Robyn V. Jamieson

Human TUBB3 Mutations Perturb Microtubule Dynamics, Kinesin Interactions, and Axon Guidance

We report that eight heterozygous missense mutations in TUBB3, encoding the neuron-specific beta-tubulin isotype III, result in a spectrum of human nervous system disorders that we now call the TUBB3 syndromes. Each mutation causes the ocular motility disorder CFEOM3, whereas some also result in intellectual and behavioral impairments, facial paralysis, and/or later-onset axonal sensorimotor polyneuropathy. Neuroimaging reveals a spectrum of abnormalities including hypoplasia of oculomotor nerves and dysgenesis of the corpus callosum, anterior commissure, and corticospinal tracts. A knock-in disease mouse model reveals axon guidance defects without evidence of cortical cell migration abnormalities. We show that the disease-associated mutations can impair tubulin heterodimer formation in vitro, although folded mutant heterodimers can still polymerize into microtubules. Modeling each mutation in yeast tubulin demonstrates that all alter dynamic instability whereas a subset disrupts the interaction of microtubules with kinesin motors. These findings demonstrate that normal TUBB3 is required for axon guidance and maintenance in mammals.

Deletion at 14q22-23 Indicates a Contiguous Gene Syndrome Comprising Anophthalmia, Pituitary Hypoplasia, and Ear Anomalies

Anophthalmia and pituitary gland hypoplasia are both debilitating conditions where the underlying genetic defect is unknown in the majority of cases. We identified a patient with bilateral anophthalmia and absence of the optic nerves, chiasm and tracts, as well as pituitary gland hypoplasia and ear anomalies with a de novo apparently balanced chromosomal translocation, 46,XY,t(3;14)(q28;q23.2). Translocation breakpoint analysis using FISH and high‐resolution microarray comparative genomic hybridization (CGH) has identified a 9.66 Mb deleted region on the long arm of chromosome 14 which includes the genes BMP4, OTX2, RTN1, SIX6, SIX1, and SIX4. Three other patients with interstitial deletions involving 14q22‐23 have been described, all with bilateral anophthalmia, pituitary abnormalities, ear anomalies, and a facial phenotype similar to our patient. OTX2 is involved in ocular developmental defects, and the severity of the ocular phenotype in our patient and the other 14q22‐23 deletion patients, suggests this genomic region harbors other gene/s involved in ocular development. BMP4 haploinsufficiency is predicted to contribute to the ocular phenotype on the basis of its expression pattern and observed murine mutant phenotypes. In addition, deletion of BMP4 and SIX6 is likely to contribute to the abnormal pituitary development, and SIX1 deletion may contribute to the ear and other craniofacial features. This indicates that contiguous gene deletion may contribute to the phenotypic features in the 14q22‐23 deletion patients.

An inherited TUBB2B mutation alters a kinesin-binding site and causes polymicrogyria, CFEOM and axon dysinnervation

Microtubules are essential components of axon guidance machinery. Among β-tubulin mutations, only those in TUBB3 have been shown to cause primary errors in axon guidance. All identified mutations in TUBB2B result in polymicrogyria, but it remains unclear whether TUBB2B mutations can cause axon dysinnervation as a primary phenotype. We have identified a novel inherited heterozygous missense mutation in TUBB2B that results in an E421K amino acid substitution in a family who segregates congenital fibrosis of the extraocular muscles (CFEOM) with polymicrogyria. Diffusion tensor imaging of brains of affected family members reveals aberrations in the trajectories of commissural projection neurons, implying a paucity of homotopic connections. These observations led us to ask whether axon dysinnervation is a primary phenotype, and why the E421K, but not other, TUBB2B substitutions cause CFEOM. Expression of exogenous Tubb2b-E421K in developing callosal projection neurons is sufficient to perturb homotopic connectivity, without affecting neuronal production or migration. Using in vitro biochemical assays and yeast genetics, we find that TUBB2B-E421K αβ-heterodimers are incorporated into the microtubule network where they alter microtubule dynamics and can reduce kinesin localization. These data provide evidence that TUBB2B mutations can cause primary axon dysinnervation. Interestingly, by incorporating into microtubules and altering their dynamic properties, the E421K substitution behaves differently than previously identified TUBB2B substitutions, providing mechanistic insight into the divergence between resulting phenotypes. Together with previous studies, these findings highlight that β-tubulin isotypes function in both conserved and divergent ways to support proper human nervous system development.

