confirm an important genotype-phenotype correlation between sclerocornea and the p.(Gly94Arg) variant, and detail intra- and inter-familial phenotypic variability, which is important for clinical assessment and genetic counselling. Megalocornea, also known as anterior megalophthalmos, X-linked megalocornea, and macrocornea, is a rare bilateral nonprogressive congenital defect that is characterized by an increased corneal diameter greater than 12.5 to 13 mm at birth and a deep anterior chamber with normal intraocular pressures. Thinning of the cornea has also been frequently associated. The defect falls under the category of anterior segment dysgenesis and is associated with several other conditions, including Axenfeld-Rieger syndrome, Peters anomaly, primary congenital glaucoma, aniridia, congenital hereditary endothelial dystrophy, and sclerocornea. In addition, it is a component of many different congenital syndromes. There are two patterns of clinical presentation. Primary megalocornea is an isolated megalocornea with no additional ocular or systemic manifestations. The second clinical presentation is megalocornea with other associated ocular and systemic abnormalities. This article will focus on primary congenital megalocornea (hereafter referred to as megalocornea) and will discuss possible comorbid conditions and syndromes in the ‘differential diagnosis’ section. Primary megalocornea was first described in 1869, but the genetic basis of this disease has only been elucidated in the past decade. The disease is usually inherited in an X-linked fashion, but other inheritance patterns have been described. The disease is generally asymptomatic in childhood though astigmatism from the large cornea may result in blurred vision. Adults may experience premature cataract formation, typically between the ages of 30 and 50, glaucoma, arcus juvenilis, lens subluxation, and mosaic corneal dystrophy. Other described comorbidities include iris atrophy, coloboma, abnormalities in zonular fibers, transillumination of the irides, lens dislocation, retinal detachment, and asymmetry in corneal size.

The forkhead transcription factor FOXE3 is critical for vertebrate eye development. Recessive and dominant variants cause human ocular disease but the full range of phenotypes and mechanisms of action for the two classes of variants are unknown. We identified FOXE3 variants in individuals with congenital eye malformations and carried out in vitro functional analysis on selected alleles. Sixteen new recessive and dominant families, including six novel variants, were identified. Analysis of new and previously reported genetic and clinical data demonstrated a broad phenotypic range with an overlap between recessive and dominant disease. Most families with recessive alleles, composed of truncating and forkhead-domain missense variants, had severe corneal opacity (90%; sclerocornea in 47%), aphakia (83%) and microphthalmia (80%), but some had milder features including isolated cataract. The phenotype was most variable for recessive missense variants, suggesting that the functional consequences may be highly dependent on the type of amino acid substitution and its position. When assessed, aniridia or iris hypoplasia were noted in 89% and optic nerve anomalies in 60% of recessive cases, indicating that these defects are also common and may be underrecognized. In dominant pedigrees, caused by extension variants, normal eye size (96%), cataracts (99%) and variable anterior segment anomalies were seen in most, but some individuals had microphthalmia, aphakia or sclerocornea, more typical of recessive disease. Functional studies identified variable effects on the protein stability, DNA binding, nuclear localization and transcriptional activity for recessive FOXE3 variants, whereas dominant alleles showed severe impairment in all areas and dominant-negative characteristics.

This project expands the disease spectrum for mutations in GJA8 to include total sclerocornea, rudimentary lenses and microphthalmia, in addition to this gene's previously known role in isolated congenital cataracts. Ophthalmic findings revealed bilateral total sclerocornea in 3 probands, with small abnormal lenses in 2 of the cases, and cataracts and microphthalmia in 1 case. Next-generation sequencing revealed de novo heterozygous mutations affecting the same codon of GJA8 : (c.281G>A; p.(Gly94Glu) and c.280G>C; p.(Gly94Arg)) in 2 of the probands, in addition to the c.151G>A; p.(Asp51Asn) mutation we had previously identified in the third case. In silico analysis predicted all of the mutations to be pathogenic. These cases show that deleterious, heterozygous mutations in GJA8 can lead to a severe ocular phenotype of total sclerocornea, abnormal lenses, and/or cataracts with or without microphthalmia, broadening the phenotype associated with this gene. GJA8 should be included when investigating patients with the severe anterior segment abnormality of total sclerocornea.

To describe 2 sporadic Mexican patients having congenital bilateral, total sclerocornea, aphakia, and microphthalmia associated with novel mutations in the FOXE3 gene. Two affected individuals with congenital bilateral, total sclerocornea, aphakia, and microphthalmia underwent detailed examinations including slit-lamp examination, visual acuity, and intraocular pressure measurements. Ocular ultrasonography and ultrasound biomicroscopy were performed. Genomic DNA was isolated from blood leukocytes in each subject, and molecular analysis of the FOXE3 gene was performed. For cosegregation analysis, presumable pathogenic variants were tested by Sanger sequencing in parental DNA. Molecular screening of FOXE3 was performed in 2 cases with congenital bilateral, total sclerocornea, aphakia, and microphthalmia. In patient 1, genetic analysis demonstrated a novel homozygous c.291C>G (p.Ile97Met) FOXE3 pathogenic variant. In patient 2, compound heterozygosity for the novel c.387C>G (p.Phe129Leu) transversion and for the previously reported c.244A>G (p.Met82Val) transition, was recognized. The sclerocornea-microphthalmia-aphakia complex is a severe malformative ocular phenotype resulting from mutations in the FOXE3 transcription factor. To date, patients from at least 14 families with this uncommon ocular disorder have been described. The identification of 2 novel pathogenic variants in our patients expands the mutational spectrum in FOXE3-related congenital eye disorders. In addition, we performed a review of the clinical and genotypic characteristics of all published patients carrying biallelic FOXE3 mutations.

Anophthalmia and microphthalmia (A/M) are developmental ocular malformations defined as the complete absence or reduction in size of the eye. A/M is a highly heterogeneous disorder with SOX2 and FOXE3 playing major roles in dominant and recessive pedigrees, respectively; however, the majority of cases lack a genetic etiology. We analyzed 28 probands affected with A/M spectrum (without mutations in SOX2/FOXE3) by whole-exome sequencing. Analysis of 83 known A/M factors identified pathogenic/likely pathogenic variants in PAX6, OTX2 and NDP in three patients. A novel heterozygous likely pathogenic variant in PAX6, c.767T>C, p.(Val256Ala), was identified in two brothers with bilateral microphthalmia, coloboma, primary aphakia, iris hypoplasia, sclerocornea and congenital glaucoma; the unaffected mother appears to be a mosaic carrier. While A/M has been reported as a rare feature, this is the first report of congenital primary aphakia in association with PAX6 and the identified allele represents the first variant in the PAX6 homeodomain to be associated with A/M. A novel pathogenic variant in OTX2, c.651delC, p.(Thr218Hisfs*76), in a patient with syndromic bilateral anophthalmia and a hemizygous pathogenic variant in NDP, c.293 C>T, p.(Pro98Leu), in two brothers with isolated bilateral microphthalmia and sclerocornea were also identified. Pathogenic/likely pathogenic variants were not discovered in the 25 remaining A/M cases. This study underscores the utility of whole-exome sequencing for identification of causative mutations in highly variable ocular phenotypes as well as the extreme genetic heterogeneity of A/M conditions.