Abstract
We identified herein additional patients with rod-cone dystrophy (RCD) displaying mutations in KIZ, encoding the ciliary centrosomal protein kizuna and performed functional characterization of the respective protein in human fibroblasts and of its mouse ortholog PLK1S1 in the retina. Mutation screening was done by targeted next generation sequencing and subsequent Sanger sequencing validation. KIZ mRNA levels were assessed on blood and serum-deprived human fibroblasts from a control individual and a patient, compound heterozygous for the c.52G>T (p.Glu18*) and c.119_122del (p.Lys40Ilefs*14) mutations in KIZ. KIZ localization, documentation of cilium length and immunoblotting were performed in these two fibroblast cell lines. In addition, PLK1S1 immunolocalization was conducted in mouse retinal cryosections and isolated rod photoreceptors. Analyses of additional RCD patients enabled the identification of two homozygous mutations in KIZ, the known c.226C>T (p.Arg76*) mutation and a novel variant, the c.3G>A (p.Met1?) mutation. Albeit the expression levels of KIZ were three-times lower in the patient than controls in whole blood cells, further analyses in control- and mutant KIZ patient-derived fibroblasts unexpectedly revealed no significant difference between the two genotypes. Furthermore, the averaged monocilia length in the two fibroblast cell lines was similar, consistent with the preserved immunolocalization of KIZ at the basal body of the primary cilia. Analyses in mouse retina and isolated rod photoreceptors showed PLK1S1 localization at the base of the photoreceptor connecting cilium. In conclusion, two additional patients with mutations in KIZ were identified, further supporting that defects in KIZ/PLK1S1, detected at the basal body of the primary cilia in fibroblasts, and the photoreceptor connecting cilium in mouse, respectively, are involved in RCD. However, albeit the mutations were predicted to lead to nonsense mediated mRNA decay, we could not detect changes upon expression levels, protein localization or cilia length in KIZ-mutated fibroblast cells. Together, our findings unveil the limitations of fibroblasts as a cellular model for RCD and call for other models such as induced pluripotent stem cells to shed light on retinal pathogenic mechanisms of KIZ mutations.
Highlights
Rod-cone dystrophy (RCD), known as retinitis pigmentosa, is a heterogeneous group of inherited retinal disorders affecting rod photoreceptors in the majority of cases with secondary cone degeneration [1]
Identification of Additional rod-cone dystrophy (RCD) Patients with Mutations in KIZ. We report another patient (CIC07875 of family F4400) with RCD showing the known M1
One of the previously reported cases with the same mutation was from North African Sephardic Jewish ancestry, and another one was of Spanish ancestry [9]
Summary
Rod-cone dystrophy (RCD), known as retinitis pigmentosa, is a heterogeneous group of inherited retinal disorders affecting rod photoreceptors in the majority of cases with secondary cone degeneration [1]. Subjects diagnosed with RCD initially complain of night blindness followed by progressive visual field constriction, abnormal color vision and eventually loss of central vision [1]. These visual symptoms indicate the gradual loss of the two photoreceptor types: rods, which mediate achromatic vision in dim-lit environments and cones, which are important for daylight color vision and fine acuity [1]. Photoreceptors are post-mitotic sensory neurons that display a polarized structure including a biosynthetically-active inner segment (IS) and a photosensitive outer segment (OS) [2]. The CC is a region with increasing relevance for ciliary homeostasis due to its implication in ciliogenesis [3] and protein trafficking [4]
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