title: Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7.
abstract: Inbreeding and population bottlenecks in the ancestry of Friesian horses has led to health issues such as dwarfism. The limbs of dwarfs are short and the ribs are protruding inwards at the costochondral junction, while the head and back appear normal. A striking feature of the condition is the flexor tendon laxity that leads to hyperextension of the fetlock joints. The growth plates of dwarfs display disorganized and thickened chondrocyte columns. The aim of this study was to identify the gene defect that causes the recessively inherited trait in Friesian horses to understand the disease process at the molecular level.
  
  
  
  We have localized the genetic cause of the dwarfism phenotype by a genome wide approach to a 3 Mb region on the p-arm of equine chromosome 14. The DNA of two dwarfs and one control Friesian horse was sequenced completely and we identified the missense mutation ECA14:g.4535550C > T that cosegregated with the phenotype in all Friesians analyzed. The mutation leads to the amino acid substitution p.(Arg17Lys) of xylosylprotein beta 1,4-galactosyltransferase 7 encoded by B4GALT7. The protein is one of the enzymes that synthesize the tetrasaccharide linker between protein and glycosaminoglycan moieties of proteoglycans of the extracellular matrix. The mutation not only affects a conserved arginine codon but also the last nucleotide of the first exon of the gene and we show that it impedes splicing of the primary transcript in cultured fibroblasts from a heterozygous horse. As a result, the level of B4GALT7 mRNA in fibroblasts from a dwarf is only 2 % compared to normal levels. Mutations in B4GALT7 in humans are associated with Ehlers-Danlos syndrome progeroid type 1 and Larsen of Reunion Island syndrome. Growth retardation and ligamentous laxity are common manifestations of these syndromes.
  
  
  
  We suggest that the identified mutation of equine B4GALT7 leads to the typical dwarfism phenotype in Friesian horses due to deficient splicing of transcripts of the gene. The mutated gene implicates the extracellular matrix in the regular organization of chrondrocyte columns of the growth plate. Conservation of individual amino acids may not be necessary at the protein level but instead may reflect underlying conservation of nucleotide sequence that are required for efficient splicing.
authors: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, PO Box 80154, NL-3508 TD, Utrecht, The Netherlands. p.a.j.leegwater@uu.nl.
authors: Peter A
authors: 0000-0002-4100-015X
authors: PA
authors: Leegwater
authors: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, PO Box 80154, NL-3508 TD, Utrecht, The Netherlands.
authors: Manon
authors: M
authors: Vos-Loohuis
authors: Animal Breeding and Genomics Centre, Wageningen University, PO Box 338, NL-6700 AH, Wageningen, The Netherlands.
authors: Bart J
authors: BJ
authors: Ducro
authors: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, PO Box 80154, NL-3508 TD, Utrecht, The Netherlands.|Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 112-114, NL-3584 CM, Utrecht, The Netherlands.
authors: Iris J
authors: IJ
authors: Boegheim
authors: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, PO Box 80154, NL-3508 TD, Utrecht, The Netherlands.
authors: Frank G
authors: FG
authors: van Steenbeek
authors: Department of Medical Genetics, University Medical Center Utrecht, PO Box 85090, NL-3508 AB, Utrecht, The Netherlands.
authors: Isaac J
authors: IJ
authors: Nijman
authors: Department of Medical Genetics, University Medical Center Utrecht, PO Box 85090, NL-3508 AB, Utrecht, The Netherlands.
authors: Glen R
authors: GR
authors: Monroe
authors: Animal Breeding and Genomics Centre, Wageningen University, PO Box 338, NL-6700 AH, Wageningen, The Netherlands.
authors: John W M
authors: JW
authors: Bastiaansen
authors: Animal Breeding and Genomics Centre, Wageningen University, PO Box 338, NL-6700 AH, Wageningen, The Netherlands.
authors: Bert W
authors: BW
authors: Dibbits
authors: Dr. van Haeringen Laboratorium B.V., PO Box 408, NL-6700 AK, Wageningen, The Netherlands.
authors: Leanne H
authors: LH
authors: van de Goor
authors: Koninklijke Vereniging "het Friesch Paarden-Stamboek", PO Box 624, NL-9200 AP, Drachten, The Netherlands.
authors: Ids
authors: I
authors: Hellinga
authors: Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 112-114, NL-3584 CM, Utrecht, The Netherlands.|Department of Surgery and Anaesthesiology of Domestic Animals, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, B-9820, Merelbeke, Belgium.
authors: Willem
authors: W
authors: Back
authors: Animal Breeding and Genomics Centre, Wageningen University, PO Box 338, NL-6700 AH, Wageningen, The Netherlands.
authors: Anouk
authors: A
authors: Schurink
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002874:Chromosome Mapping; D004392:Dwarfism / Q000662:veterinary*; D005260:Female; D005700:Galactosyltransferases / Q000235:genetics*; D056726:Genetic Association Studies; D006734:Horse Diseases / Q000235:genetics*; D006736:Horses; D007593:Joint Instability / Q000235:genetics*; D009154:Mutation; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide; D022821:RNA Splice Sites; D017422:Sequence Analysis, DNA
chemical_list: D022821:RNA Splice Sites; D005700:Galactosyltransferases
keywords: Equus caballus; Extracellular matrix; Galactosyltransferase I; Genome; Growth retardation; Hypermobile joints; Linkeropathy; Proteoglycan