Optic Fissure Closure Research Articles

Reduced TFAP2A function causes variable optic fissure closure and retinal defects and sensitizes eye development to mutations in other morphogenetic regulators.

Mutations in the transcription factor encoding TFAP2A gene underlie branchio-oculo-facial syndrome (BOFS), a rare dominant disorder characterized by distinctive craniofacial, ocular, ectodermal and renal anomalies. To elucidate the range of ocular phenotypes caused by mutations in TFAP2A, we took three approaches. First, we screened a cohort of 37 highly selected individuals with severe ocular anomalies plus variable defects associated with BOFS for mutations or deletions in TFAP2A. We identified one individual with a de novo TFAP2A four amino acid deletion, a second individual with two non-synonymous variations in an alternative splice isoform TFAP2A2, and a sibling-pair with a paternally inherited whole gene deletion with variable phenotypic expression. Second, we determined that TFAP2A is expressed in the lens, neural retina, nasal process, and epithelial lining of the oral cavity and palatal shelves of human and mouse embryos--sites consistent with the phenotype observed in patients with BOFS. Third, we used zebrafish to examine how partial abrogation of the fish ortholog of TFAP2A affects the penetrance and expressivity of ocular phenotypes due to mutations in genes encoding bmp4 or tcf7l1a. In both cases, we observed synthetic, enhanced ocular phenotypes including coloboma and anophthalmia when tfap2a is knocked down in embryos with bmp4 or tcf7l1a mutations. These results reveal that mutations in TFAP2A are associated with a wide range of eye phenotypes and that hypomorphic tfap2a mutations can increase the risk of developmental defects arising from mutations at other loci.

Novel SOX2 partner-factor domain mutation in a four-generation family

Anophthalmia (no eye), microphthalmia (small eye) and associated ocular developmental anomalies cause significant visual handicap. In most cases the underlying genetic cause is unknown, but mutations in some genes, such as SOX2, cause ocular developmental defects, particularly anophthalmia, in a subset of patients. Here, we describe a four-generation family with a p.Asp123Gly mutation in the highly conserved partner-factor interaction region of the SOX2 protein, which is important for cell-specific actions of SOX2. The proband in this family has bilateral anophthalmia and several other family members have milder ocular phenotypes, including typical optic fissure coloboma. Expression studies indicate that Sox2 is expressed in the eye at the site of closure of the optic fissure during development. The SOX2 mutation in this family implicates the partner-factor interaction region of SOX2 in contributing to the specificity of SOX2 action in optic fissure closure. Our findings indicate that investigation of SOX2 in a broad range of eye anomaly patients aids in the determination of particular functions of SOX2 in development.

Expression profiling during ocular development identifies 2 Nlz genes with a critical role in optic fissure closure

The gene networks underlying closure of the optic fissure during vertebrate eye development are poorly understood. Here, we profile global gene expression during optic fissure closure using laser capture microdissected (LCM) tissue from the margins of the fissure. From these data, we identify a unique role for the C(2)H(2) zinc finger proteins Nlz1 and Nlz2 in normal fissure closure. Gene knockdown of nlz1 and/or nlz2 in zebrafish leads to a failure of the optic fissure to close, a phenotype which closely resembles that seen in human uveal coloboma. We also identify misregulation of pax2 in the developing eye of morphant fish, suggesting that Nlz1 and Nlz2 act upstream of the Pax2 pathway in directing proper closure of the optic fissure.

Case Report and Literature Review of Inferior Rectus Muscle Aplasia in 16 Japanese Patients

