Electrophysiologic and phenotypic features of an autosomal cone-rod dystrophy caused by a novel CRX mutation

Transcriptional precision in photoreceptor development and diseases - Lessons from 25 years of CRX research

The vertebrate retina is made up of six specialized neuronal cell types and one glia that are generated from a common retinal progenitor. The development of these distinct cell types is programmed by transcription factors that regulate the expression of specific genes essential for cell fate specification and differentiation. Because of the complex nature of transcriptional regulation, understanding transcription factor functions in development and disease is challenging. Research on the Cone-rod homeobox transcription factor CRX provides an excellent model to address these challenges. In this review, we reflect on 25 years of mammalian CRX research and discuss recent progress in elucidating the distinct pathogenic mechanisms of four CRX coding variant classes. We highlight how in vitro biochemical studies of CRX protein functions facilitate understanding CRX regulatory principles in animal models. We conclude with a brief discussion of the emerging systems biology approaches that could accelerate precision medicine for CRX-linked diseases and beyond.

Occult Macular Dysfunction Syndrome: Identification of Multiple Pathologies in a Clinical Spectrum of Macular Dysfunction with Normal Fundus in East Asian Patients: EAOMD Report No. 5

Occult macular dystrophy (OMD) is the most prevalent form of macular dystrophy in East Asia. Beyond RP1L1, causative genes and mechanisms remain largely uncharacterised. This study aimed to delineate the clinical and genetic characteristics of OMD syndrome (OMDS). Patients clinically diagnosed with OMDS in Japan, South Korea, and China were enrolled. The inclusion criteria were as follows: (1) macular dysfunction and (2) normal fundus appearance. Comprehensive clinical evaluation and genetic assessment were performed to identify the disease-causing variants. Clinical parameters were compared among the genotype groups. Seventy-two patients with OMDS from fifty families were included. The causative genes were RP1L1 in forty-seven patients from thirty families (30/50, 60.0%), CRX in two patients from one family (1/50, 2.0%), GUCY2D in two patients from two families (2/50, 4.0%), and no genes were identified in twenty-one patients from seventeen families (17/50, 34.0%). Different severities were observed in terms of disease onset and the prognosis of visual acuity reduction. This multicentre large cohort study furthers our understanding of the phenotypic and genotypic spectra of patients with macular dystrophy and normal fundus. Evidently, OMDS encompasses multiple Mendelian retinal disorders, each representing unique pathologies that dictate their respective severity and prognostic patterns.

The Value of Electroretinography in Identifying Candidate Genes for Inherited Retinal Dystrophies: A Diagnostic Guide

Inherited retinal dystrophies (IRDs) are a group of heterogeneous diseases caused by genetic mutations that specifically affect the function of the rod, cone, or bipolar cells in the retina. Electroretinography (ERG) is a diagnostic tool that measures the electrical activity of the retina in response to light stimuli, and it can help to determine the function of these cells. A normal ERG response consists of two waves, the a-wave and the b-wave, which reflect the activity of the photoreceptor cells and the bipolar and Muller cells, respectively. Despite the growing availability of next-generation sequencing (NGS) technology, identifying the precise genetic mutation causing an IRD can be challenging and costly. However, certain types of IRDs present with unique ERG features that can help guide genetic testing. By combining these ERG findings with other clinical information, such as on family history and retinal imaging, physicians can effectively narrow down the list of candidate genes to be sequenced, thereby reducing the cost of genetic testing. This review article focuses on certain types of IRDs with unique ERG features. We will discuss the pathophysiology and clinical presentation of, and ERG findings on, these disorders, emphasizing the unique role ERG plays in their diagnosis and genetic testing.

A survey of protein interactions and posttranslational modifications that influence the polyglutamine diseases

The presence and aggregation of misfolded proteins has deleterious effects in the nervous system. Among the various diseases caused by misfolded proteins is the family of the polyglutamine (polyQ) disorders. This family comprises nine members, all stemming from the same mutation—the abnormal elongation of a polyQ repeat in nine different proteins—which causes protein misfolding and aggregation, cellular dysfunction and disease. While it is the same type of mutation that causes them, each disease is distinct: it is influenced by regions and domains that surround the polyQ repeat; by proteins with which they interact; and by posttranslational modifications they receive. Here, we overview the role of non-polyQ regions that control the pathogenicity of the expanded polyQ repeat. We begin by introducing each polyQ disease, the genes affected, and the symptoms experienced by patients. Subsequently, we provide a survey of protein-protein interactions and posttranslational modifications that regulate polyQ toxicity. We conclude by discussing shared processes and pathways that bring some of the polyQ diseases together and may serve as common therapeutic entry points for this family of incurable disorders.

