Progression of phenotype in Leber's congenital amaurosis with a mutation at the LCA5 locus

Leber's Congenital Amaurosis: Current Concepts of Genotype-Phenotype Correlations

Leber’s congenital amaurosis (LCA), one of the most severe inherited retinal dystrophies, is typically associated with extremely early onset of visual loss, nystagmus, and amaurotic pupils, and is responsible for 20% of childhood blindness. With advances in molecular diagnostic technology, the knowledge about the genetic background of LCA has expanded widely, while disease-causing variants have been identified in 38 genes. Different pathogenetic mechanisms have been found among these varieties of genetic mutations, all of which result in the dysfunction or absence of their encoded proteins participating in the visual cycle. Hence, the clinical phenotypes also exhibit extensive heterogenicity, including the course of visual impairment, involvement of the macular area, alteration in retinal structure, and residual function of the diseased photoreceptor. By reviewing the clinical course, fundoscopic images, optical coherent tomography examination, and electroretinogram, genotype-phenotype correlations could be established for common genetic mutations in LCA, which would benefit the timing of the diagnosis and thus promote early intervention. Gene therapy is promising in the management of LCA, while several clinical trials are ongoing and preliminary success has been announced, focusing on RPE65 and other common disease-causing genes. This review provides an update on the genetics, clinical examination findings, and genotype-phenotype correlations in the most well-established causative genetic mutations of LCA.

Voretigene Neparvovec and Gene Therapy for Leber's Congenital Amaurosis: Review of Evidence to Date

Gene therapy has now evolved as the upcoming modality for management of many disorders, both inheritable and non-inheritable. Knowledge of genetics pertaining to a disease has therefore become paramount for physicians across most specialities. Inheritable retinal dystrophies (IRDs) are notorious for progressive and relentless vision loss, frequently culminating in complete blindness in both eyes. Leber’s congenital amaurosis (LCA) is a typical example of an IRD that manifests very early in childhood. Research in gene therapy has led to the development and approval of voretigene neparvovec (VN) for use in patients of LCA with a deficient biallelic RPE65 gene. The procedure involves delivery of a recombinant virus vector that carries the RPE65 gene in the subretinal space. This comprehensive review reports the evidence thus far in support of gene therapy for LCA. We explore and compare the various gene targets including but not limited to RPE65, and discuss the choice of vector and method for ocular delivery. The review details the evolution of gene therapy with VN in a phased manner, concluding with the challenges that lie ahead for its translation for use in communities that differ much both genetically and economically.

Treatment Potential for LCA5-Associated Leber Congenital Amaurosis

Purpose

To determine the therapeutic window for gene augmentation for Leber congenital amaurosis (LCA) associated with mutations in LCA5.

Methods

Five patients (ages 6–31) with LCA and biallelic LCA5 mutations underwent an ophthalmic examination including optical coherence tomography (SD-OCT), full-field stimulus testing (FST), and pupillometry. The time course of photoreceptor degeneration in the Lca5^gt/gt mouse model and the efficacy of subretinal gene augmentation therapy with AAV8-hLCA5 delivered at postnatal day 5 (P5) (early, n = 11 eyes), P15 (mid, n = 14), and P30 (late, n = 13) were assessed using SD-OCT, histologic study, electroretinography (ERG), and pupillometry. Comparisons were made with the human disease.

Results

Patients with LCA5-LCA showed a maculopathy with detectable outer nuclear layer (ONL) in the pericentral retina and at least 4 log units of dark-adapted sensitivity loss. The Lca5^gt/gt mouse has a similarly severe and rapid photoreceptor degeneration. The ONL became progressively thinner and was undetectable by P60. Rod- and cone-mediated ERGs were severely reduced in amplitudes at P30 and became nondetectable by P60. Subretinal AAV8-hLCA5 administered to Lca5^gt/gt mice at P5 and P15, but not at P30, resulted in structural and functional rescue.

Conclusions

LCA5-LCA is a particularly severe form of LCA that was recapitulated in the Lca5^gt/gt mouse. Gene augmentation resulted in structural and functional rescue in the Lca5^gt/gt mouse if delivered before P30. Retained photoreceptors were visible within the central retina in all patients with LCA5-LCA, at a level equivalent to that observed in rescued Lca5^gt/gt mice, suggesting a window of opportunity for the treatment of patients with LCA5-LCA.

