UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
Friesian Horse Dwarfism and Hydrocephalus

Introduction

Dwarfism and hydrocephalus are two genetic disorders of Friesian horses for which the causative mutations are known and genetic tests are available. These mutations arose spontaneously but have spread in the breed as a consequence of inbreeding. With genetic testing and proper breeding practices, the impact of these mutations can be reduced or avoided.

Dwarfism

Dwarfism in Friesians is characterized by a disproportionate growth with reduced bone length of limbs and ribs while the size of the head and length of back are normal. Microscopic analysis of growth plates in affected animals shows irregular transition from cartilage to bone and abnormal arrangements of chondrocytes (cartilage cells). Affected horses have hyperextension of fetlock joints of all limbs with varying degrees of severity. A mutation (c.50G>A) in exon 1 of the Beta-1,4-Galactosyltransferase 7 (B4GALT7) gene has been identified that is associated with this disorder. B4GALT7 plays an important role in proper formation of extracellular matrix, a key element for bone development. The mutation affects normal functioning of the gene which interferes with proper collagen formation.

Friesian dwarfism is inherited as a simple autosomal recessive trait. This means that affected foals have two copies of the defective gene and that the disorder occurs in males and females. It is estimated that about 12% of Friesian population are carriers (N/D) of the mutation.

The VGL offers a genetic test for Friesian dwarfism. Results from the test assist owners and breeders to identify carriers and to make informed mate selection to avoid producing affected foals. Matings between carriers should be avoided as these crosses have a 25% chance of producing affected foals. Breeding carriers to normal horses is safe and a means to maintain genetic diversity in the breed and avoid production of affected foals.

This test is recommended for Friesian and Friesian-crosses.

ORDER TEST | PRICE LIST
Allow 6-10 business days for results.

Results are reported as:

N/N

No copies of the dwarfism mutation. Horse is normal.

N/D

1 copy of the dwarfism mutation. Horse is normal but is a carrier.

D/D

2 copies of the dwarfism mutation. Horse is affected.

Hydrocephalus

Hydrocephalus is a condition characterized by excessive accumulation of fluid in the brain that results in severe cranial distension. It is thought to occur because of an abnormal narrowing of the jugular foramen (opening at the base of the skull) and can lead to dystocia in dams pregnant with affected foals. This condition often causes foals to be stillborn or to be euthanized at birth due to problems during parturition. A mutation (c.1423C>T) in exon 12 of the β-1,3-N-acetylgalactosaminyltransferase 2 (B3GALT2) gene has been identified that is associated with this disorder. The mutation does not allow for the full-length protein to be produced which compromises its normal function. Mutations in this gene also cause muscular dystrophy-dystroglycanopathy with brain abnormalities in humans.

Friesian hydrocephalus is inherited as an autosomal recessive trait. This means that affected foals have two copies of the defective gene and that the disorder occurs in males and females. It is estimated that about 17% of Friesian horses are carriers (N/H).

The VGL offers a genetic test for Friesian hydrocephalus.  Results from the test assist owners and breeders to identify carriers and to make informed mate selection to avoid producing affected foals. Matings between carriers should be avoided as these crosses have a 25% chance of producing affected foals. Breeding carriers to normal horses is safe and a means to maintain genetic diversity in the breed and avoid production of affected foals.

This test is recommended for Friesian and Friesian-crosses.

ORDER TEST | PRICE LIST
Allow 6-10 business days for results.

Results are reported as:

N/N

No copies of the hydrocephalus mutation. Horse is normal.

N/H

1 copy of the hydrocephalus mutation. Horse is normal but is a carrier.

H/H

2 copies of the hydrocephalus mutation. Horse is affected.

References:

Back W., van der Lugt J.J., Nikkels P.G., van den Belt A.J., van der Kolk J.H. & Stout T.A. (2008) Phenotypic diagnosis of dwarfism in six Friesian horses. Equine Vet J 40, 282-7.

Boerma S., Back W. & Sloet van Oldruitenborgh-Ossterbaan M.M. (2012) The Friesian horse breed: a clinical challenge to the equine veterinarian? Equine Veterinary Education 24, 66-71.

Ducro B.J., Schurink A., Bastiaansen J.W., Boegheim I.J., van Steenbeek F.G., Vos-Loohuis M., Nijman I.J., Monroe G.R., Hellinga I., Dibbits B.W., Back W. & Leegwater P.A. (2015) A nonsense mutation in B3GALNT2 is concordant with hydrocephalus in Friesian horses. BMC Genomics 16, 761.

Leegwater P.A., Vos-Loohuis M., Ducro B.J., Boegheim I.J., van Steenbeek F.G., Nijman I.J., Monroe G.R., Bastiaansen J.W., Dibbits B.W., van de Goor L.H., Hellinga I., Back W. & Schurink A. (2016) Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7. BMC Genomics 17: 839. doi 10.1186/s12864-016-3186-0.

Sipma K.D., Cornillie P., Saulez M.N., Stout T.A., Voorhout G. & Back W. (2013) Phenotypic characteristics of hydrocephalus in stillborn Friesian foals.
Vet Pathol 50, 1037-42.

 
Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL