Mutation discovered in a feline model of human congenital retinal blinding disease

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.

Canine and Feline Models of Inherited Retinal Diseases

Naturally occurring inherited retinal diseases (IRDs) in cats and dogs provide a rich source of potential models for human IRDs. In many cases, the phenotypes between the species with mutations of the homologous genes are very similar. Both cats and dogs have a high-acuity retinal region, the area centralis, an equivalent to the human macula, with tightly packed photoreceptors and higher cone density. This and the similarity in globe size to that of humans means these large animal models provide information not obtainable from rodent models. The established cat and dog models include those for Leber congenital amaurosis, retinitis pigmentosa (including recessive, dominant, and X-linked forms), achromatopsia, Best disease, congenital stationary night blindness and other synaptic dysfunctions, RDH5-associated retinopathy, and Stargardt disease. Several of these models have proven to be important in the development of translational therapies such as gene-augmentation therapies. Advances have been made in editing the canine genome, which necessitated overcoming challenges presented by the specifics of canine reproduction. Feline genome editing presents fewer challenges. We can anticipate the generation of specific cat and dog IRD models by genome editing in the future.

Animal Models in Eye Research: Focus on Corneal Pathologies

The eye is a complex sensory organ that enables visual perception of the world. The dysfunction of any of these tissues can impair vision. Conduction studies on laboratory animals are essential to ensure the safety of therapeutic products directly applied or injected into the eye to treat ocular diseases before eventually proceeding to clinical trials. Among these tissues, the cornea has unique homeostatic and regenerative mechanisms for maintaining transparency and refraction of external light, which are essential for vision. However, being the outermost tissue of the eye and directly exposed to the external environment, the cornea is particularly susceptible to injury and diseases. This review highlights the evidence for selecting appropriate animals to better understand and treat corneal diseases, which rank as the fifth leading cause of blindness worldwide. The development of reliable and human-relevant animal models is, therefore, a valuable research tool for understanding and translating fundamental mechanistic findings, as well as for assessing therapeutic potential in humans. First, this review emphasizes the unique characteristics of animal models used in ocular research. Subsequently, it discusses current animal models associated with human corneal pathologies, their utility in understanding ocular disease mechanisms, and their role as translational models for patients.

Metabolic changes and retinal remodeling in Heterozygous CRX mutant cats (CRXRDY/+)

CRX is a transcription factor essential for normal photoreceptor development and survival. The CRXRdy cat has a naturally occurring truncating mutation in CRX and is a large animal model for dominant Leber congenital amaurosis. This study investigated retinal remodeling that occurs as photoreceptors degenerate. CRXRdy/+ cats from 6 weeks to 10 years of age were investigated. In vivo structural changes of retinas were analyzed by fundus examination, confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography. Histologic analyses included immunohistochemistry for computational molecular phenotyping with macromolecules and small molecules. Affected cats had a cone-led photoreceptor degeneration starting in the area centralis. Initially there was preservation of inner retinal cells such as bipolar, amacrine and horizontal cells but with time migration of the deafferented neurons occurred. Early in the process of degeneration glial activation occurs ultimately resulting in formation of a glial seal. With progression the macula-equivalent area centralis developed severe atrophy including loss of retinal pigmentary epithelium. Microneuroma formation occured in advanced stages as more marked retinal remodeling occurred. This study indicates that retinal degeneration in the CrxRdy/+ cat retina follows the progressive, phased revision of retina that have been previously described for retinal remodeling. These findings suggest that therapy dependent on targeting inner retinal cells may be useful in young adults with preserved inner retinas prior to advanced stages of retinal remodeling and neuronal cell loss.

Cat LCA-CRX Model, Homozygous for an Antimorphic Mutation Has a Unique Phenotype

Purpose

Mutations in the CRX transcription factor are associated with dominant retinopathies often with more severe macular changes. The CRX-mutant cat (Rdy-A182d2) is the only animal model with the equivalent of the critical retinal region for high-acuity vision, the macula. Heterozygous cats (CRXRdy/+) have a severe phenotype modeling Leber congenital amaurosis. This study reports the distinct ocular phenotype of homozygous cats (CRXRdy/Rdy).

Methods

Gene expression changes were assessed at both mRNA and protein levels. Changes in globe morphology and retinal structure were analyzed.

Results

CRXRdy/Rdy cats had high levels of mutant CRX mRNA and protein. The expression of photoreceptor target genes was severely impaired although there were variable effects on the expression of other transcription factors. The photoreceptor cells remained immature and failed to elaborate outer segments consistent with the lack of retinal function. The retinal layers displayed a progressive remodeling with cell loss but maintained overall retinal thickness due to gliosis. Rapid photoreceptor loss largely occurred in the macula-equivalent retinal region. The homozygous cats developed markedly increased ocular globe length.

Conclusions

The phenotype of CRXRdy/Rdy cats was more severe compared to CRXRdy/+ cats by several metrics.

Translational Relevance

The CRX-mutant cat is the only model for CRX-retinopathies with a macula-equivalent region. A prominent feature of the CRXRdy/Rdy cat phenotype not detectable in homozygous mouse models was the rapid degeneration of the macula-equivalent retinal region highlighting the value of this large animal model and its future importance in the testing of translational therapies aiming to restore vision.

Preclinical Models of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the name for a group of phenotypically-related heritable retinal degenerative disorders. Many genes have been implicated as causing variants of RP, and while the clinical phenotypes are remarkably similar, they may differ in age of onset, progression, and severity. Common inheritance patterns for specific genes connected with the development of the disorder include autosomal dominant, autosomal recessive, and X-linked. Modeling the disease in animals and other preclinical systems offers a cost-conscious, ethical, and time-efficient method for studying the disease subtypes. The history of RP models is briefly examined, and both naturally occurring and transgenic preclinical models of RP in many different organisms are discussed. Syndromic forms of RP and models thereof are reviewed as well.

Epidemiology and clinical presentation of feline presumed hereditary or breed-related ocular diseases in France: retrospective study of 129 cats

OBJECTIVES

This study aimed to describe the epidemiology and clinical presentation of presumed hereditary or presumed breed-related ocular diseases in a population of cats in France.

METHODS

Medical records from between September 2013 and August 2017 were reviewed to identify cats with at least one presumed hereditary or breed-related ocular disease. Cats with concurrent, or a history of, ocular or systemic infectious diseases were excluded. Signalment, history and clinical findings were recorded.