NMNAT1-ASSOCIATED CONE-ROD DYSTROPHY: EVIDENCE FOR A SPECTRUM OF FOVEAL MALDEVELOPMENT

PURPOSE

To describe in detail the phenotype of two siblings with biallelic NMNAT1 mutations.

METHODS

A 4-year-old male patient (P1) and his 7-year-old sister (P2), product of a nonconsanguineous union of Egyptian ancestry, underwent a comprehensive ophthalmic examination, retinal imaging with spectral domain optical coherence tomography and near infrared (NIR) fundus autofluorescence (FAF), and full-field electroretinograms (ERG).

RESULTS

Patients had blurred vision and nystagmus at ∼3 years of age. P2 was hyperopic (+6D). Visual acuity in P1 was 20/100 at age 3 and remained at ∼20/125 at age 4; P2 visual acuity was 20/70 at age 4 and declined to ∼20/200 at age 7. ERGs recorded in P1 showed relatively large rod-mediated responses but nearly undetectable cone signals. There was foveal/parafoveal depigmentation. Spectral domain optical coherence tomography showed hypoplastic foveas, a thin outer nuclear layer centrally but normal thickness beyond the vascular arcades. At the foveal center, cone outer segments were absent and the outer nuclear layer was further hyporreflective. The inner retina was mostly within normal limits. There was central depigmentation on near infrared fundus autofluorescence. Biallelic mutations were identified in NMNAT1: One was previously reported (c.769 G>A; pGlu257Lys), and the other one (c.245T>C; pVal82Ala) was novel.

CONCLUSION

NMNAT1 mutations cause a consistent phenotype characterized by early-onset, progressive, cone>rod retinawide dysfunction and predominantly central abnormalities ranging from a hypoplastic to an atrophic fovea, supporting a critical role for NMNAT1 in central retinal development and maintenance. Relatively preserved inner retina and detectable photoreceptors may become therapeutic targets.

Knocking out lca5 in zebrafish causes cone-rod dystrophy due to impaired outer segment protein trafficking

Leber congenital amaurosis (LCA) is the most serious form of inherited retinal dystrophy that leads to blindness or severe visual impairment within a few months after birth. Approximately 1-2% of the reported cases are caused by mutations in the LCA5 gene. This gene encodes a ciliary protein called LCA5 that is localized to the connecting cilium of photoreceptors. The retinal phenotypes caused by LCA5 mutations and the underlying pathological mechanisms are still not well understood. In this study, we knocked out the lca5 gene in zebrafish using CRISPR/Cas9 technology. An early onset visual defect is detected by the ERG in 7 dpf lca5^-/- zebrafish. Histological analysis by HE staining and immunofluorescence reveal progressive degeneration of rod and cone photoreceptors, with a pattern that cones are more severely affected than rods. In addition, ultrastructural analysis by transmission electron microscopy shows disordered and broken membrane discs in rods' and cones' outer segments, respectively. In our lca5^-/- zebrafish, the red-cone opsin and cone α-transducin are selectively mislocalized to the inner segment and synaptic terminal. Moreover, we found that Ift88, a key component of the intraflagellar transport complex, is retained in the outer segments. These data suggest that the intraflagellar transport complex-mediated outer segment protein trafficking might be impaired due to lca5 deletion, which finally leads to a type of retinal degeneration mimicking the phenotype of cone-rod dystrophy in human. Our work provides a novel animal model to study the physiological function of LCA5 and develop potential treatments of LCA.

Amelioration of Neurosensory Structure and Function in Animal and Cellular Models of a Congenital Blindness

Most genetically distinct inherited retinal degenerations are primary photoreceptor degenerations. We selected a severe early onset form of Leber congenital amaurosis (LCA), caused by mutations in the gene LCA5, in order to test the efficacy of gene augmentation therapy for a ciliopathy. The LCA5-encoded protein, Lebercilin, is essential for the trafficking of proteins and vesicles to the photoreceptor outer segment. Using the AAV serotype AAV7m8 to deliver a human LCA5 cDNA into an Lca5 null mouse model of LCA5, we show partial rescue of retinal structure and visual function. Specifically, we observed restoration of rod-and-cone-driven electroretinograms in about 25% of injected eyes, restoration of pupillary light responses in the majority of treated eyes, an ∼20-fold decrease in target luminance necessary for visually guided behavior, and improved retinal architecture following gene transfer. Using LCA5 patient-derived iPSC-RPEs, we show that delivery of the LCA5 cDNA restores lebercilin protein and rescues cilia quantity. The results presented in this study support a path forward aiming to develop safety and efficacy trials for gene augmentation therapy in human subjects with LCA5 mutations. They also provide the framework for measuring the effects of intervention in ciliopathies and other severe, early-onset blinding conditions.

