Novel congenital stationary night blindness in a Tennessee Walking Horse

The high frequency of this gene (10%) in the TWH is surprising finding as night blindness is not commonly associated with the breed. likely to be a recessive gene so a horse must carry two copies of the gene for night blindness to occur partially hides the trait.
DrO

Equine Vet J. 2020 Jul 12.
Whole genome sequencing identifies missense mutation in GRM6 as the likely cause of congenital stationary night blindness in a Tennessee Walking Horse.
Hack YL1, Crabtree EE2, Avila F1, Sutton RB 3, Grahn RA1, Oh A2, Gilger B2, Bellone RR1,4.

Author information:
1. Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis, California, 95616, USA.
2. College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina, 27607, USA.
3. Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, 79430, USA.
4. Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, California, 95616, USA.
Abstract
BACKGROUND:

The only known genetic cause of congenital stationary night blindness (CSNB) in horses is a 1378 bp insertion in TRPM1. However, an affected Tennessee Walking Horse was found to have no copies of this variant.
OBJECTIVES:

To identify the genetic cause for CSNB in an affected Tennessee Walking Horse.
STUDY DESIGN:

Case report detailing a whole genome sequencing approach to identify a causal variant.
METHODS:

A complete ophthalmic exam, including an electroretinogram (ERG), was performed on suspected CSNB affected horse. Whole genome sequencing (WGS) data were generated from the case and compared to data from seven other breeds (n=29). One hundred candidate genes were evaluated for coding variants homozygous in the case and absent in all other horses. Protein modeling was used to assess the functional effects of the identified variant. A random cohort of 90 unrelated Tennessee Walking Horses and 273 horses from additional breeds were screened to estimate allele frequency of the GRM6 variant.
RESULTS:

ERG results were consistent with CSNB. WGS analysis identified a missense mutation in metabotropic glutamate receptor 6 (GRM6) (c.533C>T p.Thr178Met). This SNP is predicted to be deleterious and protein modeling supports impaired binding of the neurotransmitter glutamate. This variant was not detected in 273 horses from three additional breeds. The estimated allele frequency in Tennessee Walking Horses is 10%.
MAIN LIMITATIONS:

Limited phenotype information for controls and no additional cases with which to replicate this finding.
CONCLUSIONS:

We identified a likely causal recessive missense variant in GRM6. Based on protein modeling, this variant alters GRM6 binding, and thus signaling from the retinal rod cell to the ON-bipolar cell, impairing vision in low light conditions. Given the 10% population allele frequency, it is likely that additional affected horses exist in this breed and further work is needed to identify and examine these animals.