RESULTS

Of the 1161 cats that presented to our institution during the study period, 129 were diagnosed with at least one presumed hereditary or presumed breed-related ocular disease (11.1%, 95% confidence interval [CI] 9.3-12.9). Five ocular abnormalities had a prevalence of >1%: entropion, corneal sequestration, persistent pupillary membrane, cataract and retinal dysplasia. The prevalence of entropion was 2.2% (95% CI 1.3-3.0), with Persians (P = 0.03), Maine Coons (P <0.01) and male cats (P <0.01) being over-represented. The prevalence of corneal sequestration was 2.4% (95% CI 1.5-3.3), with Persians (P <0.01) and Exotic Shorthairs (P = 0.02) being over-represented. Persistent pupillary membranes and cataracts had the same prevalence of 2.3% (95% CI 1.5-3.2), with no particular sex or breed significantly over-represented. Retinal dysplasia had a prevalence of 1.6% (95% CI 0.8-2.3) and Persian cats were over-represented (P = 0.04). Anterior segment dysgenesis had a low prevalence (0.9%, 95% CI 0.4-1.5), with all affected cats being domestic shorthairs and this breed therefore was over-represented (P = 0.04).

CONCLUSIONS AND RELEVANCE

In a French population of cats, presumed hereditary or breed-related ocular diseases accounted for 11.1% of all ocular diseases. Cataracts, corneal sequestration, persistent pupillary membrane, entropion and retinal dysplasia were the most common conditions. Statistical breed over-representation was observed for entropion, corneal sequestration and retinal dysplasia. We recommend that more systematic screening of feline species is conducted.

VISTA Is a Diagnostic Biomarker and Immunotherapy Target of Aggressive Feline Mammary Carcinoma Subtypes

Simple Summary

Mammary tumors are common in cats, showing aggressive behavior and few therapeutic options. Recently, feline mammary carcinomas have become a reliable cancer model for human breast cancer studies, due to the similarities between the two species. Thus, the identification of new tumor biomarkers and therapeutic targets to improve cat’s prognosis is needed. VISTA is an important immune checkpoint protein that has gained importance over the past few years in women’s cancers. In this study, the serum VISTA levels and tumor expression were analyzed in cats with mammary tumors, being correlated with other immune checkpoints. In the diseased animals, VISTA is overexpressed in more aggressive tumor subtypes (HER2-positive and triple-negative), showing a positive correlation with the expression of VISTA in tumor-infiltrating lymphocytes, and is associated with an immunosuppressive status, suggesting that VISTA could be a promising non-invasive prognostic biomarker and therapeutic target in cats with mammary carcinomas, as reported in humans.

Abstract

Feline mammary carcinoma (FMC) is a common neoplasia, showing aggressive clinicopathological features, without viable therapeutic options. The study of tumor microenvironment has gained importance, due to the ability to control tumor progression by regulating the immune response. Considering the lack of knowledge, feline serum VISTA levels from cats with mammary carcinoma were compared with healthy controls, and with serum levels of PD-1/PD-L1, CTLA-4, LAG-3, IL-6, and TNF-α. In parallel, VISTA tumor expression was evaluated in FMC samples. The obtained data revealed that serum VISTA levels were significantly higher in cats presenting HER2-positive (p = 0.0025) or triple-negative subtypes (p = 0.0019), with higher serum levels in luminal A (p = 0.0025) correlated to the presence of metastasis (p = 0.0471). Furthermore, in HER2-positive or triple-negative tumors, correlations were obtained between serum VISTA levels and the serum levels of the above-mentioned molecules. In tumors, VISTA expression revealed a stronger intensity in cancer cells, when compared to TILs (p < 0.0001). Stratifying the samples by subtypes, a higher number of VISTA-positive TILs was observed in the HER2-positive subtype, compared with triple-negative tumors (p = 0.0138). In conclusion, results support the development of therapeutic strategies for HER2-positive and triple-negative FMC subtypes, reinforcing the use of cats as a human oncology model.

Emerging Biomarkers and Targeted Therapies in Feline Mammary Carcinoma

Feline mammary carcinoma (FMC) is a common aggressive malignancy with a low survival rate that lacks viable therapeutic options beyond mastectomy. Recently, increasing efforts have been made to understand the molecular mechanisms underlying FMC development, using the knowledge gained from studies on human breast cancer to discover new diagnostic and prognostic biomarkers, thus reinforcing the utility of the cat as a cancer model. In this article, we review the current knowledge on FMC pathogenesis, biomarkers, and prognosis factors and offer new insights into novel therapeutic options for HER2-positive and triple-negative FMC subtypes.

Animals Models of Inherited Retinal Disease

Inherited retinal diseases (IRDs) are an important cause of blindness worldwide. Over 270 genes have been associated with IRD. Genetic testing can determine the cause of the clinical disease in the majority of patients. However, at least 25-50% of patients with clinical diagnosis of IRD remain unsolved even after whole genome sequencing. Animal models of IRD can be useful for expanding the set of established IRD genes, to gain biological understanding of the function of these genes in the retina, and to test advanced therapeutics prior to human clinical trials. In this chapter some small and large animal models of IRD are discussed including some of the advantages and limitations of each for various forms of retinopathy.

Modeling Cone/Cone-Rod Dystrophy Pathology by AAV-Mediated Overexpression of Mutant CRX Protein in the Mouse Retina

Purpose

This study aims to evaluate the pathogenesis of cone/cone–rod dystrophy (CoD/CoRD) caused by a cone–rod homeobox (CRX) mutation, which was identified in a Chinese family, through adeno-associated virus (AAV)-mediated overexpression of mutant CRX protein in the mouse retina.

Methods

Comprehensive ophthalmologic examinations were performed for the pedigree members of a Chinese family with CoD/CoRD. Whole exome sequencing and Sanger sequencing were performed to determine the genetic cause of the disease. Furthermore, AAV vectors were used to construct AAV-CRX-mut-HA, which was transfected into mouse photoreceptor cells to clarify the pathogenesis of the mutant CRX.

Results

Fundus photography and optical coherence tomography images displayed features that were consistent with CoD/CoRD, including macular atrophy and photoreceptor layer thinning. Electroretinogram analysis indicated an obvious decrease in photopic responses or both scotopic and photopic responses in affected individuals. A frameshift variant c.611delC (p.S204fs) in CRX was cosegregated with the disease in this family. AAV-CRX-mut-HA that subretinally injected into the C57BL/6 mice generally transfected the outer nuclear layer, leading to the loss of cone and rod photoreceptor cells, abnormal expression of CRX target genes, and a decrease in electroretinogram responses.

Conclusions

AAV-mediated overexpression of CRX^[S204fs] in the mouse retina led to a CoRD-like phenotype and showed the possible pathogenesis of the antimorphic CRX mutation.

Translational Relevance

This study provides a modeling method to evaluate the pathogenesis of CoD/CoRD and other inherited retinal dystrophies caused by distinct gain-of-function mutations.