A mutation in IFT43 causes non-syndromic recessive retinal degeneration

The aim of this work is to identify the molecular cause of autosomal recessive early onset retinal degeneration in a consanguineous pedigree. Seventeen members of a four-generation Pakistani family were recruited and underwent a detailed ophthalmic examination. Exomes of four affected and two unaffected individuals were sequenced. Variants were filtered using exomeSuite to identify rare potentially pathogenic variants in genes expressed in the retina and/or brain and consistent with the pattern of inheritance. Effect of the variant observed in the gene Intraflagellar Transport Protein 43 (IFT43) was studied by heterologous expression in mIMCD3 and MDCK cells. Expression and sub-cellular localization of IFT43 in the retina and transiently transfected cells was examined by RT-PCR, western blot analysis, and immunohistochemistry. Affected members were diagnosed with early onset non-syndromic progressive retinal degeneration and the presence of bone spicules distributed throughout the retina at younger ages while the older affected members showed severe central choroidal atrophy. Whole-exome sequencing analysis identified a novel homozygous c.100 G > A change in IFT43 segregating with retinal degeneration and not present in ethnicity-matched controls. Immunostaining showed IFT43 localized in the photoreceptors, and to the tip of the cilia in transfected mIMCD3 and MDCK cells. The cilia in mIMCD3 and MDCK cells expressing mutant IFT43 were found to be significantly shorter (P < 0.001) than cells expressing wild-type IFT43. Our studies identified a novel homozygous mutation in the ciliary protein IFT43 as the underlying cause of recessive inherited retinal degeneration. This is the first report demonstrating the involvement of IFT43 in retinal degeneration.

Leber congenital amaurosis, from darkness to light: An ode to Irene Maumenee

This article is dedicated to Irene Hussels Maumenee, Professor of Human Genetics and Ophthalmology, Johns Hopkins' Wilmer Eye Institute, Ocular Genetics Fellowship director in 1994-1995. Leber congenital amaurosis (LCA) has almost come full circle, from a profound and molecularly uncharacterized form of congenital retinal blindness to one in which a large number of causative genes and disease pathways are known, and the world's first human retinal disease to be treated by gene therapy. Dr. Maumenee's insights, efforts, and leadership have contributed significantly to this remarkable scientific journey. In this manuscript, we present a short summary of the known LCA genes, LCA disease subtypes, and emerging treatment options. Our manuscript consolidates previous knowledge with current findings in an attempt to provide a more comprehensive understanding of LCA.

Available Evidence on Leber Congenital Amaurosis and Gene Therapy

Leber congenital amaurosis (LCA) is a group of severe inherited retinal dystrophies that lead to early childhood blindness. In the last decade, interest in LCA has increased as advances in genetics have been applied to better identify, classify, and treat LCA. To date, 23 LCA genes have been identified. Gene replacement in the RPE65 form of LCA represents a major advance in treatment, although limitations have been recognized. In this article, we review the clinical and genetic features of LCA and evaluate the evidence available for gene therapy in RPE65 disease.

Longitudinal clinical course of three Japanese patients with Leber congenital amaurosis/early-onset retinal dystrophy with RDH12 mutation

PURPOSE

To report the longitudinal clinical course of three Japanese patients from two families with Leber congenital amaurosis/early-onset retinal dystrophy (LCA/EORD), and the results of next-generation DNA sequences on them.

PATIENTS AND METHODS

The patients were three Japanese children: a 4-year-old girl, a 6-year-old boy, and a 3-year-old girl. Patients 1 and 2 were siblings, and patient 3 was from an unrelated family. Standard ophthalmic examinations including perimetry, electroretinography, optical coherence tomography, and ultrasonography were performed on each patient. The patients were observed for 28, 16, and 10 years. Whole exomes of the patients and their non-symptomatic parents were analyzed using a next-generation sequence technique.

