Current genetics of equine cutaneous asthenia : (Review of literature)

Abstract

Cutaneous asthenia, also known as hyperelastosis cutis is an inherited congenital skin disorder that has been reported in a variety of species, characterised by skin hyperextensibility. This degenerative connective tissue dysplasia of friable, thin, hyperelastic skin was first described in Quarter horses in 1978 by Lerner and McCracken as Hereditary equine regional dermal asthenia (HERDA). The clinical symptoms of HERDA typically present on average, at 1.5 years of age and are frequently associated with the introduction of the saddle and riding. It primarily affects the dorsal region of the body and manifests as extremely loose, hyperextensible, fragile skin that tears easily and upon stretching does not return to its normal position. Single or multiple seromas and haematomas develop leading to chronic slow healing ulcers with sloughing of the skin and atrophic scar formation. This debilitating disease is found predominantly in Quarter horses, with the highest incidence seen in the cutting horse subpopulation, however cases in other breeds have also been reported. HERDA is an inherited autosomal recessive skin disorder that almost exclusively has been found in Quarter horse lineage tracing to the famous stallion “Poco Bueno”. The genetic defect responsible for this disorder is a missense mutation in the equine genome cyclophilin B (CypB) where a Gly-6 to Arg-6 substitution at codon 115 has been identified. It is believed this mutation alters protein-protein interactions of CypB and delays the triple helix folding of collagen. Research has indicated that this CypB mutation is responsible for alterations in diverse tissues containing fibrillar collagen, thus suggesting the HERDA phenotype is not limited to the skin. Since the mapping of the horse genome sequence was completed in 2007, the genetic basis for many hereditary diseases has been discovered and diagnostic tests have been developed and are available to the public. Further to this, the first sequencing of a Quarter horse genome by next generation sequencing in 2012, has led to an increased quantity of genetic variants for use in equine genomics which will provide advantageous resources for future equine genetic studies regarding performance traits and diseases. In an effort to control further manifestation of HERDA, it would be extremely beneficial for owners and breeders to become more informed about this genetic disorder in the horse and take the necessary precautionary steps in order to selectively avoid the further production of affected foals. Mandatory testing for genetic diseases such as HERDA is not the standard among breed registries around the world, with the exception being the Australian Quarter Horse Association (AQHA). Consequently, by availing of the genetic screening against this disorder and by careful selection of breeding stock, it may be possible to slowly reduce and even eliminate HERDA from the horse population in the future.