Mining the 99 Lives Cat Genome Sequencing Consortium database implicates genes and variants for the Ticked locus in domestic cats (Felis catus)

Tabby patterns of fur coats are defining characteristics in wild and domestic felids. Historically, three autosomal alleles at one locus (Tabby): Abyssinian (Ta ; a.k.a. ticked), mackerel (Tm ; a.k.a. striped) and blotched (tb ; a.k.a. classic, blotched) were thought to control these patterns in domestic cats and their breeds. Currently, at least three loci influence cat tabby markings, two of which are designated Tabby and Ticked. The Tabby locus is laeverin (LVRN) and affects the mackerel and blotched patterns. The unidentified gene for the Ticked locus on cat chromosome B1 was suggested to control the presence or absence of the ticked pattern (Tabby - Abyssinian (Ta ; a.k.a. ticked). The cat reference genome (Cinnamon, the Abyssinian) has the ticked phenotype and the variant dataset and coat phenotypes from the 99 Lives Cat Genome Consortium (195 cats) were used to identify candidate genes and variants associated with the Ticked locus. Two strategies were used to find the Ticked allele(s), one considered Cinnamon with the reference allele or heterozygous (Strategy A) and the other considered Cinnamon as having the variant allele or heterozygous (Strategy B). For Strategy A, two variants in Dickkopf Wnt Signaling Pathway Inhibitor 4 (DKK4), a p.Cys63Tyr (B1:41621481, c.188G>A) and a less common p.Ala18Val (B1:42620835, c.53C>T) variant are suggested as two alleles influencing the Ticked phenotype. Bioinformatic and molecular modeling analysis suggests that these changes disrupt a key disulfide bond in the Dkk4 cysteine-rich domain 1 or Dkk4 signal peptide cleavage respectively. All coding variants were excluded as Ticked alleles using Strategy B.

Genetic relationships and inbreeding levels among geographically distant populations of Felis catus from Japan and the United States

Pedigreed cats have traditionally been mated with close relatives, which increases the risks for inbreeding depression and genetic disorders. We evaluated the genome-wide population structure and the degree of inbreeding of 1022 cats, including 13 pedigreed and two random bred populations from Japan and the USA, using single nucleotide polymorphism array-based data. Ancestry structure analysis revealed Japan's American Curl, Norwegian Forest, and Siamese cat populations were genetically distinct from their American counterparts. Furthermore, we found an ancestral genetic component shared between five pedigreed and random bred Japanese cats, suggesting the breeds were admixed with Japanese cats or cats of east Asian origin. Between-country differences in inbreeding estimates based on runs of homozygosity were found for Maine Coon, Siamese, and random bred cats. To reduce the risks of inbreeding depression and genetic disorders, particularly for highly inbred breeds, such as Abyssinian cats, as well as Russian Blue and Siamese cats in the USA, appropriate breeding practices must be observed, including mating practices that increase the genetic diversity.

Large Animal Models of Inherited Retinal Degenerations: A Review

Studies utilizing large animal models of inherited retinal degeneration (IRD) have proven important in not only the development of translational therapeutic approaches, but also in improving our understanding of disease mechanisms. The dog is the predominant species utilized because spontaneous IRD is common in the canine pet population. Cats are also a source of spontaneous IRDs. Other large animal models with spontaneous IRDs include sheep, horses and non-human primates (NHP). The pig has also proven valuable due to the ease in which transgenic animals can be generated and work is ongoing to produce engineered models of other large animal species including NHP. These large animal models offer important advantages over the widely used laboratory rodent models. The globe size and dimensions more closely parallel those of humans and, most importantly, they have a retinal region of high cone density and denser photoreceptor packing for high acuity vision. Laboratory rodents lack such a retinal region and, as macular disease is a critical cause for vision loss in humans, having a comparable retinal region in model species is particularly important. This review will discuss several large animal models which have been used to study disease mechanisms relevant for the equivalent human IRD.

Cellular mechanisms of hereditary photoreceptor degeneration - Focus on cGMP

The cellular mechanisms underlying hereditary photoreceptor degeneration are still poorly understood, a problem that is exacerbated by the enormous genetic heterogeneity of this disease group. However, the last decade has yielded a wealth of new knowledge on degenerative pathways and their diversity. Notably, a central role of cGMP-signalling has surfaced for photoreceptor cell death triggered by a subset of disease-causing mutations. In this review, we examine key aspects relevant for photoreceptor degeneration of hereditary origin. The topics covered include energy metabolism, epigenetics, protein quality control, as well as cGMP- and Ca²⁺-signalling, and how the related molecular and metabolic processes may trigger photoreceptor demise. We compare and integrate evidence on different cell death mechanisms that have been associated with photoreceptor degeneration, including apoptosis, necrosis, necroptosis, and PARthanatos. A special focus is then put on the mechanisms of cGMP-dependent cell death and how exceedingly high photoreceptor cGMP levels may cause activation of Ca²⁺-dependent calpain-type proteases, histone deacetylases and poly-ADP-ribose polymerase. An evaluation of the available literature reveals that a large group of patients suffering from hereditary photoreceptor degeneration carry mutations that are likely to trigger cGMP-dependent cell death, making this pathway a prime target for future therapy development. Finally, an outlook is given into technological and methodological developments that will with time likely contribute to a comprehensive overview over the entire metabolic complexity of photoreceptor cell death. Building on such developments, new imaging technology and novel biomarkers may be used to develop clinical test strategies, that fully consider the genetic heterogeneity of hereditary retinal degenerations, in order to facilitate clinical testing of novel treatment approaches.

Pre-existing antibodies to candidate gene therapy vectors (adeno-associated vector serotypes) in domestic cats

Adeno-associated virus (AAV) vectors represent promising candidates for gene therapy; however, pre-existing neutralizing antibodies (NAb) may reduce AAV vector delivery efficiency. In this study, the presence of AAV NAb was investigated in cats, which serve as a larger and outbred animal model for the prediction of gene therapy outcomes in humans but also in cats.Serum/plasma samples from 230 client-owned Swiss cats and 20 specified pathogen-free cats were investigated for NAb to AAV1, AAV2, AAV5, AAV6, AAV7, AAV8 and AAV9 using in vitro transduction inhibition and a beta-galactosidase assay. NAb to all tested AAV serotypes were found. Of the client-owned cats, 53% had NAb to one or more of the AAV serotypes. NAb (≥1:10) were found at frequencies of 5% (AAV6) to 28% (AAV7). The highest titers were found against AAV7 (≥1:160). The NAb prevalence to AAV2, AAV7 and AAV9 differed geographically. Regarding titers ≥1:10 against single AAV serotypes, age, breed and sex of the cats were not associated with the NAb prevalence. Cats with titers ≥1:20 against AAV2 and titers ≥1:40 against AAV7 were significantly younger than cats with low/no titers, and purebred cats were significantly more likely than non-purebred cats to have NAb to AAV2 (≥1:40). Additionally, regarding NAb to all AAV combined, female cats were significantly more likely than male cats to have NAb titers ≥1:40. Preliminary data using AAV-DJ indicated that less pre-existing NAb to the hybrid AAV-DJ can be expected compared to the wild-type AAV serotypes. AAV NAb will need to be taken into account for future in vivo gene therapy studies in cats.

