Congenital Stationary Night Blindness 2 (CSNB2)
Gene or Region: GRM6
Reference Variant: C
Mutant Variant: T
Affected Breeds: Tennessee Walking Horse, Standardbred, Missouri Fox Trotting Horse, and an additional breeds
Research Confidence: High - Findings reproduced in multiple studies
Explanation of Results: csnb2/csnb2 = homozygous for Congenital Stationary Night Blindness 2, trait expressed csnb2/n = heterozygous for Congenital Stationary Night Blindness 2, carrier n/n = no variant detected
General Description for Congenital Stationary Night Blindness 2
Congenital Stationary Night Blindness (CSNB) is an inherited disorder in horses characterized by an inability to see in low-light or dark conditions. This condition is present from birth and does not worsen over time. Common indicators include a horse’s reluctance to enter dark environments, difficulties in locating food and water in the dark, and a propensity for nighttime injuries. Often, owners or handlers may not immediately recognize the signs of CSNB, which can only be conclusively diagnosed through dark-adapted electroretinography (ERG) tests performed by a veterinary ophthalmologist.
The genetic basis of CSNB is believed to vary among breeds and involve multiple genes. For instance, horses with the leopard complex spotting pattern seen in Appaloosas often carry a gene variant linked to CSNB. However, cases of CSNB have also been documented in other breeds without this spotting pattern, such as Thoroughbreds and Tennessee Walking Horses, suggesting different genetic causes. Research by veterinary geneticists and ophthalmologists has identified a specific missense variant in the GRM6 gene, which is crucial for signaling in low-light vision, as a cause of CSNB in Tennessee Walking Horses, termed CSNB2.
This variant is inherited in an autosomal recessive manner, meaning that a horse must inherit two copies of the variant to be affected. Population studies estimate that approximately one in a hundred Tennessee Walking Horses carry two copies of this variant and are likely affected by night blindness. Additional research across multiple breeds has confirmed the presence of the CSNB2 allele in several other horse breeds, emphasizing its broader impact and the importance of genetic testing for informed breeding decisions.
References
Esdaile, E., Knickelbein, K. E., Donnelly, C. G., Ferneding, M., Motta, M. J., Story, B. D., Avila, F., Finno, C. J., Gilger, B. C., Sandmeyer, L., Thomasy, S., & Bellone, R. R. (2023). Additional evidence supports GRM6 p.Thr178Met as a cause of congenital stationary night blindness in three horse breeds. Veterinary ophthalmology, 10.1111/vop.13151. Advance online publication. doi: 10.1111/vop.13151
Hack, Y.L., Crabtree, E.E., Avila, F., Sutton, R.B., Grahn, R., Oh, A., Gilger, B., & Bellone, R.R. (2021). Whole‐genome sequencing identifies missense mutation in GRM6 as the likely cause of congenital stationary night blindness in a Tennessee Walking Horse. Equine Veterinary Journal, 53(2), 316–323. doi: 10.1111/evj.13318
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