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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2018) 30 (5) 274-281 doi: 10.1111/eve.12578
Review Article Genomics in equine veterinary medicine
E. N. Adam Gluck Equine Research Center, University of Kentucky, Lexington, USA. Corresponding author email:
eadam9387@gmail.com
Keywords: horse; genomics; genetics; transcriptomics
Summary Veterinary medicine is marching forward with genomics grasped firmly in one fist. Genomics has been part of our veterinary tool bag for some time in various guises, but the breadth and depth of its ramifications can be daunting. This article is designed to provide toe-holds for veterinarians to enjoy a better understanding of genomics – a truly fascinating area of science.
Introduction
The subject of genetics may bring back tormented memories of wrinkled seeds or the dreaded Punnett square diagrams. For many of us it may be a distant memory. However, now one can hardly turn around without bumping into genetics-based headlines in every form of media whether it relates to movie stars such as Angelina Jolie or a horse’s genetic predisposition for speed or disease. Veterinary medicine is marching forward with genomics grasped firmly in one fist. Genomics has been part of our veterinary tool bag for some time in various guises, but the breadth and depth of its ramifications can be daunting (Bailey 2014). This article is designed to provide the basis for enjoying the variety and plethora of genomics literature in veterinary science.
Genomes and cytogenetics
Prior to the molecular age of genetics, cytogenetics and specifically karyotyping provided the first views of the genome (Rothfels et al. 1959). The horse has 32 pairs of chromosomes: 31 pairs of autosomes and two sex chromosomes (X and Y). While we could not discern the presence, distribution or even number of genes at the beginning of this technology, we did realise that horses needed to have exactly all 32 pairs in order to be healthy and fertile. Chromosomes are identified by length, banding patterns, location of chromosome features, and from this, a naming convention developed (Bowling et al. 1997). For example, the largest chromosome (autosome) is designated ECA 1 (ECA for Equus caballus) and the smallest autosome is identified as ECA31. Chromosome abnormalities include loss or gain of sex chromosomes (X or Y), gain of one of the smaller chromosomes or rearrangement of chromosome arms. These abnormalities occur in 2–3% of live-births (Bugno et al. 2007). It is likely that severe chromosome changes result in early embryonic death or abortion while mild alterations may be carried to term. However, even mild rearrangements adversely affect fertility or even long-term viability. The most common chromosome defect is loss of the X chromosome (Lear and Villagomez 2011). In these cases, mares may
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appear normal however they have small ovaries and be infertile. Severe disease occurs when one of the smaller chromosomes (notably ECA23, -26, -27, -28, -29, -30 or -31) is duplicated. On rare occasions, a chromosome segment may detach and join another chromosome; this is called a translocation. Horses with translocations are normal and healthy but may have problems producing viable gametes. Such studies are still relevant to the practicing veterinarian; however, molecular genetics provides greater resolution for genomic studies and has provided valuable tools for veterinary research and practice.
Terminology and historical perspective of DNA
Genetics is considered the study of heredity, whereas genomics is a term with wider scope encompassing all aspects of genes, including their structure and function, with few limitations. A genome is the genetic material of a particular organism. Before we dive through the looking glass, let’s just recap on some essential groundwork:
● 1944 – Avery, MacLeod and McCarty demonstrated DNA as the molecule of heredity (Avery et al. 1944).
● 1950 – Chargaff’s rules: guanine (G) and cytosine (C); adenine (A) and thymine (T) present in equal proportions in DNA samples (Chargaff 1950).
● 1953 – published simultaneously: ○ Watson and Crick publish the helical structure of DNA with Watson-Crick base-pairing rules: G paired to C, and A paired to T (Watson and Crick 1953).
○ Wilkins (Wilkins et al. 1953) and Franklin (Franklin and Gosling 1953) publish critical x-ray crystallographic images of DNA.
DNA is deoxyribose nucleic acid and the importance of
the ‘deoxy’ component of the ribose sugar molecule in the backbone contributes to the inertness of the molecule. This is a key ingredient to its stability and hardiness. The stability of this incredible double stranded molecule is entrenched in our
modern collective psyche as we watch crime shows and films such as Jurassic Park. From a veterinary perspective the stability of DNA enables samples to be shipped with minimal extra handling requirements, availing itself as a diagnostic tool. From a scientific perspective, the robust nature of DNA is the reason ancient DNA can be studied. Recently, the genome of two horse specimens found in the permafrost of the Yukon Territory that lived approximately 560,000– 780,000 years ago was sequenced (Orlando et al. 2013; Schubert et al. 2014). This study offered some marvellous insight into which genes and mutations have been selected
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