Candidate genes for idiopathic epilepsy in four dog breeds

BMC Genetics Candidate genes for idiopathic epilepsy in four dog breeds Kari J Ekenstedt 0 Edward E Patterson 0 Katie M Minor 1 James R Mickelson 1 0 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota , 1352 Boyd Avenue, Saint Paul, Minnesota, 55108 , USA 1 Department of Veterinary and Biomedical Sciences, College of Veterinary Medicine, University of Minnesota , 295 AS/VM, 1988 Fitch Avenue, St. Paul, Minnesota, 55108 , USA Background: Idiopathic epilepsy (IE) is a naturally occurring and significant seizure disorder affecting all dog breeds. Because dog breeds are genetically isolated populations, it is possible that IE is attributable to common founders and is genetically homogenous within breeds. In humans, a number of mutations, the majority of which are genes encoding ion channels, neurotransmitters, or their regulatory subunits, have been discovered to cause rare, specific types of IE. It was hypothesized that there are simple genetic bases for IE in some purebred dog breeds, specifically in Vizslas, English Springer Spaniels (ESS), Greater Swiss Mountain Dogs (GSMD), and Beagles, and that the gene(s) responsible may, in some cases, be the same as those already discovered in humans. Results: Candidate genes known to be involved in human epilepsy, along with selected additional genes in the same gene families that are involved in murine epilepsy or are expressed in neural tissue, were examined in populations of affected and unaffected dogs. Microsatellite markers in close proximity to each candidate gene were genotyped and subjected to two-point linkage in Vizslas, and association analysis in ESS, GSMD and Beagles. Conclusions: Most of these candidate genes were not significantly associated with IE in these four dog breeds, while a few genes remained inconclusive. Other genes not included in this study may still be causing monogenic IE in these breeds or, like many cases of human IE, the disease in dogs may be likewise polygenic. - Background The prevalence of epilepsy in humans is reported to be in the range of 4 - 10/1000 in most study settings [1], with idiopathic epilepsy (IE) representing 15 - 20% of these cases [2]. It is now generally accepted that IE in humans is due to an underlying genetic origin [2], although causative mutations have been discovered in only a small subset of IEs, mostly in isolated populations. These identified epilepsy mutations are, for the most part, Mendelian or monogenic IEs [3-6], and are often termed channelopathies due to their occurrence in ion channel genes. In a recent review, 16 of 21 susceptibility genes for human epilepsy were ion channels or neurotransmitter receptors [7]. Canine epilepsy is a naturally occurring, spontaneous condition. Canine seizures exhibit a remarkable resemblance to human seizures [8] and the usefulness of naturally occurring canine epilepsy as a translational model to explore potential treatments for human epilepsy was recently proposed by Leppik, et al. [9]. The prevalence of canine epilepsy is estimated to be between 0.5% and 5.7% and it is the most common chronic neurological disorder in dogs [10]. A diagnosis of IE in the canine indicates recurrent seizures for which no cause can be identified and implies a genetic predisposition [11]. While the molecular basis of IE in canines is entirely unknown, a hereditary basis for IE has been suggested in a number of dog breeds including Beagles [12], British Alsatians [13], Keeshonds [14], Labrador retrievers [15], Golden retrievers [16,17], Bernese mountain dogs [18], Belgian Tervurens [19,20], Boxers [21], Shetland sheepdogs [22], Vizslas [23], English Springer Spaniels [24], Irish wolfhounds [25], and Standard Poodles [26]. One inherited progressive seizure disorder that has been identified in dogs is the autosomal recessive Lafora disease in Miniature Wirehaired Dachshunds [27]. This progressive myoclonic epilepsy disorder results from a mutation in the EPM2B (also known as NHLRC1-NHL repeat containing 1) gene. Modes of inheritance predicted from examination of pedigrees of dogs with IE show evidence for a gene of major effect or autosomal recessive inheritance, both of which could occur due to a founder effect. Examples include the English Springer Spaniel (partially penetrant autosomal recessive or polygenic inheritance) [24], the Vizsla (autosomal recessive or polygenic inheritance) [23], and the Golden Retriever [16] and the Bernese Mountain Dog [18] (both autosomal recessive but controlled by more than one gene). There is enough variation in mode of inheritance and clinical characteristics in dogs to presume that the underlying genetic basis is unlikely to be identical between breeds. Linebreeding and inbreeding have become commonplace to meet the very specific dog breed standards of the hundreds of dog breeds in existence today. These practices, while creating more uniform offspring, can also result in deleterious diseases due to the concentration of recessive mutations passed from the founders. Almost half of the nearly four hundred genetic disorders that have been described in dogs are recognized exclusively in one or a few breeds [28], presumably due to a strong founder effect. The aim of the present study was to take advantage of the likely founder effect occurring in purebred dogs with IE in order to investigate an underlying genetic basis for IE in four dog breeds: the Vizsla, the English Springer Spaniel (ESS), the Greater Swiss Mountain Dog (GSMD) and the Beagle. A candidate gene approach was utilized; genes known to be involved in inherited human epilepsy, along with two genes involved in mouse epilepsy models and additional related gene family members, were selected. Most candidate genes tested were ion channels. Microsatellite markers within or very close to these genes were used to generate genotypes which were then subjected to either two-point linkage analysis or association chi-square studies. Using four different breeds, which may each have different forms of IE, increased the chances of potentially finding an associated gene in at least one of the breeds. Results Call rates in each sample set for the microsatellite markers that were ultimately used in each analysis were approximately 90% or higher. Heterozygosity was recorded for each marker as a measure of that markers extent of utility for this study. Average heterozygosity standard error of all markers analyzed for Beagles was 0.574 0.024, for Vizslas was 0.569 0.026, for ESS was 0.425 0.030, and for GSMD was 0.341 0.030. As expected, the average heterozygosity varied by breed and markers that were informative in one breed were not necessarily so in another breed. Out of all tested microsatellites, there were greater numbers of low heterozygosity (cut-off of 0.3) markers in the GSMD (27) and ESS (20) compared to the Vizslas (9) and Beagles (6), resulting in the lower average Table 1 Vizsla two-point linkage anal (...truncated)