2.1.1. Dog Model

A spontaneous dog model of *RHO* adRP has been identified [18]. The c.11C>G, p.Thr4Arg mutant dog (*RhoT4R*) develops a retinal degeneration that is greatly exacerbated by light exposure [19]. The phenotype of this dog model closely resembles that of the human p.Thr4Lys *RHO* mutation which results in dominant RP [20]. Studies of the *RhoT4R* dog have advanced the understanding of the disease mechanism underlying class four *RHO* mutations (those with altered post-translational modification and stability [17]).

RHO has seven transmembrane loops with intradiscal and cytoplasmic loops and in the dark-adapted state is combined with the chromophore 11-cis-retinal. The N-terminal of the protein, which is affected by the p.Thr4Lys mutation, is positioned within the lumen of the outer segmen<sup>t</sup> discs and creates a "cap" over one of the extracellular loops. This cap contributes towards thermal stability and receptor activation of the protein; it also protects the chromophore to opsin protein covalent bond from hydrolysis. Important for the cap role of the N-terminal is glycosylation at N2 and N15. The p.Thr4Arg mutation interferes with glycosylation at the N2 site, altering the cap role. The monoglycosylated rhodopsin is expressed and is trafficked to the outer segment; however, it loses the chromophore faster than the normal meta-rhodopsin II and interacts poorly with the G-protein [21]. The *RhoT4R* dog has light dependent degeneration similar to the sector RP seen with some rhodopsin mutations. In sector RP, the inferior retina is more severely affected as this region gets more light exposure [22]. There is increasing awareness of the need to reduce light exposure to patients with certain *RHO* mutations [23]. Studies suggested that the unliganded form of the mutant opsin has a detrimental effect because of the loss of its structural integrity. Further evidence to support this was provided by cross breeding *RhoT4R* dogs with the *Rpe65*−/− dog to produce *RhoT4R*/+ *Rpe65*−/− dogs which lack 11-cis-retinal chromophore (due to the lack of Rpe65 function) and thus have only unliganded mutant rod opsin (i.e., have a lack of rod opsin combined with 11-cis-retinal) and show a greatly accelerated rate of retinal degeneration compared to *RhoT4R*/+*Rpe65*+/+ dogs (see details on the *Rpe65*−/− dog below) [21].
