An early-onset autosomal dominant macular dystrophy (MCDR3) resembling North Carolina macular dystrophy maps to chromosome 5

Pseudocoloboma-like maculopathy with biallelic RDH12 missense mutations

BACKGROUND

Hereditary maculopathy is a group of clinically and genetically heterogeneous disorders. With distinctive clinical features, subtypes of macular atrophy may correlate with their genetic defects.

METHODS

Seven patients from six families with adolescent/adult-onset maculopathy were examined in this clinical case series. A detailed medical history and eye examination were performed. Genomic DNA sequencing was performed using whole exome sequencing or direct sequencing of retinol dehydrogenase 12 (RDH12) coding exons.

RESULTS

Seven patients, including one male and six female patients, with pseudocoloboma-like maculopathy had biallelic missense RDH12 mutations. The most common mutant allele found in six of the seven patients was p.Ala269Gly. The average disease onset was at age 19.3 years, and visual acuity ranged from count fingers to 1.0. Most of the patients had mild myopic refraction. Common findings on fundus examination and spectral-domain optical coherence tomography include discrete margins of pseudocoloboma-like macular lesions with variable degrees of chorioretinal atrophy, excavation of retinal tissue and pigmentary changes mainly in the macular area. The electroretinograms were relatively normal to subnormal in all participants.

CONCLUSIONS

Progressive macular degeneration with a relatively normal peripheral retina and subsequent development of a pseudocoloboma-like appearance were the main clinical features in patients with compound heterozygous RDH12 missense mutations. Genetic testing may be crucial for early diagnosis and may play a key role in the development of future treatment strategies.

New Noncoding Base Pair Mutation at the Identical Locus as the Original NCMD/MCDR1 in a Mexican Family, Suggesting a Mutational Hotspot

Purpose

To clinically and molecularly study a newly found family with North Carolina macular dystrophy (NCMD/MCDR1) from Mexico.

Methods

This retrospective study comprised 6 members of a 3-generation Mexican family with NCMD. Clinical ophthalmic examinations, including fundus imaging, spectral-domain optical coherence tomography, electroretinography, and electrooculography, were performed. Genotyping with polymorphic markers in the MCDR1 region was performed to determine haplotypes. Whole-genome sequencing (WGS) was performed followed by variant filtering and copy number variant analysis.

Results

Four subjects from 3 generations were found to have macular abnormalities. The proband presented with lifelong bilateral vision impairment with bilaterally symmetric vitelliform Best disease-like appearing macular lesions. Her 2 children had bilateral large macular coloboma-like malformations, consistent with autosomal dominant NCMD. The 80-year-old mother of the proband had drusen-like lesions consistent with grade 1 NCMD. WGS and subsequent Sanger sequencing found a point mutation at chr6:99593030G>C (hg38) in the noncoding region of the DNase I site thought to be a regulatory element of the retinal transcription factor gene PRDM13. This mutation is the identical site/nucleotide as in the original NCMD family (#765) but is a guanine to cytosine change rather than a guanine to thymine mutation, as found in the original NCMD family.

Conclusions

We report a new noncoding mutation at the same locus (chr6:99593030G>C) involving the same DNase I site regulating the retinal transcription factor gene PRDM13. This suggests that this site, chr6:99593030, is a mutational hotspot.

Modern diagnostic and therapeutic approaches in familial maculopathy with reference to North Carolina macular dystrophy

AIMS

We present a familial hereditary macular dystrophy, resembling North Carolina Macular Dystrophy. In members of a family, we describe the development of diagnostic-therapeutic approaches and their impact on the prognosis of those whose vision was affected.

METHODS

The macular dystrophy of varying degrees of severity was diagnosed in 3 consecutive generations in different family members, both men and women. Modern therapeutic tools were used for the diagnostics. In one patient of the youngest generation, the development of secondary choroidal neovascularization (CNV) was identified and treated with an anti-VEGF (vascular endothelial growth factor) agent. DNA was isolated from venous blood and genome sequencing was performed in a proband.

RESULTS

We analysed the data of 13 members of one family of three consecutive generations. Six of them had macular dystrophy. The first were two of three siblings, a woman (73 years old) and a man (67). The offspring of the afflicted man, a female (36) and a male (80), had maculopathy. The first daughter of the woman (12) revealed findings of maculopathy but with normal electrical activity of the retina. The second girl (18), developed secondary CNV which responded well to intravitreal anti-VEGF treatment. Genetic analysis excluded mutations previously reported to be pathogenic for NCMD.

