Identification of a novel idiopathic epilepsy locus in Belgian Shepherd dogs

Abstract

Epilepsy is the most common neurological disorder in dogs, with an incidence ranging from 0.5% to up to 20% in particular breeds. Canine epilepsy can be etiologically defined as idiopathic or symptomatic. Epileptic seizures may be classified as focal with or without secondary generalization, or as primary generalized. Nine genes have been identified for symptomatic (storage diseases) and one for idiopathic epilepsy in different breeds. However, the genetic background of common canine epilepsies remains unknown. We have studied the clinical and genetic background of epilepsy in Belgian Shepherds. We collected 159 cases and 148 controls and confirmed the presence of epilepsy through epilepsy questionnaires and clinical examinations. The MRI was normal while interictal EEG revealed abnormalities and variable foci in the clinically examined affected dogs. A genome-wide association study using Affymetrix 50K SNP arrays in 40 cases and 44 controls mapped the epilepsy locus on CFA37, which was replicated in an independent cohort (81 cases and 88 controls; combined p = 9.70×10⁻¹⁰, OR = 3.3). Fine mapping study defined a ∼1 Mb region including 12 genes of which none are known epilepsy genes or encode ion channels. Exonic sequencing was performed for two candidate genes, KLF7 and ADAM23. No variation was found in KLF7 but a highly-associated non-synonymous variant, G1203A (R387H) was present in the ADAM23 gene (p = 3.7×10⁻⁸, OR = 3.9 for homozygosity). Homozygosity for a two-SNP haplotype within the ADAM23 gene conferred the highest risk for epilepsy (p = 6.28×10⁻¹¹, OR = 7.4). ADAM23 interacts with known epilepsy proteins LGI1 and LGI2. However, our data suggests that the ADAM23 variant is a polymorphism and we have initiated a targeted re-sequencing study across the locus to identify the causative mutation. It would establish the affected breed as a novel therapeutic model, help to develop a DNA test for breeding purposes and introduce a novel candidate gene for human idiopathic epilepsies.

Figure 1. An example of interictal EEG recording for an epileptic (A) and for a healthy (B) BS dog.

The epileptic and healthy dogs correspond to dogs 26 and 4C in Table 1, respectively. The Epileptic dog shows spike and slow waves in right central and posterior derivations. The control dog exhibits a high-voltage low-frequency background activity. Background activity is superimposed with focal beta bursts in frontal derivation. The EEG pattern is consistent with the sedation protocol used. Bipolar montage, time constant = 0.3 s; high filter 70 Hz; notch filter inserted.

Figure 2. A genome-wide association study reveals a locus at CFA37.

Genomic control –adjusted p-values are shown (A). Fine-mapping of a 8.3 Mb region with 96 additional SNPs on chromosome 37 defines a 1 Mb associated region (B). The region showing strongest association with epilepsy (17,525,804–18,623,591 bp) contains 12 genes including two neuronal candidates, ADAM23 and KLF7 (C).