Prevalence of Genetic Mutations in Horses With Muscle Disease From a Neuromuscular Disease Laboratory.
Authors: Aleman Monica, Scalco Rebeca, Malvick Julia, Grahn Robert A, True Alexander, Bellone Rebecca R
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Researchers from the University of California examined muscle biopsy samples and genetic data from 296 horses with suspected muscular disease to establish the prevalence of six known disease-causing mutations (HYPP, MH, PSSM1, GBED, MC, and MYHM) and their correlation with histological findings. Of the cohort, 192 horses showed myopathic changes, 41 displayed neurogenic patterns, and 63 had normal muscle histology; genetic variants explained disease in approximately one-third of cases overall, but accounted for 45% of horses with confirmed myopathic lesions on biopsy. The MYHM-associated *My* allele proved most prevalent at 62%, with homozygous carriers demonstrating markedly more severe clinical presentations than heterozygotes, whilst all horses carrying the MH allele experienced fatal outcomes when exposed to triggering factors such as anaesthesia, stress, or concurrent myopathy. Critically, the study revealed that neither histology nor genetics alone provides complete diagnostic clarity: 10% of horses with histologically normal muscle nonetheless carried pathogenic variants, whilst 63% of those with confirmed histological abnormalities had no identifiable genetic cause. These findings underscore the necessity of performing both immunohistochemical analysis and targeted genetic testing as complementary investigations in equine muscle disease cases, particularly given the breed-specific concentration of mutations and the variable phenotypic expression that genetic status alone cannot predict.
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Practical Takeaways
- •Genetic testing and muscle histology are complementary diagnostic tools—normal biopsy findings do not exclude genetic muscle disease in symptomatic horses
- •Horses carrying the MH allele face fatal risk under anesthesia or stress; identify these horses before procedures or high-stress situations
- •MYHM homozygous horses show significantly worse clinical signs than heterozygotes, informing breeding decisions and athletic expectations
Key Findings
- •One-third of horses with muscle disease had identifiable genetic variants, accounting for 45% of histologically confirmed myopathic cases
- •The MYHM-associated My allele was most prevalent (62%), with homozygotes showing more severe phenotype than heterozygotes
- •All horses with the malignant hyperthermia allele died when exposed to triggering factors including anesthesia or stress
- •10% of horses with clinically normal muscle histology harbored disease-causing genetic variants, while 63% of histologically abnormal muscle had no known genetic explanation