Retinal Dystrophies Associated With Peripherin-2: Genetic Spectrum and Novel Clinical Observations in 241 Patients

Purpose

To describe the clinical, electrophysiological and genetic spectrum of inherited retinal diseases associated with variants in the PRPH2 gene.

Methods

A total of 241 patients from 168 families across 15 sites in 9 countries with pathogenic or likely pathogenic variants in PRPH2 were included. Records were reviewed for age at symptom onset, visual acuity, full-field ERG, fundus colour photography, fundus autofluorescence (FAF), and SD-OCT. Images were graded into six phenotypes. Statistical analyses were performed to determine genotype-phenotype correlations.

Results

The median age at symptom onset was 40 years (range, 4-78 years). FAF phenotypes included normal (5%), butterfly pattern dystrophy, or vitelliform macular dystrophy (11%), central areolar choroidal dystrophy (28%), pseudo-Stargardt pattern dystrophy (41%), and retinitis pigmentosa (25%). Symptom onset was earlier in retinitis pigmentosa as compared with pseudo-Stargardt pattern dystrophy (34 vs 44 years; P = 0.004). The median visual acuity was 0.18 logMAR (interquartile range, 0-0.54 logMAR) and 0.18 logMAR (interquartile range 0-0.42 logMAR) in the right and left eyes, respectively. ERG showed a significantly reduced amplitude across all components (P < 0.001) and a peak time delay in the light-adapted 30-Hz flicker and single-flash b-wave (P < 0.001). Twenty-two variants were novel. The central areolar choroidal dystrophy phenotype was associated with 13 missense variants. The remaining variants showed marked phenotypic variability.

Conclusions

We described six distinct FAF phenotypes associated with variants in the PRPH2 gene. One FAF phenotype may have multiple ERG phenotypes, demonstrating a discordance between structure and function. Given the vast spectrum of PRPH2 disease our findings are useful for future clinical trials.

Rapid Variant Pathogenicity Analysis by CRISPR Activation of CRB1 Gene Expression in Patient-Derived Fibroblasts

Inherited retinal diseases (IRDs) are a heterogeneous group of blinding genetic disorders caused by pathogenic variants in genes expressed in the retina. In this study, we sought to develop a method for rapid evaluation of IRD gene variant pathogenicity by inducing expression of retinal genes in patient-derived fibroblasts using CRISPR-activation (CRISPRa). We demonstrate CRISPRa of CRB1 expression in fibroblasts derived from patients with retinitis pigmentosa, enabling investigation of pathogenic mechanisms associated with specific variants. We show the CRB1 c.4005 + 1G>A variant caused exon 11 skipping in CRISPR-activated fibroblasts and retinal organoids (ROs) derived from the same RP12 patient. The c.652 + 5G>C variant was shown to enhance exon 2 skipping in CRISPR-activated fibroblasts and differentially affected CRB1 isoform expression in fibroblasts and ROs. Our study demonstrates an accessible platform for transcript screening of IRD gene variants in patient-derived fibroblasts, which can potentially be applied for rapid pathogenicity assessments of any gene variant.

Towards Uncovering the Role of Incomplete Penetrance in Maculopathies through Sequencing of 105 Disease-Associated Genes

Inherited macular dystrophies (iMDs) are a group of genetic disorders, which affect the central region of the retina. To investigate the genetic basis of iMDs, we used single-molecule Molecular Inversion Probes to sequence 105 maculopathy-associated genes in 1352 patients diagnosed with iMDs. Within this cohort, 39.8% of patients were considered genetically explained by 460 different variants in 49 distinct genes of which 73 were novel variants, with some affecting splicing. The top five most frequent causative genes were ABCA4 (37.2%), PRPH2 (6.7%), CDHR1 (6.1%), PROM1 (4.3%) and RP1L1 (3.1%). Interestingly, variants with incomplete penetrance were revealed in almost one-third of patients considered solved (28.1%), and therefore, a proportion of patients may not be explained solely by the variants reported. This includes eight previously reported variants with incomplete penetrance in addition to CDHR1:c.783G>A and CNGB3:c.1208G>A. Notably, segregation analysis was not routinely performed for variant phasing-a limitation, which may also impact the overall diagnostic yield. The relatively high proportion of probands without any putative causal variant (60.2%) highlights the need to explore variants with incomplete penetrance, the potential modifiers of disease and the genetic overlap between iMDs and age-related macular degeneration. Our results provide valuable insights into the genetic landscape of iMDs and warrant future exploration to determine the involvement of other maculopathy genes.

Analysis of the Outer Retinal Bands in ABCA4 and PRPH2-Associated Retinopathy using OCT

PURPOSE

To evaluate the outer retinal bands using OCT in ABCA4- and PRPH2-associated retinopathy and develop a novel imaging biomarker to differentiate between these 2 genotypes.

DESIGN

Multicenter case-control study.

PARTICIPANTS

Patients with a clinical and genetic diagnosis of ABCA4- or PRPH2-associated retinopathy and an age-matched control group.

METHODS

Macular OCT was used to measure the thickness of the outer retinal bands 2 and 4 by 2 independent examiners at 4 retinal loci.

MAIN OUTCOME MEASURES

Outcome measures included the thicknesses of band 2, band 4, and the band 2/band 4 ratio. Linear mixed modeling was used to make comparisons across the 3 groups. Receiver operating characteristic (ROC) analysis determined the optimal cutoff for the band 2/band 4 ratio to distinguish PRPH2- from ABCA4-associated retinopathy.

RESULTS

We included 45 patients with ABCA4 variants, 45 patients with PRPH2 variants, and 45 healthy controls. Band 2 was significantly thicker in patients with PRPH2 compared with ABCA4 (21.4 vs. 15.9 μm, P < 0.001) variants, whereas band 4 was thicker in patients with ABCA4 variants than those with PRPH2 variants (27.5 vs. 21.7 μm, P < 0.001). Similarly, the band 2/band 4 ratio was significantly different (1.0 vs. 0.6 for PRPH2 vs. ABCA4, P < 0.001). The area under the ROC curve was 0.87 for either band 2 (> 18.58 μm) or band 4 (< 26.17 μm) alone and 0.99 (95% confidence interval: 0.97-0.99) for the band 2/band 4 ratio with a cutoff threshold of 0.79, providing 100% specificity.

CONCLUSIONS

We report an altered outer retinal band profile whereby the band 2/band 4 ratio was able to discriminate between PRPH2- and ABCA4-associated retinopathy. This may have future clinic utility in predicting the genotype and provide further insight into the anatomic correlate of band 2.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Driving with retinitis pigmentosa

BACKGROUND

To establish the proportion of patients with retinitis pigmentosa (RP) meeting the Australian fitness to drive (FTD) visual standards.

