Genetic mapping of a cone and rod dysfunction (Aland Island eye disease) to the proximal short arm of the human X chromosome

Genetic architecture of natural variation in visual senescence in Drosophila

Senescence, i.e., functional decline with age, is a major determinant of health span in a rapidly aging population, but the genetic basis of interindividual variation in senescence remains largely unknown. Visual decline and age-related eye disorders are common manifestations of senescence, but disentangling age-dependent visual decline in human populations is challenging due to inability to control genetic background and variation in histories of environmental exposures. We assessed the genetic basis of natural variation in visual senescence by measuring age-dependent decline in phototaxis using Drosophila melanogaster as a genetic model system. We quantified phototaxis at 1, 2, and 4 wk of age in the sequenced, inbred lines of the Drosophila melanogaster Genetic Reference Panel (DGRP) and found an average decline in phototaxis with age. We observed significant genetic variation for phototaxis at each age and significant genetic variation in senescence of phototaxis that is only partly correlated with phototaxis. Genome-wide association analyses in the DGRP and a DGRP-derived outbred, advanced intercross population identified candidate genes and genetic networks associated with eye and nervous system development and function, including seven genes with human orthologs previously associated with eye diseases. Ninety percent of candidate genes were functionally validated with targeted RNAi-mediated suppression of gene expression. Absence of candidate genes previously implicated with longevity indicates physiological systems may undergo senescence independent of organismal life span. Furthermore, we show that genes that shape early developmental processes also contribute to senescence, demonstrating that senescence is part of a genetic continuum that acts throughout the life span.

Assessment of night vision problems in patients with congenital stationary night blindness

Congenital Stationary Night Blindness (CSNB) is a retinal disorder caused by a signal transmission defect between photoreceptors and bipolar cells. CSNB can be subdivided in CSNB2 (rod signal transmission reduced) and CSNB1 (rod signal transmission absent). The present study is the first in which night vision problems are assessed in CSNB patients in a systematic way, with the purpose of improving rehabilitation for these patients. We assessed the night vision problems of 13 CSNB2 patients and 9 CSNB1 patients by means of a questionnaire on low luminance situations. We furthermore investigated their dark adapted visual functions by the Goldmann Weekers dark adaptation curve, a dark adapted static visual field, and a two-dimensional version of the “Light Lab”. In the latter test, a digital image of a living room with objects was projected on a screen. While increasing the luminance of the image, we asked the patients to report on detection and recognition of objects. The questionnaire showed that the CSNB2 patients hardly experienced any night vision problems, while all CSNB1 patients experienced some problems although they generally did not describe them as severe. The three scotopic tests showed minimally to moderately decreased dark adapted visual functions in the CSNB2 patients, with differences between patients. In contrast, the dark adapted visual functions of the CSNB1 patients were more severely affected, but showed almost no differences between patients. The results from the “2D Light Lab” showed that all CSNB1 patients were blind at low intensities (equal to starlight), but quickly regained vision at higher intensities (full moonlight). Just above their dark adapted thresholds both CSNB1 and CSNB2 patients had normal visual fields. From the results we conclude that night vision problems in CSNB, in contrast to what the name suggests, are not conspicuous and generally not disabling.

The negative ERG: clinical phenotypes and disease mechanisms of inner retinal dysfunction

Inner retinal dysfunction is encountered in a number of retinal disorders, either inherited or acquired, as a primary or predominant defect. Fundus examination is rarely diagnostic in these disorders, although some show characteristic features, and careful electrophysiological assessment of retinal function is needed for accurate diagnosis. The ERG in inner retinal dysfunction typically shows a negative waveform with a preserved a-wave and a selectively reduced b-wave. Advances in retinal physiology and molecular genetics have led to a greater understanding of the pathogenesis of these disorders. This review summarizes current knowledge on normal retinal physiology, the investigative techniques used and the range of clinical disorders in which there is predominantly inner retinal dysfunction.