Naturally Occurring Inherited Forms of Retinal Degeneration in Vertebrate Animal Species: A Comparative and Evolutionary Perspective

The ability to noninvasively monitor retinal abnormalities using imaging and cognitive and electrophysiological assessment has made it possible to carefully characterize genetic influences on retinal health. Because genetic retinal traits in animal species are not commonly detrimental to survival beyond birth, it is possible to document the natural history of retinal disease. Human quality of life is greatly impacted by retinal disease, and blindness carries a significant financial burden to society. Because of these compelling reasons, there is an ongoing medical need to study the effect of genetic mutations on retinal health and to develop therapies to address them. Transgenic animal models have aided in these missions, but there are opportunities for novel gene discovery and a development of greater understanding of retinal physiology using animal models that develop naturally occurring heritable retinal disorders. In this chapter, the advantages and disadvantages of transgenic and spontaneous vertebrate animal models of human inherited retinal disease are debated, in particular those of carnivore species, and the potential resource of spontaneous heritable retinal disorders in inbred nondomestic carnivore species is discussed.

Pluripotent Stem Cells for Retinal Tissue Engineering: Current Status and Future Prospects

The retina is a very fine and layered neural tissue, which vitally depends on the preservation of cells, structure, connectivity and vasculature to maintain vision. There is an urgent need to find technical and biological solutions to major challenges associated with functional replacement of retinal cells. The major unmet challenges include generating sufficient numbers of specific cell types, achieving functional integration of transplanted cells, especially photoreceptors, and surgical delivery of retinal cells or tissue without triggering immune responses, inflammation and/or remodeling. The advances of regenerative medicine enabled generation of three-dimensional tissues (organoids), partially recreating the anatomical structure, biological complexity and physiology of several tissues, which are important targets for stem cell replacement therapies. Derivation of retinal tissue in a dish creates new opportunities for cell replacement therapies of blindness and addresses the need to preserve retinal architecture to restore vision. Retinal cell therapies aimed at preserving and improving vision have achieved many improvements in the past ten years. Retinal organoid technologies provide a number of solutions to technical and biological challenges associated with functional replacement of retinal cells to achieve long-term vision restoration. Our review summarizes the progress in cell therapies of retina, with focus on human pluripotent stem cell-derived retinal tissue, and critically evaluates the potential of retinal organoid approaches to solve a major unmet clinical need—retinal repair and vision restoration in conditions caused by retinal degeneration and traumatic ocular injuries. We also analyze obstacles in commercialization of retinal organoid technology for clinical application.

Genetic characterization and disease mechanism of retinitis pigmentosa; current scenario

Retinitis pigmentosa is a group of genetically transmitted disorders affecting 1 in 3000-8000 individual people worldwide ultimately affecting the quality of life. Retinitis pigmentosa is characterized as a heterogeneous genetic disorder which leads by progressive devolution of the retina leading to a progressive visual loss. It can occur in syndromic (with Usher syndrome and Bardet-Biedl syndrome) as well as non-syndromic nature. The mode of inheritance can be X-linked, autosomal dominant or autosomal recessive manner. To date 58 genes have been reported to associate with retinitis pigmentosa most of them are either expressed in photoreceptors or the retinal pigment epithelium. This review focuses on the disease mechanisms and genetics of retinitis pigmentosa. As retinitis pigmentosa is tremendously heterogeneous disorder expressing a multiplicity of mutations; different variations in the same gene might induce different disorders. In recent years, latest technologies including whole-exome sequencing contributing effectively to uncover the hidden genesis of retinitis pigmentosa by reporting new genetic mutations. In future, these advancements will help in better understanding the genotype-phenotype correlations of disease and likely to develop new therapies.

CrxRdy Cat: A Large Animal Model for CRX-Associated Leber Congenital Amaurosis

Purpose

Mutations in the retinal transcription factor cone-rod homeobox (CRX) gene result in severe dominant retinopathies. A large animal model, the Rdy cat, carrying a spontaneous frameshift mutation in Crx, was reported previously. The present study aimed to further understand pathogenesis in this model by thoroughly characterizing the Rdy retina.

Methods

Structural and functional changes were found in a comparison between the retinas of Crx^Rdy/+ kittens and those of wild-type littermates and were determined at various ages by fundus examination, electroretinography (ERG), optical coherence tomography, and histologic analyses. RNA and protein expression changes of Crx and key target genes were analyzed using quantitative reverse-transcribed PCR, Western blot analysis, and immunohistochemistry. Transcription activity of the mutant Crx was measured by a dual-luciferase transactivation assay.

Results

Crx^Rdy/+ kittens had no recordable cone ERGs. Rod responses were delayed in development and markedly reduced at young ages and lost by 20 weeks. Photoreceptor outer segment development was incomplete and was followed by progressive outer retinal thinning starting in the cone-rich area centralis. Expression of cone and rod Crx target genes was significantly down-regulated. The mutant Crx allele was overexpressed, leading to high levels of the mutant protein lacking transactivation activity.

Conclusions

The Crx^Rdy mutation exerts a dominant negative effect on wild-type Crx by overexpressing mutant protein. These findings, consistent with those of studies in a mouse model, support a conserved pathogenic mechanism for CRX frameshift mutations. The similarities between the feline eye and the human eye with the presence of a central region of high cone density makes the Crx^Rdy/+ cat a valuable model for preclinical testing of therapies for dominant CRX diseases.

Top2b is involved in the formation of outer segment and synapse during late-stage photoreceptor differentiation by controlling key genes of photoreceptor transcriptional regulatory network

Topoisomerase II beta (Top2b) is an enzyme that alters the topologic states of DNA during transcription. Top2b deletion in early retinal progenitor cells causes severe defects in neural differentiation and affects cell survival in all retinal cell types. However, it is unclear whether the observed severe phenotypes are the result of cell-autonomous/primary defects or non-cell-autonomous/secondary defects caused by alterations of other retinal cells. Using photoreceptor cells as a model, we first characterized the phenotypes in Top2b conditional knockout. Top2b deletion leads to malformation of photoreceptor outer segments (OSs) and synapses accompanied by dramatic cell loss at late-stage photoreceptor differentiation. Then, we performed mosaic analysis with shRNA-mediated Top2b knockdown in neonatal retina using in vivo electroportation to target rod photoreceptors in neonatal retina. Top2b knockdown causes defective OS without causing a dramatic cell loss, suggesting a Top2b cell-autonomous function. Furthermore, RNA-seq analysis reveals that Top2b controls the expression of key genes in the photoreceptor gene-regulatory network (e.g., Crx, Nr2e3, Opn1sw, Vsx2) and retinopathy-related genes (e.g., Abca4, Bbs7, Pde6b). Together, our data establish a combinatorial cell-autonomous and non-cell-autonomous role for Top2b in the late stage of photoreceptor differentiation and maturation. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Early-Onset Progressive Retinal Atrophy Associated with an IQCB1 Variant in African Black-Footed Cats (Felis nigripes)

