Hereditary Equine Regional Dermal Asthenia (HERDA)
Hereditary equine regional dermal asthenia (Ehlers-Danlos syndrome)
Gene or region: PPIB
Reference allele: G
Mutant allele: A
Affected Breeds: Quarter Horse
Research Confidence: High confidence, findings reproduced in multiple studies
What it does:
Hereditary equine regional dermal asthenia (HERDA) is a degenerative skin disease that primarily affects the American Quarter Horse breed. Loose skin is often an early indication of the disease, and severe seromas, hematomas, ulcerations usually develop around 1.5 years of age and progressively worsen. There is no cure, and the majority of affected animals have to be euthanized within 2-4 years.
Although the exact cause of the condition is not known, it likely involves an inability to properly regulate inflammation and wound healing. The condition tracks as a recessive disease, meaning that horses can appear normal but be carriers for the condition. If two carrier horses mate, there is a 25% chance that the foal will have HERDA. Studies estimate that ~3.5% of Quarter Horses are carriers.
The particular SNP assessed here causes an amino acid substitution in the PPIB protein, which is involved in multiple processes including organization of collagen and immune activation. It is not currently known how this mutation affects PPIB function.
Tryon et al., “Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse.” (2007) Genomics 90: 93-102. PMID: 17498917