EQUINE | Equine Disease Update
EQUINE | Equine Disease Update
Pandora ’ s Box – Equine Genomics
he genetics of horses has been of great interesthe genetics of horses has been of great interest to humans for millennia . While we may not have used that term specifically , the recording of equine parentage , births , and deaths was a practice that pre-dated the compulsory recording of human births and deaths in Great Britain by over a century . The term “ genomics ” covers all aspects of genes , including their structure and function , not just the science of heredity . The genome is all of the DNA , divided up and packaged as chromosomes , in each cell . The DNA molecule , the unit of heredity , is made up of four nucleotide bases , guanine ( G ), cytosine ( C ), adenine ( A ), and thymine ( T ) in a sequence . The equine genome consists of ~ 2,700 million base pairs , which is similar in size to the human genome . Less than 3 % of the genome actually codes for proteins . The remaining 97 % was formerly termed “ junk DNA ” but we now know it orchestrates the use of the entire genome , as regulatory elements . Individuals have differences in the sequences of the nucleotides . This is pretty obvious - we have greys and bays but they are still horses . However , not all of the sequence differences are that obvious , and sequence differences can be whole sections of sequence or just single nucleotides . A Single Nucleotide Polymorphism ( SNP , usually pronounced “ snip ”) is a single nucleotide in a sequence that differs between individuals at a low population frequency . Approximately 10 million SNPs have been found in the collective equine genome .
It is important to remember that SNPs are not necessarily mutations that have any effect on the organism . Indeed , SNPs are less likely to be found in the protein coding genes of the genome as these areas have been heavily selected for functionality by evolution . Any change in DNA sequence that did not benefit the animal would reduce its chances of survival in the gene pool . Science uses selected SNPs as a crude road map of the genome . By way of analogy , a set of directions are like a SNP map . If I gave you some directions , they might be : “ Turn right at the pub , then take the second left after the church .” The pub and the church have no actual bearing on the destination , they are just guiding landmarks , and so it is with SNPs . SNPs that sit close together on a chromosome are likely to be inherited together . Using commercial SNP arrays (“ SNP chips ”), molecular biology techniques can rapidly hone in on a region of the genome that is different between horses in terms of its SNP frequency . SNP analysis is an immensely useful tool to narrow down the search for areas of the genome harboring genetic traits of interest , whether they be a trait relating to disease or a desired trait . Studies using this approach are called Genome Wide Association Studies or GWAS ( pronounced ‘ gee-waahs ’). The SNP-GWAS approach has helped identify regions of interest in the genome in diseases such as Lavender Foal Syndrome , Polysaccharide Storage Myopathy , recurrent laryngeal neuropathy , Foal Immunodeficiency Syndrome in the Fell and Dales pony , osteochondrosis dissecans ( OCD ), guttural pouch tympany in Arabians and German warmbloods , recurrent uveitis in German warmbloods , insect bite hypersensitivity , and hydrocephalus in Friesians . We have only just started to uncover the wealth of information the equine genome holds . Continued efforts for funding and research will yield more mind-blowing results than our imagination can fathom . Watch this space !
CONTACT : Emma N . Adam , BVetMed , MRCVS , DACVIM-LA , DACVS-LA , PhD emma . adam @ uky . edu ( 859 ) 218-1175 Maxwell H . Gluck Equine Research Center University of Kentucky Lexington , KY
20 • Equine Health Update •