African black-footed cats (Felis nigripes) are endangered wild felids. One male and full-sibling female African black-footed cat developed vision deficits and mydriasis as early as 3 months of age. The diagnosis of early-onset progressive retinal atrophy (PRA) was supported by reduced direct and consensual pupillary light reflexes, phenotypic presence of retinal degeneration, and a non-recordable electroretinogram with negligible amplitudes in both eyes. Whole genome sequencing, conducted on two unaffected parents and one affected offspring was compared to a variant database from 51 domestic cats and a Pallas cat, revealed 50 candidate variants that segregated concordantly with the PRA phenotype. Testing in additional affected cats confirmed that cats homozygous for a 2 base pair (bp) deletion within IQ calmodulin-binding motif-containing protein-1 (IQCB1), the gene that encodes for nephrocystin-5 (NPHP5), had vision loss. The variant segregated concordantly in other related individuals within the pedigree supporting the identification of a recessively inherited early-onset feline PRA. Analysis of the black-footed cat studbook suggests additional captive cats are at risk. Genetic testing for IQCB1 and avoidance of matings between carriers should be added to the species survival plan for captive management.

Pathogenic mutations in TULP1 responsible for retinitis pigmentosa identified in consanguineous familial cases

PURPOSE

To identify pathogenic mutations responsible for autosomal recessive retinitis pigmentosa (arRP) in consanguineous familial cases.

METHODS

Seven large familial cases with multiple individuals diagnosed with retinitis pigmentosa were included in the study. Affected individuals in these families underwent ophthalmic examinations to document the symptoms and confirm the initial diagnosis. Blood samples were collected from all participating members, and genomic DNA was extracted. An exclusion analysis with microsatellite markers spanning the TULP1 locus on chromosome 6p was performed, and two-point logarithm of odds (LOD) scores were calculated. All coding exons along with the exon-intron boundaries of TULP1 were sequenced bidirectionally. We constructed a single nucleotide polymorphism (SNP) haplotype for the four familial cases harboring the K489R allele and estimated the likelihood of a founder effect.

RESULTS

The ophthalmic examinations of the affected individuals in these familial cases were suggestive of RP. Exclusion analyses confirmed linkage to chromosome 6p harboring TULP1 with positive two-point LOD scores. Subsequent Sanger sequencing identified the single base pair substitution in exon14, c.1466A>G (p.K489R), in four families. Additionally, we identified a two-base deletion in exon 4, c.286_287delGA (p.E96Gfs77*); a homozygous splice site variant in intron 14, c.1495+4A>C; and a novel missense variation in exon 15, c.1561C>T (p.P521S). All mutations segregated with the disease phenotype in the respective families and were absent in ethnically matched control chromosomes. Haplotype analysis suggested (p<10(-6)) that affected individuals inherited the causal mutation from a common ancestor.

CONCLUSIONS

Pathogenic mutations in TULP1 are responsible for the RP phenotype in seven familial cases with a common ancestral mutation responsible for the disease phenotype in four of the seven families.

Loss of function mutations in RP1 are responsible for retinitis pigmentosa in consanguineous familial cases

PURPOSE

This study was undertaken to identify causal mutations responsible for autosomal recessive retinitis pigmentosa (arRP) in consanguineous families.

METHODS

Large consanguineous families were ascertained from the Punjab province of Pakistan. An ophthalmic examination consisting of a fundus evaluation and electroretinography (ERG) was completed, and small aliquots of blood were collected from all participating individuals. Genomic DNA was extracted from white blood cells, and a genome-wide linkage or a locus-specific exclusion analysis was completed with polymorphic short tandem repeats (STRs). Two-point logarithm of odds (LOD) scores were calculated, and all coding exons and exon-intron boundaries of RP1 were sequenced to identify the causal mutation.

RESULTS

The ophthalmic examination showed that affected individuals in all families manifest cardinal symptoms of RP. Genome-wide scans localized the disease phenotype to chromosome 8q, a region harboring RP1, a gene previously implicated in the pathogenesis of RP. Sanger sequencing identified a homozygous single base deletion in exon 4: c.3697delT (p.S1233Pfs22*), a single base substitution in intron 3: c.787+1G>A (p.I263Nfs8*), a 2 bp duplication in exon 2: c.551_552dupTA (p.Q185Yfs4*) and an 11,117 bp deletion that removes all three coding exons of RP1. These variations segregated with the disease phenotype within the respective families and were not present in ethnically matched control samples.

CONCLUSIONS

These results strongly suggest that these mutations in RP1 are responsible for the retinal phenotype in affected individuals of all four consanguineous families.

Whole genome sequencing in cats, identifies new models for blindness in AIPL1 and somite segmentation in HES7

BACKGROUND

The reduced cost and improved efficiency of whole genome sequencing (WGS) is drastically improving the development of cats as biomedical models. Persian cats are models for Leber's congenital amaurosis (LCA), the most severe and earliest onset form of visual impairment in humans. Cats with innocuous breed-defining traits, such as a bobbed tail, can also be models for somite segmentation and vertebral column development.

METHODS

The first WGS in cats was conducted on a trio segregating for LCA and the bobbed tail abnormality. Variants were identified using FreeBayes and effects predicted using SnpEff. Variants within a known haplotype block for cat LCA and specific candidate genes for both phenotypes were prioritized by the predicted variant effect on the proteins and concordant segregation within the trio. The efficiency of WGS of a single trio of domestic cats was evaluated.

RESULTS

A stop gain was identified at position c.577C > T in cat AIPL1, a predicted p.Arg193*. A c.5A > G variant causing a p.V2A was identified in HES7. The variants segregated concordantly in a Persian - Japanese bobtail pedigree. Over 1700 cats from 40 different breeds and populations were genotyped for the AIPL1 variant, defining an allelic frequency in only Persian -related breeds of 1.15%. A sub-set of cats was genotyped for the HES7 variant, supporting the variant as private to the Japanese bobtail breed. Approximately 18 million SNPs were identified for application in cat research. The cat AIPL1 variant would have been considered a high priority variant for evaluation, regardless of a priori knowledge from previous genetic studies.