Distance refractive error among Aboriginal people attending eye clinics in remote South Australia

PURPOSE

To determine the prevalence of distance refractive error among Aboriginal people attending eye clinics in remote South Australia.

METHODS

A clinic-based cross-sectional study was conducted that involved opportunistic sampling of Aboriginal people attending eye clinics in remote South Australia. There were 189 individuals who were invited to participate in the study all of whom underwent ophthalmic examination. This examination included measurement of pinhole-corrected visual acuity and non-cycloplegic autorefraction.

RESULTS

Automated refractive error examinations were performed on 148 people within this sample. The mean age was 44.8 +/- 14.5 years and women comprised 57.7% of the sample. The overall mean refractive error was -0.01 +/- 1.8 D (SD). The prevalence of myopia (spherical equivalent (SE) < -0.5 D), high myopia (SE less than or equal to -6.0 D), hypermetropia (SE > 0.5 D), astigmatism (cylinder at least -0.5 D) and anisometropia (difference in SE of >0.5 D) was 31.1%, 0.7%, 33.1%, 55.8% and 45.9%, respectively. Further analyses revealed significant age-related trends with both myopia and hypermetropia. There were no gender associations with any form of refractive error. Of those people with clinically significant refractive error, 51/148 (34%), only four people owned distance spectacles.

CONCLUSIONS

There continues to be a level of uncorrected distance refractive error within these patients. This represents a need to screen for refractive error among Aboriginal people in remote locations and to provide them with appropriate spectacle correction.

The NDUFB11 gene is not a modifier in Leber hereditary optic neuropathy

Over 95% of Leber hereditary optic neuropathy (LHON) cases are due to mutations in mitochondrial DNA-encoded subunits of NADH:ubiquinone oxidoreductase (E.C.1.6.5.3., complex I). A recessive X-linked susceptibility gene that acts synergistically with the primary mtDNA mutation to produce visual loss is suggested by the high male-to-female ratio among LHON patients. The ESSS protein is a recently isolated subunit of bovine heart mitochondrial complex I. We revisited the genomic sequence of NDUFB11, the human homolog mapping to chromosome Xp11.23, and identified two mRNA isoforms showing different expression profiles in human tissues. Cultured skin fibroblasts from four LHON patients showed a pattern of expression similar to normal controls. Moreover, NDUFB11 did not seem to influence risk and age at onset of visual loss in a total of 65 individuals from 35 Italian LHON families. Also, the gene was not affected in 11 children with a severe encephalopathy associated with decreased complex I activity in skeletal muscle.

Thirty distinct CACNA1F mutations in 33 families with incomplete type of XLCSNB and Cacna1f expression profiling in mouse retina

X-linked CSNB patients may exhibit myopia, nystagmus, strabismus and ERG abnormalities of the Schubert-Bornschein type. We recently identified the retina-specific L-type calcium channel alpha1 subunit gene CACNA1F localised to the Xp11.23 region, which is mutated in families showing the incomplete type (CSNB2). Here, we report comprehensive mutation analyses in the 48 CACNA1F exons in 36 families, most of them from Germany. All families were initially diagnosed as having the incomplete type of CSNB, except for two which have been designated as Aland Island eye disease (AIED)-like. Out of 33 families, a total of 30 different mutations were identified, of which 24 appear to be unique for the German population. The mutations, 20 of which are published here for the first time, were found to be equally distributed over the entire gene sequence. No mutation could be found in a classic AIED family previously shown to map to the CSNB2 interval. Cacna1f expression in photoreceptor-negative mice strains indicate that the gene is expressed in the outer nuclear, the inner nuclear, and the ganglion cell layer. Such a distribution points to the central role of calcium regulation in the interaction of retinal cells that mediate signal transmission.