Purpose: To describe clinical features of inferior rectus muscle aplasia in Japanese patients and to gain insight into its pathogenesis. Methods: Case presentation and literature review of 16 Japanese patients with inferior rectus muscle aplasia. Results: The age at presentation of 16 patients (9 females, 4 males, and 3 with unknown gender) varied from 7 months to 73 years (mean, 21.8 years). The inferior rectus muscle was absent on the right side in 10 patients, on the left side in 4 patients, and on both sides in 2 patients. Structural anomalies of the eye globe, such as microphthalmos, microcornea, and coloboma of varying degrees, were present on the ipsilateral side of the inferior rectus muscle aplasia in 4 patients and on the contralateral side in 1 patient. As surgical findings, anomalous inferiorly located insertion of the medial rectus muscle was discovered in 5 patients: 1 patient in association with microcornea and iris coloboma. Frequent surgical procedures were recession or tenotomy of the superior rectus muscle, combined with downward transposition of the entire or partial tendons of the medial rectus and lateral rectus muscle to the medial and lateral end, respectively, of the putative inferior rectus muscle insertion. Conclusions: The association of coloboma with inferior rectus muscle aplasia suggests that abnormal optic fissure closure during embryogenesis might underlie the muscle aplasia.

The temporal requirement for vitamin A in the developing eye: Mechanism of action in optic fissure closure and new roles for the vitamin in regulating cell proliferation and adhesion in the embryonic retina

Mammalian eye development requires vitamin A (retinol, ROL). The role of vitamin A at specific times during eye development was studied in rat fetuses made vitamin A deficient (VAD) after embryonic day (E) 10.5 (late VAD). The optic fissure does not close in late VAD embryos, and severe folding and collapse of the retina is observed at E18.5. Pitx2, a gene required for normal optic fissure closure, is dramatically downregulated in the periocular mesenchyme in late VAD embryos, and dissolution of the basal lamina does not occur at the optic fissure margin. The addition of ROL to late VAD embryos by E12.5 restores Pitx2 expression, supports dissolution of the basal lamina, and prevents coloboma, whereas supplementation at E13.5 does not. Surprisingly, ROL given as late as E13.5 completely prevents folding of the retina despite the presence of an open fetal fissure, showing that coloboma and retinal folding represent distinct VAD-dependent defects. Retinal folding due to VAD is preceded by an overall reduction in the percentage of cyclin D1 positive cells in the developing retina, (initially resulting in retinal thinning), as well as a dramatic reduction in the cell adhesion-related molecules, N-cadherin and β-catenin. Reduction of retinal cell number combined with a loss of the normal cell–cell adhesion proteins may contribute to the collapse and folding of the retina that occurs in late VAD fetuses.

Diagnosing papillorenal syndrome: see the optic papilla

Sirs, In their article “A clinico-genetic study of renal coloboma syndrome in children”, Cheong and colleagues accurately point out that a variety of PAX2 mutations, one of which they had discovered, can cause the described phenotype of associated ocular, renal and urinary tract anomalies seen in this autosomal dominant condition [1]. However, it should also be noted that many patients with the “renal coloboma” disorder do not have any identifiable mutations in the PAX2 gene [2, 3]. The syndrome is not defined by PAX2 genetic analysis, but by clinical findings. Cheong and co-workers employed the syndrome’s older “renal-coloboma” nomenclature [1] as opposed to the subsequent “papillorenal” designation. Precise language is important in this case, since confusion created by the term “coloboma” has led many investigators to consider incomplete closure of the embryonic optic fissure to be the cause of an optic nerve anomaly in cases where there is no supportive evidence for such an etiology [2]. Accordingly, the preferred OMIM appellation now is “papillorenal syndrome” [4] (OMIM #120330). It should be noted that the prefix “papillo” indicates the optic papilla as the site of the ocular abnormality and not that of the renal papillae. While the term “papillorenal” is also not fully descriptive of all the possible associated ocular or urinary tract anomalies (such as vesicoureteral reflux), it avoids the misnomer of “coloboma” and implications, thereof, of an unrelated pathophysiologic pathway. Cheong et al. correctly emphasize that there is considerable variability of ocular and urinary tract findings [1]. Among that variability, however, one constant feature exists and is clearly visible in the ocular fundus photographs they published: the lack of central retinal blood supply. Instead of central vessels within the optic papilla, the disc is variably excavated centrally; the cilioretinal vessels originate outside and at the periphery of the optic nerve supplying the retina (“vacant discs”). These disc vessels circumferentially make hairpin turns over the neuroretinal rim toward the retina. Though often asymptomatic, the absence of central retinal vasculature can be associated with retinal hypoplasia, causing visual field defects. In some instances, this can lead to secondary serous retinal detachments [5] which, if occurring early in development, can lead to relative microphthalmia. This bilaterally visible vascular anomaly of the optic papilla testifies to a systemically deficient process of angiogenesis (as opposed to vasculogenesis) [6]. Angiogenesis is most critical to the development of both eye and kidney, the most perfused tissues per gram weight of the human body. Angiogenic factors are involved in the budding of vascular endothelial cells from pre-existing vessels laid down by vasculogenesis [6]. These same factors also affect the budding of other tubular structures Pediatr Nephrol (2008) 23:1893–1894 DOI 10.1007/s00467-008-0870-6