METHODOLOGY

A prospective consecutive case series of patients with a clinical or genetic diagnosis of RP. Data on age at symptom onset, current driving status, inheritance pattern, better eye visual acuity (BEVA), binocular Esterman visual field (BEVF) parameters, genotype and ability to meet the driving standards based on BEVA and BEVF were collected. Outcome measures included the proportion of RP patients overall meeting the standards and clinical predictors for passing. A sub-analysis was performed on those RP patients who reported to drive. Change in BEVA and BEVF parameters across age in specific genotype groups was assessed.

RESULTS

Overall, 228 patients with RP had a BEVF assessment. Only 39% (89/228) met the driving standards. Younger age at the time of testing was the only significant predictor (p < 0.01) for passing. Of the 55% of RP patients who reported to drive, 52% (65/125) met the standards, decreasing to 14% in the 56- to 65-year-old age group. RP patients harbouring mutations in HK1 or RHO genes may have slower rates of decline in their VF parameters.

CONCLUSION

Nearly 40% of RP patients met the driving standards. However, almost 50% of RP drivers were unaware of their failure to meet the current standards. BEVF testing is essential in the assessment of RP patients who are still driving. Phenotype and genotype predictors for passing the standards warrant further investigation.Abbreviation: FTD, fitness to drive; IRD, inherited retinal disease; RP, retinitis pigmentosa; RHO, rhodopsin; HK1, hexokinase 1; PRPF31 pre-mRNA processing factor 31; RPGR, retinitis pigmentosa GTPase regulator; VF, visual field; BEVA, better eye visual acuity; BEVF, binocular Esterman visual field.

Microperimetry and Adaptive Optics Imaging Reveal Localized Functional and Structural Changes in Asymptomatic RPGR Mutation Carriers

Purpose

Female carriers of RPGR mutations demonstrate no significant retinal dysfunction or structural change despite a characteristic tapetal-like reflex. In this study, we examined localized changes of pointwise sensitivity (PWS) and cone density (CD) using microperimetry (MP) and adaptive optics (AO) imaging in female carriers of RPGR mutations.

Methods

In this cross-sectional case-control study, MP (MAIA, 10-2 test grid) and AO imaging (rtx1) were performed in female carriers of RPGR mutations and unrelated age-matched healthy controls. PWS at 68 loci located 1 degree to 9 degrees away from the preferred retinal locus and CD at 12 loci located 1 degree to 3 degrees away from the foveal center were measured. Severity of defect was defined by standard deviation (SD) from age-matched healthy control means: normal (<1 SD from normal average), moderate defect (1-2 SD from normal average), and severe defect (>2 SD from normal average).

Results

Twelve patients from seven unrelated families were enrolled. Seven patients were asymptomatic, 5 of whom had visual acuity 20/20 or better in both eyes. PWS and CD were available in 12 and 8 patients, respectively. Severe PWS and CD defect in at least 1 test location was observed in 10 of 12 patients and 7 of 8 patients, respectively. Among the five asymptomatic patients who had normal visual acuity, severe PWS and CD defects were observed in three of five and four of five patients, respectively.

Conclusions

MP and AO imaging revealed early functional and structural changes in asymptomatic RPGR mutation carriers and should be considered in clinical assessment of these patients.

Longitudinal Analysis of Functional and Structural Outcome Measures in PRPH2-Associated Retinal Dystrophy

PURPOSE

To establish disease progression rates in total lesion size (TLS), decreased autofluorescence (DAF) area, total macular volume (TMV), and mean macular sensitivity (MMS) in PRPH2-associated retinal dystrophy.

DESIGN

Single-center, retrospective chart review.

PARTICIPANTS

Patients with heterozygous pathogenic or likely pathogenic PRPH2 variants.

METHODS

Patients who underwent serial ultrawide-field (UWF) fundus autofluorescence (FAF), OCT, and Macular Integrity Assessment microperimetry with at least 1 year of follow-up were included. Linear correlation was performed in eyes of all patients to determine the rate of change over time.

MAIN OUTCOME MEASURES

Outcome measures included changes in TLS, DAF area, TMV, and MMS.

RESULTS

Twelve patients (mean age, 55) from 10 unrelated families attended 100 clinic visits, which spanned over a mean (SD) of 4.7 (2.0) years. Mean (SD) TLS and DAF radius expansion were 0.14 (0.12) and 0.10 (0.08) mm/year, respectively. Mean (SD) TMV change was -0.071 (0.040) mm³/year with no interocular difference (P = 0.20) and strong interocular correlation (r² = 0.88, P < 0.01). Mean (SD) MMS change was -0.10 (1.25) dB/year. Mean macular sensitivity declined in 4 and improved in 6 patients. Mean macular sensitivity was subnormal despite a TMV within the normal range.

CONCLUSIONS

Serial measurements of UWF-FAF-derived TLS and DAF showed slow expansion. Total macular volume might be a more sensitive measure than MMS in detecting disease progression.

Characterising splicing defects of ABCA4 variants within exons 13-50 in patient-derived fibroblasts

The ATP-binding cassette subfamily A member 4 gene (ABCA4)-associated retinopathy, Stargardt disease, is the most common monogenic inherited retinal disease. Given the pathogenicity of numerous ABCA4 variants is yet to be examined and a significant proportion (more than 15%) of ABCA4 variants are categorized as splice variants in silico, we therefore established a fibroblast-based splice assay to analyze ABCA4 variants in an Australian Stargardt disease cohort and characterize the pathogenic mechanisms of ABCA4 variants. A cohort of 67 patients clinically diagnosed with Stargardt disease was recruited. Genomic DNA was analysed using a commercial panel for ABCA4 variant detection and the consequences of ABCA4 variants were predicted in silico. Dermal fibroblasts were propagated from skin biopsies, total RNA was extracted and the ABCA4 transcript was amplified by RT-PCR. Our analysis identified a total of 67 unique alleles carrying 74 unique variants. The most prevalent splice-affecting complex allele c.[5461-10T>C; 5603A>T] was carried by 10% of patients in a compound heterozygous state. ABCA4 transcripts from exon 13 to exon 50 were readily detected in fibroblasts. In this region, aberrant splicing was evident in 10 out of 57 variant transcripts (18%), carried by 19 patients (28%). Patient-derived fibroblasts provide a feasible platform for identification of ABCA4 splice variants located within exons 13-50. Experimental evidence of aberrant splicing contributes to the pathogenic classification for ABCA4 variants. Moreover, identification of variants that affect splicing processes provides opportunities for intervention, in particular antisense oligonucleotide-mediated splice correction.

SIBLING CONCORDANCE IN SYMPTOM ONSET AND ATROPHY GROWTH RATES IN STARGARDT DISEASE USING ULTRA-WIDEFIELD FUNDUS AUTOFLUORESCENCE

PURPOSE

To investigate concordance in symptom onset, area of dark autofluorescence (DAF), and growth rate (GR) between Stargardt disease siblings at an age-matched time point.

METHODS

In this retrospective longitudinal study of sibling pairs with identical biallelic ABCA4 variants, age at symptom onset, best-corrected visual acuity, atrophy area, and effective radius of DAF on ultra-widefield fundus autofluorescence were recorded. Absolute intersibling differences for both eyes were compared with absolute interocular differences using the Mann-Whitney test.

