Achromatopsia in three sibling Labrador Retrievers in the UK

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.

Achromatopsia: Genetics and Gene Therapy

Achromatopsia (ACHM), also known as rod monochromatism or total color blindness, is an autosomal recessively inherited retinal disorder that affects the cones of the retina, the type of photoreceptors responsible for high-acuity daylight vision. ACHM is caused by pathogenic variants in one of six cone photoreceptor-expressed genes. These mutations result in a functional loss and a slow progressive degeneration of cone photoreceptors. The loss of cone photoreceptor function manifests at birth or early in childhood and results in decreased visual acuity, lack of color discrimination, abnormal intolerance to light (photophobia), and rapid involuntary eye movement (nystagmus). Up to 90% of patients with ACHM carry mutations in CNGA3 or CNGB3, which are the genes encoding the alpha and beta subunits of the cone cyclic nucleotide-gated (CNG) channel, respectively. No authorized therapy for ACHM exists, but research activities have intensified over the past decade and have led to several preclinical gene therapy studies that have shown functional and morphological improvements in animal models of ACHM. These encouraging preclinical data helped advance multiple gene therapy programs for CNGA3- and CNGB3-linked ACHM into the clinical phase. Here, we provide an overview of the genetic and molecular basis of ACHM, summarize the gene therapy-related research activities, and provide an outlook for their clinical application.

Impact of gene therapy for canine monogenic diseases on the progress of preclinical studies

Rapid progress in knowledge of the organization of the dog genome has facilitated the identification of the mutations responsible for numerous monogenic diseases, which usually present a breed-specific distribution. The majority of these diseases have clinical and molecular counterparts in humans. The affected dogs have thus become valuable models for preclinical studies of gene therapy for problems such as eye diseases, immunodeficiency, lysosomal storage diseases, hemophilia, and muscular dystrophy. Successful gene therapies in dogs have significantly contributed to decisions to run clinical trials for several human diseases, such as Leber's congenital amaurosis 2-LCA2 (caused by a mutation of RPE65), X-linked retinitis pigmentosa-XLRP (caused by mutation RPGR), and achromatopsia (caused by mutation of CNGB3). Promising results were also obtained for canine as follows: hemophilia (A and B), mucopolysaccharidoses (MPS I, MPS IIIB, MPS VII), leukocyte adhesion deficiency (CLAD), and muscular dystrophy (a counterpart of human Duchenne dystrophy). Present knowledge on molecular background of canine monogenic diseases and their successful gene therapies prove that dogs have an important contribution to preclinical studies.

Gene therapy for achromatopsia

The present review summarizes the current status of achromatopsia (ACHM) gene therapy-related research activities and provides an outlook for their clinical application. ACHM is an inherited eye disease characterized by a congenital absence of cone photoreceptor function. As a consequence, ACHM is associated with strongly impaired daylight vision, photophobia, nystagmus and a lack of color discrimination. Currently, six genes have been linked to ACHM. Up to 80% of the patients carry mutations in the genes CNGA3 and CNGB3 encoding the two subunits of the cone cyclic nucleotide-gated channel. Various animal models of the disease have been established and their characterization has helped to increase our understanding of the pathophysiology associated with ACHM. With the advent of adeno-associated virus vectors as valuable gene delivery tools for retinal photoreceptors, a number of promising gene supplementation therapy programs have been initiated. In recent years, huge progress has been made towards bringing a curative treatment for ACHM into clinics. The first clinical trials are ongoing or will be launched soon and are expected to contribute important data on the safety and efficacy of ACHM gene supplementation therapy.

Canine CNGA3 Gene Mutations Provide Novel Insights into Human Achromatopsia-Associated Channelopathies and Treatment

Cyclic nucleotide-gated (CNG) ion channels are key mediators underlying signal transduction in retinal and olfactory receptors. Genetic defects in CNGA3 and CNGB3, encoding two structurally related subunits of cone CNG channels, lead to achromatopsia (ACHM). ACHM is a congenital, autosomal recessive retinal disorder that manifests by cone photoreceptor dysfunction, severely reduced visual acuity, impaired or complete color blindness and photophobia. Here, we report the first canine models for CNGA3-associated channelopathy caused by R424W or V644del mutations in the canine CNGA3 ortholog that accurately mimic the clinical and molecular features of human CNGA3-associated ACHM. These two spontaneous mutations exposed CNGA3 residues essential for the preservation of channel function and biogenesis. The CNGA3-R424W results in complete loss of cone function in vivo and channel activity confirmed by in vitro electrophysiology. Structural modeling and molecular dynamics (MD) simulations revealed R424-E306 salt bridge formation and its disruption with the R424W mutant. Reversal of charges in a CNGA3-R424E-E306R double mutant channel rescued cGMP-activated currents uncovering new insights into channel gating. The CNGA3-V644del affects the C-terminal leucine zipper (CLZ) domain destabilizing intersubunit interactions of the coiled-coil complex in the MD simulations; the in vitro experiments showed incompetent trimeric CNGA3 subunit assembly consistent with abnormal biogenesis of in vivo channels. These newly characterized large animal models not only provide a valuable system for studying cone-specific CNG channel function in health and disease, but also represent prime candidates for proof-of-concept studies of CNGA3 gene replacement therapy for ACHM patients.