Next-generation Sequencing Extends the Phenotypic Spectrum for LCA5 Mutations: Novel LCA5 Mutations in Cone Dystrophy

On the Wrong Track: Alterations of Ciliary Transport in Inherited Retinal Dystrophies

Ciliopathies are a group of heterogeneous inherited disorders associated with dysfunction of the cilium, a ubiquitous microtubule-based organelle involved in a broad range of cellular functions. Most ciliopathies are syndromic, since several organs whose cells produce a cilium, such as the retina, cochlea or kidney, are affected by mutations in ciliary-related genes. In the retina, photoreceptor cells present a highly specialized neurosensory cilium, the outer segment, stacked with membranous disks where photoreception and phototransduction occurs. The daily renewal of the more distal disks is a unique characteristic of photoreceptor outer segments, resulting in an elevated protein demand. All components necessary for outer segment formation, maintenance and function have to be transported from the photoreceptor inner segment, where synthesis occurs, to the cilium. Therefore, efficient transport of selected proteins is critical for photoreceptor ciliogenesis and function, and any alteration in either cargo delivery to the cilium or intraciliary trafficking compromises photoreceptor survival and leads to retinal degeneration. To date, mutations in more than 100 ciliary genes have been associated with retinal dystrophies, accounting for almost 25% of these inherited rare diseases. Interestingly, not all mutations in ciliary genes that cause retinal degeneration are also involved in pleiotropic pathologies in other ciliated organs. Depending on the mutation, the same gene can cause syndromic or non-syndromic retinopathies, thus emphasizing the highly refined specialization of the photoreceptor neurosensory cilia, and raising the possibility of photoreceptor-specific molecular mechanisms underlying common ciliary functions such as ciliary transport. In this review, we will focus on ciliary transport in photoreceptor cells and discuss the molecular complexity underpinning retinal ciliopathies, with a special emphasis on ciliary genes that, when mutated, cause either syndromic or non-syndromic retinal ciliopathies.

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.

Molecular Screening of 43 Brazilian Families Diagnosed with Leber Congenital Amaurosis or Early-Onset Severe Retinal Dystrophy

Leber congenital amaurosis (LCA) is a severe disease that leads to complete blindness in children, typically before the first year of life. Due to the clinical and genetic heterogeneity among LCA and other retinal diseases, providing patients with a molecular diagnosis is essential to assigning an accurate clinical diagnosis. Using our gene panel that targets 300 genes that are known to cause retinal disease, including 24 genes reported to cause LCA, we sequenced 43 unrelated probands with Brazilian ancestry. We identified 42 unique variants and were able to assign a molecular diagnosis to 30/43 (70%) Brazilian patients. Among these, 30 patients were initially diagnosed with LCA or a form of early-onset retinal dystrophy, 17 patients harbored mutations in LCA-associated genes, while 13 patients had mutations in genes that were reported to cause other diseases involving the retina.

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.