CONCLUSION

If there is a maculopathy of unclear etiology in younger patients or in patients with unclear development or appearance, it is advisable to focus carefully on the family history and trace the occurrence of impaired vision in other family members.

A novel tandem duplication of PRDM13 in a Chinese family with North Carolina macular dystrophy

PURPOSES

North Carolina macular dystrophy (NCMD) is a rare autosomal dominant inherited disorder characterized by macular impairment with a variety of phenotypic manifestations. The aims of this study were to assess the clinical features of a Chinese family with NCMD and to identify the underlying genetic cause of the disease.

METHODS

Three patients from a Chinese family were included in this study. Detailed ophthalmological examinations were performed, including best corrected visual acuity (BCVA), slit lamp, dilated indirect ophthalmoscopy, fundus photography, optical coherence tomography (OCT), fundus autofluorescence, full-field electroretinography (ERG), and electrooculography (EOG). Genomic DNA was extracted from peripheral blood samples. Whole-genome sequencing and long-read genome sequencing were applied to detect the pathogenic variants. Sanger sequencing was performed to confirm the breakpoints.

RESULTS

All three patients had macular involvement ranging from patchy yellowish-white lesions to big-area thinning, which are typical for NCMD. The BCVA ranged from 20/50 to 20/20. OCT revealed varying degrees of macular structure disorganization. The ERG responses were normal, and the Arden ration of the EOG was reduced. A novel 134.6 kb (g.99932464-100067110dup) tandem duplication on chromosome 6 (NC_000006.11) encompassing the entire CCNC and PRDM13 genes and a DNase 1 hypersensitivity site in the MCDR1 locus was identified.

CONCLUSION

A novel large tandem duplication in MCDR1 locus was confirmed in a Chinese family with NCMD with a variety of macular phenotypes.

North Carolina macular dystrophy shows a particular drusen phenotype and atrophy progression

BACKGROUND/AIM

To provide a comprehensive multimodal retinal imaging characterisation of patients with North Carolina macular dystrophy (NCMD).

METHODS

Clinical evaluation and retinal imaging in six families.

RESULTS

Twenty-one subjects showed phenotypic characteristics of NCMD . Small drusen-like deposits were found in all affected individuals, either tightly grouped in the macula, or surrounding atrophic or fibrotic macular alterations. These small subretinal lesions showed an increased fundus autofluorescence and were associated with only mild irregularities on optical coherence tomography imaging. Similar drusen-like deposits were regularly seen in the peripheral fundus, predominantly temporally and often with a radial distribution. Two patients showed a bilateral chorioretinal atrophy and two had a macular neovascularisation (MNV). Findings from follow-up examinations were available from 11 patients. The retinal phenotype remained overall stable, except for two patients: one patient with atrophy showed a distinct growth of the atrophic lesions on longitudinal AF imaging over a review period of 14 years. One patient with MNV showed a unilateral decline of best-corrected visual acuity. Genetic testing identified the single nucleotide variant chr6:100040987G>C upstream of the PRDM13 gene in all family members with NCMD phenotype.

CONCLUSION

Patients with NCMD show a characteristic retinal phenotype and distribution of drusen that differ from drusen in patients with age-related macular degeneration. Although the prognosis of this developmental condition is overall better than for other macular diseases with drusen, patients may be at risk of developing MNV or enlargement of pre-existing atrophy.

Multimodal imaging of benign yellow dot maculopathy

PURPOSE

To describe the clinical features of 2 unrelated families affected with Benign Yellow Dot Maculopathy and to analyze anatomical and functional findings of this peculiar phenotype Methods: Case series Results: We retrospectively described 5 patients (3 males, 2 females) affected with Benign Yellow Dot Maculopathy. The mean age at referral was 50,8 years (range 34-69 yrs.). All patients were characterized by a good visual acuity (20/20 in both eyes) and by symmetric multiple yellow dots at the posterior pole in both eyes. In 3 patients (P1, P3, P4) the yellow dots were mainly located at the nasal side of the macula. The yellow dots appeared hyper-autofluorescent at the fundus autofluorescence (FAF) imaging. OCT examination revealed in 3 patients (P1, P3, P4) mild irregularities at the level of the retinal pigment epithelium (RPE) and at the interdigitation (IZ) and ellipsoid zone (EZ). OCT angiography (OCT-A), performed in 3 patients (P1, P4,P5), was normal. Adaptive Optics imaging (AO) showed a peculiar pattern of the cone mosaic: the yellow dots were detectable as hyper-reflective lesions at the macular region. In 2 patients (P1, P4) we reported a follow-up of 2 and 18 years respectively. Genetic examination performed on patient P1 did not reveal pathogenic variants for retinal dystrophies.