High heritability of myopia does not preclude rapid changes in prevalence

The aetiology of myopia is complex, but the major form of myopia results from an interplay between genetic factors and environmental influences. Although there are clear patterns of family inheritance and high heritability values in studies from all over the world,environmental factors are increasingly important in determining myopic outcomes in East Asia, and perhaps elsewhere. This is not paradoxical, as high heritability does not preclude strong environmental influences. The lower heritability values obtained from parent-offspring correlations in populations of East Asian origin where there are marked differences in the environmental exposures (education and urbanization) of parents and children, and where there has been a major shift in the population distribution of myopia, are consistent with this view. Despite the impact of environmental pressures in East Asia, there is evidence that parental myopia influences the position of their offspring within that new population distribution.

An integrated, functionally annotated gene map of the DXS8026-ELK1 interval on human Xp11.3-Xp11.23: potential hotspot for neurogenetic disorders

Human chromosome Xp11.3-Xp11.23 encompasses the map location for a growing number of diseases with a genetic basis or genetic component. These include several eye disorders, syndromic and nonsyndromic forms of X-linked mental retardation (XLMR), X-linked neuromuscular diseases and susceptibility loci for schizophrenia, type 1 diabetes, and Graves' disease. We have constructed an approximately 2.7-Mb high-resolution physical map extending from DXS8026 to ELK1, corresponding to a genetic distance of approximately 5.5 cM. A combination of chromosome walking and sequence-tagged site (STS)-content mapping resulted in an integrated framework and transcript map, precisely positioning 10 polymorphic microsatellites (one of which is novel), 16 ESTs, and 12 known genes (RP2, PCTK1, UHX1, UBE1, RBM10, ZNF157, SYN1, ARAF1, TIMP1, PFC, ELK1, UXT). The composite map is currently anchored with 89 STSs to give an average resolution of approximately 1 STS every 30 kb. By a combination of EST database searches and in silico detection of UniGene clusters within genomic sequence generated from this template map, we have mapped several novel genes within this interval: a Na+/H+ exchanger (SLC9A7), at least two zincfinger transcription factors (KIAA0215 and Hs.68318), carbohydrate sulfotransferase-7 (CHST7), regucalcin (RGN), inactivation-escape-1 (INE1), the human ortholog of mouse neuronal protein 15.6, and four putative novel genes. Further genomic analysis enabled annotation of the sequence interval with 20 predicted pseudogenes and 21 UniGene clusters of unknown function. The combined PAC/BAC transcript map and YAC scaffold presented here clarifies previously conflicting data for markers and genes within the Xp11.3-Xp11.23 interval and provides a powerful integrated resource for functional characterization of this clonally unstable, yet gene-rich and clinically significant region of proximal Xp.

Physical map covering a 2 Mb region in human xp11.3 distal to DX6849

A 2Mb contig was constructed of yeast artificial chromosomes (YACs) and P1 artificial chromosomes (PACs), extending from DXS6849 to a new marker EC7034R, 1Mb distal to UBE1, within the p11.3 region of the human X chromosome. This contig, which has on average four-fold cloned coverage, was assembled using 37 markers, including 13 new sequence tagged sites (STSs) developed from YAC and PAC end-fragments, for an average inter-marker distance of 55kb. The inferred marker order predicted from SEGMAP analysis, STS content and cell hybrid data is Xpter-EC7034R-EC8058R-FB20E11-DXS7804-D XS8308-(DXS1264, DXS1055)-DXS1003-UBE1-(UHX), PCTK1)-DXS1364-DXS1266-DXS337-SYN1-DXS6 849-cen. One (TC)n dinucleotide sequence from an end-clone was identified and found to be polymorphic (48% heterozygosity). The contig is merged with published physical maps both in the distal and in the centromeric direction of Xp, and provides reagents to aid in the DNA sequencing and the finding of genes in this region of the human genome.