Novel clinical presentation of a CRX rod-cone dystrophy

We describe a novel clinical presentation of a CRX rod-cone dystrophy in a single family. Two boys ages 6 and 12 years presented with clinical and optical coherence tomography features suggestive of X-linked retinoschisis, but with optic nerve swelling without increased intracranial pressure. One patient had an electronegative electroretinogram (ERG) and the other had rod-cone dysfunction. Neither had retinoschisin (RS1) gene mutations. Biological mother and sister presented with retinal pigment epithelium (RPE) changes and abnormal cone-rod ERG responses. On further testing, next generation sequencing with array comparative genomic hybridisation showed a deletion in exon 4 of the CRX gene. Cystoid maculopathy in young male children can be difficult to distinguish from RS1-associated schisis. Phenotypic variants within a family must prompt a thorough retinal dystrophy evaluation even with electronegative ERG in the presenting child. This novel phenotype for CRX presents with optic nerve swelling and cystoid maculopathy in men, and RPE changes in women.

Association of CRX genotypes and retinal phenotypes confounded by variable expressivity and electronegative electroretinogram

IMPORTANCE

A framework for understanding the phenotypic features of CRX retinopathy was established.

BACKGROUND

To perform a phenotype-genotype correlation analysis in two groups of patients with heterozygous mutations in distinct locations of the CRX gene, encoding the cone-rod homeobox.

DESIGN

Multicentre retrospective study.

PARTICIPANTS

Twenty-one Japanese patients from 14 families with a heterozygous CRX mutation.

METHODS

Retrospective data analysis.

MAIN OUTCOME MEASURES

Clinical records on CRX mutation, symptoms, best-corrected visual acuity (BCVA), visual field, fundus photography, fundus auto-fluorescence, optical coherence tomography and electroretinograms (ERGs).

RESULTS

Six different CRX heterozygous mutations were identified in the subjects. Twelve patients from 9 families shared the p.R41W mutation and 1 patient had the p.R43C mutation, both of which affect the homeobox domain of CRX. These patients often displayed adult-onset retinal dystrophy with macular degeneration. In contrast, five patients with downstream mutations (p.S204fs, p.S213fs, p.G243X and p.L299F) displayed retinal degeneration or macular degeneration with bone-spicule pigmentation. Three asymptomatic carriers with different mutations (p.R41W, p.S213fs and p.G243X) were present in both groups. Nearly all patients and carriers had an electronegative ERG in response to a bright flash under dark adaptation. There was no cross-sectional association between patients' age and BCVA, despite progressive decline in BCVA.

CONCLUSIONS AND RELEVANCE

Heterozygous mutations within or downstream of the homeobox domain in CRX relate to the difference associated retinal phenotypes, which was confounded by variable expressivity and electronegative ERGs. CRX mutations should be considered in patients with an electronegative ERG with minimal or no macular changes.

A recurrent arcuate retinopathy in familial cone-rod dystrophy secondary to heterozygous CRX deletion

Purpose: To describe an arcuate retinopathy appearance in a familial cone-rod dystrophy and the underlying genetic cause.Methods: Retrospective case series of an affected three-generation family (eight affected members, eight unaffected members)Results: The proband, a 47-year-old male, noted significant visual loss since his early thirties. In addition to central macular atrophic changes, retinal examination was notable for peripapillary atrophy and an arcuate of drusenoid deep yellow lesions in the temporal macula. Spectral-domain optical coherence tomography in the area of the lesions showed regional outer neurosensory retina loss with nasal extension (forming a ring around the central retina) but no drusen. Full-field electroretinography revealed cone-rod dysfunction with an electronegative waveform. Fifteen other available family members were examined, and seven (age range 13-71 years old) showed variable expressivity for similar phenotypic findings, most notably the arcuate lesions in all but the oldest individual who had end-stage atrophy. Exome sequencing of the proband's affected daughter uncovered a heterozygous CRX deletion [NM_000554.4: CRX: c.(100 + 1_101-1)_(c.900 + 1_?)del] that segregated with the disease.Conclusion: An unusual familial cone-rod dystrophy phenotype was associated with heterozygous CRX deletion, a pathogenic variant that had a presumed mechanism of haploinsufficiency. The consistent finding of arcuate temporal macular lesions among affected family members was striking, particularly given the variable expressivity previously associated with CRX-related retinopathy. Additional phenotypic studies are needed to assess how frequently this temporal arcuate retinopathy appearance occurs in individuals harboring a similar deletion who are not from the current family.

