Originally published as Genetics Published Articles Ahead of Print on July 27, 2008.

Genetics, Vol. 179, 1861-1870, August 2008, Copyright © 2008
doi:10.1534/genetics.108.088807

Differential Gene Expression of TRPM1, the Potential Cause of Congenital Stationary Night Blindness and Coat Spotting Patterns (LP) in the Appaloosa Horse (Equus caballus)

Rebecca R. Bellone^*,1, Samantha A. Brooks, Lynne Sandmeyer, Barbara A. Murphy, George Forsyth^**, Sheila Archer, Ernest Bailey and Bruce Grahn

^* Department of Biology, University of Tampa, Tampa, Florida 33606, Department of Veterinary Science, University of Kentucky, Lexington, Kentucky 40546, Department of Small Animal Clinical Sciences and ^** Department of Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan S7N5B4, Canada, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Dublin 4, Ireland and Quill Lake, Saskatchewan S0A3E0, Canada

¹ Corresponding author: Department of Biology, University of Tampa, 401 W. Kennedy Blvd., Box 3F Tampa, FL 33606.
E-mail: rbellone{at}ut.edu

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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