Description of the D4/D4 genotype in Miniature horses with dwarfism

Abstract

Four causative mutations (D1, D2, D3*, and D4) of chondrodysplastic dwarfism have been described in the equine aggrecan ( ACAN) gene. Homozygotes for one of these mutations and heterozygotes for any combination of these mutations exhibit the disproportionate dwarfism phenotype. However, no case description of homozygotes for D4 (D4/D4) has been reported in the literature, to our knowledge. We report 2 Miniature horses with the genotype D4/D4 in the ACAN gene. Clinically, the 2 dwarfs had a domed head that was large compared to the rest of the body, mandibular prognathism, and short and bowed limbs, mainly in the proximal region of the metatarsal bones. Radiographic examination revealed contour irregularities of the subchondral bone in the long bones and confirmed mandibular prognathism; histopathology revealed irregular chondrocyte organization. To determine the genotypes of the horses, we performed DNA extraction from white blood cells, PCR, and Sanger sequencing. Genotyping demonstrated that these 2 animals had the D4/D4 genotype in the ACAN gene. The D4/D4 dwarfs were clinically similar to animals with the other ACAN genotypes reported for this disease. Identification of heterozygous animals makes mating selection possible and is the most important control measure to minimize economic losses and casualties.

Evaluation of a new variant in the aggrecan gene potentially associated with chondrodysplastic dwarfism in Miniature horses

“…The clinical signs observed in the Miniature horses with dwarfism described here were similar to those previously reported 2 , 5 , 11 , 12 . As ACAN - D1 was not observed in the dwarf Miniature horses of the present study, the clinical signs that this variant can cause (cleft palate with protruding tongue, large abdominal hernia and embryonic or late term loss) 2 were not reported in the affected animals of the present study.…”

“…This animal presented with respiratory distress, tachypnoea, inspiratory honking noises and dysphagia. The other dwarfs had no clinical signs of tracheal collapse, but airway endoscopy revealed mild tracheal collapse in eight of ten dwarf Miniature horses (animals [5][6][7][8][9][10][11][12][13][14]. Moreover, other abnormalities were observed during airway endoscopy examination, such as dorsal displacement of the soft palate (5/10) and thickening of both arytenoid cartilages (4/10).…”

“…population screening. Genetic testing (D1, D2, D3* and D4) according to the methodology previously described 11 and using the DG_ACAN_Eq_F13 and DG_ACAN_Eq_R13 primer pair 11 for ACAN:c.6465A > T (RefSeq XM_005602799.2-EquCab3.0) was performed in 347 phenotypically normal Miniature horses randomly sampled from nine farms in Brazil. The parents of the 14 neonatal Miniature horses with dwarfism admitted to the Large Animal Internal Medicine Section of São Paulo State University (Unesp) were included in this…”

“…In Miniature horses, four ACAN gene variants ( D1 , D2 , D3* and D4 ) have already been described as causative of chondrodysplastic dwarfism 2 , 5 . Most D1 genotypes ( D1 / D1 , D1 / D2 , D1 / D3* and D1 / D4 ) cause foetal death 2 , whereas the other genotypes ( D2 / D2 , D2 / D3* , D2 / D4 , D3* / D3* , D3* / D4 and D4 / D4 ) are involved in the birth of dwarf foals 2 , 5 , 11 . Clinical diagnosis of chondrodysplastic dwarfism in Miniature horses has already been described in Brazil 12 , and the D4 / D4 genotype was recently characterized in the same country 11 .…”

“…Most D1 genotypes ( D1 / D1 , D1 / D2 , D1 / D3* and D1 / D4 ) cause foetal death 2 , whereas the other genotypes ( D2 / D2 , D2 / D3* , D2 / D4 , D3* / D3* , D3* / D4 and D4 / D4 ) are involved in the birth of dwarf foals 2 , 5 , 11 . Clinical diagnosis of chondrodysplastic dwarfism in Miniature horses has already been described in Brazil 12 , and the D4 / D4 genotype was recently characterized in the same country 11 . However, it is still not possible to associate all dwarf Miniature horses with the previously described ACAN variants 2 .…”