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Curly Hair and Hypotrichosis
| Acronym: | CHH |
| Gene: | SP6 |
| Mutation: | c.1090G>A |
| Inheritance: | Autosomal Dominant |
| Sample type: | WBE (Whole Blood EDTA), HA (Hairs - with roots) |
Genetics and characteristics
Aside from coat color, coat texture and type are other important coat characteristics in horses. Some horse breeds such as North American Curly, Missouri Foxtrotters, and American Bashkir Curly Horse show another distinctive coat characteristic – curliness. Curly hair in horses presents a variety of curl phenotypes that are associated with various degrees of curliness of coat, mane, tail, and ear hairs ranging from slightly curled, curled up to extremely dense and “micro curled” hairs. However, curliness in horses is shown to often be associated with Hypotrichosis, a form of follicular dysplasia and a rare condition in which there is little or no hair growth and hair is usually coarse, dry, and tightly curled. Both conditions, curly hair and hypotrichosis, are under the control of two genes and their variants – KRT25 and SP6. KRT25 is a protein expressed in regenerated hair follicles and is important for hair follicle neogenesis while SP6 is a protein that plays an important role as a cell- and tissue-specific transcription factor primary expressed in hair follicles, teeth and limbs.
Curly Hair and Hypotrichosis in horses have a complex mode of inheritance since two genes are involved. Curly Hair on its own has complete dominance for both gene variants, while incomplete dominance was reported for hypotrichosis. KRT25 and SP6 show epistasis, a circumstance where the expression of one gene is affected by the expression of one or more other genes. Research has shown that mutant KRT25 is epistatic to mutant SP6, meaning the effect of SP6 is overlaid due to the pleiotropy of the KRT25 variant on the structure of hair fibers. That means horses that carry one or two mutated copies of KRT25 will show curly coats and hypotrichosis, whereas horses with SP6 variant only, will exhibit curly coats without hypotrichosis. Horses that carry mutated genes of both genes will develop curly hair and hypotrichosis. Early detection by genetic testing can help identify carriers of the mutation and help breeders in further selection of mating pairs.
Results Reported As
Test Result |
Interpretation of test result |
Straight coat |
Horse does not have the gene with the mutation and has a straight coat. |
Curly coat - Carrier of straight |
Horse has one copy of the gene with curliness associated mutation and displays Curly coat. |
Curly coat |
Horse has two copies of the gene with curliness associated mutation and displays Curly coat. |
Curly coat with some hypotrichosis |
Horse has one copy of the gene with hypotrichosis associated mutation and displays Curly coat and some degree of hypotrichosis. |
Curly coat with hypotrichosis |
Horse has one copy of the gene with hypotrichosis associated mutation and displays Curly coat and hypotrichosis. |
References:
Thomer, A., Gottschalk, M., Christmann, A., Naccache, F., Jung, K., Hewicker-Trautwein, M., Distl, O., Metzger, J. (2018). An epistatic effect of KRT25 on SP6 is involved in curly coat in horses. Scientific reports, 8(1), 6374. https://doi.org/10.1038/s41598-018-24865-3
Morgenthaler, C., Diribarne, M., Capitan, A., Legendre, R., Saintilan, R., Gilles, M., Esquerré, D., Juras, R., Khanshour, A., Schibler, L., Cothran, G. (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. Genetics, selection, evolution : GSE, 49(1), 85. https://doi.org/10.1186/s12711-017-0359-5