Loss-of-function of an N-acetylglucosaminyltransferase, POMGnT1, in muscle-eye-brain disease

Muscle-eye-brain disease (MEB), an autosomal recessive disorder, is characterized by congenital muscular dystrophy, brain malformation, and ocular abnormalities. Previously, we found that MEB is caused by mutations in the gene encoding the protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), which is responsible for the formation of the GlcNAcbeta1-2Man linkage of O-mannosyl glycan. Although 13 mutations have been identified in patients with MEB, only the protein with the most frequently observed splicing site mutation has been studied. This protein was found to have no activity. Here, we expressed the remaining mutant POMGnT1s and found that none of them had any activity. These results clearly demonstrate that MEB is inherited as a loss-of-function of POMGnT1.

Phenotype of the zebrafish masterblind (mbl) mutant is dependent on genetic background

The zebrafish masterblind (mbl) mutant is characterized by the lack of olfactory placodes and optic vesicles, reduced telencephalon, an expanded epiphysis (Heisenberg et al. [1996] Development 123:191-203), and enlarged jaw. To understand the cellular events giving rise to the olfactory placode defect of this mutant, we examined the expression pattern of the distal-less-3 (dlx3) gene in mbl. In the mutant, dlx3, which is normally expressed in the developing nose and ear, showed reduced expression in the olfactory placode field, but normal expression in the developing ear. To determine whether the loss of dlx3 expression was due to cell loss, we assayed cell death by using TUNEL labeling. Although cell death in the mutant was not concentrated in the region of dlx3 expression, there was increased cell death in the forebrain, epiphysis, and jaw region, as compared with that in wild-type controls. This cell death phenotype was cyclical in nature, showing an increase and decrease in cell death on a roughly 24-hr cycle. Further analysis showed that this cyclical phenotype was specific to the genetic background. The severity of the mbl phenotype, including cell death, expanded epiphysis, and enlarged jaw, decreased when the mutation was moved from the original "TL" background to the "AB" background. Thus, the severity of developmental defects in the mbl mutant is strongly dependent on genetic background. We examined the contribution of cell death to the morphologic defects of mbl by blocking cell death by using zVADfmk, a known caspase inhibitor. We found that this treatment partially rescued the expanded jaw defect and that this rescue was dependent on the genetic background. Therefore, the mbl mutant phenotypes result, in part, from genetic background effects that alter the pattern of programmed cell death early in development.

A novel syndrome of congenital lid and punctal anomalies, corneal and chorioretinal dystrophy

A 28-year old woman had an ocular syndrome consisting of congenital lid and punctal anomalies, and corneal and chorioretinal dystrophy without facial dysmorphism. These combined malformations of the ocular adnexae and both anterior and posterior ocular segments have not been previously described and appear to represent a novel syndrome. Direct sequencing of PAX6 and the DNA-binding domain of FOXC1 failed to detect a mutation.

Novel anterior segment phenotypes resulting from forkhead gene alterations: evidence for cross-species conservation of function

PURPOSE

Mutations in murine and human versions of an ancestrally related gene usually result in similar phenotypes. However, interspecies differences exist, and in the case of two forkhead transcription factor genes (FOXC1 and FOXC2), these differences include corneal or anterior segment phenotypes, respectively. This study was undertaken to determine whether such discrepancies provide an opportunity for identifying novel human-murine ocular phenotypes.

METHODS

Four pedigrees with early-onset glaucoma phenotypes secondary to segmental chromosomal duplications or deletions encompassing FOXC1 and 18 individuals from 9 FOXC2 mutation pedigrees underwent detailed ocular phenotyping. Subsequently, mice with mutations in Foxc1 or a related forkhead gene, Foxe3, were assessed for features of the human phenotypes.

RESULTS

A significant increase in central corneal thickness was present in affected individuals from the segmental duplication pedigrees compared with their unaffected relatives (mean increase 13%, maximum 35%, P < 0.05). Alterations in corneal thickness were present in mice heterozygous and homozygous for Foxe3 mutations but neither in Foxc1 heterozygotes nor the small human segmental deletion pedigree. Mutations in FOXC2 resulted in ocular anterior segment anomalies. These were more severe and prevalent with mutations involving the forkhead domain.

CONCLUSIONS

Normal corneal development is dependent on the precise dose and levels of activity of certain forkhead transcription factors. The altered corneal thickness attributable to increased forkhead gene dosage is particularly important, because it may affect the clinical management of certain glaucoma subtypes and lead to excessive treatment. The FOXC1 and Foxe3 data, taken together with the novel ocular phenotypes of FOXC2 mutations, highlight the remarkable cross-species conservation of function among forkhead genes.