Pathogenicity discrimination and genetic test reference for CRX variants based on genotype-phenotype analysis

The cone-rod homeobox (CRX) gene is specifically expressed in developing and mature photoreceptors and is relatively conserved, with limited polymorphisms in coding regions. Rare variants in CRX are usually considered causative for different forms of retinal degeneration, but this might be problematic based on recent data. This study aimed to classify CRX variants based on a genotype-phenotype analysis of our data and the literature. Twenty-four CRX variants, including 14 novel variants, were detected in 37 Chinese families based on exome sequencing data obtained from 4971 Chinese probands with different forms of eye diseases. After detailed phenotypic analysis and cosegregation analysis in families with CRX variants, the 24 variants could be classified into three groups: benign (six), likely benign (six), and pathogenic (12). Somatic mosaicism was identified in a family with unaffected parents (the father had a mutant allele that was detected in approximately 17% of his leukocyte DNA) and two affected sons. Furthermore, a thorough reassessment was systematically performed for all 113 heterozygous variants as well as for their associated phenotypes from our cohort and patients previously reported. Two critical findings on the pathogenicity of CRX variants were obtained based on the genotype-phenotype correlation, family segregation and ensemble predicting methods: 1) approximately half of heterozygous missense variants are likely benign, and 2) heterozygous truncating variants affecting the homeodomain are likely benign. Truncating mutations after the homeodomain are likely associated with a more severe phenotype. Although most heterozygous pathogenic variants in CRX are associated with autosomal dominant retinal degeneration, a homozygous c.268C> T (p.Arg90Trp) substitution and homozygous complete deletion of CRX have been reported to cause Leber congenital amaurosis. In conclusion, many rare missense variants and some truncating variants in CRX are likely benign, although previously, they might have been predicted to be damaging by some online tools. Evaluation of the pathogenicity of a CRX variant should consider both its nature and location. The information obtained in this study is critical in the era of routine clinical genetic test, not only for CRX but also for many other genes with many more variants. Functional studies and additional genotype-phenotype analyses are expected to confirm these associations.

Characteristic Ocular Features in Cases of Autosomal Recessive PROM1 Cone-Rod Dystrophy

Purpose

To define characteristic ocular features in a group of patients with autosomal recessive (AR) PROM1 cone-rod dystrophy (CRD).

Methods

Three males and one female from three unrelated families were first seen at the ages of 15 to 22 years and diagnosed with CRD. Clinical testing available for review included full-field electroretinogram (ERG) in three patients, as well as near-infrared autofluorescence (NIR-AF), spectral-domain optical coherence tomography (SD-OCT), and color fundus photography in all four patients. Whole exome sequencing (WES) was performed on all cases, and whole genome sequencing (WGS) was performed in two families.

Results

WES found compound heterozygous PROM1 variants in one isolated male, plus heterozygous variants in the remaining patients. WGS uncovered deleterious PROM1 variants in these two families. ERG showed markedly reduced cone-isolated amplitudes and variably reduced rod-isolated amplitudes. The dark-adapted combined rod and cone responses demonstrated notably reduced a-wave amplitudes and moderately reduced b-waves, and the resultant waveform resembled the normal rod-isolated response. On fundus examination, oval-shaped macular lesions were observed, as were several small, circular hypoautofluorescent lesions within the posterior pole on NIR-AF. Three patients showed extramacular circular atrophic lesions.

Conclusions

The autofluorescence changes, peripheral retinal abnormalities, and ERG findings have not been emphasized in previous reports of AR PROM1, but they became a recognizable phenotype in this cohort of patients. A similar constellation of findings may be observed in CRD due to CDHR1, a functionally related gene. The pattern of abnormalities reported herein may help to focus genetic screening in patients with these findings.

