Secrets of white colouring in horses revealed

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A Franches-Montagnes mare with little residual pigmentation.
A Franches-Montagnes mare with little residual pigmentation.

Several different mutations within a specific gene appear to be responsible for the development of dominant white colouring in several modern horse populations, researchers have revealed.

Researchers from Europe and the United States worked together in the study into the white coat colour of horses, which has been a highly valued trait in horses for at least 2000 years.

Dominant white is one of several known depigmentation phenotypes in horses. It shows considerable variation, ranging from about 50% depigmented areas up to a completely white coat.

In the horse, the four depigmentation phenotypes – roan, sabino, tobiano, and dominant white – were independently mapped to a chromosomal region harbouring the KIT gene.

The KIT gene plays an important role in colour during embryonic development, causing an absence of melanocytes in depigmented skin areas.

The researchers determined the sequence and genomic organisation of the equine KIT gene.

They then studied the KIT gene in white Franches-Montagnes Horses and found a particular mutation that was responsible for the white colouring.

A dominant white Franches-Montagnes stallion showing partial depigmentation as a colt (B), and (C) almost complete depigmentation at four years of age.
A dominant white Franches-Montagnes stallion showing partial depigmentation as a colt (B), and (C) almost complete depigmentation at four years of age.

Researchers then analysed the KIT gene in horses characterised as dominant white from other populations and found three additional mutations within the same gene responsible for their dominant white colouring.

Three almost completely white Arabians carried a different mutation in another part of the gene while six Camarillo white horses had a mutation in different part again. Five white thoroughbreds were found to have yet another mutation responsible for the colouring.

“Our results indicate that the dominant white color in Franches-Montagnes Horses is caused by a nonsense mutation in the KIT gene and that multiple independent mutations within this gene appear to be responsible for dominant white in several other modern horse populations,” the researchers said in their study, entitled “Allelic Heterogeneity at the Equine KIT Locus in Dominant White (W) Horses”.

“Our findings will allow genetic testing and a more precise classification of horses with white coat colour.”

The study team said that, unlike horses with grey coat colouring, which are characterised by progressive greying of the hair, white horses show the depigmentation at birth and have a depigmented skin.

Such animals have always been highly valued by their human owners. Their important role in history is reflected by their widespread use as heraldic animals, including on the flags of the German states of Lower Saxony and North Rhine-Westphalia.

In various horse breeds, cases of white or almost white horses born out of solid-colored parents have been reported.

Some breed registries have restrictions on white horses, or do not allow them to be registered.

In the Franches-Montagnes horse population involved in the study, white horses are known and reported to trace back to the white founder mare Cigale, born in 1957 out of solid brown parents.

The Camarillo white horses, which have a similar depigmentation phenotype, represent another famous line of horses that can be traced back to the white founder stallion Sultan, born in 1912.

A dominant white thoroughbred stallion.
A dominant white thoroughbred stallion.

“According to anecdotal reports from breeders,” the authors said, “the dominant white phenotype appears to have originated independently on several occasions in thoroughbreds.”

The researchers said all living white Franches-Montagnes Horses are descendants of Cigale.

In horses, the four depigmentation phenotypes – roan, sabino-1, tobiano, and dominant white – were independently mapped to a region on equine Chromosome 3, which harbours the KIT gene.

The sabino-1 spotting pattern is caused by what they called an intronic mutation in the KIT gene. The tobiano spotting pattern is caused by a large chromosomal inversion that disrupts a potential regulatory element downstream of the KIT gene. In contrast to sabino and tobiano, the mutations for roan and dominant white have been reported to cause death in the homozygous state in some horse breeds.

The researchers described in detail the nature and function of the KIT gene. They said a complete loss of function of the KIT gene causes prenatal or perinatal death due to anaemia.

The study team selected four white and four solid-colored Franches-Montagnes horses from a three-generation family for the initial mutation analysis.

They also sequenced the KIT gene in 16 other white or partially depigmented horses from other breeds (Arabian, Camarillo White Horses, Thoroughbred, Miniature, and Shetland Pony).