RESULTS

Overall 39 patients from 19 families were recruited. In 16 families, age-matched best-corrected visual acuity and DAF were compared between siblings. In 8 families, DAF GR was compared. The median (range) absolute difference in age at symptom onset between siblings was 3 (0-35) years. Absolute intersibling differences in age-matched best-corrected visual acuity were greater than interocular differences ( P = 0.01). Similarly, absolute intersibling differences in DAF area and radius were greater than interocular differences ( P = 0.04 for area and P = 0.001 for radius). Differences between absolute interocular and intersibling GR were not statistically significant ( P = 0.44 for area GR and P = 0.61 for radius GR).

CONCLUSION

There was significant discordance in age-matched best-corrected visual acuity and DAF beyond the expected limits of interocular asymmetry. Lack of significant intersibling differences in GR warrants further investigation.

Genotype-Specific Lesion Growth Rates in Stargardt Disease

Reported growth rates (GR) of atrophic lesions in Stargardt disease (STGD1) vary widely. In the present study, we report the longitudinal natural history of patients with confirmed biallelic ABCA4 mutations from five genotype groups: c.6079C>T, c.[2588G>C;5603A>T], c.3113C>T, c.5882G>A and c.5603A>T. Fundus autofluorescence (AF) 30° × 30° images were manually segmented for boundaries of definitely decreased autofluorescence (DDAF). The primary outcome was the effective radius GR across five genotype groups. The age of DDAF formation in each eye was calculated using the x-intercept of the DDAF effective radius against age. Discordance between age at DDAF formation and symptom onset was compared. A total of 75 eyes from 39 STGD1 patients (17 male [44%]; mean ± SD age 45 ± 19 years; range 21-86) were recruited. Patients with c.3113C>T or c.6079C>T had a significantly faster effective radius GR at 0.17 mm/year (95% CI 0.12 to 0.22; p < 0.001 and 0.14 to 0.21; p < 0.001) respectively, as compared to those patients harbouring c.5882G>A at 0.06 mm/year (95% CI 0.03-0.09), respectively. Future clinical trial design should consider the effect of genotype on the effective radius GR and the timing of DDAF formation relative to symptom onset.

CLASSIFYING ABCA4 MUTATION SEVERITY USING AGE-DEPENDENT ULTRA-WIDEFIELD FUNDUS AUTOFLUORESCENCE-DERIVED TOTAL LESION SIZE

PURPOSE

To establish a mutation-specific age-dependent ultra-widefield fundus autofluorescence (UWF-FAF) trajectory in a large Stargardt disease (STGD1) cohort using total lesion size (TLS) and to develop a clinical method for variant classification.

METHODS

A retrospective study of patients with biallelic ABCA4 mutations that were evaluated with UWF-FAF. Boundaries of TLS, defined by stippled hyper/hypoautofluorescence, were outlined manually. Pathogenicity was assessed according to ACMG/AMP criteria, and mutation severities were classified based on the current literature. Age-dependent trajectories in TLS were examined in patients with nullizygous, mild, and intermediate mutations. Mutations of uncertain severities were classified using a clinical criterion based on age of symptom onset and TLS.

RESULTS

Eighty-one patients with STGD1 (mean age = 42 ± 20 years and mean visual acuity = 20/200) were recruited from 65 unrelated families. Patients with biallelic null/severe variants (n = 6) demonstrated an increase in TLS during their second decade reaching a mean ± SD of 796 ± 29 mm2 by age 40. Those harboring mild mutations c.5882G>A or c.5603A>T had lesions confined to the posterior pole with a mean ± SD TLS of 30 ± 39 mm2. Intermediate mutations c.6079C>T or c.[2588G>C;5603A>T] in trans with a null/severe mutation had a mean ± SD TLS of 397 ± 29 mm2. Thirty-two mutations were predicted to cause severe (n = 22), intermediate (n = 6), and mild (n = 5) impairment of ABCA4 function based on age of symptom onset and TLS.

CONCLUSION

Age-dependent TLS showed unique ABCA4 mutation-specific trajectories. Our novel clinical criterion using age of symptom onset and TLS to segregate ABCA4 mutations into three severity groups requires further molecular studies to confirm its validity.

Multimodal Retinal Imaging and Microperimetry Reveal a Novel Phenotype and Potential Trial End Points in CRB1-Associated Retinopathies

Purpose

Biallelic crumbs cell polarity complex component 1 (CRB1) mutations can present as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP), or cystic maculopathy. This study reports a novel phenotype of asymptomatic fenestrated slit maculopathy (AFSM) and examines macular volume profile and microperimetry as clinical trial end points in CRB1-associated retinopathies.

Methods

Twelve patients from nine families with CRB1 mutation were recruited. Ultra-widefield (UWF) color fundus photography and autofluorescence (AF), spectral-domain optical coherence tomography (SD-OCT), microperimetry, and adaptive optics (AO) imaging were performed. Macular volume profiles were compared with age-matched healthy controls. Genotyping was performed using APEX genotyping microarrays, targeted next-generation sequencing, and Sanger sequencing.

Results

We identified one patient with LCA, five patients with RP, and four patients with macular dystrophy (MD) with biallelic CRB1 mutations. Two siblings with compound heterozygote genotype (c.[2843G>A]; [498_506del]) had AFSM characterized by localized outer retinal disruption on SD-OCT and parafoveal cone loss on AO imaging despite normal fundus appearance, visual acuity, and foveal sensitivity. UWF AF demonstrated preserved para-arteriolar retinal pigment epithelium (PPRPE) in all patients with RP. Microperimetry documented preserved central retinal function in six patients. The ratio of perifoveal-to-foveal retinal volume was greater than controls in 89% (8/9) of patients with RP or MD, whereas central subfield and total macular volume were outside normal limits in 67% (6/9).

Conclusions

AO imaging was helpful in detecting parafoveal cone loss in asymptomatic patients. Macular volume profile and microperimetry parameters may have utility as CRB1 trials end points.

Translational Relevance

Macular volume and sensitivity can be used as structural and functional end points for trials on CRB1-associated RP and MD.

Generation of an induced pluripotent stem cell line from a patient with Stargardt disease caused by biallelic c.[5461-10T>C;5603A>T];[6077T>C] mutations in the ABCA4 gene

Mutations in ABCA4 gene are causative for autosomal recessive Stargardt disease (STGD1), the most common inherited retinal dystrophy. Here, we report the generation of an induced pluripotent stem cell (iPSC) line from a STGD1 patient carrying biallelic c.[5461-10T>C;5603A>T];[6077T>C] mutations in the ABCA4 gene. Episomes carrying OCT4, SOX2, KLF4, L-MYC, LIN28 and mp53DD were employed for the reprogramming of patient-derived fibroblasts. This iPSC line expressed comparable pluripotency markers as in a commercially available human iPSC line, displayed normal karyotype and potential for trilineage differentiation, and were negative for both reprogramming episomes and mycoplasma test.