Incomplete penetrance of CRX gene for autosomal dominant form of cone-rod dystrophy

Purpose: Cone-rod dystrophy (CRD) is an inherited retinal dystrophy that is transmitted via different modes of inheritance. Mutations in more than 30 genes have been identified to cause the disease. We aimed to investigate the genetic agents of two unrelated cone-rod dystrophy affected Iranian families with autosomal recessive inheritance patterns. Methods: Whole-exome sequencing (WES) was performed for identification of the disease-causing mutations in the probands of both families. The candidate mutations were further confirmed by Sanger sequencing. Samples from five available members of each family were then sequenced for the mutations present in the probands. Comprehensive ocular examinations for all members of the families carrying the mutations were completed by ophthalmologists. Results: We identified a novel premature stop codon c.310C>T in CRX gene in heterozygote form in two symptomatic and two non-symptomatic members of one family (family-A), and a known CRX mutation c.122G>A in homozygote form in another (family B). c.122G>A has been reported to cause late-onset autosomal dominant form of the disease in previous studies. However, the middle-aged heterozygous carriers of the mutation in this family showed normal phenotype. Conclusion: The CRX gene has been previously linked to the autosomal dominant form of cone-rod dystrophy. We report incomplete penetrance of CRX gene for autosomal dominant form of the disease. Incomplete penetrance of the mutations may be partly caused by the influence of other genes in the complex genetic network underlying retinal regulation.

A novel CRX frameshift mutation causing cone-rod dystrophy in a Chinese family: A case report

BACKGROUND

Cone-rod dystrophy (CORD) is an inherited, progressive retinal disorder with genetic and phenotypic heterogeneity. Here, we aimed to identify the pathogenic mutation in affected individuals in a Chinese family with autosomal dominant cone-rod dystrophy (adCORD).

METHODS

Genomic DNA and clinical examination results were collected from a Chinese family presenting with adCORD. The candidate disease-causing mutations were screened with whole-exome sequencing (WES) and bioinformatics analyses. Sanger sequencing was used for validation and cosegregation analysis.

RESULTS

A novel frameshift mutation (NM_000554.4; c.538dupG:p.Val180fs) in exon 4 of the CRX gene was identified in all affected individuals in the Chinese family with adCORD. Cosegregation analysis confirmed that this mutation was cosegregated with the disease. This variant, which results in premature termination of the protein, was absent from all public variant databases or internal exome databases.

CONCLUSIONS

We used whole-exome sequencing to identify a novel CRX mutation causing adCORD in a Chinese family. This study broadens the known pathogenic mutation spectrum of the CRX gene and shows the potential of WES in identifying the pathogenic mutations of CORD disease.

Autosomal Dominant Retinal Dystrophy With Electronegative Waveform Associated With a Novel RAX2 Mutation

IMPORTANCE

The patients evaluated in this study, to our knowledge, represent the first complete clinical description of a family with an autosomal dominant inheritance pattern of retinal dystrophy associated with a novel mutation in RAX2.

OBJECTIVES

To clinically evaluate 4 patients and 5 unaffected family members, characterize the disease phenotype over time, and identify the associated genetic mutation.

DESIGN, SETTING, AND PARTICIPANTS

A prospective, longitudinal, observational, case-series analysis of 9 members of an affected family at the Casey Eye Institute, Oregon Health and Science University, Portland. The dates of the study were from July 31, 1992, to August 11, 2014.

INTERVENTIONS

Clinical evaluations included eye examination, color fundus photography, autofluorescence imaging, spectral-domain optical coherence tomography, kinetic visual field testing, and electroretinography. Genetic mutation screening was performed with next-generation sequencing, and identified mutations were confirmed with Sanger sequencing.

MAIN OUTCOMES AND MEASURES

Clinical diagnosis and longitudinal characterization of retinal dystrophy and identification of genetic mutation.

RESULTS

Six members of the family were identified as having retinal dystrophy (4 were examined, and 3 were genetically tested). Five unaffected family members were clinically evaluated (2 were genetically tested). The age at onset of retinal dystrophy was variable. All affected individuals presented with declining visual acuity, central scotomas, waxy disc pallor, attenuated vasculature, small yellow macular deposits and/or macular pigment mottling, and abnormal electroretinograms demonstrating mixed cone and rod dysfunction and a scotopic electronegative response to bright flashes. There were no other causes of an electronegative electroretinogram identified in any of the affected patients. Genetic testing revealed, to our knowledge, a novel frameshift heterozygous mutation in RAX2 in the patients with retinal dystrophy.

