*4.4. CNGB3*

Two unique mutations were identified in the canine *Cngb3* gene—a genomic deletion that removes the entire *Cngb3* gene (*Cngb3*−/−) and a missense mutation (p. Asp262Asn; D262N). The missense mutation was identified in German Shorthaired Pointers, while the genomic deletion was initially found in Alaskan Malamutes and later found in other breeds [86,87]. Mutations in *CNGB3* are responsible for at least 50% of achromatopsia cases in humans, making it an ideal target for therapies [87,88].

The D262N mutation in *Cngb3* inspired investigation into the Tri-Asp motif that is conserved in CNG channels [89]. When this mutation was introduced to heterologously expressed CNGA3, it abolished homomeric channel function and was responsible for mislocalization of the protein. When the mutation was introduced in CNGB3 in coexpression studies with CNGA3, the response to cyclic nucleotides was reduced but still present and consistent with CNGA3 homomeric channel function. These experiments show the Tri-Asp motif is necessary for proper cone CNG channel formation, but raises the question why mutations in *CNGB3* result in the loss of cone function and achromatopsia when CNGA3 can form a functioning homomeric channel in the absence of CNGB3. Further work in retinal tissue and/or cone photoreceptors will be needed to elucidate this [89].

Delivery of a human *CNGB3* gene via rAAV5 vector to both *Cngb3*−/− and the D262N mutant dogs showed that the vector could be targeted to the medium and long wavelength (M/L) cones via a human red cone opsin promoter and the rescue was dependent on the age of the dog and the promotor, not on the mutation type [90].
