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

Genetic parameters of cryptorchidism and testis size in Friesian colts

In males with cryptorchidism, one or both testes do not descend into the scrotum thereby affecting among other things fertility. Testis size has been suggested to contribute to cryptorchidism. Therefore, the aim of our study was to estimate genetic parameters of cryptorchidism and testis size in Friesian colts. Data on cryptorchidism (0/1, n=1327) and testis size (cm, n=868 with size of both testes estimated) in Friesian colts of 1–7 months-of-age were gathered by a veterinarian during inspections from 2009 to 2012. Heritabilities, phenotypic and genetic correlations were estimated using ASReml4 including age of the colt (in months), location, year and month of inspection as fixed effects. Prevalence of cryptorchidism was 14.2%. Most affected colts (88.3%) were unilateral, while 11.7% were bilateral cases. Of the unilateral cases, significantly fewer colts had a left retained testis (35.5%) compared to a right retained testis (64.5%). Heritability of cryptorchidism was 0.13 (SE=0.06) and increased slightly when only cases of 4 months and older were considered. Based on literature and our findings we advise not to inspect colts at a very young age. Mean testis size significantly differed between left (3.47 cm) and right testis (3.19 cm). Heritability of left testis size (0.12±0.07) was lower compared to heritability of right testis size (0.31±0.10), where genetic correlation between left and right testis size was 0.87 (SE=0.12). The genetic correlation between left testis size and cryptorchidism was −0.94 (SE=0.15) and between right testis size and cryptorchidism was −0.64 (SE=0.23). At the age of the investigated Friesian colts, cryptorchidism genetically coincides with smaller testis size. The development of the left and right testis might differ, which could be hereditary in nature. More precise phenotyping, like recording position and size (and side) of the retained testis and age of the stallion, might contribute additionally to disentangling the genetic background of equine cryptorchidism and identifying the gene(s) affecting this disorder. For now, the continuation of the data recording as described in our study will enable the studbook to estimate breeding values and thereby select against cryptorchidism.</p

Additional file 1: of A nonsense mutation in B3GALNT2 is concordant with hydrocephalus in Friesian horses

Sanger DNA sequencing results for the nonsense mutation. A table with information on all horses that have been sequenced for the nonsense mutation. (DOCX 20 kb

Additional file 3: of A nonsense mutation in B3GALNT2 is concordant with hydrocephalus in Friesian horses

Genes located in the ECA1 region shared in a homozygous state by hydrocephalus cases. Start and stop position (in base pair; Equus caballus EquCab2.0 reference genome [29]), symbol and description of genes located in the region of 1.47 Mb in length that is shared in a homozygous state by hydrocephalus cases. (DOCX 21 kb