CONCLUSIONS AND RELEVANCE

A frameshift heterozygous mutation in RAX2 inherited in an autosomal dominant fashion was associated with mixed cone and rod dysfunction. Among the patients, there was variability in the age at onset and in the specific pattern of photoreceptor dysfunction, but the clinical course was nevertheless slowly progressive. Screening for RAX2 mutation could provide prognostic value for patients and families with scotopic electronegative responses to bright flashes.

Mechanisms of blindness: animal models provide insight into distinct CRX-associated retinopathies

BACKGROUND

The homeodomain transcription factor CRX is a crucial regulator of mammalian photoreceptor gene expression. Mutations in the human CRX gene are associated with dominant inherited retinopathies Retinitis Pigmentosa (RP), Cone-Rod Dystrophy (CoRD), and Leber Congenital Amaurosis (LCA), of varying severity. In vitro and in vivo assessment of mutant CRX proteins have revealed pathogenic mechanisms for several mutations, but no comprehensive mutation-disease correlation has yet been reported.

RESULTS

Here we describe four different classes of disease-causing CRX mutations, characterized by mutation type, pathogenetic mechanism, and the molecular activity of the mutant protein: (1) hypomorphic missense mutations with reduced DNA binding, (2) antimorphic missense mutations with variable DNA binding, (3) antimorphic frameshift/nonsense mutations with intact DNA binding, and (4) antimorphic frameshift mutations with reduced DNA binding. Mammalian models representing three of these classes have been characterized.

CONCLUSIONS

Models carrying Class I mutations display a mild dominant retinal phenotype and recessive LCA, while models carrying Class III and IV mutations display characteristically distinct dominant LCA phenotypes. These animal models also reveal unexpected pathogenic mechanisms underlying CRX-associated retinopathies. The complexity of genotype-phenotype correlation for CRX-associated diseases highlights the value of developing comprehensive "true-to-disease" animal models for understanding pathologic mechanisms and testing novel therapeutic approaches.

CRX variants in cone-rod dystrophy and mutation overview

Mutations in the cone-rod homeobox gene (CRX) are associated with cone-rod dystrophy (CORD), Leber congenital amaurosis (LCA), and, in rare cases, retinitis pigmentosa (RP). In this study, three variations were detected in 3 of 130 families with CORD, including two novel mutations, c.239A>G (p.Glu80Gly) and c.362C>T (p.Ala121Val). So far, 49 mutations in CRX were reported, affecting about 2.35% of LCA, 4.76% of CORD, and 0.80% of RP. These mutations can be classified as missense (38.78%), nonsense (4.08%), deletion (36.73%), insertion (16.33%), and indel (4.08%). They distributed in the three coding exons without mutation hot spots. No clear genotype-phenotype correlation could be established so far.

Proteolytic cleavage of ataxin-7 by caspase-7 modulates cellular toxicity and transcriptional dysregulation

Spinocerebellar ataxia type 7 (SCA7) is a polyglutamine (polyQ) disorder characterized by specific degeneration of cerebellar, brainstem, and retinal neurons. Although they share little sequence homology, proteins implicated in polyQ disorders have common properties beyond their characteristic polyQ tract. These include the production of proteolytic fragments, nuclear accumulation, and processing by caspases. Here we report that ataxin-7 is cleaved by caspase-7, and we map two putative caspase-7 cleavage sites to Asp residues at positions 266 and 344 of the ataxin-7 protein. Site-directed mutagenesis of these two caspase-7 cleavage sites in the polyQ-expanded form of ataxin-7 produces an ataxin-7 D266N/D344N protein that is resistant to caspase cleavage. Although ataxin-7 displays toxicity, forms nuclear aggregates, and represses transcription in human embryonic kidney 293T cells in a polyQ length-dependent manner, expression of the non-cleavable D266N/D344N form of polyQ-expanded ataxin-7 attenuated cell death, aggregate formation, and transcriptional interference. Expression of the caspase-7 truncation product of ataxin-7-69Q or -92Q, which removes the putative nuclear export signal and nuclear localization signals of ataxin-7, showed increased cellular toxicity. We also detected N-terminal polyQ-expanded ataxin-7 cleavage products in SCA7 transgenic mice similar in size to those generated by caspase-7 cleavage. In a SCA7 transgenic mouse model, recruitment of caspase-7 into the nucleus by polyQ-expanded ataxin-7 correlated with its activation. Our results, thus, suggest that proteolytic processing of ataxin-7 by caspase-7 may contribute to SCA7 disease pathogenesis.