Fras1 deficiency results in cryptophthalmos, renal agenesis and blebbed phenotype in mice

Loss of tight association between epidermis and dermis underlies several blistering disorders and is frequently caused by impaired function of extracellular matrix (ECM) proteins. Here we describe a new protein in mouse, Fras1, that is specifically detected in a linear fashion underlying the epidermis and the basal surface of other epithelia in embryos. Loss of Fras1 function results in the formation of subepidermal hemorrhagic blisters as well as unilateral or bilateral renal agenesis during mouse embryogenesis. Postnatally, homozygous Fras1 mutants have fusion of the eyelids and digits and unilateral renal agenesis or dysplasia. The defects observed in Fras1-/- mice phenocopy those of the existing bl (blebbed) mouse mutants, which have been considered a model for the human genetic disorder Fraser syndrome. We show that bl/bl homozygous embryos are devoid of Fras1 protein, consistent with the finding that Fras1 is mutated in these mice. In sum, our data suggest that perturbations in the composition of the extracellular space underlying epithelia could account for the onset of the blebbed phenotype in mouse and Fraser syndrome manifestation in human.

Rieger anomaly with bilateral choroidal osteoma: coincidence or association?

PURPOSE

To report a case of Rieger anomaly associated with bilateral choroidal osteoma.

CASE

An 18-year-old woman presented with corectopia, iridocorneal adhesions, and stromal hypoplasia of the iris, and was diagnosed with Rieger anomaly.

RESULTS

Fundus examination revealed bilateral yellowish-orange, placoid degenerative fundus lesions with pseudopod-like edges associated with the areas of retinal pigment epithelium atrophy, indicating choroidal osteoma. Fundus fluorescein angiography showed bilateral early patchy and late diffuse hyperfluorescence. B-scan echography revealed placoid lesion at the posterior ocular coats characterized by localized areas of high ultrasound reflectivity with a corresponding retrobulbar orbital shadowing. This indicated a dense calcium foreign body. In addition, computed tomography of the orbit demonstrated bilateral plate-like thickening with calcification of the choroid that was isodense with the normal skeletal bone.

CONCLUSIONS

Although Rieger anomaly is classically known as a disease of the anterior segment of the eye, choroidal osteoma may be a posterior segment finding of the disease that has not previously been reported.

Fox's in development and disease

Since the first forkhead (Fox) gene was identified, the importance of this family of transcription factors has increased steadily with the discoveries of the diverse range of developmental processes that they regulate in eukaryotes. Among other processes, the Fox factors are important in the establishment of the body axis and the development of tissues from all three germ layers. In this article, we present some of the recent data on this gene family with reference to selected phenotypes observed in patients and model organisms, and the sensitivity of developmental processes to alterations in forkhead gene dosage.

Extracellular matrix and nuclear abnormalities in skeletal muscle of a patient with Walker-Warburg syndrome caused by POMT1 mutation

Walker-Warburg syndrome (WWS) is an autosomal recessive disorder characterized by congenital muscular dystrophy, structural eye abnormalities and severe brain malformations. We performed an immunohistochemical and electron microscopy study of a muscle biopsy from a patient affected by WWS carrying a homozygous frameshift mutation in O-mannosyltransferase 1 gene (POMT1). alpha-Dystroglycan glycosylated epitope was not detected in muscle fibers and intramuscular peripheral nerves. Laminin alpha2 chain and perlecan were reduced in muscle fibers and well preserved in intramuscular peripheral nerves. The basal lamina in several muscle fibers showed discontinuities and detachment from the plasmalemma. Most nuclei, including myonuclei and satellite cell nuclei, showed detachment or complete absence of peripheral heterochromatin from the nuclear envelope. Apoptotic changes were detected in 3% of muscle fibers. The particular combination of basal lamina and nuclear changes may suggest that a complex pathogenetic mechanism, affecting several subcellular compartments, underlies the degenerative process in WWS muscle.

Degradation of DIAP1 by the N-end rule pathway is essential for regulating apoptosis

Some members of the inhibitor of apoptosis (IAP) protein family block apoptosis by binding to and neutralizing active caspases. We recently demonstrated that a physical association between IAP and caspases alone is insufficient to regulate caspases in vivo and that an additional level of control is provided by IAP-mediated ubiquitination of both itself and the associated caspases. Here we show that Drosophila IAP 1 (DIAP1) is degraded by the 'N-end rule' pathway and that this process is indispensable for regulating apoptosis. Caspase-mediated cleavage of DIAP1 at position 20 converts the more stable pro-N-degron of DIAP1 into the highly unstable, Asn-bearing, DIAP1 N-degron of the N-end rule degradation pathway. Thus, DIAP1 represents the first known metazoan substrate of the N-end rule pathway that is targeted for degradation through its amino-terminal Asn residue. We demonstrate that the N-end rule pathway is required for regulation of apoptosis induced by Reaper and Hid expression in the Drosophila melanogaster eye. Our data suggest that DIAP1 instability, mediated through caspase activity and subsequent exposure of the N-end rule pathway, is essential for suppression of apoptosis. We suggest that DIAP1 safeguards cell viability through the coordinated mutual destruction of itself and associated active caspases.