CONCLUSIONS

This study represents the first effort of the 99 Lives Cat Genome Sequencing Initiative to identify disease--causing variants in the domestic cat using WGS. The current cat reference assembly is efficient for gene and variant identification. However, as the feline variant database improves, development of cats as biomedical models for human disease will be more efficient, providing an alternative, large animal model for drug and gene therapy trials. Undiagnosed human patients with early-onset blindness should be screened for this AIPL1 variant. The HES7 variant should further calibrate the somite segmentation clock.

Characterization of an Early-Onset, Autosomal Recessive, Progressive Retinal Degeneration in Bengal Cats

PURPOSE

A form of retinal degeneration suspected to be hereditary was discovered in a family of Bengal cats. A breeding colony was established to characterize disease progression clinically, electrophysiologically, and morphologically, and to investigate the mode of inheritance.

METHODS

Affected and related cats were donated by owners for breeding trials and pedigree analysis. Kittens from test and complementation breedings underwent ophthalmic and neuro-ophthalmic examinations and ERG, and globes were evaluated using light microscopy.

RESULTS

Pedigree analysis, along with test and complementation breedings, indicated autosomal recessive inheritance and suggested that this disease is nonallelic to a retinal degeneration found in Persian cats. Mutation analysis confirmed the disease is not caused by CEP290 or CRX variants found predominantly in Abyssinian and Siamese cats. Ophthalmoscopic signs of retinal degeneration were noted at 9 weeks of age and became more noticeable over the next 4 months. Visual deficits were behaviorally evident by 1 year of age. Electroretinogram demonstrated reduced rod and cone function at 7 and 9 weeks of age, respectively. Rod responses were mostly extinguished at 14 weeks of age; cone responses were minimal by 26 weeks. Histologic degeneration was first observed at 8 weeks, evidenced by reduced photoreceptor numbers, then rapid deterioration of the photoreceptor layer and, subsequently, severe outer retinal degeneration.

CONCLUSIONS

A recessively inherited primary photoreceptor degeneration was characterized in the Bengal cat. The disease is characterized by early onset, with histologic, ophthalmoscopic, and electrophysiological signs evident by 2 months of age, and rapid progression to blindness.

DNA mutations of the cat: the good, the bad and the ugly

PRACTICAL RELEVANCE

The health of the cat is a complex interaction between its environment (nurture) and its genetics (nature). Over 70 genetic mutations (variants) have been defined in the cat, many involving diseases, structural abnormalities and clinically relevant health concerns. As more of the cat's genome is deciphered, less commonly will the term 'idiopathic' be used regarding the diagnosis of diseases and unique health conditions. State-of-the-art health care will include DNA profiling of the individual cat, and perhaps its tumor, to establish the best treatment approaches. Genetic testing and eventually whole genome sequencing should become routine diagnostics for feline health care.

GLOBAL IMPORTANCE

Cat breeds have disseminated around the world. Thus, practitioners should be aware of the breeds common to their region and the mutations found in those regional populations. Specific random-bred populations can also have defined genetic characteristics and mutations.

AUDIENCE

This review of 'the good, the bad and the ugly' DNA variants provides the current state of knowledge for genetic testing and genetic health management for cats. It is aimed at feline and general practitioners wanting to update and review the basics of genetics, what tests are available for cats and sources for genetic testing. The tables are intended to be used as references in the clinic. Practitioners with a high proportion of cat breeder clientele will especially benefit from the review.

EVIDENCE BASE

The data presented is extracted from peer-reviewed publications pertaining to mutation identification, and relevant articles concerning the heritable trait and/or disease. The author also draws upon personal experience and expertise in feline genetics.

Genome-wide association and linkage analyses localize a progressive retinal atrophy locus in Persian cats

Hereditary eye diseases of animals serve as excellent models of human ocular disorders and assist in the development of gene and drug therapies for inherited forms of blindness. Several primary hereditary eye conditions affecting various ocular tissues and having different rates of progression have been documented in domestic cats. Gene therapy for canine retinopathies has been successful, thus the cat could be a gene therapy candidate for other forms of retinal degenerations. The current study investigates a hereditary, autosomal recessive, retinal degeneration specific to Persian cats. A multi-generational pedigree segregating for this progressive retinal atrophy was genotyped using a 63 K SNP array and analyzed via genome-wide linkage and association methods. A multi-point parametric linkage analysis localized the blindness phenotype to a ~1.75 Mb region with significant LOD scores (Z ≈ 14, θ = 0.00) on cat chromosome E1. Genome-wide TDT, sib-TDT, and case-control analyses also consistently supported significant association within the same region on chromosome E1, which is homologous to human chromosome 17. Using haplotype analysis, a ~1.3 Mb region was identified as highly associated for progressive retinal atrophy in Persian cats. Several candidate genes within the region are reasonable candidates as a potential causative gene and should be considered for molecular analyses.

Differential targeting of feline photoreceptors by recombinant adeno-associated viral vectors: implications for preclinical gene therapy trials

The cat is emerging as a promising large animal model for preclinical testing of retinal dystrophy therapies, for example, by gene therapy. However, there is a paucity of studies investigating viral vector gene transfer to the feline retina. We therefore sought to study the tropism of recombinant adeno-associated viral (rAAV) vectors for the feline outer retina. We delivered four rAAV serotypes: rAAV2/2, rAAV2/5, rAAV2/8 and rAAV2/9, each expressing green fluorescent protein (GFP) under the control of a cytomegalovirus promoter, to the subretinal space in cats and, for comparison, mice. Cats were monitored for gene expression by in vivo imaging and cellular tropism was determined using immunohistochemistry. In cats, rAAV2/2, rAAV2/8 and rAAV2/9 vectors induced faster and stronger GFP expression than rAAV2/5 and all vectors transduced the retinal pigment epithelium (RPE) and photoreceptors. Unlike in mice, cone photoreceptors in the cat retina were more efficiently transduced than rod photoreceptors. In mice, rAAV2/2 only transduced the RPE whereas the other vectors also transduced rods and cones. These results highlight species differences in cellular tropism of rAAV vectors in the outer retina. We conclude that rAAV serotypes are suitable for use for retinal gene therapy in feline models, particularly when cone photoreceptors are the target cell.

Altered fundus appearance resulting from autofluorescence imaging with the confocal scanning laser ophthalmoscope (cSLO) in cats

PURPOSE

This paper is to report that imaging the tapetal fundus of cats with the 488 nm laser of the Spectralis(®) HRA+OCT (Heidelberg Engineering Inc., Heidelberg, Germany) can result in a pale appearance of the imaged area.

ANIMALS STUDIED AND PROCEDURES

Wild-type and Rdy kittens (CRX mutant heterozygotes-CRX(Rdy+/-) ) (8-20 weeks of age) and adult cats (1-4 years of age) were imaged by confocal scanning laser ophthalmoscope (cSLO) and spectral domain optical coherence tomography (SD-OCT) using the Spectralis(®) HRA+OCT. Color fundus photography (RetCam II(®) , Clarity Medical Systems, Inc., Pleasanton, CA) was performed after imaging using the Spectralis(®) HRA+OCT.

