Dwarfism with joint laxity in Friesian horses is associated with a splice site mutation in B4GALT7

Background: 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. Results: 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. Conclusions: 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

The prevalence of nine genetic disorders in a dog population from Belgium, the Netherlands and Germany.

The objective of this study was to screen a dog population from Belgium, the Netherlands and Germany for the presence of mutant alleles associated with hip dysplasia (HD), degenerative myelopathy (DM), exercise-induced collapse (EIC), neuronal ceroid lipofuscinosis 4A (NCL), centronuclear myopathy (HMLR), mucopolysaccharidosis VII (MPS VII), myotonia congenita (MG), gangliosidosis (GM1) and muscular dystrophy (Duchenne type) (GRMD). Blood samples (K3EDTA) were collected for genotyping with Kompetitive Allele Specific PCR (n = 476). Allele and genotype frequencies were calculated in those breeds with at least 12 samples (n = 8). Hardy-Weinberg equilibrium was tested. Genetic variation was identified for 4 out of 9 disorders: mutant alleles were found in 49, 15, 3 and 2 breeds for HD, DM, EIC and NCL respectively. Additionally, mutant alleles were identified in crossbreeds for both HD and EIC. For HD, DM, EIC and NCL mutant alleles were newly discovered in 43, 13, 2 and 1 breed(s), respectively. In 9, 2 and 1 breed(s) for DM, EIC and NCL respectively, the mutant allele was detected, but the respective disorder has not been reported in those breeds. For 5 disorders (HMLR, MPS VII, MG, GM1, GRMD), the mutant allele could not be identified in our population. For the other 4 disorders (HD, DM, EIC, NCL), prevalence of associated mutant alleles seems strongly breed dependent. Surprisingly, mutant alleles were found in many breeds where the disorder has not been reported to date

Comparisons of genotype frequencies for exercise-induced collapse in the Labrador Retriever.

<p>The reference population consists of 2 different subsets based on collection method. Source: Public = based on request by the owner to perform genetic testing for EIC, Soure: researchers = researchers went to several competitions and took samples from every dog. Est. Freq (HWE) = estimated frequencies under Hardy-Weinberg equilibrium.</p

Breed and samples per breed tested.

a<p> = not specified whether English or Welsh Springer Spaniel.</p

Breeds where mutant alleles for degenerative myelopathy were found and breed specific prevalence for breeds with at least 12 samples.

<p>NN = 2 normal alleles, NA = heterozygous, AA = 2 mutant alleles, HWE = Hardy-Weinberg equilibrium, q = mutant allele frequency, % = percent of dogs belonging to specific category, – = not applicable.</p

Breeds where mutant alleles for hip dysplasia were found and breed specific prevalence for breeds with at least 12 samples.

<p>NN = 2 normal alleles, NA = heterozygous, AA = 2 mutant alleles, HWE = Hardy-Weinberg equilibrium, q = mutant allele frequency, % = percent of dogs belonging to specific category, – = not applicable, ^a = not specified whether English or Welsh Springer Spaniel.</p

Breeds where mutant alleles for exercise-induced collapse were found and breed specific prevalence for breeds with at least 12 samples.

<p>NN = 2 normal alleles, NA = heterozygous, AA = 2 mutant alleles, HWE = Hardy-Weinberg equilibrium, q = mutant allele frequency, % = percent of dogs belonging to specific category, – = not applicable.</p

Overview of 9 disorders tested with their corresponding chromosome number, the mutation, the effect, the inheritance, the breeds where the mutation has been reported before and the animal model.

<p>CFA = chromosome number; AR = autosomal recessive, XR = X-linked recessive, MP = multifactorial; MS = missense, SP = splice variant, FS = frame shift, ES = exon skipping, SINE = short interspersed element, SNP = single nucleotide polymorphism;¹ leading to premature termination, ² reduced penetrance.</p

Breeds where mutant alleles for neuronal ceroid lipofuscinosis 4A were found and breed specific prevalence for breeds with at least 12 samples.

<p>NN = 2 normal alleles, NA = heterozygous, AA = 2 mutant alleles, HWE = Hardy-Weinberg equilibrium, q = mutant allele frequency, % = percent of dogs belonging to specific category, - = not applicable.</p