Cone photoreceptors are the main targets for gene therapy of NPHP5 (IQCB1) or NPHP6 (CEP290) blindness: generation of an all-cone Nphp6 hypomorph mouse that mimics the human retinal ciliopathy

Abstract

Leber congenital amaurosis (LCA), a severe autosomal recessive childhood blindness, is caused by mutations in at least 15 genes. The most common molecular form is a ciliopathy due to NPHP6 (CEP290) mutations and subjects have profound loss of vision. A similarly severe phenotype occurs in the related ciliopathy NPHP5 (IQCB1)-LCA. Recent success of retinal gene therapy in one form of LCA prompted the question whether we know enough about human NPHP5 and NPHP6 disease to plan such treatment. We determined that there was early-onset rapid degeneration of rod photoreceptors in young subjects with these ciliopathies. Rod outer segment (OS) lamination, when detectable, was disorganized. Retinal pigment epithelium lipofuscin accumulation indicated that rods had existed in the past in most subjects. In contrast to early rod losses, the all-cone human fovea in NPHP5- and NPHP6-LCA of all ages retained cone nuclei, albeit with abnormal inner segments and OS. The rd16 mouse, carrying a hypomorphic Nphp6 allele, was a good model of the rod-dominant human extra-foveal retina. Rd16 mice showed normal genesis of photoreceptors, including the formation of cilia, followed by abnormal elaboration of OS and rapid degeneration. To produce a model of the all-cone human fovea in NPHP6-LCA, we generated rd16;Nrl-/- double-mutant mice. They showed substantially retained cone photoreceptors with disproportionate cone function loss, such as in the human disease. NPHP5- and NPHP6-LCA across a wide age spectrum are thus excellent candidates for cone-directed gene augmentation therapy, and the rd16;Nrl-/- mouse is an appropriate model for pre-clinical proof-of-concept studies.

Overlap of abnormal photoreceptor development and progressive degeneration in Leber congenital amaurosis caused byNPHP5mutation

“…Human NPHP5-LCA shares a central retinal phenotype with certain ciliopathies but not others NPHP5-LCA patients had early onset and rapid loss of rod photoreceptor function and structure but retained central retinal photoreceptors, albeit accompanied by reduced cone function (10)(11)(12). In contrast to normal retina with preserved photoreceptor nuclear layer (outer nuclear layer, ONL) thickness across the entire cross-sectional image (Fig.…”

“…In terms of the renal disease, there can be considerable phenotypic variability with some patients developing renal disease later in life, and some not at all (10)(11)(12). We used high resolution 9.4 Tesla MRI imaging, and renal ultrasounds, to establish normal renal structures in mutant dogs.…”

“…The current work showed that NPHP5-mutant dogs recapitulate the human retinal but not the renal phenotypes. In patients, the renal disease can be mild or severe and renal transplantation, if necessary, is an available and successful therapy, although obviously a major procedure (10)(11)(12). The retinal disease, however, has no therapy.…”

“…There have been reports of NPHP5 mutations causing LCA without renal symptoms, but the broad range in age of NPHP onset leaves open the possibility that in these cases the renal disease could still develop at a later age (10). NPHP5 patients show early and rapid rod degeneration with unexpected preservation of central cones that function poorly or not at all (11,12). The gene product localizes to renal cilia and to the photoreceptor connecting cilium/transitional zone (9), and is required for early cilia assembly and ciliogenesis (13,14).…”

The role of primary cilia in the development and disease of the retina

“…Crossing of conditional knockouts to lines expressing Cre recombinase under the control of a cone-specific gene promoter have been successfully used to develop models of foveal diseases. 19 Additionally, mice photoreceptors lack calyceal processes (CPs), finger-like structures that protrude from the apical region of the inner segment and surround the base of the outer segment, further compromising their utility as models of retinal ciliopathies. This is particularly true in the case of mouse models of Usher syndrome (USH), an inherited condition involving sensorineural hearing loss and retinal dystrophy.…”