Dkk1 and Wnt3 interact to control head morphogenesis in the mouse

Loss of Dkk1 results in ectopic WNT/β-catenin signalling activity in the anterior germ layer tissues and impairs cell movement in the endoderm of the mouse gastrula. The juxtaposition of the expression domains of Dkk1 and Wnt3 is suggestive of an antagonist-agonist interaction. The downregulation of Dkk1 when Wnt3 activity is reduced reveals a feedback mechanism for regulating WNT signalling. Compound Dkk1;Wnt3 heterozygous mutant embryos display head truncation and trunk malformation, which are not found in either Dkk1+/- or Wnt3+/- embryos. Reducing the dose of Wnt3 gene in Dkk1-/- embryos partially rescues the truncated head phenotype. These findings highlight that head development is sensitive to the level of WNT3 signalling and that DKK1 is the key antagonist that modulates WNT3 activity during anterior morphogenesis.

Secondary glaucoma after paediatric cataract surgery

Aim: To determine the prevalence and risk factors associated with secondary glaucoma postcongenital cataract surgery. Methods: All children diagnosed as having congenital cataracts in a major children’s hospital between 1985 and 2005 were included in a retrospective case series. Medical records of 423 eyes among 283 patients who underwent cataract surgery with or without intraocular lens implantation at age ⩽16 for congenital cataract were reviewed. The main outcome measure was presence or absence of secondary glaucoma and time to glaucoma postsurgery. The following risk factors were evaluated: age at cataract surgery, presence of systemic anomalies, microcornea, persistent hyperplastic primary vitreous (PHPV), primary capsulotomy/anterior vitrectomy, primary intraocular lens implantation, secondary membrane surgery and duration of postoperative observation. Results: The statistical methods were the use of Kaplan–Meier survival analysis and Multivariate Cox hazards regression analysis. The mean follow-up was 6.3 (SD 5.0) years (median 4.6 years; range 0.5 to 20.3 years). Glaucoma developed in 36 of 234 patients (15.4%). Multivariate Cox proportional hazards regression analysis identified age less than 9 months at time of surgery (RR 2.9, 95% CI 1.3 to 7.7; p = 0.03), microcornea (RR 3.7, 95% CI 2.0 to 7.0; p<0.001), and follow-up time as important predictors of glaucoma. PHPV (RR 1.4, 95% CI 0.7 to 2.7; p = 0.41) and primary posterior capsulotomy/anterior vitrectomy (RR 2.2, 95% CI 0.9 to 5.5; p = 0.17) were not significantly associated with secondary glaucoma in the multivariate model. The mean time to glaucoma after congenital cataract surgery was 4.9 years (range 2 weeks to 16.8 years). Conclusion: Secondary glaucoma is an important sequela in patients who undergo surgery for congenital cataracts. It is imperative that these patients get lifelong surveillance, as glaucoma can occur years after the initial operation.

PAX6 mutations may be associated with high myopia

PAX6 is a key regulator of eye development and there are many well recognized ophthalmic sequelae of mutations at this locus. The 14 exon PAX6 gene is well conserved across species and phyla. Coding region mutations manifest in a variety of phenotypes. Predicted premature protein truncations are generally associated with classical aniridia. Missense mutations are often found in cases with variant phenotypes such as ectopia pupillae; isolated foveal hypoplasia; nystagmus and hyaloid vessel proliferation. The locus has also been implicated, through a genome-wide sib-pair scan, to be important in the normal variation of myopia. We investigated the association between identified PAX6 mutations and refractive error in Australian patients from four pedigrees. Two of eight subjects with a 1410delC PAX6 mutation had a mean spherical equivalence < −9D, whilst a mean spherical equivalence ≤ −5D was recorded in two from four subjects with an Arg240Stop PAX6 mutation and one of two subjects with a Glu93Stop mutation. One individual identified with a Pro346Ala PAX6 mutation had a mean spherical equivalence of +2.8 D. Thus, our observations generally support other incidental findings, that PAX6 mutation, particularly predicted haploinsufficiency, may be associated with extreme refractive error, although the mechanism by which this occurs is not clear.