RESULTS

Following retinal cSLO imaging using the 488 nm laser (autofluorescence imaging) in both wild-type kittens and adult cats, the imaged region appeared paler than the adjacent retina that had not been imaged. This change was probably due to retinal bleaching and was fully reversible. Imaging CRX(Rdy+/-) kittens or adults, which had very reduced levels of visual pigments, did not induce the altered fundus appearance.

CONCLUSIONS

Those using autofluorescence imaging by cSLO should be aware that it can induce a characteristic pale appearance of the tapetal fundus in the imaged area of normal cats.

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.

Gene therapies for inherited retinal disorders

Significant advances have been made over the last decade or two in the elucidation of the molecular pathogenesis of inherited ocular disorders. In particular, remarkable successes have been achieved in exploration of gene-based medicines for these conditions, both in preclinical and in clinical studies. Progress in the development of gene therapies targeted toward correcting the primary genetic defect or focused on modulating secondary effects associated with retinal pathologies are discussed in the review. Likewise, the recent utilization of genes encoding light-sensing molecules to provide new functions to residual retinal cells in the degenerating retina is discussed. While a great deal has been learned over the last two decades, the next decade should result in an increasing number of preclinical studies progressing to human clinical trial, an exciting prospect for patients, those active in research and development and bystanders alike.

Causes and consequences of inherited cone disorders

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

Mechanistically distinct mouse models for CRX-associated retinopathy

Cone-rod homeobox (CRX) protein is a “paired-like” homeodomain transcription factor that is essential for regulating rod and cone photoreceptor transcription. Mutations in human CRX are associated with the dominant retinopathies Retinitis Pigmentosa (RP), Cone-Rod Dystrophy (CoRD) and Leber Congenital Amaurosis (LCA), with variable severity. Heterozygous Crx Knock-Out (KO) mice (“+/−”) have normal vision as adults and fail to model the dominant human disease. To investigate how different mutant CRX proteins produce distinct disease pathologies, we generated two Crx Knock-IN (K-IN) mouse models: Crx^E168d2 (“E168d2”) and Crx^R90W (“R90W”). E168d2 mice carry a frameshift mutation in the CRX activation domain, Glu168del2, which is associated with severe dominant CoRD or LCA in humans. R90W mice carry a substitution mutation in the CRX homeodomain, Arg90Trp, which is associated with dominant mild late-onset CoRD and recessive LCA. As seen in human patients, heterozygous E168d2 (“E168d2/+”) but not R90W (“R90W/+”) mice show severely impaired retinal function, while mice homozygous for either mutation are blind and undergo rapid photoreceptor degeneration. E168d2/+ mice also display abnormal rod/cone morphology, greater impairment of CRX target gene expression than R90W/+ or +/− mice, and undergo progressive photoreceptor degeneration. Surprisingly, E168d2/+ mice express more mutant CRX protein than wild-type CRX. E168d2neo/+, a subline of E168d2 with reduced mutant allele expression, displays a much milder retinal phenotype, demonstrating the impact of Crx expression level on disease severity. Both CRX^[E168d2] and CRX^[R90W] proteins fail to activate transcription in vitro, but CRX^[E168d2] interferes more strongly with the function of wild type (WT) CRX, supporting an antimorphic mechanism. E168d2 and R90W are mechanistically distinct mouse models for CRX-associated disease that will allow the elucidation of molecular mechanisms and testing of novel therapeutic approaches for different forms of CRX-associated disease.

The transcription factor Cone-Rod Homeobox (CRX) plays a central role in regulating gene expression of rod and cone photoreceptors, the primary light sensing cells of the retina. Mutations in the human CRX gene have been associated with the retinal degeneration diseases Retinitis Pigmentosa (RP), Cone-Rod Dystrophy (CoRD) and Leber Congential Amaurosis (LCA). These diseases cause progressive and permanent loss of vision, vary widely in age of onset and severity, and are currently untreatable. To understand how mutations in CRX cause distinct forms of retinal disease, we have genetically engineered mice to carry human disease-causing mutations in their Crx gene. These mouse lines accurately recapitulate distinct forms of CRX-associated disease, demonstrating that different classes of CRX mutations are responsible for phenotype variability in humans. We have characterized the pathology of these mice and identified critical mechanisms of disease. In addition, we have discovered that modifying the level of mutant protein had a dramatic effect on disease pathology in one mutant model, suggesting that targeted therapy against the mutant CRX could be an effective treatment strategy. These mouse models will allow for the testing of novel therapeutic strategies for retinal diseases caused by CRX mutations.

Functional evaluation of iodoacetic acid induced photoreceptor degeneration in the cat

Iodoacetic acid (IAA) has been applied to different species to acutely induce photoreceptor degeneration. The purpose of the present study was to use this toxin to thoroughly eliminate photoreceptors and induce complete blindness in the cat. IAA was delivered by single ear vein injection (20 mg kg(-1)). Six months after the IAA treatment, functional evaluations including pupillary light reflex (PLR), electroretinogram (ERG), visual behavior tests were performed. Morphological examinations were carried out after the functional evaluation. The present result shows that, six months after the IAA application, animals lost visual functions and became completely blind. High dose IAA application via ear vein delivery created an acute and reliable complete photoreceptor degeneration model in the cat. This model can be applied to genetic and cellular therapies for visual function restoration.

Cats: a gold mine for ophthalmology

Over 200 hereditary diseases have been identified and reported in the cat, several of which affect the eye, with homology to human hereditary disease. Compared with traditional murine models, the cat demonstrates more features in common with humans, including many anatomic and physiologic similarities, longer life span, increased size, and a genetically more heterogeneous background. The development of genomic resources in the cat has facilitated mapping and further characterization of feline models. During recent years, the wealth of knowledge in feline ophthalmology and neurophysiology has been extended to include new diseases of significant interest for comparative ophthalmology. This makes the cat an extremely valuable animal species to utilize for further research into disease processes affecting both cats and humans. This is especially true in the advancement and study of new treatment regimens and for extended therapeutic trials. Groups of feline eye diseases reviewed in the following are lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerations. Each has important implications for human ophthalmic research.

Genetic testing in domestic cats

Varieties of genetic tests are currently available for the domestic cat that support veterinary health care, breed management, species identification, and forensic investigations. Approximately thirty-five genes contain over fifty mutations that cause feline health problems or alterations in the cat's appearance. Specific genes, such as sweet and drug receptors, have been knocked-out of Felidae during evolution and can be used along with mtDNA markers for species identification. Both STR and SNP panels differentiate cat race, breed, and individual identity, as well as gender-specific markers to determine sex of an individual. Cat genetic tests are common offerings for commercial laboratories, allowing both the veterinary clinician and the private owner to obtain DNA test results. This article will review the genetic tests for the domestic cat, and their various applications in different fields of science. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's genome.