CONCLUSIONS

Our work confirmed the benign nature of this peculiar macular phenotype showing a normal macular function and a stable clinical picture during a long-term follow-up. Multimodal imaging allows a detailed detection and monitoring of Benign Yellow Dot Maculopathy.

Multimodal Imaging and Functional Testing in a North Carolina Macular Disease Family: Toxoplasmosis, Fovea Plana, and Torpedo Maculopathy Are Phenocopies

PURPOSE

To describe multimodal imaging and corresponding functional studies in a newly found family with North Carolina macular dystrophy (NCMD). To our knowledge, this is an original report using OCT angiography to evaluate persons with NCMD.

DESIGN

A descriptive, retrospective study of a family with NCMD.

PARTICIPANTS

A total of 3 participants, representing 3 generations of a single family, each demonstrating a different grade of NCMD, underwent clinical and genetic testing.

METHODS

Diagnostic multimodal imaging and functional testing of the retina included color fundus photography, fundus autofluorescence, intravenous fluorescein angiography, spectral-domain OCT and OCT angiography, multifocal electroretinography, full-field electroretinography, and microperimetry. DNA sequencing was performed using Sanger sequencing techniques.

MAIN OUTCOME MEASURES

Spectral-domain OCT images, fundus photographs, fundus autofluorescence images, fluorescein angiograms, OCT angiograms, multifocal electroretinography images, full-field electroretinography images, and microperimetry maps. Sanger sequencing was performed for molecular diagnosis.

RESULTS

Multimodal imaging helped to demonstrate the nature of the retinal and choroidal lesions in each participant and the extent of visual function. Genetic testing demonstrated the variant 2 point mutation (chromosome 6: 99593111) in the deoxyribonuclease 1 hypersensitivity binding site on chromosome 6q16 causing overexpression of the retinal transcription factor PRDM13.

CONCLUSIONS

NCMD has great phenotypic variability, which can be appreciated only by examining multiple family members. To our knowledge, this is an original report that shows a correlation of functional studies with multimodal imaging. These findings are consistent with NCMD being a developmental abnormality of the macula. All layers of the retina and choroid demonstrate maldevelopment and varying degrees of malfunction. Although PRDM13 is expressed in the amacrine cells, we have yet to identify an abnormality specific to this cellular layer. The retinal vasculature appears to be surprisingly well preserved or intact by OCT angiogram compared with that shown in intravenous fluorescein angiograms. OCT angiograms suggest that foveal hypoplasia is a phenocopy of grade 1 NCMD, torpedo maculopathy is a phenocopy of grade 2 NCMD, and in this single family, congenital toxoplasmosis is a phenocopy of grade 3 NCMD.

Duplication events downstream of IRX1 cause North Carolina macular dystrophy at the MCDR3 locus

Autosomal dominant North Carolina macular dystrophy (NCMD) is believed to represent a failure of macular development. The disorder has been linked to two loci, MCDR1 (chromosome 6q16) and MCDR3 (chromosome 5p15-p13). Recently, non-coding variants upstream of PRDM13 (MCDR1) and a duplication including IRX1 (MCDR3) have been identified. However, the underlying disease-causing mechanism remains uncertain. Through a combination of sequencing studies on eighteen NCMD families, we report two novel overlapping duplications at the MCDR3 locus, in a gene desert downstream of IRX1 and upstream of ADAMTS16. One duplication of 43 kb was identified in nine families (with evidence for a shared ancestral haplotype), and another one of 45 kb was found in a single family. Three families carry the previously reported V2 variant (MCDR1), while five remain unsolved. The MCDR3 locus is thus refined to a shared region of 39 kb that contains DNAse hypersensitive sites active at a restricted time window during retinal development. Publicly available data confirmed expression of IRX1 and ADAMTS16 in human fetal retina, with IRX1 preferentially expressed in fetal macula. These findings represent a major advance in our understanding of the molecular genetics of NCMD and provide insights into the genetic pathways involved in human macular development.

