Differential gene expression of TPLPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP) in the appaloosa horse (Equus caballus)

The appaloosa coat spotting pattern in horses is caused by a single incomplete dominant gene (LP). Homozygosity for LP (LP/LP) is directly associated with congenital stationary night blindness (CSNB) in Appaloosa horses. LP maps to a 6-cM region on ECAl. We investigated the relative expression of two functional candidate genes located in the LP candidate region (TRP1 and OCA2), as well as three other linked loci(TJP1, MTMR10 and OTUD7A) by quantitative real-time RT-PCR. No large differences were found for expression levels of TJP1, MTMR10, OTUD7A, and OCA2. However, TRPM1 (Transient Receptor Potential Cation Channel, Subfamily M, Member 1) expression in the retina of homozygous appaloosa horses was 0.05% the level found in non-appaloosa horses (R = 0.0005). This constitutes a > 1800-fold change (FC) decrease in TRPM1 gene expression in the retina (FC = -1870.637, P = 0.001) of CSNB-affected (LP/LP) horses. TRPM1 was also downregulated in LP/LP pigmented skin (R = 0.005, Fc = -193.963, P = 0.001 and in LP/LP unpigmented skin (R = 0.003, FC = -288.686, P = 0.001) and was down regulated to a lesser extent ill LP/lp unpigmented skin (R = 0.027, FC = -36.583, P = 0.001). TRP proteins are thought to have a role in controlling intracellular Ca2+ concentration. Decreased expression of TRPM1 in the eye and the skin may alter bipolar cell signalling as well as melanocyte function, thus causing both CSNB and LP in horses.