Retinitis pigmentosa (RP) is a group of hereditary retinal degenerative diseases in which patients typically lose night vision during adolescence and central vision later in life due to the degeneration of rod and cone photoreceptors (Hartong et al., 2006). RP can be inherited as an autosomal dominant, autosomal recessive, or X-linked trait (Bunker et al., 1984; Grøndahl, 1987). The rhodopsin (RHO) gene was the first photoreceptor-specific gene to harbor mutations (Dryja et al., 1990a, 1990b). Rhodopsin-mediated autosomal dominant retinitis pigmentosa (RHO-adRP) accounts for approximately 25% of adRP (Hartong et al., 2006). More than 150 point mutations have been identified (Meng et al., 2020).

The gene RHO is situated on chromosome 3q22.1 and consists of five exons. It encodes a G protein-coupled receptor (GPCR) with seven transmembrane domains, which has a length of 348 amino acids. This receptor is synthesized in the inner segments (IS) of rods and then transported to the outer segments (OS) (Nathans and Hogness, 1984). The N-terminus resides in the interior of the rod discs, whereas the C-terminus is located in the cytoplasm of the OS. Previous in-depth characterization studies of many rhodopsin mutations have revealed that there are distinct consequences for protein structure and function associated with different mutations (Athanasiou et al., 2018). Based on its biochemical and cellular properties, RHO-adRPs can be classified into seven main classes (Athanasiou et al., 2018; Mendes et al., 2005). Each class owns its key features, including post-Golgi trafficking and OS targeting (class-1), misfolding, ER retention, and instability (class-2), disrupted vesicular traffic and endocytosis (class-3), altered post-translational modifications and reduced stability (class-4), altered transducin activation (class-5), constitutive activation (class-6), and dimerization deficiency (class-7) (Athanasiou et al., 2018; Newton and Megaw, 2020). Only amino acid substitutions at R135 in rhodopsin form class-3, as they have distinct defects compared with other characterized rhodopsin mutants (Aguilà et al., 2014; Chuang et al., 2004; Mendes et al., 2005). Mutations at this residue, which is highly conserved in GPCRs, result in a severe and rapidly progressive form of adRP (Aguilà et al., 2014; Iannaccone et al., 2006). To date, relatively little research has been conducted on this mutant. A study of the R135L mutant rod opsin in HEK cells suggested that it might cause disease by disrupting dynamic interactions between different endocytic compartments and causing them to become dysfunctional, thus accumulating rhodopsin-arrestin complexes in the photoreceptor IS and preventing their traffic to the OS (Chuang et al., 2004). Recently, a study has reported that the R135W mutation lying a cluster with mild misfolding, low ER retention and high ability to bind retinal that favor plasma membrane localization (Behnen et al., 2018). Due to the lack of faithful disease models for class-3 mutants, the precise mechanisms remain to be elucidated.

To a certain extent, induced pluripotent stem cells (iPSCs)-derived organoids play a key role in understanding the developmental biology of organs by their very nature (Clevers, 2016; Deng et al., 2018). It is an advanced tool in cell biology that recapitulates organ/tissue morphogenesis in the cellular hierarchy and represents the cellular and genetic heterogeneity of native tissues in vitro (Jin et al., 2019; Li et al., 2022; Liu et al., 2020).

Here, the first three-dimensional (3D) retinal organoid model of a class-3 RHO point mutation generated from patient-derived iPSCs was reported to better understand human diseases resulting from RHO mutations from a developmental perspective.