Curly coat (KRT25, SP6-related)

 

Genes: KRT25, SP6

Transmission: Autosomal dominant (complex genetics)

For an autosomal dominant genetic disease, an animal must have at least one copy of the mutation in question to be at risk of developing the disease.  Animals with two copies of the mutation generally have more severe symptoms and an earlier onset of the disease than animals with just one copy of the mutation.  One or both of the parents of an animal with the mutation has one or two copies of the mutation.  Animals that have one or two copies of the mutation can pass the mutation on to future generations.

Mutations:

KRT25 mutation: Substitution, KRT25 gene: c.266G>A, p.(Arg89His), exon1 Chr.11.

SP6 mutation: Substitution, SP6 gene: c.1090G>A, p.(Gly364Ser), exon1, Chr.11.

Medical system: Dermal

Breeds: American Bashkir Curly horse, Missouri Foxtrotter, other breeds

Age of onset of symptoms: From birth.

Curly hair is a genetic trait affecting the hair shaft that is seen in a number of domestic animals, mention the Rex phenotypes in the cat.  The genetic causes of the curly hair phenotype are variable both between species and within species.

Curly coat is the name of the curly hair phenotype in horses and is a feature of breeds such as the American Bashkir Curly Horse and the Missouri Foxtrotter.  Curliness is present in the hair shaft of the animal’s coat, main and tail, and the degree of curliness can be variable.  Curly coat can be associated with hair loss (hypotrichosis), but this is not necessarily the case.  Both dominant and recessive genetics have been ascribed to curly coat, and evidently it is a complex polygenic trait.

Molecular studies were performed on horses with curly coat that segregated with dominant genetics, and a mutation within the KRT25 gene was identified.  The KRT25 gene codes for a type of keratin protein expressed in the hair follicle.  The curly coat phenotype was variably associated with hair loss (hypotrichosis).  In a large survey study, the KRT25 mutation could account for most but not all cases of curly coat looked at, indicating that additional genetic factors could be involved.  In a separate molecular study, the KRT25 gene mutation was confirmed as a dominant genetic factor responsible for the curly coat phenotype.  In addition, a mutation within the SP6 gene was also positively associated with the phenotype, again with dominant genetics.  SP6 gene codes for a transcription factor expressed within hair follicles.  In contrast to the KRT25 mutation, the SP6 mutation was not by itself associated with hypotrichosis, although this fact was masked in animals positive for both the KRT25 and SP6 mutations.  The SP6 mutation was more associated with the Missouri Foxtrotter horse breed, while the KRT25 mutation was more associated with the American Bashkir Curly Horse breed.

Curly coat in horses remains a trait with complex genetics, involving at least two dominant mutations that have been characterized and possibly several recessive mutations that have yet to be characterized. When hypotrichosis is associated with the curly coat trait, affected horses may be more sensitive to the sun and cold, making them more susceptible to sunburn and hypothermia. There is a suggestion that horses with the curly coat trait may be hypoallergenic for people with an allergy to horses.

 

References:

OMIA links: [0245-9796], [2175-9796]

Durward-Akhurst SA, Marlowe JL, Schaefer RJ, et al. (2024) Predicted genetic burden and frequency of phenotype-associated variants in the horse. Sci Rep 14:8396. [pm/38600096]

Mitlehner A, Mitlehner C, Reißmann M, et al. (2024) Horse allergy: Curly Horses can mediate immune tolerance. Pneumologie 78:47-57. [pm/37827498]

Thomer A, Gottschalk M, Christmann A, et al. (2018) An epistatic effect of KRT25 on SP6 is involved in curly coat in horses. Sci Rep 8:6374.  [pm/29686323]

Morgenthaler C, Diribarne M, Capitan A, et al. (2017) A missense variant in the coil1A domain of the keratin 25 gene is associated with the dominant curly hair coat trait (Crd) in horse. Genet Sel Evol 49:85. [pm/29141579]

 

With contributions by: Marie-Jane Grady and Mariya Sushko, Class of 2030, Faculty of Veterinary Medicine, University of Montreal.  (Translation: DWS).