Benign Yellow Dot Maculopathy: A New Macular Phenotype

PURPOSE

To describe a novel macular phenotype that is associated with normal visual function.

DESIGN

Retrospective, observational case series.

PARTICIPANTS

Thirty-six affected individuals from 23 unrelated families.

METHODS

This was a retrospective study of patients who had a characteristic macular phenotype. Subjects underwent a full ocular examination, electrophysiologic studies, spectral-domain optical coherence tomography (OCT), and fundus autofluorescence imaging. Genomic analyses were performed using haplotype sharing analysis and whole-exome sequencing.

MAIN OUTCOME MEASURES

Visual acuity, retinal features, electroretinography, and whole-exome sequencing.

RESULTS

Twenty-six of 36 subjects were female. The median age of subjects at presentation was 15 years (range, 5-59 years). The majority of subjects were asymptomatic and presented after a routine eye examination (22/36 subjects) or after screening because of a positive family history (13/36 subjects) or by another ophthalmologist (1/36 subjects). Of the 3 symptomatic subjects, 2 had reduced visual acuity secondary to nonorganic visual loss and bilateral ametropic amblyopia with strabismus. Visual acuity was 0.18 logarithm of the minimum angle of resolution (logMAR) or better in 30 of 33 subjects. Color vision was normal in all subjects tested, except for the subject with nonorganic visual loss. All subjects had bilateral symmetric multiple yellow dots at the macula. In the majority of subjects, these were evenly distributed throughout the fovea, but in 9 subjects they were concentrated in the nasal parafoveal area. The dots were hyperautofluorescent on fundus autofluorescence imaging. The OCT imaging was generally normal, but in 6 subjects subtle irregularities at the inner segment ellipsoid band were seen. Electrophysiologic studies identified normal macular function in 17 of 19 subjects and normal full-field retinal function in all subjects. Whole-exome analysis across 3 unrelated families found no pathogenic variants in known macular dystrophy genes. Haplotype sharing analysis in 1 family excluded linkage with the North Carolina macular dystrophy (MCDR1) locus.

CONCLUSIONS

A new retinal phenotype is described, which is characterized by bilateral multiple early-onset yellow dots at the macula. Visual function is normal, and the condition is nonprogressive. In familial cases, the phenotype seems to be inherited in an autosomal dominant manner, but a causative gene is yet to be ascertained.

Structural divergence of essential triad ribbon synapse proteins among placental mammals - Implications for preclinical trials in photoreceptor transplantation therapy

As photoreceptor transplantation rapidly moves closer to the clinic, verifying graft efficacy in animal models may have unforeseen xenogeneic barriers. Although photoreceptor transplants have most convincingly exhibited functional synaptogenesis in conspecific studies, such evidence (while ruling out false-positives due to: viral graft labeling, fusion/cytosolic transfer, or neuroprotection) has not yet been shown for discordant xenografts. From this, a fundamental question should be raised: is useful xenosynaptogenesis likely between human photoreceptors and mouse retina? The triad ribbon synapse (TRS) that would normally form is unique and contains trans-synaptic proteins essential to its formation and function. Thus, could interspecific structural divergence be present that may inhibit this trans-synaptic bridge in discordant xenografts? In an effort to address this question computationally, we compared eight recently confirmed (including subcellular location) TRS specific (or predominantly expressed at the TRS) proteins among placental mammals (1-to-1 orthologs) using HyPhy selection analysis (a predictive measure of structural divergence) and by using Phyre2 tertiary structural modeling. Here, selection analysis revealed strong positive (diversifying) selection acting on a particularly important TRS protein: pikachurin. This positive selection was localized to its second Laminin-G (LG)-like domain and on its N-terminal domain - a putative region of trans-synaptic interaction. Localization of structural divergence to the N-terminus of each putative post-translational cleavage (PTC) product may suggest neofunctionalization from ancestral uncleaved pikachurin - this would be consistent with a recent counter-paradigm report of pikachurin cleavage predominating at the TRS. From this, we suggest a dual role after cleavage where the N-terminal fragment can still mediate the trans-synaptic bridge, while the C-terminal fragment may act as a diffusible trophic or "homing" factor for bipolar cell dendrite migration. Tertiary structural models mirrored the conformational divergence predicted by selection analysis. With human and mouse pikachurin (as well as other TRS proteins) likely to diverge considerably in structure among placental mammals - alongside known inter-mammalian variation in TRS phenotype and protein repertoire, high levels of diversifying selection acting on genes involving sensation, considerable timespans allowing for genetic drift that can create xenogeneic epistasis, and uncertainty surrounding the extent of xenosynaptogenesis in PPC transplant studies to date - use of distantly related hosts to test human photoreceptor graft therapeutic efficacy should be considered with caution.

