Increased Alpha-Tocopherol Metabolism In Horses With Equine Neuroaxonal Dystrophy

Background Equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy (eNAD/EDM) is an inherited neurodegenerative disorder associated with a vitamin E deficiency within the first year of life. Vitamin E consists of 8 isoforms metabolized by the CYP4F2 enzyme. No antemortem diagnostic test currently exists for eNAD/EDM. Hypothesis/Objectives Based on the association of alpha-tocopherol deficiency with the development of eNAD/EDM, we hypothesized that the rate of alpha-tocopherol, but not gamma-tocopherol or tocotrienol metabolism, would be increased in eNAD/EDM-affected horses. Animals Vitamin E metabolism: Proof of concept (POC) study; eNAD/EDM-affected (n = 5) and control (n = 6) horses. Validation study: eNAD/EDM-affected Quarter Horses (QHs; n = 6), cervical vertebral compressive myelopathy affected (n = 6) horses and control (n = 29) horses. CYP4F2 expression and copy number: eNAD/EDM-affected (n = 12) and age- and sex-matched control (n = 11-12) horses. Methods The rates of alpha-tocopherol/tocotrienol and gamma-tocopherol/tocotrienol metabolism were assessed in equine serum (POC and validation) and urine (POC only) using liquid chromatography tandem mass spectrometry (LC-MS/MS). Quantitative reverse-transcriptase PCR (qRT-PCR) and droplet digital (dd)-PCR were used to assay expression and genomic copy number of a CYP4F2 equine ortholog. Results Metabolic rate of alpha-tocopherol was increased in eNAD/EDM horses (POC,P < .0001; validation, P = .03), with no difference in the metabolic rate of gamma-tocopherol. Horses with eNAD/EDM had increased expression of the CYP4F2 equine orthologue (P = .02) but no differences in copy number. Conclusions and Clinical Importance Increased alpha-tocopherol metabolism in eNAD/EDM-affected QHs provides novel insight into alterations in vitamin E processing in eNAD/EDM and highlights the need for high-dose supplementation to prevent the clinical phenotype in genetically susceptible horses.