At left: Immunohistochemistry using a polyclonal KIT antibody on a skin biopsy from a solid-colored horse. Blue staining indicates KIT expression throughout the epidermis. Melanin produced by melanocytes is visible as brown granules. At right: Immunohistochemistry on a skin biopsy of a white horse. Note the weak blue staining and the complete absence of melanocytes and melanin. The bars correspond to 50 µm.
At left: Immunohistochemistry using a polyclonal KIT antibody on a skin biopsy from a solid-colored horse. Blue staining indicates KIT expression throughout the epidermis. Melanin produced by melanocytes is visible as brown granules. At right: Immunohistochemistry on a skin biopsy of a white horse. Note the weak blue staining and the complete absence of melanocytes and melanin. The bars correspond to 50 µm.

“Our study represents another example where different mutations in a single gene have been described for a Mendelian trait in domestic animals.

“At this time,” they said, “it is not known whether the depigmentation of the other reported white horses is caused by as-yet undescribed mutations at the KIT locus or whether mutations in other genes can also cause this phenotype.

“In line with the recent origin of these mutation events, the four proposed candidate causative mutations of this study segregate only within the four respective families.

“In contrast, all of the other 14 KIT polymorphisms that were discovered during the initial mutation analysis segregate in at least two distinct horse populations, indicating that they are much older and spread into different horse populations by the ongoing admixture, which is typical for many modern horse breeds.”

The equine KIT gene plays a central role in equine pigmentation, as at least four distinct depigmentation phenotypes are known to be associated with mutations in the gene.

While depigmentation mutations in some species can cause health issues, such as reduced fertility and anaemia, there is no evidence to suggest that is the case with horses.

“We are not aware of any specific health problems in the studied white horses. At least two of the white thoroughbreds … successfully competed in horse races, indicating a very good general fitness.

A Camarillo White Horse.
A Camarillo White Horse.

“There is very little data on fertility in white horses; however, one white Franches-Montagnes stallion was successfully used for artificial insemination, and all routine sperm parameters from this stallion were normal.

“Therefore, from the limited available data, it appears that heterozygous KIT mutations may have [fewer] detrimental effects in horses on hematopoiesis and fertility than in mice.”

The study team said most “white” horses probably carry the greying-with-age mutation, which means that they are born solid-coloured and become white at the age of four to six years.

However, there are also a number of historical reports that explicitly mention white-born horses resembling the phenotype of dominant white horses.

“Two thousand years ago, the Romans already knew of the phenotypic differences of depigmented horses, which they described as candidus (white) or glaucus (grey). The Roman historian Tacitus described the use of sacred white horses for auguries by German tribes.

“The so-called white horse of the Saxons is depicted on the flags of the German states of Lower Saxony and North Rhine-Westphalia.

“It is thus of considerable historic interest to trace the origins of white horses, particularly because the nature of their white colour can have different causes, some of which are KIT mutations such as those described here.

“We do not know whether the Roman terms candidus and glaucus actually correspond to our modern coat colour designations of white and grey. Archaeogenetics on historic DNA samples may help to identify the genetic constitution of these horses.”

The study team concluded that they had identified the probable causative mutation for the dominant white phenotype in Franches-Montagnes Horses. “We have also identified three additional candidate causative mutations in Arabians, Camarillo White Horses, and Thoroughbreds.

“The knowledge of these mutations will allow genetic testing and should help to assign more precise coat colour descriptions for partially or completely depigmented horses.”

The study was funded by a grant from the Swiss National Science Foundation, and a grant from the Swiss Federal Office for Agriculture.

It was reported in the November 2007 issue of the Public Library of Science Genetic journal.

* This article was first published on Horsetalk.co.nz on December 18, 2007. 
The researchers were:  Bianca Haase,1,2 (see below), Samantha A Brooks,3, Angela Schlumbaum,4, Pedro J Azor,1,5, Ernest Bailey,3, Ferial Alaeddine,6, Meike Mevissen,6, Dominik Burger,7, Pierre-André Poncet,7, Stefan Rieder,8, and Tosso Leeb1,2.
1 Institute of Genetics, Vetsuisse Faculty, University of Berne, Berne, Switzerland
2 DermFocus, Vetsuisse Faculty, University of Berne, Berne, Switzerland
3 M. H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky, United States of America
4 Institute of Prehistory and Archaeological Sciences, University of Basel, Basel, Switzerland
5 Department of Genetics, University of Cordoba, Gregory Mendel Building, Cordoba, Spain
6 Division of Veterinary Pharmacology and Toxicology, Vetsuisse Faculty, University of Berne, Berne, Switzerland
7 Swiss National Stud, Avenches, Switzerland
8 Swiss College of Agriculture, Zollikofen, Switzerland

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