North Carolina Macular Dystrophy Is Caused by Dysregulation of the Retinal Transcription Factor PRDM13

PURPOSE

To identify specific mutations causing North Carolina macular dystrophy (NCMD).

DESIGN

Whole-genome sequencing coupled with reverse transcription polymerase chain reaction (RT-PCR) analysis of gene expression in human retinal cells.

PARTICIPANTS

A total of 141 members of 12 families with NCMD and 261 unrelated control individuals.

METHODS

Genome sequencing was performed on 8 affected individuals from 3 families affected with chromosome 6-linked NCMD (MCDR1) and 2 individuals affected with chromosome 5-linked NCMD (MCDR3). Variants observed in the MCDR1 locus with frequencies <1% in published databases were confirmed using Sanger sequencing. Confirmed variants absent from all published databases were sought in 8 additional MCDR1 families and 261 controls. The RT-PCR analysis of selected genes was performed in stem cell-derived human retinal cells.

MAIN OUTCOME MEASURES

Co-segregation of rare genetic variants with disease phenotype.

RESULTS

Five sequenced individuals with MCDR1-linked NCMD shared a haplotype of 14 rare variants spanning 1 Mb of the disease-causing allele. One of these variants (V1) was absent from all published databases and all 261 controls, but was found in 5 additional NCMD kindreds. This variant lies in a DNase 1 hypersensitivity site (DHS) upstream of both the PRDM13 and CCNC genes. Sanger sequencing of 1 kb centered on V1 was performed in the remaining 4 NCMD probands, and 2 additional novel single nucleotide variants (V2 in 3 families and V3 in 1 family) were identified in the DHS within 134 bp of the location of V1. A complete duplication of the PRDM13 gene was also discovered in a single family (V4). The RT-PCR analysis of PRDM13 expression in developing retinal cells revealed marked developmental regulation. Next-generation sequencing of 2 individuals with MCDR3-linked NCMD revealed a 900-kb duplication that included the entire IRX1 gene (V5). The 5 mutations V1 to V5 segregated perfectly in the 102 affected and 39 unaffected members of the 12 NCMD families.

CONCLUSIONS

We identified 5 rare mutations, each capable of arresting human macular development. Four of these strongly implicate the involvement of PRDM13 in macular development, whereas the pathophysiologic mechanism of the fifth remains unknown but may involve the developmental dysregulation of IRX1.

Causes and consequences of inherited cone disorders

Hereditary cone disorders (CDs) are characterized by defects of the cone photoreceptors or retinal pigment epithelium underlying the macula, and include achromatopsia (ACHM), cone dystrophy (COD), cone-rod dystrophy (CRD), color vision impairment, Stargardt disease (STGD) and other maculopathies. Forty-two genes have been implicated in non-syndromic inherited CDs. Mutations in the 5 genes implicated in ACHM explain ∼93% of the cases. On the contrary, only 21% of CRDs (17 genes) and 25% of CODs (8 genes) have been elucidated. The fact that the large majority of COD and CRD-associated genes are yet to be discovered hints towards the existence of unknown cone-specific or cone-sensitive processes. The ACHM-associated genes encode proteins that fulfill crucial roles in the cone phototransduction cascade, which is the most frequently compromised (10 genes) process in CDs. Another 7 CD-associated proteins are required for transport processes towards or through the connecting cilium. The remaining CD-associated proteins are involved in cell membrane morphogenesis and maintenance, synaptic transduction, and the retinoid cycle. Further novel genes are likely to be identified in the near future by combining large-scale DNA sequencing and transcriptomics technologies. For 31 of 42 CD-associated genes, mammalian models are available, 14 of which have successfully been used for gene augmentation studies. However, gene augmentation for CDs should ideally be developed in large mammalian models with cone-rich areas, which are currently available for only 11 CD genes. Future research will aim to elucidate the remaining causative genes, identify the molecular mechanisms of CD, and develop novel therapies aimed at preventing vision loss in individuals with CD in the future.

