Maree Flaherty

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.

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.

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.

Microphthalmia, Anophthalmia, and Coloboma and Associated Ocular and Systemic Features Understanding the Spectrum

IMPORTANCE Microphthalmia, anophthalmia, and coloboma form an interrelated spectrum of congenital eye abnormalities. OBJECTIVE To document the ocular and systemic findings and inheritance patterns in patients with microphthalmia, anophthalmia, and coloboma disease to gain insight into the underlying developmental etiologies. DESIGN, SETTING, AND PARTICIPANTS This retrospective consecutive case series was conducted at a tertiary referral center. Included in the study were 141 patients with microphthalmia, anophthalmia, and coloboma disease without a recognized syndromic etiology who attended the Westmead Childrens Hospital, Sydney, from 1981-2012. EXPOSURE Cases were grouped on the basis of the presence or absence of an optic fissure closure defect (OFCD); those with OFCD were further subdivided into microphthalmic and nonmicrophthalmic cases. Anophthalmic cases were considered as a separate group. MAIN OUTCOMES AND MEASURES Associated ocular and systemic abnormalities and inheritance patterns were assessed. RESULTS Of 141 cases, 61 (43%) were microphthalmic non-OFCD (NOFCD), 34 (24%) microphthalmic OFCD, 32 (23%) nonmicrophthalmic coloboma (OFCD), 9 (6%) anophthalmic, and 5 (4%) were unclassified. Sixty-three (45%) had bilateral disease. Eighty-four patients (60%) had an associated ocular abnormality; of these, cataract (P < .001) and posterior segment anomalies (P < .001) were most common in the NOFCD group. Forty-eight (34%) had an associated systemic abnormality, most commonly neurological, musculoskeletal and facial, urological and genital, or cardiac. Neurological abnormalities were most common in the anophthalmic group (P = .003), while urological abnormalities were particularly seen in the OFCD groups (P = .009). Familial cases were identified in both the OFCD and NOFCD groups, with a likely autosomal dominant inheritance pattern in 9 of 10 families. CONCLUSIONS AND RELEVANCE This series indicated that the OFCD/NOFCD distinction may be useful in guiding evaluation for ocular and systemic associations, as well as the direction and analysis of genetic investigation.

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.

Chromosomal rearrangements and novel genes in disorders of eye development, cataract and glaucoma.

Disorders of eye development such as microphthalmia and anophthalmia (small and absent eyes respectively), anterior segment dysgenesis where there may be pupillary and iris anomalies, and associated cataract and glaucoma, often lead to visual impairment or blindness. Currently treatment options are limited, as much is unknown about the molecular pathways that control normal eye development and induce the aberrant processes that lead to ocular defects. Mutation detection rates in most of the known genes are generally low, emphasizing the genetic heterogeneity of developmental ocular defects. Identification of the disease genes in these conditions improves the clinical information available for affected individuals and families, and provides new insights into the underlying biological processes for facilitation of better treatment options. Investigation of chromosomal rearrangements associated with an ocular phenotype has been especially powerful for disease gene identification. Molecular characterization of such rearrangements, which pinpoints the region by physically disrupting the causative gene or its regulatory sequences, allows for rapid elucidation of underlying genetic factors that contribute to the phenotype. Genes including PAX6, PITX2, FOXC1, MAF, TMEM114, SOX2, OTX2 and BMP4 have been identified in this way to be associated with developmental eye disorders. More recently, new methods in chromosomal analysis such as comparative genomic hybridization (CGH) microarray, have also enhanced our ability in disease gene identification.

Assessment and diagnosis of suspected glaucoma in patients with mucopolysaccharidosis

The mucopolysaccharidoses (MPS) are a group of rare lysosomal storage disorders, characterized by the accumulation of glycosaminoglycans within multiple organ systems including the eye. This study aimed to determine the prevalence of glaucoma in patients with MPS, as well as the characteristics, diagnosis and management of patients with MPS and glaucoma.

Broadening the phenotype of LRP2 mutations: a new mutation in LRP2 causes a predominantly ocular phenotype suggestive of Stickler syndrome

Two siblings, from a consanguineous Iraqi family, were investigated to identify the underlying genetic cause of their high myopia, esotropia, vitreous changes and cataract. Subsequent investigation identified low molecular weight proteinuria as part of their syndrome. Exome sequencing of one of the probands revealed a new non‐synonymous variant in the LRP2 gene. Sanger sequencing confirmed the mutation and segregation in the family. No mutation was identified in COL9A1/2, COL11A1/2, or COL2A1 genes. The variant (c.11483A>G; p.Asp3828Gly) is predicted to be damaging and is conserved among vertebrate species. Mutations in LRP2 have been shown to cause the Donnai–Barrow syndrome (DBS) or facio‐oculo‐acoustico‐renal (FOAR) syndrome, a syndrome associated with facial dysmorphism, ocular anomalies, sensorineural hearing loss, low molecular weight proteinuria, and diaphragmatic hernia and absent corpus callosum, although there is variability in the expression of some features. This family shows a milder phenotype with a predominant eye phenotype similar to the Stickler syndrome and only a few features of the DBS, including microglobulinuria. The presence of microglobulinuria was only detected after molecular results were known. In conclusion, with the identification of a new mutation in LRP2 associated with a predominant eye phenotype similar to the Stickler syndrome, we have broadened the phenotypic spectrum of LRP2 mutations.

Advantage of Whole Exome Sequencing over Allele-Specific and Targeted Segment Sequencing in Detection of Novel TULP1 Mutation in Leber Congenital Amaurosis

Abstract Background: Leber congenital amaurosis (LCA) is a severe form of retinal dystrophy with marked underlying genetic heterogeneity. Until recently, allele-specific assays and Sanger sequencing of targeted segments were the only available approaches for attempted genetic diagnosis in this condition. A broader next-generation sequencing (NGS) strategy, such as whole exome sequencing, provides an improved molecular genetic diagnostic capacity for patients with these conditions. Materials and Methods: In a child with LCA, an allele-specific assay analyzing 135 known LCA-causing variations, followed by targeted segment sequencing of 61 regions in 14 causative genes was performed. Subsequently, exome sequencing was undertaken in the proband, unaffected consanguineous parents and two unaffected siblings. Bioinformatic analysis used two independent pipelines, BWA-GATK and SOAP, followed by Annovar and SnpEff to annotate the variants. Results: No disease-causing variants were found using the allele-specific or targeted segment Sanger sequencing assays. Analysis of variants in the exome sequence data revealed a novel homozygous nonsense mutation (c.1081C > T, p.Arg361*) in TULP1, a gene with roles in photoreceptor function where mutations were previously shown to cause LCA and retinitis pigmentosa. The identified homozygous variant was the top candidate using both bioinformatic pipelines. Conclusions: This study highlights the value of the broad sequencing strategy of exome sequencing for disease gene identification in LCA, over other existing methods. NGS is particularly beneficial in LCA where there are a large number of causative disease genes, few distinguishing clinical features for precise candidate disease gene selection, and few mutation hotspots in any of the known disease genes.