RESULTS

The decimal visual acuity varied between 0.07 and 0.6 at the initial visit and decreased to counting finger to hand motion in their teens. Funduscopy showed diffuse retinal and macular degeneration. During the follow-up period, a posterior staphyloma developed and the macular area became atrophic. Patient 1 developed cataracts in her early twenties. Genetic analysis revealed a homozygous A126V substitution in the RDH12 gene in all patients.

CONCLUSIONS

The three patients with LCA/EORD had a progressive decrease of their vision with the formation of a posterior staphyloma. This is the first report of Japanese patients with LCA/EORD with a RDH12 mutation.

Retinal Structure in Cobalamin C Disease: Mechanistic and Therapeutic Implications

PURPOSE

To describe the retinal structure in a patient with cobalamin C (cblC) disease.

METHODS

A 13-year-old male patient diagnosed with cblC disease during a perinatal metabolic screening prompted by jaundice and hypotony underwent ophthalmic examinations, electroretinography (ERG) and spectral domain optical coherence tomography (SD-OCT).

RESULTS

The patient carried a homozygous (c.271dupA) mutation in the methylmalonic aciduria and homocystinuria type C (MMACHC) gene. At age 3 months he had a normal eye exam. A pigmentary maculopathy progressed to chorioretinal atrophy from 5-10 months. ERG at 7 months was normal. A nystagmus remained stable since the age of 2 years. At age 13, visual acuity was 20/250 (right eye) and 20/400 (left eye), with a +5.00 D correction, a level of vision maintained since first measurable at age 5 years. SD-OCT showed bilateral macular coloboma-like lesions; there was also a thickened surface layer with ganglion cell layer thinning. Photoreceptor outer segment loss and thinning of the outer nuclear layer (ONL) transitioned to regions with no discernible ONL with a delaminated, thickened, inner retina.

CONCLUSIONS

A thick surface layer near the optic nerve resembling an immature retina and an initially normal macula that rapidly developed coloboma-like lesions suggest there may be an interference with retinal/foveal development in cblC, a mechanism of maculopathy that may be shared by other early onset retinal degenerations. Photoreceptor loss and inner retinal remodeling confirm associated photoreceptor degeneration.

Involvement of LCA5 in Leber congenital amaurosis and retinitis pigmentosa in the Spanish population

OBJECTIVE

We aimed to identify novel genetic defects in the LCA5 gene underlying Leber congenital amaurosis (LCA) in the Spanish population and to describe the associated phenotype.

DESIGN

Case series.

PARTICIPANTS

A cohort of 217 unrelated Spanish families affected by autosomal recessive or isolated retinal dystrophy, that is, 79 families with LCA and 138 families with early-onset retinitis pigmentosa (EORP). A total of 100 healthy, unrelated Spanish individuals were screened as controls.

METHODS

High-resolution homozygosity mapping was performed in 44 patients with LCA using genome-wide single nucleotide polymorphism (SNP) microarrays. Direct sequencing of the LCA5 gene was performed in 5 patients who showed homozygous regions at chromosome 6 and in 173 unrelated individuals with LCA or EORP. The ophthalmic history of 8 patients carrying LCA5 mutations was reviewed and additional examinations were performed, including electroretinography (ERG), optical coherence tomography (OCT), and fundus photography.

MAIN OUTCOME MEASURES

Single nucleotide polymorphism genotyping, identity-by-descent (IBD) regions, LCA5 mutations, best-corrected visual acuity, visual field assessments, fundus appearance, ERG, and OCT findings.

RESULTS

Four novel and 2 previously reported LCA5 mutations have been identified in 6 unrelated families with LCA by homozygosity mapping or Sanger sequencing. Thus, LCA5 mutations have a frequency of 7.6% in the Spanish population. However, no LCA5 mutations were found in 138 patients with EORP. Although most of the identified LCA5 mutations led to a truncated protein, a likely pathogenic missense variant was identified for the first time as a cause of LCA, segregating in 2 families. We also have characterized a novel splicing site mutation at the RNA level, demonstrating that the mutant LCA5 transcript was absent in a patient. All patients carrying LCA5 mutations presented nystagmus, night blindness, and progressive loss of visual acuity and visual field leading to blindness toward the third decade of life. Fundoscopy showed fundus features of pigmentary retinopathy with atrophic macular lesions.