Translational bypass of nonsense mutations in zebrafish rep1, pax2.1 and lamb1 highlights a viable therapeutic option for untreatable genetic eye disease

The extensive molecular genetic heterogeneity seen with inherited eye disease is a major barrier to the development of gene-based therapeutics. The underlying molecular pathology in a considerable proportion of these diseases however are nonsense mutations leading to premature termination codons. A therapeutic intervention targeted at this abnormality would therefore potentially be relevant to a wide range of inherited eye diseases. We have taken advantage of the ability of aminoglycoside drugs to suppress such nonsense mutations and partially restore full-length, functional protein in a zebrafish model of choroideraemia (chm(ru848); juvenile chorio-retinal degeneration) and in two models of ocular coloboma (noi(tu29a) and gup(m189); congenital optic fissure closure defects). In vitro cell-based assays showed significant readthrough with two drugs, gentamicin and paromomycin, which was confirmed by western blot and in vitro prenylation assays. The presence of either aminoglycoside during zebrafish development in vivo showed remarkable prevention of mutant ocular phenotypes in each model and a reduction in multisystemic defects leading to a 1.5-1.7-fold increase in survival. We also identified a significant reduction in abnormal cell death shown by TUNEL assay. To test the hypothesis that optic fissure closure was apoptosis-dependent, the anti-apoptotic agents, curcumin and zVAD-fmk, were tested in gup(m189) embryos. Both drugs were found to reduce the size of the coloboma, providing molecular evidence that cell death is required for optic fissure remodelling. These findings draw attention to the value of zebrafish models of eye disease as useful preclinical drug screening tools in studies to identify molecular mechanisms amenable to therapeutic intervention.

Anterior segment dysgenesis in the eyes of mice deficient for the high-mobility-group transcription factor Sox11

We describe that Sox11, a member of the group C of the Sox transcription factor family, is critically required during the morphogenetic processes of early eye development, and that lack of Sox11 results in ocular anterior segment dysgenesis (ASD). Sox11-deficient mice show a persistent lens stalk, a delay in lens formation, and the phenotypes of Peters' anomaly and microphthalmia at birth. In addition, the optic fissure does not close in the anterior halves of the eyes resulting in anterior coloboma. The delay in lens formation is associated with a reduced mitotic activity in the lens placode during its invagination into the optic cup. No changes in Pax6 expression are observed in the developing eyes of Sox11 −/− mice, whereas the expression of Sox11 is reduced in optic cup, optic vesicle and lens placode of Pax6 + / − embryos and in the optic vesicle of Pax6 −/− mice. Transfection experiments show an increase in Sox11 expression when higher doses of Pax6 are present. Considerably smaller amounts of BMP7 are expressed in lens and optic cup of Sox11 −/− mice as compared to their wild-type littermates. We conclude that Sox11 is required during separation of the lens vesicle from the surface ectoderm and the closure of the anterior optic fissure. The expression of Sox11 in early eye development is under control of Pax6, and changes in BMP7-signalling appear to be involved in the effects of Sox11 on anterior eye development. Our findings suggest that SOX11 might similarly be involved in the pathogenesis of ASD in human patients.