Genetic factors in nonsmokers with age-related macular degeneration revealed through genome-wide gene-environment interaction analysis

Relatively little is known about the interaction between genes and environment in the complex etiology of age-related macular degeneration (AMD). This study aimed to identify novel factors associated with AMD by analyzing gene-smoking interactions in a genome-wide association study of 1207 AMD cases and 686 controls of Caucasian background with genotype data on 668,238 single nucleotide polymorphisms (SNPs) after quality control. Participants' history of smoking at least 100 cigarettes lifetime was determined by a self-administered questionnaire. SNP associations modeled the effect of the minor allele additively on AMD using logistic regression, with adjustment for age, sex, and ever/never smoking. Joint effects of SNPs and smoking were examined comparing a null model containing only age, sex, and smoking against an extended model including genotypic and interaction terms. Genome-wide significant main effects were detected at three known AMD loci: CFH (P = 7.51×10(-30) ), ARMS2 (P = 1.94×10(-23) ), and RDBP/CFB/C2 (P = 4.37×10(-10) ), while joint effects analysis revealed three genomic regions with P < 10(-5) . Analyses stratified by smoking found genetic associations largely restricted to nonsmokers, with one notable exception: the chromosome 18q22.1 intergenic SNP rs17073641 (between SERPINB8 and CDH7), more strongly associated in nonsmokers (OR = 0.57, P = 2.73 × 10(-5) ), with an inverse association among smokers (OR = 1.42, P = 0.00228), suggesting that smoking modifies the effect of some genetic polymorphisms on AMD risk.

Comparison of an expanded ataxia interactome with patient medical records reveals a relationship between macular degeneration and ataxia

Spinocerebellar ataxias 6 and 7 (SCA6 and SCA7) are neurodegenerative disorders caused by expansion of CAG repeats encoding polyglutamine (polyQ) tracts in CACNA1A, the alpha1A subunit of the P/Q-type calcium channel, and ataxin-7 (ATXN7), a component of a chromatin-remodeling complex, respectively. We hypothesized that finding new protein partners for ATXN7 and CACNA1A would provide insight into the biology of their respective diseases and their relationship to other ataxia-causing proteins. We identified 118 protein interactions for CACNA1A and ATXN7 linking them to other ataxia-causing proteins and the ataxia network. To begin to understand the biological relevance of these protein interactions within the ataxia network, we used OMIM to identify diseases associated with the expanded ataxia network. We then used Medicare patient records to determine if any of these diseases co-occur with hereditary ataxia. We found that patients with ataxia are at 3.03-fold greater risk of these diseases than Medicare patients overall. One of the diseases comorbid with ataxia is macular degeneration (MD). The ataxia network is significantly (P= 7.37 × 10⁻⁵) enriched for proteins that interact with known MD-causing proteins, forming a MD subnetwork. We found that at least two of the proteins in the MD subnetwork have altered expression in the retina of Ataxin-7^266Q/+ mice suggesting an in vivo functional relationship with ATXN7. Together these data reveal novel protein interactions and suggest potential pathways that can contribute to the pathophysiology of ataxia, MD, and diseases comorbid with ataxia.

Childhood macular dystrophies

PURPOSE OF REVIEW

The aim of this review is to highlight recent advances in our understanding of the molecular genetic basis and phenotype of childhood onset macular dystrophies and to summarize current attempts to develop novel therapies for this group of disorders.

RECENT FINDINGS

The genes associated with the major causes of childhood onset macular dystrophies have now been identified and current research efforts have been focused on understanding the function of the encoded protein, how the mutant protein leads to photoreceptor cell death and investigation of the range of retinal phenotypes that result from mutations in these genes. Assessment of the phenotype has been greatly helped by improvements in retinal imaging such as spectral domain optical coherence tomography and fundus autofluorescence imaging. The development of animal models has, despite their limitations, helped understanding of disease mechanisms and allowed assessment of new therapeutic approaches such as gene replacement therapy and pharmacological treatments.

