Horse Clinical Cytogenetics: Recurrent Themes and Novel Findings.

Abstract

Simple SummaryHorse chromosomes have been studied for veterinary diagnostic purposes for over half a century. The findings show that changes in the chromosome number or structure are among the most common non-infectious causes of decreased fertility, infertility, and developmental abnormalities. Based on large-scale surveys, almost 30% of horses with reproductive or developmental problems have abnormal chromosomes. For a comparison, only 2–5% of horses in the general population have abnormal chromosomes. Most chromosome abnormalities are rare and found in one or a few animals. However, two conditions are recurrent: sterile mares with only one X chromosome, instead of two, and sterile mares with XY male sex chromosomes where the Y has lost the ‘maleness’ gene SRY. The two are signature features of chromosome abnormalities in the horse, being rare or absent in other domestic animals. The progress in horse genome sequencing and the development of molecular tools have improved the depth and quality of diagnostic chromosome analysis, allowing for an understanding of the underlying molecular mechanisms. Nevertheless, cutting-edge genomics tools are not about to entirely replace traditional chromosome analysis, which still is the most straightforward, cost-effective, and fastest approach for the initial evaluation of potential breeding animals and horses with reproductive or developmental disorders.Clinical cytogenetic studies in horses have been ongoing for over half a century and clearly demonstrate that chromosomal disorders are among the most common non-infectious causes of decreased fertility, infertility, and congenital defects. Large-scale cytogenetic surveys show that almost 30% of horses with reproductive or developmental problems have chromosome aberrations, whereas abnormal karyotypes are found in only 2–5% of the general population. Among the many chromosome abnormalities reported in the horse, most are unique or rare. However, all surveys agree that there are two recurrent conditions: X-monosomy and SRY-negative XY male-to-female sex reversal, making up approximately 35% and 11% of all chromosome abnormalities, respectively. The two are signature conditions for the horse and rare or absent in other domestic species. The progress in equine genomics and the development of molecular tools, have qualitatively improved clinical cytogenetics today, allowing for refined characterization of aberrations and understanding the underlying molecular mechanisms. While cutting-edge genomics tools promise further improvements in chromosome analysis, they will not entirely replace traditional cytogenetics, which still is the most straightforward, cost-effective, and fastest approach for the initial evaluation of potential breeding animals and horses with reproductive or developmental disorders.