CONCLUSIONS

This work reveals a higher frequency of LCA5 mutations in a Spanish LCA cohort than in other populations. This study established gene-specific frequencies and the underlying phenotype of LCA5 mutations in the Spanish population.

Screening of a large cohort of leber congenital amaurosis and retinitis pigmentosa patients identifies novel LCA5 mutations and new genotype-phenotype correlations

This study was undertaken to investigate the prevalence of sequence variants in LCA5 in patients with Leber congenital amaurosis (LCA), early-onset retinal dystrophy (EORD), and autosomal recessive retinitis pigmentosa (arRP); to delineate the ocular phenotypes; and to provide an overview of all published LCA5 variants in an online database. Patients underwent standard ophthalmic evaluations after providing informed consent. In selected patients, optical coherence tomography (OCT) and fundus autofluorescence imaging were possible. DNA samples from 797 unrelated patients with LCA and 211 with the various types of retinitis pigmentosa (RP) were screened by Sanger sequence analysis of all LCA5 exons and intron/exon junctions. Some LCA patients were prescreened by APEX technology or selected based on homozygosity mapping. In silico analyses were performed to assess the pathogenicity of the variants. Segregation analysis was performed where possible. Published and novel LCA5 variants were collected, amended for their correct nomenclature, and listed in a Leiden Open Variation Database (LOVD). Sequence analysis identified 18 new probands with 19 different LCA5 variants. Seventeen of the 19 LCA5 variants were novel. Except for two missense variants and one splice site variant, all variants were protein-truncating mutations. Most patients expressed a severe phenotype, typical of LCA. However, some LCA subjects had better vision and intact inner segment/outer segment (IS/OS) junctions on OCT imaging. In two families with LCA5 variants, the phenotype was more compatible with EORD with affected individuals displaying preserved islands of retinal pigment epithelium. One of the families with a milder phenotype harbored a homozygous splice site mutation; a second family was found to have a combination of a stop mutation and a missense mutation. This is the largest LCA5 study to date. We sequenced 1,008 patients (797 with LCA, 211 with arRP) and identified 18 probands with LCA5 mutations. Mutations in LCA5 are a rare cause of childhood retinal dystrophy accounting for ∼2% of disease in this cohort, and the majority of LCA5 mutations are likely null. The LCA5 protein truncating mutations are predominantly associated with LCA. However, in two families with the milder EORD, the LCA5 gene analysis revealed a homozygous splice site mutation in one and a stop mutation in combination with a missense mutation in a second family, suggesting that this milder phenotype is due to residual function of lebercilin and expanding the currently known phenotypic spectrum to include the milder early onset RP. Some patients have remaining foveal cone structures (intact IS/OS junctions on OCT imaging) and remaining visual acuities, which may bode well for upcoming treatment trials.

Identification of a novel LCA5 mutation in a Pakistani family with Leber congenital amaurosis and cataracts

PURPOSE

To determine the cause of Leber congenital amaurosis (LCA) and developmental cataracts in a consanguineous Pakistani family.

METHODS

The diagnosis was established in all affected individuals of a Pakistani LCA family by medical history, funduscopy, and standard ERG. We performed genome-wide linkage analysis for mapping the disease locus in this family.

RESULTS

Congenitally severely reduced visual acuity and nystagmus were reported for all patients who, in the later phase of the disease, also developed cataracts. LCA in the family cosegregated with homozygosity for a single nucleotide polymorphism (SNP) haplotype on chromosome 6p14.1. The respective candidate region contained Leber congenital amaurosis 5 (LCA5), a gene previously reported to underlie LCA. We subsequently identified a novel truncating mutation in exon 4 of LCA5, c.642delC, in homozygous state in all affected persons of the family.

CONCLUSIONS

We report a novel LCA5 mutation causing LCA in a Pakistani family. Developmental cataracts were present in two of the four patients, raising the possibility that LCA5 mutations may predispose to this additional ocular pathology.