SUMMARY

Molecular diagnosis and improvements in retinal imaging have greatly improved the accuracy of diagnosis in paediatric macular disease and allowed better genetic counselling and information about prognosis to be given to children and their families. Advances in basic understanding of disease mechanism will lead to the development of clinical trials of novel therapies in the near future.

A genome-wide linkage scan for diabetic retinopathy susceptibility genes in Mexican Americans with type 2 diabetes from Starr County, Texas

We conducted a genome-wide linkage scan for genes contributing to retinopathy risk using 794 diabetes case subjects from 393 Mexican-American families from Starr County, Texas, having at least two diabetic siblings. The sample included 567 retinopathy case subjects comprising 282 affected sibling pairs. Retinopathy was classified as none, early nonproliferative, moderate-to-severe nonproliferative, or proliferative. Using 360 polymorphic markers (average spacing 9.4 cM), we conducted nonparametric linkage analysis followed by ordered-subset analysis (OSA) ranking families by average age of diabetes diagnosis. For any retinopathy, the highest LOD scores including all families were on chromosomes 3 (2.41 at 117 cM) and 12 (2.47 at 15.5). OSA logarithm of odds (LOD) scores >2 for any retinopathy occurred on chromosomes 12 (4.47 at 13.2 cM), 15 (3.65 at 100.6), and 20 (2.67 at 54.1). Scores >2 for either moderate-to-severe nonproliferative or proliferative retinopathy occurred on chromosomes 5 (2.53 at 11.2 cM), 6 (2.28 at 30.6), and 19 (2.21 at 100.6). Thus, unconditional linkage analysis revealed suggestive evidence of linkage with retinopathy on two chromosomes, whereas OSA revealed strong evidence of linkage on two chromosomes, and suggestive evidence on four. Candidate genes were identified in most implicated regions.

A novel locus on 19q13 associated with autosomal-dominant macular dystrophy in a large Greek family

OBJECTIVE

To describe the clinical features of and genetic locus associated with autosomal-dominant macular dystrophy (MCDR5) in a large Greek family.

METHODS

26 members of a single family underwent clinical examinations and venepuncture. A genomewide linkage scan using 400 microsatellite markers distributed with an average spacing of 10 cM throughout the human genome.

RESULTS

14 members of the study family exhibited clinical features of the disease including decreased central vision and macular abnormalities in the posterior pole of the retina. Analysis of loci known to be associated with macular dystrophy did not show positive linkage. A genomewide linkage scan showed linkage to chromosome 19q, with a two-point maximum LOD score of 5.809 at theta = 0 between the disease and marker locus D19S412. On the basis of recombination events, the disease interval was localised between markers D19S420 and D19S540 on chromosome 19q, at a span of about 3.8 cM, in an area known to contain 120 known genes/transcripts. Eleven of these genes/transcripts were sequenced, and no disease-causing mutation was identified.

CONCLUSIONS

This study describes a new locus on 19q associated with autosomal-dominant macular dystrophy, designated as MCDR5. Additional study of other family members will be necessary to further narrow the interval and identify the responsible gene. The study of MCDR5 will aid in elucidation of the underlying pathogenic mechanisms for this and other macular diseases, including age-related macular degeneration.

The genetics of inherited macular dystrophies

The inherited macular dystrophies comprise a heterogeneous group of disorders characterised by central visual loss and atrophy of the macula and underlying retinal pigment epithelium (RPE). The different forms of macular degeneration encompass a wide range of clinical, psychophysical and histological findings. The complexity of the molecular basis of monogenic macular disease is now beginning to be elucidated with the identification of many of the disease-causing genes. Age related macular degeneration (ARMD), the leading cause of blind registration in the developed world, may also have a significant genetic component to its aetiology. Genes implicated in monogenic macular dystrophies are good candidate susceptibility genes for ARMD, although to date, with the possible exception of ABCA4, none of these genes have been shown to confer increased risk of ARMD. The aim of this paper is to review current knowledge relating to the monogenic macular dystrophies, with discussion of currently mapped genes, chromosomal loci and genotype-phenotype relationships. Inherited systemic disorders with a macular dystrophy component will not be discussed.