Generation of two induced pluripotent stem cell lines from a patient with Stargardt disease caused by compound heterozygous mutations in the ABCA4 gene

Stargardt disease (STGD1) is the most common inherited retinal dystrophy and ABCA4 c.546--10 T>C is the most commonly reported splice mutation. Here, we generated and characterized two induced pluripotent stem cell (iPSC) lines from a STGD1 patient with compound heterozygous mutations in ABCA4 (c.[5461-10 T > C;5603A > T];[4163 T > C;455G > A]). Episomal vectors containing OCT4, SOX2, KLF4, L-MYC, LIN28 and mp53DD were employed to conduct the reprogramming of patient-derived fibroblasts. Both lines had a normal karyotype, displayed iPSC morphology, expressed pluripotency markers and showed trilineage differentiation potential. These lines can provide a powerful platform for further investigating the pathophysiological consequences of mutations in ABCA4.

Clinical Evidence for the Importance of the Wild-Type PRPF31 Allele in the Phenotypic Expression of RP11

PRPF31-associated retinopathy (RP11) is a common form of autosomal dominant retinitis pigmentosa (adRP) that exhibits wide variation in phenotype ranging from non-penetrance to early-onset RP. Herein, we report inter-familial and intra-familial variation in the natural history of RP11 using multimodal imaging and microperimetry. Patients were recruited prospectively. The age of symptom onset, best-corrected visual acuity, microperimetry mean sensitivity (MS), residual ellipsoid zone span and hyperautofluorescent ring area were recorded. Genotyping was performed using targeted next-generation and Sanger sequencing and copy number variant analysis. PRPF31 mutations were found in 14 individuals from seven unrelated families. Four disease patterns were observed: (A) childhood onset with rapid progression (N = 4), (B) adult-onset with rapid progression (N = 4), (C) adult-onset with slow progression (N = 4) and (D) non-penetrance (N = 2). Four different patterns were observed in a family harbouring c.267del; patterns B, C and D were observed in a family with c.772_773delins16 and patterns A, B and C were observed in 3 unrelated individuals with large deletions. Our findings suggest that the RP11 phenotype may be related to the wild-type PRPF31 allele rather than the type of mutation. Further studies that correlate in vitro wild-type PRPF31 allele expression level with the disease patterns are required to investigate this association.

Generation of two induced pluripotent stem cell lines from a retinitis pigmentosa patient with compound heterozygous mutations in CRB1

Two human iPSC lines were generated from dermal fibroblasts derived from a patient with retinitis pigmentosa caused by CRB1 mutation using episomal plasmids containing OCT4, SOX2, LIN28, KLF4, L-MYC and mp53DD. These clonal iPSC lines carry compound heterozygous mutations in CRB1 (c.2555 T > C and c.3014A > T). Both lines expressed pluripotency markers, displayed a normal karyotype and demonstrated the ability to differentiate into the three primary germ layers, as well as retinal organoids.

Atrophy Expansion Rates in Stargardt Disease Using Ultra-Widefield Fundus Autofluorescence

Purpose

Design

Participants

Methods

Main Outcome Measures

Results

Conclusions

Deep clinical phenotyping and gene expression analysis in a patient with RCBTB1-associated retinopathy

Background: Mutations in the RCC1 and BTB domain-containing protein 1 (RCBTB1) gene have been implicated in a rare form of retinal dystrophy. Herein, we report the clinical features of a 45-year-old Singaporean-Chinese female and her presymptomatic sibling, who each possesses compound heterozygous mutations in RCBTB1. Expression of RCBTB1 in patient-derived cells was evaluated.Materials and Methods: The natural history was documented by a series of ophthalmic examinations including electroretinography, fundus autofluorescence imaging, spectral-domain optical coherence tomography, visual field, microperimetry, and adaptive optics retinal imaging. Patient DNA was genetically analysed using a 537-gene Next Generation Sequencing panel and targeted Sanger sequencing. Expression of RCBTB1 in lymphocytes, fibroblasts, and induced pluripotent stem cells (iPSC) derived from the proband and healthy controls was characterized by quantitative PCR, Sanger sequencing, and western blotting.Results: The proband presented with left visual distortion at age 40 due to extrafoveal chorioretinal atrophy. Atrophy expanded at 1.3 (OD) and 1.0 (OS) mm²/year. Total macular volume declined by 0.09 (OD) and 0.13 (OS) mm³/year. Microperimetry demonstrated enlarging scotoma in both eyes. Generalised cone dysfunction was demonstrated by electroretinography. A retinal dystrophy panel testing revealed biallelic frameshifting mutations, c.170delG (p.Gly57Glufs*12) and c.707delA (p.Asn236Thrfs*11) in RCBTB1. The level of RCBTB1 mRNA expression was reduced in patient-derived lymphocytes compared to controls. RCBTB1 protein was detected in control fibroblasts and iPSC but was absent in patient-derived cells.Conclusions: Atrophy expansion rate and macular volume change are feasible endpoints for monitoring RCBTB1-associated retinopathy. We provide further functional evidence of pathogenicity for two disease-causing variants using patient-derived iPSCs.

Association of Sex With Frequent and Mild ABCA4 Alleles in Stargardt Disease

Importance

The mechanisms behind the phenotypic variability and reduced penetrance in autosomal recessive Stargardt disease (STGD1), often a blinding disease, are poorly understood. Identification of the unknown disease modifiers can improve patient and family counseling and provide valuable information for disease management.

Objective

To assess the association of incompletely penetrant ABCA4 alleles with sex in STGD1.

Design, Setting, and Participants

Genetic data for this cross-sectional study were obtained from 2 multicenter genetic studies of 1162 patients with clinically suspected STGD1. Unrelated patients with genetically confirmed STGD1 were selected. The data were collected from June 2016 to June 2019, and post hoc analysis was performed between July 2019 and January 2020.

Main Outcomes and Measures

Penetrance of reported mild ABCA4 variants was calculated by comparing the allele frequencies in the general population (obtained from the Genome Aggregation Database) with the genotyping data in the patient population (obtained from the ABCA4 Leiden Open Variation Database). The sex ratio among patients with and patients without an ABCA4 allele with incomplete penetrance was assessed.

Results

A total of 550 patients were included in the study, among which the mean (SD) age was 45.7 (18.0) years and most patients were women (311 [57%]). Five of the 5 mild ABCA4 alleles, including c.5603A>T and c.5882G>A, were calculated to have incomplete penetrance. The women to men ratio in the subgroup carrying c.5603A>T was 1.7 to 1; the proportion of women in this group was higher compared with the subgroup not carrying a mild allele (difference, 13%; 95% CI, 3%-23%; P = .02). The women to men ratio in the c.5882G>A subgroup was 2.1 to 1, and the women were overrepresented compared with the group carrying no mild allele (difference, 18%; 95% CI, 6%-30%; P = .005).

