Callum Donnelly

Background Equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy (eNAD/EDM) is an inherited neurodegenerative disorder associated with vitamin E deficiency. In humans, polymorphisms in genes involved in vitamin E uptake and distribution determines individual vitamin E requirements. Hypothesis/Objectives Genetic polymorphisms in genes involved in vitamin E metabolism would be associated with an increased risk of eNAD/EDM in Quarter Horses (QHs). Animals Whole‐genome sequencing: eNAD/EDM affected (n = 9, postmortem [PM]‐confirmed) and control (n = 32) QHs. Validation: eNAD/EDM affected (n = 39, 23‐PM confirmed) and control (n = 68, 7‐PM confirmed) QHs. Allele frequency (AF): Publicly available data from 504 horses across 47 breeds. Methods Retrospective, case control study. Whole‐genome sequencing was performed and genetic variants identified within 28 vitamin E candidate genes. These variants were subsequently genotyped in the validation cohort. Results Thirty‐nine confirmed variants in 15 vitamin E candidate genes were significantly associated with eNAD/EDM ( P < .01). In the validation cohort, 2 intronic CD36 variants (chr4:726485 and chr4:731082) were significantly associated with eNAD/EDM in clinical ( P = 2.78 × 10 −4 and P = 4 × 10 −4 , respectively) and PM‐confirmed cases ( P = 6.32 × 10 −6 and 1.04 × 10 −5 , respectively). Despite the significant association, variant AFs were low in the postmortem‐confirmed eNAD/EDM cases (0.22‐0.26). In publicly available equine genomes, AFs ranged from 0.06 to 0.1. Conclusions and Clinical Importance Many PM‐confirmed cases of eNAD/EDM were wild‐type for the 2 intronic CD36 SNPs, suggesting either a false positive association or genetic heterogeneity of eNAD/EDM within the QH breed.

Congenital stationary night blindness (CSNB) is an ocular disorder characterized by nyctalopia. An autosomal recessive missense mutation in glutamate metabotropic receptor 6 ( GRM6 c.533C>T, p.(Thr178Met)), called CSNB2 , was previously identified in one Tennessee Walking Horse and predicted to reduce binding affinity of the neurotransmitter glutamate, impacting the retinal rod ON‐bipolar cell signaling pathway. Thus, the first aim was to identify the allele frequency (AF) of CSNB2 in breeds with reported cases of CSNB and breeds closely related to the Tennessee Walking Horse. The second aim was to perform ocular examinations in multiple breeds to confirm the link between genotype and CSNB phenotype. In evaluating 3518 horses from 14 breeds, the CSNB2 allele was identified in nine previously unreported breeds. The estimated AF was highest in pacing Standardbreds (0.17) and lowest in American Quarter Horses (0.0010). Complete ophthalmic examinations and electroretinograms (ERG) were performed on 19 horses from three breeds, including one CSNB2 homozygote from each breed. All three CSNB2/CSNB2 horses had an electronegative ERG waveform under scotopic light conditions consistent with CSNB. The remaining 16 horses (seven CSNB2/N and nine N/N ) had normal scotopic ERG results. All horses had normal photopic ERGs. This study provides additional evidence that GRM6 c.533C>T homozygosity is likely causal to CSNB in Tennessee Walking Horses, Standardbreds, and Missouri Fox Trotting Horses. Genetic testing is recommended for breeds with the CSNB2 allele to limit the production of affected horses. This study represents the largest across‐breed identification of CSNB in the horse and suggests that this disorder is likely underdiagnosed.