*7.2. TRPM1*

Studies of Appaloosa horses with CSNB contributed to the identification of the role of transient receptor potential cation channel subfamily M, member 1 (*Trpm1*) in bipolar cell signaling [144,145]. The night-blind Appaloosa horse was first recognized as an animal model for the Schubert–Bornschein type of CSNB in the 1970s [146]. The study published at that time reported a lack of an ERG b-wave and night blindness but no retinal degeneration or obvious morphological abnormality of the photoreceptor synapses with second order neurons [146]. The correlation of CSNB in the Appaloosa with the Leopard complex spotting coat color was recognized [147]. This coat color is governed at a single gene locus with animals homozygous for the Leopard complex spotting associated allele (LP) also having CSNB [147]. When the LP locus was mapped, *Trpm1*, which is also expressed in melanocytes, was identified as a positional candidate gene and showed markedly reduced expression in retina and

skin from a ffected animals [148]. Investigation of *TRPM1* in humans with complete CSNB identified mutations in *TRPM1* [149–151] and its role in ON-bipolar cell signaling was identified.

The LP mutation was identified as a retroviral insertion in intron 1 of equine *Trpm1* that disrupts gene transcription by causing premature polyadenylation [152].

### *7.3. Whippet Dog Model of Incomplete CSNB with Retinal Degeneration*

A dog model of cone–rod synaptic dysfunction, which has been described by some authors as a form of incomplete CSNB, has been identified [153]. The a ffected dogs lack an ERG b-wave and also lack cone OFF-bipolar cell attributable ERG components [154]. Interestingly, the dog model develops a progressive retinal degeneration, which is not reported as a feature of the condition in humans [155].