Familial retinal arteriolar tortuosity: a review

Familial retinal arteriolar tortuosity is a rare disorder with autosomal dominant inheritance. It is characterized by a pathognomic pattern of progressive, pronounced tortuosity of the arterioles of second and third order in the macular and peripapillary area developing during childhood or early adulthood. The disorder can be complicated by intra- or preretinal hemorrhages that occur spontaneously or during physical exertion. These hemorrhages can be asymptomatic if they do not involve the fovea. Clearing of the hemorrhages without treatment and recovery of full visual acuity is the rule, even if the central macula is involved. In most cases, no other vascular malformations or associated systemic diseases have been found. In some cases, retinal hemorrhages may precede the development of the tortuosity or the tortuosity may remain subtle, even if the patients suffer from recurrent hemorrhages. The ophthalmologist should be aware of familial retinal arteriolar tortuosity as a possible cause for retinal hemorrhages.

Diploid-dependent regulation of gene expression: a genetic cause of abnormal development in fish haploid embryos

A diploid-dependent regulatory mechanism of gene expression for spatial patterning of the eye in vertebrates has been determined by analyzing the phenotypes of haploid goldfish embryos. There are two gene loci in charge of eye spatial patterning during embryonic morphogenesis. The expressional probability for each copy of the two genes in a set of chromosomes is 50%. A pair of genes in two sets of homologous or heterologous chromosomes is 100% and essential for normal gene expression. The haploid condition itself would result in the obstruction of gene expression and abnormal development because the diploid-dependent regulatory apparatus will regulate gene expression in a haploid embryo according to the same rule as in the diploid embryo.

Physical map of the chromosome 6q22 region containing the oculodentodigital dysplasia locus: analysis of thirteen candidate genes and identification of novel ESTs and DNA polymorphisms

Oculodentodigital dysplasia (ODDD) is an autosomal dominant condition with congenital anomalies of the craniofacial and limb regions and neurodegeneration. Genetic anticipation for the dysmorphic and neurologic features has been inferred in a few families. Our previous linkage studies have refined the ODDD candidate region to chromosome 6q22-->q23. In an attempt to clone the ODDD gene, we created a yeast artificial chromosome contig with 31 redundant clones spanning the region and identified and ordered candidate genes and markers. Fluorescent IN SITU hybridization mapped two of these YAC clones to chromosome 6q22.2 telomeric to a known 6q21 fragile site, excluding it as a possible cause of the suggested anticipation. We performed mutation analysis on thirteen candidate genes - GRIK2, HDAC2, COL10A1, PTD013, KPNA5, PIST, ROS1, BRD7, PLN, HSF2, PKIB, FABP7, and HEY2. Although no mutations were found, we identified 44 polymorphisms, including 28 single nucleotide polymorphisms. Direct cDNA selection was performed and fifty-five clones were found to contain sequences that were not previously reported as known genes or ESTs. These clones and polymorphisms will assist in the further characterization of this region and identification of disease genes.

Worldwide distribution and broader clinical spectrum of muscle-eye-brain disease

Muscle-eye-brain disease (MEB), an autosomal recessive disorder prevalent in Finland, is characterized by congenital muscular dystrophy, brain malformation and ocular abnormalities. Since the MEB phenotype overlaps substantially with those of Fukuyama-type congenital muscular dystrophy (FCMD) and Walker-Warburg syndrome (WWS), these three diseases are thought to result from a similar pathomechanism. Recently, we showed that MEB is caused by mutations in the protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1) gene. We describe here the identification of seven novel disease-causing mutations in six of not only non-Finnish Caucasian but also Japanese and Korean patients with suspected MEB, severe FCMD or WWS. Including six previously reported mutations, the 13 disease-causing mutations we have found thus far are dispersed throughout the entire POMGnT1 gene. We also observed a slight correlation between the location of the mutation and clinical severity in the brain: patients with mutations near the 5' terminus of the POMGnT1 coding region show relatively severe brain symptoms such as hydrocephalus, while patients with mutations near the 3' terminus have milder phenotypes. Our results indicate that MEB may exist in population groups outside of Finland, with a worldwide distribution beyond our expectations, and that the clinical spectrum of MEB is broader than recognized previously. These findings emphasize the importance of considering MEB and searching for POMGnT1 mutations in WWS or other congenital muscular dystrophy patients worldwide.