Leber congenital amaurosis: clinical correlations with genotypes, gene therapy trials update, and future directions

Leber congenital amaurosis comprises a group of early onset childhood retinal dystrophies, characterized by vision loss, nystagmus, and severe retinal dysfunction. To date, 15 causative genes have been identified that account for the heterogeneous presentation and clinical course. Knowledge of key aspects of phenotype and clinical course can contribute to the determination of a precise genetic etiology using genetic testing. Gene-based therapies are emerging, and knowledge of a patient's genotype is essential. A review of clinical presentation and disease course, their correlation to specific genotypes, and underlying physiological mechanisms, coupled with the latest results of human gene therapy trials, will assist the clinician in patient diagnosis and counseling.

Leber congenital amaurosis caused by Lebercilin (LCA5) mutation: retained photoreceptors adjacent to retinal disorganization

PURPOSE

To determine the retinal disease expression in the rare form of Leber congenital amaurosis (LCA) caused by Lebercilin (LCA5) mutation.

METHODS

Two young unrelated LCA patients, ages six years (P1) and 25 years (P2) at last visit, both with the same homozygous mutation in the LCA5 gene, were evaluated clinically and with noninvasive studies. En face imaging was performed with near-infrared (NIR) reflectance and autofluorescence (AF); cross-sectional retinal images were obtained with optical coherence tomography (OCT). Dark-adapted thresholds were measured in the older patient; and the transient pupillary light reflex was recorded and quantified in both patients.

RESULTS

Both LCA5 patients had light perception vision only, hyperopia, and nystagmus. P1 showed a prominent central island of retinal pigment epithelium (RPE) surrounded by alternating elliptical-appearing areas of decreased and increased pigmentation. Retinal laminar architecture at and near the fovea was abnormal in both patients. Foveal outer nuclear layer (ONL) was present in P1 and P2 but to different degrees. With increasing eccentricity, there was retinal laminar disorganization. Regions of pericentral and midperipheral retina in P1, but not P2, could retain measurable ONL and less laminopathy. P2 had a small central island of perception with >5 log units of sensitivity loss. Pupillary responsiveness was present in both LCA5 patients; the thresholds were abnormally elevated by >or=5.5 log units.

CONCLUSIONS

LCA5 patients had evidence of retained photoreceptors mainly in the central retina. Retinal remodeling was present in pericentral regions in both patients. The NIR reflectance and NIR-AF imaging in the younger patient suggested preserved RPE in retinal regions with retained photoreceptors. Detailed phenotype studies in other LCA5 patients with longitudinal follow-up will help determine the feasibility of future intervention in this rare disease.

Molecular genetics of infantile-onset retinal dystrophies

Over the last decade there have been major advances in our understanding of the molecular pathology of inherited retinal dystrophies. This paper reviews recent advances in the identification of genetic mutations underlying infantile-onset inherited retinal disorders and considers how this knowledge may lead to novel therapeutic approaches.

Identification of a novel splice-site mutation in the Lebercilin (LCA5) gene causing Leber congenital amaurosis

PURPOSE

Leber congenital amaurosis (LCA) is one of the most common causes of hereditary blindness in infants. To date, mutations in 13 known genes and at two other loci have been implicated in LCA causation. An examination of the known genes highlights several processes which, when defective, cause LCA, including photoreceptor development and maintenance, phototransduction, vitamin A metabolism, and protein trafficking. In addition, it has been known for some time that defects in sensory cilia can cause syndromes involving hereditary blindness. More recently evidence has come to light that non-syndromic LCA can also be a "ciliopathy."

METHODS

Here we present a homozygosity mapping analysis in a consanguineous sibship that led to the identification of a mutation in the recently discovered LCA5 gene. Homozygosity mapping was done using Affymetrix 10K Xba I Gene Chip and a 24.5cM region on chromosome 6 (6q12- q16.3) was identified to be significantly homozygous. The LCA5 gene on this region was sequenced and cDNA sequencing also done to characterize the mutation.

RESULTS

A c.955G>A missense mutation in the last base of exon 6 causing disruption of the splice donor site was identified in both the affected sibs. Since there is a second consensus splice donor sequence 5 bp into the adjacent intron, this mutation results in a transcript with a 5 bp insertion of intronic sequence, leading to a frameshift and premature truncation.