Conclusions and Relevance

This study found an imbalance in observed sex ratio among patients harboring a mild ABCA4 allele, which concerns approximately 25% of all patients with STGD1, suggesting that STGD1 should be considered a polygenic or multifactorial disease rather than a disease caused by ABCA4 gene mutations alone. The findings suggest that sex should be considered as a potential disease-modifying variable in both basic research and clinical trials on STGD1.

A novel phenotype in a family with autosomal dominant retinal dystrophy due to c.1430A > G in retinoid isomerohydrolase (RPE65) and c.37C > T in bestrophin 1 (BEST1)

PURPOSE

The c.1430A > G (Asp477Gly) variant in RPE65 has been reported in Irish and Scottish families with either an autosomal dominant retinal dystrophy (adRD) that resembles choroideremia, a vitelliform macular dystrophy or an isolated macular atrophy. We report novel features on multimodal imaging and the natural history of a family harbouring this variant in combination with the BEST1 c.37C > T (Arg13Cys) variant.

METHODS

Members of a family with an adRD were examined clinically to ascertain phenotype and underwent genetic testing. Multimodal imaging included widefield colour fundus photography, quantitative autofluorescence (qAF) and spectral domain optical coherence tomography. Electrophysiology and microperimetry were also performed.

RESULTS

Vision loss was attributed to foveal atrophy in the proband and choroidal neovascularisation and a vitello-eruptive lesion in one affected son. Peripheral retinal white dots corresponding to subretinal deposits were seen in three patients. The median qAF₈ values in the proband (I:1) were low (40 and 101 in OD and OS) at age 79. Similarly, the qAF₈ values for the middle son (II:2) were also low (100 and 87 in ODS and OS) at age 60. Electrophysiology showed disproportionate reduction in Arden ratio prior to the gradual loss of full-field responses. Microperimetry demonstrated an enlarging scotoma in the proband.

CONCLUSIONS

The coexistence of the pathogenic BEST1 c.37C > T variant may modify clinical features observed in RPE65 adRD. This study expands our understanding of RPE65 adRD as a retinoid cycle disorder supported by the reduced qAF, fine white retinal dots and corresponding subretinal deposits on OCT in affected members.

Gene correction of the CLN3 c.175G>A variant in patient-derived induced pluripotent stem cells prevents pathological changes in retinal organoids

Background

Mutations in CLN3 cause Batten disease, however non‐syndromic CLN3 disease, characterized by retinal‐specific degeneration, has been also described. Here, we characterized an induced pluripotent stem cell (iPSC)‐derived disease model derived from a patient with non‐syndromic CLN3‐associated retinopathy.

Methods

Patient‐iPSC, carrying the 1 kb‐deletion and c.175G>A variants in CLN3, coisogenic iPSC, in which the c.175G>A variant was corrected, and control iPSC were differentiated into neural retinal organoids (NRO) and cardiomyocytes. CLN3 transcripts were analyzed by Sanger sequencing. Gene expression was characterized by qPCR and western blotting. NRO were characterized by immunostaining and electron microscopy.

Results

Novel CLN3 transcripts were detected in adult human retina and control‐NRO. The major transcript detected in patient‐NRO displayed skipping of exons 2 and 4–9. Accumulation of subunit‐C of mitochondrial ATPase (SCMAS) protein was demonstrated in patient‐derived cells. Photoreceptor progenitor cells in patient‐NRO displayed accumulation of peroxisomes and vacuolization of inner segments. Correction of the c.175G>A variant restored CLN3 mRNA and protein expression and prevented SCMAS and inner segment vacuolization.

Conclusion

Our results demonstrate the expression of novel CLN3 transcripts in human retinal tissues. The c.175G>A variant alters splicing of the CLN3 pre‐mRNA, leading to features consistent with CLN3 deficiency, which were prevented by gene correction.

Generation of two induced pluripotent stem cell lines from a patient with recessive inherited retinal disease caused by compound heterozygous mutations in SNRNP200

The human induced pluripotent stem cell (iPSC) lines LEIi015-A and LEIi015-B were derived from a patient with inherited retinal disease caused by compound heterozygous mutations in the SNRNP200 gene (c.[1792C>T];[3341T>C]). Dermal fibroblasts were transfected with episomal plasmids carrying transgenes encoding OCT4, SOX2, KLF4, L-MYC, LIN28, mir302/367 microRNA and shRNA for P53. The clonal iPSC lines LEIi015-A and LEIi015-B expressed iPSC markers, were free from genomic alterations and demonstrated trilineage differentiation potential.

Expanding the genetic spectrum of choroideremia in an Australian cohort: report of five novel CHM variants

Choroideremia is an X-linked chorioretinal dystrophy caused by mutations in the CHM gene. Several CHM gene replacement clinical trials are in advanced stages. In this study, we report the molecular confirmation of choroideremia in 14 Australian families sourced from the Australian Inherited Retinal Disease Registry and DNA Bank. Sixteen males (14 symptomatic) and 18 females (4 symptomatic; 14 obligate carriers) were identified for analysis. Participants' DNA was analyzed for disease-causing CHM variants by Sanger sequencing, TaqMan qPCR and targeted NGS. We report phenotypic and genotypic data for the 14 symptomatic males and four females manifesting disease symptoms. A pathogenic or likely pathogenic CHM variant was detected in all families. Eight variants were previously reported, and five were novel. Two de novo variants were identified. We previously reported the molecular confirmation of choroideremia in 11 Australian families. This study expands the CHM genetically confirmed Australian cohort to 32 males and four affected carrier females.

Progressive sector retinitis pigmentosa due to c.440G>T mutation in SAG in an Australian family

BACKGROUND

Heterozygous c.440 G > T mutation in the S-antigen visual arrestin (SAG) gene has been described as a cause of autosomal dominant retinitis pigmentosa (adRP) in a series of patients of Hispanic origin. This study presents the early and late clinical features and disease progression rates in an Australian family with SAG adRP.

MATERIALS AND METHODS

An observational case series of four family members with adRP. They were examined clinically, with multi-modal retinal imaging and electroretinography (ERG) to ascertain phenotype. Disease progression rate was measured using optical coherence tomography (OCT) and fundus autofluorescence (FAF). A retinal dystrophy panel was used for the proband and cascade testing with targeted Sanger sequencing was conducted in other available family members.

RESULTS

The proband presented at 36 years of age with profoundly reduced full-field ERG responses despite a sector RP phenotype. This progressed to a classic RP pattern over several decades leaving a small residual island of central visual field. The horizontal span of the residual outer nuclear layer and the area of hyperautofluorescent ring contracted at a rate of 8-11% and 9-14% per year, respectively. DNA sequencing confirmed the segregation of SAG c.440 G > T mutation with disease.

CONCLUSION

SAG adRP presents with a reduced full-field ERG response consistent with a rod-cone dystrophy in mid-life despite a sector RP phenotype. Centripetal progression of the disease into the macula can be tracked by OCT and FAF imaging.

