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.
A missense mutation in the equine cyclophilin B (PPIB) was determined to cause a functional defect in this protein, resulting in less effective catalysis of the rate-limiting step in collagen folding. Etalon offers diagnostic testing to determine carrier or affected status.
This disease follows an autosomal recessive mode of inheritance, so in order for the foal to be affected, both the sire and the dam must carry the allele. This also means 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. It's recommended that both carriers and clinically affected horses with HERDA be removed from breeding programs.
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
Ishikawa Y, Vranka JA, Boudko SP, Pokidysheva E, Mizuno K, Zientek K, Keene DR, Rashmir-Raven AM, Nagata K, Winand NJ, Bächinger HP. Mutation in cyclophilin B that causes hyperelastosis cutis in American Quarter Horse does not affect peptidylprolyl cis-trans isomerase activity but shows altered cyclophilin B-protein interactions and affects collagen folding. J Biol Chem. 2012 Jun 22;287(26):22253-65. doi: 10.1074/jbc.M111.333336. Epub 2012 May 3. PMID: 22556420; PMCID: PMC3381186.