Developmental genetics in ophthalmology

Much of our knowledge about the function of genes in mammalian development has been derived from the molecular analysis of spontaneous or induced mutations in the mouse. Since mutations affecting the mouse eye can be easily identified, a remarkable number of mutant lines provide animal models for congenital anomalies in man. To understand the mechanisms of lens development in detail, the isolation of the corresponding genes and the characterization of the mutations at the molecular level are important. A prerequisite for molecular analysis is the chromosomal localization of the gene. In this review, some mutants from our institute will be discussed according to the embryological time scale of the expression of the affected genes, reflecting also their genetic hierarchy. (1) In the aphakia mouse mutant, two deletions in the promoter of the homeobox transcription factor Pitx3 lead to a loss of its function and to an arrest of eye development at the lens stalk stage. Mutations in the homologous human PITX3 gene have been demonstrated to be causative of cataracts and the dysmorphology of the anterior segment of the eye. (2) Connexin50 is present in the lens vesicle. Later on, it becomes abundant in the anterior part of the fiber cells and in the lens epithelial cells. Mutations in the connexin50-encoding gene Gja8 lead to dominant cataracts. (3) alphaA-crystallin is present in the mouse lens cup, in the posterior half of the lens vesicle, and later in a high concentration in the lens fiber cells. Mutations in the alphaA-crystallin-encoding gene Cryaa lead to recessive and dominant cataracts. (4) Mutations in the gamma-crystallin -encoding genes (Cryg) are the most frequent cause of congenital, dominant nuclear, or total cataracts in the mouse. Indications from our first studies in congenital human cataracts support these data. (5) Some postnatal, progressive cataracts have been characterized by mutations in the beta-crystallin -encoding genes (Cryb). Since at least one of them is also expressed in the retina and the brain, effects on these tissues have to be considered, too.

A family with Axenfeld-Rieger syndrome and Peters Anomaly caused by a point mutation (Phe112Ser) in the FOXC1 gene

PURPOSE

Mutations of the forkhead transcription factor gene FOXC1 result in anterior segment anomalies. No description of the spectrum of defects resulting from a single point mutation of this gene exists in the ophthalmology literature. We have screened all available patients with Axenfeld-Rieger genes (PITX2 and FOXC1). In this report, we clinically characterize the spectrum of ocular and systemic manifestations in one family resulting from a previously reported point mutation (Phe112Ser) in FOXC1.

DESIGN

Observational case series.

METHODS

Ten members of a multigenerational family were examined for signs of glaucoma, anterior segment abnormalities, and systemic features of Axenfeld-Rieger syndrome. The examinations were performed in an ophthalmology examination room or in the patients' homes. Blood was obtained from 10 members and screened for mutations in FOXC1 using direct DNA sequencing.

RESULTS

A single mutation causing a T to C change in codon 112 (Phe112Ser) of FOXC1 was present in six members of the family. Five of these six patients were examined and all demonstrated anterior segment anomalies. One patient had Axenfeld anomaly, one had Rieger syndrome, and one had both Axenfeld anomaly and Peters anomaly. Additionally, some members demonstrated cardiac abnormalities, which may be secondary to their FOXC1 mutation.

CONCLUSIONS

A wide spectrum of clinical phenotypes can result from a single point mutation of FOXC1. This report confirms that Rieger syndrome (with dental and facial abnormalities) can be caused by a mutation in FOXC1. It is also the first report of Peters anomaly being caused by a FOXC1 mutation.

Mutation spectrum of FOXC1 and clinical genetic heterogeneity of Axenfeld-Rieger anomaly in India

PURPOSE

Axenfeld-Rieger anomaly (ARA) is a form of anterior segment dysgenesis of the eye, mainly caused by mutations in the FOXC1 gene. We had earlier reported a novel mutation in the wing region of FOXC1 in an autosomal dominant family. The present study was aimed to identify the spectrum of mutations in the FOXC1 gene in a cohort of Indian ARA patients from different ethnic backgrounds, and to understand its role in the disease pathogenesis.

METHODS

Two new autosomal dominant families and seven sporadic cases of ARA from different ethnic backgrounds were screened for mutations by direct sequencing of the coding region of the FOXC1 gene. Another autosomal dominant ARA family that was previously reported by us was also included for comparative analysis of clinical genetic parameters. The segregation of the mutations in the autosomal dominant families was analyzed by haplotype and restriction analysis. Genotype-phenotype correlation were also undertaken to study the role of FOXC1 in phenotypic manifestation in the patient cohort.

