Current genetics of osteochondrosis (OCD) in equines
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The aim of the study was to perform a review study regarding the current genetic background of OC in the domestic horse. OC is a commonly seen locomotor disorder in young horses (WITTWER et al, 2007). The cause of OC is still not clear, even though it is assumed that it is of multifactorial origin, affecting many breeds and with an important genetic aspect. The whole equine genome scan was the first step in the investigation of genomic areas holding genes for OC and OCD. During the last years several studies has been done regarding the genetic background of OC in horses. In some reports it has already been proven that it's a disorder carried on many different genes (WITTWER et al, 2008). The articles analyzed for QTL mapping were mostly studies from the University of Hannover done between the years 2000-2010. The first article was the; Genome scan for Quantitative Trait Loci for OC in Hanoverian Warmblood horses using an optimized microsatellite marker set by LÖHRING (2003). LÖHRING revealed; 27 chromosome-wide QTLs on thirteen equine chromosomes for the OC trait in Hanoverian warmblood horses. In Dierks study 19 chromosome-wide significant QTLs were reveled on 17 equine chromosomes. They were found on equine chromosome; 2, 3, 4, 5, 7, 8, 9, 13, 14, 15, 16, 18, 19, 21, 22, 24 and 30. In 2009 Lampe did a complete genome scan in Hanoverian Warmblood horses and refined the QTLs for OC on chromosome 5, 16, 18 and 21. The genome-wide significant QTLs on equine chromosome 5, 16 and 21 were as earlier mentioned for fine mapping, together with the release of the horse genome assembly EquCab2 it was possible to identify new microsatellites. All QTLs on theses chromosomes were confirmed. Equine chromosome 18 was investigated as well, earlier QTLs on this chromosome was only confirmed in South German Coldblood horses in a linkage study for OC (WITTWER et al, 2007), and now a further investigation was done in Hanoverian warmblood horses. A new QTLs was identified on chromosome 18, thanks to the new microsatellites which made a more evenly and denser distribution marker set. The genetic influences of the development of fetlock OC and hock OC did not show any similarity, due the hock QTL did not map at the fetlock QTL. This was assumed likely as the genetic correlation between fetlock OC and hock OC were close to zero in trotter horses (GRØNDAHL and DOLVIK 1993) and even negative in Hanoverian warmblood horses (STOCK et al, 2005) WITTWER preformed a whole genome scan to confirm the QTLs identified in Hanoverian warmblood horses in South German Coldblood. A scan was preformed in 216 coldblood using 250 polymorphic microsatellite markers. WITTWER identified 17 putative QTLs on 17 equine chromosomes for the OC / OCD traits. The aim of WITTWER’s study was to confirm the QTLs by using single nucleotide polymorphisms (SNPs) of these genomic regions. SNPs are used as genetic markers for large scale genetic mapping projects and have been used with good result to identify chromosome regions associated with polygenic human and animal diseases. In 2008 a commercial SNPs microarray became available, the Equine SNP50 Genotyping BeadChip. It makes it possible to do, genome-wide analyses, quantitative trait loci identification and validation. It enables the development of a DNA test that can determine a horse’s genetic risk for susceptibility to a genetic disease, improving horse breeding programs since it enables the development of new diagnostic methods to upgrade equine health and welfare. The aim of KOMM’s study was to make a refine mapping of the already identified QTL on equine chromosome 2 and 4 in Hanoverian warmblood horses using dense marker sets, and also identify new potential candidate genes. Another aim of KOMM’s study was to confirm already identified QTLs and to detect new potential QTL by doing a whole genome scan with SNPs using the newly developed equine SNP50 BeadChip. 154 unrelated Hanoverian warmblood horses were used in the whole genome scan. 313 significant associated SNPs for the different phenotypic traits were observed. The QTL on equine chromosome 2 was confirmed at 17.55Mb and on equine chromosome 4 two QTL were revealed (at 7.61Mb and 39, 26 MB) these two QTL were located within the same QTL as identified before. In total ten new QTL were detected on equine chromosome 3, 5, 7, 16, 19, 20, 22, 26 and 29.