Deep learning segmentation of hyperautofluorescent fleck lesions in Stargardt disease

Stargardt disease is one of the most common forms of inherited retinal disease and leads to permanent vision loss. A diagnostic feature of the disease is retinal flecks, which appear hyperautofluorescent in fundus autofluorescence (FAF) imaging. The size and number of these flecks increase with disease progression. Manual segmentation of flecks allows monitoring of disease, but is time-consuming. Herein, we have developed and validated a deep learning approach for segmenting these Stargardt flecks (1750 training and 100 validation FAF patches from 37 eyes with Stargardt disease). Testing was done in 10 separate Stargardt FAF images and we observed a good overall agreement between manual and deep learning in both fleck count and fleck area. Longitudinal data were available in both eyes from 6 patients (average total follow-up time 4.2 years), with both manual and deep learning segmentation performed on all (n = 82) images. Both methods detected a similar upward trend in fleck number and area over time. In conclusion, we demonstrated the feasibility of utilizing deep learning to segment and quantify FAF lesions, laying the foundation for future studies using fleck parameters as a trial endpoint.

Exploring microperimetry and autofluorescence endpoints for monitoring disease progression in PRPF31-associated retinopathy

BACKGROUND

Mutations in the splicing factor pre-messenger RNA processing factor 31 (PRPF31) gene cause autosomal dominant retinitis pigmentosa 11 (RP11) through a haplo-insufficiency mechanism. We describe the phenotype and progression of microperimetry and autofluorescence endpoints in an Indigenous Australian RP11 family.

PATIENTS AND METHODS

Ophthalmic examination, optical coherence tomography, fundus autofluorescence and microperimetry were performed at baseline and every 6-12 months. Baseline and annual change in best-corrected visual acuity (BCVA), microperimetry mean sensitivity (MS) and number of scotoma loci, residual ellipsoid zone (EZ) span and hyperautofluorescent ring (HAR) area were reported. Next-generation and Sanger sequencing were performed in available members.

RESULTS

12 affected members from three generations were examined. Mean (SD, range) age at onset of symptoms was 11 (4.5, 4-19) years. MS declined steadily from the third decade and EZ span and HAR area declined rapidly during the second decade. Serial microperimetry showed negligible change in MS over 2-3 years. However, mean EZ span, near-infrared and short-wavelength HAR area reduction was 203 (6.4%) µm/year, 1.8 (8.7%) mm²/year and 1.1 (8.6%) mm²/year, respectively. Genetic testing was performed on 11 affected and 10 asymptomatic members and PRPF31 c.1205 C > A (p.Ser402Ter) mutation was detected in all affected and two asymptomatic members (non-penetrant carriers).

CONCLUSIONS

Our findings suggest that in the studied cohort, the optimal window for therapeutic intervention is the second decade of life and residual EZ span and HAR area can be considered as efficacy outcome measures. Further studies on larger samples with different PRPF31 mutations and longer follow-up duration are recommended.

Characterization of CRB1 splicing in retinal organoids derived from a patient with adult-onset rod-cone dystrophy caused by the c.1892A>G and c.2548G>A variants

BACKGROUND

Mutations in the human crumbs homologue 1 (CRB1) gene are associated with a spectrum of inherited retinal diseases. However, functional studies demonstrating the impact of individual CRB1 mutations on gene expression are lacking for most variants. Here, we investigated the effect of two CRB1 variants on pre-mRNA splicing using neural retinal organoids (NRO) derived from a patient with recessive rod-cone dystrophy caused by compound heterozygous mutations in CRB1 (c.1892A>G and c.2548G>A).

METHODS

The patient received ophthalmological examinations including multimodal imaging. NRO were differentiated from induced pluripotent stem cells (iPSCs) derived from the patient and a control subject. CRB1 transcripts were characterized by RT-PCR and Sanger sequencing.

RESULTS

The Patient displayed retinal thickening with disorganization of retinal layers and preservation of para-arteriolar retinal pigment epithelium. Both patient and control iPSC produced NRO containing photoreceptor progenitor cells expressing CRB1 mRNA. Patient NRO expressed a novel CRB1 transcript displaying skipping of exon 6. CRB1 transcripts containing the c.2548G>A substitution in exon 7 were expressed in patient NRO.

CONCLUSIONS

Together, these results confirm the pathogenicity of the c.1892A>G and c.2548G>A CRB1 variants in a family with recessive adult-onset rod-cone dystrophy and further demonstrate the effects of these variants on pre-mRNA splicing. This data provide important insights into the pathogenic mechanisms associated with these variants.

Edge of Scotoma Sensitivity as a Microperimetry Clinical Trial End Point in USH2A Retinopathy

Purpose

Microperimetry is commonly used to assess retinal function. We perform cross-sectional and longitudinal analysis on microperimetry parameters in USH2A retinopathy and explore end points suitable for future clinical trials.

Methods

Microperimetry was performed using two grids, Grid 1 (18° diameter) and Grid 2 (6° diameter). In Grid 1, four parameters (number of nonscotomatous loci, mean sensitivity [MS], responding point sensitivity [RPS], and edge of scotoma sensitivity [ESS]) were analyzed. In Grid 2, number of nonscotomatous loci and MS were examined. Interocular symmetry was also examined. Longitudinal analysis was conducted in a subset of eyes.

Results

Microperimetry could be performed in 16 of 21 patients. In Grid 1 (n = 15; average age, 35.6 years), average number of nonscotomatous loci, MS, RPS, and ESS were 46.6 loci, 10.0 dB, 14.7 and 9.6 dB, respectively. In Grid 2 (n = 13; average age, 37.4 years), 12 eyes had measurable sensitivity across the entire grid. Average MS was 23.8 dB. Interocular analysis revealed large 95% confidence intervals for all parameters. Longitudinally, Grid 1 (n = 12, average follow-up 2.6 years) ESS showed the fastest rate of decline (–1.84 dB/y) compared with MS (–0.34 dB/y) and RPS (–0.90 dB/y).

Conclusions

Our data suggest that ESS may be more useful than MS and RPS in test grids that cover a large extent of the macula. We caution the use of contralateral eye as an internal control.

Translational Relevance

ESS may decrease the duration or sample size of treatment trials in USH2A retinopathy.

Phenotype-genotype correlations in a pseudodominant Stargardt disease pedigree due to a novel ABCA4 deletion-insertion variant causing a splicing defect

BACKGROUND

Deletion-insertion (delins) variants in the retina-specific ATP-binding cassette transporter gene, subfamily A, member 4 (ABCA4) accounts for <1% in Stargardt disease. The consequences of these delins variants on splicing cannot be predicted with certainty without supporting in vitro data.

METHODS

Candidate ABCA4 variants were revealed by genetic and segregation analysis of a family with pseudodominant Stargardt disease using a commercial panel and Sanger sequencing. RNA extracted from patient-derived fibroblasts was analyzed by RT-PCR to evaluate splicing behavior of the ABCA4 variants.