CONCLUSIONS

We report a missense mutation functionally altering the splice donor site and leading to a truncated protein. This is the second report of LCA5 mutations causing LCA. It may also be significant that one affected child died at eleven months of age due to asphyxia during sleep. To date the only phenotype unambiguously associated with mutations in this gene is LCA. However the LCA5 gene is known to be expressed in nasopharynx, trachea and lungs and was originally identified in the proteome of bronchial epithelium ciliary axonemes. The cause of death in this child may therefore imply that LCA5 mutations can in fact cause a wider spectrum of phenotypes including respiratory disease.

Mutations in LCA5, encoding the ciliary protein lebercilin, cause Leber congenital amaurosis

Leber congenital amaurosis (LCA) causes blindness or severe visual impairment at or within a few months of birth. Here we show, using homozygosity mapping, that the LCA5 gene on chromosome 6q14, which encodes the previously unknown ciliary protein lebercilin, is associated with this disease. We detected homozygous nonsense and frameshift mutations in LCA5 in five families affected with LCA. In a sixth family, the LCA5 transcript was completely absent. LCA5 is expressed widely throughout development, although the phenotype in affected individuals is limited to the eye. Lebercilin localizes to the connecting cilia of photoreceptors and to the microtubules, centrioles and primary cilia of cultured mammalian cells. Using tandem affinity purification, we identified 24 proteins that link lebercilin to centrosomal and ciliary functions. Members of this interactome represent candidate genes for LCA and other ciliopathies. Our findings emphasize the emerging role of disrupted ciliary processes in the molecular pathogenesis of LCA.

Exclusion of LCA5 locus in a consanguineous Turkish family with macular coloboma-type LCA

BACKGROUND

Leber's congenital amaurosis (LCA) is an inherited retinal dystrophy, which causes severe visual impairment in early childhood. Recent molecular genetic studies have linked 11 loci (AIPL1, CRB1, CRX, GUCY2D, RPE65, RDH12, RPGRIP1, TULP1, LCA3, LCA5, and LCA9) to LCA. LCA5 is a new locus, which maps to the 6q11-q16 chromosomal region and was found to be associated with macular coloboma-type LCA in a Pakistani family. Herein, we describe the molecular genetic features of a consanguineous Turkish family in which four children have macular coloboma-type LCA.

METHODS

Haplotype analysis was performed on the DNA of the family members using microsatellite markers against GUCY2D, RPE65, and LCA5. Genomic DNA was screened for mutations by means of single-strand conformational polymorphism (SSCP) analysis in exons of the RPE65 and CRX genes.

RESULTS

In haplotype analysis, no linkage to LCA5 or GUCY2D loci was detected. None of the tested markers showed homozygosity or segregation between affected siblings. PCR-SSCP mutation analysis revealed no mutations in the screened RPE65 and CRX genes.

CONCLUSION

We excluded LCA5 as the genetic cause of macular coloboma-type LCA in this Turkish family. Macular coloboma-type LCA shows genetic heterogeneity and it is not possible to establish a phenotype-genotype correlation with LCA5 and macular coloboma.

Leber congenital amaurosis: a genetic paradigm

Leber congenital amaurosis (LCA; estimated prevalence 1 : 50,000-100,000) is an early-onset inherited cause of childhood blindness characterized by a severe retinal dystrophy immediately after birth. Variants in at least six genes, AIPL1, CRB1, CRX, GUCY2D, RPE65, and RPGRIP1, have been associated with a diagnosis consistent with LCA or early-onset retinitis pigmentosa and together account for less than 50% of all LCA cases. Genetically heterogeneous inheritance has complicated the molecular analysis of LCA cases, especially sporadic ones where conventional methods are of limited value. Until recently, the management of patients with LCA relied mainly on clinical examination, electrophysiology, and other ancillary tests. Genotyping, i.e., determining the exact genetic defect causing LCA in each specific case, was not routinely performed since the comprehensive screening of six genes by SSCP and/or direct sequencing is relatively inefficient and cost-prohibitive. Patients, therefore, were often left with no specific information on their disease status. Recent advances in genotyping technologies have allowed the introduction of comprehensive and affordable screening procedures to determine causal genetic variation, resulting in precise molecular diagnosis, more accurate visual prognosis, and suggestions towards treatment options.