Background: Variation in the ABCB1 gene is believed to play a role in drug resistance in epilepsy. Hypothesis/Objectives: Variation in the ABCB1 gene encoding the permeability-glycoprotein could have an influence on phenobarbital (PB) resistance, which occurs with high frequency in idiopathic epileptic Border Collies (BCs). Animals: Two hundred and thirty-six client-owned BCs from Switzerland and Germany including 25 with idiopathic epilepsy, of which 13 were resistant to PB treatment. Methods: Prospective and retrospective case-control study. Data were collected retrospectively regarding disease status, antiepileptic drug (AED) therapy, and drug responsiveness. The frequency of a known mutation in theABCB1 gene (4 base-pair deletion in the ABCB1 gene [c.296_299del]) was determined in all BCs. Additionally, the ABCB1 coding exons and flanking sequences were completely sequenced to search for additional variation in 41 BCs. Association analyses were performed in 2 case-control studies: idiopathic epileptic and control BCs and PB-responsive and resistant idiopathic epileptic BCs. Results: One of 236 BCs (0.4%) was heterozygous for the mutation in the ABCB1 gene (c.296_299del). A total of 23 vari- ations were identified in the ABCB1 gene: 4 in exons and 19 in introns. The G-allele of the c.-6-180T4G variation in intron 1 was significantly more frequent in epileptic BCs resistant to PB treatment than in epileptic BCs responsive to PB treatment (Praw 5.0025). Conclusions and Clinical Importance: A variation in intron 1 of the ABCB1 gene is associated with drug responsiveness in BCs. This might indicate that regulatory mutations affecting the expression level of ABCB1 could exist, which may influence the reaction of a dog to AEDs.
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