RESULTS

Three of the nine ARA cases harbored mutations in FOXC1, of which two novel nonsense mutations Q2X and Q123X, resulted in haploinsufficiency of the gene product. The missense mutation (M161K) that we previously reported in an autosomal dominant family was also found in another family. Haplotype analysis of these two families suggested multiple founders in the same ethnic group. The mutations resulted in variable expressions of phenotype among the patients as assessed from their prognosis based on visual outcomes.

CONCLUSIONS

Significant genetic heterogeneity of FOXC1 was observed in a multi-ethnic population studied in this region of India resulting in variable ARA phenotypes. The different visual outcome seen in the patients suggest a variable expression of FOXC1 and also provide some insight for understanding the gene functions in this population.

Evidence for a direct functional antagonism of the selector genes proboscipedia and eyeless in Drosophila head development

Diversification of Drosophila segmental and cellular identities both require the combinatorial function of homeodomain-containing transcription factors. Ectopic expression of the mouthparts selector proboscipedia (pb) directs a homeotic antenna-to-maxillary palp transformation. It also induces a dosage-sensitive eye loss that we used to screen for dominant Enhancer mutations. Four such Enhancer mutations were alleles of the eyeless (ey) gene that encode truncated EY proteins. Apart from eye loss, these new eyeless alleles lead to defects in the adult olfactory appendages: the maxillary palps and antennae. In support of these observations, both ey and pb are expressed in cell subsets of the prepupal maxillary primordium of the antennal imaginal disc, beginning early in pupal development. Transient co-expression is detected early after this onset, but is apparently resolved to yield exclusive groups of cells expressing either PB or EY proteins. A combination of in vivo and in vitro approaches indicates that PB suppresses EY transactivation activity via protein-protein contacts of the PB homeodomain and EY Paired domain. The direct functional antagonism between PB and EY proteins suggests a novel crosstalk mechanism integrating known selector functions in Drosophila head morphogenesis.

The tumor-suppressor and cell adhesion molecule Fat controls planar polarity via physical interactions with Atrophin, a transcriptional co-repressor

Fat is an atypical cadherin that controls both cell growth and planar polarity. Atrophin is a nuclear co-repressor that is also essential for planar polarity; however, it is not known what genes Atrophin controls in planar polarity, or how Atrophin activity is regulated during the establishment of planar polarity. We show that Atrophin binds to the cytoplasmic domain of Fat and that Atrophin mutants show strong genetic interactions with fat. We find that both Atrophin and fat clones in the eye have non-autonomous disruptions in planar polarity that are restricted to the polar border of clones and that there is rescue of planar polarity defects on the equatorial border of these clones. Both fat and Atrophin are required to control four-jointed expression. In addition our mosaic analysis demonstrates an enhanced requirement for Atrophin in the R3 photoreceptor. These data lead us to a model in which fat and Atrophin act twice in the determination of planar polarity in the eye: first in setting up positional information through the production of a planar polarity diffusible signal, and later in R3 fate determination.

Screening for PAX6 gene mutations is consistent with haploinsufficiency as the main mechanism leading to various ocular defects

PAX6, a paired box transcriptional factor, is considered as the master control gene for morphogenesis of the eye. Human PAX6 mutations have been associated with a range of eye abnormalities, including aniridia, various anterior segment defects and foveal hypoplasia. We carried out a mutational analysis of the PAX6 gene in 54 unrelated patients with aniridia or related syndromes. A deleterious variation was evidenced in 17 sporadic cases (50%) and in 13 (72%) familial cases. Twenty-four different mutations, 17 of which are novel, were found. The spectrum of PAX6 mutations was highly homogeneous: 23 mutations (96%) leading to premature stop codons (eight nonsense and four splice site mutations, 11 insertions and deletions) and only one (4%) missense mutation. Twenty-two mutations were associated with aniridia phenotypes whereas two were associated with atypical phenotypes. These latter encompassed a missense mutation (R19P) in an individual with a microphthalmia-sclerocornea and a splice site mutation (IVS4+5G > C) in a family presenting with a congenital nystagmus. Both represented the most probably hypomorphic alleles. Aniridia cases were associated with nonsense or frameshifting mutations. A careful examination of the phenotypes did not make it possible to recognise significant differences whenever the predicted protein was deprived of one or another of its functional domains. This strongly suggested that most of the truncating mutations generated null alleles by nonsense mediated mRNA decay. Our observations support the concept of dosage effects of the PAX6 mutations as well as presenting evidence for variable expressivity.