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 ECA1. We investigated the relative expression of two functional candidate genes located in this LP candidate region (TRPM1 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 downregulated to a lesser extent in 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 signaling as well as melanocyte function, thus causing both CSNB and LP in horses.
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