A high-resolution 15,000(Rad) radiation hybrid panel for the domestic cat

The current genetic and recombination maps of the cat have fewer than 3,000 markers and a resolution limit greater than 1 Mb. To complement the first-generation domestic cat maps, support higher resolution mapping studies, and aid genome assembly in specific areas as well as in the whole genome, a 15,000(Rad) radiation hybrid (RH) panel for the domestic cat was generated. Fibroblasts from the female Abyssinian cat that was used to generate the cat genomic sequence were fused to a Chinese hamster cell line (A23), producing 150 hybrid lines. The clones were initially characterized using 39 short tandem repeats (STRs) and 1,536 SNP markers. The utility of whole-genome amplification in preserving and extending RH panel DNA was also tested using 10 STR markers; no significant difference in retention was observed. The resolution of the 15,000(Rad) RH panel was established by constructing framework maps across 10 different 1-Mb regions on different feline chromosomes. In these regions, 2-point analysis was used to estimate RH distances, which compared favorably with the estimation of physical distances. The study demonstrates that the 15,000(Rad) RH panel constitutes a powerful tool for constructing high-resolution maps, having an average resolution of 40.1 kb per marker across the ten 1-Mb regions. In addition, the RH panel will complement existing genomic resources for the domestic cat, aid in the accurate re-assemblies of the forthcoming cat genomic sequence, and support cross-species genomic comparisons.

Characterization of feline hereditary retinal dystrophies using clinical, functional, structural and molecular genetic studies

Only in recent years have specific mutations been elucidated for feline hereditary retinal dystrophies. Molecular genetic characterization of feline diseases has so far been a slow process but with a full genome sequence for the cat recently completed and the development of a feline single nucleotide polymorphism chip, the characterization of feline monogenic disorders will be significantly simplified. This review summarizes current knowledge with regard to specific hereditary retinal dystrophies in cats and gives an overview of how cats can be used as models in translational research.

Removal of potentially confounding phenotypes from a Siamese-derived feline glaucoma breeding colony

Feline breeding colonies face genetic constraints involving founder effects. A Siamese-founded colony used to study primary congenital glaucoma displayed coat colors additional to the Siamese coat. Genes affecting pigment can exhibit pleiotropy on ocular development and function. To remove potentially confounding phenotypes from our colony, we documented the source and frequency of the Siamese allele at the gene for tyrosinase (TYR), the dilution allele at melanophilin (MLPH), and the brown allele at tyrosinase-related protein 1 (TYRP1). We used PCR-RFLP diagnostics to genotype cats in our colony for the published alleles. A commercially acquired phenotypically normal tom was the source of the dilute allele. A founding Siamese queen was the source of the brown allele. Founders also were blood-typed and screened for disease-associated alleles segregating in Siamese cats at 3 loci (ASB, GLB1, and CEP290). Siamese founders were normal at all loci except ASB, at which both animals carried the hypomorpic allele. Current stock is being managed to limit production of glaucomatous cats with brown, dilute, or Siamese phenotypes or homozygosity for the ASB hypomorphic allele. Genotyping will aid in the elimination of these alleles. The clinical effect of these phenotypes and alleles on the glaucoma phenotype is uncertain, but their elimination will remove potentially confounding effects. In conclusion, when founding a colony, stock should be selected or screened to limit potentially confounding phenotypes. When studying the immune, nervous, and visual systems, screening stock for alleles known to be associated with coat color may be warranted.

The domestic cat as a large animal model for characterization of disease and therapeutic intervention in hereditary retinal blindness

Large mammals, including canids and felids, are affected by spontaneously occurring hereditary retinal diseases with similarities to those of humans. The large mammal models may be used for thorough clinical characterization of disease processes, understanding the effects of specific mutations, elucidation of disease mechanisms, and for development of therapeutic intervention. Two well-characterized feline models are addressed in this paper. The first model is the autosomal recessive, slowly progressive, late-onset, rod-cone degenerative disease caused by a mutation in the CEP290 gene. The second model addressed in this paper is the autosomal dominant early onset rod cone dysplasia, putatively caused by the mutation found in the CRX gene. Therapeutic trials have been performed mainly in the former type including stem cell therapy, retinal transplantation, and development of ocular prosthetics. Domestic cats, having large human-like eyes with comparable spontaneous retinal diseases, are also considered useful for gene replacement therapy, thus functioning as effective model systems for further research.

Feline genetics: clinical applications and genetic testing

DNA testing for domestic cat diseases and appearance traits is a rapidly growing asset for veterinary medicine. Approximately 33 genes contain 50 mutations that cause feline health problems or alterations in the cat's appearance. A variety of commercial laboratories can now perform cat genetic diagnostics, allowing both the veterinary clinician and the private owner to obtain DNA test results. DNA is easily obtained from a cat via a buccal swab with a standard cotton bud or cytological brush, allowing DNA samples to be easily sent to any laboratory in the world. The DNA test results identify carriers of the traits, predict the incidence of traits from breeding programs, and influence medical prognoses and treatments. An overall goal of identifying these genetic mutations is the correction of the defect via gene therapies and designer drug therapies. Thus, genetic testing is an effective preventative medicine and a potential ultimate cure. However, genetic diagnostic tests may still be novel for many veterinary practitioners and their application in the clinical setting needs to have the same scrutiny as any other diagnostic procedure. This article will review the genetic tests for the domestic cat, potential sources of error for genetic testing, and the pros and cons of DNA results in veterinary medicine. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's internal genome.

Efficient transduction of feline neural progenitor cells for delivery of glial cell line-derived neurotrophic factor using a feline immunodeficiency virus-based lentiviral construct

Work has shown that stem cell transplantation can rescue or replace neurons in models of retinal degenerative disease. Neural progenitor cells (NPCs) modified to overexpress neurotrophic factors are one means of providing sustained delivery of therapeutic gene products in vivo. To develop a nonrodent animal model of this therapeutic strategy, we previously derived NPCs from the fetal cat brain (cNPCs). Here we use bicistronic feline lentiviral vectors to transduce cNPCs with glial cell-derived neurotrophic factor (GDNF) together with a GFP reporter gene. Transduction efficacy is assessed, together with transgene expression level and stability during induction of cellular differentiation, together with the influence of GDNF transduction on growth and gene expression profile. We show that GDNF overexpressing cNPCs expand in vitro, coexpress GFP, and secrete high levels of GDNF protein—before and after differentiation—all qualities advantageous for use as a cell-based approach in feline models of neural degenerative disease.