Shedding Light on Night Blindness in Appaloosas

Congenital stationary night blindness (CSNB) in Appaloosas has been linked to the leopard complex spotting in these horses. But how are these two apparently independent genetic features related?

According to researchers, both CSNB and spotting pattern in Appaloosas are thought to be caused by a gene called "transient receptor potential cation channel, subfamily M, member 1," or simply, TRPM1.

Rebecca Bellone, PhD, and fellow researchers from the University of Tampa's Department of Biology examined horses with zero, one, or two copies of the dominant form of the LP gene and compared expression level of five genes located near the mapped LP locus, including TRPM1, in skin and retina samples.

They found that the TRPM1 gene expressed itself more in horses with two copies of the LP allele.

"In retinal tissues obtained from Appaloosas with two copies of the dominant LP allele, expression of the TRPM1 gene was 0.05% the level measured in non-Appaloosas," reported Bellone.

Further, TRPM1 gene expression was almost 200 times lower in skin samples from Appaloosas with two dominant copies of the LP gene.

Together, the available data on CSNB in Appaloosas suggest that the gene TRPM1 is involved in both the leopard complex spotting patterns in Appaloosas and night blindness at the molecular level.

According to Bellone, "The protein encoded by the TRPM1 gene might alter intracellular calcium concentrations, which may affect cellular signaling pathways in the retina and melanocyte function in Appaloosa horses."

The study, "Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness (CSNB) and coat spotting patterns (LP) in the Appaloosa horse (Equus caballus)" is scheduled to appear in an upcoming edition of the journal Genetics. The full-length ePub ahead of print version is available for free at PubMed.  


Appaloosa spotting pattern by LP gene

This diagram shows the full spectrum of patterning observed in Appaloosas. The horses in the upper row have coat patterns that indicate they are are heterozygous for LP, and therefore are not affected by CSNB. Note that they have white patterning with moderate to plentiful dark spotting. The lower row of this diagram shows Appaloosas with coat patterns that indicate they are homozygous for LP, and therefore affected by CSNB. Unlike the upper row horses, these have white patterning with few or no dark spots.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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