Congenital Stationary Night Blindness type 2 (CSNB2)
Congenital Stationary Night Blindness (CSNB) is an hereditary, non-progressive disorder of the retina where individuals have reduced or no vision in poor lighting conditions. A mutation in the GRM6 gene leads to Congenital Stationary Night Blindness Type 2 (CSNB2) when two copies of the mutation are present. This mutation is different than the one associated with Leopard Complex Spotting (TRPM1).
Horses with CSNB may be nervous or difficult to handle when in areas of low light, for example when entering a barn or horse trailer, or in areas which are unfamiliar to them. Due to their inability to see obstacles in the dark, they may be more prone to injuries, or have difficulty finding food or water buckets at night.
Symptoms
- Severely impaired vision in low light
- Avoidance of poorly lit areas (not wanting to enter a dark barn or horse trailer, or unfamiliar area)
- May be prone to injuries due to being unable to see obstacles in the dark
Vision is unimpaired in normal light conditions
General Information
- Congenital Stationary Night Blindness (CSNB) is an hereditary, non-progressive disorder of the retina where individuals have reduced or no vision in poor lighting conditions.
- Gene mutations can disrupt the signalling pathway that prompts the eye to respond to low light conditions, resulting in the affected animal having very poor vision in the dark and in dimly lit environments.
- Two CSNB mutations have been identified in horses to date:
- CSNB associated with Leopard Spotting: caused by having two copies of the mutation in the TRPM1 gene, which also causes the leopard complex spotting phenotype (genotype: LP/LP)
- CSNB2: caused by having two copies of a mutation in the GRM6 gene (csnb2/csnb2)
- Both of these mutations are in genes encoding proteins in the same signalling pathway in the eye.
- The mutation was discovered in aTennessee Walking Horse, and the mutation frequency is estimated to be 10% in that breed.
Breeds affected
- Tennessee Walking Horse
- Standardbreds
- possibly related gaited breeds
Affected Gene
It is a c.533C>T mutation in the GRM6 gene.
Genotype and Lab Report
Inheritance: autosomal recessive
→ Only animals with two copies of the variant (csnb2/csnb2) are affected. Animals with only one copy (N/csnb2) are clinically normal carriers.
Genotype:
N/N = normal
→ The horse has no copies of the mutations causative for CSNB2 and therefore cannot pass it on to its offspring.
N/csnb2 = Träger
→ The horse is a clinically normal carrier. The variant will be passed on to its offspring with a probability of 50 %.
csnb2/csnb2 = betroffen
→ The horse is affected. The variant will be passed on to all offspring. All offspring will be carriers (N/csnb2).
Recommendations
- Carriers may be bred to normal animals (N/csnb2 x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
- Breeding two carriers (N/csnb2 x N/csnb2) is not recommended due to the possibility of 25% of the offspring being affected with night blindness.
- Affected animals (csnb2/csnb2) should only be bred to normal tested horses (n/n) to avoid producing affected foals.
Literature
Hack et al., 2020,: 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, 2021 Mar;53(2):316-323. doi: 10.1111/evj.13318. Epub 2020 Aug 3.
https://doi.org/10.1111/evj.13318
Esdaile et al., 2022: Additional Evidence Supports GRM6 p.Thr178Met As a Cause of Congenital Stationary Night Blindness in Two Horse Breeds. Presentation Abstract: PAG International Plant and Animal Genome Conference, 2022, Equine Workshop 1.: Additional Evidence Supports GRM6 p.Thr178Met As a Cause of Congenital Stationary Night Blindness in Two Horse Breeds
Further information is available at: Online Mendelian Inheritance in Animals