Port-wine vascular malformations and glaucoma risk in Sturge-Weber syndrome

PURPOSE Treatment of the capillary vascular malformation (port-wine stain) in Sturge-Weber syndrome with the use of a laser is helpful cosmetically. However, concerns have been raised that laser obliteration of port-wine stains may result in ocular hypertension. The aim of this study was to review clinical features and management of ocular complications of SWS and assess the effects of dermatological laser treatment on the incidence of glaucoma or ocular hypertension. METHODS This retrospective cohort study was conducted in an institutional setting. All patients had involvement of the face. Patients who underwent skin laser to the port-wine vascular malformation were analyzed further. Ocular involvement, glaucoma, and skin laser treatment and the relationship to ocular hypertension/glaucoma were observed. RESULTS Forty-one Sturge-Weber syndrome patients with port-wine vascular malformation were analyzed. Glaucoma was observed in 24 patients (58.5%) at mean age of 2.9 years (range, 0.0-16.5). Of these, 18 (75.0%) were treated with medical therapy, and 10 (41.7%) required trabeculectomy, with 2 of these requiring Seton implant. Of the 41 patients, 28 (68.3%) underwent laser to face/forehead. Mean age of laser commencement was 5 years (range, 0.4-16.5). Thirteen did not undergo laser treatment. Fourteen of the 28 and 10 of the 13 developed ocular hypertension/glaucoma. CONCLUSIONS This retrospective review did not find evidence to suggest that laser treatment of port-wine vascular malformations causes glaucoma or that it can worsen a preexisting ocular hypertension or glaucoma. Statistical analysis was inconclusive.

Exome sequencing in developmental eye disease leads to identification of causal variants in GJA8 , CRYGC , PAX6 and CYP1B1

Developmental eye diseases, including cataract/microcornea, Peters anomaly and coloboma/microphthalmia/anophthalmia, are caused by mutations encoding many different signalling and structural proteins in the developing eye. All modes of Mendelian inheritance occur and many are sporadic cases, so provision of accurate recurrence risk information for families and affected individuals is highly challenging. Extreme genetic heterogeneity renders testing for all known disease genes clinically unavailable with traditional methods. We used whole-exome sequencing in 11 unrelated developmental eye disease patients, as it provides a strategy for assessment of multiple disease genes simultaneously. We identified five causative variants in four patients in four different disease genes, GJA8, CRYGC, PAX6 and CYP1B1. This detection rate (36%) is high for a group of patients where clinical testing is frequently not undertaken due to lack of availability and cost. The results affected clinical management in all cases. These variants were detected in the cataract/microcornea and Peters anomaly patients. In two patients with coloboma/microphthalmia, variants in ABCB6 and GDF3 were identified with incomplete penetrance, highlighting the complex inheritance pattern associated with this phenotype. In the coloboma/microphthalmia patients, four other variants were identified in CYP1B1, and CYP1B1 emerged as a candidate gene to be considered as a modifier in coloboma/microphthalmia.

Sporadic and Familial Congenital Cataracts: Mutational Spectrum and New Diagnoses Using Next‐Generation Sequencing

Congenital cataracts are a significant cause of lifelong visual loss. They may be isolated or associated with microcornea, microphthalmia, anterior segment dysgenesis (ASD) and glaucoma, and there can be syndromic associations. Genetic diagnosis is challenging due to marked genetic heterogeneity. In this study, next‐generation sequencing (NGS) of 32 cataract‐associated genes was undertaken in 46 apparently nonsyndromic congenital cataract probands, around half sporadic and half familial cases. We identified pathogenic variants in 70% of cases, and over 68% of these were novel. In almost two‐thirds (20/33) of these cases, this resulted in new information about the diagnosis and/or inheritance pattern. This included identification of: new syndromic diagnoses due to NHS or BCOR mutations; complex ocular phenotypes due to PAX6 mutations; de novo autosomal‐dominant or X‐linked mutations in sporadic cases; and mutations in two separate cataract genes in one family. Variants were found in the crystallin and gap junction genes, including the first report of severe microphthalmia and sclerocornea associated with a novel GJA8 mutation. Mutations were also found in rarely reported genes including MAF, VIM, MIP, and BFSP1. Targeted NGS in presumed nonsyndromic congenital cataract patients provided significant diagnostic information in both familial and sporadic cases.

Retinal dystrophies, genomic applications in diagnosis and prospects for therapy

Retinal dystrophies (RDs) are degenerative diseases of the retina which have marked clinical and genetic heterogeneity. Common presentations among these disorders include night or colour blindness, tunnel vision and subsequent progression to complete blindness. The known causative disease genes have a variety of developmental and functional roles with mutations in more than 120 genes shown to be responsible for the phenotypes. In addition, mutations within the same gene have been shown to cause different disease phenotypes, even amongst affected individuals within the same family highlighting further levels of complexity. The known disease genes encode proteins involved in retinal cellular structures, phototransduction, the visual cycle, and photoreceptor structure or gene regulation. This review aims to demonstrate the high degree of genetic complexity in both the causative disease genes and their associated phenotypes, highlighting the more common clinical manifestation of retinitis pigmentosa (RP). The review also provides insight to recent advances in genomic molecular diagnosis and gene and cell-based therapies for the RDs.