Molecular genetics of human retinal dystrophies

Retinal dystrophies are a heterogeneous group of diseases in which the retina degenerates, leading to either partial or complete blindness. The severe and clearly hereditary forms, retinitis pigmentosa (RP) and various macular degenerations, affect approximately 1 in 3000 people, but many more suffer from aging macular dystrophy in later life. Patients with RP present with narrowing visual fields and night blindness, while those with diseases of the macula lose central vision first. Even before the advent of molecular genetics it was evident that these were heterogeneous disorders, with wide variation in severity, mode of inheritance and phenotype. However, with the widespread application of linkage analysis and mutation detection techniques, a complex underlying pathology has now been revealed. In total, 66 distinct non-overlapping genes or gene loci have been implicated in the various forms of retinal dystrophy, with more being reported regularly in the literature. Within the category of non-syndromic RP alone there are at least 22 genes (and probably many more) involved, with further allelic heterogeneity arising from different mutations in the same gene. This complexity presents a problem for those involved in counselling patients, and also compounds the search for therapies. Nevertheless, several lines of research raise the hope of generic treatments applicable to all such patients, while the greater understanding of normal visual function that arises from genetic studies may open up new avenues for therapy.

Evidence for genetic heterogeneity in X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder characterized by disturbed or absent night vision; its clinical features may also include myopia, nystagmus, and impaired visual acuity. X-linked CSNB is clinically heterogeneous, and it may also be genetically heterogeneous. We have studied 32 families with X-linked CSNB, including 11 families with the complete form of CSNB and 21 families with the incomplete form of CSNB, to identify genetic-recombination events that would refine the location of the disease genes. Critical recombination events in the set of families with complete CSNB have localized a disease gene to the region between DXS556 and DXS8083, in Xp11.4-p11.3. Critical recombination events in the set of families with incomplete CSNB have localized a disease gene to the region between DXS722 and DXS8023, in Xp11.23. Further analysis of the incomplete-CSNB families, by means of disease-associated-haplotype construction, identified 17 families, of apparent Mennonite ancestry, that share portions of an ancestral chromosome. Results of this analysis refined the location of the gene for incomplete CSNB to the region between DXS722 and DXS255, a distance of 1.2 Mb. Genetic and clinical analyses of this set of 32 families with X-linked CSNB, together with the family studies reported in the literature, strongly suggest that two loci, one for complete (CSNB1) and one for incomplete (CSNB2) X-linked CSNB, can account for all reported mapping information.

Summary of ocular genetic disorders and inherited systemic conditions with eye findings

Of the close to 10,000 known inherited disorders that affect humankind, a disproportionately high number affect the eye. The total number of genes responsible for the normal structure, function, and differentiation of the eye is unknown, but the list of these genes is rapidly and constantly growing. The objective of this paper is to provide a current list of mapped and/or cloned human eye genes that are responsible for inherited diseases of the eye. The ophthalmologist should be aware of recent advances in molecular technology which have resulted in significant progress towards the identification of these genes. The implications of this new knowledge will be discussed herein.

Long-range map of a 3.5-Mb region in Xp11.23-22 with a sequence-ready map from a 1.1-Mb gene-rich interval

Most of the yeast artificial chromosomes (YACs) isolated from the Xp11.23-22 region have shown instability and chimerism and are not a reliable resource for determining physical distances. We therefore constructed a long-range pulsed-field gel electrophoresis map that encompasses approximately 3.5 Mb of genomic DNA between the loci TIMP and DXS146 including a CpG-rich region around the WASP and TFE-3 gene loci. A combined YAC-cosmid contig was constructed along the genomic map and was used for fine-mapping of 15 polymorphic microsatellites and 30 expressed sequence tags (ESTs) or sequence transcribed sites (STSs), revealing the following order: tel-(SYN-TIMP)-(DXS426-ELK1)-ZNF(CA) n-L1-DXS1367-ZNF81-ZNF21-DXS6616- (HB3-OATL1pseudogenes-DXS6950)-DXS6949-DXS694 1-DXS7464E(MG61)-GW1E(EBP)- DXS7927E(MG81)-RBM- DXS722-DXS7467E(MG21)-DXS1011E-WASP-DXS6940++ +-DXS7466E(MG44)-GF1- DXS226-DXS1126-DXS1240-HB1- DXS7469E-(DXS6665-DXS1470)-TFE3-DXS7468E-+ ++SYP-DXS1208-HB2E-DXS573-DXS1331- DXS6666-DXS1039-DXS 1426-DXS1416-DXS7647-DXS8222-DXS6850-DXS255++ +-CIC-5-DXS146-cen. A sequence-ready map was constructed for an 1100-kb gene-rich interval flanked by the markers HB3 and DXS1039, from which six novel ESTs/STSs were isolated, thus increasing the number of markers used in this interval to thirty. This precise ordering is a prerequisite for the construction of a transcription map of this region that contains numerous disease loci, including those for several forms of retinal degeneration and mental retardation. In addition, the map provides the base to delineate the corresponding syntenic region in the mouse, where the mutants scurfy and tattered are localized.

