*4.3. CNGA3*

About 25% of human achromatopsia cases are caused by damaging mutations in the alpha subunit of the cone CNG channel (*CNGA3*) [81]. Two canine models of achromatopsia were identified with mutations in *CNGA3*. The mutations, p.Arg424Trp and p.Val644del (R424W and V644del, respectively), provided intriguing mutation sites for in vitro testing. Modeling of these mutations lead to further insights into CNG channel gating and subunit interactions [82]. The residue R424 is located in the S6 transmembrane helix and forms a salt bridge with the residue E306 in the S4–S5 linker. Protein modeling of the canine R424W mutation found the disruption of this salt bridge plays an important role in protein folding, subunit assembly and channel gating. In vitro expression studies of the R424W mutation showed increased mislocalization of the mutant CNGA3 protein and the mutant channels did not produce cyclic nucleotide-activated currents [82].

The canine V644del mutation is in the C-terminal leucine zipper (CLZ) domain, which is involved in channel assembly and stability. The one amino acid deletion shifts all subsequent residue interactions within the domain. In vitro studies in a heterologous expression system showed evidence of mislocalization and a decrease in the total number of active channels, but some mutant channels (~60%) reached the cell membrane and had normal cyclic nucleotide-activated currents [82].

Two spontaneous sheep models of *Cnga3* achromatopsia have been identified [83]. The first identified has a nonsense mutation (p.Arg236Ter) and the second a missense mutation (p.Gly540Ser) [84]. Adeno-associated virus gene therapy was able to restore cone function in both models [84,85].