Clinical findings in early onset cone-rod dystrophy in the Standard Wire-haired Dachshund

OBJECTIVE

To describe the clinical findings and the age of onset of cone-rod dystrophy (crd) in the Standard Wire-haired Dachshund (SWHD) and to evaluate which clinical tests could be used to obtain a reliable diagnosis.

ANIMALS

Sixty-eight SWHD and SWHD-derived dogs were used, including 23 affected with crd and 45 controls, respectively.

PROCEDURES

The dogs were subjected to behavioral testing, examination of pupillary light reflexes (PLRs), indirect ophthalmoscopy and bilateral full field electroretinography (ERG).

RESULTS

The majority of affected puppies (5-10 weeks) displayed pin-point sized pupils upon examination with focal light. All dogs in the control group, except one, displayed normal PLRs upon examination. In all crd-affected dogs there was a great variation both in age of onset and in clinical appearance of retinal changes upon fundoscopy. Two siblings displayed panretinal degeneration at the age of 10 months while other affected dogs showed early changes at the age of 3 years. Generalized bilateral retinal atrophy was the end stage of the disease. The maze test revealed no obvious differences among affected and unaffected groups. ERG recordings showed only slightly reduced rod, and mixed rod-cone responses, but severely reduced cone single flash a- and b-wave amplitudes, and cone flicker amplitudes were observed in all affected dogs.

CONCLUSION

Presence of pin-point sized pupils in young SWHDs was found to be an important indicator of early onset crd. Fundoscopic changes and progression of disease at later stages resembled those previously described in the majority of progressive retinal atrophies in dog. ERG was found to be the most reliable diagnostic procedure to clinically diagnose crd in the SWHD.

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

Genotype-phenotype correlation in a German family with a novel complex CRX mutation extending the open reading frame

PURPOSE

To describe the genotype-phenotype correlation in a German family with a novel CRX mutation and to perform a comparative analysis of published cases.

DESIGN

Retrospective observational case series, systematic review, and comparative analysis of the literature.

PARTICIPANTS

Four related patients with progressive retinal degeneration.

METHODS

Mutation screening by single-strand polymorphism analysis and direct sequencing. Clinical examination included kinetic visual fields (VFs), 2-color threshold perimetry (2CTP), full-field electroretinography, fundus photography, optical coherence tomography, and fundus autofluorescence (FA) recording.

MAIN OUTCOME MEASURES

Visual fields, subjective and objective cone- and rod-specific function, fundus aspect, retinal stratification, and FA.

RESULTS

A novel heterozygous complex mutation (c.816delCACinsAA) in CRX predicting the substitution of 27 C-terminal amino acids by 44 novel amino acids, thus abolishing the OTX tail, was identified in a 2-generation family finally diagnosed with cone-rod dystrophy (CRD), which was confirmed by 2CTP. Patients presented with variability in progression, nystagmus, and nyctalopia. Most of the patients were hyperopic. Electroretinography recordings showed residual rod and mixed cone-rod responses in 2 of the subjects. Age-dependent VF losses followed funduscopic changes of progressive atrophy of the retinal pigment epithelium and neuroretina in the macula and midperiphery marked by disturbed FA. Optical coherence tomography showed decreased central retinal thickness. Comparative analysis of the 131 published data sets revealed 2 groups: patients with early and late onset.

CONCLUSIONS

We described a 2-generation family with a novel mutation in CRX. The resulting phenotype is that of CRD with variable age at onset and progression. The phenotype description of previously published cases is conclusive only for CRD.

Progressive cone and cone-rod dystrophies: phenotypes and underlying molecular genetic basis

The cone and cone-rod dystrophies form part of a heterogeneous group of retinal disorders that are an important cause of visual impairment in children and adults. There have been considerable advances made in recent years in our understanding of the pathogenesis of these retinal dystrophies, with many of the chromosomal loci and causative genes having now been identified. Mutations in 12 genes, including GUCA1A, peripherin/RDS, ABCA4 and RPGR, have been described to date; and in many cases detailed functional assessment of the effects of the encoded mutant proteins has been undertaken. This improved knowledge of disease mechanisms has raised the possibility of future treatments for these disorders, for which there are no specific therapies available at the present time.

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.