Hereditary Equine Regional Dermal Asthenia

Rob Tryon, Tom Famula, Stephen White, Danika Bannasch

Funding provided by the UC Davis Center for Equine Health and the American Quarter Horse Foundation

Hereditary equine regional dermal asthenia (HERDA) is a genetic skin disease predominantly found in subpopulations of the American Quarter Horse. The disease is characterized by hyperextensible skin, scarring, and severe lesions along the dorsal aspect of affected horses. Presentation typically occurs by the age of two, most notably when the horse is first being broke to a saddle. There is no cure and the majority of diagnosed horses are euthanized because they are unable to be ridden and inappropriate for future breeding.

HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions. Due to the genetic nature of the disease, a homozygosity mapping approach was used to identify the gross chromosomal location of the HERDA locus. The location of the locus was confirmed using a sample of 53 affected animals and fine structure mapping was performed across the equine chromosome. Since the horse genome sequence has only recently been released, a comparative genomic approach was taken to identify candidate genes using the most current equine genome maps and known homologies to human, mouse, and canine genomes. Upon sequencing of the candidate genes, a mutation was identified in affected horses.  A genetic test is now available for HERDA that can identify normal, affected and carrier individuals.  Information about this test, as well as submission information and pricing is available through the Veterinary Genetics Laboratory at the University of California, Davis (http://www.vgl.ucdavis.edu/services/index.php).  It is our hope that breeders will utilize this test to identify carriers of HERDA, thereby providing a means to control further expansion of the disease allele within the Quarter horse population.

Relevant publications:

White, S. D., Affolter, V.K., Bannasch, D.L., Schultheiss, P.C., Hamar, D.W., Chapman,P.L., Naydan, D., Spier, S.J., Rosychuk, R.A.W., Rees,C, Veneklasen, G.O., Martin, A., Bevier, D., Jackson, H.J., Bettenay,S., Matousek, J., Campbell, K.L. and P.J. Ihrke. Hereditary equine regional dermal asthenia (hyperelastosis cutis) in 50 horses: clinical, histologic, immunohistologic and ultrastructural findings. Veterinary Dermatology; 15 207-215.

Tryon, R.C., White, S.D., Famula, T.R., Schultheiss, P.C., Hamar, D.W., Bannasch, D.L. Inheritance of hereditary equine regional dermal asthenia in Quarter Horses.  Am J Vet Res. 2005 Mar; 66(3):437-42.

White SD, Affolter VK, Schultheiss PC, Ball BA, Wessell MT, Kass P, Molinaro AM, Bannasch DL, Ihrke PJ. Clinical and pathological findings in a HERDA-affected foal for 1.5 years of life. Vet Dermatol. 2007 Feb: 18(1) 36-40.

Tryon, RC, White, SD, Bannasch, DL. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse.  Genomics. 2007 Jul;90(1):93-102. Epub 2007 May 11


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