RESULTS

Affected members carrying the novel c.6031_6044delinsAGTATTTAACCAATATTT variant in exon 44 presented with contrasting phenotypes; from early-onset cone-rod dystrophy to late-onset macular dystrophy. This variant resulted in a 56-nucleotide deletion in the mutant allele by activation of a cryptic splice acceptor site which disrupts the reading frame and results in a premature termination codon (p.Ile2003LeufsTer41). If translated, the crucial functional domains near the C-terminus would be truncated from the ABCA4 protein.

CONCLUSION

This work demonstrates the intrafamilial phenotypic variability in a pseudodominant Stargardt disease pedigree and the use of patient-derived fibroblasts to evaluate the effect of a novel ABCA4 delins variant on splicing to complement in silico pathogenicity assessment.

Development of High-Throughput Clinical Testing of RPGR ORF15 Using a Large Inherited Retinal Dystrophy Cohort

Purpose

Mutations in the ORF15 region of RPGR account for approximately half of all X-linked retinitis pigmentosa cases. However, a robust high-throughput method for the detection of ORF15 mutations has yet to be validated. We set out to develop the first clinically validated next-generation sequencing (NGS) method for the detection of mutations in this difficult-to-sequence region, including test accuracy and coverage data.

Methods

As part of a blind-test, 145 research samples, previously tested by Sanger sequencing, and 81 clinical samples were evaluated using NGS of long-range PCR products fragmented with Illumina's Nextera library preparation kit (method 1), or with Centrillion's OneTube technology, supplemented with duplication analysis using an ORF15-specific in-silico array (method 2). DNA fragments were analyzed using Agilent's DNA 1000 assay, and sequencing was done on Illumina's MiSeq 2×150 or HiSeq2500 2×100. NextGENe by SoftGenetics was used for data analysis and variant calling.

Results

The Nextera library preparation method produced 24 cases of discordance due to (in order of decreasing occurrence) false-negatives, incorrectly called variants, and a false-positive. Subsequent use of a new, OneTube NGS library preparation method, supplemented with duplication analyses, resolved discordance between Sanger and NGS data in all cases. This improvement in variant detection accuracy was largely attributed to improvement in random fragmentation offered by the enzymatic OneTube method, resulting in more complete coverage of the highly repetitive ORF15 region. Minimum coverage was roughly 320 reads for Nextera and 6800 reads for OneTube (normalized for total read counts).

Conclusions

This paper documents the first clinically validated NGS method for reliable, high-throughput sequencing of RPGR ORF15. Sensitivity and specificity of the new method were 100%, with the caveat of unclear zygosity calling for one large duplication case. These findings demonstrate a reliable and practical implementation for NGS-based diagnosis of RPGR ORF15 mutations. They also provide the foundation for targeted, high-coverage sequencing of any other repetitive regions within the genome.

Analysis of the ABCA4 c.[2588G>C;5603A>T] Allele in the Australian Population

Inherited retinal diseases (IRDs) are genetically and phenotypically diverse, and they cause significant morbidity worldwide. Importantly, IRDs may be amenable to precision medicine strategies, and thus the molecular characterisation of causative variants is becoming increasingly important with the promise of personalised therapies on the horizon. ABCA4, involved in the translocation of visual cycle derivatives, is a well-established, frequent cause of IRDs worldwide, with pathogenic variants implicated in phenotypically diverse diseases. Identification of causative ABCA4 variants in some individuals, however, has been enigmatic, and resolution of this issue is currently a hotbed of research. Recent evidence has indicated that hypomorphic alleles, which cause disease under certain conditions, may account for some of the missing causal variants. It has been postulated that the ABCA4 c.5603A>T (p.Asn1868Ile) variant, previously considered benign, be reclassified as hypomorphic when in cis configuration with c.2588G>C (p.Gly863Ala/Gly863del), a variant previously considered to be pathogenic in its own right. We are exploring this relationship within an Australian cohort to test this theory.

Maternal uniparental isodisomy of chromosome 6 unmasks a novel variant in TULP1 in a patient with early onset retinal dystrophy

Purpose

Inherited retinal dystrophies are a clinically and genetically heterogeneous group of disorders. Molecular diagnosis has proven utility for affected individuals. In this study, we report an individual enrolled in the Australian Inherited Retinal Disease Registry and DNA Bank diagnosed with clinical features overlapping between Leber congenital amaurosis and retinitis pigmentosa.

Methods

DNA from the proband was sequenced using a gene panel for inherited retinal disorders, and a single nucleotide polymorphism (SNP) array was conducted to detect the presence of deletions and uniparental disomy.

Results

We identified a novel homozygous variant (c.524dupC, p.(Pro176ThrfsTer7)) in TULP1 resulting from maternal uniparental isodisomy of chromosome 6. The patient had clinical features consistent with biallelic pathogenic variants in TULP1, including congenital nystagmus, night blindness, non-recordable electroretinogram, mild myopia, and mild peripheral pigmentary changes in the fundus.

Conclusions

This is the first report of uniparental disomy 6 and a homozygous variant in TULP1 associated with a rod-cone dystrophy. Molecular diagnosis of inherited retinal dystrophies is essential to inform the mode of transmission and clinical management, and to identify potential candidates for future gene-specific therapies.

The genetic profile of Leber congenital amaurosis in an Australian cohort

Background

Leber congenital amaurosis (LCA) is a severe visual impairment responsible for infantile blindness, representing ~5% of all inherited retinal dystrophies. LCA encompasses a group of heterogeneous disorders, with 24 genes currently implicated in pathogenesis. Such clinical and genetic heterogeneity poses great challenges for treatment, with personalized therapies anticipated to be the best treatment candidates. Unraveling the individual genetic etiology of disease is a prerequisite for personalized therapies, and could identify potential treatment candidates, inform patient management, and discriminate syndromic forms of disease.

Methods

We have genetically analyzed 45 affected and 82 unaffected individuals from 34 unrelated LCA pedigrees using predominantly next‐generation sequencing and Array CGH technology.

Results

We present the molecular findings for an Australian LCA cohort, sourced from the Australian Inherited Retinal Disease Registry & DNA Bank. CEP290 and GUCY2D mutations, each represent 19% of unrelated LCA cases, followed by NMNAT1 (12%). Genetic subtypes were consistent with other reports, and were resolved in 90% of this cohort.

Conclusion

The high resolution rate achieved, equivalent to recent findings using whole exome/genome sequencing, reflects the progression from hypothesis (LCA Panel) to non‐hypothesis (RD Panel) testing and, coupled with Array CGH analysis, is a highly effective first‐tier test for LCA.

Establishment and evolution of the Australian Inherited Retinal Disease Register and DNA Bank

BACKGROUND

Inherited retinal disease represents a significant cause of blindness and visual morbidity worldwide. With the development of emerging molecular technologies, accessible and well-governed repositories of data characterising inherited retinal disease patients is becoming increasingly important. This manuscript introduces such a repository.