Phactr4 Regulates Neural Tube and Optic Fissure Closure by Controlling PP1-, Rb-, and E2F1-Regulated Cell-Cycle Progression

Here we identify the humpty dumpty (humdy) mouse mutant with failure to close the neural tube and optic fissure, causing exencephaly and retinal coloboma, common birth defects. The humdy mutation disrupts Phactr4, an uncharacterized protein phosphatase 1 (PP1) and actin regulator family member, and the missense mutation specifically disrupts binding to PP1. Phactr4 is initially expressed in the ventral cranial neural tube, a region of regulated proliferation, and after neural closure throughout the dorsoventral axis. humdy embryos display elevated proliferation and abnormally phosphorylated, inactive PP1, resulting in Rb hyperphosphorylation, derepression of E2F targets, and abnormal cell-cycle progression. Exencephaly, coloboma, and abnormal proliferation in humdy embryos are rescued by loss of E2f1, demonstrating the cell cycle is the key target controlled by Phactr4. Thus, Phactr4 is critical for the spatially and temporally regulated transition in proliferation through differential regulation of PP1 and the cell cycle during neurulation and eye development.

Uveal coloboma: clinical and basic science update

To integrate knowledge on the embryologic and molecular basis of optic fissure closure with clinical observations in patients with uveal coloboma. Closure of the optic fissure has been well characterized and many genetic alterations have been associated with coloboma; however, molecular mechanisms leading to coloboma remain largely unknown. In the past decade, we have gained better understanding of genes critical to eye development; however, mutations in these genes have been found in few individuals with coloboma. CHD7 mutations have been identified in patients with CHARGE syndrome (coloboma, heart defects, choanal atresia, retarded growth, genital anomalies, and ear anomalies or deafness). Animal models are bringing us closer to a molecular understanding of optic fissure closure. Optic fissure closure requires precise orchestration in timing and apposition of two poles of the optic cup. The relative roles of genetics and environment on this process remain elusive. While most cases of coloboma are sporadic, autosomal dominant, autosomal recessive, and X-linked inheritance patterns have been described. Genetically, colobomata demonstrate pleiotropy, heterogeneity, variable expressivity, and reduced penetrance. Coloboma is a complex disorder with a variable prognosis and requires regular examination to optimize visual acuity and to monitor for potential complications.

JNK initiates a cytokine cascade that causes Pax2 expression and closure of the optic fissure.

The c-Jun NH(2)-terminal kinase (JNK) group of mitogen-activated protein kinases is stimulated in response to a wide array of cellular stresses and proinflammatory cytokines. Mice lacking individual members of the Jnk family (Jnk1, Jnk2, and Jnk3) are viable and survive without overt structural abnormalities. Here we show that mice with a compound deficiency in Jnk expression can survive to birth, but fail to close the optic fissure (retinal coloboma). We demonstrate that JNK initiates a cytokine cascade of bone morphogenetic protein-4 (BMP4) and sonic hedgehog (Shh) that induces the expression of the paired-like homeobox transcription factor Pax2 and closure of the optic fissure. Interestingly, the role of JNK to regulate BMP4 expression during optic fissure closure is conserved in Drosophila during dorsal closure, a related morphogenetic process that requires JNK-regulated expression of the BMP4 ortholog Decapentaplegic (Dpp).

National study of microphthalmia, anophthalmia, and coloboma (MAC) in Scotland: investigation of genetic aetiology

We report an epidemiological and genetic study attempting complete ascertainment of subjects with microphthalmia, anophthalmia, and coloboma (MAC) born in Scotland during a 16 year period beginning on 1 January...

The optic fissure in the normal and microphthalmic mouse

The gene for microphthalmia in the homozygous cinnamon mouse produces colobomatous microphthalmia due to failure of closure of the optic fissure. Optic fissure formation and closure were examined in control cinnamon and homozygous microphthalmic foetuses from the 10th to the 13th gestational day using light and electron microscopy. This study showed that basal lamina degeneration and cell death occurred in the area of fusion in the control eyes. Examination of the optic fissure in homozygous microphthalmic foetuses showed cell death in the fissure margins, but there was failure of optic fissure closure associated with persistence of the basal lamina. It is therefore suggested that absence or abnormality of programmed disintegration of the basal lamina prevents fusion in the mutant leading to the development of colobomatous microphthalmia.