Conclusive evidence for a distinct congenital stationary night blindness locus in Xp21.1

X linked congenital stationary night blindness (CSNBX) is a non-progressive retinal disorder characterised by decreased visual acuity and disturbance of night vision. CSNBX appears to be not only clinically but also genetically heterogeneous. On studying a single large family, we recently suggested the presence of a distinct locus for CSNBX in Xp21.1. Here, we describe the results of a linkage analysis in another large CSNBX family, which confirms this finding. Thus, the data presented here provide conclusive evidence for a distinct CSNBX locus in Xp21.1, closely linked to the X linked retinitis pigmentosa type 3 gene. The results combined with other published results indicate the order Xpter-DXS451-DMD-DYS1-(DXS1110, CSNBX1, XLRP3)-DXS7-(CSNBX2, XLRP2)-DXS14-Xcen.

Aplasia cutis congenita, high myopia, and cone-rod dysfunction in two sibs: a new autosomal recessive disorder

We report on a brother and a sister with congenital nystagmus, cone-rod dysfunction, high myopia, and aplasia cutis congenita on the midline of the scalp vertex. To our knowledge this familial oculocutaneous condition, transmitted as an autosomal recessive trait, has not been reported previously.

Duchenne muscular dystrophy: negative electroretinograms and normal dark adaptation. Reappraisal of assignment of X linked incomplete congenital stationary night blindness

Aland Island eye disease (AIED) and X linked congenital stationary night blindness (CSNB) have been mapped to Xp11.3. Patients have been described with deletions of the Duchenne muscular dystrophy (DMD) gene who also had a negative electroretinogram (ERG) similar to that seen in patients with CSNB and AIED. This seems to confirm that some cases of AIED and CSNB map to Xp21. We examined 16 boys with DMD/BMD (Becker muscular dystrophy) of whom 10 had negative ERGs, eight of them having deletions downstream from exon 44. Normal dark adaptation thresholds were observed in all patients and there were no anomalous visual functions. Hence, CSNB cannot be assigned to Xp21 and negative ERG in DMD/BMD is not associated with eye disease. Six boys with DMD/BMD had normal ERGs. We speculate that a retinal or glial dystrophin may be truncated or absent in the boys with negative ERGs.

Aland island eye disease: clinical and electrophysiological studies of a Welsh family

Clinical and molecular genetic studies were performed on a single, large, white family, in which congenital nystagmus and moderate to high refractive error segregated as a sex linked trait with manifestation in some female carriers. In this family, affected males demonstrate myopia, but a high proportion of female carriers, and some of the possibly affected males, show hypermetropia. Clinical ophthalmic examination and electrodiagnostic studies of retinal function were fully compatible with a diagnosis of either incomplete congenital stationary night blindness or of Aland island eye disease. Previous studies have mapped both disorders to the proximal short arm of the X chromosome: our molecular studies support this localisation. Incomplete congenital stationary nightblindness and Aland Island eye disease could be considered as a single entity.

Identification of a key recombinant which assigns the incomplete congenital stationary night blindness gene proximal to MAOB

The gene for complete congenital stationary night blindness (CSNB1) has been assigned to the Xp11.3 region. However, little evidence has been provided for the assignment of the incomplete congenital stationary night blindness gene (CSNB2). Here we present the clinical and molecular data from a CSNB2 family which show a key recombinant assigning the CSNB2 gene proximal to MAOB.