DESIGN

Participants were recruited from the Retina Australia membership, through the Royal Australian and New Zealand College of Ophthalmologists, and by recruitment of suitable patients attending the Sir Charles Gairdner Hospital visual electrophysiology clinic.

PARTICIPANTS

Four thousand one hundred ninety-three participants were recruited. All participants were members of families in which the proband was diagnosed with an inherited retinal disease (excluding age-related macular degeneration).

METHODS

Clinical and family information was collected by interview with the participant and by examination of medical records. In 2001, we began collecting DNA from Western Australian participants. In 2009 this activity was extended Australia-wide. Genetic analysis results were stored in the register as they were obtained.

MAIN OUTCOME MEASURES

The main outcome measurement was the number of DNA samples (with associated phenotypic information) collected from Australian inherited retinal disease-affected families.

RESULTS

DNA was obtained from 2873 participants. Retinitis pigmentosa, Stargardt disease and Usher syndrome participants comprised 61.0%, 9.9% and 6.4% of the register, respectively.

CONCLUSIONS

This resource is a valuable tool for investigating the aetiology of inherited retinal diseases. As new molecular technologies are translated into clinical applications, this well-governed repository of clinical and genetic information will become increasingly relevant for tasks such as identifying candidates for gene-specific clinical trials.

Application of a high-throughput genotyping method for loci exclusion in non-consanguineous Australian pedigrees with autosomal recessive retinitis pigmentosa

PURPOSE

Retinitis pigmentosa (RP) is the most common form of inherited blindness, caused by progressive degeneration of photoreceptor cells in the retina, and affects approximately 1 in 3,000 people. Over the past decade, significant progress has been made in gene therapy for RP and related diseases, making genetic characterization increasingly important. Recently, high-throughput technologies have provided an option for reasonably fast, cost-effective genetic characterization of autosomal recessive RP (arRP). The current study used a single nucleotide polymorphism (SNP) genotyping method to exclude up to 28 possible disease-causing genes in 31 non-consanguineous Australian families affected by arRP.

METHODS

DNA samples were collected from 59 individuals affected with arRP and 74 unaffected family members from 31 Australian families. Five to six SNPs were genotyped for 28 genes known to cause arRP or the related disease Leber congenital amaurosis (LCA). Cosegregation analyses were used to exclude possible causative genes from each of the 31 families. Bidirectional sequencing was used to identify disease-causing mutations in prioritized genes that were not excluded with cosegregation analyses.

RESULTS

Two families were excluded from analysis due to identification of false paternity. An average of 28.9% of genes were excluded per family when only one affected individual was available, in contrast to an average of 71.4% or 89.8% of genes when either two, or three or more affected individuals were analyzed, respectively. A statistically significant relationship between the proportion of genes excluded and the number of affected individuals analyzed was identified using a multivariate regression model (p<0.0001). Subsequent DNA sequencing resulted in identification of the likely disease-causing gene as CRB1 in one family (c.2548 G>A) and USH2A in two families (c.2276 G>T).

CONCLUSIONS

This study has shown that SNP genotyping cosegregation analysis can be successfully used to refine and expedite the genetic characterization of arRP in a non-consanguineous population; however, this method is effective only when DNA samples are available from more than one affected individual.

Isolation and expression analysis of a Pax group III gene from the crustacean Cherax destructor

Pax genes encode transcription factors that are critical regulators of key developmental processes in evolutionarily diverse animal phyla. Here we report the first isolation of a Pax gene from a crustacean: a Pax group III gene we have termed CdpaxIII that contains highly conserved DNA-binding domains, the paired domain and homeodomain. CdpaxIII is expressed in the embryo, in adult limb muscle during both quiescence and regeneration, and during the distinct process of epimorphic limb regeneration. Interestingly, CdpaxIII is expressed as two distinct alternate transcripts, one of which is novel in lacking a large portion of its paired domain.

Pax genes in myogenesis: alternate transcripts add complexity

Pax3 and Pax7 are powerful myogenic inducers and hence play a critical role in skeletal muscle development and regeneration. In this paper we discuss the role of Pax3 and Pax7 in dorsal patterning of the somite with subsequent determination of myogenic precursor cells for muscle formation within the developing embryo and in adult muscle. Recent evidence of the ability of stem cells to contribute to muscle regeneration in adult tissues, and the role of Pax7 in conversion of multipotent stem cells to the myogenic lineage are also discussed. Several tissue specific Pax7 transcripts that encode isoforms with different DNA binding characteristics and potentially distinct transactivation specificities are identified. The expression of a range of transcripts in the determination of different tissue lineages and distinct cell populations both in the embryo and in the adult indicates an extraordinary level of complexity. A detailed understanding of these molecules and their functions during embryogenesis and adult muscle formation is imperative for future stem cell therapies.

Two fast-type fibers in claw closer and abdominal deep muscles of the Australian freshwater crustacean,Cherax destructor, differ in Ca2+ sensitivity and troponin-I isoforms

One type of fast fiber and two types of slow (slow-twitch, S1 and slow-tonic, S2) fibers are found in decapod crustacean skeletal muscles that differ in contractile properties and myofibrillar protein isoform compositions. In this study the structural characteristics, protein isoform compositions, and Ca2+-activation properties of fast fibers in the claw closer (F1) and abdominal deep flexor (F2) muscles of Cherax destructor were analyzed. For comparison, myofibrillar protein isoform compositions of slow (long-sarcomere) fibers from claw and abdomen were also determined; our results indicate that the slow fibers in the claw closer were the slow-twitch (S1) type and those in the abdominal superficial flexor were primarily slow-tonic (S2) type. F1 fibers had shorter resting sarcomere lengths (2.93 microm in unstretched fibers and 3.06 microm in stretched fibers) and smaller fiber diameter (256 microm) than F2 fibers (sarcomere lengths 3.48 microm in unstretched and 3.46 microm in stretched; 747 microm diameter). Moreover, F1 fibers showed a narrower range in sarcomere lengths than F2 fibers (2.81 to 3.28 microm vs. 2.47 to 4.05 micro m in unstretched fibers). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting showed that the fast fibers from claw and abdomen differed in troponin-I composition; F1 fibers expressed two isoforms of troponin-I (TnI1 and TnI2) in approximately equal amounts, whereas F2 fibers expressed primarily TnI3 and lower levels of TnI1. F1 fibers were more sensitive to Ca2+, as shown by higher pCa values at threshold activation (pCa(10)=6.50+/-0.07) and at 50% maximum force (pCa(50)=6.43+/-0.07) than F2 fibers (pCa(10)=6.12+/-0.04 and pCa(50)=5.88+/-0.03, respectively). F1 fibers also had a greater degree of co-operativity in Ca2+ activation, as shown by a higher maximum slope of the force-pCa curve (n(Ca)=12.98+/-2.27 vs. 4.34+/-0.64). These data indicate that there is a greater fast fiber-type diversity in crustacean muscles than was previously supposed. Moreover, the differences in activation properties suggest that the TnI isoform composition influences the Ca2+ sensitivity of the contractile mechanism.